摘要:

DISCUSSIONS OF THE FARADAY SOCIETY No. 19, 1955 MICROWAVE AND RADIO- FREQUENCY SPECTROSCOPY THE FARADAY SOCIETY Agents for the Society’s Publications : The Aberdeen University Press Ltd. 6 Upper Kirkgate AberdeenThe Faraday Society reserves the copyright of all Communications published in the ‘‘ Discussions ” PUBLISHED . . . 1955 PRINTED IN GREAT BRITAIN AT THE UNIVERSITY PRESS A B E R D E E NA GENERAL DISCUSSION ON MICROWAVE AND RADIO-FREQUENCY SPECTROSCOPY A GENERAL DISCUSSION on Microwave and Radio-frequency Spectroscopy was held in the Department of Zoology, Cambridge University (by kind permission of the Vice-Chancellor) on the 4th, 5th and 6th April, 1955. The President, Prof, R. G. W. Norrish, F.R.S., was in the Chair at the opening session and about 250 members and visitors were present.b o n g the distinguished overseas members and guests welcomed by the President were the following :- Dr. A. Abragam (France), Mr. J. R. Andersen (Denmark), Dr. and Mrs. W. Anderson (Switzerland), Mr. C. 0. Andersson (Sweden), Dr. J. Arnold (Switzerland), Dr. B. Bak (Denmark), Mr. A. Bassompierre (France), Prof. G. J. Ben6 (Switzerland), Prof. J. Benoit (France), Mr. J. Bonanomi (Switzer- land), Dr. J. C. van den Bosch (Holland), Prof. C. J. F. Botticher (Holland), Monsieur M. Buyle-Bodin (France), Mr. B. P. Combrisson (France), Mr. A. F. Corsniit (Netherlands), Dr. L. V. Coulter (U.S.A.), Prof. B. P. Dailey (U.S.A.), Dr. H. G. Dehmelt (U.S.A.), Dr. G. Del Re (Italy), Dr. P. M. Denis (Switzer- land), Mr. G. Dijkstra (Holland), Moiisieur B.Dreyfus (France), Dr. J. and Madame Duchesne (Belgium), Dr. A. Dymanus (Holland), Dr. G. Erlandsson (Sweden), Prof. Richard Extermann (Switzerland), Dr. P. Favere (Italy), Dr. T. Gaumann (Switzerland), Dr. L. van Gerven (Belgium), Prof. W. Gordy (U.S.A.), Dr. L. Grifono (Italy), Prof. H. S. GLI~OWS~C~ (U.S.A.), Dr. W. D. Gwinn (U.S.A.), Miss Lise Nansen (Denmark), Mr. J. Herrmann (Switzerland), Dr. A. Honig (France), Prof. and Mrs. D. F. Hornig (U.S.A.), Prof. and Mrs. G. B. Kistiakowsky (Cambridge), Dr. M. Kofler (Switzerland), Dr. D. M. Kozyrev (U.S.S.R.), Mr. D. J. Kroon (Holland), Dr. H. Kruger (Germany), Prof. W. N. Lipscomb (U.S.A.), Dr. Ralph Livingston (U.S.A.), Mr. J. H. Lupinski (Holland), Dr. and Madame Lurcat (France), Dr. C. McLean (Holland), D.M. Mjasin-Mickailev (U.S.S.R.), Dr. A. Monfils (Belgium), Monsieur S. Montagner (France), Prof. R. S. Mulliken (London), Dr. G. W. Nederbragt (Holland), Mlle. M. Neuilly (France), Mr. J. A. Nikolaev (U.S.S.R.), Prof. R. A. Ogg, Jr. (U.S.A.), Prof. C. T. O’Konski (Holland), Prof. L. J. Oosterhoff (Holland), Prof. G. E. Pake (U.S.A.), Dr. P. Perio (France), Ir. J. Ph. Poley (Holland), Prof. E. C. Pollard (U.S.A.), Mr. H. Primas (Switzerland), Dr. A. M. Prokhorov (U.S.S.R.), Dr. C .Reid (Canada), Prof. Rogers (U.S.A.), Dr. J. M. van Santen (Holland), Prof. R. L. Scott (U.S.A.), Prof. G. Semerano (Italy), Mr. T. M. Shaw (U.S.A.), Dr. and Mrs. Shoolery (U.S.A.), Dr. M. Skogh (Sweden), Prof. H. G. Thode (Canada), Dr. H. Thurn (Germany), Prof. C. H. Townes (U.S.A.), Mr.W. Versnel (Holland), Dr. J. H. van Waals (Holland), Dr. and Mrs. J. S. van Wieringen (Holland), Mr. E. de Wolf (Belgium), Dr. W. Zeil (Germany). 3A GENERAL DISCUSSION ON MICROWAVE AND RADIO-FREQUENCY SPECTROSCOPY A GENERAL DISCUSSION on Microwave and Radio-frequency Spectroscopy was held in the Department of Zoology, Cambridge University (by kind permission of the Vice-Chancellor) on the 4th, 5th and 6th April, 1955. The President, Prof, R. G. W. Norrish, F.R.S., was in the Chair at the opening session and about 250 members and visitors were present. b o n g the distinguished overseas members and guests welcomed by the President were the following :- Dr. A. Abragam (France), Mr. J. R. Andersen (Denmark), Dr. and Mrs. W. Anderson (Switzerland), Mr.C. 0. Andersson (Sweden), Dr. J. Arnold (Switzerland), Dr. B. Bak (Denmark), Mr. A. Bassompierre (France), Prof. G. J. Ben6 (Switzerland), Prof. J. Benoit (France), Mr. J. Bonanomi (Switzer- land), Dr. J. C. van den Bosch (Holland), Prof. C. J. F. Botticher (Holland), Monsieur M. Buyle-Bodin (France), Mr. B. P. Combrisson (France), Mr. A. F. Corsniit (Netherlands), Dr. L. V. Coulter (U.S.A.), Prof. B. P. Dailey (U.S.A.), Dr. H. G. Dehmelt (U.S.A.), Dr. G. Del Re (Italy), Dr. P. M. Denis (Switzer- land), Mr. G. Dijkstra (Holland), Moiisieur B. Dreyfus (France), Dr. J. and Madame Duchesne (Belgium), Dr. A. Dymanus (Holland), Dr. G. Erlandsson (Sweden), Prof. Richard Extermann (Switzerland), Dr. P. Favere (Italy), Dr. T. Gaumann (Switzerland), Dr. L. van Gerven (Belgium), Prof.W. Gordy (U.S.A.), Dr. L. Grifono (Italy), Prof. H. S. GLI~OWS~C~ (U.S.A.), Dr. W. D. Gwinn (U.S.A.), Miss Lise Nansen (Denmark), Mr. J. Herrmann (Switzerland), Dr. A. Honig (France), Prof. and Mrs. D. F. Hornig (U.S.A.), Prof. and Mrs. G. B. Kistiakowsky (Cambridge), Dr. M. Kofler (Switzerland), Dr. D. M. Kozyrev (U.S.S.R.), Mr. D. J. Kroon (Holland), Dr. H. Kruger (Germany), Prof. W. N. Lipscomb (U.S.A.), Dr. Ralph Livingston (U.S.A.), Mr. J. H. Lupinski (Holland), Dr. and Madame Lurcat (France), Dr. C. McLean (Holland), D. M. Mjasin-Mickailev (U.S.S.R.), Dr. A. Monfils (Belgium), Monsieur S. Montagner (France), Prof. R. S. Mulliken (London), Dr. G. W. Nederbragt (Holland), Mlle. M. Neuilly (France), Mr. J. A. Nikolaev (U.S.S.R.), Prof. R. A.Ogg, Jr. (U.S.A.), Prof. C. T. O’Konski (Holland), Prof. L. J. Oosterhoff (Holland), Prof. G. E. Pake (U.S.A.), Dr. P. Perio (France), Ir. J. Ph. Poley (Holland), Prof. E. C. Pollard (U.S.A.), Mr. H. Primas (Switzerland), Dr. A. M. Prokhorov (U.S.S.R.), Dr. C .Reid (Canada), Prof. Rogers (U.S.A.), Dr. J. M. van Santen (Holland), Prof. R. L. Scott (U.S.A.), Prof. G. Semerano (Italy), Mr. T. M. Shaw (U.S.A.), Dr. and Mrs. Shoolery (U.S.A.), Dr. M. Skogh (Sweden), Prof. H. G. Thode (Canada), Dr. H. Thurn (Germany), Prof. C. H. Townes (U.S.A.), Mr. W. Versnel (Holland), Dr. J. H. van Waals (Holland), Dr. and Mrs. J. S. van Wieringen (Holland), Mr. E. de Wolf (Belgium), Dr. W. Zeil (Germany). 3CONTENTS PAGE General Introduction. By H. C. Longuet-Higgins . I. MICROWAVE SPECTROSCOPY- Introductory Paper : Quadrupole Couplings, Dipole Moments and the Chemical Bond.By Walter Gordy . . Microwave Spectra of Deuterated Furans. Structure of the Furan Molecule. By €3. Bak, L. Hansen and J. Rastrup-Andersen. . The Microwave Spectrum and Structure of Methyl Diacetylene. By . G. A. Heath, L. F. Thomas, E. I. Sherrard and J. Sheridan . Information Pertaining to Molecular Structure, as obtained from the Microwave Spectra of Molecules of the Asymmetric Rotor Type. By W. D. Gwinn . Millimetre Wave Spectrum of Methyl Chloride. By J. T. Cox, T. Gaumann and W. J. Orville Thomas . Connections between Molecular Structure and Certain Magnetic Effects in Molecules. By C. H. Townes, G. C. Dousmanis, R. L. White and R. F. Schwarz . GENERAL DrscussroN.-Dr.B. Bleaney, Prof. C. A. Coulson, Dr. D. J. Millen and Mr. K. M. Sinnott, Prof. C. H. Townes, Dr. W. J. Orville-Thomas . II (a). MICROWAVE ABSORPTION- The Use of Microwaves in the Study of Ionic and Chemical Equilibria at High Temperatures. By T. M. Sugden . The Ions Produced by Traces of Alkaline Earths in Hydrogen Flames. By T. M. Sugden and R. C. Wheeler . The Determination of the Electron Affinity of the Hydroxyl Radical by Microwave Measurements on Flames. By F. M. Page . . The Theory of a Molecular Oscillator and a Molecular Power Amplifier. 5 By N. G. Bassov and A. M. Prokhorov . 9 14 30 38 43 52 56 64 68 76 87 966 CONTENTS TI (b). PARAMAGNETIC RESONANCE- GENERAL DIscuss1oN.-Prof. A. R. Ubbefohde, Dr. T. M. Sugden, Mr. R. P. Bell, Dr. J. N.Agar, Dr. E. Warhurst, Mr. F. M. Page, Prof. F. S. Dainton, Dr. D. J. E. Ingram, Prof. C. H. Townes . Introductory Paper. By J. H. E. Griffiths . Paramagnetism of the Actinide Group. By B. Bleaney . Paramagnetic Resonance of Divalent Manganese Incorporated in Various Lattices. By J. S. van Wieringen . The Magnetic Evidence for Charge Transfer in Octahedral Complexes. By J. Owen . Paramagnetic Resonance in Solutions of Electrolytes. By B. M. Kozyrev . Paramagnetic Resonance in Phthalocyanine, Haemoglobin and other . Organic Derivatives. By D. J. E. Ingram and J. E. Bennett . Paramagnetic Resonance of Free Radicals. By G. E. Pake, S. I. Weissmann and J. Townsend . Paramagnetic Resonance in X-Irradiated PIastics and in Plastic Solu- tions of Free Radicals. By E. E.Schneider . Paramagnetic Resonance Studies of Atomic Hydrogen Produced by Ionizing Radiation. By R. Livingston, H. Zeldes and E. H. Taylor GENERAL DIscussIoN.-Dr. E. E. Schneider, Dr. B. Bleaney, Mr. M. Tinkham, Dr. D. J. E. Ingram, Mr. W. A. Runciman, Dr. P. George, MI-. J. Stanley Griffith, Prof. G. E. Pake, Dr. J. Combrisson and Dr. J. Uebersfeld, Prof. W. Gordy, Prof. F. S. Dainton, Dr. R. Livingston, Dr. H. Zeldes and Dr. E. H. Taylor, Dr. C. R. Extermann, P. Denis and G. Ben&, Prof. C. H. Townes . 111. NUCLEAR MAGNETIC RESONANCE Nuclear Magnetic Resonance Applied to Chemical Problems. By H. S. Gutowsky . Proton Resonance Spectra and the Structure of Diketene. By P. T. Ford and R. E. Richards . A Proton Magnetic Resonance hvestigation of the Structure of Urea, By E.R. Andrew and D. Hyndman . Nuclear Magnetic Resonance in Ammonium Fluoride. By L. E. Drain A Nuclear Resonance Investigation of Polytetrafluoroethylene. By J. A. S. Smith . PAGE 99 106 112 118 27 35 140 147 158 166 173 I87 193 195 200 207CONTENTS 7 PA08 The Relation of High Resolution Nuclear Magnetic Resonance Spectra to Molecular Structures. By J. N. Shoolery . High Resolution of Proton Magnetic Resonance Spectra. By W, A. Anderson and J. T. Arnold . Proton Magnetic Resonance Spectra of Crystalline Borohydrides of Sodium, Potassium and Rubidium. By P. T. Ford and R. E. Richards . Nuclear Magnetic Resonance Spectrum and Molecular Structure of . Aluminium Borohydride. By R. A. Ogg, Jr. and 5. D. Ray. GENERAL DISCuSSIoN.-~rof. H. S. Gutowsky, Dr. N. Sheppard, Prof. W. N. Lipscomb, Dr. Peter Gray, Prof. D. F. Hornig, Dr. Manse1 Davies, Prof. E. R. Andrew, Dr. J. A. S. Smith, Prof. G. E. Pake, Dr. N. Sheppard, Dr. J. N. Shoolery, Mr. R. P. Bell . 1V. QUADRUPOLE SPECTROSCOPY The Interpretation of Quadrupole Spectra. By B. P. Dailey . . An Analysis of the Gradient of the Electric Field in HCN. By A. Bassompierre . Nuclear Quadrupole Resonance in Solids. By H. G. DehmeIt . . GENERAL DISCuSSIoN.-Prof. B. P. DaiIey and Prof. C. H. Tomes, Mr. B. Dreyfus, Dr. H. 0. Pritchard, Prof. C. A. Coulson, Prof. D. F. Hornig, Dr. Lucrat . . 21 5 226 230 239 246 255 260 263 274 Author Index . . 282

ISSN:0366-9033    

DOI:10.1039/DF9551900001

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

MICROWAVE AND RADIO-FREQUENCY SPECTROSCOPY GENERAL INTRODUCTORY PAPER BY H. C. LONGUET-HIGGINS University Chemical Laboratory, Cambridge 1. INTRODUCTION The intention of the Faraday Society in holding this discussion is to catalyze the employment by chemists of the new techniques of microwave and radio- frequency spectroscopy recently developed by physicists. These techniques have enabled us to probe into h e details of atomic and molecular structure which would have been thought inaccessible to observation as little as 15 years ago. The least recent and most extensively applied of these new developments is the microwave spectroscopy of gases, but this subject represents only one of the five topics which are to be discussed at this conference. These topics are : (1) the microwave spectroscopy of gases : (2) the absorption of microwave power by ionized gases, particularly flames ; (3) paramagnetic resonance spectroscopy ; (4) nuclear magnetic resonance spectroscopy ; (5) nuclear quadrupole spectroscopy. In this list the second heading covers a topic which differs somewhat from the others in that the absorption of microwave power in flames is not associated with quantized transitions.What is measured is essentially the electrical conduc- tivity of the flame as a function of temperature and composition. This con& ductivity, which arises almost entirely from the free electrons, is determined by their concentration and their collision frequency with the molecular species present. The method therefore enables one to measure both these quantities separately, and in some cases to determine ionization potentials, electron affinities, and equilibrium constants.Our first topic of discussion, the microwave spectroscopy of gases, is a subject already thoroughly familiar to physical chemists. Suffice it to say that the transitions responsible for microwave absorption lines are usually due either to changes in rotational quantum number or to changes in internal configuration- for example, the inversion of ammonia and the hindered rotation of methanol. Microwave spectroscopy provides, in fact, the most accurate method for deter- mining the geometry of simple molecules, and is becoming increasingly useful as a tool for measuring barriers to internal rotation. There is, however, one aspect of microwave spectroscopy which promises to provide detailed information also about the electronic structures of simple molecules, namely, the study of hyperfine structure.This hyperhe structure arises from the different possible orientations of certain nuclei in the molecule, and provides both qualitative information about the molecular symmetry and quantitative information as to the electrical envison- ment of the nuclei in question. As the last three subjects in this discussion are also closely concerned with nuclear effects it may be as well at this point to digress at some length on the properties of nuclei which are relevant to low-frequency spectroscopy. 910 GENERAL INTRODUCTION 2. THE ELECTROMAGNETIC PROPERTIES OF THE NUCLEUS For most chemical purposes it is sufficient to regard the nucleus as having only three properties, position r, mass M, and charge Ze.However, in order to interpret the low-energy transitions which arise in radio-frequency spectroscopy, it is necessary to take into account three more properties, namely, spin I, magnetic moment p , and electric quadrupole moment Q. The spin of the nucleus determines its total angular momentum, which is given by the expression h - 271. - Z/I(I + i). If I differs from zero the nucleus can take up any one of 21 + 1 orientations with respect to a given axis in space. These orientations may differ in energy for either of two quite different reasons. First, a nucleus with spin greater than zero in- variably possess a magnetic moment p ; that is to say, the nucleus behaves as a little magnet whose axis coincides with the axis of spin.Hence if the nucleus is placed in a magnetic field H there will be an additional contribution to the potential energy equal to pNcos 8. When this potential is added to the Hamiltonian it turns out that each (214 1)-fold degenerate level is split into a set of 2 1 3 1 sublevels between which it may be possible to induce transitions by the application of a fluctuating magnetic field of suitable frequency. FIG. 1. (a) Nucleus of magnetic dipole moment p in magnetic field H. Magnetic potential energy = pH cos 8. Splitting of levels in (b) Nucleus of electric quadrupole moment Q in inhomogeneous electric field E = V V. If field is axially symmetrical, quadrupole q = 32V,W. Splitting of levels in pure pure magnetic field when I = 2: energy = teQq(3 cos2 6 - 1) where electric field when I = 4 : Secondly, if the nuclear spin I is greater than 3, the nucleus will possess an electric quadrupole moment Q.What this means is that the positive charge in the nucleus is not spherically distributed, but that the nucleus is rather to be thought of as elongated (Q > 0) or flattened (Q < 0), while still retaining its symmetry about the axis of spin. Such deviations from spherical symmetry make no difference to the energy, provided that the electric field at the nucleus is uniform; however, if the electric lines of force are not parallel in the neighbourhood of the nucleus then the electrical energy of the nucleus will depend on its orientationH. C. LONGUET-HIGGINS 11 relative to the line of force which passes through it.This may be expressed by adding to the Hamiltonian a potential-energy term of the form QeQq (3 C O S ~ 8 - I), where 4 represents the inhomogeneity of the electric field arising from all the other charges (electron and nuclei included) in the rest of the molecule.* In short, the energy of a nucleus may depend on its orientation either for mag- netic or for electrical reasons, or both. These two situations are illustrated diagrammatically in fig. 1. Table 1 gives the electromagnetic constants of some important nuclei. It may be noticed, as already remarked, that if I = 0 there is no magnetic moment or electric quadrupole moment; if I = -$ there is a magnetic moment but no quadrupole moment and that if I > 3 there is a quadrupole moment also, but that its magnitude varies widely from isotope to isotope.TABLE 1 .-PHYSICAL CONSTANTS OF SOME NUCLEI (from Gordy, Smith and Trambarulo, Microwave Spectroscopy) spin z 1 3 - 1 a I 8 a 0 magnetic moment p (nuclear magnetom) 2.8 0.9 1.8 2.6 0 0.7 0.4 - 0.3 0 0 2.6 3.6 1.1 0 0.6 0.8 0.7 2.1 2.3 2.8 quadrupole moment Q (units of 10-24 cm2) 0 0.003 0.06 0.03 0 0 002 0 0 0 0 0.16 0 0 - 0.08 - 0.08 - 0.06 0.34 0.28 - 0.7 3. PARAMAGNETIC RESONANCE SPECTROSCOPY The phenomenon of paramagnetic resonance, or electron magnetic resonance, as it is sometimes called, is essentially simple in principle. The phenomenon is exhibited only by substances whose molecules possess electronic angular momentum. This angular momentum may arise from the presence of either electrons of uncompensated spin or electrons with finite orbital momentum.In either case the ground state of the molecule will be degenerate in the absence of an external magnetic field. However, we know that both the spin and the orbital angular momentum of an electron give rise to magnetic moments: the spin magnetic moment t is eh/$mnc and its component in a chosen direction may be either & ehl4~mc. Leaving aside the orbital contribution, the theory of which is discussed in later papers, one can see that if a molecule with an unpaired electron * If the field at the nucleus is not axially symmetrical, a more complicated expression -f More accurately, the magnetic moment is 20023 X 4 x eh/4~mc ; this small correction is required.also affects the equation for the resonance frequency.12 GENERAL INTRODUCTION is subjected to a magnetic field H its doublet ground state will be split into two components separated by an energy gap of AE = ehH/2nmc. If now an oscillating magnetic field is applied, transitions will be induced between these two levels If v = eH/2nmc and energy will be absorbed, since at low tem- peratures the lower level will be more highly populated. In practice one keeps v constant and varies H until resonance absorption occurs, but this is only a matter of convenience. The occurrence of absorption is a definite indication of de- generacy in the unperturbed ground state; the interest of the phenomenon, however, lies primarily in the interpretation of the hyperfine structure.Such structure may be observed if the molecule contains nuclei with permanent magnetic moments. The magnetic field of a magnetic nucleus will add to, or subtract from, the external magnetic field in its neighbourhood according to the orientation of the nucleus; hence, if the unpaired electron spends much time near this nucleus it will experience a net magnetic field which depends on the nuclear orientation. This results in a splitting of the resonance lines, and the splitting can be used for determining the extent to which the odd electron is associated with the nucleus in question. I t should be added that more information can be obtained from studies of crystalline materials than from fluid specimens, both because the magnetic field of a nucleus averages to zero if the molecule is rotating rapidly, and because in a crystalline material one can determine the resonance absorption for different directions of the applied fields relative to the crystal axes.4. NUCLEAR MAGNETIC RESONANCE There are two distinct types of measurement which come under the general title of nuclear magnetic resonance. Basically the same phenomenon is observed in both, but rather different types of information are obtained from the two sorts of experiment. In low resolution nuclear magnetic resonance spectroscopy one places a crystal in a permanent magnetic field H and subjects it to an oscillating magnetic field of frequency v. The external field has the effect of splitting the energy levels of the magnetic nuclei in the manner described in 6 2, and the oscillating magnetic field then induces transitions between these levels. If we consider a particular magnetic nucleus, the magnetic field in its neighbourhood may be modified by the magnetic fields of neighbouring nuclei, but there are various possible orientations for each of these nuclei.From a statistical point of view the magnetic nuclei of a particular type will therefore not all be experiencing the same magnetic field and the ob- served resonance spectrum will be spread out from a line into a fairly wide band. The width of this band will be determined by the distribution of other magnetic nuclei around the nucleus whose re-orientation is being studied, and by making use of this fact one can determine the distances between magnetic nuclei in the molecule. This method has proved particularly valuable for determining the positions of protons in crystalline solids, and proton magnetic resonance provides a useful supplement to X-ray crystallography in this respect.High-resolution nuclear magnetic resonance is a more recent development. In this technique one works with a fluid sample in which each molecule is rotating rapidly in the external magnetic field. As indicated above, this rotation has the effect of averaging out to zero the direct magnetic field of any nucleus at any other nucleus. It might be thought, therefore, that rather little information could be obtained about the molecular structure from the high-resolution method. This, however, is not so for two reasons. First, the average magnetic field at a chosen nucleus is nearly but not exactly equal to the externally applied field.This is because the electrons associated with the nucleus are magnetically susceptible toH. C. LONGUET-HIGGINS 13 the field and their susceptibility has the effect of partially screening the nucleus from the permanent field. The resonance frequency of a nucleus therefore depends slightly on its chemical environment and experience has shown that one can distinguish, for example, protons bonded to oxygen from protons bonded to nitrogen OF carbon. Indeed, the so-called " chemical shifts " in proton resonance can be used as an analytical device in much the same way as infra-red spectroscopy. Secondly, although as just indicated the direct magnetic field of one nucleus at another averages to zero, a given nucleus is not, so to speak, entirely unaware magnetically of its neighbours. This is because the electrons in the bond between two nuclei have the effect of coupling their magnetic moments.The theory of this effect is somewhat complicated but it is quite certain that the resonance fre- quency of a particular nucleus depends slightly on the spin orientations of neigh- bouring magnetic nuclei. This fact enables one to derive from the high-resolution measurements much useful information about the relative disposition of different magnetic nuclei, as later papers in this Discussion fully illustrate. Mention should be made of two more points. First, if a nucleus has a large quadrupole moment, account must be taken of this in the quantitative interpreta- tion of the resonance frequency, though this effect is absent in proton resonance since the proton has no quadrupole moment.Secondly, fine structure can only be resolved if the nucleus in question maintains its position in the molecule for a time substantially greater than l/Av, where Av is the frequency separation of the lines to be resolved. This fact makes it possible to identify mobile protons in hydrogen compounds and even to obtain limits for the rate constants of certain exchange reactions. 5. NUCLEAR QUADRUPOLE SPECTROSCOPY This branch of radiofrequency spectroscopy is somewhat simpler in principle than magnetic resonance spectroscopy. No permanent magnetic field need be applied : the crystalline sample is simply placed in a coil carrying a radiofrequency current and the absorption of energy is measured as a function of frequency.Such absorption will occur if the sample contains one or more nuclei with a permanent electric quadrupole moment. In $ 2 we remarked that the energy of such a nucleus will depend on its orientation in the molecule, if the lines of electric force at the nucleus are not parallel. Such a nucleus can then take up 21 + 1 different orientations whose energies will not be all equal, and one may expect under favourable conditions to observe transitions between these states. Now the nucleus has no electric dipole moment, so an oscillating electric field will be ineffective in inducing such transitions. However, a nucleus with a quadru- pole moment always has a magnetic moment and can be induced to alter its orienta- tion by an oscillating magnetic field of suitable radiofrequency. What one measures, then, is the radiofrequency at which transitions are induced, and this tells one how much energy is needed to re-orient the nucleus in the electric field of the rest of the molecule. Consequently, if one knows the nuclear quadrupole moment Q one can determine the value of q, that is to say, the inhomogeneity of the electric field at the nucleus due to all the other charged particles in the molecule. This measurement of q can in some cases be checked against values obtained from microwave spectra, in which the hyperfine structure also arises from quadrupole effects; it is then a matter for the theoretical chemist to interpret the magnitude of q in terms of the electronic structure of the molecule.

