摘要:

PARAMAGNETIC RESONANCE OF FREE RADICALS BY G. E. PAKE,* S. I. WEISSMANN AND J. TOWNSEND Depts. of Physics and Chemistry, Washington University, St. Louis, Missouri, U.S.A. Received 24th January, 1955 Paramagnetic resonance spectra of free radicals are discussed, with emphasis upon observed hyperfine patterns. A brief description of a paramagnetic resonance spectro- meter is given, and examples of applications to problems of physical, chemical, and biological interest are presented. The polyatomic free radicals may be contrasted in their paramagnetic resonance properties to the ions of the iron and rare-earth groups of transition elements. The free radicals possess spectroscopic splitting factors which usually differ only in the third decimal place from the free electron value, 2.0023, whereas the much stronger spin-orbit coupling which characterizes the transition elements may lead to values from one to about six.Both paramagnetic types exhibit exchange coup- ling which may render magnetic dipolar interactions between neighbouring ions less effective,l but this phenomenon is generally more striking for the polyatomic radicals in that resonance widths of as little as a few oersteds (10 Mc/s) or less for crystalline samples are not uncommon. Even the narrower lines of undiluted samples of the transition elements often have widths measured in hundreds of oersteds. The likelihood of a relatively sharp line quite near g = 2.0023 for each poly- atomic free radical does not promise to make its study either very interesting or very fruitful. However, the strong exchange interaction often existing between neighbouring free radicals in an undiluted crystal or in a concentrated liquid solution reflects the extension of the unpaired electron wave function widely enough over space to provide regions of appreciable overlap between neighbouring mole- cules. Although the exchange interaction obscures possible intcresting line structure in more condensed samples, the spatial extent of the wavc function assures at least some magnetic coupling between the electron spin and many of the nuclear spins which may bc present in the polyatomic structure.The result, for sufficiently dilute samples, is a richness of hyperfine structure which for many free radicals has thus far proved to be overwhelming. Not only is this hyperfinc structure of some intrinsic interest, but it also plays an important role in a number of applications of the technique.The free radicals studied by paramagnetic resonance techniques are often extremely stable, in that lifetimes of the unpaired electron spin state may be in- definitely long, or at least a few hours. Others may rcquire certain precautions, such as protection from contact with oxygen or maintenancc of a certain solution pH, to achieve adequately long life. There are, of coursc, short-lived free radicals, in which there is usually considcrable interest, but they are less accessible to study for two reasons growing out of their rapid decay : first, it evidently becomes more difficult to maintain in the apparatus a significant number of free radicals (the spectrometer at Washington University requires at least 10-11 mole of a material with line width 1 oersted in order for resonance to be barely detectable at room temperature) and, second, the short lifetime may so broaden the energy levels of the electron spin that the resonance line is in turn broadened and the maximum absorption at the line centre correspondingly weakened.* on leave at Stanford University, Stanford, California, during 1954-55. 147148 FREE RADICALS EXPERIMENTAL PROCEDURE The paramagnetic resonance condition is, for a simple absorption line, Itv = &H, (1) where p = ea/2mc is the Bohr magneton, H is the applied magnetic ficld, and v the frequency. For the free electron ( g = 2.0023) resonance occurs at about 2-80 Mc/s per oersted of applied field.Thus laboratory magnetic fields of a few thousand oersteds throw paramagnetic resonance spectra into the centimetre wave region; frequencies near 10,OOO Mc/s or 25,000 Mcls are commonly used. Because it is difficult, or at best inconvenient, to sweep such microwave radiation sources and circuit components over appreciable frequency ranges, paramagnetic resonance is aImost always observed by passing the externally applied magnetic field through resonance. FIG. 1 .--Block diagram of the paramagnetic resonance spectrometer designed by Townsend. As an illustration, fig. 1 presents a block diagram of the experimental arrange- ment developed by one of US (J. T.). The transmission-type resonant cavity is excited in the TE 012 mode by a 723 A/B klystron tube operating near 9000 Mcls.Placed between the radiation source and the cavity is a " gyrator ",2 which employs the Faraday rotation by its ferrite element to isolate the klystron from cavity reflections. No further regulation of the klystron frequency proved nccessary after the klystron was thermally lagged and placed in a ferromagnetic shield. Modulation of the magnetic ficld with amplitude small compared to the line width provides response of the output meter to the derivative of the absorption line shape as the magpetic field is swept through resonance. This procedure permits use of a phase-sensitive detector, which effectively narrows the bandwidth and thus accepts a smaller portion of the noise spectrum, in order to increase sensitivity .It should perhaps be explicitly noted that the spectra to be presented in this paper are all derivativcs (with respect to the magnetic field as it is varied at fixed microwave frequency) of the absorption curve. Some practice may be required to identify individual lines in complicated spectra, such as that of fig. 6, but this inconvenience is offset by the greater sensitivity of the first derivative than of theG . E . PAKE, S . I . WEISSMANN A N D J . TOWNSEND 149 absorption curve itself to partially resolved splittings. Relative intensity measure- ments, which rcquire two integrations, are of course less readily obtained from the derivative. The sensitive element for r.f. detection is a bolometer (Sperry 821 barretcr), which is, for thc Washington University spectrometer, superior to a crystal in signal-to-noisc characteristics because the modulation frequency of thc magnetic ficld (90 c/s) falls in a region of considerable crystal noise.Increasing thc modulation frequency introduces more pick-up originating with thc magnetic forces of the constant field on the eddy currents induced in the cavity walls by the alternating field component. With sufficient effort at reducing these eddy currents, such as the use of non-conducting waveguides with plated interior surfaces, it may well be possible to take advantage of the bctter signal-to- noise characteristics of crystals at higher frequencies. HYPERPINE SPLITTING IN FREE RADICAL RESONANCES If a frce radical sample containing one nuclear moment of spin I is placed in a magnetic field large compared to thc magnetic field of the nucleus at the un- paired electron, a splitting of the electron resonance into 21 + 1 hyperfine components, evenly spaced in first order, will occur. The corresponding modification of eqn.(1) is 3 (2) where nil may take on values - I, - I t - 1, . . ., I. In general, both g and K depend upon the direction of the external field relative to cither crystalline or molecular axes. However, the very small spin-orbit coupling of the polyatomic free radicals may place their spectroscopic splitting factor so close to 2.0023 that angular dependence of the difference is lost within the relatively narrow line width. hv = g/3H -t- KmI, FIG. 2.-Hyperfine triplet arising from the nitrogen 14 nucleus in the free radical ion ON(S03)22-.Furthermore, the spectra prcsentcd here are derived from liquid samples, in which the moleculcs usually are tumbling at a sufficient rate to average away4 the anisotropic component of K and to present a similar kind of average of g over the tumbling motion. Thus the simplified interpretation of eqn. (2) is often justifiable for free radical resonance spectra. Whether the 21 -1- 1 hyperfine lilies will be experimentally resolvable depends upon several factors : (i) the exchange interaction and the magnetic dipole-dipole coupling must both be small compared to the hyperfine interaction K, a condition which can always be achieved upon sufficient dilution of the sample (which may,150 FREE RADICALS however, be inconvenient, or result in too few spins for detection, or lead to a sample of less scientific interest); (ii) both the natural lifetime of the electron’s magnetic spin state (denoted T I ) and the lifetime of the free radical must be longer than the inverse of the hyperfine splitting expressed as a frequency; (iii) the amplitude of the r.f.magnetic field in oersteds must be small compared to the hyperfine interval expressed in oersteds ; and (iv) the inhomogeneity of the external field over the sample must be smaller than the hyperfine interval in oersteds. Fig. 2 presents a simple example of such a pattern of hyperfine lines. The centres of the outermost lines are separated by 26 oersteds (corresponding to 73 Mc/s) and the pattern centre occurs near 3200 oersteds for the 9ooo Mc/s radiation frequency.The substance from which the spectrum is obtained is the free radical ion, ON(S03)p, in 0.001 M chloroform solution. The nitrogen 14 nucleus, with I - 1, is responsible for the structure, no other abundant nucleus in the ion having a nuclear moment. FIG. 3.-Hypefine structure of the paramagnetic resonance of the negatively charged m-dinitrobenzene ion in liquid solution. Other hyperfine splittings are lcss simple. Numerous aromatic compounds, when treated with a strong reducing agent, such as sodium, in the presence of certain solvents, accept an electron and become negative free radicals. An example is rn-dinitrobenzene so treated in 1 : 2-dimethoxyethane. At adequately low concentrations one observes the spectrum of fig. 3, covering about 25 oersteds. Similarly treated, nitrobenzene and trinitrobenzene give, respectively, spectra of at least 10 peaks and of 8 peaks (but differently distributed in spacing and intensity from fig.3). Naphthalene (fig. 9) and anthracene negative ions also exhibit complicated hyper- fine patterns. Evidently hydrogen nuclei are involved ; however, detailed explanations for these spectra have not been made. Fig. 4 shows the hyperfine spitting of sodium trimesityl boron in tetrahydro- furan, the unpaired electron spin belonging to the ion, - - IG . E. P A K E , S. I , WEISSMANN AND J . TOWNSEND 151 Here the four peaks arise from B11 with I = 3/2. Thc less abundant isotope BlO has spin 3, but such a small moment that no splitting is observable. The interval between peaks is about 14 oersteds. Fig.5 presents a portion of the absorption curve derivative for Wurster’s blue ion, [ (CH3)z N<>N (CHdz]+ At least 39 lines have been resolved in this spectrum, consisting of 13 triplets with about 7.4 oersteds between triplet centres and 2-1 oerstcds between members of each triplet. This spectrum, observed in aqueous solution of the perchlorate, FIG. 4.-Paramagnetic resonancelof theitrimesityliboron negative ion in liquid solution. ( D * ... a - 9 , . 7: FIG. 5.-A portion of the paramagnetic resonance spectrum of Wurster’s blue ion, [(CH~)~N(C~H~)N(CH~)Z]+. The total spectrum contains at least thirteen triplets and covers about 100 oersteds. is unaltered in D20. The spectrum does change, howwcr, if the solvent is tetrahydrofuran ; upon addition of a few per cent.of water the pattern has again become completely water-like. Since D20 had no effect, this phenomenon must reflect a distortion of the free radical molecule by a nearby solvent molecule rather than direct interaction between solvent nuclear moments and the unpaired electron. Efforts to disentangle by chemical substitution the complicatcd pattern of fig. 5 have been made by Weissman,6 who has preparcd, among others, the following relatives of thelwurster’s blue ion and observed the hyperfine spectrum : [ CHJH N-N HCH3]’152 FREE RADICALS The spectra vary considerably, as exhibited in fig. 6. A difficulty in analyzing such spectra is that, even with large signal-to-noise in the centre of the line, one may not detect a numbcr of the weakest outermost lines. Thus there is often uncertainty as to how many lines actually constitute the spectrum. These fcw cxamplcs indicate, on one hand, thc simplicity of the pattern if but one nuclear moment is involved and, on the other hand, thc richness of structural detail which can arise if a number of nuclear moments contribute to thc hyperfine splitting.FIG. 6.-l?aramagnetic resonances of three relatives of the Wurster’s blue ion : top : spectrum of [CH3HN(C6H41NHC&]+ ; centre : spectrum of [(CH&N(C6H4)NH2] + ; bottom : spectrum of [(CH&N(CGH~)ND~] +. It was pointed out earlier that, for most liquid solutions at room temperatures, the rate of tumbling of free radical molecules generally averages to zero the anisotropic part of thc hyperfine interaction and only the isotropic part of the splitting is observed.This part of the hyperfine interaction is proportional to the probability density I $(O) 12 of the unpaired electron at the nucleus. This concept of motional narrowing, which is derived in part from analogy with the narrowing of nuclear resonance lines, is checked by additional measurements of Weissman and Banfill7 on the ion responsible for the simple triplet of fig. 3. They prepared a solid solution of 0-25 mole per cent. K2(SO&NO in a single crystal of the diamagnetic salt K2(SO&NOH. A maximum splitting slightly greater than twice the splitting in the liquid was observed for one orientation, and a minimum splitting about half as great as that of the liquid occurred for another orientation.At most other orientations as many as five lines were ob- served, presumably because of different molecular orientations in the unit cell. Evidently observation of the pattcrn in a sufficiently mobile liquid provides considerable simplification of the spectrum as well as, in principle, direct in- formation on I $(O) 12 for the electron at the various nuclear moments. Fortun- gtely these simpler spectra occur for the solutions easier to prepare, liquids.G . E. PAKE, S . I . WEISSMANN A N D J . TOWNSEND 153 The remainder of this discussion will mention a number of potential applica- tions of free radical resonances, and in particular their hyperfine splittings, to problems of physical, chemical and even biological interest. SPINS AND MOMENTS OF RADIONUCLEI The burden here falls upon the chemist, for, if he can place a radionucleus in a suitable free radical, the physicist may find a paramagnetic rcsonance and associated hyperfine splitting from which he can measure both the spin and nuclear magnetic moment.Thc former requires only the counting of lines, and the magnitude of splitting may be compared with that produced by a stable iso- tope of the same element in order to determine the moment to perhaps two or three significant figures. The potential advantages of the technique over a direct measurement of nuclear spin resonance are obvious. First, advantage is taken of the electron’s larger magnetic dipole matrix element to increase sensitivity, and second, the necessity for prolonged searching in frequency is eliminated by the occurrence of free radical resonances very near g = 2.FIG. 7.-Spectrum of the beryllium benzoyl acetonate negative ion, showing the four components from Beg. Efforts not thus far successful have been made at Washington University to find a free radical which might provide the spin and momcnt of Be7. In addition to requirements of adequately long free radical life, there must of course be suitable chemical yield from the initial supply of the radio-isotope. Fig. 7 shows that the hyperfine quartet arising from stable Be9 ( I = 3/2) has at lcast been observed in the negatively charged beryllium benzoyl acetonate ion. INFORMATION ON THE LOCATION OF THE ELECTRON IN TRIPHENYL METHYL The free radical dcrived from CH3 by substitution of three phenyl groups is relatively stablc.When its spectrum is observed in a 10-3 M hexaphenylcthane solution, the resonance is a single peak about 5 oerstcds wide.* No hypedine splitting from the methyl carbon is ordinarily expected, since carbon 12 is an “ cvcn-even ” isotope having zero nuclear spin. However, Weissman and Sowden 8 prepared a solution with 53 atom per cent. carbon 13 (nuclear spin +) in the methyl position. The resulting spectrum (fig. 8) is then a triplet consisting of a superposed doublet and singlet, the central singlet arising from thc 47 % of the ions with carbon 12 in the methyl position. The interval between the * In very homogeneous magnetic fields this spectrum has been observed by Jarrett 9 to contain nearly 100 closely spaced lines.154 FREE RADICALS doublet lines from carbon 13 is about 22 oersteds arid integration of the curve gives intensities consistcnt with the relative abundances of carbon 12 and carbon 13 in the methyl position.This indicates an appreciable probability density for the unpaired electron at the carbon nucleus. An interesting comparison is afforded by the isoelectronic free radical ion (C&T5)3N-I. The nitrogen 14 moment is somewhat smaller than that of carbon 13, but a triplet resulting from the unit spin would be resolved if thc unpaired electron spcnt a comparable fraction of its time near the nitrogen nucleus. No hyperfine splitting is obscrvcd, howevcr, and the unpaired electron of this free radical apparently has a greater preference for the phenyl groups. FIG. &-Paramagnetic resonance from triphenyl methyl with 53 atom per cent of carbon 13 in the methyl position.Recently a solid solution of in (C6Hs)sP has been prepared, The spectra from polycrystalline samples consist of, in addition to the central line of carbon 12, broad shoulders arising from the two carbon 13 satellites which are spread over a range of some 70 oersteds. Thus an anisotropic component of the hyperfine splitting is present which exceeds the 22-oersted isotropic splitting from liquids, in general agreement with the notion of many chemists that the unpaired electron orbit contains mostly p character. STUDY OF ELECTRON TRANSFER RATES A recent application of paramagnetic resonance hyperfine structure has been made by Ward and Weissmanlo to the study of electron transfer between naphthalene negative ion and naphthalene.As mentioned earlier, a number of aromatic structures, of which naphthalene is one, form free radicals with one negativc charge when treated with alkali metals in certajn solvents. The upper resonance curve in fig. 9 shows the spectrum of 5 x 10 -4 M naphthalene negative ion (Cl()H8-) in tetrahydrofuran, in which at least twentyeight lines are resolved. The lower spectrum of fig. 9 is obtained with the same concentration of CloH8- but with the solution also containing 0.35 M neutral CloH8. When neutral naphthalene is added in larger concentrations the individual lines broaden further and the entire spectrum becomes a single peak with broad tails extending beyond the region of the original spectrum. This is interpreted as a broadening, in consequence of the uncertainty principle, of the energy levels of the electron spin states through the life-time limitation resulting from increasingly frequent electron transfer.The broadening may be observed by direct measurement ofG . E . PAKE, S . I . WEJSSMANN A N D J . TOWNSEND 155 the component line width in fig. 9, by the merging of close componcnts, and by the decrease in amplitudc of the spectrum. The experimental data indicate that the mcan lifetime of a naphthalcnc negativc ion in the prescncc of 0.8 M naphthalene is 1-2 x 10-6 sec, and the rate constant is readily calculated, undcr assumption of a second-order rate law, tolbcllO x 1C6 1. mole--1 sec-1 at 30" C . FIG. 9. furan. ,-Upper: spectrum of 5 x lO-4M naphthalene negative ion in tetrahydro- Lower : spectrum of 5 x 10-4 M naphthalene negative ion in the presence of 0.35 M neutral naphthalene, illustrating the effect of electron transfer.The method is of interest in that, unlike most rate determinations, it does not depend upon some method of distinguishing reactants from products. The mean life is directly measured. DISSOCIATION CONSTANTS OF MANGANESE COMPLEXES Although the paramagnetic properties of manganese arc associatcd with a partially filled inner shell of electrons, it will not be out of place in this discussion of the application of free radical resonance to mention the mcasurement of dis- sociation constants of manganese, since thc mcthod has general applicability to paramagnetic species. The measurement depends upon the expectation that the hyperfinc interval, which in liquids reflects the unpaired electron density at the nucleus, should be different for different complcxes of the paramagnetic ion.The example cited is a somewhat degenerate case, in that nonc of the complexes has thus far provided a detectable resonance. The mcthod hcre depends entirely upon the fact that the Mn2+ ion gives a strong and casily resolved spcctruin of six lincs. In tests on controllcd solutions of the ion, the amplitude betwcen maxima and minima of the derivativc of a Mn2-k hypcrfine component was directly proportional to concentration bclow 10-2 M (which indicates that the width and shape of each component is independent of concentration). By obscrving the decrease in the Mn2-k spectra1 intensity as the complexing agent is increased in initial concentration, one can determine the dissociation constant of thc complex.In this way Dr. Mildred Cohn of the Washington University School of Medicine, Department of Biological Chemistry, has measured thc dissociation constants of manganese complexes with such substances as glucose monophosphates, malonic acid, glycylglycine, and histidine, compounds which may not often be the subject of discussion among physicists but which illustrate the scope of the applications of their paramagnetic resonance technique. Similar considerations indicate promise for paramagnetic resonance in disentangling the valence states of vanadium in solution.12156 FREE RADICALS FREE RADICALS IN BIOLOGICAL MATERIALS The last potentiality of free radical paramagnetic resonance to bc discussed here lies even farther afield from physics.Free radicals have for some tinle been considered by the biological scientist to play a significant part in a number of processes of interest to him, among thcm (i) biological oxidations and reductions,l3 (ii) the action of ionizing, visible, and ultra-violet radiation on biological systeiiis,14 and (iii) the chemical induction of abnormal growth.15 If these hypotheses are correct, one expects free radicals to be present in living systems. However, their presence has only been inferred from indirect evidence or model experiments. It is natural then to raise the question whether free radicals can be detected directly in biological systems by means of paramagnetic resonance.To answer this question, one would like simply to insert the biological material into the resonant cavity. Unfortunately, the dielectric properties of water, always present in appreciable proportions in such material, lead to a large non-resonant absorption of microwave energy, and full sensitivity of the spectrometer is not reached, 0 c 0 L U w Y V - I a, Y c x R ~ I I n c r e m e n t i n moq field Gauss FIG. 10.-Paramagnetic resonance from digitalis seedlings. R ~ I . increment i n mag. f i e l d . GO US^ FIG. 11 .-Resonances from barley leaves which have been blanched by exclusion of sunlight and then illuminated by 100 foot candles of fluorescent lamp radiation for various periods of time. The resonance from a normal sunlight- grown leaf is also shown.In the experiments made by Prof. B. Commoner and co-workers 16 at Washington University, the material is lyophilized and placed in the spectrometer cavity. Numerous tissues were found to possess detectable free radical content. It is notable that relatively higher free radical content is found in metabolically active tissues, such as green leaf, liver, and kidney. Melanin, which is responsible for the dark pigmentation of squid ink, frog eggs, and a number of other tissues, gives a paramagnetic resonance slightly narrower than 10 oersteds and is apparently a very stable free radical. It has been shown that melanin formation in animals may be induced by ultra-violet and ionizing radiation.17G . E . PAKE, S . I . WEISSMANN AND J. TOWNSEND I57 Somewhat more spectacular correlation of free radical content with important biological processes is evidenced by fig.10, which shows spectra obtained from ungerminated and germinated digitalis seedlings. No resonance was detectable before germination ; the observable resonance is taken from seedlings just after emergence of the primary root. Normal green barley leaves exhibit a resonance about 5 oersteds wide. If such leaves are blanched by the exclusion of sunlight, the resonance intensity drops to perhaps 20 % of the normal value. When these blanched leaves are exposed to 100 foot candles of illumination from fluorescent lights, the resonance intensity grows after 6 h of illumination to about 50 % of the normal value. After 24 h, during which time the leaves become fully green, the resonance has returned to the intensity characteristic of the normal light-grown leaves.Fig. 11 presents the experimental curves illustrating this effect. The occurrence of paramagnetic resonance in melanin suggested examination of similar polymeric products of biological origin : gum guaicum, licorice, humin, various plant resins, caramelized glucose, charcoals of organic origin, various types of coal and tars. All of these products exhibit paramagnetic resonance and the free radicals seem to be extremely stable. These substances possess in common condensed polymeric ring structures. Paul 18 has shown that electron affinity of such ring structures as benzene, naphthalene, and anthracene increases ap- preciably with the size of the molecule.In relation to theories that free radical mechanisms may explain abnormal growth, Lipkin and Paul 5 have suggested that the carcinogenic activity of certain larger ring structures is perhaps related to their ability to form negative free radicals with mild reducing agents, whereas non-carcinogenic hydrocarbons, such as naphthalene, require very strong reducing agents. Thus free radical content has been correlated with a number of biological processes of fundamental interest. The experiments performed to date are of a most preliminary nature, and they do not offer an explanation of these phenomena. Perhaps the most one can state at the present stage is that free radicals are im- plicated only on the basis of a ‘‘ guilt by association ” type of argument ; proof or disproof, therefore, rests on further extensive experimentation. It can perhaps be safely stated that paramagnetic resonance of free radicals provides a new and powerful technique for such future investigations.It has been the aim of this discussion to survey some of the results of research on paramagnetic resonance of free radicals at Washington University. In addition to the perhaps expected implications for the physicist in the study of magnetic interactions between electrons and nuclei and for the chemist in the study of molecular structure, these preliminary researches indicate that the technique also holds promise for investigation of reaction rates, solvent effects in solutions, and problems of biochemical and biological importance. The authors take pleasure in acknowledging numerous stimulating and highly educational discussions with many of their colleagues throughout the science departments of the University, particularly Prof. B. Commoner, L. Helmholtz, D. Lipkin and H. PrimakoR. 1 Van Vleck, Physic Rev., 1948,74, 1168. 2 Hogan, Rev. Mod. Physics, 1953, 25, 253. 3 Bleaney, Phil. Mag., 1951, 42, 441. 4 Weissman, J. Chem. Physics, 1954, 22, 1378. 5 Lipkin, Paul, Townsend and Weissman, Science, 1953, 117, 534. 6 Weissman, J. Chem. Physics, 1954, 22, 1135. 7 Weissman and Banfill, J. Amer. Chem. Suc., 1953, 75, 2534. 8 Weissman and Sowden, J. Amer, Chem. SOC., 1953, 75, 503.158 X-IRRADIATED PLASTICS 9 Jarrett and Sloan, J. Chem. Physics, 1954, 22, 1783. 10 Ward and Weissman, J. Amer. Chem. SOC., 1954, 76, 3612. 11 Cohn and Townsend, Nature, 173, 1990. 12 Pake and Sands, Bull. Arner. Physic. Soc., 1954, 8, 18. 13 Michealis, The Enzymes (ed. Sumner and Myrbach, Academic Press, New York, 14 Weiss, Nature, 1946, 157, 584. 15 Brues and Baron, Annual Rev. Biochem. (Annual Reviews, Stanford, California, 16 Commoner, Townsend and Pake, Nature, 1954, 174, 689. 17 McIwain, Nature, 1946, 158, 898. 18 Paul, Thesis (Washington University, St. Louis, 1953). 1951), vol. 2, chap. 44. 1951), p. 343.

