摘要:

J . Chem. SOC., Faraday Trans. I, 1984,80, 225-235 Spectroscopic Studies of Hydrogen Adsorbed on Zinc Oxide (Kadox 25) BY JOSEPH HOWARD* AND IAN J. BRAID Chemistry Department, University of Durham, South Road, Durham DH 1 3LE AND JOHN TOMKINSON Neutron Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX Received 13th June, 1983 The vibrational spectra (320-2230 cm-l) of ZnO and of hydrogen adsorbed on ZnO have been obtained by incoherent inelastic neutron-scattering spectroscopy (INS). The v, and v6 modes of type I Zn-H surface species are observed at 829 and 1708 cm-l, in accord with published i.r. data. The recent assignments by Boccuzzi et a f . of the v6 mode of type I 0-H species aIld the v,, mode of type I1 species to weak i.r. bands at ca.850 and 1475 cm-l, respectively, are not corroborated by the INS results. No resolved bands assignable to type I1 species are observed. The INS spectrum of ZnO contained a previously unreported, intense feature at 1346 cm-l which was unchanged on hydrogen adsorption. This feature is assigned to impurities within the bulk. Raman and i.r. data of ZnO are also discussed. H / \ Zn Zn There have been numerous studies of zinc oxide and of hydrogen adsorbed on it.l Although these systems are complex, four main types of hydrogen adsorption processes have been identified.l Type I adsorption is rapid and reversible at room temperature and gives two i.r.-active species, ascribedl to surface Zn-H and 0-H species. The stretching modes of these species are generally accepted to occur at ca.1710 and ca. 3490 cm-l, respective1y.l Recently, the bending modes were assigned by Boccuzzi et aL2 in the i.r. spectrum of hydrogen adsorbed by ZnO to a band of moderate intensity at 817 cm-l [vg(Zn-H)] and to a broad, weak feature at ca. 845-850 cm-l [va(O-H)]. Type I1 adsorption is irreversible at room temperature and was first reported by Boccuzzi et aL2 to give weak, broad i.r. bands at 1475 and ca. 3400 cm-l, assigned to / \ and 0-I+---0 bridged species, respectively. H Zn Zn Type 111 adsorption occurs at low temperatures (77 K), giving rise to i.r.-active species, and is reversible and non-dissociative. The internal modes and vibrations relative to the ZnO surface of type I11 hydrogen occurs outside the frequency range of our incoherent inelastic neutron-scattering (INS) experiment^.^ Type I V adsorption occurs principally at high temperatures but also, to a lesser extent, at room temperature.The purpose of the present work, which was motivated by statistical thermodynamic studies of the same system, was to observe and assign the normal modes of the types I and I1 species using a combination of i.r., Raman and I N S spectroscopies. 225226 SPECTROSCOPIC STUDY OF H, ON ZnO EXPERIMENTAL The INS measurements (320-2230 cm-l) were made at 77 K using the INlB spectrometer at the Institut Laue Langevin in Grenoble in its beryllium filter detector mode.4 Data were normalised to constant monitor counts and it has been shown that the resultant spectrum is proportional to the amplitude-weighted density of states multiplied by the Debye-Waller f a ~ t o r .~ The transition frequencies have been calculated from the positions of the band maxima using standard correction factors.6 For all of our measurements the ZnO used was Kadox 25 (obtained from the New Jersey Zinc Co.), which is produced by burning zinc in air. For the INS experiment the ZnO was calcined, in a silica tube, by heating to 723 K in a static, but periodically refreshed, dry-oxygen atmosphere. The sample was then cooled to room temperature under oxygen and subsequently evacuated to 7 x Torr (1 Torr = 133.3 N m-,). The ZnO was then transferred, under vacuum, to a thin-walled cylindrical aluminium cell (3.8 cm diameter) via a glass-metal seal and the cell sealed off from the vacuum line. After the INS spectrum of the ZnO had been obtained hydrogen was admitted to the cell via a glass break-seal and the INS spectrum of the ZnO + H, obtained.Hydrogen (99.9995%, Masonlite, Chatham) was adsorbed by exposing the sample to 390 Torr of H, for 10 min followed by cooling of the sample to 77 K. This method is expected to produce7-'0 types I, I1 and 111 surface hydrogen with concentrations in the order type 111 > I1 > I. However, type 111 is thought to be molecular3 and so will probably not contribute bands to the spectral region measured in the INS experiments. The volume adsorbed was 85 cm3 and this quantity is calculated to scatter 1.4% of the incident neutron beam. The ratio of the incoherent cross-sections of the adsorbed H to that of the ZnO is 8.8 to 1.0 (at an incident neutron energy of 204 cm-l).Before pretreatment the catalyst had a surface area of 10 m2 g-' (N,, B.E.T.) and afterwards a surface area of 7.5 m2 g-l. This is in agreement with the results of Baranski and Cvetanovic." Using these values the uptake of hydrogen in our INS experiment corresponds to minimum surface coverage, 8, of 0.7 and maximum of 0.8 monolayers. We have obtained the i.r. spectra (4000-750 cm-l) of Kadox 25 during and after a calcination procedure similar to that used to prepare the INS samples. In the i.r. experiment, the sample was heated in 0, (10-20 Torr) to a maximum of 725 K. At various temperatures the sample was evacuated to ca. 5 x lop5 Torr and the i.r. spectrum then recorded, followed by re-admission of 0, to the ZnO and further heating.The i.r. spectrum was also recorded at 305 K after cooling in U ~ C U O at the end of the calcination procedure. 1.r. data were obtained using a Perkin-Elmer 580B spectrophotometer with data station. The sample was in the form of a self-supporting disc and the heating, adsorption and evacuation operations were carried out in situ with the sample in an all-metal infrared cell equipped with KRS5 windows. Because, for the self-supporting disc, the sample was totally absorbing below 700 cm-l, a spectrum of ZnO diluted with KBr was also obtained so that the region 700-300 cm-' could be studied. The Raman data were obtained using a Cary 82 spectrophotometer and the 514.5 mm line of an argon laser. No pretreatment of the sample was carried out. RESULTS AND DISCUSSION The space group of ZnO (wurtzite) is C,, and there are 4 atoms per unit cell.There are therefore 3 acoustic and 9 optic modes12 and the optic modes are classified as: 1 x A , mode 1 x E, mode 2 x E, mode 2 x B, mode inactive i.r. + R active i.r. + R active R active onlyJ. HOWARD, I. J. BRAID AND J. TOMKINSON 227 However, according to the Lyddane-Sachs-Teller (LST) theory,13 splitting of the formally i.r.-active optic phonons leads to transverse (t.0.) and longitudinal (1.0.) branches of which the t.0. branch generally occurs to lower wavenumbers. These branches retain the same i.r./R activity as the parent phonon mode with one exception: all the 1.0. branches are i.r. inactive regardless of their symmetry. RAMAN SPECTRUM OF ZnO The data and assignments from our Raman study of ZnO are summarised in table 1 together with the results of previously published work on similar l5 Our data on a polycrystalline sample are closer to those obtained from single-crystal studies by Damen et aI.l4 than to those obtained by Mitra and Bryant.15 In particular, our spectrum does contain a band at 100 cm-l and so provides some support for the assignment by Damen et al.of a band at 101 cm-l to an E, optic mode. Mitra and Bryant assigned a band at 180 cm-1 to this mode but we, like Damen et al., did not observe a transition at this value. Table 1. Frequencies (cm-l) and assignments of bands in the vibrational spectrum of ZnO this work uncalcined ZnO, room temperature calcined ZnO, Raman15 Raman14 77 K assignment assignment Raman INS assignment - _ 180 t.0.(E,) _ _ - - 373 t.0. (E,) 420 t.0. ( A , ) 438 t.0. (El) 538 1.0. 