ISSN:0366-9033    

DOI:10.1039/DF9551900009

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

I. MICROWAVE SPECTROSCOPY INTRODUCTORY PAPER: QUADRUPOLE COUPLINGS, DIPOLE MOMENTS AND THE CHEMICAL BOND * BY WALTER GORDY Dept. of Physics, Duke University, Durham, North Carolina, U.S.A. Received 22nd February, 1955 It is particularly fitting that a discussion of microwave and radio-frequency spectroscopy be conducted under the auspices of the Society which bears the name of Michael Faraday. Faraday’s discoveries in electromagnetism led Maxwell to predict, and later Hertz to discover, radio waves. Faraday’s experiments in electrolysis revealed the quantization of electric charge and the electrical aspects of matter which are responsible for the interaction of these radio waves with material particles. Faraday’s ice-pail experiments presaged the invention of microwave cavities and wave-guide.It was Faraday’s search for a common denominator for all physical manifestations which led him to his discoveries of basic relations between electricity and magnetism, between electricity and matter, between magnetism and light. One day in 1822 Faraday set out to find a relation between magnetism and light. Twenty-three years later, twenty-two years and some odd months after all ordinary men would have lost heart and quit, Faraday succeeded. Thus Faraday’s work led to the development of the implements or tools of microwave and radio-frequency spectroscopy. His belief in the unity of physical reality gives philosophical purpose to this discussion. His patient persistence in his endeavours demonstrates the proper psychological attitude for those who would do the precise measurements and careful analysis of modern spectroscopy. Considering the early discovery of radio waves, it is puzzling that their use for spectroscopy came so late.In 1946 both microwave and radio-frequency spectroscopy were just beginning, yet the radio waves of Faraday, Maxwell and Hertz were known and had been used in numerous practical applications for half a century. We “ pure ” scientists can take no pride in this. We can only point with pride to the rapid progress we made once we did start to tune in the atoms and molecules on our radios. Probably never before in the history of science has so much highly precise and diverse information about the structure and properties of matter been obtained in a comparable span of time by any other method as has been accumulated within the past decade from microwave and radio-frequency spectroscopy.If you doubt this statement, it may be that your senses have been dulled by the bang of the hydrogen bomb which occurred during the same period. By inserting the modifiers “ highly precise and diverse ”, I have sought to dis- qualify the great bulk of information obtained with X-rays and electron diffraction We in this Discussion are not concerned with the structure of the nucleus, and I shall not speak about the large amount of nuclear information which has been gained via radio waves. We are not concerned with astronomy, and I shall not tell how one microwave spectral line is unravelling the structure of our galaxy and informing us of the composition of interstellar space.I am not supposed to *This research was supported by the United States Air Force through the Office of Scientific Research of the Air Research and Development Command. 14WALTER GORDY 15 tell you of the new knowledge of the solid state being gained fron electron spin resonance and cyclotron resonance at microwave frequencies, nor of the invasion of biology by microwave spectroscopists. I was asked to give an introduction only to the microwave spectroscopy of gases. Even that is too much to introduce properly in a single speech. Hence, I shall attempt a proper introduction to only one aspect of the subject-that of nuclear quadrupole couplings-but I shall first call your attention to certain other aspects. I I I I INFRARED RAYS I MICROWAVES RADIOWAVES I I I I I I I I I I I I I l04CPS ! ,----r---& I lolo i rde I& i 10'4 1012 I I 3 O / i , I 1 1 3mra 3Bcm t 30m 3km I 1 I I ' -+-J--1/32,000 IN 1/32 IN.3 FT 20 MILES MOLECULAR ROTATIONS NUCLEAR PRECESSIONS MOLECULAR VIBRATIONS ELECTRONIC PRECESSIONS FIG. 1 .-Spectral space of the microwave region. The spectral space of the microwave region is indicated in fig. 1. With micro- wave electronic methods we at Duke have measured spectral lines down to 0.77 inm wavelength.1 In Czerny's infra-red laboratory at Frankfort, Germany, an infra- red grating spectrometer has been used by Genzel and Eckhardtz to measure rotational lines up to 0.99 mm wavelength. The microwave and infra-red frontiers have thus overlapped. No virgin territory remains in the entire electro- magnetic spectrum except at its boundless ends.The last territory to be explored, our last spectral reserve, is indeed a fertile one. Here molecular lines are strong and abundant-orders of magnitude more so than in the centimetre wave region -yet the high resolution and precise measuring methods of microwave electronics are still applicable (see fig. 2). FIG. 2.-Microwave rotational line (J = 27 -> 28) of ClCN in sub-millimetre region A = 0.89 p. Measured frequency 334,2193 Mc/s ; line width, 0.4 Mcjs ; cell volume, 0-4 cm3 (from Burrus and Gordy).l Just as molecular vibrational frequencies fall in the infra-red region and nuclear precessional frequencies in the conventional radio region, rotational transitions of nearly all molecules fall in the microwave region.Most of the molecular information from microwave spectra has been obtained from rotational spectra- but not from simple, unperturbed rotational spectra. Superimposed on the16 QUADRUPOLE COUPLINGS rotational motions of molecules are nuclear precessions and also molecular pre- cession when an external magnetic or electric field is applied. The structure of rotational spectra arising from nuclear or external field interaction allows the microwave spectroscopist to obtain considerably more information about the molecule than he could obtain from an unsplit rotational linc. The type of in- formation obtainable from the rotational spectra alone, bond angles and bond lengths, is widely appreciated and needs no special attention in this introduction.Microwave structural evaluations are generally more accurate than those from other sources, and they are being accumulated at a rapid rate. The same applies to electric dipole moment determinations from the Stark effect of microwave rotational spectra. Resolvable Zeeman splitting of rotational lines of practically any molecule can be produced by fields of 5 to 20 kilogauss. Numerous gaseous free radicals should be detectable with microwave spectroscopy, and a few have been so detected. Molecular electric quadrupole moments and collision dia- meters can be ascertained from microwave line shape measurements. Prof. A. Kastler has pointed out to me that molecular twisting vibrational transitions of many large organic molecules in solids should be observable in the one-milli- metre-wave region.The marriage * of microwave and optical spectroscopy has been brought about by ingenious proposals of Bitter 3 and of Kastler 4 whereby microwave or radio-frequency energy is used to change the distribution of the Zeeman levels of an optically excited substance so that observable light of a particular polar- ization is radiated. This is a development truly in the spirit of Faraday. QUADRUPOLE COUPLINGS, DIPOLE MOMENTS AND THE CHEMICAL BOND Because the information which nuclear quadrupole coupling can (or cannot) give about the chemical bond is not widely appreciated, I shall treat this subject in some detail. The manner in which these coupling constants are deduced from the hyperfine structure 5 of rotational spectra need not be described.The coupling constant which is obtained directly from an analysis of the hyperfine structure of symmetrical molecules is eQq, in which e is the charge on the electron, Q is the electric quadrupole moment of the coupling nucleus, and q is the electric field gradient at the nucleus with reference to the charge-symmetric axis. From atomic beam measurements, Q is now accurately known for the halogens Cl, Br, and I, and for B. Whenever the coupling of one of these nuclei is measured in a molecule, the field gradient q is determined. An electron i at a distance ri from the nucleus gives rise to a potential, V = e/r at the nucleus and a field gradient, In the last form 8 is the angle between r and the z axis. To obtain the contribution of the ith electron to the observed q, one must average the above quantity over the orbital of the electrons of the molecule.The total q can thus be expressed as i with the normalization, Eqn. (2) suggests at once how quadrupole coupling might give information about the electronic structure of a molecule : it links the measurable quantity, q, to the wave functions of the molecular electrons. It also reveals at the outset * The marriage ceremony, I understand, was performed by Bitter and Brossel (Physic. Rm., 1950,79, 775).WALTER GORDY 17 the difficulties facing anyone who woufd arrive at chemical bond information in this way. The difficulties are the same ones that always plague those who seek to solve the chemical bond problem in an elegant and exact manner. There are many more electrons in the molecule than there are observables, and we do not really know the correct #i for any of them.We must make assumptions, some questionable, to apply our new datum. Nevertheless, we are proud of our number q. It is more accurately measured than are most other quantities around which assumptions are constructed. We can ignore all electrons except those in the valence shell of the coupled atom, A. This represents a sweeping simplification of the problem, and yet it is not hard to justify, at least to the semi-practical man, namely, the chemical physicist, Electrons on other atoms are too far away and the inner-shell clouds of A have too much symmetry to make significant contributions to qA. Our problem resolves to one of guessing what happens to the electronic cloud of its valence shell when the atom forms a chemical bond.P consider this an attractive way of putting the problem, for I know that there is no greater sport for the chemist or chemical physicist than that of guessing what happens when a chemical bond is formed. For a halogen atom which forms an orthodox single bond, the simplest first guess is that nothing much happens to any of the orbitals except the one which gets mixed up with an orbital of the neighbouring atom. Let us suppose that the A-orbital in question is represented by the wave function, $a, and the B-orbital with which it gets involved by &. By a popular convention we then express the new molecular orbital as the mixture, (4) with the yet undetermined mixing coefficients a and b.We assume that the two electrons are in the bonding orbital represented by # and express their combined contribution to q A by d' = a$a + b$b, The last term of eqn. (5) represents the contributions to q A of electronic charge density in the atomic orbital of B, which, because of the inverse cube variation of q with r are negligible. The term next to the last, while larger than the last, is- less than 1 % of the first term for halogens considered here. We, therefore, throw away both of the last terms and have left only which, except for the factor 2a2, is simply the contribution of a single electron in the atomic orbital #a. Let us first assume that #a is a p orbital of A. The other p orbitals are then filled with unshared pairs and the d orbitals are empty.The total qA can then be obtained most simply by treating it as arising from a pz electron deficit of (2 - 2a2) electrons in an otherwise spherical cloud. The atomic coupling, on the other hand, arises from a deficit of exactly one p electron in an otherwise closed shell. Therefore, we can write (7) in which the factor of - 2 corrects for the difference in orbital orientation of the unbalanced p electron in the molecule and in the free atom. By multiplication of both sides of eqn. (7) by eQ it is found on rearrangement that (8) where for future convenience we designate molecular qtotal = (2 - 2a2)(- 2) atomic qtotal, a2 = 1 - (pq/2), molecular eQ9 2 atomic eQq (9) Let us see if the a2 values thus obtained have any meaning.18 QUADRUPOLE COUPLINGS We consider first the homopolar molecule for which2 -.- 6 .For it, one obtains (1 0) in which Sub is the overlap integral J+&bdr. The measured couplings substituted in eqn. (8) gives a2 = 0.50 for both C12 and Br2. These values with eqn. (10) indicate Sub = 0. The overlap integral obtained with Mulliken’s tables 6 indicates the very different values &, = 0.34 for C12 and &b = 0.31 for Br2, and hence the values a2 = 0.36 and 0.38. The inconsistency can be removed by postulation of large amounts of s hybridization, 30 to 35 % for the atomic orbitals if one likes to postulate hybridization. It is a little more cumbersome to handle, but if one employs the unnormalized Heitler-London wave function for the pure covalent bond, from the normalizatioii of eqn. (4), 1 a2 = 2(1 + sub)’ and follows through the evaluation of qA in the same way by assuming pure p orbitals, one finds that which yields with the observed couplings, c2 = 0.50 for Cl2 and for Br2. With the normalizing requirement of the Heitler-London function, c2 = 1 - (pq/2), (12) the couplings again require sub = 0.Mulliken’s values of Sd with eqn. (13) give c2 = 0-43 and 0.46 for C12 and Br2. Because of the different expression for the normalization, a postulation of only 10 to 12 % s character for the atomic orbital z,hu would this time make harmony. Nevertheless, it is fair to ask: if the 1.c.a.o. molecular orbital approximation can be 35 % wrong here, may not the Heitler-London approxiniation be at least 12 % wrong? The difficulties brought out in the above discussion originate principally, I think, from the following conditions.Because of inverse cube variation with Y , the major part of the nuclear quadrupole interaction arises from the relatively small part of the valence orbitals which lies within the major lobes of the inner shells. Contrary to the implications of the usual normalizations, significant charge density is not lifted from regions near the nuclei to help form the cloud which I do not deny is piled up between the nuclei when the covalent bond is formed. The orbital overlap distortions occur mainly in the outer regions of the atoms where the nuclear coupling is insensitive to charge redistributions. Both the Heitler-London (H.-L.) and the 1.c.a.o. molecular wave functions as conven- tionally normalized appear to over-emphasize distortions near the nucleus-the latter more than the former.For dealing with nuclear quadrupole coupling it appears permissible to ignore the overlap integral7 and to normalize the wave functions by setting sub = 0. The normalization then gives a2 or c2 = 3 for homopolar bonds, or a2 + b2 ;= 1 for polar ones. In treating dipole moments, bond energies, or other properties which are sensitive to charge redistributions in the outer parts of the atom, we cannot of course do this.8 The setting of s u b = 0 seems to free the nuclear coupling from the chemical bond and thus to end our story. Actually, this is no more than one-third true. So far we have considered only pure covalent bonds (no ionic character) formed out of unhybridized atomic orbitals.Unlike orbital overlap, ionic character can decrease significantly the electron density of the bonding atomic orbital in regions close to one nucleus because this density is in effect transferred to a com- parably low potential region near the other nucleus. Furthermore, a pure covalentWALTER GORDY 19 bond through scrambling (hybridization) of the atomic orbitals can alter the angular distribution of the electronic charge cloud near the nucleus and thus can influence the coupling without lifting charge density appreciably away from the nucleus, although some lifting of course accompanies the hybridization. To examine the effects of bond orbital hybridization and ionic character let us go back to eqn. (6) and consider a + b, with (14) $a = as#s + ap$p -k ad#d.If cross-terms are neglected, eqn. (6) then becomes The first integral here is zero because the s electronic cloud is spherically symmetric. The second represents the contribution of the p electron already described. The third integral is small because the dorbital is non-penetrating. It is further reduced by the factor ad2 which should insure that its contribution to q A in the halogens is less than 1 %. We, therefore, drop both the first and the last terms on the right and have left, (qdbond = 2a2ap2[#pqi#idT. (1 6 ) This expression gives only the contribution of the hybridized bonding orbital. To get the total q A we must consider other orbitals, some of which will be counter- hybridized. Wc choose the p orbital involved to be the pz. The counter-hybrid- ized spz orbital will have as2 amount of p character and will contain an unshared pair of electrons, while the counter hybridized pZd orbital will have ad2 amount of pz character but will be empty.As before, the px and pv orbitals contain un- shared pairs. There will be a resultant pz population of 2a2up2 + 2us2. The total q A will arise from a pz electron deficit of (2 - 2a2ap2 - 2as2) in an otherwise spherical cloud. With the normalizations as2 + ap2 + ad2 = 1 and a2 + 62 = 1, and with the representation, ionic character = p = a2 - b2, one obtains molecular cQq = [I - as2 + ad2 - p(l - as2 - adz)](- 2 atomic eQq), or fl(1 - as2 - ad2) + as2 - ad2 = 1 - pq. (1 8) A more exact expression which contains an averaged correction for the changes in nuclear screening in the halogen is (19) In these equations as2 represents the s character and ad2 the d character of the bond orbital of A.A negative sign of corresponds to a positive charge on the coupling atom. Because we have set Sab as zero, our designation of ionic character is equivalent in magnitude to ionic character as conventionally defined in terms of ionic-covalent resonance concepts. The difficulty with eqn. (18) or (19) is that it contains three parameters and only one observable. A fortunate circumstance exists, however. If we revert to the homopolar molecules C12 and Br2, with their observed pq = 1.00 and fl = 0, we learn from eqn. (18) that either there is no hybridization or as2 = adz. If correction is made for the cross-bonding 9 in the macromolecule of crystalline I2 as indicated by its asymmetry parameter, we learn similarly that for 12, as2 = a$, to a very close approximation.Although we are compelled again to empIoy solid state data for the pure homopolar bonds, the pq values for the gaseous state of the nearly homopolar molecule BrCl, indicates as2 m ad2 for any reason- able value of /3. Furthermore, a consistent interpretation of the C1 and I coupling /3(1 + 0 . 1 3 ~ ~ - as2 - a$) + as2 - ad2 = 1 - pa.20 QUADRWPOLE COUPLINGS in ICl requires either no hybridization for either atom or nearly equal s and d hybridization. Now if we consider completely ionic molecules such as KCl or KBr for which = 1 and pq = 0 (observed), we find from eqn. (18) that ad2 = 0, and that possible effects of s hybridization drop out. In other words, the experimental evidence is that hybridization does not exist either in the pure covalent or in the pure ionic state X+Hal- unless it exists in such form as to have no observable effects on the coupling.Therefore we suspect that there is probably 0.9 - “li 01 ELECTRO&GATI\IITY DIFFERENCE FIG. 3.-Plo t of ionic character (as indicated by nuclear coupling) against electronegativity difference. The x values 16 for : H, 2.13 ; Li, 0.95 ; T1, 1.3 ; CI, 2-98 are from ref. (16). When rounded off to 2 figures, these coincide with Pauling’s, except for Tl, for which Pauling gives no value. The value for CH3 is from force constants (see table 3). Other values are from Pauling 12 except that Haissinsky’s value (J. Physique, 1946, 7, 7) of 2.6 for I is employed. The hydrogen halide couplings are from ref.(1 1) ; the T1 halide couplings from Mandel and Barrett (Bull. Physic. SOC., 1955, p. 20); and the alkali halide couplings from Hornig et al. (Physic. Rev., 1954,96,621). The other couplings are listed in ref. (5). little, if any, of the detectable kind in mixtures of the two. Later I shall state other reasons for suspecting this. With this evaluation of the constants from the neutral and negative end points, as2 = ad2 = 0, eqn. (18) becomes B = 1 - Pq, or more precisely, p = (1 - PqYU 4 0.13Pq). Eqn. (20) or (21) should give a rather good measure of the ionic character of the type which puts the negative pole on the coupling halogen. In the approximate considerations to follow, the screening effects of the negative charge will be neglected, since at most they cause only 0.03 charge in p.For a positive charge, the screening effects are more serious and must be considered. Eqn. (21) applies also for positively charged atoms when there is no hybridization; but for a large positive charge, hybridization effects may not be negligible.WALTER GORDY 21 The ionic characters obtained with eqn. (20) for a number of simple molecules are compared with the electronegativity difference of the bonded atoms in fig. 3 and 4. These plots indicate that the relation between ionic character and electro- negativity difference is approximately but not exactly linear. Because of the inexactness of the best available electronegativities, one cannot quibble over the non-linearity except, of course, where the curve levels off at the top.It reveals that for electronegativity differences greater than two the bonds are purely ionic ECECTRONEGATIVITY DIFFERENCE FIG. 4.-Plot of ionic character against electronegativity difference with Mulliken’s scale (open circles) and with Pauling’s scale (solid dots). The Mulliken scale for H and for. the first and second row elements is that obtained by Skinner and Pritchard (Trans. Faraday SOC., 1953,49, 1254). In both plots the Haissinsky value 2 6 for I is used. The only significant difference in the two scales is for H, 2-28 (M) and 2.1 (P). and that a small electronegativity difference leads to significant ionic character. The convenient approximate rule, (22) for estimating ionic character derived earlier,los 7 from meagre coupling data is borne out by the more complete data now available.The plot of data on hundreds of polyatomic molecules, including molecules in the solid state shows a wider scattering of points about the same line if Pauling’s electronegativity scale is used. This mass agreement occms because Pauling’s scale represents mean values chosen from a variety of molecules and because various deviating factors tend to balance in the aggregate. The 7~ character, about 10 %, expected in the SiHal bonds,s if taken into account, would improve the agreement of fig. 3. In contrast to the mass approach there is the discriminating approach which includes only those molecules for which the parameters involved are most ac- curately known. Fortunately, both lead to the same relation here.Fig. 4 shows the result of the discriminating approach. Even here there are two very respectable electronegativity scales, Pauling’s and Mulliken’s, which must be considered, but again the weighted average agrees with fig. 3. The couplings of the hydrogen halides recently measured in the one-to-two millimetre wave region 11 are particularly significant for the above relation. It ionic character = (1 x,, - xB I )/2 for I xA - xB I < 2 ,22 QUADRUPOLE COUPLINGS was with the dipole moments of the hydrogen halides that Pauling 12 estimated the ionic character values which he used to obtain the first ionic character against electronegativity relation, Because he did not correct for the large overlap moment (then unknown) which in the hydrogen halides opposes the primary moment, it is understandable that his relation (see fig.5 ) predicts much lower values for ionic character than the nuclear couplings and dipole moments now indicate. FIG. 5.