ISSN:0366-9033    

DOI:10.1039/DF9551900147

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

158 X-IRRADIATED PLASTICS PARNMAGmTIC RESONANCE IN X-IRRADIATED PLASTICS AND IN PLASTIC SOLUTIONS OF FREE RADICALS BY E. E. SCHNEIDER King’s College, University of Durham, Newcastle-upon-Tyne Received 1 1 fh February, 1955 Paramagnetic resonance, occurring as a result of X-irradiation (105-107 r) in plastics such as polymethyl methacrylate, polystyrene, polyethylene, and polytetrafluoroethylene, has been investigated at 3 cm wavelength. A dominant triplet structure of resonance lines is observed in all cases, consisting of a strong central line and two symmetrical satellites. In irradiated polymethyl methacrylate, where the resonance for a given X-ray dosage is very inuch stronger than in other plastics, this triplet structure is well resolved and superimposed on a very symmetrical pattern of at least another six lines.It is suggested that the unpaired electrons responsible for the resonance are created as a final result of the X-irradiation by a breaking of C-C bonds in the polymer chain and are essentially localized in p-orbitals on C atoms. On this basis the complex structure of the spectra is explained as a hyperfine splitting arising from the interaction of the localized p-electron with protons or fluorine nuclei in its immediate neighbourhood. Resonance experiments on DPPH are reported, in which plastics are used as solid solvents. The results are shown to lend indirect support to the ideas of localization of electrons on the plastic polymer chains, Magnetic centres giving rise to paramagnetic resonance absorption can be produced in plastics by ionizing radiation or by incorporating paramagnetic molecules as impurities.The peculiar interest of plastics as a medium for magnetic resonance studies lies in their rigid and yet irregular molecular structure which makes the dominant magnetic interactions quite different from those in liquids or crystalline solids. The major part of this paper is devoted to a discussion of the paramagnetic resonance spectra of several X-irradiated plastics, namely, polymethyl methacrylate (Perspex or Lucite), polystyrene, polyethylene and poly- tetrafluoroethylene (Fluon or Teflon). The experimental work at 3 cm wavelength are a continuation of earlier work by Schneider, Day and Stein 1 (SDS). Reson- ance studies at 1 cm of X-irradiated Teflon have been reported by Schneider.2 Resonance of y-irradiated plastics have also been reported by Combrisson and Uebersfeld.3~ 4 A brief discussion of the resonance of Perspex and polystyrene containing the diphenylpicrylhydrazyl (DPPH) in solid solution, (based on experi- ments carried out some years ago but so far unpublished), is included to illuminate ome general aspects of resonance in the plastic state.E .E . SCHNEIDER 159 EXPERIMENTAL The resonances were investigated at 9,500 Mc/s (3.16 cm wavelength) with a micro- wave bridge-type apparatus whose essential features have been described previously.5 Improvements in sensitivity have been achieved particularly by the inclusion of a stabil- ization system of the signal klystron referred to the measuring cavity. The plastic specimens in the form of discs, 1-3 mni thick, 10-25 mm diam., were irradiated in a Maximar X-ray therapy set (200 kV, 15 mA, beryllium window) at the bottom of the " displacement cone " (dose rate 8000 r/min) or at a position 10 cm up the cone (2000 rlmin).Small crystals of DPPH served both as " g-markers " and as intensity standards allowing an estimate of the total number of magnetic centres to be obtained from the total area under the resonance absorption curves. In the later experiments proton resonance was used for determining the relative field position of the resonance lines. Most irradiations and measurements were carried out at room temperature. A few irradiations were carried out at liquid nitrogen temperature and the specimen subsequently transferred to the microwave cavity cooled to about 120" I< by thermal conduction through a 1-in.diam. copper rod submerged into liquid nitrogen. The plastic solutions of DPPH were prepared by dissolving both the plastic and the DPPH in benzene and pouring the viscous solutions on a glass plate. The thin films obtained after evaporation of the benzene were folded and compressed into discs for reson- ance investigation.* RESULTS MAGNETIC RESONANCE SPECTRA OF X-IRRADIATED PLASTICS 2 : ~-POLYMETHYL METHACRYLATE Fig. 1 and 2 show the resonance spectra of commercial Perspex at different dosages. Most experiments were carried out with clear uncoloured Perspex. The spectra in coloured Perspex or in specimens polymerized in the laboratory are not significantly different.? The interesting effect of the presence of dyestuffs on the chemical processes involved in the build-up and decay of magnetic centres and optical absorptions reported in SDS and in the optical work of Day and Stein 7 are not directly relevant to the theme of this paper.The centre of all resonance patterns corresponds to a g-factor of 2 . 0 0 3 0 , very close to that of DPPH. A further striking feature is the existence of two main satellite lines at +. 2 2 . 5 gauss from the centre. A closer study of the spectra shows that three further pairs of lines at 11, 34 and 45 gauss are common to all spectra and that the changes in the resonance with concentration is due to a change in the intensity ratio of different lines while the width and shape of individual lines appears to have the same value of - 8 gauss throughout.In experiments in which the specimens were irradiated at liquid nitrogen temperature and resonance was observed subsequently at low temperature without intermediate warming, only a single peak resonance at g = 2-00 and width - 20 gauss showed up instead of the complex spectrum just discussed. The complex spectrum identical with that observed in specimens irradiated at room temperature appeared, however, after warming up to room temperature and remained essentially unchanged both as regards the width and the position of the various lines when later observed at low temperature. 2 : 2-POLYSTYRENE The intensity of the paramagnetic resonance in coinmercial polystyrene is about 100 times smaller than that in Perspex at comparable X-irradiation at room temperature, and experiments are made more difficult by the very much faster decay of the radiation effects at room temperature.Fig. 3 shows the spectrum after 20-11 irradiation (107 r), with intensity 3 X 1015 spins/cm3. It consists of a centralkeak at g = 2.003 with satellites at 5 17 gauss. The width of individual lines is 17 gauss, * A similar technique for the preparation of Wurster's base in Perspex has recently TThe difference in g-factor reported in SDS has not been confirmed in later more been described by Bijl and Rose Innes.6 accurate experiments.160 X-IRRADIATED PLASTICS 2 : %POLYETHYLENE Fig. 4 shows the resonance in irradiated commercial Polythene, in the form of discs cut from the insulator in high frequency cables, which is even weaker than that of poly- styrene but has a similar appearance.The triplet splitting is 15 gauss and the width of individual lines 20 gauss. 5 x 1 0 5 r s p l n s 7 FIG. 2.-Resonance spectra of X-irradiated Perspex at different dosages. B gauss FIG. 3.-Resonance spectrum of X-irradiated polystyrene. I I I I I I I 50 6 0 70 8 0 90 3,400 10 B gauss FIG. 4.-Resonance spectrum of X-irradiated polythene. 2 : 4-POLYTETRAFLUOROETHYLENE This material is very suitable for resonance studies since, due to its low dielectric loss, larger specimens can be used than with Perspex without unduly loading the microwave cavity, and hence larger signal to noise ratio for given concentration of magnetic centres can be obtained. The concentration of magnetic centres at given X-ray dose is about 1/3 of that in Perspex. The characteristic feature of Fluon is the large positive g-shift.Otherwise the appearance of the spectra is similar to that in the other plastics, although due to the large line width of 23 gauss, the triplet structure is hardly resolved. The spectrum at 106 r (1016 centreslcm3) can be compared with that obtained at 1.25 cm wavelength by irradiation with soft X-rays2 The splitting of 19 gauss and the half- width of individual lines agree satisfactorily for the two wavelengths, but there is a small difference between the g-values of the central line 2.021 at 1-25 cm and 2.016 at 3.16 cm. Comparison of the spectra at low and intermediate concentration indicates that changes of intensity ratio of the various lines with concentration occur also in this case.It is believed that the absence of structure at the highest concentration is due to the same cause, namely, an increase of the central line relative to side lines. DISCUSSION INTERPRETATION OF THE RESONANCE SPECTRA GENERAL MODEL OF MAGNETIC CENTRES The most striking feature of the resonances is their structure which in the early work of SDS suggested mutual interaction between regularly spaced magneticE. E. SCHNEIDER 161 centres. Abandoning this idea the structure will now be ascribed to hyperfine iuteraction of individual magnetic centres with their surroundings. The similar- ities of the normal spectra, in particular the appearance of a dominant triplet, would thcn imply that the nature of the magnetic centrcs is the same in all plastics.Thc normal behaviour contrasts sharply with that at low temperature. There the structure is absent indicating that low-temperature irradiation leaves electrons in shallow traps in which they do not interact magnetically with the surroundings. At the same time it must be assumed that in this case the positive holes, i.e. the ionized atoms on the polymer chain from which the trapped electrons have come, becomc stabilized so that the situation at low temperature resembles that in X- irradiated ionic solids where F- and V-centres are formed. The more permanent magnetic centre responsible for the normal complex resonance spectrum, on the other hand, is likely to be in the nature of a radical, i.e. an end- or side-group on the polymer chain associated with an unpaired electron interacting in a specific fashion with nuclear spins in the group.Positive holes are extremely unlikely to survive at room temperature. This is confirmed by the uniform decrease in intensity of the resonance spectrum without change of its shape during the slow decay at room temperatwe of the radiation effects. This decay is due to a reaction with oxygen diffusing in from the surface 1,’ and is thus entirely different from thc thermal bleaching of X-irradiated ionic crystals through recombination of clectrons and positive holes which shows up as a gradual decrease of the colour centre concentration. If positive holes are present after low-temperature irradiation and not present after subsequent warming to room temperature the thermal foimation of the normal magnetic centres must involve the recombination of positive holes with electrons, and it suggests itself that the magnetic centres are formed by a breaking of a chcmical bond through the energy released in this recombination process.The actual mechanism could involve the emission of a photon during recombin- ation of the positive hole and its re-absorption leading to photodissociation of the bond. Recent experiments by Jarrett8 on the production of magnetic resonance spectra in Lucite of the type discussed here by irradiation with ultra- violet light would lend support to this idea. The bonds most likely to be affected are the C--C links of the polymer chains. It is therefore proposed that the magnetic centres in irradiated plastics consist of the electronic systems resulting from an excitation of the main G-C bonds.Schematically all the plastics considered in this paper can be represented by X Y X I l l -GC-C- I l l x z x where CX2 is a CF2 group in Fluon and CH2 in the other plastics and the two bonded groups are identical in the polyethylenes (Z=Y=X). The magnetic centre formed by excitation of the bond would then correspond to the configuration X Y X Y 1 I.? r.1 I I t I 1 4 - c C-C- x z x z and as far as electron spins are concerned could be regarded as a metastable triplet state of the C-C link. It is rcasonable that in this state the newly formed end groups would rearrange themselves into a set-up of bonds resembling that of the monomer with one extra electron localized on the C atoms of each end F1 62 X-IRRADIATED PLASTICS group.While it cannot so easily be predicted what happens at the CZY ends in Perspex and polystyrene, the situation on the CX2 groups would certainly have to be an electron in a p orbital normal to the trigonal arrangement of carbon sp2 orbitals. THE TRIPLET SPLIlTlNG Considering the CX2 cnd of thc magnetic ccntrc, the splitting of the magnetic resonance spectrum into a triplet must be associated with the interaction of the localized clectron with the X nuclei, i.e. with the energy of alignment of the nuclear spins in the magnetic field B,, produced at the X nuclci by the electron on the C atom. Both F and H have I = 112, and the total nuclear quantum numbcr mT = Ern1 can have values & 1, 0, so that the interaction would indeed lead to a splitting into a triplet.However, both the shape of the satellitc lines and the magnitude of the splitting indicate that the simple model for the interaction is not inadequate. For a simple " distant " interaction, B, would be given essentially by the field of a point magnet of magnetic moment equal to 1 Bohr magneton /lX concentrated on the C atom," herc r, = C-X distance, po permeability of vacuum, so that with the magnetic moment Px of the X nuclei the field positions of the triplet of resonance lines would be That means the positions of the sidelines would depend on the orientation 8 of the normal to the CX2 plane with respect to the external magnetic field. For a statistical distribution of orientations of the groups Gaussian lines of half-width w and height h would be drawn out into an asymmetrical shape (see fig.7 bclow) having9 a maximum at A(l - w/3A) of a height 064(w/A)*h. The observed spectra show no evidence of any distortion of line shape and the intensity ratio of satellites to central line is far largcr than the value 0-32(w/A)* expected from the simple theory (note that for Pcrspex, w -*A). There is a further discrepancy between experimental values of the triplet splittings of 15-23 gauss and the values of A of 10.5 and 5.6 gauss derived from the above formula from the known values of 2.8 and 2.6 for the nuclear magnetic moments and of 1-1 and 1.33 8, for the bond lengths of F and H rcspectively. To account for the magnitudc of the splittiiigs and gaussian shape of the satellitc lines it must be assumed that the field at thc X nuclei arises from an unbalanccd x-electron centred on the X nuclei themselves.A partial delocalization of thc p electron could not easily give thc desired effect (in Perspex, 5 % of an unbalanced s-electron would be required) since a mere redistribution of the hybridization of C orbitals would not affect the magnetic balancing on thc X nuclei. It seems more Iikely that the s-unbalance arises from a direct spin-spin interaction between the p-electron and the orbitals on the X atoms. A theoretical estimate of the magnitude of such effccts would be of extreme interest. Whatever the final theoretical explanation, it appears that there exists here a new type of magnetic coupling which may be of wider importancc whenever unpaired electrons interact with non-local nuclei.Since this coupling would affect all thc orbitals of thc X-atoms, it would possibly also explain as a spin orbit coupling the large g-shift observed in the resonance of Fluon, which, in contrast to the small shift in the othcr plastics and in radicals like DPPH, can hardly be accounted for by diamagnetic effects. * the finite extension of the p wave-function leads to a small negative correction not exceedinrz 10 %.E. E . SCHNEIDER 163 ELECTRON INTERACTION AND THE PERSPEX SPECTRUM So far the fact has been disregarded that according to the proposed model the magnetic centre is a bi-radical. Whether or not the presence of interaction between the two electrons has an effect on the resonance spectra should depend on the magnitudc of the exchange interaction energy relative to the resonance energy.If the frequency vo at which resonance is investigated is large compared with the electron exchange frequency, the electrons act essentially independently and the magnetic resonance spectra can be considered as transitions between energy levels charac- terized by electronic quantum numbers M = rt 3 which for hyperfme interaction with a CX2 group are indicated in fig. 6a. At the other extreme, that is when the exchange frequency is large compared with YO, the electrons act like a system of spin 1 with three electronic energy levels M = 0, f 1. The energy level scheme accounting for hyperfine interaction with two equal end groups CX2 would then be that given in fig.6d. The spectrum of transitions AM = f 1, Am, = O in this case would be a quintuplet with lines at f A/2, and f A around the central line. The experimental results in polythene and Fluon where only a triplet spectrum has been observed so far, would point to an cx- / I I I I 1 I I I I 3 0 4 0 3.350 bo 70 80 90 - B gauss 1 I I I I I I I 1 3 0 40 3,350 bo 7 0 80 90 - 8 gauss 1 I I I I I I I ] 30 4 0 3,350 6 0 7 0 80 90 - B qauss FIG. 5.--R esonance spectra of X-irradiated Fluon at different dosages. change interaction small coinpared with 10 kMc/s. The same is likely for polystyrene. The case of Perspex is of spccial interest. At first sight it would appear most natural to regard the complex rcsonancx spectrum as a superposition of two single electron spectra and to associate the dominant triplet with the CH2 end-group and the remaining lines with the other more complex end-group resulting from the excitation of the C-C link.However, the fact that the addition lines lic at 3, # and 2 times the main triplet distancc points to a common origin of the whole spectrum. Thus it will now be assumed that in Perspex, just as in thc other plastics, the magnetic centres are symmetrical, i.e. that the breaking of the C-C link results in some structural rearrangements leaving both bonding electrons localized on C atoms of CH2 groups, but that Perspex is distinguished by a larger exchange frequency. Hence the greater complexity of the Perspex spectrum is ascribed to a situation just intermediate between the extreme cases of small and large electron interaction discussed above.In order to understand how this would lead to the observcd 4 pairs of satellite lines it is useful to consider the cnergy level diagram, fig. 6b, which gives the levels for the combined energy of the two independent electrons in terms of the total electronic quantum number, MT = MI 4- M2. It is entirely cquivalent to that of fig. 6a and the allowed transitions AM= == & 1, Am, == 0 lead to the same triplet of lines at Bo and Bo 3: A . It differs from the energy scheme for large interaction in so f a as there164 X-IRRADIATED PLASTICS is a Set of central levels for MT = 0 corresponding to different mT in place of the single level for M = 0 in fig. 6d. At an intermediate interaction, the exchange effect will lead to a mixing of the central set of levels so that all 5 of them will in part belong to all 5 values of m~ (fig.64. Transitions still governed by the same selection rule Am,= 0 become therefore possible from all M = 0 levels to all kf = 4- 1 or A4 = - 1 levels which gives the spectrum with four pairs of satellites at zk *A, rt A , f #A, f 2A just as observed in Perspex. In the light of this inter- pretation it is very significant that experiments by the author carried out at Duke +2- ‘I1 t2- 0 ) b) c> d) FIG. 6.-Magnetic energy levels in plastic. University at 1.25 cm wavelength (23.7 kMc/s) gave spectra in Perspex in which the “ bi-radical lines ” at f *A and the “ transition ” lines at f SA and f 2A if present at all were too weak to be resolved, indicating that the resonance fre- quency critically determines the nature of the spectrum.At the time the subject was not considered to be of sufficient interest to be followed up. Further work on the frequency dependence of these resonances must now appear to be very re- warding, in particular the extension of the measurements on irradiated Fluon to lower frequencies (larger wavelengths) which may reveal the “ bi-radical ”. transitions. RESONANCE IN PLASTIC SOLUTIONS OF DPPH The magnetic resonance of DPPH in solid solution in Pyrex and polystyrene differs from that in liquid solutions in two important respects. (i) The solid spectra show hyperfine structurc up to the highest concentrations investigated (10 % in Perspex) while in liquid solutions, exchange effects trans- mitted through the solvent rnoleculcs, suppress the hyperfine structure down to small concentrations 10 (in bcnzene down to at least 0.5 ”/,).E .E. SCHNEIDER 165 (ii) The shape and intensity ratio of the hyperfine lines and the magnitude of the hyperfine splitting is diffcrent in solid and liquid solutions. The structure in liquid solutions (see for instance Cohen and Kikuchi 11) can be analysed into 5 components, 10 gauss apart, of gaussian shape, intensity ratio 1 : 2: 3 : 2: 1, compatible with a hyperfine interaction of the radical electron with two nitrogen nuclei. It must be stressed that this interaction can be due only to that part of the electron orbital on the nitrogens which has s-character. The major p-part of the orbital is ineffective since for sufficiently fast molecular motion its magnetic field at the nucleus averages out to zero.The structure of the resonance in solid solutions on the other hand can be decomposed into a gaussian central line and two pairs of asymmetrical satellites (see fig. 7). It is the pattern which would - 8 qwrr FIG. 7.-Resonance spectrum of solid solution of DPPH in Perspex. be expected for a statistical distribution of magnetic systems having the direction- dependent hyperhe interaction of a p-electron. The analysis using the formulae given above leads to an h.f. splitting of 22 gauss. These results are highly relevant to the previous discussions. The inability of the plastic host structure to support long-range exchange effects demonstrates that the electrons are strictly confined to their special positions on the solute molecules just as the electrons in the irradiated plastic have been assumed to be localized on the CX2 end-groups of the magnetic centres. The direction-dependent hyperfine structure due to the radical p-electrons, stabilized in direction with respect lo a fixed plane, confirms the high degree of rigidity which is implied in the ideas on the nature of the radiation induced magnetic centres developed here. The author wishes to express his thanks to Mr. P. A. Forrester for help with some of the resonance measurements, to Dr. F. T. Farmer of the Royal Victoria Infirmary, Newcastle, for making the X-ray set available and to Mr. J. F. Fowler and Mr. M. J. Day for help with the irradiations. 1 Schneider, Day and Stein, Nature, 1951, 168,645. 2 Schneider, J. Chem. Physics, 1955 (in press). 3 Combrisson, Uebersfeld, Compt. rend., 1954, 238, 1397. 4 Uebersfeld, Compt. rend., 1954, 239,240. 5 Schneider and England, Physica, 1951, 17, 221. 6 Bijl and Rose-Innes, Nature, 1955, 175, 82. 7 Day and Stein, Nature, 1951, 168, 643. 8 Jarrett, private communication. 10 Hutchison, Pastor and Kowalsky, J . Chem. Physics, 1952, 20, 534. 11 Kikuchi and Cohen, Physic. Rev., 1954,93, 394. 9 unpublished calculations by the author.