588 1.0. - - 101 E, 208 mp 334 mp 380 t.0. ( A , ) 407 t.0. ( E l ) 437 E, 574 1.0. ( A , ) 583 1.0. ( E l ) 986 mp 1084 b mp 1149 b mp - - 100 vs 204, 21 1 vw - - - - - 335 m 383 w 4 1 4 ~ , sh - 440 vs 437 vs 543 w, b 553 vs 588 w - 635 sh 758, 765 w, b 749 m 877 m 1007 m - - - - - - - - - 1072 sh, w 1105 sh - 1150 s, b - - 1346 vs, b t.0. phonon mP mP t.0. phonon t.0. phonon 1.0. phonon ( A , ) 1.0. phonon (El) mp (1 00 + 543 = 643 cm-l) mp(2 x 383 = 766 cm-l) mp (2 x 440 = 880 cm-') mp (440 + 588 = 1028 cm-l) mp (2 x 543 = 1086 cm-l) mp(543+588 = 1121 ern-') mp (2 x 588 = 1 176 cm-l) ? - - - - l.o., longitudinal optical phonon mode; t.o., transverse optical phonon mode; mp, multi- phonon mode; b, broad; m, medium; s, strong; sh, shoulder; v, very; w, weak.INFRARED SPECTRA 750-300 cm-l Our i.r. spectrum (750-300 cm-l) of unpretreated ZnO (KBr disc) showed strong absorptions at 440,490 and 525 cm-l which, in accord with other authors,16 we assign to surface optic phonons. The i.r. band at 440 cm-l is not assigned to the E, bulk228 SPECTROSCOPIC STUDY OF H, ON ZnO phonon which was also observed at 440 cm-l in our Raman data. Our reasons are: (i) group-theory arguments predict that the E, mode is Raman active but i.r. inactive and (ii) particle-size arguments13- l6 predict that only surface phonons will be observed in the i.r. spectra of our sample. Measurements using a Coulter counter indicate that 90% of the particles had a diameter < 0.5 pm.The original assignment of the E, mode was made from a polarised Raman study of a single crystal14 in which the polarisation characteristics of a Raman band at 437 cm-l lead to its unambiguous assignment to the E, bulk phonon. Therefore, the Raman band at 440 cm-l and the i.r. maximum at 440 cm-l have different physical origins. 4000-750 cm-l Before starting the calcination treatment, but after mild evacuation to 4.5 x lop2 Torr, the ZnO sample at 299 K showed three sharp i.r. bands in the 3000-2800 cm-l region, at 2960,2930 and 2875 cm-l. Calcination completely removed these bands, which have not been repolded elsewhere for ZnO. The frequencies are typical of the C-H symmetric stretching mode of aliphatic hydrocarbons. We therefore assign the bands to the v,(C-H) mode of surface contaminants on the ZnO. Some support for our assignment is provided by the i.r.spectrum of SiO, (Cabosil) obtained by McDonald." An untreated SiO, sample showed bands at 2960, 2959 and 2875 cm-l which disappeared after heating for several minutes at 773 K in 0,. These bands were also assigned to the C-H stretch of surface impurities. With the exception of the 3000-2800 cm-l region, the i.r. spectra (4000-750 cm-I) of ZnO obtained during the calcination treatment to 725 K in 0, were in accord with the results of other w ~ r k e r s . l ~ - ~ ~ After cooling the calcined sample in vacuo to 305 K the i.r. spectrum showed the presence of only hydroxyl and carbonate groups on the calcined ZnO surface. Such species are therefore the only likely contaminants on the ZnO samples used in our INS experiments, these samples having been prepared using a similar calcination technique. INS SPECTRUM OF ZnO The INS spectrum obtained at 77 K on IN 1 B of Kadox 25 after calcination is shown in fig.1 and the results are summarised in table 1. Two very intense bands are observed at 437 and 553 cm-l. By comparison with our Raman data (table 1) these bands are assigned to t.0. and 1.0. phonon modes, respectively. Although, in the INS technique both bulk and surface phonons will be active, the measured spectra of our sample will be dominated by bulk modes because of the large bu1k:surface ratio. The bands at 635, 749, 877, 1007 and 1072 cm-l in the INS spectrum (fig. 1) are tentatively assigned to multiphonon processes (table 1).In addition to these features the INS spectrum of ZnO (fig. 1) shows an intense, broad band at 1346 cm-l. No i.r. or Raman bands have been reported for ZnO in this region. On adsorption of H, the scattered-neutron intensity due to this transition remains constant whereas the intensity increases at all other frequencies (see fig. 1). This suggests that the 1346 cm-l band is not due to phonon processes. Also, the highest value at which multiphonon bands have been observed in the Raman data is 1155 cm-l. It is unlikely that the 1346 cm-l band is due to surface carbonate because the cross-sections of carbon and oxygen and their likelyz3 concentration (ca. 0.05 0) are too low to explain a band of such high intensity. Moreover, surface hydroxyls are not reported to give rise to bands near 1346 cm-l nor are they likely to be present in high enough concentration to explain the observed intensity.In summary, we are unable to make a definite assignment of the band at 1346 cm-'.J. HOWARD, I. J. BRAID AND J. TOMKINSON 229 I I I I 500 1000 1500 2000 incident neutron energylcm-' Fig. 1. INS spectra at 77 K of (a) ZnO after calcination and (b) ZnO after adsorption of H,. + and x indicate data collected using the (200) plane and 0 and 0 those collected using the (220) plane of the copper monochromator. However, because its intensity is unchanged on H, adsorption, it is probably related to transitions within the bulk rather than at the surface of the ZnO particles. Since incorporation of impurities within the lattice during manufacture is conceivable there is a chance that the 1346 cm-l INS band is due to contaminants, e.g.carbonate or water, trapped within the bulk of the sample. INS SPECTRUM OF ZnO + H, The spectrum of ZnO+H,, 68 pmol g-l coverage, obtained on IN1B at 77 K is shown without any background subtraction in fig. 1 . The result of subtracting the ZnO background is shown in fig. 2. Table 2 lists the frequencies of bands observed after the subtraction and includes the i.r. data of Boccuzzi et aL2 for comparison. The two optical phonon modes which arose at 437 and 553cm-l in the ZnO (fig. 1) now appear at 458 and 538 cm-l on adsorption of H, (fig. 2) and the lower- frequency, t.0. band apparently gains in intensity relative to the higher-frequency, 1.0.band. However, a change in the sloping background is the most likely cause of the change in relative intensities of these two phonon modes upon hydrogen adsorption. Modification of the bonding at the ZnO surface on hydrogen adsorption may be the cause of the shift in frequency of the two phonon modes. The frequency shift is notSPECTROSCOPIC STUDY OF H, ON ZnO 400 600 800 1000 1200 1400 1600 1800 2000 2200 incident neutron energy/cm-' Fig. 2. Difference spectrum : INS spectrum of ZnO + H, minus INS spectrum of ZnO. Symbols as for fig. 1 . due to plasmon effects since the adsorbed hydrogen does not form an accumulation layer2, at temperatures as low as 77 K. A shoulder at 584 cm-l in the ZnO + H, INS spectrum (fig. 2) may arise from either the bulk phonon mode at 588 cm-l (table I ) or from the bending mode of surface H /' \ species.The assignment to the phonon mode is preferred for the reasons Zn Zn given later. vg(Zn-H) The intense band at 829 cm-I in the subtracted spectrum (fig. 2 ) is not present in the ZnO data. This frequency is typical of metal-hydrogen bending modes, e.g. vg for terminally bound H in transition-metal hydridocarbonyls lies in the region 600-800cm-l, and the bending modes in GeH,, SnH, and GeH, (matrix isolated) occur at 931, 758 and 928 cm-l, re~pectively.