-Comparison of various ionic character electronegativity relations. Pauling’s relation is from ref. (12) ; Hannay and Smyth’s from J. Amer. Chem. Sac., 1946, 68, 171 ; Dailey’s from J. Physic. Chem., 1953, 57, 490. A relation similar to that of Dailey’s is recommended by Townes.26 Although I consider the values from quadrupole coupling the more reliable, I shall attempt to calculate ionic character from the dipole moments of the hydrogen halides and alkaline halides just to show that there is not necessarily any disagree- ment between the two methods.According to modern concepts, the dipole moment can be expressed as the sum of four parts, The first term on the right is the primary moment which arises from ionic character ; it is given by fled. This is the term which is of interest to us, but in order to find it from the observed moment we must evaluate the other terms. The second term on the right is the overlap moment, the significance of which was first recog- nized by Mulliken. It can be expressed 13 as EL = PP + PS $- Ph Pi- (23) where r is the covalent radius of the larger atom, and d is the internuclear distance. The third term represents the atomic hybridization moment of Coulson.14 It is zero when there is no hybridization, as we shall here assume. The last term pi is the polarization or induced moment.In the ionic or nearly ionic alkali halides we can neglect both ps and ph, and have only to evaluate pi. This we do in an elementary manner. Classically the polarization moment can be expressed Pi == Ea%a + Ebab, (25) where Eb are the polarizing fields assumed to be uniform over the ions. and ccb are the polarizabilities of the ions A and B, and where Ea and The latter is,WALTER GORDY 23 of course, not exactly true in our case. To obtain an estimate of Eu, we assume it to arise from an effective charge on B, pObS./d, equal to its observed pole. Similarly, the field Eb is obtained from the effective pole on A. Quite simply, but only approximately, With eqn. (23) and (26) and with ph = 0, ps = 0 the ionic character is Eqn. (27) has been used to calculate ionic characters for the alkali halides, which are compared with those from nuclear coupling in table 1.If, instead of the effective pole, the full electronic charge is used with the classical method of Debye 15 to calculate pi, ionic characters greater than unity are obtained. Al- though we think we know that ionic characters greater than unity do not occur here, the results are still in agreement with the quadrupole coupling in that com- plete ionicity is indicated when I X, - x, I > 2. TABLE 1 .-PREDICTED IONIC CHARACTER from from from molecule dipole moment Hal. coupkg electronegativity P*"pld 1 - Pq I xA-xB l 2 HCl HBr HI LiBr LiI Nacl KC1 KBr KI cscl 0.4 1 0.35 0.18 0.90 090 0.97 1-01 097 0.98 099 0.40 0.3 1 021 0.95 0.91 1-00 1-00 0.99 0.98 1.00 042 0.34 0.23 093 0 8 3 1.00 1-00 1-00 0.90 1.00 In the hydrogen halides, where the proton is essentially embedded in the side of the halogen, it is very difficult to calculate the induced moment, but we know from geometrical reasoning that it will be much smaller than that given by eqn.(26) and therefore not very large. Although I do not know how he did it, Mulliken 13 calculated pi for HCl as 0.3 D. I have used this value with the pi ratios obtained from eqn. (26) to estimate the Pj for HBr and HI given in table 2. TABLE 2.-Eh"IMATION OF IONIC CHARACTERS FROM DIPOLE MOMENTS HCi - 1.14 - 0.3 0 1 *08 2.52 0.41 HBr - 1.38 -. 0 2 0 0.80 238 0.35 HI - 1.70 - 0.1 0 - 0.42b 1-38 0.18 a assumes pure p orbitals for halogen, pure s for H. b The sign of pp is taken as positive, whereas the observed moment of HI is assumed to be in opposition and hence negative.To calculate the overlap moment I have used eqn. (24) with Mulliken'sd §'& values for a pure p bonding orbital of the halogens. The primary moment used to estimate ionic character is then obtained by addition of these quantities to the observed moment, with ph = 0, as shown in table 2. It is interesting that in P-II the overlap moment is so large that the observed moment is in opposition to the primary moment, i.e. the negative pole is at the hydrogen end.24 QUADRUPOLE COUPLINGS In addition to coupling evidence against it, dipole moment considerations do not favour hybridization when the ionic resonance puts the negative charge on the halogen. Suppose that a bond X-Hal of length 2 A has 50 % ionic character, of the kind X+Hal-.There would then be a primary moment of 4.8 19, with the negative pole on A. This large primary moment would strongly tend to quench any sp atomic hybridization moment on A which would of necessity be in the same direction as the primary moment when the negative charge is on Hal. Furthermore, in the diatomic interhalogens, where the smaller atom is always the more electronegative, the overlap moment would be in the same direction as the primary moment and would assist in the quenching. Actually, in a molecule like ICl if there is 15 % s character or even 10 % in the C1 bond orbital and no hybridization in the I orbitals, an atomic hybridization moment of the order of 2 I9 would add to the primary and overlap moments to give a total of several Debye to be cancelled down to the observed moment of @54D by the induced atomic moment, which here must be in opposition to the atomic hybridization moment.Also, this type of hybridization would increase the electronegativity of Cl but not of I and would hence increase the already too large moment still further. All these factors, on the other hand, favour hybridization on the positively charged atom; and both the quadrupole couplings and the dipole moments of the molecules FCl and FBr indicate that some hybridization may exist at the positive end, possibly as much as 10 %, if the bonds are normal single bonds, as they may not be. No s character is in evidence for either the Br or C1 orbitals in BrCl, nor for either halogen of IC1.In BrCl the Br coupling indicates 10 % ionic character, and the C1 coupling indicates only 6 %. This disagreement is not serious, and is only made worse by postulation of s character on either halogen. Small amounts of d hybridization could clear up the discrepancy, which could also be caused by small errors in the coupling and estimated correction for screening. If we choose the average value of 8 % for the ionic character, the primary moment is 0.08 x 2.14 x 4.8 = 0-82 D. The overlap moment for pure p orbitals is 0.17 D in the direction of pp, whereas the opposing induced moment is estimated from eqn. (26) to be 0.45 D. The total moment predicted in this way is 0.54 D, whereas the observed moment is 0-57D. A similar but less certain analysis for FBr and FCl leaves a sizeable difference to be cancelled by n- feed-back and possibly by a hybridization moment on the positive halogen.The ionic character of ICI predicted without hybridization by eqn. (21) with the C1 coupling is 23 % and with the 1 coupling is 24 %. This agreement is excellent. A postulation of s hybridization (without d ) on either or both atoms would (as in BrCl) make the agreement worse, not better. A consideration of the fact that more s charge would be lifted out of a given s hybridized orbital of A when a2 < b2 than when a2 > b2 might lead one to con- clude that s hybridization in the halogens would occur at the negative but not at the positive pole. However, this is by no means the only consideration. Already atomic dipole effects have been mentioned as favouring hybridization on the positive pole.Other things being equal, it would seem reasonable that the type of hybridization would be favoured which would lower rather than raise the electronegativity difference of the bonded atoms. This appears to be borne out in FCl for which the bond energy with Pauling’s method gives (XF - xcl) = 0.76, whereas MuEliken’s method for pure p orbitals gives (xp - xcl) = 0.90. A little s character on C1 would clear up the discrepancy, whereas s character on F would only make it worse. As a general rule, the dipole moment would tend to sustain the hybridization on the positive atom and to quench it on the negative atom. Probably never in a molecule is there hybridization of the type which would significantly increase an already large total moment. In such molecules as PF3, AsF3, AsC13 and SbGI3 a large atomic hybridization moment on P, As, or Sb is in opposition toWALTER OORDY 25 the primary moment as well as the overlap moment.Note that in these molecules s hybridization occurs on the positively charged atom. In NF3, where the overlap moment is negligible and the polarization moment small, the large primary moment of the NF bonds is almost cancelled by the comparably large atomic hybridization moment on N, to give the small observed moment of only 0.23 D. If the nega- tively charged F atoms had significant hybridization, this would not be true, and a large moment would be expected. In AsH3 the primary moment is very small (Ax = 0*1), but there is a large overlap moment which opposes the comparably large hybridization moment of As so that the observed moment is only 0.16D.In H2S the overlap and primary moments are opposed but leave a resultant of approximately 0.6D pointing in the same direction as the atomic hybridization of pS to give the observed moment of about 1 D. Like H2S, H20 and NH3 are rather exceptional in that an atomic hybridization moment points in the direction of the resultant moment, but the large bond angle, short bond length, and opposing overlap moment keep down the resulting moment to reasonable size. If the quadrupole coupling data are interpreted correctly here, then ionic character is a much more sensitive function of electronegativity difference than has been previously supposed (see fig.5 for example). It follows that the moderate variations in electronegativity with chemical bonding state have much more in- fluence on ionic character, and hence upon physical and chemical properties of molecules, than has been previously supposed. Furthermore, with the electro- negativity quadrupole coupling relation shown in fig. 3 we can use nuclear quadru- pole coupling of the halogens to evaluate effective electronegativities of atoms bonded in various chemical groups regardless of whether we admit nuclear coupling as measuring ionic character. The relation can be expressed approximately as where x, is the effective electronegativity of the atom bonded to the coupling halogen. In using this relation one must always be sure that no significant double bond character exists in the bond.An application is made in table 3. TABLE 3.-EFFECTIVE ELECTRONEGATIVITY OF c IN THE METHYL GROUP a from from molecule force constant Hal eQq eqn. (30$’ eqn. (28) CH3Cl 237 238 CH3Br 2.34 2.30 CH31 228 229 av. 2.33 av. 2.32 a When xcl = 3.0, xBr = 2.8, and x, = 2.6. b With force constants by Linnett (J. Chern. Physics, 1940, 8, 91). Noether’s force constants (J. Chem. Physics, 1942, 10, 664) for CD3CI and CD3Br yield the x values 2.38 and 2-39, respectively, for the methyl group. When, as is most often the case, the T bond component in the double bond character is formed through donation of an unsharedp pair by the coupling halogen, the unbalanced pz charge is reduced and the coupling is lowered. If the amount of such m- character is designated by 3/n then it is easily shown that the above relations become approximately in which Po represents the ionic character of the (T component. When the effective electronegativities are known, these relations can be used to estimate the ionic character of the (T component as well as the rr character of the bond.26 QUADRUPOLE: COUPLINGS With the help of other relations to estimate effective electronegativities, 1 shall now apply eqn.(29) to certain types of bonds in polyatomic molecules. The force constant against electronegativity relation,l6 with the force constants given by Herzberg 17 for C2H2, 5.92 and for C2D2, 5.99, and with the interatomic distance d = 1.057A, the electronegativity of the sp orbital of C is obtained as 2.7. A somewhat higher value is expected for the sp hybrid in XCN because of the effects of the electronegative N.From the decreased screening of C in the latter compound I have estimated the C in XCN as 0.1 higher, or 2.8. With eqn. (29) and these x values of carbon, I have estimated the n character of the C-Hal bond adjacent to triple bonds as given in table 4. As expected, the TABLE 4.-DOUBLE BOND (n) CHARACTER AND IONIC CHARACTER PREDICTED FROM QUADRUPOLE COUPLINGS AND ELECTRONEGATIVITIES ionic character resultant ionic coupling ratio n character of of 0 component character mo a u k for Hal C- Hal ofC--Hald of C - Hal OQCObS.) (%I <f%> % (Pa - Yn>% ClCN 0-76a HCCCl 0.73b BrCN 049a CH3 CCB r 0.84c ICN 1.06a CH3CCI 0.98c 28 10 - 18 (Hal.+)e 25 15 - 10 22 0 - 22 22 5 - 17 8 - 10 - 18 14 - 5 - 19 a Smith, Ring, Smith and Gordy, Physic.Rev., 1948, 74, 370. Townes, Holden and b Westenberg, Goldstein and Wilson, J . Chem. Physics, 1949, 17, 1319. c Sheridan and Gordy, J. Chem. Physics, 1952,20,735. dElectronegativities employed are: 3-0 for C1, 2-8 for Br, 2.6 for I, 2.7 for CEC-, e A minus sign for (P0-yn) corresponds with a positive charge on the halogen. Merritt, Physic. Rev., 1949, 74, 1113. 2.8 for NGC-. 7~ character decreases from C1 to I. In all cases it is in satisfactory agreement with that predicted from a consideration of bond lengths. Goldstein and Bragg 18 have shown that the asymmetry parameter of the quadrupole coupling gives an unam- biguous evaluation of double bond character in asymmetric molecules. This method is developed in a valuable paper by Bersohn.~g The results of these workers show that the CCl bond has about 5 % double bond character when conjugated with a double bonded CC system.Thus it appears from table 4 that the T character is much greater when the conjugation is with a triple rather than with a double bond. One relation, earlier evolved,20 measures electronegativity in terms of the effective nuclear charge and the covalent radius, zeff.. elr. The screening constant per valence electron which makes this scale agree with Pauling’s was found to be 0.5. For a positive charge of c electron units on the atoms, the relation can be expressed as + 0.50, (n + 1 + c j x = 0.31 r where n is the number of electrons in the valence shell of the neutral atom and r is the covalent radius.Let us now consider the spherical XY4 molecules in which X is C, Si, Ge or Sn, and Y is Cl, Br or I. Quadrupole coupling data for all these combinations are now available. Although the data are for solid state, the fact that couplings in molecules such as CF3C1 are almost identical in the solid and gaseous statesWALTER GORDY 27 suggests that we can safely neglect effects of solid state interactions in these com- pletely spherical molecules. If we let yn represent the amount of T character we can write c = 4(pu - 7) and transform eqn. (31) to in which n = 4 has been substituted for the C, Si, Ge and Sn. Since yn will not be large here, we can regard xHa1 as constant and can neglect effects of any changes in Hal screening on pq. We cannot likewise neglect changes in xx.Electrons are being sucked away from X in four directions, and its electronegativity will change according to eqn. (32). With the known values of xHa1, r, and pq, eqn. (29) and (32) were solved for yn, Bu and xx. The results are shown in table 5. TABLE 5.-BOND PROPERTIES PREDICTED PROM QUADRUPOLE COUPLINGS AND ELECTRONEGATIVITES coupling effective ionic charge on ratio electro-negativity chagcter z:m;;i c&t2er central atom - m) in electron obs. of central Pq units (%I (%) (%I atom B(CH3) O*lOa 2.06 0 12 12 + 0.36 BCl3 0*39b 2.08 29 46 17 + 0.51 CC4 0.79 2.5 8 9 21 12 + 0.48 CI4 0*93d 2-50 4 5 1 + 0.04 CBr4 0.83~ 2-55 8 13 5 + 0.20 Sic14 0.376 2.04 30 48 18 + 0.72 SiBr4 0.46~ 1-98 26 41 15 + 0.60 siI4 0.58d 1 a96 20 32 12 + 0.48 GeC14 0.47b 2.08 14 46 32 + 1-28 GeBr4 0*54e 2-03 12 39 27 + 1-08 GeT4 0 .6 9 2.00 9 30 21 + 0.84 SnC14 0.446 1 *97 9 51 42 + 1.68 SnBr4 0-50e 1.91 8 45 37 + 1-48 SnI4 0.61d 1.87 6 37 31 + 1-24 a Molecular coupling by Dehmelt (see tabulation in ref. (5)) with atomic coupling by Wessel, Physic. Rev., 1953, 92, 1581. 6 Livingston (see tabulation in ref. (5)). c Schawlow, J. Chem. Physics, 1954,22, 1211. d Robinson, Dehmelt and Gordy, J. Chern. Physics, 1954,22, 51 1. e Kojima, Tsukada, Ogawa and Shimanchi, J. Chern. Physics, 1953,21, 1415. The values in table 5 appear entirely consistent and reasonable. The T character of the individual bonds is shown diagrammatically in fig. 6. The trends agree in every respect with the qualitative predictions of Pauling21 based upon considerations of interatomic distances alone-distances which were often very inaccurate.We might say that these results tend to substantiate the remarkable intuition of Pauling. Perhaps some will be reluctant to accept relation (22) on the basis that it seems to require too much formal charge on certain atoms in polyatomic molecules. That molecules have a way of avoiding excessive charges without violating this relation can be seen from an examination of table 5. The methods seemingly employed by these molecules are electronegativity variation and negative feed back ( 7 ~ character). Note that the resultant ionic character (Po - yn) given in column 6 is down to that to which people are accustomed.28 QUADRUPOLE COUPLINGS One would suppose that the bonding in A(Hal)3 molecules is similar to that of X(Hal)4 molecules, but I cannot treat these molecules in the same way from solid state couplings because of their strong intermolecular dipole interactions which probably alter the couplings 10 to 15 %.I have ventured treatments similar to the above for the non-polar molecules RCl3 and B(CH3)3 from solid state coupling. The results are given in table 5. That of B(CH3)3 is based on the B11 coupling for which distortion effects, not taken into account, may be significant. Note that n- feed back is not detected for B(CH3)3 but is strong in BCl3. 3 0 - r a.. !! W XCI X Br: Borine carbonyl, BH3CO, is a strange sort of molecule. From its early micro- wave structural determination in our laboratory 22 it was concluded that the BC link iii it is only about half a bond.Now that the atomic coupling perp electron has been measured,23 we can conclude with fair assurance that the BC bond order is 0.6. This prediction still depends partly on the structure in that the bond orbital hybridization cniployed jn the estimate is calculated from the HBH bond angle of 113" 58'. Although T have found no quadrupole coupling nor dipole moment requirements for hybridization in bond orbitals of the halogens except possibly on the C1 and Br in FCl and FBr, where there is a positive (not a negative) charge on the halogen, the quadrupole couplings do not exclude the possibility of small amounts, the order 2 or 3 % s or d character, nor even larger amounts if there is approximately equal s and d hybridization.The evidence is against d hybridization on the negative ion, however. The quadrupole coupling of S33 in H2S shows that unquestionably there is significant hybridization 24 of the S bonding orbitals despite the approxi- mate right-angle of its bonds. The same applies to AsH3 and SbH3. To reconcile the bond angles in these molecules with the hybridization required by the coupling, it appears necessary to assume both s and d contribution or to treat the bonding in terms of delocalized molecular orbitals. These cases are treated elsewhere.25 The programme committee asked that we stress our personal interpretations. I have given here some of my own notions on what nuclear quadrupole couplings show us about the chemical bond. 1 think it appropriate to tell you that mine is not the only treatment.Tomes and Dailey, who took the first look at quadru- pole couplings of the halogens,26 interpreted them as indicating 18 to 20 % s hybridization of the C1 bonding orbital. They later revised the estimates to include Br and 1, but only when there is ionic character which gives the negative charge to the coupling C1, Br or 1. This latest interpretation is condensed in a rule given by Townes.27 While confessing that the rule is a bit arbitrary, he states that quadrupole couplings appear to indicate the following type of variation : '' The halogen bonds are hybridized with 15 % s character whenever the halogenWALTER GORDY 29 is more electronegative by 0.3 unit than the atom to which it is bonded. Other- wise there is no hybridization.”* As you can see, Tomes and Dailey and 1 are at opposite poles on hybridization. As a result, our ionic characters are different.In wrangling with these problems I have tried to keep hi mind the power of the simple intuitive approach of Faraday, but I have been unable to forget a remark by Mulliken that molecules just aren’t simple. I regret that limitations of space prevent my giving proper reference to all the material 1 have used. It is a pleasure to acknowledge many helpful discussions of quodrupole couplings with Dr. H. Dehmelt. I wish to thank the U.S. National Science Foundation for a travel grant to attend this Discussion. * In a paper published after this was written, Townes and Dailey keep the rule but change the dividing line to 0.25 unit (J. Chem. Physics, 1955, 23, 118). 1 Burrus and Gordy, Physic. Rev., 1954,93,897. 2 Genzel and Eckhardt, Z. Physik, 1954,139, 592. 3 Bitter, Physic. Rev., 1949, 76, 833. 4 Kastler and Brossel, Compt. rend., 1949, 229, 1213. 5 Gordy, Smith and Trambarulo, Microwave Spectroscopy (John Wiley and Sons, 6 Mulliken, J. Arner. Chem. Soc., 1950,72,4493. 7 This has been emphasized earlier by the author, and the inadequacy of the H.-L. and 1.a.c.o. functions for calculating nuclear coupling has been pointed out (J. Chem. Physics, 1954, 22, 1470). 8 In effect, we here regard the atomic orbitals as divided into inner and outer parts. The inner parts, which account essentially for the nuclear coupling, have no overlap (S,b = 0 for these parts) and suffer no reduction in the normalization of the total molecular orbital function, whereas the outer parts have overlap (Sab = 0) and suffer distortions. New York, 1953). 9 Robinson, Dehmelt and Gordy, J. Chem. Physics, 1954, 22, 51 1. 10 Gordy, J. Chem. Physics, 1951, 19, 792. 11 Burrus and Gordy, Physic. Rev., 1954,92,1437 ; Physic. Rev., 1954,93,419. Burrus, 12 Pauling, Nature of the Chemical Bond (Cornell Univ. Press, Ithaca, N.Y., 1939), 13 Mulliken, J. Chim. Phys., 1949, 46, 497. 14 Coulson, Valence (Clarendon Press, Oxford, 1952), p. 206. 15 Debye, Polar Molecules (reprint by Dover Publications, New York, 1945), p. 60. 16 Gordy, J. Chem. Physics, 1946, 14, 305. 17 Herzberg, Infra-red and Raman Spectra (D. van Nostrand Co., New York, 1945), 18 Goldstein and Bragg, Physic. Rev., 1950, 78, 347. 19 Bersohn, J. Chem, Physics, 1954, 22, 2078. 20 Gordy, Physic. Rev., 1946, 69, 604. 21 Pauling, ref. (12), chap. 7. 22 Gordy, Ring and Burg, Physic. Rev., 1950, “1, 512. 23 Wessel, Physic. Rev., 1953, 92, 1581. 24 Burrus and Gordy, Physic. Rev., 1953, 92, 274. 25 Weissberger and West (ed.), Chemical AppZications of Spectroscopy (Interscience Publishers, New York, in press), chap. 2 (W. Gordy). 26 Townes and Dailey, J. Chem. Physics, 1949, 17, 782. 27Townes in Symposium on Molecular Physics (Maruzen Go., Led., Tokyo, 1954), p. 105. This rule -is adhered to in a more recent publication (Townes and Dailey, J. Chem. Physics, 1955, 23, 118). Gordy, Benjamin and Livingston, Physic. Rev., 1955 (in press). chap. 2. p. 180.