ISSN:0366-9033    

DOI:10.1039/DF9551900158

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

PARAMAGNETIC RESONANCE STUDIES OF ATOMIC HYDROGEN PRODUCED BY IONIZING RADIATION * BY RALPH LIVINGSTON, HENRY ZELDES AND ELLISON H. TAYLOR Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A. Received 18th January, 1955 Free radicals produced in various substances by C060 gamma rays have been observed by the paramagnetic resonance method. A sufficient concentration has been obtained by performing the irradiation and observation at liquid nitrogen temperature, where the radicals are stably trapped. One pair of lines observed in irradiated HzSO4, HC1O4 and H3P04 has been identified as arising from atomic hydrogen as shown by deuterium substitution experiments and a consideration of the strength of the hypexfine interaction. Atomic hydrogen is also formed from water adsorbed on glass surfaces.Atomic hydrogen was not found in irradiated ice. The presence of additional, weak paramagnetic resonance lines gives information on the environment of the atomic hydrogen, while warming experi- ments give rate data that indicate second order kinetics for the disappearance of atomic hydrogen. Free radicals have been postulated as intermediates in almost all reactions induced by ionizing radiation. The evidence for their participation has been indirect, and particularly unsatisfactory in reactions in condensed phases, con- sisting largely of agreement of kinetics deduced from proposed free-radical mechanisms with the observed kinetics. The existence of radicals in gaseous thermal and photochemical reactions has been demonstrated more directly, for instance, by optical absorption spectroscopy and by mass spectroscopy.The present work was undertaken in order to try to use paramagnetic resonance absorption to identify some of the free-radical species believed to be important in the radiation chemistry of condensed systems. This offers the possibility of studying free radical intermediates in a rather direct manner since the phenomenon depends primarily on the microscopic properties of the unpaired electron that characterizes a paramagnetic substance such as a free radical. The method chosen for the first approach was to conduct the irradiation and the observation of paramagnctisrn at temperatures low enough to prevent the free diffusion and interaction of such species, and thus to permit the accumulation of reasonable concentrations of the frec radicals.It is naturally hoped that the technique can be extended to observe free radicals in more mobile systems during irradiation. Pararnagnctic resonance absorption lines have been observed in a large number of substances after irradiation at 77" K with gamma rays from C060, indicating the presence of numerous interesting free-radical species. The transitory nature of these species is shown by the disappearance of the lines as the samples are warmed, and accompanying the reaction of disappearance there are often glow and colour bleaching phenomena similar to those that have been observed in irradiated ionic crystals. Only a few of the paramagnetic lines observed in various substances have been firmly.identified with free radicals, and it is possible that some of thcm may be associated with trapped charges similar to the colour centres in alkali halide crystals. This paper is devoted to the information which has been obtained about one particular radical species, which was revealed early in this work by a characteristic pair of fairly sharp lines, and which was identified as atomic hydrogen.1 * This work was performed for the United States Atomic Energy Commission.166R . LIVINGSTON, H. ZELDES A N D E . H. TAYLOR 167 EXPERIMENTAL The paramagnetic resonance spectrometer had a rectangular microwave transmission cavity excited in the TE102 mode. All work, except where otherwise noted, was carried out at 23,000 Mc/s with a nominal magnetic field of 8000 gauss and an inhomogeneity of less than 0.1 gauss over the sample.The magnet was sinusoidally modulated at 60 c/s and the absorption lines were displayed directly on an oscilloscope. The magnetic field was measured by the proton magnetic resonance method, the circuit being arranged to superimpose the proton resonance signal on the paramagnetic resonance line on the oscillo- scope. Approximate microwave frequencies were determined with a cavity wavemeter and, in the absence of a microwave frequency standard, final values determined by measuring the magnetic field for the paramagnetic resonance of the hydrazyl radical (a : a-diphenyl-p-picryl hydrazyl) taking g = 2.0037.2 Samples were usually contained in &-in. diameter, thin-wall, Pyrex glass capillaries.After being filled and cooled to liquid nitrogen temperature the capillaries were placed in test tubes, for case of handling, and immersed in liquid nitrogen in a pint-sized glass Dewar flask, This flask was placed next to a 10oO curie Coa source for the desired irradi- ation tinie, usually a few days. The dose rate was roughly 3000 roentgenlmin. Following the irradiation, the samples were transferred to a special Dewar, the tip of which could be inserted in the +in. bole of the microwave cavity. In the little work done at 9OOO Mc/s the tip of the Dewar was in. in diameter and correspondingly larger samples could be used. In the few cases where the hydrazyl radical was used as a standard it was placed simultaneously in the cavity beside the tip of the Dewar vessel holding the sample.For some studies, to be described later, the temperature and rate at which free-radical species disappeared were desired. The general plan was to observe the intensity of the paramagnetic line at liquid nitrogen temperature, then to place the sample in a cryostat where it very quickly warmed to the preset cryostat temperature. After the desired time interval the sample was removed from the cryostat by withdrawing it into a transfer device that quenched it to a lower temperature. Finally, the new line strength would be deter- mined at liquid nitrogen temperature. This process could be repeated for as many points as were needed on the rate curve. The cryostat consisted essentially of a massive copper cylinder about 24 in. in diameter and 6 in.tall with provisions for extracting heat to a liquid nitrogen bath through a fixed heat leak, and provisions for supplying a variable amount of heat electrically so that the temperature could be set at a predetermined value. The copper cylinder was placed in a tall Dewar flask, and screwed into the bottom of the cylinder was a post of iron that served as the heat leak to liquid nitrogen in the bottom of the Dewar. Different diameter posts were available for different heat leak rates. The bottom of the iron post was actually joined to copper and this copper contacted the liquid nitrogen. This made the unit less sensitive to variations in the nitrogen level. Heat was supplied to the copper cylinder electrically by means of a one-turn nichrome ribbon wound at the bottom of the cylinder.Although heat inputs up to 12 W could be used, the iron heat leak was usually chosen so that very much smaller electric powers would balance the copper cylinder temperaturc at the desired value. A number of holes were drilled into the top of the copper cylinder. A copperconstantan thermocouple was placed in one of these holes, and with the help of a potentiometer the temperature could be set and read to 0.05" C. Other holes were used for samples in capillary tubes. The sample capillaries were so small that they would rather quickly equilibrate to the cryostat temperature, and the thermal shock was hardy readable with the thermocouple. The transfer device was essentially a metal cylinder with a carrying handle. The cylinder had an axial hole that would just accommodate the &-in.diameter capillary. The transfer into the cryostat was made by placing the capillary into the hole of the transfer device while the unit was immersed in liquid nitrogen. The transfer was then quickly made to the cryostat. In removing the capillary the procedure was reversed and the capillary was quenched as soon as it was quickly withdrawn from the cryostat into the precooled transfer device. RESULTS AND DISCUSSION Concentrated H2SO4, irradiated and examined at 77" K, showed four strong absorption lines. Two of these were near the g vaIue of the free electron, one rather sharp and one very broad and at slightly lower field. The other two, shown to arise from atomic H, were symmetrically placed on either side of the1 68 ATOMIC HYDROGEN central pair, were of about equal strength and width (about 4 gauss width at half-height) and were about 505 gauss apart. The relative strcngths of the lines varied with dilution of the acid.As progressively more dilute acid was used the sharp central line diminished in intensity while the outer pair increased, the former almost disappearing at a water-to-acid mole ratio of about 2/1 while the latter increased to a maximum at 511. Upon further dilution the entire spectrum weakened. Since the outer two lines always appeared as a matched pair it was originally suspected that they might result from a free radical that contained a hydrogen nucleus that interacted with the unpaired electron. In order to test this concept' the concentrated acid was diluted with D20 to a 2/1 water-to-acid mole ratio.Three new lines (thc expected number for D with a nuclear spin of 1) appeared. Fig. 1 shows these three lines as well as the central broad line. The hydrogen lines are displaced farther from the centre and do not show in this picture. The central deuterium line appears on top of the central sharp line mentioned above. This sharp, unidentified line is presumably relatively weak at this D20 dilution of the acid. The same outer pair of hydrogen lines was also found in HClO4 and H3P04, and the same behaviour on adding D20 was noted. In HC104 the hydrogen yield reached a maximum at a water-to-acid 7/1 mole ratio while in H3P04 the yield was essentially at a maximum for the 100 % acid with little change for the usual 86 % acid.The central lines in these acids, however, were considerably different from each other and from H2SO4. H3P04 showed two broad lines in the region of g = 2, while HC104 showed a number of lines of various strengths and widths, all (except the hydrogen ones) being vcry much broader in the dilute acid. Quantitative measurements (in magnetic field units) of the hydrogen lines were made in each of the three acids and the microwave frequency was determined by measuring the hydrazyl radical resonance. A hyperfine separation VH and electron g-factor were computed using the Breit-Rabi formula.113~ 4 The results appear in table 1. The VH values are strikingly close to Nafe and Nelson's 4 TABLE HYPERFIN FINE SEPARATTONS VH AND ELECTRON g FACTORS FOR HYDROGEN IN IRRADIATED ACIDS acid * mole ratio microwave high field low field HzO/acid freq., Mc/s line, gauss lfne, gauss "HI Me's 6 ~ ~ 1 0 ~ 7.0 23,03 3.5 8462-7 7960.2 1407.1 2.0022 23,033.7 8462-6 7960.6 1405.6 2.0022 23,033.7 8462.9 7960.2 1407.3 2.0022 HzS04 2.1 23,0451 8467.0 796 1 *4 1415.7 2.0025 23,042.7 8466-8 79613 1414-9 2.0023 23,043-7 8466.6 7961.4 1414.5 20024 H3PO4 0.0 23,0652 8475.3 7967.0 1423.5 2.0025 23,065.5 8475-8 7967-5 1423.4 2,0024 23,064-7 8475.6 7967-3 1423.3 2*002,4 * The measurements on each acid were made in succession on a single sample.atomic beam value of 1420.410 Mc/s. This indicates that the unpaired electron is represented very well by an atomic hydrogen 1s wave function, and that the hydrogen atom can have no chemical bond. The g values of table 1 are satisfac- torily close to the unpaired electron value of 2.0023.The deuterium lines in a diluted sulphuric acid sample were also measured and the computed hyperfine separation for deuterium was also in good agreement with the atomic beam value. Very shortly before the preliminary report 1 on the above work appeared, Smaller, Matheson and Yasaitis 5 reported on paramagnetic resonance in irradiated ice. They found one pair of lines with a splitting of 85.5 Mc/s which was attributedR . LIVINGSTON, H . ZELDES AND E . H . TAYLOR 1 69 to atomic hydrogen. Thc considerably smaller value than the 1420 Mcls free-atom value was attributed " to the polarization effect of the ice ". Other lines were found that were attributed to the OH radical. Their measurements were carried out with a spectrometer operating at 350 Mc.This work was of considerable concern for two reasons. First, on the experi- mental side, observations of irradiated ice at 23,000 Mc/s at this laboratory did not show lines corresponding to those reported at 350 Mc/s, but rather a weak, broad smudge, even aftcr long irradiations. Second, the interpretation of the small separation observed in ice at 350 Mc/s as a hyperfine separation of H modified by a polarization was difficult to reconcile with thc constancy of this separation at the free-atom value in the three acids. Experiments were designed to help give additional information on these two points. A series of dilutions of H2SO4 was examined for atomic H after irradiation. The lines, weakening with dilution, were observable up to a water-to-acid molc ratio of 2000/1.No significant vari- ation in the hyperfine separation could be found. This very strongly indicated that in the limiting case of pure water any atomic hydrogen present should give essentially the free atom value. The above irradiations were carried out at 3000 roentgen/min for 24 h. Longer irradiations might have allowed higher dilutions to be measured, but there was danger of interference from atomic hydrogen produced from adsorbed water on the Pyrex capillary as described below. Atomic hydrogen formed from such adsorbed water does give the free-atom separation. A second series of experiments centred upon a re-examination of irradiated ice. Very long irradiation (several weeks) at 77" K gave extremely weak atomic hydrogen lines, but an examination of D20 ice irradiated during the same time showed atomic H of the same strength and no atomic D.This effect was found to result from moisture that was adsorbed on the capillary. There was no evidence for atomic H in the bulk ice. In a final ice experiment a single crystal was irradiated for several days. Paramagnetic absorption was found near g = 2.' As the crystal was rotated a definite, but extremely weak, close-spaced doublet appeared and with further rotation it merged into a broad, just detectable smudge. Such lines, showing anisotropy, are quite reasonable to expect for a radical such as, say, OH. Anisotropy might be expected in cases where the unpaired electron is not in an S state. These results on ice seemed to give a new kind of information without obvious relation to the reportcd 350 Mc/s results.Although the sensitivity of the 23,000 Mcls spectrometer was moderately high (0.07 pg of hydrazyl radical gave a signal to noise ratio of 3/1) the concentration limit of a radical that could be detected was not very low since the sample size was only about 10mg. An effort is being made to improve the sensitivity greatly and thus decrease the con- centration limit in the hope that the lines corresponding to those reported at 350 Mc/s will be found. The discovery of atomic H lines from H20 adsorbed on glass led to further experiments using high-porosity glasses made by the leaching of soft glass (Corning Glass Co.). The pore diameters in these glasses are supposed to be quite uniform.Two sizes were used, 36A and 71 A. After one-day's irradiation at 77" K, strong atomic H lines were seen. Tf the glasses were covered with D20 at room temperature and allowed to stand for a period of time before cooling to 77°K both atomic D and H lines were seen. In a first experiment the ratio of D to H line intensities was greater for the 71 A pore-sized glass, presumably because of the shorter time required for the D20 to diffuse into the pores. This suggested following the rate of D20-diffusion and exchange by allowing the D20 covered glass to stand for varying lengths of time at room temperature, Such an experiment was carried out with the 71 A pore-size glass. D20 was placed on several samples of the crushed glass in capillaries.The samples were allowed to stand at room tem- perature for different time intervals and then were quenched in liquid nitrogen. All samples were irradiated together for the same period of time. The D to H intensity ratio was estimated for each sample and taken as a measure of the170 ATOMIC HYDROGEN progress of the diffusion and exchange. The results are summarized in fig. 2. Here the percentage D of total D + H paramagnetic absorption is plotted as a function of time at room temperature so that the asymptotic behaviour can be seen. The identification of the H atoms with the glass-water interface in these experi- ments seems unequivocal, but some difficulties in interpretation remain. Although Pyrex capillaries occasionally showed very weak atomic H lines, the strength was not enhanced by increasing the surface by using powdered Pyrex or Pyrex rod with a surface increased by mechanical roughening. The high silica content of the high-porosity glass suggested the use of powdered silica, but irradiation of wetted Cab-0-Sil, a very finely divided silica powder (0.015 to 0.020 micron particles) gave no evidence for atomic H. TIME HOURS FIG.2.-The progress of diffusion and exchange of D20 for H20 in 71 A porous glass. In the course of examining very intense atomic H lines in the three acids, relatively weak satellite lines were observed. These are shown in fig. 3 for the high field atomic H line in HClO4. The essential features of this work have been given in a preliminary report.6 These extra satellite lines originate from the flipping of nearby proton spins in conjunction with the flipping of the electron spin as a consequence of a weak magnetic interaction. The satellite spacing cor- responds to the proton magnetic resonance energy in the prevailing magnetic field.The satellites were also examined for H lines in the three acids at 9O00 Mc/s and the satellite spacing was found appropriately smaller. The difference in ap- pearance of the satellites was the only gross change seen in the features of the spectra at 9000 Mc/s. The satellite intensity relative to the main line varies with the second power of the field strength and essentially with the inverse sixth power of the distance of the hydrogen atom to a nearby perturbing proton. Preliminary measurements and calculations gave 1.74 A as the distance for a single nearby interacting proton, or somewhat greater than this distance for more than one (the weighting is roughly l/rq.The H atom must be essentially at rest for this phenomenon to exist. This would probably be concluded independently from chemical grounds since motion of the H atoms should lead to recombination. Correction of an error reduced the previous value of 1-85A to 1*74A. This new value must still be regarded as preliminary. As indicated earlier, the yield of stably trapped atomic hydrogen is at a maximum in H2S04 for a water-to-acid mole ratio of 5/1, for HC104 at a ratio of 7/1, while the commercial 86 % H3P04 gives about the maximum yield. No effort was made to determine accurate absolute yields. However, an order ofR.LIVINGSTON, H. ZELDES A N D E. H. TAYLOR 171 magnitude estimate could be made. A 24-h irradiation of the 5/1 H2SO4 at 3000 roentgen/min gave atomic H lines of about l5/l signal-to-noise ratio. A comparison with a known sample size of hydrazyl radical where the number FIG. 0 2 5 5 0 H2SO4. T I M E , MINUTES FIG. 5.-The disappearance of atomic H in 7/1 water-to-acid mole ratio HClO4. of unpaired electrons present was known indicated a yield in the order of 1015 H atoms. This corresponds to a radiation chemical yield of the order 0.04 atoms/100 eV. H3P04 of the above concentration gave thc same yield while the HC104 was 3 times larger.172 ATOMIC HYDROGEN A series of saturation tests was made in which acid samples of the above concentrations were given large radiation dosages and the signal-to-noise ratio for the atomic hydrogen lines was periodically measured.In order to assure reliability of the results one set of samples was kept as reference samples after a one-day irradiation and were measured each time the test samples were examined. The H2SO4 and H3P04 behaved alike, being saturated at about 50/1. The HC104 was saturated at about the same time giving a signal-to-noise ratio estimated as 300/1. The samples were given an accumulated dosage over a period of 523 h TIME, MINUTES FIG. 6.-The disappearance of atomic H in 86 % H3P04. (108 roentgen), but there was no signi- ficant change after the first 150 h. In all cases the lines showed evidence of broadening. This was particularly notice- able for the HC104.The effect was probably due to magnetic dipole-dipole interactions from the high concentration of H atoms and other radicals. The 7/1 HC104 at saturation is estimated to have of the order 2 x 1016H atoms in a 0.01 g sample which would amount to one H atom for every 104H20 or Features of the chemical system that account for the stability of the H atoms at low temperatures and the mechanism by which the H atoms are destroyed at high temperatures are suggested by the following experiments. All samples stored at liquid nitrogen temperature appeared to be stable indefinitely. Some samples were examined after several months of storage and there was no obvious change in the atomic H content. Higher temperature measurements, where moderately fast rates were encountered, were made with the cryostat described earlier and are summarized in fig.4, 5 and 6. These data do not fit first-order 103 HC~O~. kinetics but are represented reasonably well by second order. This suggests that the process for atomic H disappearance is reaction with itself. A second possibility, however, is that the atomic hydrogen reacts with another species that was originally formed in one-to-one correspondence with the hydrogen. The latter possibility includes migration of a positive hole at the elevated temperature. Moving charges should make the sample electrically conducting. A crude test, however, on irradiated H2SO4 that was quickly warmed to - 160°C, at which temperature the hydrogen will very rapidly vanish, gave no indication of con- ductivity.Work is continuing on obtaining more extensive rate data and it is hoped that activation energies can be determined. Generally, the atomic H Iines disappeared at lower temperatures than the other absorption lines in the samples. In H2SO4, for example, after the atomic H had completely disappeared most or all of the broad central absorption line shown in fig. 1 remained. At a much higher temperature, near - 110" C, this central line quickly vanished and the amber colour of the irradiated acid bleached. The sample also showed electrical conductivity in this temperature region. With regard to the nature of the systems giving atomic hydrogen it is interesting to note that for the oxygen acids only a small sequence of elements in the periodic table show the behaviour. These are elements of atomic number 15, 16 and 17,R . LIVINGSTON, H. ZELDES AND E. H. TAYLOR 173 and possibly the oxygen complexes of 14 (Si) which appears borderline in its behaviour. Atomic H was not seen in irradiated boric, chromic, arsenic or chloric acids. One consideration that seems significant is that hydrogen bonding may play an important part, presumably in forming an inert environment by tending to saturate residual electric interactions that could contribute to bond formation. Several fluorine compounds, where hydrogen bonding would be expected, were examined, and 48 % HF was found to give a moderate yield of atomic H while fluosilicic and fluoboric acids gave excellent yields. The possi- bility has not yet been completely eliminated that the lines in 48 % HF were due to silica contaminant. 1 Livingston, Zelda and Taylor, Physic. Rev., 1954,94,725. 2 Hutchison and Pastor, Physic. Rev., 1951,81,282. 3 Breit and Rabi, Physic. Rev., 1931, 38, 2082. 4 Nafe and Nelson, Physic. Rev., 1948, 73, 718. 5 Smaller, Matheson and Yasaitis, Physic. Rev., 1954, 94, 202. 6 Zeldes and Livingston, Physic. Rev., 1954, 96, 1702.