~~ We assign the 829 cm-l band to v6 of surface Zn-H at a type I site, in agreement with previous work.2 INS SPECTRUM OF TYPE I SPECIES v, (Zn-H) The subtracted spectrum (fig. 2) shows a broad, strong band at ca. 1665 cm-l which is also absent from the ZnO spectrum (fig. 1).This band may be assigned immediately to the symmetric stretch of Zn-H at type I sites. It has been reported, at 1708 cm-' in the i.r. spectrum (table 2). The discrepancy between the frequencies observed by INS and i.r. spectra is explained by the temperature and coverage dependence of the frequency of this mode and the difficulty of locating the exact centre of the broad INS transition. The INS spectrum of ZnO+H, with background subtracted (fig. 2) shows unresolved bands in the region between the 584 cm-l shoulder and the 829 cm-l band. This intensity we assign to scattering from multiphonon bands (observed at 635 and 749 cm-l in the INS spectrum of ZnO).Table 2. Frequencies and assignments of bands in the vibrational spectrum of ZnO plus adsorbed hydrogen (cm-l) assignment (Boccuzzi et ~ 1 .~ ) INS i.r. Boccuzzi2 type of assignment (this work) mode type of H adsorption INS ZnO ZnO + H,-background ZnO + H, mode adsorption t.0. phonon 1.0. phonon 1.0. phonon mP mP v,(Zn-H) mP mP mP - - vs(0-H) ? - v, (Zn-H) - __ 437 vs 553 vs - 635 sh I - - - - 749 - - - - - 877 m - 1007 m - 1027 sh, w - - - I 1346 vs, b - 458 vs 538 vs 584 sh, m - - 829 s - - - - - 1125 sh (1346 vs, b)" - - - - - 817 m 845 to 850 b, w 870 s 990 s - ca. 1100 sh, m - 1475 vb 1708 s ca. 3400 b 3498 a The band at 1346 cm-l occurs with equal intensity in the background spectrum of ZnO and in the spectrum of ZnO + H, before subtraction of the background. A decrease in intensity is therefore observed at 1346 cm-l in the spectrum of ZnO + H, -background.232 SPECTROSCOPIC STUDY OF H, ON ZnO The INS spectrum in the region 829 to 1125 cm-l (fig.2) shows the high-frequency edge of a band with a maximum at ca. 1125 cm-l and some unresolved intensity in the region down to ca. 850 cm-l. There are three possible (not necessarily mutually exclusive) origins of this intensity. (i) There is the possibility of a symmetric stretching mode of a / \ surface species, formed by type I1 adsorption, occurring in this region. This is discussed below. (ii) Multiphonon bands were observed in this region in the INS spectrum of the ZnO background (fig. 1, table 2). Thus the first overtones of the intense t.0. (458 cm-l) and 1.0. (538 cm-l) bulk phonons in the INS spectrum of ZnO + H (fig.2) are therefore expected at ca. 916 and 1076 cm-l. (iii) v6 of type I (0-H) was assigned2 to a broad i.r. band at ca. 850 cm-l (table 2). Although no band was resolved in our INS spectrum of ZnO+H2 at 850 cm-l (fig. 2), this mode should be INS active. Thus we have calculated the predicted intensities of an INS band due to vs(O-H) in the region 850 to 1125 cm-l relative to v,(Zn-H). In general, the vg modes of metal-OH bonds in organometallic and inorganic compounds are found between 700 and 1200 cm-l in the Now since type I adsorption is said to occur at Zn-0 pair sites, the surface concentration of type I Zn-H and 0-H species will be equal. vg(Zn-H) was assigned above to an intense band at 829 cm-l (fig. 2) and as a first approximation we might expect to observe the vd(O-H) mode with similar intensity.There are two possible assignments of a band to vs(O-H) in fig. 2; the first at 1125 cm-l (shoulder) and the second at ca. 850 cm-l [this being unresolved from vg(Zn-H) at 829cm-lI. We recall that the vs(O-H) band in the i.r. spectrum was broad.2 Because type I Zn-H and 0-H have equal concentrations, the intensities ( I n ) in the INS spectra of the fundamental modes of these species can be estimated (in the harmonic approximation) from:26 H Zn Zn where fie is the momentum transfer during the scattering process, Z(C,) is the vibrational density of states, Fn is the normal mode frequency in cm-l and exp [ - 2 W, (PA)] is the mode-dependent anisotropic Debye-Waller factor. The relative INS intensities predicted for the v, and vg fundamentals of type I Zn-H species are compared in table 3 with those of the proposed vs(O-H) fundamentals at 850 and 1125 cm-l.In the calculations we have taken 3498 cm-l as the frequency of the v,(O-H) mode.27 We need also to consider the intensity in the region 1500-2300 cm-l Table 3. Relative INS intensities predicted for the normal modes of type I hydrogen on ZnO type I surface species Zn-H 0-H 0-H Zn-H frequency/cm-' 1665 1125* 850* 829 relative Ih'S intensity 1 .o 1 .5 3.0 3.0 a The frequencies 1125 and 850 cm-I are taken as the two most likely possibilities for vs(,O--H) (see text). The large difference between the two predicted relative INS intensities for v,(O-H) is a consequence of the use of a mode-dependent Debye-Waller factor.J. HOWARD, I.J. BRAID AND J. TOMKINSON 233 from the first overtones of vs(Zn-H) and vs(O-H). Using published formulae26 we estimate the intensity of each overtone to be ca. that of its fundamental. When the overtone contribution is included, we find that the intensities of the observed bands (fig. 2) cannot be measured with sufficient accuracy to distinguish between the alternative assignments of vb(0-H). This is largely due to the uncertainty in the baseline position of fig. 2. However, because we expect to observe a strong INS band due to vs(O-H), we tentatively assign this mode to the shoulder at 1125 cm-l rather than to the region at ca. 850 cm-l where our INS data show no resolved features (fig. 2). Our assignment of vs(O-H) disagrees with that of Boccuzzi2 who ascribed a broad i.r.band at 845-850 cm-l to this mode. The published i.r. spectra, of hydrogen adsorbed on ZnO in the region ca. 845-775 cm-l show a general increase in i.r. absorbance with increasing hydrogen coverage. We submit that the feature at 850-845 cm-l in the published spectra, is not sufficiently well differentiated from the general decrease in i.r. transmission to be definitely ascribed to a band arising from a vibrational mode of a particular surface species. The previous assignment, of the vs mode of type I (0-H) to the i.r. feature at 850-845 cm-l should therefore also be regarded as tentative. No bands appear in the i.r. spectra in the region of 1125 cm-l on adsorption of H,., (A band at ca. llOOcm-l in the ZnO spectrum of Boccuzzi et al.before H, adsorption was not assigned by the authors. From our i.r. resuits for calcined ZnO we suggest that this is a multiphonon band.) The discrepancy between the values for vs(O-H) (type I) as assigned by Boccuzzi et al., in their i.r. data and by us using INS cannot be resolved in the present work. We submit that both assignments must be regarded as tentative. An INS study of HD adsorbed on ZnO may help to clarify the situation, though it would be difficult with current neutron sources. INS SPECTRUM OF TYPE I1 SPECIES The INS data show no strong bands assignable to the symmetric or antisymmetric stretches of the species proposed by Boccuzzi et aL2 for adsorption at type I1 sites. A very broad band in the i.r. spectrum of ZnO+H, at 1475 cm-l was assigned23 to the asymmetric stretch. The symmetric stretch, although not observed in the i.r.data of Boccuzzi et al., is expected to occur in the region 895-1300 cm-l on the basis of hydridocarbonyl [@,-H) M,] data2* (M is a transition-metal atom). We cannot discount the possibility of a low concentration of a / \ species on our ZnO surface. It is possible that a weak asymmetric stretching band might be present at ca. 1475 cm-l in the INS spectrum of ZnO + H, (fig. 2), unresolved from the broad 1665 cm-l band. The INS spectrum also shows unresolved intensity in the /H\ Zn Zn H Zn Zn H range 829-1 125 cm-l which may include a contribution from a / \ symmetric Zn Zn stretch. There are two possible assignments for the shoulder at 584 cm-1 (fig. 2): (i) a bulk ). If we assume (a) that the 584 cm-l feature is / \ , expected in the range 490-630 cm-l from H / \ Zn Zn phonon mode and (ii) vg( H Zn Zn due to the bending mode of234 SPECTROSCOPIC STUDY OF H, ON ZnO hydridocarbonyl data,28 (6) that the symmetric stretch occurs within the range 829-1 125 cm-l and (c) that the antisymmetric stretch occurs at 1475 cm-l, then using the modified Katovic equation2g we calculate the / \ bond angle (a) to lie in the range 99-108'.Since type I1 adsorption is said to occur on the (1010) planes1 (Zn- - - -Zn distance30 of 3.25 A unaffected by surface reconstr~ction~l), this corresponds to a Zn-H distance of 2.G2.1 A. We consider this to be too long. Since the charge of the ions at the ZnO surface are less than those in the bulk,31 we expect the surface Zn-H bonds to be predominantly covalent. The predicted Zn-H bond length is longer than that usually found in covalent h y d r i d e ~ ~ ~ and is also greater than the sum of the covalent radii (1.57 A).34 This indicates that the assignment of the 584 cm-l band to the vs mode of &-H) Bn, is probably incorrect.The alternative assignment to a bulk phonon mode is therefore preferred. In view of the above discussion, and the fact that the 1475 cm-l i.r. band has not been reported by other workers, we submit that its assignment, and the bridge structure of type I1 hydrogen, proposed by Boccuzzi et aL2 should be regarded as tentative. H Zn Zn CONCLUSIONS The adsorption conditions used in the INS experiment are expected to give rise to type I, I1 and I11 hydrogen.The modes of type 111 hydrogen and the stretching modes of surface hydroxyls lie outside the wavenumber range (320-2230 cm-l) of our data. The bending and stretching modes of Zn-H at type I sites were assigned at 829 and 1665 cm-l in the INS data. These frequencies are in close agreement with the published i.r. results.'. From our INS spectra, we were unable to confirm the recent assignments by Boccuzzi et aL2 of the vs mode of type I (O-H) to a broad i.r. band at 845-850 cm-l and of the v,, mode of type I1 hydrogen at zinc sites to an i.r. band at 1475 cm-l. We suggest that these assignments2 of the i.r. bands are to be regarded as tentative. An intense broad band was observed at 1346 cm-l in the INS spectrum of ZnO itself. The intensity of this band was unchanged on H, adsorption and since it is not observed in the optical spectra it is suggested that this band arises from modes of carbonate impurities in the bulk.We thank the S.E.R.C. and A.E.R.E. Harwell for the award of a CASE studentship to one of us (I. J. B.) and for the provision of access to neutron-beam facilities. We also thank the New Jersey Zinc Co. for the gift of the Kadox 25. C. S. John, in CatafyAis, ed. A. Dowden and C . Kemball (Specialist Periodical Reports, ThqChemical Society, London, 1980) vol. 3, p. 169. F. Boccuzzi, E. Borrello, A. Zecchina, A. Bossi and M. Camia, J . Cataf., 1978, 51, 150. C. C. Chang, L. T. Dixon and R. J . Kokes, J . Phys. Chert?., 1973, 77, 2634. B. Maier. Neutron Beam Facilities Acaifahle for Users (Internal Report, lnstitut Laue Langevin, Grenoble, France, 198 1 ).J. Howard and T. C. Waddington, J . Phjys. Chem., 1981, 85, 2467. A. D. Taylor and J . Howard, J . Phys. E, 1982, 15, 1359. 153. A. L. Dent and R. J. Kokes, J . Phys. Chem., 1969, 73, 3781. R. J. Kokes, A, L. Dent, C. C . Chang and L. T. Dixon. J . Am. Chem. Soc., 1972,94,4429. A. Baranski and R. J . Cvetanovic, J . Phys. Chon., 1971, 75. 208. ' B. Fubini, E. Giamello, G. D. Gatta and G. Venturella, J . Chem. Soc., Faraday Trans. 1, 1982, 78, l o A. Baranski and J. Galvszka. J . Catal., 1976, 44, 259.J. HOWARD, I. J. BRAID AND J. TOMKINSON 235 l 2 W. G. Fateley, F. R. Dollisn, N. T. McDevitt and F. F. Bentley, infrared and Raman Selection Rules l 3 P. M. A. Sherwood, Vibrational Spectroscopy of Solids (Cambridge University Press, Cambridge, l4 T.C. Damen, S. P. S. Port0 and B. Tell, Phys. Rev., 1966, 142, 570. l5 S. S. Mitra and J. I. Bryant, Bull. Ann. Phys. Soc., 1965, 10, 333. l6 K. Yamamoto, K. Veda, M. Yamada and K. Abe, J. Phys. SOC. Jpn, 1979,46, 1792. l7 R. S. McDonald, J. Phys. Chem., 1958, 62, 1168. l8 F. Boccuzzi, C. Monterra, R. Scala and A. Zecchina, J. Chem. Soc., Faraday Trans. 2, 1981,77,2059. l9 J. Savssey, J-C. Lavalley and C. Bovet, J. Chem. SOC., Faraday Trans. I , 1982, 78, 1457. 2o G. L. Griffin and J. T. Yates Jr, J . Catal., 1982, 73, 396. 21 K. A. Atherton, G. Newbold and J. A. Hockey, Discuss. Faraday Soc., 1971, 52, 33. 22 K. Morishige, S. Kittaka and T. Moriyasu, J. Chem. SOC., Faraday Trans. I , 1980, 76, 728. 23 F. Boccuzzi, E. Borrello, A. Chiorino and A. Zecchina, Chem. Phys. Lett., 1979, 61, 617. 24 D. Narayana, V. S. Subrahmanyam, J. Lal, M. Mahmood Ali and V. Kesavulv, J. Phys. Chem., 1970, 25 K. Nakamoto, Infrared and Raman Spectra of inorganic and Co-ordination Compounds (Wiley 26 T. C. Waddington, J. Howard, K. P. Brierley and J. Tomkinson, Chem. Phys., 1982, 64, 193. 27 R. P. Eischens, N. Pliskin and M. J. D. Low, J. Catal., 1962, 1, 180. 28 J. Howard, I. J. Braid and J. Tomkinson, J. Chem. Soc., Faraday Trans. 2, 1983, 79, 253. 2n V. Katovic and R. E. McCartey, Inorg. Chem., 1978, 17, 1268. 30 H. Schulz and K. H. Thiemann, Solid State Commun., 1979, 32, 783. 31 C. B. Duke, R. J. Meyer and A. Paton, Phys. Rev. B, 1978, 18,4225. 32 R. E. Watson, M. L. Perlman and J. W. Davenport, Surf. Sci., 1982, 115, 117. 33 J. W. Turley and H. W. Rinn, inorg. Chem., 1969, 8, 18. 34 J. E. Huheey, Inorganic Chemistry: Principles of Structure and Reactivity (Harper and Row, London, For Molecular and Lattice Vibrations (Wiley Interscience, New York, 1972). 1972). 74, 779. Interscience, New York, 3rd edn, 1978) and references therein. 1972). (PAPER 3/988)

ISSN:0300-9599    

DOI:10.1039/F19848000225

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1984, 80, 237-248 X-Ray Photoelectron Spectroscopic Study of Montmorillonite Containing Exchangeable Divalent Cations BY HARUHIKO SEYAMA* AND MITSUYUKI SOMA National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 305, Japan Received 23rd June, 1983 Montmorillonite containing divalent Mg, Ca, Sr, Ba or Cd as an exchangeable cation has been studied by X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy. The relative atomic abundances of the exchangeable cations in the montmorillonite samples, obtained by X.P.S. measurements, are consistent with the cation-exchange capacity of the montmorillonite. A comparison of the photoelectron binding energies and Auger electron kinetic energies of the exchangeable cations with those of corresponding halides and oxides reveals that they fall among those of corresponding cations in typical ionic compounds such as fluorides and chlorides.On the other hand, the bonding state of the non-exchangeable Mg ion in montmorillonite lattice, as indicated by the Mg 1s and KL,, 3L2, Auger electron energies, is similar to that of magnesium oxide and distinguishable from that of the exchangeable Mg ion. Clay minerals are important constituents of soil and sediment and greatly influence the geochemical behaviour of various cations by their cation-exchange capabilities. Many investigations on the cation-exchange reactions of clay minerals have been undertaken and many data, e.g. concerning cation-exchange capacities, have been accumulated.l* However, there has been a lack of systematic knowledge concerning the bonding states of exchanged cations in clay minerals simply because experimental methods applicable to probing the bonding states of various elements in solids such as minerals have not been available.X-ray photoelectron spectroscopy (X.P.S.), which has advanced rapidly in recent years, is a suitable method for this purpose. It detects all elements except H and He and determines the abundance of elements from the intensity of core-electron emission. Detailed spectral features such as the chemical shift in the electron binding energy, satellite structure etc. give information about the bonding states of elements. Adams et al. attempted the quantitative analyses of clay minerals and other aluminosilicate minerals by X .P . S . ~ ~ ~ The valence state of lead adsorbed on mont- morillonite was studied by Counts et al. based on the binding energies of Pb 4f electrons5 The spectral changes of Fe 2p3/2 and 0 Is photoelectrons of nontronite and biotite with oxidation and reduction of these clay minerals were investigated by Stucki et al.' More recently, Koppelman et al. systematically investigated the adsorption of transition-metal ions and their complex ions on chlorite, illite and kaolinite.'