ISSN:0366-9033    

DOI:10.1039/DF9551900014

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

MICROWAVE SPECTRA OF DEUTERATED FURANS STRUCTWRE OF THE FURAN MOLECULE BY BBRGE BAK, LISE HANSEN AND JOHN RASTRUP-ANDERSEN Chemical Laboratory of the University of Copenhagen, Copenhagen Received 17th January, 1955 6- and P-monodeutero-, and a : a’-dideuterofuran have been prepared and their micro- wave spectra have been recorded and analyzed. Values of the rotational constants so obtained in connection with known values of the rotational constants of ordinary furan 1 are insufficient for a complete calculation of the 8 geometrical parameters of the molecule but additional reasonable assumptions concerning the length of the C-H bonds result in 5 molecular models among which a further choice may, be made by means of valence theory. This microwave work was undertaken in order to establish the molecular structure of furan with less ambiguity and uncertainty than has been achieved by electron-diffraction technique? Since furan is planar, the principal moments of inertia obey the relation la -1- I’ = I,, i.e.only two quantitative results are obtained which is quite insufficient to establish the magnitude of the 8 geo- metrical parameters of the molecule. Therefore, isotopic molecules must be studied. Various isotopic species are, however, of unequal value for the purpose, Generally, the most precise calculation of the molecular geometry follows from a study of species derived from the parent molecule by isotopic substitution of one atom. For that reason a- and P-monodeuterofurans were investigated, a : a’- dideuterofuran being included in the study only as an easy accessible means of control.In this way, 4 additional quantitative results were obtained. The total number of 8 necessary measured quantities now could have been procured by investigating isotopic species as, e.g., 12C4H4180, 12C313CH4160, etc., but the cost and the difficulties connected with the preparation of such compounds made this route impassable. 31111 order to state definite, possible models of furan, com- patible with the microwave measurements, assumptions as to the lengths of the C-H bonds were made. EXPERIMENTAL The a- and p-monomercurials (chlorides) and the a : a’-dimercurial (chloride) of furans were prepared according to Gilman and Wright.3 The yields reported by these authors were very satisfactorily reproduced.Mercury equivalent weights for the three compounds, found by boiling ca. 300 mg of the mercurial with 5 cm3 4MHC1, diluting with 20-30 cm3 hot water and precipitating Hg2+ as HgS before cooling, were, respectively, 300.1, 302.2, 283.0; calc. : 303.1, 303-1, 269.2. Potassium bromide discs were prepared from the three inercurials by pressing ca. 3 mg compound with 500 mg KBr4. The infra-red spectra of the discs, taken by a Beckmann I.R.2 instrument (700-4000 cni-1) showed that the mercurials were not contaminated with each other. Also, the absence of other im- purities seemed probable except in the case of a : a’-dimercurifuranylchloride where the low line intensity in the spectrum suggests that some spectroscopically rather inactive impurity is present (compare the Hg analysis).The conversion of these mercurials to deuteroderivatives took place by the following standard procedure. 0.065 mole DCL prepared from 12 g benzoyl chloride and 0.60 cm3 D20 was led into 24 cm3 ice-cooled D20, placed in a 100 cm3 flask, by means of a stream of dry hydrogen. In advance, 10 g mercurial (0.033, 0.033, 0.017 mole) had been placed in the same flask 30B. BAK, L. HANSEN A N D J . RASTRUP-ANDERSEN 31 which was now heated slowfy with a free flame to 100" C under continued flushing with dry hydrogen that passed a 30cm, air-cooled Vigreux column and a trap, immersed in liquid air, before leaving the system. Almost from the beginning the heating caused the evolution of (deutero) furan vapours which were separated from most of the water in the Vigreux column before being condensed in the trap.The connection between the column and the trap was now interrupted, and the latter was evacuated at about - 190" C . Under pressure control the contents of the trap were afterwards distilled at room temper- ature. The initial equilibrium pressure was approximately 400 mm. Fractions were taken until this pressure had dropped to 20mm, the water vapour pressure. The dis- tillate was dried for 30 min over anhydrous CaS04 and redistilled into a dry flask from which it was distilled in 25 cm3 fractions at constant temperature (ca. .20° C) and p = ca. 480 mm Hg. In this way we got 1.60 g a-deutero-, 1.10 g 13-deutero-, and 0-50 g a : a'-di- deuterofuran. The two first-mentioned distilled within the 470-490 mm pressure interval at 19.5-20*0" C while the dideutero sample, collected after three distillations when con- siderable " tails " were discarded, had p = 460-495 mm at 20" C.The yields are, there- fore, 70, 48 and 19 %. The infra-red absorption curves of the vapours, taken on a Beckmaiin I.R.2 instrument, showed that the three samples were neither contaminated with each other nor with ordinary furan (except for traces). The preparations have, therefore, been carried out without undesired exchange of H with D. This was fully confirmed by the following examination of the inicrowave spectra. These spectra were taken at room temperature in a conventional Stark modulation apparatus, working in the 17500-26000 Mcls interval, with a crystal-controlled frequency measuring system carefully calibrated against microwave lines of well-established frequency.Initially, Sirvetz's determinations of the line frequencies of ordinary furan 1 were checked and confirmed, Our observed and calculated microwave absorption frequencies are given in table 1 together with Sirvetz's results. This table does not include all observed frequencies ; a complete list would include about 1000 observations. TABLE 1 .-OBSERVED AND CALCULATED MICROWAVE ABSORPTION FREQUENCIES (in Mc/s) FOR FUR AN,^ a- AND 13-MONODEUTERO-, AND a : E'DIDEUTEROFURAN furan @-deuterofuran transition obs. (pa - lines) frequency 11.1321.2 23259.30+ 10.1320.2 23453*13+ 31.3+31.2 23352.476 42,3352.2 23305.88 53.3353.2 23213.45 64.3364.2 23055.80 ~ 75.3475.2 22810.92 86.33864 22458.99 97.3-fg7.2 2198430 108.3-+108.2 21377.91 1 1g.3-fllg.2 20637.71 1210.3-t 1210.2 1976798 30.3-332.2 23384.46 41.3343.2 23402.53 52,3354.2 23440.06 63.3t65.2 23507.71 74.3+76.2 23619.06 85.3437.2 23790.73 107.3+10g.2 24399.197 96.3+98.2 24043.08 ll8.3-+ll10.2 24888-97 42.2-+44.1 129.3-2.1211.2 25541 *64 53.2+55.1 calc.frequency 23259.29a 23453.1 3a 2335218 23305.76 23214.19 23056.96 228 1257 22461.20 21987.37 21 38 1.92 20642.69 19773-89 2338442a 23402.68 23440.46 23508.42 23620-09 23792.15 24044-82 24402.04 2489 1 *60 25544-73 * 8 centrifugal correction 0.01 0 0.29 0.12 - 0.74 - 1.16 - 1.65 - 2-21 - 3.07 - 4.01 - 4.98 - 5.91 0.04 - 0.15 - 0.40 - 0.71 - 1.03 -- 1.42 - 1.74 - 2.27 - 263 - 3.09 0 - 0.1 0.3 - 0.1 - 0.8 - 1.1 - 2.7 - 2.1 - 3.9 calc. frequency 21 856.8a 22617.60 21 840.9 20996-8 19572.2 * 8 * * * * * 22498.0a 22907.3 2379 1.1 25439.5 * 'i: * .I: * * 19941.8 24405.6 obs.frequency 21856.8+ 22617*5+ not obs. not obs. 19572.5 22497.9 22906.5 23790.0 25436.8 19939.7 + 24401 *7+ * frequency outside the 17500-26000 Mcls range investigated, f identified by their Stark pattern. b for the method of picking out Q-type transitions compare ref. (5). parameters adjusted to give agreement at these points.DEUTERATED FURANS TABLE 1 (cont.) furan Is.. deuterofuran obs. calc. frequency frequency 19011*46+ 19009.36 2062434 20622054 22540.27 22536.39 2476730 24763.04 * * * a-deuterofuran centrifugal correction 2.10 2.80 3.88 4.46 0.1 0.6 1.3 CalC. obs. frequency fiequency * 8 -k * 24757.0 24757.1 21684.7 21685.3 18212.1 18213.4 a : u'-dideuterofuran obp1.frequency 22696.3" 21 986.4+ 22144.0 21706.9 20961 -2 19942.6 18798.6 1915066 22477.6 22654.8 22998.2 23565.7 24410.9 25575.1 21557:6+ 25628~6~ 18489*2+ 20541.8' 2291 8*6+ 25584.7+ 19666.2 23481.7 25427.9 23934.4 calc. frequency 22696.30 21 986-4a 22144.9 21707.7 20963.4 19945.6 18801.6 * * 19 147.5 22477.7a 226554 22998.5 23565.9 244 10.8 25574.5 21549.8 25620.5 18487.7 20539-9 22915.8 25580.8 19658.9 23472.7 * ;k * * 25436.4 23943.3 centrifugal correction 0 0 - 0.9 - 0.8 - 2.2 - 3.0 - 3.0 3.1 - 0.1 - 0.6 - 0.3 - 0.2 0.1 0.6 7.8 8.1 1 3 1.9 2.8 3.9 7.3 9.0 - 8.5 - 8.9 0 0.1 0 - 0.2 - 0.1 - 2 6 - 3-5 - 5.0 - 0.5 - 0.7 - 1.5 - 1.8 0.1 - 0.9 - 1.4 - 2.1 - 2.2 - 1.7 - 2.0 - 2.6 - 3.3 calc. obs. frequency frequency 21 890.9a 20886.5a 21006.70 20246.7 19095.3 * * 18207.8 2 1027.0 24996.9 21633.2 21965.6 22593.3 23604.0 25063.1 22436.7 * * * 19852.6 22648.5 25830.7 21685.3 * * * 24722.0 22968.5 21 7 10.4 21363.4 21890*9+ 20886*6+ 21006.7 202463 19095.2 18205.2 21023.5 24991 -9 not obs.21965.1 22592.6 23602.5 25061.3 22436.8 19851-7 22647.1 25828.6 21683.1 24720.3 22966-5 21707.8 21360.1 * frequency outside the 17500-26000 Mc/s range investjgated. -1- identified by their Stark pattern. a parameters adjusted to give agreement at these points b for the method of picking out Q-type transitions compare ref. (5). DISCUSSION CALCULATED FREQUENCIES The calculated frequencies were obtained by means of rotational constants which reproduce the two 1 -+ 2 transitions and one of the 3 --f 3 transitions exactly. Since it can be assumed that rotational constants corresponding to rotational states with very low J's come close to the rotational constants for the '' rigid " molecule (undistorted by centrifugal forces) we are justified in labelling the difference between observed and calculated frequency, centrifugal correction.B. BAK, L.HANSEN A N D J . RASTRUP-ANDERSEN 33 The effect is seen to be rather small throughout. In the literature, e.g. StrandbergP has reported centrifugal stretching effects for low J transitions that are very high about 100 Mc/s or more) for molecules like NH3 and H2S. These molecules are, however, of an " open " type, i.e. the distortion of one valence angle can take place fairly independent of the rest of the molecule. Furan cannot be distorted solely by changing the valence angles.Changes in bond lengths have to take place which diminishes the co-ordinate shifts. Whether the rotational constants (given in table 2) derived as explained above are fully correct or not it is worth while noting that the rotational constants for all the isotopic species have been derived by the same procedure so that they are of comparable value. The furan model derived from them at worst corresponds to a not completely rotationless molecule but it may be even better, since to a large extent diflerences between moments of inertia enter the calculations which minimizes the possible error. Our coididence in the approximate correctness of the rotational constants of table 2 may, however, also be strengthened by showing that the centrifugal corrections of table 1, now positive, now negative, are in harmony with a simple physical picture of the rotating molecule.Taking, for example, the three 11 -+ 11 transitions for furan (table l), two of the corresponding centrifugal corrections are negative, one is positive. The general expression for the rotational energy W of a rigid rotor is : Centrifugal distortion changes the rotational constants by amounts AA, AB, and AC and the rotational energy by an amount A W : (2) By differentiation we find (K = (2B - A - C)/(A - C) : W = &(A + C)J(J + 1) + 4 (A - C)E(K). (1) Aw = (3 W ~ A ) A A -t (a w/~B)AB + (3 w p q A c . 3 W/M = +J(J + 1) + +E(K) + +(A - c ) ( ~ E ( ~ ) / ~ A ) ; aE(K)jbf == [2(c - @/(A - c)2](3E(K)/3K) ; 3 W/3B = +(A - C ) ( ~ E ( K ) / ~ B ) ; 3E(K)/bB = [2/(A - c ) ] ( a E ( K ) / a K ) ; a W / ~ C = +J(J + 1) - +E(K) + +(A - C)(JE(K)/JC) ; 3E(K)/>C = [2(B - A)/(A - C)2](3E(K)/aK) ; so that in general AW = (+J(J + 1) + W(K) I- [(C - @/(A - C ) ] ( ~ E ( K ) / ~ K ) ) A A -I- ( ~ E ( ~ K ) / ~ ( K ) A B ( 3 ) For planar molecules coplanari ty is approximately conserved during rotation, so that, since C-1 = B-1 + A-1, AB = (B/c)~Ac - (B/A)2AA.AW then becomes A W 5 [+J(J + 1) + +E(K) + ((C - B)/(A - C) - ( B / A ) ~ } ( ~ E ( K ) / ~ K ) ] A A For furan, A = 9447; B = 9247; C = 4671 ; and K = 0.91614. Therefore, + (+J(J + 1) - +E(K) + [(B - A)/(A - C ) ] ( ~ E ( K ) / ~ K ) ) A C . + [+J(J + 1) - +E(K) -t {(B/C)2 + (B - A)/(A - C))(~E(K)/~K)]AC. A W = [#J(J + 1) + +E(K) - 1.9161 (~E(K)/BK)]AA (4) + [&J(J + 1) - +E(K) f 3.877 ( ~ E ( K ) / ~ K ) ] A ~ .(5) Frequency changes for Q-lines due to centrifugal stretching, are consequently AVQ = vobs. - vcalc., AvQ = [+(E(K)" - E(K)') - 1.9161 ( ( ~ E ( K ) " / ~ K ) - ( ~ E ( K ) ' / ~ K ) ) ] A A - [+(E(K)" - E(K)') - 3*877((3E(~)"/3~) - (~E(K)'/~K))]AC (6) B34 DEUTERATED FURANS where double-prime quantities belong to the upper rotational level. The ap- proximate validity of (5) can only be maintained for cases in which AA (and AC) are almost the same for the two states in question. This condition is fulfilled for the three 11 -+ 11 transitions here considered, i.e. 119.3 -+ 119.2 ; 118.3 -+ 1110.2 ; and 119.2 --+ 1111.1. For all five states involved J = 11. The approximate magnitude of J‘s component on the approximate symmetric-top axis (perpendicular to the plane of the molecule) is 1, 2 and 3 so that 5.is oriented at small angles to the molecular plane. AA and AC, therefore, mean some sort of average corrections to Afigid and Brigid, valid for J = 11 and K+1 = 1, 2 and 3. Values of E(K) and ~ E ( K ) / ~ K are taken from Turner’s tables.7 For the three 11 -+ 11 transitions we get AVa(I19.3 -+ 119.2) = - 4.98 = - 44.783 AA + 95.040 AC, A ~ ~ ( l l 8 . 3 -+ 1110.2) = - 2.63 = 31.384AA - 58.170 AC, AV~(l19.2 + 1111.1) = 4.46 = 133.603 AA - 265.033 AC. No rigorous solutions exist since the equations are only approximately correct. But the values AA = - 1.385 Mc/s, AC = - 0.713 Mc/s give AVQ = (in the order above) - 5-73 ; - 2-00 ; 3.92 Mc/s with correct sign and correct order of magnitude (compare table 1).The calculation may be checked by repeating it for the three 8 -+ 8 transitions, 86.3 -+ 86.2; 85.3 -+ 87.2 and 86.2 --f 88.1. Ap- propriate values of b l and AC may again be found, and we can furthermore predict that AA for the J = 8 levels here involved should be approximately - 1.385 X (8/11)2 = - 0.73, while AC (J = 8) - - 0.713 x (8/11)2 = - 0.377. By insertion we actually find AA = - 0.62 and AC = - 0.33. The calculated centrifugal corrections are - 2-41 ; - 1.26 ; and 1.65 Mc/s (compare table 1). The magnitude of the centrifugal effect on the two 1 --f 2 transitions so important for this calculation could now be estimated by means of (5). Approximately AA(J= 2)/AA(J= 1) -4. Also, &(J= 2) must be about 25 times smaller than AA(J = 11).We then find that the two 1 -+ 2 lines could be in error by 0.2 -+ 0.3 Mc/s due to centrifugal stretching. To find approximately correct values of (A - C ) and K we had initially made the usual plots 5 of AE(K)/V~~~. against K. All the points in the thick swarm of points of intersection had been determined and an average ( A - C ) and K had been taken. It turns out that these values only deviate insignificantly from the rotational constants based on the two 1 -+ 2 transitions and one of the 3 -+ 3 lines (see table 2). We can now see why. Since for a planar molecule AK = @/(A - C))[- (B/A)2AA + (@(A + B))/A2)ACI, we calculate AK = - 0.00005 for the J = 11 levels, which is confirmed by looking at the graph. For the J = 8 levels it is about - 0.00004.Since levels from J = 3 to J = 12 are involved in the graphical method we see that K found by this pro- cedure would not be likely to deviate much from the true value. Also, the value of ( A - C) found graphically was astonishingly correct. We can see now that A(A - C) - - 0.7 Mc/s for the J = 11 levels and - 0.3 Mc/s for the J = 8 levels. Again, only a small deviation from the rigid rotor values is to be expected. It is interesting to note that, at least for furan, the influence of the zero-point fluctuations is of comparable magnitude to the effect of centrifugal distortion. In the calculations to follow we are forced to assume that, e.g. the C-D distance in or-deuterofuran is identical with the corresponding C-H distance in furan.However, we know from other examples (methyl halides) that the hydrogen x-co-ordinate (fig. 1) may be about 0402A greater than for deuterium. In equalizing these two co-ordinates we are ignoring in 1 ~ . Since x = 2.047 A and 1~ - 54.6 a.m.u.& the percentage error in I’ is 0.014. The percentage error in A is the same, so that AA = 0.76 Mc/s, i.e. rnH[(X + 0.002)2 - x2] - rn,x(0.004)B . BAK, L. HANSEN A N D J . RASTRUP-ANDERSEN 35 close to the effect of centrifugal stretching. The consideration shows why one should be careful not to overload a molecule with deuterium atoms since the effect of zero-point energy increases to a not unimportant extent. RESULTING ROTATIONAL CONSTANTS In table 2 we have summarized the calculated values of K, rotational constants, etc.derived (column 1) from the two 1 -+ 2 lines and one of the 3 --+ 3 lines, and by taking ( A - C)/2 and K from the Q-line plots and finding ( A + C)/2 in connection with the two 1 -+ 2 lines (column 2). TABLE 2.-cALCULATED VALUES OF ROTATIONAL CONSTANTS (A, B, c) MC/S, ( A - c)/2, ASYMMETRY PARAMETER ( K ) AND PRINCIPAL MOMENTS OF INERTIA (ZA, IB, IC) a.m.u. w2 OF FURAN AND DEUTERATED FURANS TOGETHER WITH THE QUANTUM DEFECT (q.d.) FOR Q-LINE PLOTS, ETC. (see text) ALL FOUR SPECIES. COLUMN (I), FROM THE THREE LOW-J LINES; COLUMN (2), FROM (1) A 9447.04 B 9246.77 C 467044 ( A - C)/2 2388.10 K 0.9 1 6 14 ZA 53.5067 IB 5 4 - 6 6 5 6 IC 108.22Ofl q.d. 0,048 1 furan (2) 9446.98 9246.72 4670.86 2388-06 0.91614 53-5070 54.6659 108.2199 0.0470 a-deuterofuran (1) - (2) 0.06 0.05 - 0-02 0.04 0*00000 - 04003 - 0.0003 0.0005 0.0011 (1) 9280.1 5 8638.74 4472.05 2404.05 0.73320 54.4689 58.5132 113-0309 0-0488 (2) 9280.15 8638.56 4472.09 2404.03 0.73312 544689 58.5144 1 13.0299 0.0466 (1) (2) 0.00 0.18 - 0.04 0.02 0~00008 o*oooo - 0.0012 0.0010 0.0022 B-deuterofuran a : a'-dideuterofuran A 9383.77 B 8490.39 C 4455.47 ( A - C)/2 2464.1 5 K 0.63745 I A 53.8675 ZB 59.5355 ZC 113.4516 q.d.0.0486 938355 8490.36 445549 2464.03 0.63751 53.8687 59.5358 1 13.4510 0.0465 0.22 0.03 - 0.02 0-12 - 0.00006 - 0.0012 - 0.0003 0.0006 0.0021 9033950 8 160.64 428 5.80 2373-85 0.63230 55.9562 61.9412 117.9430 0.0456 9033.38 8 160.68 4285.82 2373.78 0.63236 55.9569 61.9409 1 17.9424 0.0446 012 - 0.04 - 0.02 0.07 - 0*00006 - 0.0007 0.0003 0.0006 0~0010 RESULTING MOLECULAR MODELS In the calculation of the molecular models compatible with these constants we started to find the co-ordinates of the hydrogen atoms in the system shown in fig.1. By insertion in the formulae given in our paper on pyridine,s we get the results of table 3, based on the principal moments of inertia for furan, a- and P-deuterofuran. TABLE 3.-cALCULATED POSITIONS OF THE HYDROGEN ATOMS IN FURAN ( X , y ) ; MOMENTS OF INERTIA OF THE HYDROGENS (IH) AND THE CARBON-OXYGEN RING (14c.0) from rotational constants (1). u-hydrogen fi-hy drogen a-hydrogen j3-hydrogen 2.048 1.376 2.047 1.377 4.192 1.894 4.1 89 1.896 - 0.811 1.839 - 0.813 1.838 0,657 3.380 0.660 3.378 8.14 8.14 46.53 46.53 from rotational constants (2). 12.27 41.24 12.27 41.2436 DEUTERATED FURANS These differences are so small that they can be ignored in view of the error and the experimental uncertainty in the rotational constants.5 From the co- ordinates of the hydrogens and the rotational constant B for a : a'-dideuterofuran ' c l A , FIG.1 .-Co-ordinate system used through- out. The x, y and z axes are the principal inertia axes for furan. The origin is the centre of mass, one calculates I;'*" = 41.23 which agrees excellently with I;'* " from table 3. By properly taking into account the difference in the position of the centre of mass when comparing furan with a : a'-dideuterofuran, one calculates 1x4's " = 46.52 which also fits with the value given in table 3. We shall only proceed with the co-ordinates and corresponding moments of inertia given under (1) in table 3.Following the same line of thought as in our paper on pyridine 5 we finally arrive at the models given in table 4. VALENCE THEORY ASPECTS In the last three years furan has twice been subject to extensive theoretical treatments 8 9 9 in which its resonance energy, dipole moment, ultra-violet ab- sorption, etc., have been calculated. Both papers build upon the electron- diffraction data by Beach,2 where d(C-C) = 1.46 & 0.03 A, d(C=C) = 1.35 A (assumed), and d(C-0) = 1.40 f 0-03 A. Whichever of the models (I)-(V) above one chooses it is seen that a renewed treatment is highly justified and we intend to carry this out. Without these detailed calculations, however, quite a few important features may be stated. TABLE 4.-FURAN MODELS (1-v), COMPATIBLE WITH THE MICROWAVE SPECTRA OF FURAN, GIVEN AT THE TOP OF THE TABLE a- AND P-MONODEUTEROFURAN, AND a : a'-DIDEUTEROFURAN ASSUMING THE C-H DISTANCES I I1 V I11 IV 1 -070 1 a070 1.356 1.370 1 -444 107" 30' 110" 29' 105" 46' 116" 41' 127" 45' 1.080 1.080 1.387 1.337 1.438 104" 24' 111" 26' 106" 22' 113" 54' 127" 33' 1.075 1.075 1.371 1.354 1 a440 106" 00' 110" 55' 106" 05' 115" 20' 127" 40' 1 -070 1,080 1.374 1.357 1.426 106" 24' 110" 14' 106" 34' 115" 28' 127" 56' 1.080 1 -070 1.367 1.350 1.458 105" 38' 111" 43' 105" 28' 115" 18' 127" 17' First, the choice of models (I)-(V) seems reasonably representative. It is seen that consideration of C-H distances shorter than 1.07A would result in a C-0 distance less than 1-356 A.Since the normal C-F distance is 1.385 A, (CH3F10), such a small C-0 distance could only be accepted very reluctantly. Also, taking C-H distances longer than 1-08A would mean that the C=C distance would decrease still further below 1.353 A, the " unperturbed " C=C distance.11 Since all the models agree in having the C-C distance rather short (- 1.44A) it follows from well-established bond-order, bond-distance curves that about 24eB.BAK, L. HANSEN AND J . TASTRUP-ANDERSEN 37 must be present in the C-C “single” bond. The charge +e could have come from (i) the two C=C bonds, (ii) the oxygen atom, but not from the C-0 bondi which are so abnormally short (the C-0 distance 12 in CH30H is 1.434A). Considerations concerning the dipole moment p to be presented later show that the alternative (i) must be the more correct.Therefore, each of the C=C bonds contains only about 32e which means that they must be slightly longer than a normal C=C bond. Thus, models like (I) and (11) can be rejected. It is hard to dis- tinguish between models (III), (IV) and (V), although (V) may be slightly less probable because of its short C=C distance. The models (III) and (IV) essentially agree in all interatomic distances and their correctness may be verified by con- sidering the p of furan. Since the C-0 distance in these models is close to 1.372 A there must be about 22e in each of the C-0 bonds. If the “extra” ge is taken from oxygen this atom gets the charge + 3e. Fig. 2 shows the approximate distribution of what may be termed the redistributed n-electron charges in concordance with the above considerations.While tetrahydrofuran has p = 1.68 D,13 the electric moment of furan 1 is only 0.661 D. Furan may now be thought of as having (i) a pa equal to that of tetrahydro- furan (so-called a-moment), presumably directed downwards in fig. 2 (drawing the vector from + to - charge), (ii) a super- imposed so-called pmig, the moment due to the migration of the n-electrons. The m--charges in the three C-C bonds have been placed in the middle of the bonds and a simple calculation shows that this part of b i g approximately outbalances the 0-moment. A resulting moment of 0.66 D may now easily be explained by means of the charges on the oxygen and in the two C C C C FIG. 2.-Migrated n-electron charges in furan as derived from bond length considerations.C-0 bonds (where we need not locate the charge in the middle). A necessary consequence is, however, that the resulting p-vector points upwards in fig. 2. Confirmation of this result may be found in that the p of 2-methylfuran14 is larger than the p of furan. Its p is 0.74D. Models like (III) and (IV), therefore, explain the microwave spectra of 4 iso- topic furan species and they seem to fit reasonably well with the measured dipole moment. It may be added that a renewed electron-diffraction investigation 15 (rotating-sector method) has shown that d(C-H), -- 1.074-1-077 A while which fits excellently with our models. The older model of Beach 2 from electron- scattering has this average equal to 1.392A. It remains to show by means of thorough calculations how the models may be fitted to the observed ultra-violet spectra 16 and with current ideas on the resonance energy, etc., of furan. (2d(C-0) + 2d(C= C) + d(C-C))/S = 1.377 A 1 Sirvetz, J. Chem. Physics, 1951, 19, 1609. 2 Beach, J. Chem. Physics, 1941, 9, 54. 3 Gilman and Wright, J. Amer. Chem. SOC., 1933,55, 3302. 4 Clauson-Kaas, Nedenskov, Bak, Rastmp-Andersen, Acta Chem. Scand., 1954, 8, 5 Bak, Hansen and Rastrup-Andersen, J. Chem. Physics, 1954,22,2013. 6 Strandberg, Ann. N. Y. Acad. S., 1952, 55, 808. 7 Turner, Hicks and Reitwiesner, J. Chem. Physics, 1953, 21, 564. 8 Simonetta, J. Chim. Phys., 1952, 49, 68. 1088.38 METHYL DIACETYLENE 9 Nagakura and Hosoya, Bull. Chem. SOC. Japan, 1952,25, 179. 10 Gilliam, Edwards and Gordy, Physic. Rev., 1949, 75, 1014. 11 Gallaway and Barker, J. Chem. Physics, 1942, 10, 88. 12 Ivash and Dennison, J. Chem. Physics, 1953, 21, 1804. 13 de Vries Robles, Rec. trav. chim., 1939, 58, 111. 14Nazarova and Syrkin, Izvest. Akad. Nauk. S.S.S.R. Otdel. Khim. Nauk., 1949, 35; 15 Almenningen, Bastiansen, and Hansen, to be published. 16 e.g. Mackinney and Temmer, J. Amer. Chem. SOC., 1948, 70, 3586. Chem. Abstr., 1949, 43, 4913.