ISSN:0366-9033    

DOI:10.1039/DF9551900166

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

R . LIVINGSTON, H. ZELDES AND E. H. TAYLOR 173 GENERAL DISCUSSION Dr. E. E. Schneider (Durham University) said : In the actinide group a reson- ance process seems to be of great importance in which the microwave magnetic field is parallel to the constant magnetic field. May I ask Dr. Bleaney whether there is a simple model to explain this process similar to the model of precessing magnets used to describe the normal paramagnetic resonance process with the microwave field normal to the constant field. Dr. B Bleaney (Oxford University) said : I do not know of any simple classical model which corresponds to this case mentioned by Dr. Schneider, but there is a certain similarity with the well-known microwave absorption of ammonia. In the ammonia molecule, there are two equivalent configurations, one with the nitrogen atom above the plane of the hydrogens and the other below it.A de- generacy exists between these two states, which is lifted by interaction between them (through the so-called “ tunnelling” of the nitrogen atom through the potential barrier from one position to the other). This resonance effect splits the energy levels slightly, the correct wave functions being the symmetrical and anti- symmetrical combinations of the two separate wave functions, and electric dipole transitions are possible between the two levels giving rise to the absorption band near 1 cin-1. In the magnetic case, states without Kramers’ degeneracy may have a double degeneracy when the crystalline electric field has axial symmetry, but this degeneracy is removed by distortions from axial symmetry.In zero magnetic field the correct wave-functions are again the symmetrical and anti-symmetrical combinations, and transitions are possible when the oscillating magnetic field is directed along the crystal axis. If a steady magnetic field is applied along this axis, the degeneracy is lifted by the field, and the strength of the transitions de- creases as the steady field becomes stronger, because the resonance phenomenon becomes less effective in mixing the states. In a sense the system has an oscillating magnetic dipole moment along the axis, with which the oscillating field can inter- act, in the same way that the ammonia molecule has an oscillating electric dipole moment along its symmetry axis through the periodic tunnelling of the molecule from one equilibrium position to the other.The phenomenon cannot occur with ions which have an odd number of electrons, and which have true Kramers’ degeneracy. Thus no transitions would be allowed in rubidium neptunyl nitrate if the value of g1 were accurately zero. The transi- tions are allowed in plutonium, since the ion has two magnetic electrons, and similar174 GENERAL DISCUSSION transitions have been observed in praseodymium salts.1 In some cases (terbium ethyl sulphate,2 and ferrous fluoride 3) the crystal field is responsible for the effect without requiring any distortion due to the Jahp-Teller effect. Dr. E. E. Schneider (Durham University) said: I should like to supplement the data presented in Dr. van Wieringen’s paper by some results on the h.f.s of Mn2+ obtained in the Newcastle laboratories.Hexagonal * ZnS 0.001 % Mn powder, A = 68.0 f 0-2 gauss, cubic ZnS 0.001 % Mn powder, A = 68.2 f 0.2 ,, cubic ZnS 0.1 % Mnpowder, A = 67.7 f 0.2 ,, cubic NaCl 0.01 % Mn single crystal, A = 88 f 1 ,, * fine structure D - 10 gauss (Schneider and England, Physica, 1951, 17, 221.) The difference between the two concentrations of cubic ZnS is believed to be significant. On thc basis of the ideas put forward by Dr. van Wieringen it would indicate that exchange effects are operative even at concentrations of 0.1 % Mn. Of greatest interest is the value for the splitting of Mn in NaCl (referring to isolated Mn ions on cation sites in perfectly cubic surroundings) which is more than 10 % lower than the splitting in the Tutton salts or aqueous solutions.This points to the existence of a considerable amount of covalent binding in a lattice which is usually regarded as predominantly ionic. Mr. M. Tinkham (Oxford University) said: At Oxford we have observed paramagnetic resonance of Mi$+ ions present as substitutional impurities in single crystals of ZnF2 grown from the melt. Since the axes of the two cations per unit cell are orthogonal, the macroscopic symmetry is tetragonal although each cation has surroundings of only rhombic symmetry. The nearest neighbours to any cation are four F- ions arranged in a rcctangle whose plane includes the macro- scopic c-axis. There are two more F- ions, only slightly further away, on an axis through the cation perpendicular to the plane of the rectangle. With the Mn2+ ions in this environment, we find each of the usual h.f.s lines split into a number of “ super-hyperfine ” Components by the interaction of the magnetic electrons with the nuclear magnetic moments of the neighbouring fluoride ions.This is the same sort of intcraction as described by Owen, but in this case a-bonding of the magnetic electrons is more important than .rr-bonding, which gave the h.f.s coupling in the iridium complexes. Experimental difficulties have allowed the spectrum to be analyzed in detail in only two cases. When the external magnetic field is applied along the c-axis, each h.f.s line is split into 15 components whose shifts from the position of the centre of the unsplit line are given by H - HQ = A l m l i - A2m2, where ml and rn2 arc the total components along H of the nuclear spins of the 4 fluorine nuclei on the rectangle and the 2 on the axis, respectively; A1 = 19.5 gauss and A2 = 13.4 gauss.If the external field is applied along a principal axis perpendicular to the c-axis, each h.f.s line is split into 7 equidistant components. The relative intensities coiifii m that this is the rcsult of having A1 N A2 CI 17 gauss in a similar formula. Although this work is not yet final, we can make several observations. First, charge transfer, a characteristic of covalent bonding, gives a sizeable effect even in a crystal as ionic as a fluoride. Estimates based on the properties of F- atomic orbitals indicate that the 5 magnetic electrons are in orbitals about each F- centre for the order of I % of the time.In any case, the approximate nature of the 1 Bleaney and Scovil, Phil. Mag., 1952,43,999. 2 Baker and Bleaney, Proc. Physic. SOC. A , 1955, 68, 257. 3 Tinkham, Proc. Physic. SOC. A, 1955,68,258.GENERAL DISCUSSION 175 assumption of pure ionic binding in fluorides made by van Wieringen is demon- strated. Secondly, the anisotropy of this super-h.f.s shows that although the principal coupling is through s-electrons on the F-, there is an appreciable con- tribution from the p-bonding orbitals as well. Dr. B. Bleaney (Oxford University) said: The spectrum of the Mn2+ ion is not completely isotropic even in a cubic crystal, because the spin Hamiltonian would then contain a term of the form,l (1) In a single crystal where the external magnetic field is oriented in a direction with cosines (I, rn, n) with respect to the cubic axes the spectrum should consist of five lines (apart from hyperfine structure) at the positions 2 %? == 3a(Sx4 + Sv4 I- Sz4 - 6S(S + 1)(3S2 + 3s - l)}, hv = gpH -f 2pa In = gpH f (5pa/2) hv = gpH (relative intensity 5), (relative intensity 8), (relative intensity 9), wherep = 1 - 5(22m2 + m2n2 -1- nV).The effects of such a term have been ob- served in noncubic crystals by Bleaney and Ingram 3 and Trenam 4 for manganese, and by Blcaney and Trenam 5 for the ferric ion. In a powder it is necessary to average the function p over all values of the direction cosines (compare the cor- responding case of anisotropy in a cubic ferromagnetic material treated by Bagguley 6).This gives a root-mean-square line width of 4a/2/21 for a powder, which is additional to any other source of line width. The overall spread of the line should be & (5/2)a, and the smeared-out satellite lines would have maximum intensity at points 3r (5/8)a and f $a for the satellites of intensity 8 and 5 respectively. It may be that this accounts for the shoulders visible on each hyper- fine line in the spectrum of cubic ZnS+Mn at high dilution (fig. 3 of Dr. van Wieringen’s paper). If the constant a in this substance has about the same value of 10 gauss as measured in hydrated manganese ~alts,3~4 then these shoulders (assumed to correspond to the points of maximum intensity) should come about &- 5 gauss from the centre of the line, and the overall spread of each line would be about f 25 gauss.If this interpretation of the line width is correct, an experi- mental measurement of the root-mean-square line width would give an estimate of the value of the splitting parameter a. The fact that the colours of paramagnetic salts do not change markedly on solution in water shows that the ion must retain an octahedron of water molecules round it in solution, as pointed out by Prof. B. M. Kozyrev7 in his paper on other grounds. The reason for this is that the colour is principally due to transitions between orbital levels whose splitting depends on the cubic crystalline electric field sct up by these neighbouring water molecules. One might therefore expect that the theory given above for a powder of manganese would also apply to a liquid.However, the work reported by Prof. Kozyrev on resonance in manganese solutions at frequencies below 200Mc/s seems to rule this out, since in the presence of a splitting due to term, of the type (l), the resultant angular momentum F would no longer be a good quantum number, and a broad smeared- out resonance would result instead of a single peak at gF = 1. To reconcile this observation with the point about the colour requires that the crystalline electric field fluctuates sufficiently rapidly to approximate to spherical symmetry for 1 Bieaney and Stevens, Report Prog. Physics, 1953, 16, 108. 2 Kronig and Bouwkamp, Physica, 1939, 6, 290. 3 Bleaney and Ingram, Proc. Roy. SOC. A , 1951, 205, 336. 4 Trenam, Proc. Physic. SOC., A , 1953, 66, 118.5 Bleaney and Trenam, Proc. Roy. SOC. A, 1954, 223, 1. 6 Bagguley, Proc. Roy. SOC. A , 1955, 228, 549. 7 Kowrev (this Discussion).1 76 GENERAL DISCUSSION radio-frequencies but insufficiently rapidly to do this for optical frequencies. Since the fluctuations would have a characteristic time of the order of the Debye relaxation time (10-11 sec in pure water), this is a very reasonable result. It is just possible that the failure to observe resonance due to the ferric ion in solution is connected with the fact that the cubic field splitting for this ion in the solid is about 14 times larger than that for the manganous ion, and that the fluctuations are insufficient to average this out in solution. Dr. E. E. Schneider (Durham University) said : The asymmetrical structure of individual hyperfine lines in cubic ZnS + Mn which Dr.Bleaney suggests is due to cubic field splitting, is well resolved at concentrations of 10-5 and 10-6Mn. The accompanying figure of the two central hyperfine lines 3 and 4 (68 gauss apart) and of line 4 at reduced sweep shows that the structure is complex and cannot be associated easily with the smeared-out cubic splitting discussed by Dr. Bleaney. The question is whether the powder specimens are exclusively cubic or whether there are possibly remnants of hexagonal structure or distortions at the crystallite boundaries causing the fine structure of the resonance lines. Dr. D. J. E. Ingram (University uf Southamptun) said : The way in which the hyperfine structure obtained from divalent manganese can be employed to indicate any sudden change in crystal structure is also well illustrated by the spectra ob- served on freezing solutions of manganese salts. The accompanying figure shows the series of spectra that are obtained as a solution of manganese sulphate is cooled through 0" C.The four photographs are from the same sample in the same resonator and were taken within a few scconds of each other. The fist (at top left of the figure) is for the manganese solution at room temperature, when the " averaging " or " tumbling " effect of the molecules in the liquid state pro- duces a symmetrical field at the manganese ion, resulting in six well-resolved hyperfine components. The next three photographs show the effect of cooling this soIution through O'C, when the hyperfine structure is seen to disappear rapidly, and only a single broad line is observed once the solution has frozen.This can be explained by the anisotropic crystalline field associated with the ice lattice, which may cause an anisotropy in both the g value and hyperfine splitting and thus smears the spectrum out into one broad line. This sudden change in the observed spectrum could be used to indicate change in crystal structure under other conditions, too, and give information on such points as the temperature at which a crystal changed from a cubic lattice to one of different symmetry. Another result of interest, which we have recently obtained at Southampton, is the absorption spectrum associated with Mn6+ in potassium manganate. This can only be observed at temperatures of 20" K or below, the centre of the absorption corresponding to a g value of 1.98.This suggests that the concentrations of manganese compounds of different valency in a phosphor or other substance could be measured by paramagnetic resonance absorption experiments at different temperatures. Mr. W. A. Runciman (Harwell) said : It would be heIpful if the fluorescence colours of the phosphors listcd in table 1 of the paper by v. Wieringen were given as divalent manganese can cause fluorescence of varying wavelength in a way which is imperfectly understood. Since fluorescence involves both the ground state and an excited state it would be interesting to find the effect of illuminating these materials with ultra-violet radiation during the resonance experiment as discussed by Hershberger and Leifer.The effect might well be obscured by the resonance absorption from the ground state, and hence it would be attractive to try a phosphor which only becomes paramagnetic in the excited state. Rb(U02)(N03)3 is an ideal substance for this purpose as single crystals can be gown and the half-life is fairly long, of the order of a millisecond. Information obtainable in this way might be useful in connection with the theoretical work on the U02 group discussed by Bleaney.OENERAL DISCUSSION 177 Dr. J. S . v. Wieringen (Philip Res. Lab., Eindhoven) (communicated) : In reply to Dr. Ingram, a change in the paramagnetic resonance spectrum of Mn2+ when the surrounding crystal changes from hexagonal to cubic symmetry, has actually been observed in ZnS.Under pressure, hexagonal ZnS-which is metastable at room temperature-is transformed into the stable cubic modification.lc In reply to Mr. Runciman, we have found no correlation between fluorescence and paramagnetic resonance of Mn2+. In the compounds listed in table 1 the fluorescence was (in the same order) orange, bluish green, green, no fluorescence red, red, red, faint blue, red brick colour, red, red, whereas in the black-coloured CdTe : Mn it was impossible to detect any fluorescence. Dr. P . George (Cambridge University) (communicated) : The g-values we have obtained for the ferrihaemoglobin and fcrrimyoglobin derivatives have an important bearing upon the generally accepted classification of the bonding of the iron as " essentially ionic " or " essentially covalent " in these compounds.As with other co-ordination complexes this classification has been made on the basis of paramagnetic susceptibility measurements. With ferrous compounds there is apparently an obvious distinction. The CO complex of haemoglobin is diamagnetic, which would be expected if the bonding were covalent involving d2sp3 hybridization as in the ferrocyanide ion. The parent compound ferrohaemo- globin is, however, paramagnetic with p = 5.43 Bohr magnetom, quite similar to that of the Fe2+ aquo-ion (p = 5.2 to 5.3 Bohr magnetons) and the bonding of the iron has accordingly been regarded as essentially ionic. With ferric com- pounds, the parent ferrihaemoglobin and its fluoride complex have paramagnetic susceptibilities of 5-80 and 5.92 Bohr magnetons respectively, compared with the value of 5.92 for the Fe3-1- aquo-ion, and the bonding has likewise been classified as essentially ionic.The azide and cyanide complexes on the other hand have much lower paramagnetic susceptibilities of 2-84 and 2-50 Bohr magnetons respectively, which, by comparison with the value of 2.33 Bohr magnetons for polycrystalline potassium ferricyanide, have been taken to indicate essentially covalent bonding. It is to be noted that these values are very much larger than that calculated on the basis of electron spin alone (1.73 Bohr magnetons); and for the ferrihaemoglobin hydroxide complex the difference is larger still, its susceptibility of 4.77 Bohr magnetom is in excess of that required for 3 unpaired electrons (3437 Bohr magnetons) and is very nearly that required for 4 unpaired electrons (490 Bohr may- etons).These discrepancies have been attributed to contributions to the susceptibility from partially unquenched orbital angular momentum. The results of the paramagnetic resonance absorption measurements on the azide complexes substantiate the conclusion that the bonding is covalent as in the ferricyanide ion. But for ferrihaemoglobin and its fluoride complex, the identical g-value of 5.95 f 0.05 reveals that the bonding is not essentially ionic in the same sense that it is in the hydration shell of the Fe3+ aquo-ion, which, being in an orbitally singlet state, L = 0, has a g-value of 2.0. It would appear that in these " ionic " haemoprotein complexes, as Mr.J. Stanley Griffith has suggested below, there are probably less than five unpaired electrons and it is fortuitous that contributions to the susceptibility from partially unquenched orbital angular momentum lead to values so close to that required for five unpaired electrons neglecting orbital contribution entirely. Because of experimental difficulties, notably the rapid combination with oxygen, measurements have not yet been made on ferrohaemoglobin or ferromyoglobin, but it now seems possible that g-values very unlike those for the Fe2+ aquo-ion would be obtained. If this were so, the distinction between essentially ionic and essentially covalent bonding would like- wise lose its original significance for these ferrous haemoprotein compounds as it now has for the ferric haemoprotein compounds.Any correlation that has been suggested between bond type and the stability of haemoprotein complexes, or their speed of formation and dissociation, or the acid strength of haem-linked178 GENERAL DISCUSSION ionizing groups, will thus need evaluating afresh in the light of this new information about their electronic structures. Another problem in haemoprotein chemistry to which paramagnetic resonance absorption measurements may make a valuable contribution is that of establishing the chemical nature of the compounds formed when the ferric haemoproteins react with peroxides or other strong oxidizing agenl.1 These important deriva- tives, which are the catalytically reactive intermediate compounds in the catalase and peroxidase enzyme reactiom are one and two oxidation equivalents respectively above the ferric state of the parent compound.The former are either straight- forward quadrivalent iron compounds, or produced by electron or hydrogen atom removal from the porphyrin ring system. The latter are either peroxide complexes, e.g. Fe3’ 02H-, or straightforward quinquevalent iron compounds, or are produced by the removal of another electron or hydrogen atom from the former class of compound. Chemical evidence favours quadrivalent and quin- quevalent iron structures, especially for the oneequivalent higher oxidation state of ferrimyoglobin, the reactions of which are explained very satisfactorily in terms of a complex “ ferry1 ion ”, Fe02+.2 However, physical methods for identifying higher oxidation states formed by electron or hydrogen atom removal from the ring system are clearly very desirable.Tetrasulphonated copper phthalocyanine (TSCP) and the Zn, Mg and Al derivatives give higher oxidation states which considerations of ionization potentials suggest are formed by electron removal from onc of the v-orbitals of the ring system,3 i.e., TSCP - v-electron -+ TSCP+. Preliminary paramagnetic resonance absorption measurements show a very marked difference between TSCP and TSCP+, and so a comparison with the haemoprotein compounds should provide very useful information about the structure of their higher oxidation states. Mr. J. Stanley Griffith (Cambridge University) (communicated) : The ob- servation of a nearly isotropic g-value of 5.95 for certain ferrihaemoglobin derivatives is surprising, both in itself, and also because it conflicts with current interpretations of susceptibility data. Taking ferrihaemoglobin fluoride (HbF), thc observed susccptibility x corresponds to a Bohr magneton number p = 5.92 in excellent agreement with the theoretical value of 5.92 for an ion with a free spin S of 5/2 and no orbital moment. For this reason it has been supposed that the iron in HbF is in just such an ionic state.With this interpretation, there may be considerable covalent bonding between the ion and its surroundings but the five electrons originally in 3d orbitals are still each in a separate orbital and still all have parallel spins. However, in such a case, g would be expected to be very close to 2. In discussing this paradox, 1 would commence by pointing out that the direct conflict between the values of p and g may not be quite so serious as it seems at first.The value of p is obtained from x first by subtracting a large diamagnetic term to give xp and then using the Curie-Weiss law, However, x is only known at one temperature and so the assumption has been made that A = 0. A for other compounds is sometimes small, but rarely zero, so it may be that the true value of p is not so close to 5.92. There are, in principle, three possible values in simple ferric compounds for S of 1/2, 3/2 and 5/2. Experimentally there is only definite evidence for S = 1/2 and 5/2. In an octahedral field S = 3/2 is unlikely to occur, but it is more possible 1 George and Lrvine, Brit. J.Radiology, 1954, 27, 131. 2 George and Irvine, Symp. on Co-ordination Chemistry (Danish Chemical Society, 3 Cahill and Taube, J. Amer. Chem. SOC., 1951,73,2847. Copenhagen, 1954), p. 135.GENERAL DISCUSSION 179 in HbF. In HbF, the symmetry around the Fe3+ ion is &. We take axes centred on Fe3+ with OZ passing through the fluorine nucleus. Then three d orbitals (dxy, dyZ, dzx) do not form o-bonds with the adjacent atoms (we neglect .rr-bonding here). The remaining two are non-degenerate, one (dx2-y2) pointing towards the porphyrin nitrogen atoms and the othcr (&) mainly away from them. This splitting of the d orbitals is shown diagrammatically, where the lowest / / / / / / OH‘ € * d l * // &---- FIG. 1. level is triply degenerate.If, now, the separation El is sufficiently large and E2 sufficiently small, then four electrons would occupy dl and one d2 in a configuration which may be written d14d2. (In Pauling’s notation, the “essentially ionic” case is d13d2d3 with El, E2 both small and the “ essentially covalent ” case is dl5, with E2 large.) Since we may expect nitrogen to form stronger covalent bonds than fluorine with iron, we may expect d2 to be 4 2 and d3 to be dx2-,,2. These remarks are made mainly’in ofder to explain the ef€ect 6f the eniriron- ment of the iron atom in haemoglobin derivatives in general. It is not suggested that states with S= 3/2 necessarily occur in HbF or other derivatives, but that such states are at least a theoretical possibility. It is doubtful, though conceivable, that such states could explain g = 5.95.Further, though d14d2 has a threefold orbital degeneracy and in spite of the difficulty of interpreting susceptibility data, a considerable orbital moment would seem necessary to explain the high sus- ceptibility of HbF if S is in fact 312. Prof. G. E. Pake (Washington University, St. Louis) said: In answer to the questions : (i) Is the sensitivity inadequate to permit the study of trapped photo- produced free radicals with a lifetime of perhaps 100 microseconds? (ii) Is it feasible to irradiate optically in the cavity a substance in a glassy organic solvent at liquid nitrogen temperature ? A 100-microsecond lifetime in itself would certainly be more than amply long to avoid broadening of the resonance.zf no other interaction process unduly broadens the resonance, the only problem is the sufficiently rapid photochemical production of free radicals to assure a steady-state population in the cavity of more than 1013 free radicals, as indicated by the sensitivity quotcd in the paper. -Irradiation of a substance at optical frequencies while it is in the cavity and at low temperatures is certainly possible, although we have not yet performed such experiments. Our sample containcr is usually a capillary tube placed in the rectangular cavity in such a way that it pierces the narrow face of the waveguide and spans the broad dimension. Passing radiation through the length of the capillary should be possible at low temperatures if a small portion of the glass Dewar is unsilvered to permit a pencil of light to reach the sample tube end.A reflection-type cavity is presumably easier to place in a Dewar system than is the transmission cavity of fig. 1. Dr. D. J. E. Ingram (University of Southampton) said: In connection with the observation of free-radicals in charcoals and coals, mentioned in the paper of Pake et al., a brief summary of detailed work which we have carried out on the same subject may be of interest. We have made a systematic study of the de- pendence of the free-radical concentration on (i) the percentage of carbon present, and (ii) the carbonizing temperature. Fig. 1 shows the variation for the first case, where the ordinate represents free-radical concentration in units of 1018180 GENERAL DISCUSSION per g, and the abscissa is the percentage carbon content.A steep rise in free- radical concentration is seen between 85 % and 90 % carbon content, followed by an even steeper fall, meta-anthracites with greater than 94 % carbon having very few free radicals. A similar type of variation is seen in fig. 2, where the free- radical concentration is plotted against carbonizing temperature. Curve A is that for a medium-grade coal, while curve B is that for residues from oil distillates. 0 FIG. 2. In both cases the very marked maximum in free-radical concentration at 550" C carbonizing temperature is seen. These curves are typical of any charred hydro- carbon, carbohydrate, or other organic matter, and the disappearance of the concentration above 600" C may be explained by the formation of large graphitic planes at these temperatures, as is shown by the sudden increase in electrical conductivity.The above results were obtained from samples that had becn pyrolyzed in air, however, and more recent measurements show that the free-radical con- centration is considerably increased if the sample is charred in vacuo, and its paramagnetic resonance absorption studied before the admission of air. One ofGENERAL DISCUSSION 181 the most striking effects then is the reversible decay of free-radical concentration on the admission of oxygen. This is illustrated in fig. 3, which consists of a series of photographs of the sigaal obtained from sugar charred at 400°C in vacuu, taken at 5-sec intervals after admission of oxygen to the sample. The first (top left-hand comer) shows the signal obtained before admission of oxygen, and the sudden decay of the signal, and noticeable broadening of the line width, can be clearly seen in the following traces.The final photograph (bottom right-hand corner) was taken 80 sec after admission of oxygen. No such change occurs when argon or nitrogen are admitted to the sample, and it would appear that the oxygen must be forming a weak absorption-type of bond with the carbon surface, this removing the unpaired electrons. The most striking feature of these experi- ments is that the free-radical concentration can be restored by simply pumping off the oxygen gas, when the signal goes through exactly the reverse process of that shown in fig. 3. This suggests that the free-radicals associated with the low- temperature carbons are probably located at the edges of the condensed ring systems, and only very weak bonds with the oxygen are necessary to saturate them.Further work is in progress on the effect of other gases and vapours. We have also investigated a rather different kind of free-radical system, in which the radicals responsible for polymerization become trapped in the resulting polymer.1 Fig. 4 is a photograph of such a signal obtained from polyacrylonitrile. This test was suggested by Dr. Bamford and Dr. Jenkins of Courtaulds Ltd., the polymerization is initiated photochemically, and the precipitated polymer containing the trapped radicals is sealed in a glass tube in vacuu. This tube was inserted directly into the cavity resonator and the spectrum shown in the figure was obtained, corresponding to 1017 free radicaIs/g, which confirmed previous chemical estimations.The signal was again found to decay on admission of air to the sample, though not so quickly as in the previous case. These pre- liminary experiments suggest that paramagnetic resonance may prove a very useful tool in the study of polymer chemistry. Prof. G. E. Pake (Washington University, St. Louis) said: Cancerous liver has been compared with normal liver tissue using paramagnetic resonance. Although there appeared to be a slightly greater resonance intensity from the cancerous sample, we are not sure it is meaningful. Whatever the relation may be, if any, betwcen resonance intensity and either abnormal or normal growth, it is not necessarily to be expected that the free radical content of the cancerous tissue should exceed that of normal samples of this metabolically very active material, Dr.J. Combrisson and Dr. J. Uebersfeld (Park) said: In connection with Dr. Schneider's paper we should like to point out that we have studied pararnagnctic resonance in several irradiated aliphatic amino acids (for example, glycine) and plastics2 and that a theoretical interpretation of the observed triplet on the assumption of a -CH2-R free radical was given.3 We have also studicd the lines in irradiated carbohydrates (sugars and cellulose). In all cases the intensity of the lines was a function of the size of the irradiated crystals. When those are kept in the prescnce of oxygen, the smaller the crystals are the faster the line vanishes.This could be attributed to a destruction by oxygen of the free radicals which had been created. This action of oxygen can be compared with that on coal radicals which we have studied 4 and we should like on this subject to comment on Prof. Pake's article and Dr. Ingram's contribution. (i) We have observed a strong and narrow line in sugar coals sealed under vacuum. It can be found in any coal obtained by heating a carbohydrate in thc absence of oxygen up to 900" C. By breaking the tube and letting air into it, 1 Bamford, Jenkins, Ingram and Symons, Nature, 1955, 175, 894. 2 Combrisson and Uebersfeld, Compt. rend., 1954,238,. 1,397. 3 Uebersfeld, Compt. rerzd., 1954,239, 240. 4 Ubbersfeld and Erb, J . Physique Rad., 1955,16,340.182 GENERAL DISCUSSION the amplitude of the line gets smaller but does not disappear in low temperature coals (carbonization temperature, T below 650" C) while it disappears completely in high temperature coals (T between 650" and 900" C).When the coal is sealed again under vacuum, the line comes back as strongly as it was previously. (ii) When sealing a coal (T below 900" C) in the presence of a limited amount of oxygen, we could see a line appearing after some time, but its amplitude was smaller the larger the amount of oxygen in the tube. This proves the important role played by physical adsorption of oxygen ; the action of oxygen is much quicker when the temperature of carbonization is higher which is correlated with smaller particles offering a larger surface.(iii) The concentration of free radicals in a medium can thus be modified and this enables us to confirm the exchange narrowing. The width of the lines decreased from 10 to 2 gauss when the concentration of paramagnetic centres increased from 5 x 1018 to 1020 centres/g. The Lorentzian shape of the curves is a good confirmation of Anderson and Weiss' theory of exchange narrowing. (iv) The same type of experiments was done at 60 Mc/s and gave comparable results. Prof. W. Gordy (Duke University, N. Carolina) said: Mr. W. B. Ard, Mr. H. Shields, and I, have examined the paramagnetic resonance of X-rayed Teflon (or Fluon).* Instead of the unresolved triplet which Dr. Schneider reports, we find immediately upon irradiation a complex structure which consists of at least eight components, approximately equally spaced and spreading over ap- proximately 200 oersteds with a gaussian distribution of intensities.We believe this structure arises from F19 nuclear interaction and that it proves that the odd electron migrates over several F atoms probably by exchange with the normally unshared pairs on the F to form v bonds. Superimposed upon this structure and near the centre of the group is another resonance which at the suggestion of Dr. D. M. McQueen, of DuPont, has been proved to arise from absorbed oxygen. When the sample is allowed to age in air, the oxygen line gets stronger at the ex- pense of the other components and gradually swallows them up. In a vacuum or over N2, the F fine structure remains unchanged.At 9ooo Mcls the oxygen line consists of a single peak, whereas at 30,000 Mc/s it splits into a doublet or a triplet depending on the orientation of the sample. The oxygen we believe ties 1 I 1 on to -C* radicals in the irradiated sample to form -Cr\EO and on to --C+ I I to form --G-o-O. The splitting at 30,000 Mcls appears to be a crystalline field effect dn the resonances of 0 2 bound in the two forms indicated. Several different samples of Teflon have been examined and all have been found to exhibit the above behaviour. It was also found possible to collapse the F19 structure into a unresolved broad rescmance by cooling the Teflon to 90" K. When the sample was allowed to warm up, the resonance regained its original structure. Evidently the phase change through which the Teflon passes upon cooling breaks up the migration of the odd electron and leaves only the broadening effects of the dipole-dipole interaction with the F19 nuclei.It may be of interest to the bio-scientist present to know that we have obtained paramagnetic resonance in numerous proteins (including my toe-nail and a hair from the tail of a black horse, both of which gave the same beautiful structure!), and several of their constituent amino acids after irradiation with soft X-rays. Fine structure was observed in practically all samples investigated. Although the results are too new and too complex for us to venture interpretation here, it ' I + * This work which is supported by the Office of Ordnance Research and by the O h of Scientific Research of the Air Research and Development Command will be published in more detail elsewhere.GENERAL DISCUSSION 183 is our hope this method will prove to be a useful way of investigating proteins and other biological substances. We have also found resonances in cotton, straw, and other forms of cellulose after irradiation with X-rays.Dr. E. E. Schneider (Durham University) said: We have found recently that the resonance in X-rayed Fluon depends on the nature of specimen, particularly on the degree of crystallinity. Some specimens especially when cold-worked show the type of resonance spectrum discussed in the papers, others showed after prolonged irradiation resonance effects similar to those reported by Prof. Gordy, namely, a well-defined but asymmetrical structure of at least 7 peaks with intervals up to nearly 40 gauss.It is difficult to reconcile such a large splitting with hyper- fine interaction with fluorine nuclei on the normal polymer chain. In connection with the resonance effects in irradiated proteins mentioned by Prof. Gordy the case of X-rayed nylon where resonance had been first reported by Combrisson and Uebersfeld m a y be of interest. Mr. Forrester of the Newcastle Laboratory has found that this resonance has a very symmetrical structure consisting of a central well-resolved triplet and two outer satellites of larger splitting. Prof. F. S. Dainton ( h e & University) (communicated) : Dr. Livingston, Dr. Zeldes and Dr. Taylor, are to be warmly congratulated on providing the first acceptable and unambiguous physical evidence for the formation of hydrogen atoms in y-irradiated aqueous solutions.Their secondary tindings are also of great interest, especially the conclusion (i) that the act of creation of a hydrogen atom also involves its spatial displacement over a distance of the order of an hgstrom, (ii) that the radiation yield in frozen 16 % aqueous sulphuric acid solution is of the order O W H atoms/100 eV, and (iii) that the atomic hydrogen disappears according to a 2nd-order law without development of electrical con- ductivity. Conclusion (i) has a relevance to theories of the primary act which requires no elaboration. Conclusions (ii) and (iii) raise further points on which the authors' views would be extremely valuable. If, as their observations suggest, the atomic hydrogen is largely destroyed by formatian of molecular hydrogen, then with the accumulated dosage which is mentioned, namely lo%, measurable amounts of hydrogen gas should be liberated on warming.Thus an order-of- magnitude calculation indicates that there would be a yield of about 1 mm3 H2 per 106 of sample irradiated. Have the authors attempted to collect, identify and measure any gas released on warming and if so with what result? The mechanism of the hydrogen atom removal in solids, if it is by recombina- tion, raises the question of their mobility in the lattice. Taking the two curves fig. 5 as representative of the same saturation dose, i.e. with an initial hydrogen atom concentration of -2 x 1016 per sample assumed to have a volume of - 0 5 cm3 and half-lives for recombination of 5 and 75 min at - 149 and - 156.8" C respectively, the bimolecular constant may be calculated to be about 1020 exp (- 10 kcal/RT). Both parameters in the Arrhenius equation have a magnitude which is not readily accommodated by the hypothesis of simple bimolecular com- bination of hydrogen atoms.Dr. R. Livingston, Dr. H. Zeldes and Dr. E. H. Taylor (Oak Ridge National Laboratory) (communicated) : We wish to thank Prof. Dainton for his comments. In our original manuscript we reported a preliminary value of 1.85 A as the distance of the nearest environmental proton to the hydrogen atom. Since, say in aqueous HCIO4, H20 or OH3-t. were the only likely sources for H atoms the atom must have moved at the time of formation.After correcting a computational error the new value of the distance was 1.74 A, not far from the H-H distance in OH3+. The conclusion that the atom moved, however, is still valid since the calculation of the distance from satellite line intensities is on the basis of a single nearest neighbour. For two nearest neighbours, the OH3+ case, the effective distance to give the observed line intensities would be roughly the sixth root of two times the 1-74 A or roughly 1.95 A, substantially greater than the H-H distance in OH3+.184 OENERAL DISCUSSION We, too, have felt that sufficient gaseous hydrogen may be formed upon melting the irradiated acids that gas measurements can be made on the gross H2 yield. We are just now starting such studies and have no results to report.We do have the view, however, that there may not necessarily be a simple one-to-one corres- pondence between atoms of hydrogen observed and molecules of gas collected from the warmed samples. We hope to obtain much more extensive rate data on the hydrogen atom disappearance before attempting to discuss mechanisms. As Prof. Dainton has indicated, the 7 : 1 HC104 rate data do lead to a 10 kcal activation energy. The 5 :1 I-E2S04 data leads to about 6.6 kcal. Dr. P. George (Cambridge University) (communicated) : Dr. Livingston has reported strong paramagnetic resonance absorption showing a marked g-value variation in irradiated frozen hydrogen peroxide, and has suggested that the HO- radical may in part be responsible. Some of the absorption may, however, be due to the H02* radical, which would also show a marked g-value variation, since if the HO.radical is formed initially in close proximity to another H202 molecule the H02- radical could be formed by the reaction HO. + H202 + H02* + H20. This species would probably persist because there is good reason to believe that the further reaction with another H202 molecule regenerating the HO. radical, i.e. H02* + H202 + HO* + 0 2 + H20, is very significantly slower.le2 We have studied the absorption due to 02-, the conjugate base of the H02. radical, in polycrystalline samples of Na02 and K02, and obtained a marked g-value variation from which the following values3 have been calculate 811 = 2.175 f 0.005 and gl = 2.002 & 0.005. The absorption by the HO2 radical would probably be of a similar type, so these values may be of use in deciding which species is responsible for the absorption by the irradiated hydrogen peroxide.Dr. C. R. Extemann, P. Denis and G. Bhd (University of Geneva) said : We have been highly interested to find in the paper of Dr. Kozyrev a reference to the work of himself and Rivkind.14 We have been aware for some time of the in- hibition of the relaxation time-shortening by paramagnetic salts when these go into a complex form. Our experiments were undertaken with an aqueous solution of chrome alum : on raising the temperature of a suitably concentrated solution of this salt, the building-up of a complex leads to an increase of the longitudinal relaxation time T1 of protons in the solvent. This work was published in Arch.Sci. Physiques Naturelles (Gen&e).4 Prof. G. E. Pake (Washington University, St. Louis) said : I should like to commend Dr. Kozyrev for the excellent work upon which he has just reported. I have, unfortunately, not seen the paper (Dr. Kozyrev’s ref. (3)) by Garifyanov and Kozyrev, which happens to parallel in some respects the measurements of Dr. R. H. Sands and myself5 on paramagnetic resonance of vanadium ions in solution. We studied solutions of vanadium chloride in water and ether at room tem- perature. Although attempts to prepare the various valence states of the vanadium ion were made, we are uncertain as to the valence state responsible for the spectrum, which is shown in fig. 2. Our measurement of the hyperfine coupling parameter 1 George, Faraday SOC.Discussions, 1947, 2, 196. 2Barb, Baxendale, George and Hargrave, Trans. Faraday SOC., 1951, 47, 462. 3 Bennett, Ingram, Symons, George and Stanley Griffith, Phil. Mag., 1955, 46, 443. 4 Extermann, Denis and BCnC, Arch. Sci. Physique Naturelles (GenPve), 1949, 2, 369. 5 Pake and Sands, Bull. Amer. Physic. SOC., 1954, 29 (S), p. 18.GENERAL DISCUSSION 185 A fcr aqueous solution is also 116 oersteds, and we obtain 110 oersteds for ether solutions. VOS04 in aqueous solution gives the familiar spectrum at room temperature . As indicated in our published abstract,l a feature of fig. 2 which has commanded our attention is the peculiar variation of the widths of the expected 21 3. 1 = 8 hyperfine component lines. This variation is not symmetric with respect to positive and negative values of my the nuclear magnetic quantum number.We have tentatively explained this by supposing that a small fine structure tern2 of the form - D‘(M - 1/2), where M is the electron spin quantum number for the unpaired electron configuration of the ion, must be retained in eqn. (1) of Dr. Kozyrev’s paper, and that the ion is in a 3d2 configuration so that values 1 and 0 are to be considered for M, Then the coefficient of nz in eqn. (1) is some- what different for electron transitions originating at M = 1, from that for those originating at MI’- 0. The resulting spectrum would be a superposition of two eight-line spectra of somewhat different hyperfine splitting. The narrowest line of the resultant spectrum would arise from that m value for which the two spectra have components falling most nearly on top of each other.The fact that the third line of fig. 2 is the narrowest indicates that a small shift of the M = 1 spectrum centre reIative to the M = 0 spectrum centre is necessary. The D’ term mentioned above provides this shift. The possible necessity for considering a fine structure term fortifies the con- clusion that ‘‘ near order ”, as Dr. Kozyrev calls it, must exist in the immediate neighbourhood of the ion. An effective crystalline electric field axis is necessary if the fine structure term is to be included as we propose : we think that D‘ may represent a nearly averaged-out value of the fine structure splitting constant be- cause of the random tumbling in the liquid of the ionic complex consisting of a vanadium ion and a well-ordered atomic arrangement situated about it.Suf- ficiently rapid tumbling would average the fine structure term to zero if an aniso- tropic spectroscopic splitting factor g = (g$ cos 28 + g,.z sin 2@* with 811 $: gL did not exist. If the difference between 811 and g,. is small, the resultant average value D’ will be small. The existence of an anisotropic g for the complex can also explain the lack of symmetry of each hyerpfine component in aqueous solution. For, if the tumbling rate is less than the vanadium spin resonance frequency in the applied magnetic field, one should expect the line to have an asymmetric distribution of intensity from a point corresponding to 811 resonance to one corresponding to gL resonance. Simple estimates of the tumbling rate likely in a solvent with the viscosity of water indicate that the tumbling may well be slower than the 1010 sec-1 resonance fre- quency.Ether provides a less viscous solvent, and as shown in fig. 2, the individual lines are symmetric. On cooling to acetone + dry ice temperatures, the ether lines become individually asymmetric, consistent with this hypothesis. I should like to know whether Dr. Kozyrev’s vanadium spectra exhibit the same variation of component line widths and, in aqueous solutions at room temperature, the same asymmetry of the individual component lines which we have observed. Prof. C. H. Townes (Columbia University) said: It is interesting to find how closely the independent work of Dr. Prokhorov parallels our efforts and ideas on this type of oscillator. A considerable amount of experimental and theoretical work on such an oscillator (which we call a “ maser ”, for microwave amplification by stimulated emission of radiation) has been carried out at Columbia University by Messrs. Gordon, Zeiger, Wang, Shimoda and myself. This type of oscillator provides the most monochromatic radiation which has yet been produced. Theoretical reasoning shows that the output frequency of 1 Pake and Sands, Bull. Amer. Physic. SOC., 1954, 29 (8), p. 18. ?l3leaney and Stevens, Reports Prog, Physics, 1953,16, 108.186 GENERAL DISCUSSION the masers which have been built at Columbia should have a spectral width of about 10-2 CIS, or 1/3 x 1012 of the frequency emitted. Two masers have been experimentally compared, and a low-frequency audio beat obtained between them. This beat appears on an oscilloscope to be a good sine wave with no visible jitter or noise, so that the spectral width of each can be said to be at least as small as 10-1 c/s from these observations. One maser oscillator was found to vary with respect to the other by about f 2.5 c/s over runs of +-1 h. This corresponds to a stability of about one part in 1010, which is better than the proven stability of other known frequency standards. There is good evidence that these variations are due to temperature variations of the cavities, which results in pulling of the oscillator frequency. Their temperature was controlled only to about 0-1" C, and it can easily be more closely controlled. It can be hoped that this type of device will afford a very useful frequency and time standard, as well as a speciali7d amplifier, of very narrow band width and exceptionally low noise figure.