-ll The origins of the negative charge on the particles of these three clay minerals are considered to be due to a small amount of isomorphic substitution, to lattice imperfection and to broken bonds at the edges of the particles. Therefore the cation-exchange capacities (c.e.c.) of them are low (1040 mequiv.per 100 g for chlorite and illite and 3-15 mequiv. per 100 g for kaolinite).2 In this paper we report an X.P.S. study of montmorillonite, a typical smectite clay 237238 X.P.S. STUDY OF MONTMORILLONITE mineral containing a series of divalent exchangeable cations (Mg, Ca, Sr, Ba and Cd). In contrast to the above-mentioned chlorite, illite and kaolinite, sufficient isomorphic substitution occurs in smectite clay minerals. In order to compensate the negative charge originating from isomorphic substitution, exchangeable cations are held between the aluminosilicate layers, resulting in high c.e.c. values (80-1 50 mequiv. per 100 g).2 The choice of exchangeable cations was based not only on systematic considerations (the alkaline-earth elements) but also on geochemical, agricultural and environmental importance.The emphasis is on demonstrating the capability of X.p.s. for the non-destructive quantitative analysis of clay minerals and for characterizing the bonding states of cations by core-electron binding energies and the kinetic energies of X-ray-induced Auger electrons. A preliminary account of Mg-montmorillonite has been reported previously.12 EXPERIMENTAL MATERIALS The montmorillonite used in this study was Kunipia F, a processed natural bentonite mined in the Tsukinuno mine, Yamagata, Japan and obtained from Kunimine Industries. The nominal composition of Kunipia F in wt% was as follows: SiO,, 57.96; A1203, 21.87; Fe,O,, 1.92; MgO, 3.44; CaO, 0.54; Na,O, 2.98; K,O, 0.14.It was transformed to Na-montmorillonite containing only Na as an exchangeable cation in accordance with the method used by Posner and Quirk.I3 The original powdered clay was washed with 1 mol dm-, NaCl aqueous solution and separated by decantation, this treatment being repeated six times. Then the montmorillonite was suspended in 1 mol dmP3 NaCl aqueous solution at pH 3 adjusted with hydrochloric acid for ca. 1 h, followed by separation from the solution by centrifuging. After this procedure was repeated four times, the montmorillonite was resuspended in 1 mol dm-, NaCl aqueous solution at pH 3, stirred for cu. 36 h in order to replace completely the exchangeable cations in the montmorillonite with Na cation, separated again by centrifuging and washed twice with distilled water.It was then dialysed against deionized water for ca. 2 days and freeze-dried. M-montmorillonite containing an exchangeable divalent cation of M (M = Mg, Ca, Sr, Ba or Cd) was prepared by cation exchange of Na-montmorillonite. Na-montmorillonite was suspended in 7 x lo-, mol dmP3 M(NO,), aqueous solution for > 24 h. The total amount of M2+ in the solution was about four times as large as the c.e.c. of montmorillonite. The initial and final concentrations of M2+ in solution were determined by EDTA chelatometry. The c.e.c. of montmorillonite for each M cation was calculated from the concentration change of M cation in the solution. After the cation-exchange reaction, M-montmorillonite was separated from the solution by centrifuging, washed by resuspending in distilled water, separated again by centrifuging and dried in a vacuum desiccator.Metal salts used for X.P.S. measurements were commercial materials of guaranteed purity and used without further purification. X. P.S. MEASUREMENTS Photoelectron and Auger electron spectra were recorded on a Vacuum Generators ESCALAB 5 instrument with magnesium and aluminium X-ray sources. Typical measuring conditions were: X-ray power, 13 kV x 10 mA; electron pass energy, 50 eV; width of the entrance slit of the analyser, 4 mm. The instrumental resolution determined by the pass energy and the slit width was 1.25 eV.14 In order to improve the signal to noise ratio, the signals for weak intensity lines were accumulated on a Nicolet 1070 signal averager. All samples except metal oxides were ground to powders with an agate mortar and fixed onto stainless-steel sample holders of 10 mm diameter using double-sided sticky tape.Powder samples of metal oxides were directly deposited onto sample holders from acetone suspension. The sample of CdO was heated (600 OC, 1 day) in the analyser chamber of the instrument for the purpose of dehydration and decarbonation of Cd(OH), and CdCO, contaminants prior to making X.P.S. measurements according to the procedure previously described by Hammond et aI.l5 The samples of alkaline-earth oxides wereH. SEYAMA AND M. SOMA 239 exposed to 5 keV argon-ion bombardment at 10 pA for several tens of minutes in the analyser chamber of the instrument in order to remove hydroxide and carbonate contaminants on the sample surface.The photoelectron binding energies and the Auger electron kinetic energies of the alkaline-earth metal ions decreased and increased, respectively, as a result of the bombard- ment, which was continued until the metal-ion spectra became stationary. Before bombardment, the 0 1s spectrum of the alkaline-earth oxides consisted of only one line derived from the oxygen atom(s) of the surface contaminants. After bombardment, the same spectrum was broad or accompanied by a shoulder or other peak on the high-binding-energy side due to contributions from oxygen atoms in the residual hydroxide and/or carbonate ion contaminants, perhaps in the bulk. Therefore, the spectra of alkaline-earth oxide samples treated in this way were still not free from impurities.Photoelectron binding energies and Auger electron kinetic energies were determined relative to the Au 4f,,, binding energy (83.8 eV) of a gold film vacuum evaporated onto the sample as a primary standard. For the samples of montmorillonite, the Si 2s binding energy (1 53.4 eV) determined by this method was used as an internal standard for calibration. All the photoelectron binding energies and Auger electron kinetic energies, except for the Ca 2p,/, binding energy, were determined by A1 Ka excitation. The Ca 2p3/, binding energy was determined by Mg Ka excitation to avoid the overlapping of Ca 2p and Mg Auger KL,L,,, lines that occurs when using A1 Ka. RESULTS AND DISCUSSION ATOMIC COMPOSITION The c.e.c. values of montmorillonite for Mg, Ca, Sr, Ba and Cd cations are given in table 1.They were all identical (99 mequiv. per 100 g) within experimental error, and hence it was confirmed that there was no difference in the exchangeable-cation content of montmorillonite for the various ions. The wide-scan X-ray photoelectron spectrum of Ba-montmorillonite excited by A1 Ka radiation is shown in fig. 1. Photoelectron and Auger electron peaks due to the constituent elements of the montmorillonite lattice (Si, Al, Mg and 0) and to carbon present as a surface contaminant were found in the wide-scan X-ray photoelectron spectra of all the montmorillonite samples. In addition, characteristic peaks due to the exchangeable cation, such as the Ba 4d, 3d and M4,5N4,5N4,5 Auger lines in fig. 1, were observed for each montmorillonite sample.Table 1. Cation-exchange capacities (c.e.c.) of