ISSN:0366-9033    

DOI:10.1039/DF9551900030

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

38 METHYL DIACETYLENE THE MICROWAVE SPECTRUM AND STRUCTURE OF METHYL DIACETYLENE BY G. A. HEATH, L. F. THOMAS, E. I. SHERRARD AND J. SHERIDAN Dept. of Chemistry, The University, Birmingham Received 3 1 st January, 1955 Pure rotation spectra of H3C5H, HsC~D, D3C5H and D3CsD show that they are symmetric-top molecules with strictly linear C5H (or C5D) arrangements. The spectro- scopic constants Bo and distortion constants DJ and DJK are evaluated for these molecules. If the structural parameters concerned with the hydrogen and deuterium atoms are the same as the analogous parameters in methyl acetylene, preliminary carbon-carbon distances computed are : C (methyl)-C = 1.459 A, CE C (both assumed equal) = 1.212 A, C-C = 1.366A. Spectra are also measured for these molecules in the first excited level of a degenerate vibrational mode.These spectra are interpreted, and spectroscopic constants assigned for these states, on the basis of the theoryof I-type doubling in symmetric-top molecules. The methyl diacetylene (penta-1 : 3-diyne) structure, H3C-C=C-CfC-H, has been attributed to at least two different substances. The evidence, reviewed by Jones and Whiting,l favours allocation of this structure to the substance made by Armitage, Jones and Whiting.;! The truth of this is conclusively proved by the microwave spectrum of the substance made by this method.3 The inter- nuclear distances in this molecule are of interest, and the object of the work of which this paper forms part is to determine as fully as possible the structural parameters from microwave spectra of various isotopically substituted species.In the present communication, data are given for the molecules H3CCCCCH, H3CCCCCD, D3CCCCCH and D&CCCCD, from which preliminary deductions concerning the structure of the conjugated carbon chain can be made. With the extension of measurements to species containing carbon-13 it is expected that alI the atoms can be accurately located, with the probable exception of the central carbon atom, which is very close to the centre of gravity of the molecule. Rotation spectra of molecules in the first excited level of a degenerate vibrational mode have also been measured, and interpreted to give molecular information concerning this vibrational state. EXPERIMENTAL Methyl diacetylene was made by the method of Armitage, Jones and Whiting,z and purified by vacuum fractionation.The D3CCCCCH was obtained similarly, highly deuterated methyl iodide being used in the methylation stage. Samples of H3CCCCCD and D3CCCCCD were prepared from HJCCCCCH and D3CCCCCH respectively by treatment with an excess of 99.7 % D2O containing NaOD, for two days at room tem- perature, with frequent mixing. The spectra of samples obtained on subsequent fraction- ation showed that a large proportion of the acetylenic hydrogen was exchanged by this treatment.G . A . HEATH, L. F. THOMAS, E. I . SHERRARD AND J . SHERIDAN 39 The spectra were measured with a sweep spectrometer and frequency standard which have been described previously.4 Gas pressures of about 5 x 10-3 mm and temperatures of - 70" C were used.RESULTS The frequencies measured in the spectra of molecules in their ground states are listed, with their assignments, in table 1, The spectroscopic constants and moments of inertia, ZB, are given in table 2. The IB values are calculated from the relationship ZB (g cm2) = [839102/Bo(Mc/s)] x 10-40, derived from constants listed by DuMond and Cohen.5 K 3 2 3 4 5 6 7 9 K 0 1 2 3 4 5 6 7 K 0 1 2 3 4 5 6 7 8 9 TABLE ME MEASURED FREQUENCIES FOR GROUND STATES (MC/S rt 0.1 MC/S) HjCCCCCH J = 4 + 5 J = 5+6 J = 8 4 9 24,428.82 3 6,643.08 20,357.38 24,428.60 3 6,642-77 20,35656 24,427.85 36,641 -70 20,35555 24,42669 36,639.90 20,354.1 8 24,425.03 36,637-49 - 24,422.8 3 36,634-20 - - 36,630.24 - - - - - - H3CCCCCD J=9-+10 40,7 14.56 40,714.14 40,71296 40,7 10.96 40,708.20 40,704.62 40,700.28 40,695.1 0 - J - lo-+ 1 1 44,78592 44,785.48 44,784.16 44,782.02 44,778-98 44,770-20 44,764.52 44,75052 44,775.04 DsCCCCCH J = 4 4 5 J=5+6 19,297.70 23,157.21 19,297.52 23,156-99 19,296.98 23,15634 19,29608 23,155.25 - 23,153.71 - 23,151-67 - - - - - DjCCCCCH (ctd.) J = 8 - + 9 J=11+12 33,027.09 44,03580 33,026-86 44,035.48 33,026.08 44,034-42 33,02477 44,03264 33,022-94 44,030.18 33,020-60 44,027-08 33,017.74 44,023.24 33,014-30 - 33,010.35 - - - J = 1 0 4 11 42,454-66 42,454-25 42,45 3 -05 42,451.03 42,448.24 42,444.61 42,440.20 42,434.89 J=6+7 24,39045 24,390-68 24,390.12 24,389.17 24,387.83 24,386.10 I - - - J = 11412 J = 6 4 7 46,31411 25,687-84 46,313.66 25,687.66 46,3 12.36 25,687.03 46,3 10.1 6 25,686.05 46,307.07 25,68457 46,303-13 - 46,298.32 - - - DjCCCCCD J=9-+10 34,843.89 34,843-65 34,842.86 34,841 -5 3 34,839.63 34,837-21 34,834-26 34,830.70 34,82667 34,82204 J = 12-t 13 45,296-66 45,296.40 45,295.30 45,29356 45,29 1-10 45,287438 45,284.10 - - - TABLE 2.-sPECTROSCOPIC CONSTANTS FOR GROUND STATES molecule Bo DJ DJK IB (kc/s f 0.1) (g X 10-40 (Mcls f 0.02) (kcls f 0.1) f 0.004) H3CCCCCH 2,03 5.741 0.07 19.84 412.185 H3CCCCCD 1,929.772 0.06 18-30 434820 D3CCCCCH 1,834.856 0.1 14-54 457.3 11 D3CCCCCD 1,742.21 5 0.1 13-54 481 *628 IPossible error in Planck's constant is not included in the errors quoted for IB.The errors quoted for the values of DJK are thought to be conservatively estimated. The frequencies calculated from the constants given agree with those measured with an average deviation of 0.03 Mc/s, a discrepancy of 0.1 Mc/s being found only in a few weak lines.40 METHYL DIACETYLENE TABLE 3.-oBSERVED AND CALCULATED FREQUENCIES FOR VIBRATIONAL STATES (MC/S) H3CCCCCH K I r t l z t l r t l r t l rt2 r t l 0 *l r t l 71 *3 r t l &2 7 1 rt4 f l &3 F 1 rt5 f l f 4 F 1 rt5 7 1 unassigned r t l &l rt1 r t l +2 *1 0 ztl &3 -+1 &l F1 f 2 T l 1 4 f 1 f 3 T l rt5 r t l rt5 r l &7 r t l rt6 rl rt8 rtl unassigned J = 4 4 5 obs.calc.* 20,411.95 412.00 20,390.87 -390.96 J = 5 + 6 obs. calc. 24,494.38 -49440 24,469.1 1 469.16 -481.52 24,481'52 481.55 247480'78 -480.75 480.88 24,479.62 i Tz",:",:z 24,475.95 -476.00 H3CCCCCD J = 8-9 obs. calc. 36,74 1-44 -741 -48 36,703'62 -703.60 -722.23 36,722*30{ -722.14 36,713'83 -713.86 36,710.05 -709.98 36,70740 J=4+5 obs.calc. 19,348.50 -348.49 19,328-94 -328.93 -337.92 -337.10 19,335.80 -335.83 K *l f l f l & l f 2 &l 0 f l & l r l +3 *l s 2 r l rt4 &l zt3 r l f 5 &1 zt4 F l &5 r l +8 &l unassigned J = 5 4 6 obs. calc. 23,218.1 7 -21 8.1 9 23,194-67 -194.71 -206.22 -206.21 23,206-23 .( 2 3,202.9 6 -202 97 D3CCCCCH J = 8 4 9 obs. calc. 33,112.67 -1 12.64 33,080.20 -080.16 33,096.27 -096.26 33,096.06 -096.01 -095.36 -094.12 -492.34 -092.04 33,092-23 { 33,089.98 -090.04 33,087.11 -087.19 33,083.30 -083.21 J = 1 1 4 12 obs. calc. 46,436.04 -436.03 46,389.10 -389'09 46,412.42 -412.38 46,411.82 -41 1.84 405.57 46,405.40( -405.20 46,396.90 .-396*75 46,396'24 -396.05 46,390.82 -391.00 (4h,389*94 46,386.78 -390.14 46,387.80 D3CCCCCD J = 9 4 10 obs.calc. 34,932.59 -932.61 34,898.93 -898.93 34,915'70 -915.67 34,9 15.35 -9 15.3 1 34,911.46 L-9 11.44 - 34,909.06 -908.98 34,900.65 34,901.48 * The dash before the calculated figures indicates that the number of thousands has been omitted, for brevity.G. A . HEATH, L . F. THOMAS, E. r. SHERRARD AND J . SHERIDAN 41 The observed frequencies listed in table 3 are due to molecules in the first excited level of a degenerate vibrational mode. The form of these spectra is that predicted by Nielsen’s theory of I-type doubling in symmetric-top molecules.6 Each transition consists of two widely spaced lines (K = I = 4 1) about a central group of lines which become more widely spaced towards lower frequencies. The frequencies predicted theoretically for given values of J, K and I have been expressed 7 by general formulae involving also the rotational constants BV and A v for the vibrational state, the distortion constants BJ and DJK for that state, the Coriolis coupling coefficient for the vibration, 5, and the quantity q, equal to 2aBe2/w, where Be is the equilibrium rotational constant, w is the frequency of the vibration, and a is a factor not far removed from unity.In fitting the spectra it is convenient to assign values to the expression denoted by X in table 4, which lists values of this and other constants giving the best fit with observation. The small constants DJ are taken to be the same as for the ground states. While accurate values of Bvand q are obtainable, and DJK is shown to have values close to those for the ground states,( and X are less precisely determined.This is partly because accurate measurements are somewhat hindered by near-coincidences among the TABLE 4.-sPECTROSCOPIC CONSTANTS FOR VIBRATIONAL STATES 4 t- molecule BV DJK b U a (Mc/s) (Icc/s f 10) (kcls) (kcls) (cm-9 (Mcls) H3C5H 2,040.14 2104 20.0 0.9 0.15 151 -4.40 H3C5D 1,933.86 1956 18.7 0.92 0.20 146 -4.09 D3C5H 1,838.69 1804 14.6 0.9 0.23 143 -3.84 DSsD 1,745.80 1684 14.0 0.9 0.23 138 -3.58 central lines, especially with the heavily deuterated molecules, the lines for which are broader than those for the light species. The most probable values for f are slightly under + 1, and are thus in the theoretically permitted range for the Coriolis coupling coefficient. It is noteworthy that infra-red spectra indicate that the Coriolis coupling factors for vibrations, similar to the present one, in methyl acetylene,s.9 methyl cyanide 109 11 and methyl isocyanide 11 are also slightly under + 1.Some weak unassigned lines are noted also in table 3. They may be due to molecules in other excited vibrational states, some of which may well be appreciably populated for such a molecule under the experimental conditions. We do not, however, regard the present assignment of the spectra as final, and hope later to reexamine them using higher sensitivity. Also included in table 4 are the constants a, for the vibration concerned, which occur in the formulae 12 connecting Bv or Bo with Be. Under the heading 1.15w{a in table 4 are the frequencies of the vibration obtained from q, if a is taken as 1-15, as proposed for methyl acetylene in a similar vibrational state ; 13 Bo is used as an approximation to Be in deriving these frequencies.Since a is uncertain, their absolute values are approximate, but their relative values should be more reliable. The relative intensities of the spectra of the vibrational and ground states are in rough accord with vibration frequencies as low as those listed. Insuflicient is known of the vibration spectra of methyl diacetylene for the vibration concerned to be identified, but its low frequency and degeneracy accord with its involving bending of the C5H (or C5D) chain. Further close groups of lines due to vibrationally excited molecules are observed at frequencies higher, for a given transition, than those in table 3 ; the separations of these further groups from the spectra of the ground states are roughly twice the corresponding separations for the first vibrational state.This fact, and the relative intensities of the spectra, suggest that these further lines are due to molecules in the second excited level of the vibration discussed above. DISCUSSION The spectra of the ground states are those of strictly symmetric-top molecules. This indicates beyond doubt that, as expected, all five carbon atoms and the acetylenic hydrogen atom are located linearly on the figure axis of the molecule.42 METHYL DIACETYLENE As discussed in detail below, the moments of inertia are in excellent agreement with those expected for the methyl diacetylene structure.The other substance, or substances, to which this structure has been attributed 1 must presumablybe unsaturated five-carbon compounds, containing more than four hydrogen atoms, and will be asymmetric-top molecules. A preliminary analysis of the structural parameters can be made by assuming those associated with the hydrogen atoms, and computing information about the carbon skeleton. It is reasonable to suppose that the parameters associated with the H or D atoms will closely resemble the analogous parameters in methyl acetylene.13~ 14 Accordingly we assume the following : 14 d& (methyl) = 1.1 12 A, d c ~ (methyl) = 19108A, d c ~ (acetylenic) = 1.060 A, d c ~ (acetylenic) = 1.058 A, LHCH = 108" 25', L DCD = 108" 32'. Any three IB values can now be combined to give the total length of the Cs chain, a value of 5248A being consistently obtained from each of the four possible combinations of moments of inertia. This length is 0-23A shorter than the sum of two normal single and two normal triple bonds, and a shortening of this order of magnitude, due to conjugation, is expected from a knowledge of related structures.To proceed further, it is convenient to assume that the two triple bonds are equal in length. This is suggested by the equality of such bond lengths found crystallographically in dimethyl triacetylene,ls and by the relatively small variations found in the lengths of C r C bonds in different molecules. Any three IB values can then be combined to obtain the following carbon chain structure (distances in A) : 1'459 1'366 H 3 c - c ~ C-CE C--H 1'212 1'212 Internally consistent results are obtained from the four possible combinations of IB values.This preliminary structure is supported by the similarity of the distances to those found in related molecules. The bond length of 1.459A is close to the lengths of the analogous bonds in methyl acetylene,l3, 14 and in dimethyl tri- acetylene.15 The lengths of the triple bonds resemble those found in various acetylenic substances, and the bond length of 1.366A is similar to that found for other "single" C-C bonds located between two triple bonds, as in dia- cetylene,l6 cyanoacetylene 17 and dimethyl triacetylene.15 The influence of variations in the assumed parameters on the computed chain structure was examined.If the reasonable variations of f 1" in LHCH and of f 0.01 A in the CH and CD distances are allowed, the resulting probable uncer- tainty in the length of the CJ chain is about f 0.02A. A similar uncertainty results for the distance C (methyl)-C. The remaining parameters, however, are individually uncertain by at least twice this amount, which makes desirable the determination of spectra of other isotopic species. It appears, none the less, that considerable shortening of bonds below their normal lengths occurs in the conjugated chain. This is presumably due to contributions of structures which give double bond character to all the carbon- carbon bonds, similar to those proposed for diacetylene and methyl acetylene.16 In keeping with such a similarity, the dipole moment of methyl diacetylene appears, from the intensities of its spectra, to be of the same order of magnitude as that of methyl acetylene (0075D).We hope to determine the dipole moment of methyl diacetylene from measurement of the Stark effect on its rotational transitions of low 3. One of us (G. A. H.) is indebted to the Department of Scientific and Industrial Research for a Maintenance Grant.G . A. HEATH, L. F. THOMAS, E . I . SHERRARD AND J . SHERIDAN 43 1 Jones and Whiting, J. Chem. SOC., 1953,3317. 2 Armitage, Jones and Whiting, J. Chem. SOC., 1952, 1993. 3 Heath, Thomas and Sheridan, Nature, 1953, 172,771. 4 Heath, Thomas and Sheridan, Trans. Faraday SOC., 1954, 50,779. 5 DuMond and Cohen, Rev. Mud. Physics, 1953, 25, 691. 6 Nielsen, Physic. Rev., 1950, 77, 130. 7 Anderson, Trambarulo, Sheridan and Gordy, Physic. Rev., 1951, 82,58. 8 Boyd and Thompson, Trans. Faraday SOC., 1952,48,493. 9 Grisenthwaite and Thompson, Trans. Faraday Soc., 1954, 50,212. 10 Venkateswarlu, J. Chem. Physics, 1951, 19, 293. 11 Thompson and Williams, Trans. Farad~y SOC., 1952,48, 502. 12 Gordy, Smith and Trambarulo, Microwave Spectroscopy (J. Wiley and Sons, Inc., 13 Trambarulo and Gordy, J. Chem. Physics, 1950,18,1613. 14 Thomas, Sherrard and Sheridan, Trans. Faraday SOC. (in press). 15 Jeffrey and Rollett, Proc. Roy. SOC. A, 1952, 213, 86. 16 Pauling, Springall and Palmer, J. Amer. Chem. Suc., 1939, 61, 927. 17 Westenberg and Wilson, J. Amer. Chem. SOC., 1950, 72, 199. 1953), p. 103.

ISSN:0366-9033    

DOI:10.1039/DF9551900038

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

G . A. HEATH, L. F. THOMAS, E . I . SHERRARD AND J . SHERIDAN 43 INFORMATION PERTAINING TO MOLECULAR STRUCTURE, AS OBTAINED FROM THE MIICRO- WAVE SPECTRA OF MOLECULES OF THE ASYM- METRIC ROTOR TYPE BY WILLIAM D. GW~NN Dept. of Chemistry and Chemical Engineering, University of California, Berkeley 4, California, U.S.A. Received 4th February, 1955 For some time microwave spectra have been used to obtain the three moments of inertia of molecules of the asymmetric rotor type. These data have been utilized to obtain structural information about the molecule. In a number of examples this method has been very successful in determining not only the positions of the heavy atoms with high accuracy, but also the positions of the hydrogens in complex molecules. In later work the microwave spectrum has been employed to determine the potential barriers to internal rotation with high precision, and the Stark effect and statistical weights of levels have been useful in deciding such questions as the planarity of rings where double minimum vibrations are possible.The above problems will be discussed, along with their difficulties and limitations. In 1950 Prof. E. Bright Wilson, Jr., presented a review gaper 1 at the meeting of the Faraday Society. In that paper he discussed the methods of determining molecular structure with microwave spcctroscopy. He also presented certain examples which had been studied prior to that time. The purpose of my paper is to discuss certain of the more recent applications of this method and to illustrate how some of these methods work out in practice.I will limit my remarks prin- cipally to the work we have done with molecules of the asymmetric rotor type. Probably the most important information concerning molecular structure derived from microwave spectroscopy is the average position of the atoms in the molecule. The data from microwave spectra serve to determine the moments of inertia of the molecule, and the structure of the molecule is calculated from the moments of inertia. One characteristic of asymmetric top molecules is that they possess three different moments of inertia and the microwave spectrum is44 ASYMMETRIC ROTOR dependent upon all three, whereas the microwave spectrum of a symmetric top or linear molecule is dependent upon only one moment of inertia.Thus three structural parameters are determined for each isotopic species of an asymmetric top, as compared to one for each isotopic species of a symmetric top or linear molecule. The moments of inertia and structural parameters are the effective or average values for the vibrating molecule in its ground vibrational state. Since the structure of most asymmetric tops cannot be determined by only three structual parameters, it is necessary to combine data from several isotopic species in order to determine the structure of the molecule. Since the different isotopic species of the molecule will have different zero-point vibrations, the effective structural parameters will differ slightly for the various isotopic species. At present the only practical method for determining the structure is to ignore this difference and to assume that all bond lengths and angles are invariant with isotopic substitution.An alternate approach to the problem of vibration-rotation interaction might be to observe the spectrum of the molecule in several successive excited states of each vibrational mode and then to extrapolate back to the hypothetical non- vibrating state of the molecule. This method is not practical at present. The molecule has too many vibrational modes and, moreover, the population of molecules in upper vibrational levels for high frequency vibrations such as carbon- hydrogen stretching motions, would be far too small to be observed with present equipment. Still another approach would be to calculate the correction arising from vibration-rotation interaction. Although this calculation cannot be carried out in practice because the anharmonicity of the force fields of the atoms are not known, the principles of this calculation are well known.The vibration-rotation interaction may be divided into two principal parts, the first being the interaction if the vibrations were harmonic and the second being a result of the anharmonicity of the vibrations. The effect of the harmonic contribution would be to make the molecule appear small since the average reciprocal of the moments of inertia are measured. The anharmonicity usually makes the molecule appear larger. For diatomic molecules the anharmonic term usually dominates, but for more complex molecules the vibration-rotation interaction may be of either sign.As a result it is not possible to predict whether the vibration-rotation interaction contributed by a high frequency vibration will be larger or smaller than that contributed by a low frequency vibration. Since there seems to be no other course, we have neglected vibration-rotation interaction. We have instead studied as many isotopic species as possible in order to determine the inconsistencies involved in neglecting this interaction. For example, in the work on ethylene oxide,Z we have studied five different isotopic species, From the fifteen moments of inertia we need to determine only five bond distances and angles to determine the structure of the molecule. Also in the work on methylene chloride,3 we have TABLE 1 .-DISTANCE (IN A) OF THE studied seven different isotopic species, giving HYDROGENS FROM THE PLANE OF THE twenty-one moments of inertia from which to C-0-c RING IN ETHYLENE OXDE calculate only four structural parameters. In order to see how this works out in C2C13H40 0.9202 practice, a part of the experimental data for CHDCHDO (cis) 0-9213 ethylene oxide and methylene chloride is CMDCHDO (trans) 0.9215 presented.For ethylene oxide the three CZD40 0.9217 moments of inertia of a single isotopic species may be combined to give the dis- tance of the hydrogens above or below the plane of the three-membered ring. These are given in table 1. Another combination of the moments of inertia of a single isotopic species gives a function of two other bond projections. A third relation gives another function of the remaining two structural parameters. 0.9203 C2H40WILLIAM D .GWINN 45 These equations may be solved simultaneously in pairs to determine the bond projections, or they may be solved graphically. We prefer the graphical method because one can look at the data from all of the isotopic species at once and can better see the small inconsistencies which result from the neglected vibration- rotation interaction. Fig. 1 shows the values of one bond projection for each isotopic species plotted against the corresponding value of the other bond projec- tion, which gives the correct moments of inertia. The second set of two structural parameters gives a plot very similar to fig. 1. Inspection of table 1 and fig. 1 shows that the inconsistencies which result from the neglected vibration-rotation interaction are only of the order of 0.001 A.FIG. 1.-Plot of the values of ZH and Zo for ethylene oxide which are consistent with the determined moments of inertia. The intersection of the various lines gives the struc- tural parameters for ethylene oxide. ZH and 20 are projections of the CH and CQ bond, respectively, upon a line perpendicular to the CC bond and through the oxygen. A, C2H40; C, CHDCXPIDO (cis) ; D, CHDCHDO (trans); E, C2D4O. B, C2C13H40; 1 I 1 1.231 1.233 1.23 2 0 Another interesting observation is that the effect of substituting C13 for C12 in ethylene oxide is comparatively valueless in determining the structure of ethylene oxide. The lines in fig. 1 corresponding to C212H4O and C1C13H40 are very nearly parallel and coincident so that very small vibration-rotation interactions would cause a relatively large discrepancy in the bond distances. TABLE 2.-THE PROJECTIONS OF THE c-cl AND C-H BONDS PERPENDICULAR TO THE SYMMETRY AXIS IN METHYLENE CHLORIDE 1.4675 1 -4674 1.4673 1.4675 1.4673 1.4677 1.4676 0.8794 0.8801 0*881 0.888 0.887 0.891 0.891 The data for methylene chloride are presented in table 2 and fig.2. The data are very similar to the data for ethyleme oxide except for one feature. The distance YH in table 2 is obtained as the small difference between large moments of inertia, and it is somewhat sensitive to the vibration-rotation interactions.0 0.0 I A-- 0 and C 46 ASYMMETRIC ROTOR Even so, the greatest variation in this parameter is only 0-012A and this variation results in only an O-OOSA variation in the C-€3 distance and a 20’ variation in the H-C-H angle.It is also interesting to notice that making an isotopic substitution for chlorine alone is of little value in the determination of molecular structure. The lines corresponding to changes in the chlorine isotope alone are again nearly parallel and coincident. The data for ethylene sulphide are very similar to the data for ethylene oxide, giving discrepancies of about 0.001 A. Likewise, changing the sulphur isotope alone results in nearly parallel and coincident lines, In view of these and other similar data, we have tentatively come to the conclusion that, although the hydrogen-deuterium substitution may (but does not necessarily) affect the vibration-ro tation iiiteractions more than FIG.2.-Plot for methylene chloride similar to fig. 1. ZH and Zcl are distances along the two-fold symmetry axis. The curves for the isotopic species containing different chlorine isotopes alone are superimposed for CD2C12 and are closely parallel for CDHC12 and CH2C12. A, CH2C1235; B, CH~C135C137 ; C, CH2C1237; D, CDHC1235; F, CD2C1235; G, CD2CPsCP7. E, CDHCPC137 ; a heavy atom substitution, studying the various deuterated species of a molecule is much more fruitful than studying the species of the molecule with the heavy atoms substituted. Probably the main reason for this is that the hydrogens are usually near the periphery of the molecule and a substitution there creates a much larger change in the moments of inertia than does a substitution of the heavier atoms which are usually located near the centre of gravity of the molecule.We also conclude that interatomic &stances, including distances involving hydrogens, can often be given which are self-consistent to a few thousandths of an hgstrom. If these distances deviated from the actual average distances in the molecule, we believe that it would show up in the self-consistency of the data. At present there is no other method by which to compare these dis- tances. Electron diffraction technique allows the determination of the distances between the heavy atoms of a molecule, but only to within a few hundredths of an hgstrom. In general, there has been excellent agreement between distances determined by the use of electron diffraction and those determined by the use of microwaves.There have been a few discrepancies but they have usually been with early electroil diffraction work and most of them have been removed by more recent work. Electron diffraction technique is improving rapidly, and it will be very interesting when we can compare the methods to a few thousandths of an Angstrom.WILLIAM D. GWINN 47 It should be emphasized that it is not always possible to determine with high precision the structure of simple asymmetric tops. In some, the moments of inertia are simply not sufficiently sensitive to certain bond distances. An example is nitromethane, which is to be discussed later with respect to internal rotation. In this particular molecule, the microwave spectrum is determined primarily by the potential barrier to internal rotation and by two moments of inertia.The axial moment of inertia of the methyl group would be determined only if the barrier to internal rotation were very high. Of the two moments of inertia which are determined, one gives directly the 0-0 distance and the other could be used to determine the other four distances and angles in nitromethane. Thus we have the problem found so often in the more complex symmetric tops and linear molecules, the four distances must be derived froin four equations (each from a separate isotopic species) in four unknowns. The equations are rather similar and small vibration-rotation interactions seriously limit the accuracy of the solution. Partially deuterated species would make the axial moment of inertia show up in the microwave spectrum, but still not sufficiently to allow its deter- mination.It was estimated that we could not determine the C-H distance to better than 0.1 A. Therefore, since the experiments and calculations were very difficult, we decided that it was not worthwhile to attempt to determine the structiire of nitromethane by studying other isotopic species. The value of knowing bond distances and angles to a higher precision needs no elaboration. The hydrogen distances and angles are especially interesting since there were no previous values available for more than a few of the simplest of the asymmetric tops. In ethylene oxide and ethylene sulphide we were inter- ested in systems with bent bonds. We were also interested in the H-C-H angle and C-C distance in order to compare them with ethylene.The H-C-H angle turns out to be 116" 41' (ethylene oxide) and 116" 0' (ethylene sulphide), just intermediate between the extremes of 120" (as ethylene) and 109" 28' (the more conventional tetrahedral value). In methylene chloride we were interested again in the possibility of bent bonds. The C1-C-Cl angle was known to be about 112" & 2" (electron diffraction). If the bonds were not b e t , then the H-C-H angle would be less than tetrahedral. Experimentally, both angles are greater than tetrahedral (Cl-C-Cl angle = 11 1" 47' and H-C-H angle = 112" 0'). We regarded this as good evidence for the presence of bent bonds in methylene chloride. INTERNAL ROTATION There are two general methods of microwave spectroscopy for studying the barrier to internal rotation.The first is most applicable where the barrier is high and the motion of internal rotation approximates to torsional oscillation for the first few energy levels. In this case small satellite lines arising from the excited torsional vibrational state appear near the corresponding line of the un- excited molecule. The method consists of measuring the intensities of these lines and calculating by Boltzmann's statistics the energy of the excited levels. This method is straightforward but is subject to the disadvantage that the inten- sities of microwave lines still cannot be measured with high precision. The second method is to measure the frequency of lines involving a combination of internal rotation and over-all rotation.This method is extremely time- consuming but yields a precise value for the barrier height. The disadvantage of this method is that the investigator starting such a piece of research can, at present, give no prognosis for the outcome of the investigation. It may be that the spectrum is too complicated to be interpreted or that the lines critical for the barrier determination do not lie in the region of his spectrograph. Several mole- cules have been successfully studied. Dennison 4D 5 ~ 6 and co-workers have determined the barrier in methyl alcohol as being 1070 cal/mole, thus clearing up a long-standing problem in chemistry. Very recently Shimoda, Nishikawa48 ASYMMETRIC ROTOR and Itoh 7 have determined the barrier in methyl amine to be 1950 cal/mole.We have studied nitromethane and deuteronitromethane,S and this work will be discussed. The barrier to internal rotation in nitromethane had been investigated pre- viously 9 9 10 by thermodynamic methods and had been found to be something between 0 and 1100 cal, probably about 800 cal. If each oxygen interacted with the methyl group with a barrier which could be represented by a three-fold cosine function, then the barrier in nitromethane would be zero. The measurement of the actual barrier in nitromethane was measured by us in order to test the validity of assuming cosine functions to represent the barrier to internal rotation, Con- sidering the interaction of a single oxygen with the methyl group, the barrier to internal rotation could be given as (1) E3 E6 V = -cos 3$ + -cos 693 + $cos 993, 2 2 where the terms in &, Eg, E12, etc., represent the deviations from a simple cosine barrier.When the interactions of both oxygens are added together, the barrier for nitromethane would become (2) Y = E6 cos 64 -t El2 cos 1256 + &8 cos 18$, (3) V = - v6 cos 65b + F c o s 124 + y c o s 185b. 2 or The barrier V6 in nitromethane would represent the deviations from a simple cosine barrier for the interaction of a methyl group with a single oxygen, and V12 would represent the deviations from a simple cosine barrier in nitromethane. The interpretation of the spectrum followed the usual technique of microwave spectroscopy. The Stark effects were resolved where possible, lines with low J quantum numbers were selected, and trial assignments were made until every- thing was in agreement.The energy levels could have been calculated from the theory of Burkhart and Dennison,s but not easily. Our work was greatly facilitated by a theory worked out by Myers,ll in which the matrix elements are obtained as a solution of the matrix commutation rules. The resulting energy matrix was obtained in a form much more convenient for determining energy levels for molecules with low potential barriers. The theory of the Stark effect was also extremely important, because various types of transitions have different and very characteristic Stark effects. Most of the lines studied were rather inseiisitive to the height of the barrier. There were, however, four lines which, in the case of a free rotor, arose from two sets of doubly degenerate levels.These corresponded to the set K = + 1, k = + 3, a n d K = - l , k = - 3 ; andthesetIC=-l,k=+3,andK=+l,k=-3, where K corresponds approximately to the angular momentum of the whole molecule about the axis of internal rotation, and k corresponds approximately to the angular momentum of the methyl group about the same axis. Some of the terms involving the barrier v6 in the energy matrix connect these degenerate levels. As a result, the barrier splits the degeneracy of the levels, and the frequencies of the corresponding lines are very sensitive to, and approximately linear in, the height of the barrier. The splitting of these lines was approximately 1800 Mc/s and this corresponded to a barrier height v6 of 6-00 f 0.03 cal/mole (2.10 cm-l/ mole).In the same way, the splitting of the K = 0, k = f6 lines could be used to determine the V12 component in the barrier. Experimentally the k = k 6 lines were not split, so there is no evidence for any V12 term. From the resolution of the spectrograph and the sensitivity of the method, the upper limit to V12 can be set as 0.03 cal/mole. It is difficult to estimate the magnitude of the interaction of a single oxygen with a methyl group, but it must be in the order of magnitude of 1-2 kcal. SinceWILLIAM D. G W I N N 49 the barrier in nitromethane is much smaller than this, and since the V12 term is so much smaller than the v6 term, we conclude that the series given by eqn, (1) converges rapidly and that a simple cosine barrier is an excellent approximation to the barrier in nitromethane.Deuteronitromethane has also been studied, and the barrier in this molecule was found to be 5.17 f 0.03 cal. At first this 12 % decrease from nitromethane seems a bit large, but it does not appear unreasonable when one considers the zero-point vibrations of some of the vibrations of the methyl group. The sym- metrical H.C.H bending mode of the methyl group would change the distance between the oxygens and the hydrogens, and the shortest distances would be most effective in determining the barrier. The barriers are in the right order, the higher barrier (nitromethane) being associated with the higher amplitude motions and the lower barrier (deuteronitromethane) being associated with lower amplitude motions, The splitting of the lines used to determine the barrier height was about 1800 Mc/s. The splitting was measured to f 0.1 Mc/s and could be measured more accurately. Since the splitting is approximately linear with the barrier, the barrier could possibly be measured to an accuracy of 1 part in 18,0QQ, or 0.0003 cal. However, effects of centrifical distortion and vibration are present and the precision is limited to 0.03 cal.INVERSION DOUBLING AND PLANARITY OF RINGS The question of planarity of rings often arises in chemistry and there are several ways of attacking this problem with the techniques of microwave spectroscopy. !C) (a) ( b) FIG. 3.-Possible potential cnergy functions and energy levels for ring puckering vibra- tions. For some rotational levels the statistical weights for -t- levels are gl, for - levels they are g2, and for & levels they are gl + g2.For other rotational levels gl and g2 are interchanged. If the complete structure of the molecule has been determined with microwave spectroscopy, then the question of planarity of rings has already been settled. There are, in addition, several techniques which allow the problem of planarity of rings to be settled without a complete determination of structure. The question of planarity of rings centres about the potential energy function and the corresponding energy levels associated with the ring puckering motion. Fig. 3 represents the various possibilities. The molecule may have a planar ring, as represented in fig. 3a, or it may have such a high central maximum that it is convenient to think of the molecule as having a rigid puckered ring, such as indicated in fig.3c, or the height of the central. maximum may be low enough so that it is convenient to think of the molecule as having inversion doubling. Probably the best way to demonstrate that a molecule does not have a planar ring or a plane of symmetry in the plane of the ring is to measure the component50 ASYMMETRIC ROTOR of the dipole moment perpendicular to the ring. From the Stark effect of several lines of an asymmetric rotor it is possible to determine the three individual com- ponents of the dipole moment along the three principal axes of the molecule. The existence of a component of the dipole moment which is perpendicular to a supposed plane of symmetry is conclusive evidence that the plane of symmetry does niat exist.The fact that the dipole moment of ethylenimine 12 makes an angle of about 30" with the plane of the ring leads to the positive, yet not surprising, conclusion that the N-H bond does not lie in the plane of the C-C-N ring. If the molecule is planar, the sum of the two small moments of inertia is approximately the third. (There is a small inertial defect as a result of the vibration- rotation interaction.) If the molecule is not planar but has a plane of symmetry with a two-fold axis (2) of rotation in that plane (& symmetry), then perhaps the best method is to make use of the different statistical weights of various rota- tional levels. These different statistical weights arise from the various hydrogen nuclear spin states.Experimentally this requires the measurement of the in- tensities, but does not demand a very precise measurement. Part of the rotational levels in an asymmetric rotor (classes A and Ba) have one statistical weight and the rest (classes Bb and Bc) have another statistical weight. If the rotational line of a molecule is in an excited vibrational level of the ring puckering motion, these statistical weights alternate with the symmetry of the vibrational level, which is indicated in fig. 3. The method of utilizing the symmetries of the ground vibrational state has been applied to pyrrole by Wilcox and Goldstein,l3 who found that pyrrole was planar. They report that the spectrum of pyrrole contains several pairs of lines in which the two members of the pair are very close to each other and have different statistical weight.Measuring these intensities to decide between a 6 or 10 statistical weight is easy and reliable. In trimethylene 0xide,l4 CH2-CH2-CH2, the statistical weights were 7 and 9 and the appropriate lines were not close to each other, so use was made of the alternating statistical weights of the vibrational levels. If the ring were planar, the intensities of the lines arising from the various vibrational levels should be in the ratio of 7 : 9 exp (- El/kT) : 7 exp (- E2/kT), etc., for some lines, and for others the ratios should be 9 : 7 exp (-El/kT) : 9 exp (- E2/kT), etc. The lines arising from the excited vibrational states appear as satellites to the lines arising from the ground state, and it is easy to measure the intensities to sufficient accuracy to know that the statistical weights are necessary and that the first two vibrational separations are about 60 cm-1.If the potential function could be changed at will and a potential function were gradually changed from that represented in fig. 3a through the case of inversion doubling (fig. 3b) to the rigid puckered molecule of fig. 3c, we would observe the satellite line of the excited vibrational levels with their statistical weights to coincide in pairs, as indicated in fig. 3c, where all lines would have equal statistical weights. Since in trimethylene oxide we find from the statistical weights that the lowest vibrational level is symmetric and the next level is antisymmetric, and the third level is symmetric, we know that the vibrational levels in trimethylene oxide are single levels.Since the levels are approximately equally spaced, we know that there is no inversion doubling and that the trimethylene oxide sing is planar. The planarity of trimethylene oxide could also have been deduced from the Stark effect. If the molecule were puckered with a high central maximum, the Stark effect would be very sensitive to the component of the dipole moment perpendicular to the ring. The Stark effect shows no contribution from such a dipole. From the accuracy of the Stark measurement, an upper limit can be set to the perpendicular component of the dipole. From this it can be estimated that the upper limit to the ring bending is only 0" 20' (dihedral angle between the !-,AWILLIAM D .GWINN 51 C-C-C plane and the C-0-C plane). Such a high barrier separating two minima only 0" 40' apart would be physically unreasonable and need not be considered as a possibility. If the central maxima were lower, the degeneracy of the two vibrational levels would be removed (as in fig. 3b) and the Stark effect would not always be so sensitive to the perpendicular component of the dipole moment. If the separation of the lowest two vibrational levels were between 0 and 2cm-1, then the upper limit of this angle might be sometimes less than 0" 20', and sometimes greater than 20' (as the separation of the vibrational levels change) but it would never be larger than 4". Any separation of the first two levels between 0 and 2 cm-1 would still require too high a barrier to have the two minima only 8" apart, and this possibility can be eliminated.If the splitting of the ground state were greater than 2cm-1, the Stark effect would be of little value but we should observe the splitting of the satellite lines (at least in the upper states). The satellite lines show no such splitting, therefore the independent conclusion is reached that trimethylene oxide is planar and that there is no inversion doubling. If a molecule had inversion doubling, there would be a possibility that combina- tion inversion doubling and rotational transitions would be present, but it would only be chance that these lines would appear in the microwave region. Thus their absence is no evidence against inversion doubling. Our interest in trimethylene oxide stems from the recent work in infra-red and Raman spectroscopy which indicates that cyclobutane has a puckered ring. In the four-membered rings the classical ring strain would tend to make the rings planar while the preferred staggered positions of the hydrogen in internal rotation would tend to make the rings puckered. Trimethylene oxide, having fewer hydrogens, would have less tendency to have a puckered ring. The ring strain is the predominating term and trimethylene oxide is planar. 1 Wilson, Faraday SOC. Discussions, 1950,9, 108. 2Cunningham, Boyd, Myers, Gwinn and LeVan, J. Chem. Physics, 1951, 19, 676. 3 Myers and Gwinn, J. Chem. Physics, 1952,20, 1420. 4 Burkhard and Dennison, Physic. Rev., 1940, 71,408. 5 Hughs, Good and Coles, Physic. Rec., 1951, 84,418. 6 Ivash and Dennison, J. Chem. Physics, 1953,21, 1804. 7Shimoda, Nishikawa and Itoh, J. Chem. Physics, 1954, 22, 1456; J. Physic. Chem. 8 Tannenbaum, Johnson, Myers and Gwinn, J. Chem. Physics, 1954, 22, 949 ; and 9 Pitzer and Gwinn, J. Amer. Chem. SOC., 1941, 63, 3313. 10 Jones and Giauque, J. Amer. Chem. SOC., 1947, 69, 983. 11 in preparation for publication by Myers, Univ. of California, Berkeley, California. 12 Johnson, Myers and Gwinn, J. Chem. Physics, 1953,21, 1425. 13 Wilcox and Goldstein, J. Chem. Physics, 1952, 20, 1656. 14in preparation for publication by J. Fernandez and Gwinn, Univ. of Calif., Smith, M.Sc. Thesis (Univ. of Calif., Berkeley, Calif., 1953). Japan, 1954,9, 974. material in preparation for publication. Berkeley, Calif.