ISSN:0366-9033    

DOI:10.1039/DF9551900173

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

III. NUCLEAR MAGNETIC RESONANCE NUCLEAR MAGNETIC RESONANCE APPLIED TO CHEMICAL PROBLEMS BY H. S. GUTOWSKY Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois Received 5th April, 1955 The basic nuclear magnetic resonance experiment is described and a summary is given of applications to problems in chemical structure. The absorption line shapes and their temperature dependence in solids provide information on internuclear distances and the types of molecular motion in solids. The spin-lattice relaxation time TI gives detailed values for the frequencies of lattice motions, and is useful in evaluating potential barriers or activation energies associated with the motions. The electronic environment of nuclei causes several effects of which the most iniportant are probably the chemical shifts and the indirect spin-spin multiplets.In liquids, these effects give resolvable complex spectra which are being used increasingly for structural analysis. Another very useful phenomenon is provided by chemical exchange which averages out these electronic interactions so that chemical lifetimes of the order of 1 to 10-3 sec can be measured. The general future of nuclear magnetic resonance in chemical research is discussed. INTRODUCTION The magnetic properties of nuclei have been known for more than 25 years. However, it was not until 1946 that simple but precise and sensitive experimental methods were developed for observing directly various effects of the nuclear magnetism in bulk samples. Similar methods were discovered independently and simultaneously by Bloch’s group1 at Stanford University and by Purcell’s group 2 at Harvard.Purcell’s electronic black boxes were somewhat different ikom Bloch’s; P~~-cell called his experiment nuclear magnetic resonance while Bloch called his nuclear induction. But the two methods are equivalent in their essential features. My own experience in the field has been East of the Mississippi River so I use the nuclear magnetic resonance terminology. The physicists have learned a great deal about the magnetic and electrical properties of nuclei by using these methods. And the results have added signifi- cantly to our understanding of the structure of nuclei.3 More recently, the chemists have become aware of the field and they are developing a wide range of applications to their own problems.4~ h fact, commercial instruments for nuclear magnetic resonance experiments are now available in America.They are rather expensive, but even so a considerable number have been sold, primarily for chemical research. These applications are possible because the detailed characteristics of the magnetic resonance are very sensitive to various features of the nuclear environ- ment. That is, the characteristics of the resonance depend upon the structure of the sample, and here the term structure is used in the most general sense. So if one observes the resonance and has an adequate theory for the effects observed, then conclusions can be drawn about the structure of the sample. And the value of nuclear magnetic resonance lies in the fact that it often can give information which is more difficult or even impossible to obtain by other methods.In this introductory paper, a brief description will be given of the nuclear magnetic 187188 CHEMICAL PROBLEMS resonance experiment. This will be followed by a summary of the various interactions, which influence the magnetic resonance absorption, and the types of structural information provided by the interactions. THE NUCLEAR MAGNETIC RESONANCE EXPERIMENT If nuclei with a magnetic moment p and a nuclear spin 1 are placed in a static magnetic field thenla set of (21 -t- 1) nuclear Zeeman energy levels are established. These energy levels correspond to different orientations of the nuclear magnetic moment with respect to the magnetic field. The selection rules for magnetic dipoles limit transitions to adjacent levels, and the basic equation for nuclear magnetic resonance is v = gjSHo/h.v is the frequency of electromagnetic radiation required to induce transitions, g is the magnetogyric (or, incorrectly, gyromagnetic) ratio p/I, jS is the nuclear magneton, HO is the magnetic field actually at the nucleus, and h is Planck’s constant. Magnetic fields normally used are the order of several thousand gauss, for which the resonance frequencies of most magnetic nuclei fall between 0-1 and 40 Mcls, so this is radiofrequency rather than microwave spectroscopy. An excellent introduction to the fundamentals of nuclear magnetic resonance has been given by Pake;6 and Andrew7 has a book in preparation which promises to give thorough coverage of most of the chemical as well as the physical features of nuclear magnet ism.Nuclear magnetic resonance is a novel combination of simplicity and com- plexity. The resonance equation is undoubtedly one of the simplest spectroscopic equations. On the other hand, some of the interactions perturbing the resonance line shapes are very complex. One feature of the resonance equation must be kept in mind; the resonance frequency is directly proportional to the static magnetic field, so frequency and magnetic field units are interchangeable. A number of methods have been developed for observing nuclear magnetic resonance. The particular instrumentation used depends largely on the charac- teristics of the resonance in the sample in question and on the type of resonance phenomenon being measured.Certain features are common to all resonance experiments. The sample is placed in the coil of a radiofrequency circuit tuned to the resonance frequency. This coil is mounted perpendicular to the main magnetic field, so that the electromagnetic radiation produced in the coil will have the proper polarization to induce transitions. A radiofrequency oscillator supplies energy to the tuned circuit and the reson- ance condition is detected as a change in the energy associated with the circuit. In most experiments the radiofrequency is kept constant and the resonance absorption line is plotted by sweeping the magnetic field through the resonance condition. Strong lines may be displayed on an oscilloscope, but weak lines require amplification with a narrow band system, and in this case the derivative of the absorption is usually recorded automatically.69 7 Sample volumes are ordinarily about 1 cm3, but in some cases have been as large as a litre and in others as small as 10-3 cm3.Experiments can be performed on gases, liquids, and solids, but not on ferromagnetic or strongly paramagnetic substances. The observables in nuclear magnetic resonance are much the same as in other types of spectroscopy : (i) the positions of the resonance lines, (ii) the resonance line intensities, (iii) the line shapes : and of course one can observe the dependence of these features upon conditions such as temperature, pressure, the applied magnetic fields, and the state and composition of the system.A particular interaction of a nucleus with its environment may affect more than one of these observables, so it is somewhat simpler to discuss the types of interaction separately rather than the observables.H. S. GUTOWSKY 189 ABSORPTION LINE SHAPES AND THEIR TEMPERATURE DEPENDENCE IN SOLIDS One of the first types of interactions studied and found to give structural in- formation is the direct magnetic dipole-dipole interaction in solids.8 The magnetic nuclei themselves produce local magnetic fields. If we take a proton in the neighbourhood of another proton, both at fixed positions, then the magnetic field each proton sees is the sum of the externally applied field and the local field from its neighbour. At 1 8 , this local field is about 15 gauss, but it falls off rapidly with distance.Moreover, the way in which the fields add up depends on the angle 8 between the internuclear vector and the external field. The contribution Hiw to the total field is given as where Y is the distance from the magnetic nucleus producing the local field. The net effect is to give a spread of several gauss in the total fields at different nuclei in a solid sample. In consequence, the external field will have to be varied over this range in order to bring nuclei with different local fields to the resonance condition. Such broadening of the resonance absorption can be used to determine internuclear separations with good precision. Van Vleck9 has given a general theory for the mean square deviation or second moment of the absorption about the line centre, AH22 = (3/2)1(1+ 1)N-lg2p2 2 (3 COS2 ejk - 1)2r’k-a j>k + ( 1 / 3 ) N - p c I,(Q + 1)(3 cos2 ejf - iprjf--6.j , f N is the number of nuclei at resonance over which the calculation is made, and the subscripts f refer to other magnetic species. For crystal powders the angular dependence averages to 4/5 and the com- putation is simply the r-6 summation. If, in the resulting equation, one structural parameter is particularly important, then the experimental second moment of the absorption can be used to evaluate that parameter. This sort of approach, and its value, are well illustrated in the following papers by Ford and Richards in which the B-H distance is determined in the borohydrides of sodium, potassium and rubidium and in which diketene is shown to have a particular structure.Normally, only one structural parameter can be obtained from the resonance absorption for a crystal powder. However, Drain’s paper on ammonium fluoride gives not only the N-H distance in the N b + ion but also the H-F distance. This is possible because he was able to observe the fluorine as well as the proton resonance. But if one can grow a single crystal, about 1 cm3 in volume, then the angular dcpendence of the second moment, and of the line shape, can give con- siderable additional information. The work of Andrew and Hyndman on a single crystal of urea is an excellent examplc of this type of study, These methods are particularly valuable for hydrogen for which diffraction methods are not well suited. A rclatcd phenomenon is the temperature dependcnce of the resonance line widths.If a broad resonance is observed in a sample at some temperature, then usually a higher temperature will be found at which the line width decreases. And in the liquid state the line widths are often very narrow, less than a milligauss for most hydrogen and fluorine compounds investigated. And unless special precautions are taken, the apparent width may be due mainly to inhomogeneities in the externally applied magnetic field. This temperature dependence of the line widths is called “motional narrowing”. Changes in the distances between nuclei or in the orientation of the internuclear vectors with respect to the applied field cause fluctuations in the local fields, including reversal in direction. And if these fluctuations are rapid and random enough they average out the direct line broadening effects of the local fields.190 CHEMICAL PROBLEMS Low frequency motions, the order of 10 to 100 kc/s, are effective in narrowing the resonance, and a considerable number of interesting studies of molecular motions in solids have bcen made in this manner.If the motions are not com- pletely random but arc restricted in some manner, say to reorientation of a mole- cule about a specific axis, then the narrowing of the resonance is incomplete; and from the extent of the line narrowing the nature of the hindered motion can sometimes be inferred.10 Smith’s paper, in this Discussion, is such an attempt to analyze the complex motions in polytetrafluoroethylene. In favourable cases the frequencies of the motions can be estimated from the line widths, and the temperature dependence can then be used to approximate the potential barriers hindering the motions.10 In his paper at this meting, Drain has made such an analysis of the NH4+ reorientations in solid NH4F.TI-THE SPIN-LATTICE RELAXATION TIME A second major class of interactions includes those responsible for spin- lattice relaxation. The term spin-lattice relaxation refers to the exchange of energy between thc nuclear spin system and the other degrees of freedom in the surroundings, i.e. the !attice. When nuclei absorb radiofrequency energy the temperature of the nuclear spin system increases above that of the lattice. The spin-lattice relaxation time TI is the time constant for the attainmcnt of thermal equilibrium between nuclei and lattice.You might say that T1 describes the rate at which a hot spin system cools off. Measurements of TI involve more or less directly thc intensity of the resonance absorption; a heated spin system will absorb lcss cnergy from the radiofrequency field than will a cold spin system. Spontaneous emission of the radiofrequency energy by nuclei in higher niagnetic statcs is negligible so the attainment of thermal equilibrium depends only on the spin-lattice relaxation induced by the thermal motions of the lattice. A radio- frcquency ficld at the resonance condition is required to change the orientation and energy of a nuclear spin. And in many substances the spin-lattice relaxation is a result of the oscillating local magnetic fields generated by the magnetic nuclei themselves as they are carried about in the various lattice motions.So measurc- ments of T I can bc used to investigate lattice motions. Bloenebergcn, Purcell and Pound 11 have made a thorough theoretical analysis of spin-lattice relaxation. A result of particular importance relates TI to a cor- relaxation time 7c for the molecular motions. In the case where T1 is determined by one particular type of motion, the theory gives a relatively simplc equation of the form, 1/T1 = K[TJ(I + 47r2vkC2) -t- 27,/(1 + 16.rr2v2rC2)], where K is a constant which can be calculated from the structure of the sample and a model for the thermal motions. This theory is very useful in analyzing the temperature dependence of the niolecular motions.T I measurcrnents are more time-consuming but have an advantage over line width measurements in that TI’S can give the frequencies of molecular motions over a much greater range of values. and more accurate activation energies Eu for thermally activated motions can be obtained by fitting the data to an cquation of the usual form, T~ -= 70 exp (Eu/RT). A discontinuity in T~ indicates a co-operativc change in the motions, useful in- formation in studying phasc transitions. It should bc mentioned that paramagnetic centres are particularly effective in producing spin-lattice relaxation. This is because the electronic magnetic moment is the order of a thousand timcs greater than nuclear magnetic moments; so the oscillating local fields are correspondingly greater and more effective in inducing nuclcar magnetic transitions.This feature has been used by Kozyrev in his interesting experiments reported here on the solvation of paramagnetic ions in aqueous solutions.H . S . GUTOWSKY 191 ELECTRONIC EFFECTS The last main class of interactions includes a wide variety of effects associated with the electronic environment of nuclei. Among these are the chemical shift, the indirect spin-spin coupling, conduction electron effects in metals, the effects of paramagnetic ions and the interactions of electric field gradients on nuclei with electric quadrupole moments ( I > 1/2). Space does not permit discussing all of these. However, two of the effects provide a very powerful means of structural analysis, and I should like to comment on them briefly.These are the chemical shifts and the indirect spin-spin interactions. These effects occur in solids as well as in liquids and gases. But they are small and therefore usually obscured in the solid by the larger direct dipole-dipole coupling which broadens the reson- ance. So the chemical shifts and indirect spin-spin interactions are usually ob- served in the liquid state, in magnetic fields of high homogeneity, where they form the basis for " high resolution NMR spectroscopy ". The chemical shifts arise from diflerences in the electronic environment of the nuclei. The electrons in their motions interact with the applied magnetic field and cancel out a small fraction of it, or even add to it. Thus where Ho is the magnetic field actually at the nucleus, Ha is the field in the bulk of the sample and a is an internal magnetic susceptibility of the electron distribu- tion.The effect is similar to the magnetic susceptibility of the bulk sample. However, since the electrons are concentrated about the nuclei, the effects are up to several thousand times greater than differences in bulk diamagnetic suscepti- bility, ranging from 10-5 for protons to 10-3 for thallium. If a sample has nuclei with different electronic environments, then the reson- ance line will have different components corresponding to the different electronic environments ; and the intensities of the different components correspond to the fraction of the nuclei in each electronic environment. It should be noted that the separations of these components are directly proportional to the applied magnetic field.The chemical shifts provide a very useful tool, not only for structural 12 and quantitative analysis in the usual sense, but also for learning more about the distribution of electrons in chemical bonds. The latter appli- cation is restrained by the complexity of the general theory for the effect, as developed by Ramsey.13 But there is hope that valid simplifications can be made. Thc indirect coupling of the magnetic nuclei by the electrons in the sample gives rise to a multiplet structure14 in the resonance which, like the chemical shift, is rather small and most readily observed in liquids. The effect may bc described somewhat as follows. Consider two nuclei, A and B. When an electron is in the vicinity of nucleus A, the electron becomes polarized by the local magnetic field of A.When this polarized electron travels over to nucleus B it changes the magnetic field seen by that nucleus. In different molecules A will have different orientations with respect to the main magnetic field. So in the bulk sample, the resonance of nuclei B is split into a number of Components given by the orientations of nuclei A in different molecules. And the intensities of the com- ponents depend on the statistical weights of the orientations of A. This indirect interaction is a mutual coupling, so the resonances of both A and B arc split by equal amounts in terms of energy. The splitting is independent of applied magnetic field so that it can often be unscrambled from chemical shifts by observing the resonances at different fields. The multiplets, like the chemical shift, provide information about molecular structure and electron distribution.14 A number of excellent high resolution spectra illustrating the use of chemical shifts as well as the multiplets for structural analysis are given in the papers by Shoolery and by Ogg.192 CHEMICAL PROBLEMS Under certain conditions dynamical processes such as chemical exchange perturb the appearance of a high resolution spectrum.15 Take a case in which protons can exchange between two different structural sites.In the absence of exchange two separate resonance lines are observed. But when exchange OCCU~S at a “ fast enough ” rate the different electronic environments average out and a single resonance line occurs at an intermediate position.The rate required for the averaging is determined by the difference to be averaged. If the resonance components are separated in the absence of exchange by an amount 8v, say 100 c/s, then the correlation time T for the exchange must be the order of 1/8v or sec to average out the chemical shift. The same argument holds for the indirect spin-spin multiplets and is the basis for the ingenious double resonance experiment used by Shoolery to remove the coupling. The range of lifetimes which can be studied by observing the collapse of a multiplet or chemical shift is roughly from 1 to 10-4 sec. Rates in this range are rather difficult to investigate by other means so this can be a very im- portant application. Ogg and Ray describe some interesting results of this type in their paper on the aluminium borohydrides.CONCLUSION In conclusion I wish to emphasize the sensitivity of nuclear magnetic resonance measurements. Anderson and Arnold report in their paper the resolution of interactions as small as + c/s, which is the order of 10-11 cal/mole, and it is this really incredible sensitivity which makes nuclear magnetic resonance as Useful as it is. I hesitate to predict the future development of these chemical applications. Extrapolation is always dangerous and in this case there are only five years’ experience on which to draw. However, only a moderate fraction of nucIei have properties well suited to magnetic resonance experiments. And for this reason 1 doubt that nuclear magnetic resonance will ever be as widely applic- able, for instance, as infra-red spectroscopy, But it is certainly clear at this time that nuclear magnetic resonance will be used increasingly in the study of many chemical and physical problems. It is a pleasure to acknowledge the support by the U.S. Office of Naval Research and by the E. I. duPont de Nemours and Company of the research programme in radiofrequency spectroscopy at the University of Illinois. 1 Bloch, Science, 1953, 118,425. 2Purcell, Science, 1953, 118,431. 3 Ramsey, Nuclear Moments (John Wiley and Sons, Inc., New York, 1953). 4 Smith, Quart. Rev., 1953, 7,279. 5 Gutowsky, Ann. Rev. Physic. Chem., 1954, 5, 333. 6 Pake, Amer. J. Physics, 1950, 18,438, 473. 7 Andrew, Nuclear Magnetic Resonance (Cambridge University Press, London, 1955). 8 Pake, J. Chem. Physics, 1948, 16, 327. 9 Van Vleck, Physic. Rev., 1948,74, 1168. 10 Gutowsky and Pake, J. Chem. Physics, 1950, 18, 162. 11 Bloembergen, Purcell and Pound, Physic. Rew., 1948,73, 679. 12 Meyer, Saika and Gutowsky, J. Amer. Chem. SOC., 1953, 76,4567. 13 Ramsey, Physic. Rm., 1950, 78, 699 ; 1952, 86, 243. 14 Gutowsky, McCall and Slichter, J. Chem. Physics, 1953, 21, 279. 1s Gutowsky and Saika, J. Chem. Physics, 1953,21, 1688.

ISSN:0366-9033    

DOI:10.1039/DF9551900187

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

PROTON RESQNANCE SPECTRA AND THE STRUCTeTRE OF DIMETENE BY P. T. FORD AND R. E. RICHARDS Physical Chemistry Laboratory, South Parks Road, Oxford Received 17th January, 1955 The proton resonance absorption spectrum of crystalline diketene shows a doublet structure with a central minimum. The shape and width of the absorption line is shown to agree closely with the theoretical curve for a CH2 group with an intermolecular broaden- ing of 3.7 gaussz. These facts provide strong support for the Structure, chosen by Katz and Lipscomb on the basis of X-ray measurements, CH2=C-CH2 I I 0-c=o Various structures have been proposedl-5 for diketene, and until 1952 the evidence was ambiguous, Chemical and spectroscopic evidence,6-12 which was concerned mainly with the liquid state, could best be interpreted by structures I or TI or a mixture of them : 1 6 CH2=C--CH2 0-c=o 3 4 2 1 1 5 o-&o 3 4 I I1 Katz and Lipscomb 13 showed that X-ray diffraction measurements on a crystalline samplc could be explained in tcrms of structure I but not by structure 11, since the values givcn for the C-C distance 1 is 1*35A, and for the C-C distance 6 is 1-48 A.The measurements described below provide confirmation of this result by showing that the protons all occur as pairs in crystalline diketene, as in structure I, whereas this is not so in structure 11. EXPERIMENTAL Ketene was prepared by pyrolysis of acetone 14 and condensed in acetone at the temperature of solid carbon dioxide. Polymerization occurs when the solution is allowed to warm to room temperature.15 Some difficulty was experienced in removing all im- purities by fractional distillation and two succcssive low temperature recrystallizations from ether were carried out before the final fractional distillation.The final sample melted sharply at - 6.5" C. A further check on the purity could be obtained from the ultra-violet spectrum of a solution in cyclohexane. This was similar to that given by Roberts, Armstrong, Trimble and Burg 16 except that whereas a weak maximum was obscrved by them at 315 nip, our sample showed a vcry weak one at 275 mp. By further purification, a small sample was obtained which showed no maximum in this region at all, so it seems likely that this maximum is due to a small amount of some impurity. The diketene was distilled in O ~ C U O into a thin-walled Pyrex tube and sealed off.The nuclear resonance spectrometer has been described.17 Measurements were made at 20" K using liquid hydrogen as refrigerant because the most satisfactory ratio of signal to noise was obtained at this temperature. G 193194 STRUCTURE OF DlKETENE RESULTS Fifteen traces were obtained and the second moment of the proton resonance spectrum was 14.77 gauss2 with a standard deviation of 0.92 gauss2. The continuous curve in fig. 1 is the absorption curve obtained by integration of the best six derivative curves obtained. DISCUSSION In structure 1 thc protons occur in pairs in the two CH2 groups, whereas in structure I1 the protons occur at the corners of an equilateral triangle in the methyl group and singly in thc CH groups.The proton resonance spectrum of structure T would therefore be cxpected to show a doublet structure with a central niinimum,l* whereas structure I1 would give rise to a curve with three maxima of which the central one is much taller than the outer 0 n d 9 C ~ u r s FIG. 1 .-Continuous curve is observed proton resonance absorption curve for crystalline diketene at 20" K. Circles are theoretical points calculated for pairs of protons 1.784 A apart (a = 3.72 gauss) with a broadening function, 182 = 3.7 gauss2. Fig. 1 shows a clear central minimum and this alone provides strong evidence for the occurrence of the protons only in pairs. Other configurations of protons give a central maximum, and the presence of thcse to more than a few per cent would completely mask the central minimum characteristic of the pairs.The points plotted in fig. 1 are calculated theoretically 18 for pairs of protons 1.784 apart with an additional gaussian broadening of 3-7 gauss2 to take account of intermolecular interactions, field inhomogeneities, etc. This corresponds to a C-H distancc of 1.093 A as in methane, and a tetrahedral HCH angle. The agreement with the experjmental curve is good and provides strong evidence for structure I. The second moment of the theoretical curve is the same as the valuc found experimentally. The only other proposed structure for diketene which would be consistent with the abovc measurement is CH2-CO CO -CH2 I I and this is excluded on other grounds.6-13 We should like to thank Dr. D. F. Evans for help with the ultra-violet spectro- scopic measurements, and the Department of Scientific and Industrial Rcsearcli for a Maintenance Allowance to one of us (P.T. F.).P. T. FORD AND R . E . RICHARDS 195 1 Chick and Wilsmore, J. Chem. SOC., 1908, 93, 946. 2 Staudinger and Bereza, Ber., 1909, 42, 4908. 3 Chick and Wilsmore, J. Chenz. SOC., 1910, 97, 1978. 4 Hurd and Williams, J. Amer. Chem. SOC., 1936, 58, 962. 5 Boese, Ind. Eng. Chem., 1940, 32, 16. 6 Angus, Leckie, Le Fbvrc, Le Fbvre and Wasserman, J . Chem. SOC., 1935, 1751. 7 Rice and Roberts, J. Amer. Chem. Sac., 1943, 65, 1677. 8 Calvin, Magel and Hurd, J. Amer. Chem. Soc., 1945, 67, 754. 9 Taufen and Murray, J. Amer. Chem. Soc., 1945, 67, 754. 10 Whiffen and Thompson, J. Chem. SOC., 1946, 1005. 11 Miller and Koch, J. Amer. Cltem. SOC., 1948, 70, 1890. 12 Hurd and Blanchard, J. Arner. Chem. SOC., 1950, 72, 1461. 13 Katz and Lipscomb, Acta Cryst., 1952, 5, 313. 14 Williams and Hurd, J. Org. Chem., 1940, 5, 122. 15 Williams and Krynitsky, Organic Syntheses (Wiley and Son, 1941), 21, 64. 16 Roberts, Armstrong, Trimble and Burg, J . Amer. Chent. SOC., 1949, 71, 843. 17 Pratt and Richards, Truns. Furuduy SOC., 1953, 49, 744. 18 Pake, J. Chem. Physics, 1948, 16, 327. 19 Richards and Smith, Truns. Furaduy Soc., 1951, 47, 1261.