montmorillonite for divalent cations c.e.c. cation /mequiv. per 100 g Mg 96 Ca 101 Sr 100 Ba 102 Cd 96 mean 99 The relative atomic abundances of Al, Si, Mg and of the exchangeable cation in each montmorillonite sample were calculated from area intensities of the photoelectron and Auger electron spectra on the basis of the relative sensitivity of each line. The relative sensitivities used in the calculation were experimentally determined from the240 X.P.S. STUDY OF MONTMORILLONITE 12s 1 I 500 1000 binding energy/eV Fig. 1. Wide-scan X-ray photoelectron spectrum of Ba-montmorillonite excited by A1 Ka radiation. relative area intensities of the photoelectron and Auger electron spectra of compounds of known chemical composition, i.e.zeolites, halloysite, fluorides, chlorides and sulphates. The atomic abundances in the montmorillonite samples relative to A1 (atomic ratios) determined in this way are given in table 2. The calculated average atomic abundances of Si and Mg in the aluminosilicate layers of montmorillonite relative to A1 were 2.17 and 0.22, respectively. The bulk atomic abundances of Si and Mg relative to A1 calculated from the nominal composition of Kunipia F were 2.25 and 0.20, respectively, which are within 10% of the average atomic abundances obtained by X.P.S. This result suggests that the bulk chemical composition is maintained at the surface of the particle. Consequently, X.P.S. is a useful bulk quantitative analytical technique for aluminosilicate minerals, as pointed out by Adams et aL3 Since Mg-montmorillonite contains exchangeable Mg ions between the aluminosilicate layers, in addition to the non-exchangeable Mg ions as constituents of the aluminosilicate layers, the atomic abundance (0.34) of Mg relative to A1 in Mg-montmorillonite is larger than that (0.22) in other montmorillonites containing only non-exchangeable Mg ions.Thus the difference (0.12) between the relative atomic abundances of Mg in Mg-montmorillonite and in other montmorillonites is assigned to the relative atomic abundance of the exchangeable Mg ion in Mg-montmorillonite. The atomic abundances thus determined of exchangeable divalent cations relative to A1 in the montmorillonite samples range from 0.1 1 to 0.15 (mean = 0.12) and are equal to approximately half the amount of Na ion in Na-montmorillonite.The same quantity calculated from the c.e.c. (99 mequiv. per 100 g) and the nominal composition of Kunipia F is 0.12, which is in good agreement with that determined by X.P.S. The relative atomic abundance (0.15) found for Ba ions in Ba-montmorillonite isH. SEYAMA AND M. SOMA 24 1 Table 2. Atomic abundances in M-montmorillonite samples relative to A1 (atomic ratios) determined by X.P.S. M X-ray element line source Na Mg Ca Sr Ba Cd mean A1 Si Mg Na Ca Sr Ba Cd 2P 2s Auger Auger 2P 3d (KL'2,3L2,3) ( K L 2 , 3L2, 3) 3d 3d A1 Ka Mg Ka A1 Ka Mg Ka A1 Ka A1 Ka Mg Ka Mg Ka A1 Ka Mg Ka A1 Ka Mg Ka A1 Ka Mg Ka 1 1 2.20 2.13 0.2 1 0.25 0.27 1 1 2.26 1.99 0.34 (0.12)a - 1 1 1 1 1 1 2.15 2.22 2.00 2.06 2.33 2.28 0.22 0.21 0.23 0.23 0.22b a Relative atomic abundance of exchangeable Mg [0.34 (total Mg) - 0.22 (non-exchangeable Mg) = 0.121; Mean value for montmorillonite samples except for Mg-montmorillonite.25% larger than the average value for the exchangeable divalent cations in the montmorillonite, and requires comment. Adams and Evans4 used X.P.S. to estimate the atomic abundances of exchangeable cations relative to Si in beidellite and reported that the relative atomic abundance of Ba ions in Ba-beidellite was ca. 50% greater than that of Ca ions in Ca-beidellite. Preferential external surface adsorption of Ba ions was suggested as an explanation for the excess Ba detected by X.P.S. The same phenomena were observed for K- and Pb-beidellites.Although both beidellite and montmorillonite are smectite clay minerals, the deviation of the amount of Ba found by X.P.S. from that calculated from the c.e.c. in our case is not so pronounced as in their result. We should also note the shallow escape depths of Ba 3d photoelectrons, which have relatively low kinetic energies (ca. 700 and 470 eV for A1 Ka and Mg Ka excitation, respectively), as compared with the other cations. Thus the intensity of the Ba 3d line would be rather sensitive to the amount of surface contaminant, which makes the accurate determination of Ba concentration based on that line difficult. It may therefore be presumed that in the present case the preferential external surface adsorption of exchangeable divalent cations (Mg, Ca, Sr, Ba and Cd ions) does not occur and that they are held exclusively between the aluminosilicate layers of mon tmorilloni te.Mg IS AND KL,,,L,,, AUGER SPECTRA OF Mg-MONTMORILLONITE Fig. 2 shows the Mg Is and KL,,,L,,, Auger spectra of Mg-montmorillonite excited by A1 Ka X-rays. They are broad in comparison with those of montmorillonite containing only non-exchangeable Mg ions. In Mg-montmorillonite the exchangeable and non-exchangeable Mg ions have different locations, i.e. the former exist between the aluminosilicate layers while the latter form one of the constituents of the aluminosilicate layers. Accordingly, the broad character of the spectra of Mg-242 X.P.S. STUDY OF MONTMORILLONITE .-... . . ..-. .. .. . . (a ) non-exchangeable Mg . . .. . 1300 1305 1310 binding energy/eV .......I. -, .. _. ( b 1 exchangeable Mg I I I 1185 1180 1175 kinetic energy/eV Fig. 2. Resolution of (a) Mg 1s and (b) Mg KL,,,L,,, Auger spectra of Mg- montmorillonite. montmorillonite can be ascribed to the existence of two kinds of Mg ions. The observed Mg 1s and KL2,&2,3 Auger spectra were each resolved into two components as follows. The Mg KL,,,L,,, Auger kinetic energy for non-exchangeable Mg ion and its intensity relative to the A1 2p line due to skeletal A1 were determined for the montmorillonite samples which did not contain Mg as an exchangeable cation. This component was then subtracted from the Mg KL,,,L,,, Auger spectrum of Mg- montmorillonite and the residual spectrum was assigned to the exchangeable Mg ion. The result of the resolution is shown in fig.2. The relative intensities of the two components were consistent with the relative abundances of the non-exchangeable skeletal Mg ion and the exchangeable cation as determined with other montmorillonite samples. The Mg 1s line was also resolved similarly but with readjustment of the intensity factor in order to make the relative intensity of the resolved spectra consistent with the Mg KL2,,L2,, Auger result, as the Mg Is intensity relative to the A1 2p line varied from sample to sample. This variation is attributed to the shallow escape depth of Mg 1s photoelectrons, which have fairly small kinetic energies (ca. 180 eV) so thatH. SEYAMA AND M. SOMA 243 their intensity is very sensitive to the amount of surface contaminant. The resolved Mg 1s spectra are also included in fig.2. The Mg 1s binding energy and KL2,3L2,3 Auger kinetic energy resolved for the exchangeable Mg ion are 1305.1 and 1179.2 eV, respectively, while those for the non-exchangeable Mg ion are 1303.4 and 1181.2 eV, respectively. Thus it is apparent that there are differences of > 1 eV in both electron energies between the exchangeable and non-exchangeable Mg ion in montmorillonite. PHOTOELECTRON BINDING ENERGIES AND AUGER ELECTRON KINETIC ENERGIES The measured photoelectron binding energies and Auger electron kinetic energies of the Mg, Ca, Sr, Ba and Cd compounds are shown in tables 3-4, where also the same energies of exchangeable cations in Mg-, Ba- and Cd-montmorillonites are compared with those of the corresponding halides and oxides.A comparison of Mg Is binding energies and KL,,&2,3 Auger kinetic energies of the exchangeable and non-exchangeable Mg ions in montmorillonite with those