ISSN:0366-9033    

DOI:10.1039/DF9551900043

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

MILLIMETRE WAVE SPECTRUM OF METHYL MERCURY CHLORIDE * BY J. T. Cox, T. GAUMANN -f AND W. J. ORVILLE THOMAS $ Received 21st January, 1955 Dept. of Physics, Duke University, Durham, N.C., U.S.A. The J = 16 -+ 17 rotational transition has been studied for CH3HgCW for the most abundant mercury isotopes. These measurements lead to new values for the spectral constants Bo and Djk and to values for Djj the distortion constant associated with end- over-end rotation. Previous studies 1. 2. 3 on mercuric compounds have indicated that the a-bonding orbitals of the mercury atom are sp-hybridized. It was shown by Gordy and Sheridan 4 from the pure rotation spectra of methyl mercury chloride and bromide that these molecules are symmetrical tops. This indicated un- equivocally that in them the C-Hg-Halogen grouping is strictly linear.In their study the Bo's were calculated from the general synvnetric top formula neglecting the centrifugal distortion term 4Djj(J + l)3. This term has now been measured for the methyl mercury chloride molecules containing Hg (198, 199, 200 and 202), and new values obtained for the rotational constants. Hg201 is the only Hg isotope with a nuclear quadrupole moment. Because of its small natural abundance (13.2 %) and the splitting of the lines by nuclear quadrupole coupling the intensity of the lines is expected to be small and the assignment of the CH3Hg201C1 spectrum difficult. An attempt has been made to unravel the spectrum and an indication of the probable value of the Hg201 nuclear quadrupole coupling constant obtained. YO 2B(J + 1) - 4Djj(J + 1)' - 2Djk(J + 1)K2, EXPERIMENTAL The sample of methyl mercury chloride was the one used in the previous microwave study by Gordy and Sheridan.4 The microwave spectra were observed with a frequency sweep spectrometer employing a video type detector.5 Crystal harmonic generators 6 driven by reflex Klystron tubes were used as energy sources.Frequencies were measured with a standard monitored by comparison with the standard 5-Mc/s signal of the National Bureau of Standards station WWV. Methyl mercury chloride is a solid with a vapour pressure of 1.6 x 10-2 mm at 27" C.7 The substance was allowed to evaporate into the evacuated wave-guide cell at room temperature. SPECTRAL CONSTANTS The J = 16 --t 17 transition of CH3HgCP was studied.This particular transition was chosen because the splitting by the chlorine coupling is negligible and only one major C1 line is obtained for each K value when AF = 4 1. A large number of frequencies were observed owing to the large number of abundant mercury isotopes and the nuclear "This research was supported by The United States Air Force through the Office f' Present address : Organisch-Chemisches Laboratorium, Eidg. Technischen Hoch- $ Visiting Fulbright Scholar, 1953-54. Present address : The Edward Davies Chemical 52 of Scientific Research of the Air Research and Development Command. schule, Zurich, Switzerland. Laboratories, University College of Wales, Aberystwyth.J . T. cox, T. GAUMANN AND w. J . ORVILLE THOMAS 53 quadrupole hyperfine structure due to Hg201.Several observed lines not accounted for in the course of this work probably arise from molecules in an excited (bending) vibrational state. In tables 1 and 2 are listed the lines observed4 for the J= 8 --t 9 and for the J = 16 -+ 17 rotational transitions of methyl mercury chloride, together with the calculated values obtained using the spectral constants determined from these transitions and given in table 3. TABLE 1 .-FREQUENCIES OF mercury isotope 198 199 200 202 THE LINES OF THE J = 8 3 9 ROTATIONAL TRANSITION OF CH3HgC135 FOR K = 0 vo (obs.) vo (calc.) Mcls Mc/s 37394.00 37393.99 37388.40 37388.40 3738280 37382.80 3737 1 * 60 37371.60 CENTRIFUGAL DISTORTION CONSTANTS Interaction of the rotations of the molecule about the A and B axes (fig.1) is responsible for the energy term containing Djk. Since Djk > 0 the effective moment of inertia Ib of the molecule about the B-axis increases with K. This effect is opposite to that expected B FIG. 1 .--The methyl mercury chloride molecule. on simple grounds, since an increase in K would be expected to decrease a and thus decrease I&. For the methyl halides it has been shown by Thomas, Cox and Gordy 8 that the positive sign of ojk is explicable in terms of a shifting of the internal energy oi the molecule from one bond to another. The mechanism of distortion of the molecule as it rotates is described thus. As K increases so does the angular momentum about the A-axis. The increased centrifugal force acting on the hydrogens of the methyl group tends to make the CH3 group more nearly planar. Effectively the s-character of the three carbon orbitals which bond to hydrogen is increased.To offset this the s character of the carbon orbital bonding to the mercury must be reduced for proper normalization. The combined effect of the hybridization changes at the carbon atom is to increase the bond stretching force constant of the C-H bonds and to decrease that of the G-Hg bond. The effect of changes in size and shape of the CH3 group upon Ib can be neglected in comparison with the effect of changes in the C-Hg bond length. An increase of only 0.0012A in the C-Hg bond length would suffice to account for the 17-89 kc/s separation of the K = 0 and K = 5 lines in the J = 16 -+ 17 transition of CH3HgzooC135. If this increase is due entirely to a change in the covalent radius of the carbon atom it corresponds to a decrease of about 0.7 % in s character (or increase in p character) of the carbon orbital.This small hybrid- ization change would be sufficient to account for the needed C-Hg bond lengthening.01.0 1.12 10.0 =F SPZ.0 10.0 PZ.9LOZ zo$H 01.0 JF 1.12 01.0 ?= 0-12 01.0 0.12 10.0 'f 6SZ.O 10.0 T 9sz.o 10.0 =F TPZ-0 10.0 'f 98.9LOZ 10-0 81-LLOZ 10-0 =F 8P-LLOZ oozZH 661sH 861SHJ. T . cox, T . GAUMANN AND w. J. ORVILLE THOMAS 55 From this change in hybridization an increase of about 22’ in ,/- HCH and a decrease of 0.0004A in the C-H bond length respectively would be expected. The latter would be partly compensated by the centrifugal force tending to lengthen the C-H bonds.Since the molecule is a symmetric top, in the ground vibrational state, the CHgCl grouping is linear and no hybridization changes occur at the Hg atom. This implies that the HgCl bond length does not vary with K and hence the effects decribed are confined to the CH3-Hg grouping. Similar arguments applied to the end-over-end rotation lead one to conclude that the C-Hg would be shortened and DJ negative. Since Djj is positive in value it seems clear that the much more rapid rotation of the molecule about the A axis is more effective in distorting the HCH angles and thus varying the state of hybridization of the carbon atom than is the end-over-end rotation. The effect of the end-over-end rotation would be to decrease the magnitude of Djj. It should be pointed out that the effect of the HGH-angle distortion on the C-Hg bond-length will also tend to be cancelled by the lengthen- ing of the C-Hg-Cl grouping due to centrifugal distortion.This picture of a change in the hybridization ratios of the carbon orbitals as the molecule spins faster around the A axis receives some support from the ideas of Linnett and Wheatley 9910 who have indi- cated the possibility of the bonding orbitals of a central atom changing their hybridization in such a manner as to follow the outer atoms during FIG. 2.-Bending vibration of CH4 molecule dur- ing which orbital following by change of carbon hybridization is possible. bending vibrations. Linnett and Wheatley concluded that bending vibrations will occur more easily if the bonding orbitals are able to follow the movement of the atoms by a change of hybridization.They showed that in methane it is possible for the bonding orbitals to follow the atoms during certain vibrations by change of hybridization. In particular, it was found that for the bending vibration illustrated in fig. 2 the carbon bonding orbitals were able, by change of hybridization, to follow the attached hydrogen atoms. When we examine the motion of the methyl mercury chloride molecule as Kincreases it is seen that the hydrogen atoms move in precisely the same direction with respect to each other as they do in the vibrational bending mode illustrated in fig. 2. This problem was suggested by Prof. Walter Gordy to whom we are very grateful for much useful discussion and encouragement. Dr. Albert Jache is thanked for his technical assistance. One of us (T. G.) thanks the Sweizerischen Nationalfonds und der Gesellschaft fur Stipendium auf dem Gebiete der Chemie for assistance during this work. 1 Braekken and Scholten, 2. Krist., 1934, 89, 448 ; 1932, 81, 152. 2 Braune and Linke, 2. physik. Chem. B, 1935,31, 12. 3 Gutowsky, J. Chem. Physics, 1949, 17, 128. 4 Gordy and Sheridan, J. Chem. Physics, 1954,22,92. 5 Gordy, Smith and Trambarulo, Microwave Spectroscopy (John Wiley & Sons, 6 King and Gordy, Physic. Rev., 1953,90,319 ; 1954,93,407. 7 Charnley and Skinner, J. Chem. SOC., 1951, 1921. SThomas, Cox and Gordy, J. Chern. Physics, 1954,22, 1718. 9 Linnett and Wheatley, Trans. Furaday SOC., 1949, 45, 33. 10 Linnett and Wheatley, Trans. Furuduy SOC., 1949, 45, 39. Inc., New York, 1953), p. 6.