ISSN:0366-9033    

DOI:10.1039/DF9551900193

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

P. T. FORD AND R. E . RICHARDS 195 A PROTON MAGNETIC RESONANCE INVESTIGATION OF THE STRUC'ITURE OF UREA. BY E. R. ANDREW* AND D. HYNDMAN~ Received 25th January, 1955 The magnetic resonance spectrum of the protons in a single crystal of urea has been recorded for a series of oricntations of the crystal relative to thc applicd niagnetic field. A study of the variation with orientation of the mean square width of the spectrum shows that the symmetrical non-planar model of the urea molecule is incorrect, and that within expcrimcntal error the moleculc is planc with symmetry C2v. Parameters of the planar urea moleculc in best agreement with the experimental values of mean square spectral Nidth are : N-El bond lengths of 1-046 + 0.01 A, HNH angles of 119.1" -i 2", and outer CNH angles of 120.5" -1.2". The procedure for growing single crystals of urea of mass 1-2 g is briefly described. lt is wcll known that a study of the proton mabmetic rcsonance absorption spectruni of crystals containing hydrogen atoms frcquently enablcs tlic positions of thc protons to bc located in the unit ccll of the crystal Irrtticc. Such work complements that of the X-ray crystallographer who can provide accurate ccll co-ordinates for thc heavier atoms, but who can a t best give only a very rough indication of thc positions of the hydrogen atoms. In ii crystal containing hydrogen atoms, the magnctic dipolar interaction betwecn the protons slightly displaccs the energy levels which they occupy in the strong mpgnetic field of several kilogauss which i s applied.Ti1 conscquencc the magnctic resonance spcctrum is broadcned, the breadth and structwc of the spcctrum depending upon the strength and spatial disposition of thc intcractions. Thc cnergy of intcraction betwcen two p~otons depends upon the invcrsc cube of thcir separation, so that near neighbows haw a dominant cffcct. ?'he intcr- action encrgy depcnds also on the orientation of tlic proton pair rclativc to thc applied magnetic field ; thc perturbation produced by the interaction is thercforc anisotropic. A full examination of thc rcsonance spectrum of a single cIystal * Dept. of Physics, University College of North Wales, Bangor, Wales t Dept. of Natural Philosophy, The University, St. Andrews, Scotland196 STRUCTURE OF UREA can thus furnish information concerning the configuration of near proton neighbours in relatively simple systems.We have applied this mcthod to urea, OC(NH2)z. It is known from X-ray analysis that the four heavy atoms, lie in a pIane.1-4 Fig. 1 shows the arrange- ment of the heavy atoms, which has C2, symmetry; the interatomic distances and angles which are shown are those found by Vaughan and Donohue 4 in their recent accurate X-ray investigation. It has not been clear, however, whether the four hydrogen atoms lie in the same plane as the heavy atoms as shown in fig. 1 (a), or whether they lie symmetrically above and below this plane as shown in fig. 1 (b), or whether the molecular con- figuration has lowcr symmetry than C~J. In carrying out an analysis of the vibrations of the molecule, Kellner 5 assumed the symmetrical non-planar model of fig.1 (b) and interpreted her infra-red spectra on this basis. However, later analyses of infra-red spectroscopic measurcmcnts by Kcller? and by Waldron and Badger,7 have suggcsted that thc entirely planar model of fig. 1 (a) is the correct one. It sccmcd that this was a good case where the nuclear magnetic resonance method could distinguish between two rival structures and furthermore provide quantitative information conccrning thc positions of the protons. EXPERIMENTAL The measurements were made on two single crystals of urea of mass 1.0 and 1.6 g, which were grown from methyl alcohol solution by a modification of Moore’s cooling method. Small seed crystals were mounted on rotating supports in the solution, which was contained in an inner tank made from a Kilner jar.The inner tank was immersed in an outer tank containing water, whose temperature was automatically controlled. The temperature of the outer bath was lowered at a rate of about to C per day for about two weeks. The inner jar held about 1.5 1. of methyl alcohol saturated with urea at 40” C initially. The crystals have tetragonal symmetry and tend to crystallize as ncedles roughly square in section and elongated along the direction of the tetragonal axis. Addi- tion of a small quantity of ammonium bromide to the solution discouraged the growth along this symmetry axis. About 8 g of ammonium bromide was added to the solution, and the crystals obtained had a length/width ratio of about four. A chemical test, using Nessler’s reagent, on a solution of a part of one urea crystal, showed that no detectable amount of ammonium bromide was taken up in crystallization.The proton magnetic resonance absorption was detected by a bridge method similar in general form to that described by Bloembergen, Purcell and Pound.8 The steady horizontal magnetic field was provided by a permanent magnet having a field strength of 5500 G between polefaces 20 cm in diameter separated by a gap of 5 cni. By means of auxiliary coils the field was modulated with small amplitude at 25 c/s and was traversed through the spectrum at a constant rate. The amplified signal from the bridge was fed through a phase-sensitive amplifier to a recording current meter, which thus registered the first derivative of the spectrum.In order to get a good filling factor, the radio-frequency specimen coil was made rectangular in section to fit very closely round the crystal. The coil was rigidly mounted inside the lower end of a 35 cm long, 2.5 cm diameter, vertical brass tube, the lower end of the coil being soldered to the tube, and the upper end to an inner conductor passing axially up the tube into a brass box at the top containing other components of the radio- frequency bridge. The coaxial assembly projected vertically downwards into the magnetE. R . ANDREW A N D D. HYNDMAN 197 gap, with the crystal in its radio-frequency coil located in a homogeneous part of the magnetic field. The assembly could be rotated about its vertical axis, the orientation of the crystal relative to the magnetic field direction being read off on a circular scale.It is of course important to carry out the measurements at a temperature which is low enough to suppress any molecular motion which might reduce the width of the reson- ance spectrum. Preliminary experiments at room temperature and at about 90" K using polycrystalline urea disclosed no significant difference in the mean square width of the spectrum. The single crystal measurements were therefore all made at room temperature. Taking the two single crystals in turn, each crystal was first mounted with its tetrad [Ool] axis along the vertical axis of rotation, and perpendicular to the steady magnetic field. The resonance spectrum derivative was then recorded for various azimuth angles Urea H.38 8=180° 2 71 Y54 - FIG.2.-Typical 4 proton resonance absorption derivative curve, taken with the applied steady mabmetic field parallel to the tetrad axis of the urea crystal. The pips on either side of the record are for calibrating the scale in terms of magnetic field. I I boo 90. 0' b 0' 110. 1.30. PI. Po ( 0 ) ( b) FIG. 3.-Experimental values of second moment and theoretical curves, A for the non- planar model and B for the planar model. The experimental points and theoretical curves in (a) refer to rotation of the crystal about [Ool] ($0 = ~/2), while those in (6) refer to rotation about [110] ($0 = q'4). 40 between the field direction and the [loo] direction. The crystal was then mounted with its [110] axis along the vertical axis of rotation. Since the crystal was long in the [OOl] direction it was necessary to cut the crystal into four pieces and stack them.The resonance spectrum was then recorded for various angles $0 between the field direction and the [Wl] direction. A typical record is shown in fig. 2. Several runs were made for each setting. The second moment (mean square width) of each spectrum was cal- culated and the mean for each setting taken. A small correction was applied for broadening of the spectrum by finite modulation amplitude ; 9 the correction for broaden- ing caused by inhomogeneity of the magnetic field was small enough to be neglected. The variation of the corrected second moment with crystal setting is shown in fig. 3. Maximum and minimum values are given in table 1. The calculated probable errors are about 0.3 G2.As the record in fig. 2 shows, the spectrum exhibited a fine structure. This structure was most pronounced when the magnetic field was directed along the tetragonal axis.108 STRUCTURE OF UREA TABLE 1 parameters used for calculation of theoretical values of second moment (G2) values of second moment. ---_____ __ N-H bond H,NIHZ CNIH, field along field along field along (A) [ 1001 [W11 [WlI 1 440 120" 120" 13.5 18-6 29.3 1 *020 120" 120" 13.9 19.6 31.8 1 -040 122" 120" 12.9 19.2 27.2 1.040 120" 118" 13.8 17-8 31.8 mean experimental values 13.6 18.2 29.0 DISCUSSION Theoretical values of the second moment have been calculated for both planar and non-planar structures using the formula of Van Vlcck.10 This calculation requires a knowledge of the cell coordinates of all the nuclear magnetic dipoles, namely the protons and the N14 nuclei.This in turn means that the X-ray information concerning the coordinates of heavy atoms in the unit cell must be supplemented by assumptions concerning the lengths of the N-H bonds aiid the values of the CNH angles, and also, for the non-planar model (fig. 1 (6)) conccrning the orientation of the planes dcfined by the NH2 groups. After consideration of the bond lengths and angles in other compounds, we adopted as trial values N-H lengths of 1.OOA and CNH angles of 120". For the non- planar model it was further assumed that the NH2 planes contained the carbon atom of the molecule. The sccond moment depends most sensitively on the contribution of the close proton pairs in the NH2 groups so that any incorrectness in the last assumption is unimportant in preliminary calculations.The theoretical values are shown as a function of crystal setting in fig. 3 for both models. The curve for the planar model is actually based on more refined molecular parameters to be discussed later; the values of second moment ob- tained with the trial parameters differed from those shown by less than 10 % for all orientations. It is seen from the figure that the experimental points are in good accord with the theoretical curves for the planar model, while there is no agreement with the markedly different curves for the non-planar model ; indeed, for rotation about the [110] axis even the qualitative form of variation is quitc different. Having concluded that the non-planar model of fig.1 (b) was incorrect, the next step was to improve the parameters of the planar model of fig. 1 (u) jn order to get theoretical values of second moment in closer accord with experiment. In practice this meant obtaining better agreement with thc three extremum values in table 1, since these were more accurately determined than intermediate values and, providcd agrecment is obtained for them, all intermediate experimental and theoretical valucs are in tolerably good agreement. In order to determine precisely the positions of the four protons relative to the known skeleton of heavy atoms of the model of fig. 1 (a), four parameters are needed : (i) lcngth of bond NI-HI , (ii) Iength of bond Nl-EI2, (iii) angle HINIH~, (iv) angle CNlHl. Sincc only three parameters can be uniquely determined by fitting the three experimental values, it was assumed that the lengths of the NI-HI and N1-H2 bonds were equal.Values of this bond length and of the two angles were then sought which brought the theoretical values into agreement with the experimental values. By trial it was found that by increasing the N-H lengths to 1.04 A, and keeping the angles at 120°, quite good agreement was obtained (see table 1). The sum of the squares of the differences between the three theoretical and experimcntal pairs of second moments was used as a measure of disagreement. Having obtained this close agreement, a small change was made in each of the three parameters separately and theoretical values were in each case calculated.TheE. R . ANDREW AND D . HYNDMAN 199 results are given in tablc 1. The theoretical values of second moment were then written as a function of the threc parameters and expanded by Taylor's theorem to a first order about the closest values obtaincd by trial. The three partial first differential coefficients were obtained from the calculated cRect of changing the three parameters scgarately. Tt remained only to solve three sirnultancous linear equations in thrcc variables to obtain exact agreement with cxperiment. Thc values of the threc variablcs giving the exact agreement were : N-H bond length 1446A, NlNlH2 angle of 119-1", CNlHl angle of 120.5". Using the estimated errors of the experimental moments thc accuracy of the bond length was found to be & 0.01 A, and of the angles f 2".The proton-proton separation in each amino group is 1.803 0.015A. A check on these conclusions may bc obtained from a study of the structure of the spectrum obtaincd when the applied field is parallel to the tctragonal axis of the crystal. As fig. 4 shows, the FIG. 4.-The tetragonal unit cell of crystaIline urea. The diagram shows a projection on the xz plane; the numbers alongside the heavy atoms are their fractional y co-ordinates. The planes of the molecules all in- clude the z direction and are inclined alternately at 45" and 135" to the plane of the diagram; the planes of alternate molecules are thus at right- angles to each other and lie along diagonal planes of the unit cell. The lattice constants 4 are a0 = 5*66A, co = 4-71 A.two molecules of urea in each unit cell are 43 both arranged with the C-0 bonds parallel to the tetragonal axis. When the applied field is parallel to this axis the close proton-proton pairs, namely, those in the NH2 groups, are oriented with the same angle 8 with respect to the field. The spectrum should therefore consist of the usual two lines obtained from similar pairs,ll broadened by the interactions of next nearest neighbours. The separation bctwcen the two lincs would be 3pr-3 (3 cos2 8 - I), whcre p is the proton magnctic moment and r is the interproton distance. Taking for Y the value 1.803 f 0.01 5 8, and for 8 the value 3 1 =t 14" based on molecular parametcrs obtained above, the separation of the two lines should be 8-7 4 0.5 gauss.The actual mean value between the resonance absorption maxima, from derivative records such as that shown in fig. 2, is 8.9 f 0.2 gauss. This agree- ment lends further support to the structure deduced from these second moment measurements. If, on the other hand, the non-planar model of fig. 1 (b) had been correct, all the NH2 proton pairs would have been oriented perpendicular to the applicd field, with 8 = 90"; the separation of the component lines would then have been only 7-2 gauss for a proton pair separation of 1-80A, arid in order to get agreement with the observed value the pair separation would have to be decreased to 1.688,. The 1.046 8, N-H bond found in urea is longer than the 1.014 8, bond found in ammona,l2 and rather greater than the values ranging from 1-03 to 1.035A found for the ammonium ion.13.14~ 15 This increase in length is probably to be attributed to hydrogen bonding with the oxygen atoms of adjacent molecules. Hydrogcn bonding to oxygen causes the bond to be lengthened to 1.048 8, in certain hydrazine salts,l6 and the stronger hydrogen bonds in hydrazine fluoride cause a lengthenng 17 to 1.075 A.200 AMMONIUM FLUORIDE 1 Hendricks, J, Amer. Chem. SOC., 1928,50,2455. 2 Wyckoff, 2. Krist., 1930,75, 529 ; 1932, 81, 102. 3 Wyckoff and Corey, Z. Krist., 1934, 89,462. 4 Vaughan and Donohue, Acta Cryst., 1952,5, 530. 5 Kellner, Proc. Roy. SOC. A , 1941, 177, 456. 6 Keller, J. Chem. Physics, 1948, 16, 1003. 7 Waldron and Badger, J. Chem. Physics, 1950, 18, 566. 8 Bloembergen, Purcell and Pound, Physic. Rev., 1948, 73, 679. 9 Andrew, Physic. Rev., 1953, 91, 425. 10 Van Vleck, Physic. Rev., 1948, 74, 1168. 11 Pake, J. Chem. Physics, 1948, 16, 327. 12 Herzberg, Infra-red and Raman Spectra of Polyatomic Molecules (Van Nostrand, 13 Levy and Peterson, Physic. Rev., 1952, 86, 766. 14 Bersohn and Gutowsky, J. Chem. Physics, 1954, 22,651. 15 Gutowsky, Pake and Bersohn, J. Chem. Physics, 1954, 22, 643. 16 Pratt and Richards, Trans. Faradzy SOC., 1953, 49, 744. 17 Deeley and Richards, Trans. Faraday SOC., 1954,50, 560. 1945), p. 439.

ISSN:0366-9033    

DOI:10.1039/DF9551900195

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

200 AMMONIUM FLUORIDE NUCLEAR MAGNETIC RESONANCE IN AMMONIUM FLUORIDE BY L. E. DRAIN Chemistry Dept., University College, London * Received 26th January, 1955 The line widths of the proton and fluorine magnetic resonances in powdered ammonium fluoride have been measured at temperatures between 140" K and 360" K. Line-width transitions were found in both resonances at about 280" K. From the second moments of the low temperature absorption lines, the N-H distance in the ammonium ion was determined to be 1.04 3 0.01 A and the H-F distance in the crystal, 1.64 f 0.02A. The widths of the resonances at high temperatures were consistent with the supposition of a hindered rotation of the ammonium ion. From the dependence of mean square line-width on temperature, the barrier to this rotation was estimated to be about 10,000 cal/mole.The ammonium salts are interesting subjects for nuclear magnetic resonance investigations as the proton resonances are readily observable and there are problems connected with the orientation and motion of the ammonium ion. Several ammonium salts havc been examined by this method and in particular proton resonances from ammonium chloride and bromide have been studied in considerable detail.192 In this paper, thc rcsults of a similar study of ammonium fluoride are presented. The main results of intercst from thew measurements concern the dimensions and motion of the amnionium ion. The determination of the N-H distance in thc ammonium ion is based on the fact that a t low tem- peratures, the main contribution to the proton resonance line width arises from the magnetic interactions betwecn protons in the same ion.If wc assume that the ammonium ion is tetrahedral, a determination of thc mean square width of the proton resonance leads to an accurate value of the N-H distance. At higher temperatures, the motion of the ammonium ion lcads to a narrowing of thc resonance. In ammonium fluoride it is found that the transition in the linc- width occurs at a much higher tcmperature than in thc other ammonium halides indicating that a higher barrier hinders the rotation of the ammonium ion. * Present address : Metallurgy Division, A.E.R.E., Harwell, Didcot, Berks.L. E. DRAIN 201 Additional information can be obtained by the observation of the fluorine resonance. The 19F nucleus has a spin of 3 and its magnetic resonance is thus free from quadrupole broadening effects and the resonance line width may be interpreted in the same way as that of the proton.In ammonium fluoride, the line-width is sensitive to the distance of the fluorine atom from neighbouring hydrogen atoms and provides useful evidence for the positions of the latter in the crystal structure. The gyromagnetic ratio of 19F is close to that of the proton and thus measurements may be made with minor readjustments to the proton resonance apparatus. Unlike the other ammonium halides, which undergo transitions between a number of cubic or pseudo-cubic modifications, NH4F has a single hexagonal crystalline form whose structure 3 is similar to that of ice. Each ammonium ion is surrounded tetrahedrally by four fluoride ions and it may be supposed that the tetrahedral ammonium ion is so orientated that the hydrogen atoms lie on the lines joining nearest neighbour nitrogen and fluorine atoms forming strong N-H-F bonds.This assumption is supported by the present work. It will be notcd that in this structure there are no alternative orientations of the am- monium ions of almost equal energy which could lead to an order-disorder trans- formation like those found in thc other halides. The fluoride in fact possesses no A-transition although a small peak in the heat capacity against temperature curve has been found.4 In any casc, it is found that these transitions have very little influence on thc proton magnetic resonance. This fact is in agreement with the view established by neutron diffraction 5 and other methods that these transi- tions are due to a change from an ordered to a disordered arrangement of the ammonium ions, there being, however, no marked change in their rate of rotation.EXPERIMENTAL The main magnetic field of about 4,200 gauss was provided by a permanent magnet. The pole pieces were in the form of truncated cones of semi-angle 30" tapering from 5 in. to the pole face diameter of 4 in. The gap was lAin. For maximum homogeneity of the magnetic field the pole faces had shims 0.05 in. high and 0.16 in. wide in accordance with the calculations of Andrew and Rushworth.6 At the position of maximum homo- geneity, the maximum variation of magnetic field over a 1 cm3 specimen was about 0.5 gauss.The widths of liquid resonances were about 0.2 gauss. The method of detecting the nuclear resonance follows that of Bloch, Hansen and Packard,7 in that two radio frequency coils are employed, one for providing the r.f. field and the other for receiving the signal due to the precessing nuclear moments. By dis- posing the coils so that their axes are at right-angles, the direct coupling between them is made small. In the original apparatus,7 a semi-circular piece of metal placed in the r.f. field was used to adjust the r.f. voltage picked up in the receiving coil. This arrangement was, however, inconvenient for the present experiments and the alternative balancing network shown in fig. 1 was devised. The subsidiary coils S1 and S2 wound in opposite directions and loosely coupled to the receiving coil are placed in series with the trans- mitting coil SO which is wound in two sections to facilitate the insertion of the specimen.By adjusting the 100 pF differential condenser C1C2, the ratio of the r.f. currents flowing through S1 and S2 can be altered and a high degree of balance obtained. Complete balance may be achieved by the use of the variable condenser C3 (100 pF) in series with the resistance R (150 D). To observe the absorption component of resonance, the differential condenser C1C2 is mis-set to give a convenient input to the receiver. The transmitting and receiving coils are wound on the same former and to eliminate interference from nuclear resonances from the former, two coil systems were used with formers made of polytetrafluoroethylene for observing proton resonances and of Perspex for fluorine resonances.To minimize frequency drift, the oscillator employed the modified Colpitts circuit described by Gouriet 8 and the frequency was checked at intervals against a crystal con- trolled oscillator. The frequencies used were around 17.95 Mc/s for the proton resonance and 16.88 Mc/s for the fluorine resonance. The receiver consisted of 2 r.f. tuned ampli- fying stages employing the E.F. 95 valve, followed by a detector and a.f. amplifier. The202 AMMONIUM FLUORIDE oscillator and receiver were operated from batteries. To increase discrimination against noise and stray pick-up, the method of a.c. modulation 9 , 1 was uscd. The magnetic field was modulated by a small fraction of the line width at a frequency of 40 cis by means of I 1 FIG.1.-Diagram of the coil system and network. balancing sweep coils around the pole pieces of the magnet. The out- put of the rcceiver was fed into a narrow pass-band lock-in mixer-amplifier. The d.c. out- put of this amplifier plotted against magnetic field gives the derivative of the resonance ab- sorption. The main magnetic field was varied by means of coils wound on the permanent magnet. The field could be varied over a range of about 100 gauss. Hysteresis was observed but normally cor- rections for this were negligible. The field was calibrated by varying the frequency of the oscillator in steps of 10 kc/s and observing the changes in current in thc coils required to bring the field exactly to the resonance value for protons in a water sample.Temperature control was achieved by a thermostat working on the heat-leak prin- ciple.10 The specimen and coils were surrounded by a copper can 2 mm thick which was fixed to a copper bIock by means of Rose's metal. A can above the copper block could be filled with various refrigerants (liquid air or solid C02 and methylated spirits) and the whole system was surrounded by a vacuum jacket which was continuously pumped. The copper block is in moderate thermal contact with the can containing refrigerant through German Silver supporting tubes and by varying the power supplied to a heater in the block any temperature up to about 150" C above that of the refrigerant could be main- tained. The temperature was measured by copperanstantan thermocouples placed in the copper block and inside the can near the specimen or when practicable in contact with the specimen. The readings of these thermocouples always agreed to within 2" C and in general the estimated error in the measurement of the temperature of the specimen was f 1°C.The ammonium fluoride powder used was commercial A.R. material compressed into suitably shaped specimens and dried over calcium chloride. Below about 220" K the signals that could be observed were very weak owing to relaxation times of the order of 10 min or more. To avoid saturation effects r.f. fields lcss than 10-3 gauss were used. In order to obtain a good signal at these temperatures the specimens used were as large as possible and this could not be protected from the atmosphere, thc coppcr can containing the coils was, however, moderately airtight and very little moisture was absorbed.At higher temperatures, where greater signal strength was availablc, many of the measure- ments were done with specimens sealed in glass tubes. The results were generally in agreement except that above about 250" K, the unprotected samples often showed a narrow fine structure at the centre of the resonance due presumably to the absorption of water. The effect of this on thc mean square width of the line was, however, always negligible. All measurements of mean square width were corrected for sweep amplitude, H, by subtracting t. H,2 and for field inhomogeneity (0.1 gauss2). RESULTS AND DISCUSSION The line-widths of the proton and fluorine resonances are plotted against temperature in fig.2. For the proton resonance, the line-width is defined in the usual way as the separation in gauss between the maximum and minimum of the derivative of the absorption curve but owing to the shape of the fluorineL. E. DRAIN 203 resonance at low temperatures, this is not a satisfktory definition for this case. Instead, the half-width of the integratcd absorption curve was used. The line shape of the resonanccs at 200" K, i.e. below the line-width transition, are shown in fig. 3. These curves are averages of sevcral independent curves. The proton resoiiance is rather similar in shape to those in the other ammonium halides. The rather different form at the centre of the line is probably due to the presence of a significant H-F interaction in the fluoridc.At temperatures below the line-width transition, it is assumed that the ions may be considered to be stationary (except for a correction for the torsional oscillation of the ammonium ion). With this assumption Van Vleck 11 has shown that the mean square width of the resonance may be related simply to the magnetic interactions between neighbouring nuclei, I 5 - 4 - A I50 200 2 5 0 300 3 5 0 T E M P E R A T U R E (OK) I I I I I 1 1 I 1 1 I I I 5 0 200 2 5 0 300 3 5 0 T E M P E R A T U R E (OK) FIG. 2.-The line widths of (a) the proton resonance and (b) the fluorine resonance in ammonium fluoride as functions of temperature, The mean square width of the proton resonance will be considered in two parts.Firstly, there is a contribution arising from interactions between nuclei in the same ion. The major part of this arises from proton-proton interactions but there is a small term due to the interaction of the protons with the nitrogen nucleus. If we assume that the ammonium ion is tetrahedral, the total contribution is found to be 58.90(1 - ~ ) / r 6 gauss2, whcre r is the N-H distance in thc ammonium ion cxpressed in A. (1 - E) is a corrcction term for the zero-point torsional oscillation of the ion discussed in detail by Gutowsky, Pake and Beitsohn.2 They show that (1 - E ) may be obtained from the relation where I is the moment of inertia of the ammonium ion and v, the frequency of204 AMMONIUM FLUORIDE torsional oscillation. Unfortunately, infra-red spectroscopic measurements of this frequency are not available for ammonium fluoride, but in the next section it is shown that this frequency is probably close to that in ammonium chloride and a value of 1.4 x 1012 sx-1 is suggested.This leads to (1 - E ) == 0.92. It must be remembered, however, that this is only an estimate and the uncertainty here affects the accuracy of the measuremcnt of the N-H distance. The second contribuion to the mean square width arises from the magnetic interactions of protons with nuclei not in the same ion. This may be regarded as a correction term and so does not have to be known with high accuracy. 1 I- 2 0 10 0 I 3 2 0 (H-H,) GAUSS FIG. 3.-Derivatives of the magnetic resonance absorption at low temperatures, (a> 1H resonance at 200" K (mean of 7 curves), (b) 19F resonance at 205" K (mean of 5 curves).The double arrows indicate the field modulation. The absorption intensities are arbitrary. However, a knowledge of the crystal structure is important and for this calculation it was assumed that the H atoms lie on the N-F bonds and that the ammonium ion has the same dimensions as in ammonium chloride. Thc contributions to the line-width of protons in the nearest 12 N& groups and of the nearest 34 fluorine nuclei were summcd individually and the remainder treated as a continuum and their contributions found by integration. The total was 13.1 gauss2 of which 6.6 gauss2 was due to the interaction of a proton with its nearest neighbour fluorine nuclcus. The experimental second moment of the proton resonance was 55 f 3 gauss2.This leads to a valuc of 1.042 3: 0.01 A for the N-H distance. This value agrees with the N-H distance obtained 2 for powdered NH4Cl 1.038 f 0.004 A, within experimental error and shows that the changa in dimensions of the ammonium ion produced by hydrogen bonding is small. To obtain this experimental value of the second moment the avcrage was taken of 11 independent curves at temperatures between 199" K and 205" K. The standard deviation was 2 gauss2. It was considered that at these temperatures the mean square width had reached its true value for a stationary lattice. Ideally, it would be desirable to makc measurements at still lower temperatures to make the assumption of a stationary lattice more certain. However, it was difficult to obtain adequate signal strength at lower temperatures and 200°K was the lowest temperature at which reliance could be placed on measurements of theL.E. DRAIN 205 mean square width. At 95" K, Gutowsky, Kistiakowsky, Pake and Purcell12 obtained the value 58.5 gauss2. The mean square width of the fluorine resonance at 205" K was found to be 32 &- 2 gaud. This value was the average of five determinations at temperatures from 203" K to 210" K. The major contribution to the width of this resonance arises from the interaction of a fluorine nucleus with its neighbouring protons. A rough calculation shows that the mean square width can only have as high a value as that observed if the ammonium ion is orientated so that the hydrogen atoms are as close as possible to the fluorine atoms, i.e.lie on or close to the N-F bonds. For example, an alternative structure in which the ammonium ion is rotated by 60" about the hexagonal crystal axis leads to a mean square width of only 17 gauss2. The contribution of the four nearest neighbour protons to the mean square width may bc shown to be 636.8 (1 - E ' ) / Y ' ~ gauss2, where r' is the H-F distance in A and (1 - E') a correction factor for the rotational oscillation of the am- monium ion. This correction is a little more complex than that for thc proton resonance since thc variation of the H-F distance during an oscillation has to be taken into account in addition to the narrowing due to the variation of the direction of thc H-F vector. If we aqsume that thc amplitude of oscillation is small, geometrical considerations lead to the result that With the same assumptions that were made when considering the proton resonance, we obtain (1 - e') = 0.90.The remaining contribution to the mean square width was computed to be 3.2 gauss2 and thus we derive r' = 1-64 f 0.02 A and by the addition of Y and r' a value for the N-F distance of 2.68 f0.03 A. This may be compared to the mcan N-F distance calculated from the crystal structure.3 This is 2.67 f 0.03 A. The agreement obtained here is evidence for the essential correctness of the assumptions made. The mean square widths of the proton and fluorine resonances are plotted against temperature in fig. 4. Except for the measurements around 200" K each point represents a mean square width calculated from a single curve.The limits of error shown arc estimatcd from the uncertainties in the shape of the observed curve and arc not intended to indicate thc maximum possible error. The reduction of the line-width in a solid may often be attributed to some form of motion of a molecular group. It is natural to assume that in ammonium fluoride this is a hindered rotation of the ammonium ion as this explanation has proved to be satisfactory in the other ammonium halides.112 We cxpect that above the transition, the mean square width reaches a value governed by the magnetic interactions between nuclei averaged over the different orientations of the ammonium ion. In particular, the interaction between nuclei in the same ion averages t o zero. The remaining interactions lead to a calculated mean square width of 6-2 gauss2 for the proton resonance.The agreement with the experi- mental value of 5.9 f 0.3 gauss2 at 360" K is sufficiently good to show that this interpretation of the mechanism of line narrowing is essentially correct. The corresponding theoretical line width for the fluorine resonance was 13.8 gauss while the experimental value was 10 & 1 gauss2 at 359" K. In this case the line width does not scem to reach a steady value but continues to fall above 340" K. There is possibly some other source of line narrowing. An expression of the following type has been proposed 13 to represent the line narrowing duc to hindered rotation in a solid, where E is the activation energy for the re-orientation of the ammonium ion,206 AMMONIUM FLUORIDE b2 and (b2 + c2) are the mean square widths at high and low temperatures respec- tively and y the gyroniagnetic ratio of the nucleus whose resonance is observed, YO is a frequency factor of the order of the torsional oscillation frequency of the ion.This factor should have very similar values in all the ammonium halides. I t was not found pqssible to fit the proton rcsonance line widths shown in fig. 4(a) exactly to an equation of this form. The best fit was obtained for values of E between 7,000 and 10,000 cal/mole. Theoretical curves corresponding to these two values are shown. The frequency factors used for these curves were 6.5 x lO9sec-1 and 1.2 x 1012sec-1 whilst for the other ammonium halides they range from 8.0 x 1011sec-1 for NH4Br to 6.1 x 1012 sec-1 for NH4C1.2 If it is assunied that YO for NH4F has a similar value, a value for E of 10,000 f 1,OOO cal/mole may be obtained.Fig. 4(b) shows that the same values of E and vo can be used to represent the transition in the fluorine resonance verifying that the same mechanism is responsible for the narrowing of both resonance lines. 40- 30- N 2 0 - IO- - v, a U v , 2 0 0 2 SO 300 3 5 0 TEMPERATURE ( O K ) FIG. 4.-The mean square widths of (a) the proton and (b) the fluorine resonance as functions of temperature. The full lines are theoretical curves for (1) E = 7 kcal/mole, vo = 6.5 x 109 sec-1 and (2) E = 10 kcal/mole, vo = 1-2 x 1012 sec-1. A detailed analysis of the motion of the ammonium ion has not been made, but it will be noted that whereas in N&C1, the ammonium ion is situated in a ficld of cubic symmetry, in NI-14F its surroundings are approximately tetrahedral.In the simplest ficld of this type, the barrier to reorientation is least for a rotation about the 3-fold axis, the barricr around the 2-fold axis being 9/4 times as great. It is probablc that the activation energy E involved in the nuclear resonance line-width transition corresponds to the minimum potential barrier less the zero- point cncrgy of torsional oscillation of the ammonium ion. With this assumption: the frequency of rotational oscillation of the ammonium ion in NH4F was estimated to be 1.4 x 1013 sec-1, a value slightly higher than that in the chloride. Morc accurate information about the potential barrier hindcring thc rotation of the ammonium ion could be obtained from measurements of the variation of relaxation time with temperature. It \vas found that the proton relaxationL. E. DRAIN 207 time decreased with increasing temperature over the range covered in these measurements having a value of about + sec at room temperature. It would also be desirable to make measurements on a single crystal of ammonium fluoride but so far it has not proved possible to grow one large enough. Thanks are due to Prof. Lonsdale for her continuing interest and to the Department of Scientific and Industrial Research for the provision of a Senior Research Award. 1 Cooke and Drain, Proc. Physic. Soc. A , 195.2, 65, 894. 2 Gutowsky, Pake and Bersohn, J. Chem. Physics, 1954, 22, G43. 3 Zachariasen, Z. physik. Chenz., 1927, 127, 218. 4 Simon, von Simson and Ruhemann, Z . physik. Chem., 1927, 129, 339. 5 Levy and Peterson, Physic. Rev., 1952,86,766 ; J. Amer. Chem. SOC., 1953,75,1536. 6 Andrew and Rushworth, Proc. Physic. Soc. B, 1952, 65, 801. 7 Bloch, Hansen and Packard, Physic. Rev., 1946,70,474. 8 Gouriet, Wireless Engr., 1950, 27, 105. 9 Bloembergen, Purcell and Pound, Physic. Rev., 1948, 73, 679. 10 Alpert, Physic. Rev., 1949, 75, 398. 11 Van Vleck, Physic. Rev., 1948, 74, 1168. 12 Gutowsky, Kistiakowsky, Pake and Purcell, J . Chem. Physics, 1949, 17, 972. 13 Gutowsky and Pake, J . Chem. Physics, 1950, 18, 162.