in the magnesium halides and magnesium oxide is shown in fig. 3. Such a two-dimensional plot of photoelectron binding energy and Auger electron kinetic energy was proposed by Wagner et al. and called a chemical-state plot .16 The measured starting materials for magnesium chloride and bromide were the hexahydrates. However, the observed 0 Is peak heights for the compounds were lower than those expected for the hexahydrates, so it was considered that the water of crystallisation of these samples was partly lost in the high vacuum of the instrument. The same phenomena were also observed for X.P.S.measurements of other halides containing water of crystallisation. - non-exchangeable Mg in montmorillonite Ms 0 Mg Br2-6H20 0 i( 0 exchangeable Mg in montmorillonite I I 1307 1305 1303 Mg 1s binding energy/eV Fig. 3. Chemical-state plot for Mg. As shown in fig. 3, the position in the chemical-state plot of exchangeable Mg in Mg-montmorillonite falls between those of magnesium chloride and fluoride. There are only small differences (ca. 1 eV or less) in the Mg Is binding energy between the 9 FAR 1244 X.P.S. STUDY OF MONTMORILLONITE Table 3. Photoelectron binding energies of Ca and Sr compounds fluoride (CaF,, SrF,) 349.0 270.2 exchangeable cation 348.6 270.3 in montmorillonite a chloride (CaC1,) 348.5 - oxide (CaO, SrO) 345.9 268.8 a This value was not able to be determined owing to over- lapping of Sr 3p3/2 and Cl 2s lines.Table 4. Mg 1s binding energies, Mg KL,,,L,,, Auger kinetic energies and AEr values of Mg compounds Mg KL293L2,3 compound Mg ls/eV Auger /eV AEr/eVa MgFz 1306.3 exchangeable Mg 1305.1 non-exchangeable Mg 1303.6 MgCl, * 6H,O 1304.6 MgO 1303.7 MgBr, 6H,O 1305.1 in montmorillonite in montmorillonite 1177.0 1 179.2 1181.2 1 180.4 1181.5 1180.9 0.0 1 .o 1.5 1.7 1.9 2.7 Table 5. Cd 3d5/, binding energies, Cd M4N4,5N4,5 Auger kinetic energies and AEr values of Cd compounds Cd M4N4,5N4,5 compound Cd 3d5,,/eV Auger /eV AEr/eVa CdF, 405.8 exchangeable Cd 406.2 CdC1, * 2iH20 405.5 CdBr, * 4H,O 405.5 CdI, 405.6 CdO 404.0 in montmorillonite 378.4 378.5 380.0 380.2 380.7 382.7 0.0 0.5 1.3 1.5 2.1 2.5 exchangeable Mg ion in Mg-montmorillonite and each magnesium halide.The difference in the Mg KL,,3L2,3 Auger kinetic energy between the exchangeable Mg ion in Mg-montmorillonite and each magnesium halide is larger than the corresponding difference in the Mg 1s binding energy. Such larger differences in the Mg KL2,,L2,3 Auger kinetic energy may be attributed to the effect of extra-atomic relaxation,H. SEYAMA AND M. SOMA 245 Table 6. Ba 3d5/, binding energies, Ba M4N4,5N4,5 Auger kinetic energies and AEr values of Ba compounds Ba M4N4,5N4,5 compound Ba 3d5,2/eV Auger/eV AEJeV" BaF, 781.5 exchangeable Ba 781.0 BaC1, 2H20 781.4 BaO 779.5 in montmorillonite 595.1 0.0 595.4 - 0.2 595.1 -0.1 598.0 0.9 i.e. a screening of the final-state ion in the Auger transition by electrons from neigh- bouring atoms (see below).In contrast to the position of exchangeable Mg, the position in the chemical-state plot of non-exchangeable Mg, which is surrounded by four oxygen ions and two hydroxide ions (in octahedral coordination) in the aluminosilicate layers of mont- morillonite, is located close to that of magnesium oxide. The Mg 1s binding and KL,,,L,,, Auger kinetic energy of the non-exchangeable Mg ions are 1.5 smaller and 2.0 eV larger, respectively, than those of the exchangeable ions. These large differences may be attributed to the effect of neighbouring atoms, i.e. the flow of electronic charge to the Mg ion and the extra-atomic relaxation from surrounding oxygen atoms are larger for the non-exchangeable Mg ion, indicating its stronger interaction with oxygen.The success in differentiating between the exchangeable and non-exchangeable Mg ions by locating them among the reference compounds in the chemical-state plot demonstrates the usefulness of the chemical-state plot in characterizing the bonding states of cations in minerals. Chemical-state plots for Ba and Cd (3d5,, binding energy and M4N4,5N4,5 Auger kinetic energy) are shown in fig. 4 and 5. The M4N4,5N4,5 Auger line is weaker than the M5N4,,N4,, Auger line but is chosen for both chemical-state plots because the M5N4,5N4,5 Auger line is not sharp and therefore less suitable for a chemical-state plot, as pointed out previ0us1y.l~ Shifts in the Auger lines among barium compounds are smaller than those found among magnesium compounds, and the proximity of exchangable Ba ion in Ba-montmorillonite to barium halides is more pronounced.The differences in both Ba 3d5/, binding energy and M4N4,5N4,5 Auger kinetic energy between the exchangeable Ba ion and each barium halide are 0.5 eV or less, while there are differences of > 1 eV in both electron energies between the exchangeable Ba ion and barium oxide, as in the case of the exchangeable Mg ion. The chemical-state plot for Cd (fig. 5) shows a tendency similar to that for Mg (fig. 3). Thus the differences in Cd 3d5/2 binding energy between several cadmium halides and the exchangeable Cd ion in Cd-montmorillonite are small, whereas the differences in Cd M4N4,5N4,5 Auger kinetic energy between them are larger. The differences in both Cd 3d5,, binding energy and M4N4,5N4,5 Auger kinetic energy between the exchangeable Cd ion and cadmium fluoride are -= 0.5 eV, while the Cd 3d5,, binding energy of exchangeable Cd ion is significantly higher (2.2 eV) than that of cadmium oxide and the Cd M4N4,5N4,5 Auger kinetic energy of exchangeable Cd ion is significantly lower (4.2 eV).In the X.P.S. measurements of calcium compounds, the Ca L2,3M2,3M,,3 Auger line is observed but is not very intense. Further, it is difficult to determine its precise peak 9-2246 X.P.S. STUDY OF MONTMORILLONITE exchangeable Ba in montmorillonite 1 I I I 783 781 779 Ba 3d512 binding energy/eV Fig. 4. Chemical-state plot for Ba. exchangeable Cd in montmorillonite 407 405 403 Cd 3dS12 binding energy/eV Fig. 5. Chemical-state plot for Cd.H.SEYAMA AND M. SOMA 247 position (kinetic energy) because the Ca L2,3kf&,,, Auger spectrum is complex and its shape varies from compound to compound. Neither is an intense Sr Auger line observed in the X.P.S. measurements of strontium compounds. Therefore only photoelectron (Ca 2p3/, and Sr 3pSl2) binding energies of exchangeable Ca and Sr ions in montmorillonite were compared with those of calcium and strontium compounds. The measured Ca 2p3/, and Sr 3p3/2 binding energies are shown in table 3. In those cases also, the differences in the photoelectron binding energies between the exchangeable cations in montmorillonite and the halides are small, whereas the differences in the photoelectron binding energies between the exchangeable cations and oxides are large.EXTRA-ATOMIC RELAXATION The sum of the photoelectron binding energy (Eb) and the Auger electron kinetic energy (Ek) of a certain atom has been defined as the modified Auger parameter (a’) by Wagner et a1.l6* l7 Kowalczyk et al. have examined the relative effect of extra-atomic relaxation on Auger electron kinetic-energy and photoelectron binding-energy shifts in metals and salts.’* Extra-atomic relaxation occurs through a flow of electronic charge toward the final-state ion in the Auger transition from neighbouring atoms, and is one of the important factors in determining the Auger electron kinetic energy. The difference in the value of a’ (Act‘) for the same element between two compounds is a good approximation to the difference in the extra-atomic relaxation energy (AE,) for the element between these compounds.’* Thus x AE,.