ISSN:0366-9033    

DOI:10.1039/DF9551900052

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

CONNECTIONS BETWEEN MOLECULAR STRUCTURE AF?D CERTAIN MAGNETIC EFFECTS IN MOLECULES BY c. H. TOWNES," G. C . DOUSMANIS,* R. L. WHITE,* AND R. F. SCHWARZ Received 25th March, 1955 Several types of magnetic effects in molecules and their relation to molecular structure are discussed. Hyperfine structure in molecules with electronic angular momentum can give experi- mental determination of three independent parameters of the distribution of electronic angular momentum. These parameters are rather simply and directly related to the molecular electronic structure and hence afford critical tests for proposed structures. Fine structure for such molecules can also give useful information on electron distribution. 0 2 , NQ and OH are examples which will be discussed. A systematic investigation has been made of I J interactions in 12 molecules.Al- though these second-order effects are not as easily interpretable as are magnetic inter- actions in molecules with electronic angular momentum, it now appears possible to make an approximate prediction of the magnitude of I J interactions in common types of molecules. The interaction between an external magnetic field and rotation of a 12 molecule may be used to determine the orientation (sign) of the molecule's electric dipole moment. In order to eliminate obscuring effects such as L-uncoupling, it is necessary to measure the molecular g-factor for two isotopic species. Determination of the sign of molecular dipole moments by this method requires rather precise measurement of Zeeman effects, but appears practical for certain molecules.It is our purpose to ctiscuss several types of magnetic effects in molecules which may be observed by the techniques of microwave or radiofrequency spectroscopy, and which can be related to structural parameters of the molecules. These effects involve magnetic fine and hyperfine structure in ,molecules having electronic angular momentum, magnetic hyperfine (I J) interactions in molecules which are in 1C electronic states, and a method for determining the sign or orientation of the electronic dipole moments of a molecule by its interaction with an external magnetic field. 1. INTERPRETATION OF MAGNETIC FINE AND HYPERFINE INTERACTIONS IN MOLECULES Magnetic hyperfine structure in diatomic or linear molecules with electronic angular momentum has been discussed by Frosch and Foley 1 and shown to depend primarily on four parameters of the distribution of electronic angular momentum of the molecule. For a molecule in a 2na state, the interaction has the form IN TERMS OF THEIR ELECTRON STRUCTURE b + c 1 .J d(J+ %).)I* J J+',=.$ a - - _____ ( 2 ) J(J+ 1) * W(J+ 1) The positive sign applies to the A-doubled state of higher energy, and the negative sign to that of lower energy. For a 2ns state * Columbia University. Harvard University. 56C. H . TOWNES, G . C. DOUSMANIS, k . L. WHITE, A N D R . F . SCHWARZ 57 In these expressions, - pop1 3 cos2x + 16niuoc”’p2 (O), 31 b = - (L r3 - ’) I av. c = - ( 3popz 3 COS2 X - I r3 po is the Bohr magneton and p~ and I are the nuclear magnetic moment and spin respectively, @(O) is the density of electron spin (the probability density for an average electron spin of +) at the nucleus whose hyperfine interaction is being considered.X is the angle between the molecular axis rand the radius vector r from the nucleus to the electron which carries net angular momentum. The averages are to be taken over only the electron or electrons which contribute the angular momentum. For coefficients b, c and d, the average is over the net electron spin distribution, whereas for a, the average is over the net orbital angular momentum. The quantity d in (3) differs by a factor of two from that given by Frosch and Foley, but otherwise expressions (1) and (2) can be obtained directly from their work. Expressions equivalent to (1) and (2) for a 3C molecule have already been given.2 In this case the hyperfine structure does not depend on orbital angular momentum and there is no A-type doubling so that only the parameters b and c appear.The parameters a, b, c and d for magnetic hyperfine structure can afford rather precise and definite tests of proposed electronic structures for molecules. They are similar in many ways to the quadrupole coupling constant, eQ32V/3z2, which has already been widely used to examine the electronic structure of molecules. AS does 32V/3z2, they depend primarily on the part of the electronic distribution very near the nucleus. The magnetic interaction constants are, however, still more specific than the quadrupole coupling constant, and can give more detailed information.This is in part because there are three constants which may be experimentally determined rather than one, as in the quadrupole case, and partly because the magnetic hyperfine structure depends on the behaviour of only the electrons which carry angular momentum rather than on the entire charge dis- tribution about the nucleus. Hypesfine structure in 0 2 , due to the magnetic movement o f 0 1 7 has already been discussed.2.3 Experimental result for b and c in this molecule give 3 COG x - 1 ( -rr)av. = - 17.8 x 1024 cm-3, and $2(0) = 1.26 x 1024 cm-3. (4) The most reasonable structure for the O2 molecules involves one unpaired p r electron about each oxygen nucleus, For an electron in a pure atomic orbital of this type, the following values are expected (l/r3)av.= 34.5 x 1024 cm-3, (3 C O S ~ X - I)av, - 2 6, (3 “0”; - = -13.8 x 1024 cm-3, $2(0) = 0. (5) The measured values (4) and the theoretical ones (5) are in good qualitative agree- ment, but clearly show some quantitative disagreement.58 STRUCTURE AND MAGNETIC EFFECTS Consider first the value of $2(0) in (4). This is only 1/40 as large as the #2(0) which would be produced by an atomic 2s electron, and has what may be said to correspond to an admixture of 2.5 % 2s wavefunction. This small amount is not very different from the amount of mixing of configurations in atomic states,4 and hence is probably not a surprising detail of the molecular wavefunction. The discrepancy in the value of (3 coszp, - ljav. appears to be definitely larger than any error in evaluating (l/r3)av.for an atomic wavefunction, and corresponds to a molecular electronic wavefunction which is somewhat more flattened than expected in a plane through the oxygen nucleus and perpendicular to the molecular axis. Addition of pa orbitals or a few more highly excited atomic states would only increase the discrepancy. Overlap effects can increase the estimate of ( 73------) given in (5), but after these have been allowed for, the experi- mental value in (4) is still about 15 % larger than the value obtained from a prr orbital. Hence, although the hyperfine structure of 0 2 indicates that the expected electronic structure of 0 2 is largely correct, it gives good evidence that wave- functions for the unpaired electrons are somewhat more flattened in a plane than expected, and that they have a small amount of s character.A similar behaviour is found in NO, which affords the advantage of determining the value of ( l / ~ 3 ) ~ ~ . directly from experimental results. Hyperfine structure of N14O has been measured for both the n& and ns states,ss697 and that for NlsO in the n+ state.* These results give, from (l), (2) and (3), 3 cos2 x - 1 av. (l/r3)av. = 14.9 x lO24cm-3, 3 cos2 x - 837 + - +(O) = -3.3 x 1024~~-3. ( r3 l)av. 3 It must be remembered that the quantity (l/r3)av, applies to the distribution of orbital angular momentum, while the other two quantities apply to the spin distribution. If it is assumed that the spin and orbital angular momentum have the same value of (l/r3)av., then the first two quantities in (6) can be used in com- bination with the last to give (7) A first approximation to the structure of NO would be a combination of the N = O (4 and -N = O+ (b) where the unpaired electron is in a prr orbit about the N or the 0 atom for structures (a) and (b) respectively.The experimental value (7) of $2(0) indicates that the unpaired electron has at least a small amount of s character. In striking similarity to 0 2 , this corresponds to about 2.5 % of the value of #2(0) for a 2s electron. If the electron is in a pn atomic orbit about N, the value of (1/r3),. should be close to 22.5 x 10% cm-3, while it would be only 0.5 x 1024 cm-3 if the electron were in a pn orbit about the 0 atom. From the experimental value (6) of (l/r3)av., the impaired electron must have a probability of 0.65 of being found on the N atom.This corresponds to 65 % importance for structure (a) if only the two structures (a) and (b) are assumed. This is not unreasonably far from previous estimates 9 of the structure of NO. $2(0) = 0.85 x 1024 cm-3. two structuresc. H. TOWNES, G . C. DOUSMANIS, R . L. WHITE, AND R. F . SCHWARZ 59 From the measured value of (ll143)~~. and ~ ~ ~ x ) a v ~ - sin2X may be evaluated as 0.9 if the variables X and Y are considered separable. For a pn- atomic orbit, sin2 X = 0-8. As with 0 2 , it appears that the molecular electronic wavefunction is not far different from an atomic wavefunction, but is distinctly more flattened in a plane through the nucleus and perpendicular to the molecular axis. Information about the electronic structure of NO from the known quadrupole coupling constant of N14 in this molecule5 is not very precise.However, it is consistent with the structure indicated above. Magnetic hyperfine structure of OH due to the magnetic moment of H has also been studied. In this case it is clear that the electronic wavefunction near the H nucleus is considerably different in the OH molecule than in the H atom. Such a result is not unexpected because of the small charge on the H nucleus. It would be interesting to know the magnetic hyperfine interactions of 017 in both OH and in NO. Another type of magnetic interaction which depends on (l/r3)av. is the fine structure. Its possible usefulness was pointed out to one of the authors by R. S. Mulliken. Atomic fine structure is given by an expression A(W7 where Zi is an effective value of 2 near the nucleus.Since this f i e structure increases so rapidly near the nucleus, one finds that the molecular fine structure for a diatomic molecule is approximately A@*$) = a2Al + (1 - (9) where a2 is the probability for the electron to be in an atomic orbital about the fist atom with fine structure constant A1, and 1 - a2 is the probability that it is on the second atom with fine structure constant A2. Expression (9) assumes, of course, that the molecular wavefunction is a combination of two such atomic wavefunctions. Relation (9) affords by no means as specific or precise information as does magnetic hyperfine structure, partly because the fine structure depends on the atomic orbitals about both nuclei in the molecule.Furthermore, it measures (&/143)~~. rather than (l/r3)av. which is given by hyperhe structure. However, it allows some interesting comparisons with other information. Table 1 shows the type TABLE LATION TI ON BETWEEN ELECTRONIC STRUCTURES OF NO, OH, AND SH AND (The fine structure constant for a neutral 0 or S is multiplied by 1-20 or by 1/1-20 when the atom is positively or negatively charged 10 FTNFl STRUCTURJJ CONSTANTS fine structure constant % g ~ $ p ~ ! ~ ~ ~ ~ observed fine structure fine structure constant in cm-1 electronic structure N = O 868 63 0 - H - 153.9 46 S - H - 4087 56 -N = o+ 185 37 123.8 -0 H+ - 127.5 54 - 139.7 -s H+ - 341 44 - 378.6 of structures for the molecules NO, OH, and SH which predict fine struc- tures in agreement with the observed values.It may be seen that the fine stxucture gives more precise information for NO than for OH or SH because of the greater difference between the fine structure of the two proposed structures.60 STRUCTURE A N D MAGNETIC EFFECTS In addition, the H has such a small nuclear charge that expression (9) may be a rather poor approximation. The percentage importance obtained for the two NO structures is in remarkable agreement with the values obtained from hyperfine s tmcture. 2. SOME OBSERVATIONS ON MAGNETIC HYPERFINE (FJ) INTERACTIONS IN 11 Interactions between the magnetic fields produced by rotation of 12 molecules and the magnetic moments of nuclei in the molecules have been subject to measurements for some time.Their interpretation is rather complex since they depend in part on I-uncoupling or excitation of valence electrons by molecular rotation. A systematic study of this type of interaction indicates, however, that some general rules about its behaviour can be stated. The magnetic field at a given nucleus in a molecule may be considered as the sum of the fields produced by motion of the valence electrons, and that due to rigid rotation of the remaining positive ions considered as point charges at the positions of the appropriate nuclei. These two sources produce fields of opposite sign because of the opposite sign of the charges involved. In a linear molecule, this energy of interaction between the molecular rotation and the magnetic moment of a nucleus may be written 11 MOLECULES where B = the molecular rotational constant, pz = the nuclear magnetic moment, po = the Bohr magneton, Lx = operator for a component of the electronic angular momentum of the valence electrons which is perpendicular to the molecular axis, r = distance from nucleus to valence electron, W, - WO = difference in energy between ground state and nth excited electronic state, qs = net charge on sth atom after removal of valence electrons, rs = distance from nucleus to sth atom, c = velocity of light, J = angular momentum in units of h due to molecular rotation, I = spin of nucleus.The first term in the brackets of (10) is due to the valence electrons and the second to the ions or rigid frame of the molecule. Again, because of the appearance of l/r3, the most important contributions to hyperfine structure are usually due to the electron distribution very near the nucleus. Furthermore, the valence electrons usually produce an effect which is much larger than that of the ions or “rigid frame ” of the molecule.It will be seen from table 2 that the valence electrons dominate in all cases except for hydrogen, where l/r3 is not very large for the valence electrons because the nuclear charge is so small. Expression (10) has the form AW= CTI J, ( 1 1 ) where CZ is a coupling constant which is a measure of the hyperfine energy. One may expect in most cases that for the lowest excited electronic states, the will be approximately the average value of l/r3 for aC . H. TOWNES, G . c. DOUSMANIS, R . L . WHITE, AND R .F. SCHWARZ 61 CII ~BPIPO (1 /r3>av. valence p electron. Therefore the reduced coupling constant CR = should be approximately equal to 2 I (O I Lx and depend on the molecular structure in a somewhat predictable way. Examination of measured values of n wz- wo CI indicates that such an expectation is correct. Table 2 lists the various hyperfine interaction constants CI for linear molecules which have been measured by the authors or by others. With the help of this table the following observations may be made. TABLE 2.-MAG"E HYPERFINE STRUCTURE OF LINEAR 1x MOLECULES DUE TO ROTATION. Theenergyis AW= CII* J. molecule ref. nucleus sign of @I CI (14s) (rigid frame) I-p2 a H + -113.90 - 203 Li7F19 b F19 + 32.9 f 0-1 -2.1 1.87 x 10-44 CsF19 d F19 + 16 + 2 -0.2 6.65 X 10-44 C135F19 f C135 + 22 + 3 -0.8 3.3 x 10-43 cs g s33 + 19 f 15 -0.6 4.1 X 10-43 T1207C135 h ~ 1 2 0 7 + 73 rt 2 + 0.6 6.35 X T1207C135 h C135 + 1.4 &O-1 -O*IO 1-1 x 10-43 017C12S32 f 017 - -40 & 1.5 2.8 X 10-43 016C12S33 f S33 + 2 r t l 1-8 x 10-43 016C12Se79 f Se79 -3.2 f 1.0 3.8 X 10-43 HCN14 i N14 + 10 & 4 3.0 X 10-43 C135CN14 f N14 + 2.5 & 0.8 5.6 x 10-43 CP*CNls f C13S + 3-5 f 0.6 2.0 x 10-43 RbssF19 C F19 4- 11 r t 3 -0.24 3.85 X 10-44 DI e I127 + 140 - 1-03 6.7 X 10-44 a Harrick, Barnes, Bray and Ramsey, Physic.Rev., 1953,90,260 b Schwartz and Trischka, Physic. Rev., 1952, 88, 1085. c Hughes and Grabner, Physic. Rev., 1950,79, 314. d Trischka, Physic. Rev., 1948,74,718. e Burrus and Gordy, Physic. Rev., 1953, 92, 1437. f R. L. White, Thesis (Columbia University, 1954).g Mockler and Bird, Physic. Rev., to be published. h Carlson, Lee and Fabricand, Physic. Rev., 1952,85, 784. i Klein and Nethercot, Quart. Report (Columbia Rad. Lab., Oct. 30, 1953). (i) The predominant contribution to Cz in all cases except for H comes from the valence electrons. (ii) Values of the quantity CR do not differ widely for different types of mole- cules or atoms. The relatively small values of this quantity for the alkali halides is probably connected with the large amount of energy required for electronic excitation of these molecules, (iii) For a given atom it may be assumed that the matrix elements (0 1 LX I n) or (0 I aL, I n) are approximately the same in a series of chemically similar mole- cules. The regular increase in CR - 2 1 (' I Lx ') Iz indicated by the value of CI for F19 in LiF, RbF, and CsF is hence evidently due to the decreasing separations w, - Wo of the energy levels which can be expected for the larger molecules. Another similar series is provided by values of CZ for C135 in CH3C1, SiH3Cl and GeH3C1, which are not listed in the table, but which show the same behaviour, n wn- wo62 STRUCTURE AND MAGNETIC EFFECTS (iv) For 017 and S33 in the same molecule OCS, one would expect CR to be essentially the same, since 0 and S are very similar chemically.The table shows that such an expectation is correct. On the other hand T1 and C1 in the same molecule TlCl and C1 and N in ClCN have very different values of this quantity, since their electronic surroundings are quite dissimilar.These differences between nuclei can give a rough measure of the distribution of orbital angular momentum in the molecule due to I-uncoupling. 3. A POSSIBLE TECHNIQUE FOR DETERMINATION OF THE SIGNS OF MOLECULAR ELECTRIC DIPOLE MOMENTS FROM ZEEMAN EFFECTS Although the orientation or sign of dipole moments in many molecules is believed known from theoretical or indirect reasoning, there seems to be no way by which the sign of a molecular electric dipole moment has been directly measured. A fairly direct determination of the signs of electric dipole moments is poss- ibk from precise measurement of Zeemaii effects and use of the technique described below, Consider as a simple example a linear molecule composed of a series of fixed charges Nse arranged along the x axis, The magnetic moment of such a system when rotating with angular momentum Q about the centre of mass is where A is the moment of inertia and xs the distance of the charge Ns from the centre of gravity.The molecular g-factor, i.e. the ratio of p in nuclear magnetons to 52 in units of h, is (1 3) NsMXs2 g = Z 7 9 S S =?A'-- A' s A' s g ' = C A' where M is the proton mass. is shifted by an amount Ax and A is changed to A', the new g-factor becomes If, now, the mass of one of the charges is changed so that the centre of gravity 2 N . s . (14) NsM(Xs - AX)2 NSMXs2 2MAX cNsxs + M(Ax)2 Since e z N s is the total moiecular charge which is zero, and e z NsXs is the molecular dipole moment Dx, (14) becomes S S gfz7-.--. Ag 2MAxDx, A eA' or NOW, if g and g' are determined by measurement of Zeeman effects, and A, A' and Ax are known from the molecular structure, expression (15) allows a deter- mination of Dx. Basically the same relation as (15) can be proved for an actual molecule.The magnetic moment of a molecule due to rotation may be described in terms of a tensor .A? of second rank. Thus the magnetic moment along a principal axes x of inertia may be written E l 3 px = Ax&% + A x y Q y + =.+?TzQz where dxx, etc., are components of the tensor and Qx, etc., are components of the angular momentum along the principal axes of inertia. Ix, etc., are the prin- cipal moments of inertia and hJx, etc., are the corresponding angular momenta.c. H . TOWNES, G . c. DOUSMANIS, R . L . WHITE, AND R . F . SCHWARZ 63 In many simple molecules the principal axes of dl coincide with the principal axes of inertia, so that expressions (16) can be simplified.The components of &referred to its principal axes have the form 12913 where eZk is the charge on nucleus k, ylc and Zk its co-ordinates with respect to the centre of mass, and m is the electron mass. Lx is the operator for electronic angular momentum and W, - WO the difference in energy between the ground state and some excited state indicated by n. Consider now the change in AXx due to a change in the centre of mass which might be produced by an isotopic substitution in the molecule. The change in 2 ’ (O I I’ may be readily obtained from the fact that the diamagnetic sus- n wn- wo ceptibility X has been shown to be independent of the origin,l4 and X has the form I where c is the velocity of light and other symbols have the same meaning as in (10).The first summation is over all electrons. Letting the new co-ordinates be Y’ = JJ + AY z~ = + aZ, we have from (17) and the invariance of X, - A x x = 2MEZk(YkAYk $. ZkAZk) k 2M - I ( 0 I ~ i A y + ziAz I O)] = - -(DyAy + DzAz), (19) i e where D, and Dz are components of the electric dipole moment due to all charges in the molecule. The electron charge e has, as usual, a negative sign. If the principal axes of & or of A d o not coincide with the principal axes of inertia, a rotation of axes is necessary which transforms these tensors in the usualy way. For simplicity, we shall assume that no such rotation is necessary in the following discussion.From (16) and (17), g, and g,’ may be easily related : This is essentially the same relation as (15), which was derived above from less rigorous assumptions. For the isotopic substitution H20 -+ HDO, changes in g-factors due to the molecular dipole moment are about 0.01, or about 2 % of the total g-factors. This appears to be just slightly larger than errors in the available measurements of the g-factors for H2O and HDO, so that a definite determination of the dipole moment sign from this small effect has not yet been obtained. For a diatomic molecule, the change in g-factor which must be detected in order to determine the dipole moment sign is from (20) approximately (21) where M is the proton mass, MI and M2 are masses of the two atoms, and r the internuclear distance.Consider as an example the molecules C12016 and C12018, for which the dipole moment has the very small value 0.1 D. In this case Ag w 0.0002. The g-factor for CO probably has a magnitude near 0.05, so that SL fractional accuracy near 11500 is needed to detect the desired effect. 2MAMI D (Ml + M2)Mler Ag w64 GENERAL DISCUSSION One may raise the objection that an isotopic substitution in CO will slightly change the intemuclear distance and the electronic wavefunctions, and hence produce another type of change in g which may mask the dipole moment effect. Similarly changes in average internuclear distance may affect the dipole sign determination from the isotopic substitution H20 -+ HDO. This type of change due to a variation in average internuclear distance alone can be measured in simple cases such as CO by determining the g-factor of CO in an excited vibrational state, In the excited vibrational state, the change in average intemuclear distance is much larger than that for isotopic substitution in the ground vibrational state. Hence any such variation in g-factor with isotopic substitution may be determined and taken into account. Measurement of the signs of dipole moments of molecules by the technique described above is not easy, but it appears practical in a number of cases. One of the authors (R. F. S.) is much indebted to Prof. J. H. van Vleck for discussion and aid. 1 Frosch and Foley, Physic. Rev., 1952, 88, 1337. 2 Miller and Townes, Physic. Rev., 1953, 90, 537. 3 Miller, Townes and Kotani, Physic. Rev., 1953, 90, 542. 4 cf. Hartree, Hartree and Swirles, Phil. Trans. Roy. Soc. A, 1939,238, 229. 5 Beringer and Castle, Physic. Rev., 1950, 78, 581. Beringer, Rawson and Henry, 6 Gallagher, Bedard and Johnson, Physic. Rev., 1954,93,729. 7 Burrus and Gordy, Physic. Rev,, 1953,92, 1437. 8 Gallagher, King and Johnson, Bull. Amer. Physic. Soc., 1955,30, no. 2, 28. 10 Dailey and Townes, J. Chem. Physics, 1955, 23, 118. 11 To~nes and Schawlow, Microwave Spectroscopy (McGraw-Hill, New York), to be 12 Eshbach and Strandberg, Physic. Rev., 1952,85,24. 13 R. F. Schwarz, Thesis (Harvard University, 1952). 14 van Vleck, Electric and Magnetic Susceptibilities (Oxford University Press, 1932). Physic. Rev., 1954,94, 343. Pauling, m e Nature of the Chemical Bond (Cornell Univ. Press, Ithica, N.Y., 1945). published.

ISSN:0366-9033    

DOI:10.1039/DF9551900056

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

64 GENERAL DISCUSSION GENERAL DISCUSSION Dr. B. Bleaney (Oxford University) said : It is important to emphasize the distinction between the amount of " s-character " introduced by hybridization in molecules in the formation of directive bonds, and that introduced in atoms through " configurational interaction ". In an atom with only one electron, such as hydrogen, there is a central field with a pure Coulomb potential (V=Ze/r) and the wave equation can be solved exactly. In a many-electron atom the potential variation is more complex and no exact solution is possible. An accurate representation of the electronic state requires the superposition of several combinations of one-electron wave functions (configurations) each of which corresponds to the same term. Thus the ground state of the Mn2+ ion, normally written as (3~)2(3p)6(345, 6S5,2, may be more correctly represented by a small admixture of the state (%)(3~)6(3d>5(4~), 6S5,2.Such an admixture has been postulated by Abragam and Pryce 1 to explain the hyperfine structure observed in the paramagnetic resonance spectrum of this ion, which would otherwise be expected to be zero, since there is no resultant orbital momentum, and a spherically symmetrical distribution of electron spin magnetism (not containing any unpaired s-electrons) would give zero magnetic field at the nucleus. Since the wave-function of an s-electron does not fall to zero at the origin, the magnetic moment of its spin 1 Abragam and Pryce, Proc. Roy. Sac. A, 1951,205, 135.GENERAL DISCUSSION 65 produces a finite magnetic field at the origin.(The difference in this respect be- tween s-electrons, and other S-terms whose wave functions vanish at the origin, corresponds to the classical result that there is a finite magnetic field at the centre of a solid pzrmanently magnetized sphere, but not at the centre of a hollow one.) Although the 3s and 4s electrons in the admixed configuration must have their spin moments parallel, the 3s electron has a much greater density at the nucleus than the 4s electron and produces most of the resultant magnetic field, giving a non- zero hyperfine structure. It should be noted that configurational interaction may give admixtures with states with promoted p or d electrops, as well as ones with promoted s-electrons, but only the latter are important as regards hyperfine structure.The effect is not confined to configurations with half-filled shells, but it is most noticeable there since it gives a finite hyperfine structure where none would be expected. In other configurations it gives a change in the size of the structure. A similar effect has been found by Heald and Beringer 1 in the nitrogen atom, which has also a half-filled shell, with the configuration (ls)2(2S)2(2p)3, 4S3/2. This atom is found to have a small hyperfine structure, presumably because of a small admixture of a (ls)2(2s)(2p)3(3s), 4S3p state. The observed size of the hyperfine structure is only about 0.6 % of that which a 2s electron would give, so that the admixture required is small. In the NO molecule, the h.f.s attributed to this admixture is some four times bigger, and thus is distinctly greater than in the nitrogen atam.A similar effect is to be expected in the oxygen atom, but no measurements have been made; it is probably again rather smaller than that ob- served in the oxygen molecule. Prof. C. A. Coulson (Oxford University) said : The work of Pryce and Eisenstein, described by Dr. Bleaney, provides an answer to a question of much interest to 0 FIG. 1 .-Co-ordination of NO3 groups around the uranyl axis U89+ in uranyl nitrates. All three NO3 groups lie in the equa- torial plane shown, and the dots indicate possible partial covalency. I 0 chemists : do f electrons take part in bonding? On the basis of orbital bond strength calculations by Pauling, they would be expected to do so strongly, since Pauling’s strengths of s, p , d and f orbitals are 1, 4 3 , 4 5 and 2/7 respectively, for o-type bonds.Yet many of the chemical and spectroscopic properties of the rare-earths and the transuranic elements suggest that the f-electrons are really “ inner ” electrons unsuited to the formation of normal chemical bonds. Now, however, Dr. Bleaney shows that 5f electrons must be involved in bonds. It is interesting to point out that Dr. G. R. Lester and the present writer have obtained quite alternative (unpublished) evidence pointing to a similar con- clusion. Chemical measurements by Glueckauf and others show plainly that in solution the uranyl group U022+, which is linear, possesses a marked ability to co-ordinate six atoms (usually oxygen atoms from surrounding water or nitrate groups), and these six atoms lie either exactly or nearly in the equatorial plane defined by the U02 axis (fig.1). This regular geometrical arrangement argues strongly for directional properties such as those to be associated with chemical 1 Heald and Beringer, Physic. Rev., 1954, 96, 645. C66 GENERAL DISCUSSION bonds. But no central atom can form six simultaneous bonds of this kind without using f atomic orbitals, and to the extent, therefore, that these may be called ’‘ bonds ”, we are led to suppose that f electrons participate in forming them. But it seems as if the conventional language of bonds is not really adequate, and we have a situation in which the old-fashioned electrovalence and the more modern covalence are merging into one.As Griffiths and Owen have shown, many molecular complexes may be regarded as essentially ionic though the polarization of the migrating electrons is sufficiently great as to correspond to a partial covalence. But as Lester and the present writer have shown in their work on the uranyl nitrate complexes, the hybrids from the uranium atom which may be used in forming the a-bonds to the equatorial ligands have exceedingly strongly directed density patterns, such as that whose polar diagram is shown in fig. 2. This is even more directed than the familiar sp3 tetrahedral or the sp3d2 octahedral hybrids of Pauling. Now an elec- tron in an orbital of this kind has its mean centre of position at some place such as P in the figure. In this it differs from a pure unhybridized s, p , d, .. . orbital whose centre is at the nucleus. One way of describing this is to say that an electron in a hybrid orbital such as fig. 2 already confers a partial ionic character on the ‘‘ bond ” to the appropriate ligand. Thus starting with electrons on the central atom we find their orbits moving towards the outer atoms: alternatively starting with electrons on the outer atoms (ionic model) we find their orbits creeping in towards the central atom. We have a situation in which electrovalence and covalence each have character- istics conventionally associated with the other. Dr. D. J. Millen and Mr. K. M. Sinnott (University College, London) (cam- municated) : In the interpretation of the microwave spectrum of nitryl chloride, NOzCI, we have used an extreme case of one of thc methods described by Prof.Gwinn for establishing planarity. By making use of the nuclear quadrupole fine structure, a number of lines were assigned to low J transitions. The rotational constants of the molecule were evaluated and the positions of further lines cal- culated. In no case could a line be found corresponding to a transition involving states whose rotational eigen-functions would be antisymmetric with respect to the operation C f . This is consistent with these states having a statistical weight of zero. The only structure which satisfies this requirement is planar \N-CI with C2v symmetry. For this structure the operation C2a interchanges the oxygen nuclei, which have zero nuclear spin. Prof. C. H.Townes (Columbia University) said : It seems to me rather difficult to be sure from theoretical arguments whether deuterium or hydrogen should have the higher barrier in nitromethane. The effect of the several normal vjbra- tional modes on the barrier height gives some of difficulty. In addition, the barrier height measured, which depends on cos 66, represents a small difference in the main features of the barrier for one hydrogen. On the other hand, Prof. Gwinn’s experimental measurements of barrier height for the two cases are, of course, quite definite. Dr. W. J. Orville Thomas (Aberystwyth) said : In answer to Prof. H. C. Longuet- Higgins, centrifugal distortion constants, obtained from microwave data, can be combined with infra-red measurements to yield information about the structure FIG.2.-polar diagram of Possible SY P, d, f hybrid orbital. 0 O/GENERAL DISCUSSION 67 of valence bonds. In principle, information concerning force constants (including interaction constants) is given by the displacement of rotational lines due to centrifugal distort ion. It is hoped in the near future to obtain the infra-red spectrum of methyl mercury chloride and to assign frequencies to the stretching modes of the linear 6-Hg-C1 grouping. If the methyl group is treated as a single particle it will then be possible to obtain the allowed solutions for the two bond-stretching force constants f(C-Hg), f(Hg-Cl) and the bond-bond interaction constant f(CHg/HgCl), occurring in the potential function. The allowed solutions for the force constants represent an i w t e number of sets for the three force constants. Using the known value for the centrifugal distortion constant Dij it might be possible to determine which set of force constants reproduces the infra-red data and the microwave data best thus yielding an explicit solution for the force constants. It is clearly important, then, to obtain accurate values for the centrifugal distortion constants of molecules since although at present the calculations are difficult a certain amount of progress is being made in utilizing this comparatively new source of information about the behaviour of valence bonds.