ISSN:0366-9033    

DOI:10.1039/DF9551900200

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

L. E. DRAIN 207 A NUCLEAR RESONANCE INVESTIGATION OF POLYTETRAF'LUOROETHYLENE BY J. A. S. SMITH Dept. of Inorganic and Structural Chemistry, The University of Leeds Received 15th February, 1955 The nuclear resonance spectra of various samples of oriented and unoriented poly- tetrduoroethylene have been examined in the temperature range 77" K to 334" K. The changes in line shape and second moment of the absorption curve are discussed in terms of the structure of the polymer and the transitions previously observed by other methods. The structure of polytztrafluoroethylcnc (abbreviated in this paper to PTFE) has reccntly been shown 1 to be that of a chain of -CF2-- groups in which the fluorine atoms lie on a helix which repeats every thirteen carbon atoms. Between 288" K and 308" K, X-ray diffraction studies reveal the existence of two closely- spaced transitions, one at 293" K and thc other at 303" K.The first can be inter- preted either as a sudden increase in thc rotational freedoin of the chains, or as an order-disorder transition involving longitudinal displacements of adjacent chains in steps equal to the pitch of the helix. This qualification is necessary because both the sixth and seventh layer line reflections on photographs taken about the fibre axis remain sharp at 298O K. Above 303" K, however, these layer lines become diffusc so that the longitudinal displacements are now no longer ordercd. The specific heat curves of PTFE show lambda-typz transitions at 293" K and 303" K but also suggest the possibility of a glass transition (presumably in the amorphous regions) at about 160" K.2 The present work was undertaken to discover what inforniation could be de- rived from the nuclear resonance spectra of PTFE in the region of these transition tcrnperatures.Thc gencral features of the magnetic resonance of 1H and 19F nuclei in molecular crystals at different temperatures arc relatively well known.3208 POLYTETRAPLUOROETHYLENE At low temperatures, the absorption curve tends to be broad and may show fine structure. The second moment of the curve, or its normalized moment of inertia about the central axis of symmetry, depends in a known way on the inverse sixth power of the distances between the resonating nuclei themselves and all other nuclei present with non-zero magnetic moment.4 In practice, this inverse sixth- power dependencc means that only nearest and next-nearest neighbours make appreciable contributions to the second moment.As the temperature is raised, the absorption line narrows over a certain range of tcmperature. In many materials, thcre is a considerable amount of evidence to show that this usually occurs when a molccular reorientation frequency, which moves the nuclei over a considerable distance, approaches a certain critical frequency, which is the frequency width of the line.5 As the temperature is raised further, other transi- tions may occur, sometimes at temperatures at which physical properties such as the specific heat or the density undergo changes. In crystalline materials, the second momcnt and the line width generally change over a comparatively narrow range of temperature; in c2F6, for example, the line width drops from 11 gauss to 2.5 gauss bctween 95" K and 110" K.5 Crystal- line polymers like PTFE, however, usually contain a certain amount of amorphous material filling in the regions bctween the crystallites.In particular, in PTFE the percentage crystallinity has becn estimated to lie betwccn 70 and 80 % and is almost independent of the previous history of the polymer, i.e. whether it has been moulded or qucnched.6 Relativcly few studies have been made of nuclear resonance transitions in amorphous polymers, but usually the line-width transi- tion covers a wide temperature range, as, for example, in polystyrene 7 from 350" K to 400" K, and may sometinies decrease almost continuously with rise in tem- perature, as in cured butadiene-acl-ylonitrile copolymcr.8 This broadening of the transition region would be expected to occur in amorphous materials, in which there should be a much broader distribution of relaxation times than in crystalline substances.In polystyrene, the glass transition (Tg = 370° K) falls in the middle of the first nuclear resonance transition region, but this is not generally true, e.g. in polymcthylmethacrylate, for which T' = 360" K,9 a nuclear resonance transition occurs in the range 230" K to 290" K.7 After the glass transition, how- ever, the line width drops to the very smali valucs characteristic of liquids rather than solids, e.g. 0.08 gauss in natural rubbcr,g compared with a " rigid lattice " value of 9.45 gauss below 141" K.10 This behaviour holds for all rubber-like polymers so far examined.It will be recognized that nuclear resonance studies of transitions in solids give similar information to diclectric loss measurements. The method has the ad- vantage of being applicable to non-polar polymers like PTFE which show very small dielectric loss. lt has the disadvantage of operating at one " effective " frequency only. Thc frequcncies at which dielectric loss or internal friction have maxima correspond directly to the most efficient relaxation frequencies in the polymcr. In nuclear resonance measurements of the type discussed above, the frequency of the measurement, which is the Larmor frcquency of thc nucleus in the applied field, is not related to the relaxation frcqucncy.In general, the first nuclear resonancc transition occurs when the molecular rcorientation frequency approaches the frequency width of the line, and this is largely indcpcndent of the resonance frequency at which thc measurements are made. EXPERIMENTAL The nuclear resonance signals were detected by the r.f. bridge method of Bloembergen, Purcell and Pound,ll as modified subsequently by Anderson.12 Power from a 16.435 Mc/s crystal oscillator is fed into a twin-T r.f. bridge in one arm of which is the resonance coil. The output from the bridge passes directly to a preamplifier of the cascade type mounted on the same casing as the bridge, and from there to a three-stage r.f. amplifier using low- noise pentodes. The signal is then demodulated by a conventional half-wave detectorJ . A .S. SMITH 209 and has its band-width reduced, an indication of carrier-level being provided at this point. The modulation component (25 c/s) then passes into the presentation unit where it is further amplified before being shown on a cathode ray tube, the time-base of which is derived from the same 25 c/s source as the magnetic field modulating current. The main field is stabilized to within one part in 10,000; the estimated inhomogeneity over the sample is 0.7 gauss. For broad lines of low signal-to-noise ratio, an alternative presentation is provided by feeding the modulation component through a filter which further restricts the band- width before it is applied to a phasc sensitive rectifier. The output from the latter passes through smoothing circuits before being fed into a Kent recorder.By reducing the modulation amplitude to a fraction of the line width and sweeping slowly through the line, the derivative curve is plotted on the recorder. The slow sweep of the mean magnetic field is obtained by varying the d.c. current through modulating coils mounted on the pole pieces of the magnet, this variation being controlled by a potentiometer driven by a small synchronous motor. The magnetic field variation was linear to within 0.5 gauss with respect to both the coil current and the coil voltage over a range of about 100 gauss. A second method, giving direct presentation of weak signals, has also been used in this work.13 The method is similar to that used by Beard and Skomal.14 The equipment consists basically of a Siemens no.17 system 52-point telephone uniselector. The uni- selector wipers rotate over the 52 sets of contacts at 4 c/s; an RC integrating circuit of time constant 10 sec ( R = 10 M, C = 1 pF) is wired across the input and output of each set of contacts. While one wiper switches input to output for each set of contacts, the other generates the scanning current which is fed into the modulating coils. A sample of the same current is used to scan the cathode ray tube, and direct presentation of low intensity signals then takes about 30 sec. The first technique of recQrding the derivative curve was generally used in this work, the second device using the intcgrator being used to locate the temperature regions in which the line-width transitions occurred.Preliminary measurements with the integrator established the existence of a line-width transition at about 220" K. Nuclear resonance derivative curves were then recorded over a range of temperatures from 77" K to 334" K. The unoriented PTFE samples were machined from currently manufactured Teflon rod, and the r.f. coil was wound on a PTFE former made from similar material. The oriented sample consisted of a bundle of short lengths of oriented PTFE fibre held together by a coat of low molecular weight polythene. The coil, screened by a copper cylinder, was connected to the bridge by german silver tubing. The measurements were made at different temperatures by placing the coil assembly in a Dewar containing various " slush baths ", consisting either of melting solids or solid + liquid systems at a transition temperature.Spectra were also recorded at 77" K and 195" K. The coil assembly was left in the constant temperature bath for at least 30 min before taking measurements, for the bridge balance was sensitive to changes in temperature of about 1". No definite indication of hysteresis effects was obtained in this work. RESULTS AND DISCUSSION Between 200°K and 260"K, the line width of both unoriented samplcs of PTFE dropped from 9.7 gauss to 2.7 gauss and thereafter slowly diminished to 2.3 gauss at 334" K. These results are plotted in fig. 1, in which the line width, AHmsl., is defined as the distance between the points of maximum slope on the absorption curve or, as experimentally observed, the distance between the prin- cipal maxima on the derivative curve.The line shape undergoes a number of changes between these temperatures. Three examples of actual traces of the derivative curve arc shown in fig. 2. At 195" K, the curve has a width of 9.70 gauss and covers about 22 gauss. At 305" K the line shape is similar but thc width is now 2.60 gauss and the line covers about 13 gauss. Between these two tem- peratures, the curve develops an inflexion which occurs invariably between 8 and 10 gauss and whose relative height with respect to the main peak drops as thc temperature rises. At a temperature of 273"K, the derivative curve has the shape of the second trace in fig. 2. A rather similar curve for PTFE a t 275" K has recently been reported by Wilson and Pake.15 The variation with temperature of the ratio of the height of the inflexion to that of the principal maximum in the2 10 POLYTETRAFLUOROETHY LENE derivative curve, called the inflexion-peak ratio, is given in fig.3. Experimentally, this ratio is not easy to nieasure accurately because of the uncertainty in finding the cxact centre of the inflexion, so the points show a considerable scatter. The two crosses are values recorded from other samples of PTFE. More information can be derived from the variation of the second moment of the absorption line with temperature, which is shown in fig. 4. The peak-to- peak modulation amplitude was 0.5 gauss in all experiments so the finite sweep correction 16 (which has been allowcd for) is very small. The rather large scatter of the experimental values is attributcd mainly to the difficulty of recording the long "tails" of thc PTFE curve, which being furthest from the centre of the line make a considerable contribution to the second moment.Two transitions now appear, the first between 210" K and 230" K, followed by a gradual drop in second moment similar to that observed second between 280" K and 310" K, after which the line width appears to become constant at 2.61 gaussz. Two other sorts of transition observed in PTFE have been inserted in fig. 4. The dotted linc shows the temperature variation of the logarithm of the dynamic modulus7 in dyncs/cm2 measured at 1 1 1 to 11 5 c/s, in which two notable transitions occur, both in the vicinity of the nuclear resonance transitions.At 293" K and 303°K occur the two first-order transitions observable by specific heat and X-ray diffraction measurements, which are indicated by the num- bered arrows. I I I I 200 2 5 0 so0 s 5 0 T(OK 1 FIG. 1.-Variation of the line width of PTFE with tcrnperature. for the line width, and the FIG. 2.-Actual traces of the derivative curve of PTFE at 195"K, 273"K, and 305°K. The peak-to-peak modulation was 0.5 gauss, as shown by the two vertical lines. The chart speed was 0.416 in./min and the calibration 5.11 gauss/in. The time constant on the recorder channel was 9 sec. Below 200" K, the second nomcnt of PTFE is consistent with a " rigid lattice " of -CF2-chains. The four cxpsrimental values obtained at 77" K and 195" K arc 11.51, 11.18, 11.60 and 11.48, giving a mean of 11-44 gaussz. We adopt the configuration proposed by Bunn and Howells 1 for the crystalline polymer, in which the fluorine atoms lie on a helix of radius 1.64A and pitch 2-80A.FromJ . A . S. SMITH 21 1 an equation first derived by van Vleck,4 it can be shown that the second moment contribution in gauss2 of any pair of fluorine nuclei distance d (A) apart is given by AH^> = 317*2/d6. No other nuclear interactions need be considered, for the main carbon isotope has zero spin. Second moment contributions can be divided into two groups; T ( O K ) FIG. 3.-Variation of the inflexion-peak ratio with temperature. FIG. 4.-Variation of the second moment of PTFE from 77" K to 334" K. The dotted line shows the variation of the logarithm of the dynamic modulus from 123" K to 350" K measured at frequencies between 111 and 115 cis.17 The arrows numbered 1 and 2 indicate the two transition temperatures of 293" K and 303" K.those between pairs of fluorine nuclei in the same polymer chain, called (Lhff2>cha~, and adjacent polymer molecules, called (nH2)interchah. The first term can be calculated by determining the co-ordinates of all near fluorine atoms on the helix, assuming a reasonable value for the fluorine distance between the two nuclei212 P 0 L Y TET R A F LUO RO ETH Y L E N E attached to the same carbon atom, which we call F1F2. The co-ordinates of all remaining fluorine atoms are then fixed by the radius and repeat length of the helix. If FlF2 is taken as 208& <AH2)chah is calculated as 8.36 gauss2, which includes interactions from all the fluorine nuclei up to Flo; for F1F2 equal to 217& (AfZ2)c~m is 7.66 gauss2.In the former case, the broadening contribu- tions between F1 and successive fluorine nuclei in one direction along the helix up to Flo are 3.50, *16, -21, -81, -19, -02, -01, *01 and -01 gauss2, showing the importance of the F1F5 interaction. The considerable broadening contributions of fluorine nuclei other than F2 along the chain also seem to remove any fine structure that a 4 F 2 - group might be expected to show. Thus line shapes and second moments of the oriented fibre measured at 77" K were effectively identical with those obtained from the unoriented polymer. It is considerably more difficult to calculate the interchain broadening con- tributions.Two models were used in these calculations. In the first, the fluorine helices of radius 1-64A were stacked parallel to cach other along the c-axis of a hexagonal cell, the transverse separation of the centre of each helix being 5-54 A. Assuming F1F2 = 2.17 A in each helix (the calculation should not be very sensitive to this figure) a scale projection of the model was constructed and the distances of all fluorine atoms less than 7.50A from the particular nucleus considered were mcasured. This procedure gave a total broadening of 2.52 gauss2, which we assume to be an average value for all 26 fluorine nuclei in the repeat unit. In the second model, the nuclei F3 and F4 of a neighbouring polymer chain were placed at the van der Waals distance (2.70A) from F1 and F2 of the molccule considered and above the plane containing them; another pair then appears at an equal distance below the plane.Summing these contributions and those of likcly next-nearest neighbours gives a value for (AH2)hterchh of 3-89 gauss2. These two values subtracted from the mean experimental second momcnt of 1 1 . 4 4 give values for (AH2)chh of 8.92 and 7-55 gauss2, which by interpolation from the two previously calculated values are consistent with fluorine-fluorine distances in the - C F r group of 2.08 and 2.18 A. In diphenyltetrafluoroethane, the X-ray diffraction results 18 are consistent with a C-F distance of 1.36 A ; in CH2F2, microwave spectroscopy 19 gives the same value. Assuming therefore a C-F distance of 1-36& the nuclear resonance evidence suggests a FCF angle of between 100" and 106", appreciably less than the tctrahedral angle, so that the opening out of the CCC angle in the polymer chain to 116" is accompanicd by a contraction of the FCF angle to 103 =k 3".Because of the complcxity of the structure, it is difficult to make a more precise analysis ; in addition, we have neglected the complicating factors of the broadening contributions from the amorphous regions or possible -CF or -CF3 groups, which may occur in chain branching or chain terminating units. However, the degree of branching in PTFE is generally believed to be considerably smaller than in polythene. The agreement between the experimental and calculated values of the second moment based on a reasonable model for PTFE show that up to 200" K molecular oscillations have not suficient amplitude nor molecular reorientations a sufficiently high frequency to have an appreciable effect on the results.This renders very improbable any occurrence of a glass transition in the amorphous regions at about 160" K, as had been tentatively suggested from specific heat measurements.2J . A. S. SMITH 21 3 Above the glass temperature, the line width of a polymer should be extremely small, values of less than 0.08 gauss having been recorded for some natural and synthetic rubbers.8 Such a line width transition in PTFE involving about 30 % of the solid polymer would certainly have been detected. The nuclcar resonance results for polythene21 also seem to exclude such a possibility in this case, at least up to 293°K.Above 210" K, the second moment of PTFE drops from 11.44 gauss2 to about 6.0 gauss2. Andrew20 has shown that a similar transition occurs in some long chain hydrocarbons, notably dicetyl, at temperatures much nearer the melting point ; thus in dicetyl (map. 342" K) the second moment drops from 27 to 15 gauss2 between 240" K and 315" K and then from 15 to about 5 gauss2 at the transition point (336" K). The ratios of the second moments before and after the first nuclear resonance transition are 1.9 for PTFE and 1.8 for dicetyl. The mechanism proposed by Andrew to account for the second moment values was that only two-thirds of the hydrocarbon chains could actually rotate with a sufficiently high frequency to affect the nuclear resonance line.The molecules are imagined to rotate like a closed set of meshed gears, which is only possible if the set contains an even number. In PTFE, a projection down the c-axis should give a nearly hexagonal array of molecules, in which those at the corners form part of the set of meshed gears and those at the centres of the hexagons are stationary. A chain of -CF2- groups undergoing hindered rotation at a frequency greater than about 40,000 c/s should have its second moment reduced by a factor of about 6.6 ; 5 the interchain broadening would be reduced by a factor of about three.20 The second moment of the rotating chain should therefore be 8*36/6.6 + 3.08/3, or 2.30 gauss2, or for a set of chains two-thirds of which are rotating, 11.44/3 + 2*30/1.5, or 5.3 gauss2. This is in reasonable agreement with the experimental value of 6.0 gauss2, so that the proposed mechanism is consistent with the results.Between 230°K and 270"K, the second moment appears to drop from about 6.8 to 5.7 gauss2, and part if not all of this variation could be attributed to the amorphous regions of the polymer, in which we have reason to suppose that the transition would be more gradual. The existence of relatively rigid molecules in the crystalline portions of the polymer would also explain qualitatively the line shapes obtained in the transition region. The derivative curve has a sharp maximum at 2.5 gauss and a broad inflexion which falls in all cases around 9.2 gauss. The line width of the " rigid lattice " below 200" K is 9.70 gauss, so that the inflexion could be interpreted as due to the one-third of the molecules that had remained rigid from the nuclear resonance point of view.Probably the frac- tion is only approximate becausc of the contributions of the amorphous regions to the inflexion, which we may expect to vary over a wide temperature range. This would thcn explain the gradual fall with rise in temperature of the inflexion- peak ratio shown in fig. 3. There are signs that this ratio becomes constant over a short temperature range around 280" K, but from the experimental scatter it is difficult to bc sure of this. An attempt was made to reproduce the actual derivative curve at 287" K by superimposing two curves, one recorded at 195" K to account for the rigid chains, and the other at 318" K for the rotating chains.The curves were suitably weighted so as to give the correct inflexion-peak ratio, but failed to reproduce other parts of the line, notably the tails of the curve. Below 200" K the line covers about 22 gauss, but above 210" K, the tails contract and the curve covcrs only 17 gauss. Wilson and Pake 15 have given an alternative interpretation of the line shape in terms of the superposition of two curves, a narrow one due to the amorphous regions of the polymer and a broad one due to the crystalline regions. However, as fig. 3 shows, this ratio of the contributions of the two curves apparently varies with temperature, except possibly near 280" K, and at that temperature the superposition of the two curves described above gives an area ratio of 2.29 or a percentage crystallinity of 44, differing markedly from their value of 72 f 5 %.214 POLY TETRAFLUOROETHY LENE Measurements on two different samples of PTFE do not appear to diverge from the curves given in fig.1 and 3. The relatively sharp and considerable fall in the second moment around 220" K suggests a change in the bulk of the polymer rather than in a small portion. Also transitions in amorphous polymers, as already discussed, generally produce extremely narrow lines with widths of the order of 0.1 gauss, whereas PTFE has a line width of about 2.4 gauss even above the thermodynamic transition temperatures near 300" K. However, this proposal cannot bc regarded as definitely excluded sincc the nuclear resonance transition at 200°K may well start in the amorphous regions and affect the crystalline regions later.Thc difficulty would then be to know if there were a temperature at which the majority of the amorphous polymer had passed through the transition leaving most of the crystalline polymer unaffected. More information on the effect of crystallinity on nuclear resonance transitions is certainly needed. The second nuclear resonance transition stretches from 270" K to 310" K and reduces the second moment from about 5.5 gauss2 to 2.6 gauss2, at which value it remains constant up to at least 334" K. Little change, however, is apparent in thc line width, showing the importance of second moment measurements in this work. The second moment expected for -CF2- groups undergoing hindcrcd rotation about the axis of the helix is about 2.3 gauss2 in reasonable agrcement with thc experimental values above 310" K.The nuclear resonance results do not therefore settle the exact nature of the transition at 293" K observed in the X-ray diffraction pattern, but do show that if an order-disorder transition occurs, it must also be accompanied by a marked increase in the rotational freedom of the polymer segments. A further attempt is being made to settle this point by measurements on other oriented specimens of the polymer. One of the most interesting features of the present work is the correspondence of nuclear transitions with changes in the mechanical properties of the polymer, such as the dynamic modulus. These in turn can be correlated with changes in the dielectric dispersion.Table 1 below lists some of the temperatures at which TABLE 1 .-COMPARISON OF THE MECHANICAL, DIELECTRIC, AND NUCLEAR RESONANCE PROPERTIES OF POLYTHENE AND PTFE polymer expt. method temp. ("K) frequency (c/s) polythene mechanical loss 22 156 324 dielectric loss 22 193 11,Ooo nuclear resonance 21 213 60,000 PTFE mechanical loss 17 200 115 21 3 683 nudear resonance 210 41,000 the mechanical and dielectric loss factors of polythene and PTFE show maxima, compared with the temperatures at which nuclear resonance transitions begin. The fourth column lists the frequency to be associated with each method; that for the nuclear resonance transitions is the frequency width of the line. For polythene, the correlation of the frequency with temperature is good, but poor for PTFE.In the former case, a graph of In (frequency) against l / T gives a good straight line with a slope corresponding to an activation energy of 5.8 kcal/mole, so that there is reason to suppose that all three methods are measuring a relaxation phenomenon Df the same kind. Oakes and Robinson22 have associated the loss maxima with increased flexibility in some of the main chain - C H r groups, and such an interpretation would be consistent with the nuclear resonance results. The reason for the lack of correlation for PTFE is not understood at present. The author would like to thank Prof. E. G. Cox for making facilities available for research into nuclear resonance and for his interest and encouragement in the work. Part of the expenses were defrayed by a grant from the D.S.I,R. HeJ . A . S. SMITH 21 5 would also like to acknowlcdgc his debt to Mr. R. J. Weir, who constructed most of the apparatus used in this work, and to I.C.L. for the gift of PTFE samples, in particular to Dr. C. W. Bum and Dr. E. R. Howells who suggested the problem, and to Dr. A. H. Willbourn for the benefit of most helpful discussions. 1 Bunn and Howells, Nature, 1954, 174, 549. 2 Furukawa, McCoskey and King, J. Res. Nut. Bur. Stand., 1952, 49, 273. 3 Smith, Quart. Rev., 1953, 7, 279. 5 Gutowsky and Pake, J. Chem. Physics, 1950, 18, 162. 6 Renfrew and Lewis, Ind. B i g . Chem., 1946, 38, 870. 7 Holroyd, Codrington, Mrowca and Guth, J. Appl. Physics, 1951, 22, 696. 8 Honnold, McCaffrey and Mrowca, J. Appl. Physics, 1954, 25, 1219. 9 Values of Tg are taken from Flory, Principles of Polymer Chemistry (Cornell Uni- 10 Gutowsky and Meyer, J. Chein. Physics, 1953, 21, 2122. 11 Bloembergen, Purcell and Pound, Physic. Rev., 1948,73, 679. 12 Anderson, Physic. Rev., 1949, 76, 1460. 14 Beard and Skomal, Rev. Sci. Instr., 1953, 24, 276. 15 Wilson and Pake, J. Polymer Sci., 1953, 10, 503. 16Perlman and Bloom, Physic. Rev., 1952, 88, 1290. Andrew, Physic. Rev., 1953, 17 Dr. D. W. Robinson, private communication. 18 Cruickshank and Nyburg, private communication. 19 Gordy, Smith and Trambarulo, Microwave Spectroscopy (John Wiley, 1953). 20 Andrew, J. Chem. Physics, 1950, 18, 607. 21 Newman, J. Chem. Physics, 1950, 18, 1303. 22 Oakes and Robinson, J. Polymer Sci., 1954, 14, 505. 4 van Vleck, Physic. Rev., 1948, 74, 11 68. versity Press, Ithaca, 1953), pp. 52-53. 13 Weir, unpublished work. 91,425.