ISSN:0366-9033    

DOI:10.1039/DF9551900064

出版商:RSC    

年代:1955

数据来源: RSC

摘要:

IPa. MICROWAVE ABSORPTION THE USE OF MICROWAVES IN THE STUDY OF IONIC AND CHEMICAL EQUILIBRIA AT. HIGH TEMPERATURES BY T. M. SUGDEN Dept. of Physical Chemistry, University of Cambridge Received 1st February, 1955 An account is given of the methods available for measuring the concentration of free electrons in gases at high temperatures by the absorption of microwave radiation. The applications of these to the study of ionic and chemical equilibria in the hot gases produced by burning various combustible mixtures are described. The simplest problem dealt with is that of the thermal ionization of the alkali metals. This is modified by the inter- action of the metal and of free electrons with the free hydroxyl radicals in the hot gases to produce gaseous alkali hydroxides and negative hydroxyl ions respectively.These reactions may be elucidated and the thermochemical data characteristic of them deter- mined, Introduction of halogens into flame-gas sys tems containing traces of alkali metal gives rise to halogen ions and gaseous alkali hydroxides, whose equilibria may be studied: Brief accounts of other work, on the ionization of alkaline earths, and on the electrons liberated by carbon particles in luminous flames, are given. The purpose of this introductory paper is to outline the basis of the application of microwaves to the study of some chemical reactions in the gas phase at high temperatures. Subsequent papers in this section will deal with two particular aspects-the ionization of the alkaline earths, and the electron affinity of the hydroxyl radical.The principle of the measurements is very simple, in that the absorption of microwave radiation by free electrons is used to obtain the concentration of these electrons in gases at high temperature, and inferences made from the results concerning reactions jm which either electrons enter directly as a component, or in which one of the components also takes part in reactions involving them. Examples of these are the simple ionization of an atom of inetal A A % A+ -I- E , and the formation of the hydroxide of a inetal AOH + A + OH. In the latter case, the amount of metal available for the direct ionization is effectively reduced by removal of a proportion of it as the hydroxide. The techniques are somewhat different from those of conveiitional microwave spectro- scopy, in that careful estimates of attenuation are rcquired, whereas frequency measurements are of much less importance.Most of the work in this field has been carried out at Cambridge, where it has been especially applied to flame-gas systems, i.e. systems in which a medium for the study of thermal ionization at high temperatures is obtained by burning a combustible mixture in a suitably designed apparatus. These systeins will be described briefly, followed by an outline of the microwave methods used, and a discussion of the problems which may be studied, with some remarks on the scope of the results. 68T. M. SUGDEN 69 FLAME-GAS SYSTEMS In order that the interaction of microwaves with free electrons in hot gases (> 1500" K) may be studied in the laboratory, it is necessary to produce a zone of such gases which is reasonably extended (a few centimetres) and uniform.One very convenient method of doing this is to burn a mixture of combustible and air (or oxygen) in a M&er type of burner, in which the main reactions of combustion take place in small cones of primary reaction a few millimetres in height at the surface of the burner. Above these cones, there is an extended region of hot gases in which a marked degree of thermodynamic equilibrium obtains between the various chemical components, in contrast with the state of affairs in the primary cones. This hot gas gradually cools by entrainment of the surrounding air, although local rises of temperature at the boundaries may occur from burning of excess fuel. Even if these effects are for the moment neglected, the temperature of the burned gas is somewhat different from that calculated on the basis of thermal equilibrium determined solely by the release of heat of the various reactions of combustion involved, on account of heat losses to the burner, for the greater part.The most reliable estimate of temperature can be obtained by the method of sodium D-line reversa1,l which is now thoroughly established as suitable for burned gases at atmospheric pressure. It depends upon the establishment of thermal equilibrium between unexcited 2s atoms of sodium and atoms in the first excited (2P3p9 112) states when a trace of sodium is added to the gases. Good agreement is obtained between temperatures ob- tained by use of the resonance lines of other alkali metals, but sodium is the most convenient to use.The composition of the burned gas can then be calculated from the lmown composition of the fuel and the measured temperature. Thus, if hydrogen is used as fuel, with insufficient air for complete combustion, the burned gas consists mainly of N2, H2 and H20, with up to 0.1 % of free OH radicals and hydrogen atoms, with very minor traces of atomic oxygen and nitrogen. The amounts of these minor constituents may be calculated from the data given in standard works, such as that of Lewis and von Elbe,2 for the equilibria H20 % OH + 4H2, M2% 2H, etc. It will be seen later that the hydroxyl radicals and hydrogen atoms often play important parts in reactions affecting the amount of ionization.In the earlier work in Cambridge,3~ 4 coal-gas was used as fuel, but later work has been done with hydrogen.5-9 Other workers have used acetylene and hydrogen.10 With the gas mixtures usually employed (< 2500" K), the ionization potentials of the normal constituents are too high (> 10 eV) for sensible production of free electrons to occur, and therefore for marked attenuation of microwaves, but an amount of ionization suitable for study may be obtained by the addition of traces of easily ionizable metals, or their compounds. The simplest of these are the alkali metals. They may conveniently be added as traces of fine spray of salt solutions from an atomizer operated by the supply to the burner. It has been found that in most cases all salts of a given metal used lead to the same ionization, so that the anions must be completely broken up or reacted with the bulk of the gases, the salt being merely a vehicle for conveying the metal into the flame.The additive can readily be provided in the range of 1 in 105 to 1 in 109 of the total gases. In order to circumvent the difficulties of inhomogeneity in the column of gases, various devices are used. One useful method is to surround the test flame, to which salt is added, by another flame of the same composition, but without salt. Thus, burning of excess fuel with entrained air occurs in the outer zone, without affecting the salted gases in the first few cm of height. In this way, it has been found possible to obtain columns of burned gas in the inner region in which the temperature does not vary by more than f 10" C over a height of about 12 cm, and over a cross-section of 1.5 cm diameter.Another method 5 9 9 is to surround70 IONIC A N D CHEMICAL EQUILIBRIA the salted flame with a slow stream of nitrogen, which prevents entrainment of air until a suitable height has been reached. This gives a boundary, now con- taining the additive, in which there is a rapid fall of temperature outwards, but since the amount of ionization falls rapidly with decreasing temperature, the effects introduced are not serious. Fuel-rich mixtures are used in general since it is found that these give more uniform conditions in the burned gas. THE RELATION BETWEEN ELECTRON CONCENTRATION AND CONDUCTIVITY The absorption of microwave radiation by gases containing free ions occurs because the ions acquire directed momentum by interaction with the electric field of the radiation, which is then randomized by collisions between the ions and molecules of gas.This effect, which determines the electrical conductivity Qf the gases, will be a function of three variables, the number of ions per cm3, the frequency of the radiation, and the collision frequency of the ions with molecules. Simple electromagnetic theory shows that the dielectric constant and the conductivity may be expressed in terms of these by c = 1 - - 477ne2( ~ 1 ); ( T = . - n:( - 01 ) m o2+u12 o2+w12 ’ in which n is the number of ions per cm3 of mass rn and charge e. o = 2~ (fre- quency of radiation) and w i is the collision frequency of an ion with molecules.It can be seen that the conductivity will be determined almost entirely by electrons on account of their small mass, unless there is an overwhelming preponderance of atomic and molecular ions, which is never the case in this work. These relationships are only of an approximate nature, incorrect assumptions having been made in their derivation. In particular, the effect of collisions on the ions has been replaced by a viscous force proportional to their velocity. Much more complicated formulae have been derived by Margenau 11 using accurate kinetic methods, but provided that the electrical field of the radiation is weak, so that it does not alter the Maxwellian distribution of velocities among the electrons in thermal equilibrium with the rest of the gases to an appreciable extent, the differ- ence between his more complicated formulae and the simple ones above is very slight. In particular, experimental studies of the variation of (T with the frequency of the imposed radiation w have been shown to obey a law of the form given by the simple formula.3 These studies also lead to a value for the collision frequency w I , the values obtained being of the right order of magnitude from the point of view of simple kinetic considerations.Theory is not able to provide accurate quantitative predictions of the collisional cross-section of thermal electrons with molecules of the types occurring in flame gases,12 and thus give good theoretical values of w1, but the experimental values appear to be very satisfactory.On this account, the use of measured (T as a measure of relative numbers of electrons is more accurate than its use as an absolute measure, but the latter is still probably correct within a factor of two. It will be seen below that the very simple behaviour given by sodium in its ionization can often be used in calibration, thereby avoiding some of the difficulties. THE EXPERIMENTAL MEASUREMENT OF ELECTRICAL CONDUCTIVITY Most early work on the electrical conductivity of flame gases has been done by methods which involved the insertion of electrodes or probes in the gases (for a summary, see ref. (13)). This is unsatisfactory, on account of the disturbances introduced, and the measurement of the conductivity at microwave frequencies, where no solid object need be inserted in the gases, seems much more satisfactory, although measurements at intermediate frequencies have also been found to be useful.5 Early, but not very accurate, comparisons of the microwave and d.c.conductivity methods by Andrew, Axford and Sugden 14 in a transient flame showed compatibility between the two sets of results.T . M. SUGDEN 71 Two basic experimental methods have been adopted. The first consists of measuring the attenuation in dblcm of a beam of microwaves in passing through a known thickness of flame gases, usually from a burner of rectangular cross-section. The coefficient x of reduction of the electric field of the radiation in passing through conducting gases is given by where p is the magnetic permeability (which may be set equal to unity) and c is the velocity of light in vacuo.x is related to the attenuation of microwave power per cm /I by /I = 8.7~. At centimetric frequencies, and with the values of electron concentration and collision frequency usually encountered in flame-gas systems, the dielectric constant does not depart appreciably from unity and (4n0/w)* < 1, so that a simple binomial expansion may be performed, leading to Thus, knowing w and wl, the number of electrons cm-3 may be found from a measurement of /3 in db/cm. In a given flame p is directly proportional to n. In the experimental arrangement, microwave radiation from a klystron oscil- lator (usually in the 3 cm or 1.25 cm band) passes through an attenuator of at least 20 db to the column of flame gases, placed between suitably designed horns.It reaches a rectifying crystal via another, similar, attenuator. To facilitate amplification of the detected signal, the output of the klystron is usually modulated by a low frequency square wave. The line must be carefully tuned to eliminate standing waves. The flame gases are made attenuating by the introduction of the additive to be studied, and this attenuation measured by removing the additive, and introducing corresponding attenuation either with a calibrated rotary vane microwave attenuator, or, since the effect is practically purely a resistive one, with a calibrated resistive attenuator placed beyond the crystal. This type of system, with flames about 1 cm thick, is suitable for electron concentrations in the range of 1010 to 1012cm-3. The proportionality with attenuation does not hold for higher values, and the method is too insensitive for lower ones.Its sensitivity has been improved by at least a factor of 10 by using differential systems by Shuler and Weber,lo who employ a method of measuring the change in standing-wave ratio at a crystal fed by two beams, one of which passes through a flame, and by Page,g who has used a double-beam method in which the micro- wave power from two crystals, one of which only is affected by the flame, is balanced by a calibrated differential resistive attenuator. These modifications overcome troubles arising from long-term fluctuations in klystron output. The second method of measuring the electrical conductivity, and hence the concentration of electrons, is to measure the change in Q due to dielectric losses when a conducting column of flame gases is introduced into a cavity resonating at microwave frequencies.This method was first used by Adler 15 to study ion- ization in discharge tubes, and first applied by Sugden and Thrush 16 to a flame problem. It may readily be shown that if Qo and Ql are the values of the Q of such a cavity in the absence and presence of a column of conducting gas of conductivity o occupying part of the cavity respectively, then where g is a numerical factor determined by the dimensions of the cavity and those of the flame, and by its situation inside the cavity. In a simple disposition, such as a cylindrical flame concentric with a cylindrical cavity, this factor takes simple analytical forms.For example, in a TEo, 1,1 mode it is given for this disposition72 IONIC A N D CHEMICAL EQUILIBRIA by a2[- Jo(ka)J2(ka)J/c2([J1(kc)J2 - Jo(kc)J&c)>, where a is the radius of the cavity, c that of the column of flame gases, ka = 3.83 and J’s are Bessel functions of the first kind. For more complicated arrangements g may be estimated by graphical integration. Experimentally, a cylindrical cavity, coupled by irises to a klystron and to a waveguide system suitable for measurement of the transmitted power, is tuned by a plunger to resonate in the TEo,~, 1 or other convenient mode. The frequency of the klystron is swept by a low frequency saw-tooth voltage applied to its reflector so as to cover the resonance characteristic of the cavity.The change of Q on introduction of the conducting column of gases is measured from the changes in form of the characteristic. This method is more sensitive than the simple attenuation one, being able to deal with electron concentrations down to 108 cm-3. The principal experimental drawbacks are connected with introduction of hot gases into such a system. This has been done 16 by making holes of sufficient size in the two ends of the cavity, and preventing the escape of radiation from them by covering them with very coarse gauzes of thick platinum wires, spaced about 1/10 wavelength apart. The column of hot gases may be admitted inter- mittently by interrupting it with a sector disc rotating a few times a second placed between the burner and the lower gauze.This prevents overheating of the gauzes, which tends to distort them and lower Q. The whole cavity may be water-cooled, but not to a sufficient extent to cause condensation of moisture on its inner surface. Another arrangement which has been used is actually to incorporate the burner in the lower end of the cavity. Although this means that the primary cones of combustion are inside the cavity, they are in a region of low electric field where electrons have little effect on the resonant characteristics. In this case, the com- parison is made between “ clean ” (non-conducting) and “ salted ” flame gases. The results for the numbers of electrons cm-3 are usually converted for chemical purposes into concentrations expressed as atmospheres of partial pressure at the measured temperature of the gases, and are then denoted by [ E ] .THE IONIZATION OF ALKALI METALS IN FLAMES Since this problem has received a good deal of study in the past, and since many of the results which have been obtained for it using one or other of the microwave methods illustrate well the kind of information which can be derived, it will be discussed briefly. The nature of the anion of the salt of an alkali metal A having been established to be of no consequence in determining the con- centration of electrons produced, the equilibria which are considered to be im- portant in flame gases are the following : A % A+ + E ; K = [A+][e]/[A], K’= [A][OH]/[AOH], AOH % A + OH; OH- % OH + E ; K” = [OH][E]/[OH-]. To these equations must be added that for charge balance [A+] = [el + [OH-], and the mass balance for A [Ad = [A1 + [AOHI + [A+].For small ionization, which is often found below 2200” K, the last term of the last equation may be ignored. [Ao] represents the total alkali metal added (free or combined), Solution of these equations with small ionization leads to Eel2 = KCAol/(l + COHIIK’)(1 + [OHllK”), or [El2 = K[Aol/(l + + 49T. M. SUGDEN 73 in which $ = [OH]/K’ and $’= [OH]/R”. The value of [OH] will be held constant by the buffering action of the bulk of the flame gases. This law has been found to hold good over a wide range of conditions. It must be emphasized that the equilibria set out do not necessarily indicate the actual processes by which such equilibria may be reached and maintained. The actual processes must fulfil certain kinetic conditions, namely, that there shall be sufficient effective collisions for near equilibration in the time available between the burner and the measuring system.This is of the order of a few milliseconds, during which, at a pressure of one atmosphere, and at about 2000” K, a molecule will make about lO7collisions with others, and an electron about lo9 with molecules. In a bimolecular reaction between simple molecules the proportion of effective collisions is given by exp (- E/RT), where E is the energy of activation of the reaction. Thus to satisfy the required conditions, a reaction between a molecule containing one atom of alkali metal and a major constituent of the gases must have E < 50 kcal, and a correspondingly lower value of E if the flame-gas con- stituent involved is a minor one such as OH or €3.This clearly rules out the for- ward processes of all the equilibria set up above, since the ionization reaction X + A -+ X + A+ + E , in which X is any molecule present, requires E to be in the region of 100 kcal. Similar considerations apply to the decomposition of the hydroxide by X + AOH -+ X + A + OH, and also to the decomposition of OH-, if the electron affinity of the OH radical is as high as 65 kcal.9 It is possible however, to find a set of processes which fulfil the required conditions. They are X + A % X + A * A* + OH + A+ + OH- A+H20 % AOH+H OH- + H % H20 + E. The first pair of reactions is the formation and deactivation of excited atoms of alkali metal in the resonance state, involving not more than 50 kcal in the forward reaction in all cases, and which is known to be equilibrated from the consistency of line reversal measurements of temperature made on the various resonance lines.The second of these pairs involves heats of reaction in the neighbourhood of 10-20 kcal, and will involve little, if any, energy of activation in the exothermic direction, since atoms or free radicals take part in the reactions. The third pair is very similar to the second in this respect. The back reaction in the last pair will be endothermic to the extent of about 50 kcal. Again, its energy of activation will not be very much larger than this, and in any case, electrons make many more collisions than molecules. Thus there is ample scope for establishment of chemical and thermal equilibrium between A, AOH, OH- and E .The predicted variation of [el2 cc [Ao] has been found in many instances (see, e.g., 5). The change of the simple atomic ionization with temperature is given by the thermodynamic equation of Saha 17 where Y is the ionization potential in electron volts, T is in OK, and K is in atm. Thus, if the effects of hydroxide and hydroxyl ion formation are slight (both I# and I#’ < l), a plot of the logarithm of the proportionality constant between [el2 and [Ao] against 1/T for a given metal in a series of flames should be a straight line of slope - 5050 V. Similarly, a plot of this logarithm against Vfor various metals in the same flame should be a straight line of slope - 5050/T. Neither of these predictions is found to be the case,6 and it has been concluded that the hydroxyl effects are important.The way in which these effects arising from the formation of alkali hydroxide and of hydroxyl ions have been worked out will74 IONIC A N D CHEMICAL EQUILIBRIA be outlined, and followed by brief accounts of the other systems to which the methods have been applied. THE STABILITY OF THE ALKALI HYDROXIDES IN THE GAS PHASE Measurements of the relative stabilities of the hydroxides can be made by studying the ionization of various alkali metals in a particular flame, which will give results independent of any hydroxyl ions formed. The ratio of electron concentrations for the same total amount of two metals added to a given flame is where $ = [OH]/K’, and K’ is the equilibrium constant of AOH % A + OH.There is sufficient thermodynamic evidence available to show that, under flame-gas conditions, NaOH is formed in negligible amounts, on account of its low heat of formation from Na and OH. Hence rpNa -0, and $ can be deduced for the other alkali metals, and therefore K’. This equilibrium constant will take the form K’ = Cexp (- AE,/RT), where C is a function of the properties of the molecules (masses, interatomic distances and frequencies of vibration) taking part in the equilibrium, and is relatively independent of temperatures, and A E ~ is the heat of reaction at 0°K. C may be estimated with fair accuracy from known data for the atom A and for OH, together with reasonable assumed properties of the gaseous molecules AOH, and thus AEo calculated from the measured K‘.This has given heats of formation for CsOH, KOH and LiOH from the corresponding atoms and OH radicals in the gas phase of 91, 86 and 102 kcal/mole respectively. These fall in the same order as the gaseous alkali fluorides and chlorides, with the same differences between the metals. On this basis, NaOH may be concluded to have a heat of formation of 81 kcal/mole, which justifies the original assumption about its instability. When &i is of the order of 10, as is usually the case, qhNa < 0.1. An important pojnt is that values of AEo obtained in this way, i.e. from absolute values of K’, and hence on the basis of the third law of thermodynamics, agree with values obtained from the variation of K’ with temperature (by com- paring various flames), this being an independent method based on the second law of thermodynamics.This consistency is regarded as a valid test of the approach adopted to problems of this type. THE ELECTRON AFFINITY OF HYDROXYL 9 As has been seen above, the electron concentration given by alkali metals in flame gases may also be influenced by the formation of hydroxyl ions. The earlier work on microwaves has suggested that this OCCU~S,~, 6 and has led to a value for the electron affinity of OH of about 62 kcal/mole. A much more elaborate study of this effect has been made by Page, and is described in a subsequent paper in this Discussion.9 It is based largely on the behaviow of sodium, whose hydroxide can be ignored, leading to [el2 = “aoll(1 + [OHIIK”), where K” is the equilibrium constant of OH- S OH + E.The main contribution made by Page has been to use various initial combustible mixtures which produce flame gases at the same temperature, but wi.th different values of [OH]. The above law is shown to be obeyed, and K” deduced at various temperatures. Consistency is obtained between electron affinities deduced from absolute K” and its variation with temperature, to give a value of 65 f 1 kcal/mole for this important thermochemical quantity. The method therefore supplies a new technique for the determination of electron affinities, in addition to those reviewed recently by Pritchard.18 It is currently being applied to other substances,T. M . SUGDEN 75 THE ELECTRON AFFINITlES OF THE HALOGENS AND THE HEATS OF FORMATION OF GASEOUS ALK,ALI HALIDES The introduction of about 0.1 % of a halogen into the flame-gas supply produces a significant reduction in the electron concentration produced by traces of alkali metals (< 0.01 %).Unpublished work of Smith and Sugden indicates that this should follow a law where K2 and K3 are the equilibrium constants of the reactions AY % A + Y, Y- % Y + E respectively, in which Y is an atom of halogen and A one of alkali metal. [YO] is the total halogen present, expressed as a concentration of atoms. The parameter 8 = [HI/&, where K4 is the equilibrium constant of the dissociation of the halogen acid HY, which may be formed in significant amounts. [q] is the electron concentration in the absence of halogen. This equation, which is found to be obeyed in form, may be applied in different ways, depending on which of the six equilibria which govern it are best known.The second term on the right-hand side is independent of the alkali metal used in a given flame. It can be seen that it offers a new way of determining the electron affinities of the halogens, and the heats of formation of the alkali halides in the gas phase. THE IONIZATION OF ALKALINE EARTHS The ionization of this group of elements in flame gases presents a much more difficult problem than do the alkali metals. The law [el2 (total concentration of added alkaline earth) is not obeyed. It is discussed in the next paper in this Discussion 8 where it is shown that positive ions such as (BaOH)-l- are likely to be very stable, and much more important than Baf.Evidence for negative ions containing alkaline earth atoms is obtained. A very peculiar feature 19 is that addition of extremely small amounts of alkaline earths to flame-gas systems containing alkali metals in reasonably larger quantities causes a marked increase in the concentration of free electrons, to values much greater than would be expected from the separate ionizations. This effect is very specific to given pairs of metals, and indicates a specific interaction between them in the gas phase. Since the alkaline earths exist overwhelmingly in the form of the diatomic oxides such as BaO in flame gases,20 it is considered that the observed effects spring from the formation of a compound positive ion such as (BaONa)+, with basically electrovalent structure Ba2+02-Na+, formed from BaO and Naf.Calculations of the energy changes of this reaction, based on the methods outlined by Rittner,zl show that this is a reasonable assumption. IONIZATION IN “ PURE ” FLAME GASES At the higher temperatures realizable with flame gases (2500-3500” I<), the number of electrons which can be provided by ionization of the normal con- stituents of the flame gases becomes increasingly significant, and has recently been studied by Shuler and Weber 10 for hydrogen flames. The great experi- mental difficulty arises from the ionization of adventitious traces of alkali metals in the gases, which make it very difficult to estimate the residual ionization, The results, however, are not inconsistent with what would be expected.A much more accessible problem is that of the free electrons produced by ionization from particles of carbon such as are present in acetylene flames. Graphite has a work function22 in the region of 4eV, which is low enough to give a measurable ‘‘vapow pressure” of electrons at ordinary flame-gas tem- peratures. This has received preliminary studies by Sugden and Thrush 16 using the cavity method, and more recently by Shuler and Weber 10 using a refined76 IONS OF ALKALINE EARTHS attenuation system. Their measurements indicate a variation of ionization with temperature requiring a rather high work function (- 8 eV). Further study of this important phenomenon is desirable, particularly with regard to the size of the particles, and to the marked increase of ionization which sets in a t the sharp boundary between luminous flames (which contain particles of carbon) and non-luminous ones, in that light might be thrown on the vexed question of how these particles are formed. 1 Gaydon and Wolfhard, Flames, Their Structure, Radiation and Temperature (Chapman 2 Lewis and von Elbe, Combustion, Flames, and Explosions of Gases (Academic Press, 3 Belcher and Sugden, Proc. Roy. SOC. A,, 1950, 201,480. 4 Belcher and Sugden, Proc. Roy. SOC. A , 1950,220, 17. 5 Smith and Sugden, Proc. Roy. SOC. A , 1952,211, 31. 6 Smith and Sugden, Proc. Roy. SOC. A , 1952,211, 58. 7 Smith and Sugden, Proc. Roy. SOC. A , 1953, 219, 204. 8 Sugden and Wheeler, this Discussion. 9 Page, this Discussion. 10 Shuler and Weber, J . Chem. Physics, 1954, 22, 491. 11 Margenau, Physic. Rev., 1945, 69, 508. 12 Mott and Massey, The Theory of Atomic Collisions (Oxford University Press, 1949). 13 Wilson, Modern Physics (Blackie and Co., London, 1944). 14 Andrew, Axford and Sugden, Trans. Faraday SOC., 1948, 44, 427. 15 Adler, J . AppI. Physics, 1949, 20, 1 125. 16 Sugden and Thrush, Nature, 1951, 168, 703. 17 Saha, Phil. Mag., 1920, 40,472. 18 Pritchard, Chem. Rev., 1953, 52, 529. 19 Sugden and Wheeler, to be published. 20 Huldt and Lagerqvist, Ark. Phys., 1950, 2, 333. 21 Rittner, J . Chem. Physics, 1951, 19, 1030. 22 Reimann, Proc. Physic. SOC., 1938, 50,496. and Hall, 1953). Inc., New York, 1951).

ISSN:0366-9033    

DOI:10.1039/DF9551900068

出版商:RSC    

年代:1955

数据来源: RSC