ISSN:0366-9033    

DOI:10.1039/DF9551900207

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

J . A . S. SMITH 21 5 THE RELATION OF HIGH RESOLUTION NUCLEAR MAGNETIC RESONANCE SPECTRA TO MOLECULAR STRUCTURES BY JAMES N. SHOOLERY Varian Associates, Palo Alto, California Received 24th January, 1955 Nuclear magnetic resonance experiments under fractional milligauss resolution fre- quently yield complex spectra consisting of a number of sharp resonances for each nucleus studied. This occurs because the diamagnetic shielding of the nucleus by the surrounding electrons is different at each structurally non-equivalent site in a molecule, and this effect is not averaged out by the random molecular motions. In addition, indirect nuclear spin-spin interactions, coupled through the electron spins, complicate the spectra and split certain of the resonances into multiplets. Two experimental techniques are described for unravelling the complex spectra which arise.The first, which involves obtaining the spectra at two or more values of the magnetic field, has been found useful in correctly assigning groups of lines to structural features in fluorocarbon molecules. The second is called a " double resonancc technique " and has been employed in studies of some boron hydrides. Substitution of two fluorine nuclei on a cyclic carbon atom frequently leads to a unique multiplet structure in the spectrum. The effect of a doubly bonded carbon atom is similar. Several such compounds have been studied and their spectra are interpreted in terms of the molecular configurations. Several of the boron hydrides have been examined and it has been found that the spectra can be interpreted in terms of certain structural configurations.For example, the bridge model for diborane is shown to be in accord with the nuclear magnetic resonance data.216 MOLECULAR STRUCTURES The atomic nuclei in molecules are well shielded from their surroundings, even during molecular collisions. Their relatively small magnetic dipole moments interact only weakly with the magnetic moments of neighbouring particles. Frequently, this results in long spin-lattice relaxation times associated with the nuclear magnetic resonance transitions between the Zeeman levels which arise in an applicd magnetic field. In liquid samples, the direct dipole-dipole per- turbations of the energy levels are averaged nearly to zero, leaving the homogeneity of the applied field Over the sample dimensions as the principal contribution to line width. Under these circumstances, it becomes possible to detect and measure extremely small alterations in the magnetic fields at the nuclei in an ensemble of molecules.If discrete shifts in the magnetic field occur due to the chemical environment, a spectrum of sharp resonance lines is obtained. In this work the nucleus is not studied per se, but rather is employed as a minute probe to sense ts chemical environment. The chemical environment can affect the magnetic field at the nucleus pi-in- cipally in two ways; first, by altering the distribution probabilities and angular momenta of the surrounding electrons, particularly those involved in bonding, and second, by determining the number and grouping of other magnetic nuclei in the molecule which are capable of coupling their spins to the spin of the nucleus being studied.The energy of interaction of two spin-coupled nuclei i and j is S& 11. The magnitudes of these two effects are frequently designated respectively by the symbols 8, the chemical shift, and J, the spin-spin coupling factor. Interactions of the electrons with the applied field, leading to the relative shielding, 6, of the nucleus, are proportional to the streagth of the field, while J is independent of the field. This suggests that in complex cases in which one spin-spin multiplet falls atop another, one might be guided in the assignment of the various lines by performing experiments at more than one field and frequency.It should be noted, however, that such a technique will be successful only if the centres of gravity of the multiplets do not coincide; otherwise, no relative shift will be observed. In some cases it is not within the scope of attainable field strengths to vary the field enough to scparatc lines which mutually obscure one another. It is then sometimes possible to " collapse " the spin-spin multiplets and thus obtain a simplification of the spectrum which permits of interpretation. To do this, it is necessary to interrupt the spin orientations of thc perturbing nuclei frequently enough so that the observed nuclei see only an average value. This can be done by inducing transitions between the various spin states of the pcrturbing nuclei, using a strong r.f.(radio-frequency) field of the correct frequency. The detailed behaviour of the nuclear magnetic energy levels under the application of such an r.f. field has been worked out in some detail elsewhere; 1 however, the spin- spin multiplet can be regarded here as essentially collapsed completely when the strength of the r.f. field, H I , is such that yH1 > J, where y is the magnetogyric ratio for the perturbing nuclei and J is the spin-spin coupling constant expressed in cycles/sec. In this investigation, more complex fluorine nuclear magnetic resonance spectra were obtained from some organic fluorine-substitutcd compounds than would be expected on the basis of chemical or structural nohequivalefice alone. Similar complexities were found in the proton spectra of some of the boranes.It is the purpose of this paper to describe these spectra and relate them to the appropriate molecular configurations. EXPERIMENTAL A crossed-coil nuclear magnetic resonance high resolution spectrometer was used in conjunction with an electromagnet having 12-in. diameter pole faces and 1.75411. air gap,J. N. SHOOLERY 217 The field homogeneity over the sample in the range 7 to 10 kilogauss is of the order of one part in 107 or 108, and the short time stability of the energizing current can be main- tained to the same degree. Radio frequency units and crossed-coil induction heads, or probes, were available at 1 2 ~ 3 ~ 3 0 and 40 Mc/s, permitting studies at more than one magnetic field strength to be made. The fluorinated compounds were sealed in cylindrical Pyrex cells of about 3 mm inner diameter.Purification was not attempted because of the small quantities of the samples which were available. The zero of reference for the magnetic sweep axis was taken as the resonance field for the fluorine nuclei of the CF:! groups adjacent to the oxygen atom in the five-membered ring compound C4FsO. Equivalent frequency shift (c/s) FIG. 1 .-FI9 magnetic resonance spectrum of compound A at 30 Mc/s ; (b) compound B ; (c) compound C ; (cl) compound D. Diborane was sealed at 700mm pressure in a cylindrical Pyrex glass container of approximately 200 cm3 volume provided with a long freeze-out tip of 3 mm inner diameter, into which the diborane could be condensed with liquid nitrogen. Although the com- pound boils at - 92.5" C, no difficulty was encountered in observing signals for several minutes at a time, due to the incorporation of a small, double-walled, evacuated vessel in the probe as a support for the encircling receiver coil.A 10 % solution of NaBH4 in D20 was simply sealed in 3 mm Pyrex tubing. Pentaborane and a saturated solution of decaborane in CS2 were handled in similar fashion. A sample of (CH3)z PHBH3 prepared by Prof. Anton Burg was received sealed in a Pyrex cell of 3 mm diameter at one end and 6 mm diameter at the other. For the work with boranes, the 30 Mc/s probe was modified to convert the transmitter section into a double-tuned circuit, resonant at both 30.0013 Mc/s, the proton precession frequency in a field of 7050 gauss, and 9.6257 Mc/s, the precession frequency for B11 in the same field.A free-running oscillator capable of supplying several watts of r.f. energy21 8 MOLECULAR STRUCTURES was constructed and coupled to the transmitter coils of the probe in a manner that per- mitted irradiation of the sample with an alternating r.f. magnctic field up to 12 gauss. The receiver coil was Icft undisturbed since it is tuned to 30 Mc/s, and rcsponds only to thc proton signals. The proton spectrum of diborane was observed at 30 Mc,’s while the oscillator frequency was slowly varied. At 9.6257 Mc/s the expected changes in the spectrum occurred. A crystal was ground to stabilize thc oscillator on this frequency for greater ease in carrying out subsequent experiments with other boranes. The crystal controlled oscillator can be tuned far enough to compensate for changes in the boron frequency due to variations in chemical environment.RESULTS Fig. 1 and 2 show the fluorine nuclear magnetic resonance spectra at 30 and 40 Mc/s of four compounds in which the fluorine atoms are attached to carbon atoms belonging to a four-membered ring. In each case, there are more spectral lines than chemically non-equivalent fluorine nuclei. Fig. 3 shows the fluorine spectrum of CF2-CHCl at 40 Mc/s. Certain features of the spcctra are common to both types of ~nolecules. 0 4.200 9 4 0 0 + 600 + 800 + 1000 + 1200 t 1 4 0 0 Equivalent f r e q u e n c y shift (c/s) FIG. 2.-(u) F19 magnetic resonance spectrum of compound A at 40 Mc/s; (b) com- pound B ; (c) compound C ; (d) compound D ; 6’ = (J2 + S2)* - J.The proton spectra of the boranes were obtained with the boron spin system first in equilibrium with the static magnetic field and then in equilibrium with the additional alternating magnetic field at the B11 resonance frequency. These spectra are displayed in fig. 4, 5 and 6. The resonances of the B11 nuclei in diborane, pentaborane, and deca- borane were studied at 123 Mc/s in a field of 9000 gauss. The spectra obtained under these conditions are reproduced in fig. 7. Finally, the HI, B11 and P31 spectra of (CH3)2 PHBH3 were examined and are presented in fig. 8.J . N. SHOOLERY 219 DISCUSSION FLUORINE COMPOUNDS A preliminary examination of the F19 spectra of compounds A, B, and C in fig. 1 and 2 reveals that the lines occur in pairs, separated by about 200 cycles (50 niilligauss).Furthermore, it will be noted that thc intensities of the lines 0 FIG. 3.- I nc r e a r i n g magnetic f i e l d FIG. 4.-(u) H1 magnetic resonance spectrum of diborane at 30 Mc/s ; (6) protons directly bonded to B11; (c) protons directly bonded to €31" ; (d) bridge protons. are unequal, and that each pair of lines is accompanied by a similar pair with the same spacing and intensity ratio, but whose intensities appear in reverse order. The right-hand (high field) pair also shows a triplet structure. Comparison of the spectra at 30 and 40 Mc/s shows that the spacing of the doublets is fieldIncreirir?q ccqnetic f i e l d FIG. 5.-(u) H1 magnetic resonance spectrum of diborane at 30 Mc/s; (b) same as (a) but with B11 excited at 9.6257 Mc/s; (c) H1 magnetic resonance spectrum of NaBH4; (d) same as (c) but with B11 excited at 9.6257 Mcjs.lacrearinq s o p ~ a t i c f i e l d FIG. 6 . 4 0 ) H1 magnetic resonance spectrum of pentaborane at 30 Mc/s; (b) same as (a) but with B11 excited at 9.2576 Mc/s; (c) H1 magnetic resonance spectrum of decaborane at 30 Mc/s; (d) same as (c) but with B11 excited at 9.6257 Mc/s.FIG. 7.-(a) B11 magnetic resonance spectrum of diborane at 12.3 Mcls ; (b) B11 magnetic resonance spectrum of pentaborane at 12.3 Mc/s; (c) B11 magnetic resonance spectrum of decaborane at 12.3 Mc/s. , . . . -240 -100 -110 - 6 0 0 +do +I20 +I60 +ti0 +3bO - --I--- 1 , ~~ -zio -180 -iio -bO o +60 +GO +tao + 2 4 0 +X>Q - 5 0 0 -IS0 0 +IS0 + 3 0 0 - 4 0 0 -260 0 +2bO+4dO Equivalent frequency s h i f t (c/s) FIG.8.-(a) H1 magnetic resonance spectrum of (CH3)2 PHBH3 at 30 Mc/s ; (b) same as (u) but with B11 excited at 9.6257 Mc/s ; (c) B11 magnetic resonance spectrum of (CH3)2 PHBH3 at 12.3 Mc/s ; (d) P31 magnetic resonance spectrum of (CH3)z PHBH3 at 12.3 Mc/s. 0 0 r m222 MOLECULAR STRUCTURES independent, while the separation of their centres is proportional to the field strength. The pattern therefore indicates two chemically non-equivalent types of fluorine whose spins arc coupled together through the electron cloud. Since 21 + 1 = 2 for a doublet, the total spin angular momentum of each fluorine type is -JJ2/27~, which means that there is just one fluorine of each type. Although the electronic coupling is complcx, it is generally true that it is attenuated by distance, and thc coupling of 200 cycles is a strong indication that the two fluorine atoms are bonded to the same atom.If they are to be non-equivalent, a lack of freedom to rotate rclative to some other part of the molecule is implied. This kind of restriction occurs in cyclic molecules, molecules with multiple bonds, and as a result of steric hindrance. The compounds studied here are all of either type (J) or (11) below : H CF2--CH2 CC12-CH (1) I I R CF2---CR (11) I I CC12-CH2 where R is -Si(OC2Hs)3, -Si(CH3)3, and -Si(CH&R’ for the compounds A, B, and C respectively. R‘ is a silicone linkagc through oxygen to a duplicate arrangemciit of atoms. The chemical non-cquivalence of the fluorine nuclei arises through the substitution of R for H so that the chemical environment on the two sides of the ring is different.Comparison of thc spectra at 30 and 40 Mc/s also reveals a variation in the intensity ratio of the double lines. As the ratio of the chemical shift to the spin- spin coupling increascs, the intensities become more nearly equal. Hahn and Maxwell 2 have shown how to calculatc: thc intensity ratios, which, in the general case of a multiplet with 21 -1- 1 components, approach values determined by the statistical wcights of the various spin orientations when 6 becomes much larger than J. The measured intensity ratios for the spectra in fig. 1 and 2 are in satis- factory agreement with those calculated by the method of Hahn and Maxwell, using the values of J and 8 obtaincd from the spectra.The spectrum of compound D at 30 Mc/s is not readily interpreted, but at 40 Mc/s it is clcar that we are dealing with the overlapping spectra of two CF2 groups similar to those encountered in the compounds already discussed. The first, second, fifth and seventh lines in fig. 2(d) belong to one CF2 group, and the remainder of lines arisc from the presence of the other CF2 group. Both groups show almost the same value of J but quite different values of &,#, the chemical shift between the two fluorine atoms on the same carbon atom but on opposite sides of the ring. This is presumably due to the different distance from the perturbing silicon atom for the adjacent and diagonally opposite CF2 groups. The structural formula for this molecule is F2C--CH2 I 1 F2C-CSi(CH3)3 H Compounds A, B and C have similar values of 8, as can be seen by comparing the scparations of thc centres of the doublets.The value of 6 changes by a factor of approxirnatcly 1.6 going from adjacent to diagonally opposite CF2 and CH;! groups, on the basis of thc two values of 8 found for compound D. The fluorine rcsoiiancc spcctra, thcrefore, provide evidencc that compounds A, B and C are all of the same structural type (l) or (11). A better understanding of the details of thc spin-spin coupling mechanism might peimit a choice between (I) and (IT) to bc madc, since an additional triplet structurc of the high field pair of lines is observed in each compound. Present experience permits oiic only to say that one of the fluorine nuclei is measurably spin-spin coupled to a pair of protons in the molecule, whilc the other is weakly coupled, if at all.J .N. SHOOLERY 223 CF2=CHCl The spectrum of this compound is also characteristic of two non-equivalent fluorine nuclei bonded to the same carbon atom. The low-field doublet is further split by spin-coupling with the proton. Thc values of Jpp and JpH are respcctivcly 45 and 20 cycles. DIBORANE Proton magnetic resonance spectra of scveral of the boranes have been reported 3 s 4 and havc, in gcneral, yiclded interesting results. The data obtained here confirm and extend these results. The major features common to most of the spectra are illustrated by diborane, shown in fig. 4(a). The lack of sym- metry of the absorption lines indicates the existence of more than one bonding situation for the protons in this molecuie.Four of the protons in diborane are believed to be attached directly to the two boron atoms, while the other two share the remaining boron valences by forming the celebrated proton bridge bond.% 6 This would be expected to result in two proton magnetic resonance peaks, with amplitudes in the ratio 4/2. Howcvcr, the 21+ 1 orientations of the B11 spin of 3/2 result in a splitting of the resonance of the protons bonded directly to B11 (abundancc 81.17 %) into four equally spaced rcsonances. Similarly, thc spin of 3 of the less abundant BlO isotope splits the resonancc of the attached protons into seven resonance?, the spacing being diminished by the ratio of thc resonance frequencies of the boron isotopes.These contributions to the spectrum are shown in fig. 4(6) and 4(c). Finally, by performing a subtraction of 4(b) and 4(c) from 4(a) one arrives at the group of lines shown in fig. 4(d), which arise from inter- action of the bridge protons with the magnetic fields due to the seven orientations of the two B11 atoms equidistant from them. This multiplet represents 65 % of the molecules, the remainder of the absorption being due to unresolved lines from Blo-Bll and Blo-Blo molecules. To verify these assignments, the B11 spin orientations were “stirred” by the application of a strong r.f. field at the B11 resoiiance frcquency while tho proton resonance was recorded at 30 Mcls. Fig. 5(a) and (b) show thc spectrum before and during irradiation. This results in two peaks with the expected amplitude ratio.Further confirmation is obtained from the B11 rcsoiiancc spcctrurn, shown in fig. 7(a). This pattcrn of lines arises from the strong coupling of two indis- tinguishable protons with each €311 and, in addition, a weaker coupling of each B11 with two other indistinguishable protons, a situation reprcsentcd correctly by the bridge model. NaBH4 Ogg4 has examined NaBH4, interpreting the spectrum of fig. 5(c) in terms of complete equivalence of the four protons, the four strong lines and several weak lines being attributed to spin-coupling of the protons with thc B11 and BlO nuclei respectively. Irradiation experiments similar to those for diborane are reported here in fig. 5(d) and verify the proposed interpretation. In fig.5(c) and 5(d), all seven of the BlO components are visible. PENTABOR ANE This molecule has been reported to have a tetragonal pryamidal structure7 with one proton attached to the apical boron, one for each basal boron, and a bridge proton joining each pair of basaI boron atoms. The H1 spectrum, shown in fig. 6(a), possesses four large lines due to the spin of each B11 nucleus coupling with that of the basal proton bonded to it. There are also four small, slightly shifted lines due to the apical proton, and a large hump superimposed on the224 MOLECULAR STRUCTURES spectrum due to the bridge protons. Irradiation at the B11 frequency of the basal boron atoms yields the result of fig. 6(6), in which the two lines of nearly equal amplitude indicate that the basal protons are evenly divided between directly bonded and bridge bonded types. The " chemical shift " of the apical boron prevents the apical proton spin multiplet from being morc than partially collapsed by the irradiation process; howevcr, this multiplet can be brought togethcr by tuning the irradiation oscillator a few hundred cycles, at which time the base proton spectrum becomes only partially collapsed.The B11 spectrum is consistent with the pyramidal structure in that it indicates only two typcs of boron present in the ratio of at lcast 4/1. The large doublet is due to the interaction of the directly attached protons with the basal boron atoms, and the small doublet results from a similar interaction of the apical boron and hydrogen nuclei. The nuclear magnetic resonance spectra thus support the tetragonal pyramidal model for penta borane. DECABORANE Decaborane has been studicd by X-ray diffraction 8 and found to possess a remarkable structure.It can be visualized as two slightly distorted pentaborane molecules whose bases share a common edge and, with two additional boron atoms, form a basltct-shaped molecule. One proton is bonded directly to each boron and the remaining four protons form bridge bonds which arc laced around the open rim of the basket. The symmetry of the molecule is such that the most natural classification of the structurally non-equivalent boron nuclci is that they occur in three bonding classes: (I) four atoms form 5 B-€3 bonds; (IT) four atoms form 4 B-B bonds; (III) two atoms form only three B-I3 bonds. The proton spectrum in fig.6(c) shows a great similarity to that of pentaborane. From relative areas under the peaks in this and in the spectrum of the irradiated sample of fig. 6(d) the assignment of protons into structurally equivalent groups of 2, 4 and 8 nuclei can be made. If we assume that each boron forms one direct B--H bond, then the three proton groups can be assigned as follows. Two protons directly bonded to class 111 boron, four protons formii?g bridge bonds, and eight protons directly bonded to class I and II boron but indistinguishable in the H1 spectrum. However, in the B11 spectrum of fig. 7(c), class I and I1 boron nuclei might be expected to be distinguishable. Each should be split into a doublet by the directly bonded proton, and it is proposed that the triplet character of the large resonance is causcd by two overlapping doublets due to the four B11 nuclci of class I and the four slightly shifted B11 nuclei of class 11.Interpretation of the triplet character of this resonance as indicating two equivalent protons directly bonded to boron is unsatisfactory because the area under the peak rcpresents at lcast cight of thc ten boron atoms and would require sixteen hydrogen atoms, while the moleculc possesses only fourtcen. Thc two B11 nuclei of class 1x1 give a well shifted doublet of appropriate amplitude. It is thcrcfore possible to assign the nuclcar magnetic resonance spectra in a reasonable way to a model conforming to the structure established by the X-ray studies. (CH3)2 PHBH3 Twelve of the fourteen nuclci in this molecule are magnetic, and eleven are spin-coupled to the P31 nucleus.This results in the P31 spectrum being composed of 224 lines in two groups of 112 each. Fig, 8(d) shows that only the largest coupling, that due to the directly bonded hydrogen, can be resolved. Other data for this molecule were obtained from the H1 spectrum of fig. 8(a), the irradiated spectrum of fig. 8(6), and the B11 spectrum of fig. 8(c). They can best be summarizcd in a brief table of coupling constants (table 1). The lines of the H1 spectrum were assigned on the basis of the irradiation experiment. The four lines at 0, -1- 90, + 180 and -I- 270 cycles collapsed underJ . N . SHOOLERY 225 B11 irradiation, proving that they reprcsent the protons bonded to boron.The lines at + 180 and - 170 cycles became sharper upon B11 irradiation, revealing the coupling of the phosphine proton with the six equivalent protons of the two methyl groups. This sharpening indicates an unresolved coupling between the boron nucleus and the phosphine proton. The 350 cycle splitting is, of course, the same as the one found in the P31 spectrum and indicates a direct P-H bond. TABLE ~.-SPIN-SPIN COUPLING CONSTANTS IN c/s FOR THE MOLECULE (CH& PHBH3” spectrum JH:P JH :H’ JP:H’ JB:H” JB:P JP:H’’ HI 12 6 350 90 - 12 B11 - - - 93 50 P31 - - 350 - - - The B11 spectrum contains four pairs of lines with 1 : 3 : 3 : 1 intensity ratios. This is characteristic of a spin of 3/2 due to three identical particles of spin 3, and verifies thc existence of a BH3 group in the molecule.The nuclear resonance spectra of this compound establish the structural equivalence of the methyl groups and the existence of P-H and BH3 groups, so that one can write for the structural formula only CH3 H” \ / PH ’-B-H” CH3 / d/, The techniques described have been shown to be useful in verifying assignments of the lincs in the complex high resolution spectra from molecules with several magnetic nuclei, Since the number and relative amplitudes of spin-spin multiplet peaks permit a rather detailed picture to be obtained of the numbcr of chemically equivalent nuclei connected by covalent bonds, the correct assignment of spectral lines is of direct structural valuc. Although numerical values of bond distances and angles cannot be obtained in this way, the structural information which is available from nuclear magnetic resonance studies can provide powerful con- firmation of the correctness of the model chosen as a starting point in X-ray, electron diffraction, and microwave studies. The kind co-operation of the Westinghouse Research Laboratories in providing the fluorinated ring compounds is gratefully acknowledged. The generous contribution of the diborane and pentaborane samples by Prof. George Pimentel of the University of California, and their careful preparation by Dr. E. Whittle is greatly appreciated. The sample of dimethyl phosphine borine owes its exist- ence to Prof. Anton B. Burg of the University of Southern California and the author is indebted to him for making it available for this study. 1 Bloom and Shoolery, to be published in the Physic. Rev., March, 1955. 2 Hahn and Maxwell, Physic. Rev., 1952, 88, 1070. 3 Kelly, Ray and Ogg, Physic. Rev., 1954, 94, 767(A). 4 Ogg, J. Chem. Physics, 1954, 22, 1933. 5 Price, J. Chent. Physics, 1948, 16, 894. 6 I-Iedberg and Schomaker, J. Amer. Chem. Soc., 1951,73, 1482. 7 Hedberg, Jones and Schomaker, J. Amer. Chem. Soc., 1951,73, 3538. 8 Kasper, Lucht and Harker, Acta Cryst., 1950, 3, 436.

ISSN:0366-9033    

DOI:10.1039/DF9551900215

出版商:RSC    

年代:1955

数据来源: RSC