( 2 ) By use of eqn ( 2 ) the differences in the extra-atomic relaxation energies for Mg, Ba and Cd between fluorides and other compounds were calculated. The calculated extra-atomic relaxation energy differences (AE,) are included in tables 4-6. The AE, values of exchangeable Mg and Cd ions in montmorillonite are intermediate between those of the corresponding fluorides and chlorides, as shown in tables 4 and 5. On the other hand, the AE, value of exchangeable Ba ions in Ba-montmorillonite is approximately the same as those of barium fluoride and chloride (AE, x 0), although the overall variation of AE, for Ba among barium compounds is smaller than those for Mg and Cd (see table 6).In addition, AE, values of exchangeable Mg, Cd and Ba ions are smaller (ca. 1 eV or more) than those of the corresponding oxides. Thus the effects of extra-atomic relaxation for the exchangeable Mg, Cd and Ba ions in montmorillonite are similar to those for the corresponding cations in fluorides and chlorides but not as strong as those for the same cations in oxides. This result suggests that these exchangeable cations in montmorillonite are not subject to the strong extra-atomic relaxation from the 0 ions which constitute the aluminosilicate layers of montmorillonite. On the other hand, it is considered that the non-exchangeable Mg ion in the aluminosilicate layers is more subject to the extra-atomic relaxation than the exchangeable Mg ion, as indicated by AE, of non-exchangeable Mg ion, which is 0.5 eV larger than that of the exchangeable one.The effect of extra-atomic relaxation for the non-exchangeable Mg ion, however, is not as strong as that for the Mg ion in magnesium oxide because AE, of the non-exchangeable Mg ion is smaller by 0.4 eV than that of the Mg ion in magnesium oxide.248 X.P.S. STUDY OF MONTMORILLONITE CONCLUSIONS The bonding states of exchangeable alkaline-earth and cadmium divalent cations between the aluminosilicate layers of montmorillonite are similar to those in the typically ionic chlorides and/or fluorides, as deduced from the chemical-state plots. These results indicate that the exchangeable cation is not strongly influenced by the negative charge of the 0 ion which is part of the montmorillonite lattice.The present result can be compared with X.P.S. measurements on zeolite, an aluminosilicate mineral which also holds exchangeable cations to compensate the negative charge originating from the replacement of silicon by aluminium in the lattice. It does not have a layered structure such as possessed by clay minerals but rather the three- dimensional network structure of an aluminosilicate. It has been shown by X.P.S. that the exchangeable metal cations in zeolite also have a highly ionic bonding character.l9? 2o Thus it is suggested that the exchangeable cations which compensate the negative charge originating from isomorphic substitution in aluminosilicate minerals form nearly pure ionic bonds with the minerals. X.P.S. is very informative in characterizing the bonding state of cations in these minerals, especially when combined with Auger electron spectroscopy. S. L. Swartzen-Allen and E. Matijevic, Chem. Rev., 1974, 74, 385. J. M. Adams, S. Evans, P. I. Reid, J. M. Thomas and M. J. Walters, Anal. Chem., 1977, 49, 2001. J. M. Adams and S. Evans, Clays Clay Miner., 1979, 27, 248. M. E. Counts, J. S. C. Jen and J. P. Wightman, J. Phys. Chem., 1973,77, 1924. J. W. Stucki, C. B. Roth and W. E. Baitinger, Clays Clay Miner., 1976, 24, 289. M. H. Koppelman and J. G. Dillard, ACS Symp. Ser., 1975, 18, 186. M. H. Koppelman and J. G. Dillard, Clays Clay Miner., 1977, 25, 457. M. H. Koppelman and J. G. Dillard, J. Colloid Interface Sci., 1978, 66, 345. * R. E. Grim, Clay Mineralogy (McGraw-Hill, New York, 2nd edn, 1968). lo M. H. Koppelman, A. B. Emerson and J. G. Dillard, Clays Clay Miner., 1980, 28, 119. l1 M. H. Koppelman and J. G. Dillard, Clays Clay Miner., 1980, 28, 21 1. l2 H. Seyama and M. Soma, Chem. Lett., 1981, 1009. l3 A. M. Posner and J. P. Quirk, Proc. R. SOC. London, Ser. A, 1964, 278, 35. I4 M. P. Seah, Surf. Interface Anal., 1980, 2, 222. l5 J. S. Hammond, S. W. Gaarenstroom and N. Winograd, Anal. Chem., 1975,47, 2193. l6 C. D. Wagner, L. H. Gale and R. H. Raymond, Anal. Chem., 1979,51,466. l7 C. D. Wagner, J. Electron Spectrosc. Relat. Phenom., 1977, 10, 305. S. P. Kowalczyk, L. Ley, F. R. McFeely, R. A. Pollak and D. A. Shirley, Phys. Rev. B, 1974,9, 381. Kh. M. Minachev, G. V. Antoshin, E. S. Shpiro and Yu. A. Yusifov, Proc. 6th Int. Congr. Catal. (The Chemical Society, London, 1976), p. 621. 2o H. Vinek, H. Noller, M. Ebel and K. Schwarz, J. Chem. Soc., Faraday Trans. 2, 1977, 73, 734. (PAPER 3/ 1074)

ISSN:0300-9599    

DOI:10.1039/F19848000237

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984, 80, 249-253 Carbon Dioxide-mediated Decomposition of Hydrogen Peroxide in Alkaline Solutions BY JOSE A. NAVARRO, MIGUEL A. DE LA ROSA, MERCEDES RONCEL AND FRANCISCO F. DE LA ROSA* Departamento de Bioquimica, Facultad de Biologia y CSIC, Universidad de Sevilla, Apartado 1095, Sevilla, Spain Received 24th June, 1983 Rapid hydrogen peroxide decomposition in aerated alkaline solutions is described, the maximum rate being attained at pH values between 11.5 and 11.7, where the peroxide @K, = 11.7) is ca. 50% unprotonated. The reaction proceeds with the release of protons and is strictly dependent upon the continuous presence of carbon dioxide, but not of carbonate anions, in the peroxide solutions. The following two-step mechanism is proposed: (1) formation of percarbonic acid (H,CO,) by condensation of CO, with the undissociated peroxide (H,O,) and (2) reduction of the acid by perhydroxyl anions (HO,).In previous papers1* describing a photochemical system for hydrogen peroxide production with flavin as photosensitizer, semicarbazide as electron donor and oxygen as electron acceptor, we found an anomalous and drastic decrease in the efficiency of the photoproduction of H202 at pH ca. 12, and the stability of H202 was found to be a function of pH. Hydrogen peroxide decomposition is a very complex reaction and it has been studied extensively for many years. The decomposition may take place spontaneously, but it is accelerated by the presence of catalysts such as metal ions (Fe, Cu, Mn),3-5 metal oxide (OSO,),~ metal complexes (metallotetraphenylporphyrins, iron-EDTA)'? * or enzymes (catalase, peroxidases).Two mechanisms have been proposed for the spontaneous decomposition of H202,3-57 but neither has been readily accepted. This paper deals with the effect of carbon dioxide on the decomposition of H,02 in alkaline solutions, which, to the best of our knowledge, has not been described previously. EXPERIMENTAL The kinetics and rates of the decomposition of hydrogen peroxide were studied in a cylindrical glass vessel of 2 cm diameter. The hydrogen peroxide (30%, analytical grade, Merck) solutions were prepared immediately prior to the experiments at a final concentration of 20-22 mmol dm-3 in 10 cm3 of doubly distilled water. The H202 solutions had a continuous stream of air, nitrogen (99.9973, oxygen (99.9%) or carbon dioxide (99.5%) passed through them, the flow rate of the gas stream being controlled by a Century Vit flowmeter and maintained at 0.1 dm3 min-l.In some experiments, the air was previously passed through a saturated Ba(OH), solution to remove CO,. The pH of the H,O,-containing solutions, especially that of the unbuffered ones, was continuously monitored during the experiments with a Radiometer pH-meter and kept at the desired value by addition of a small amount of concentrated NaOH or HCl solution. The H,O, content was determined by enzymatic reduction of the peroxide with o-dianisidine, as described previously,1o the absorbance of the complex so formed being measured at 440 nm with a Pye Unicam SP8-100 spectrophotometer. All the chemicals were reagent grade and used without further purification. 249/ * 10 11 12 13 PH Fig.1. Dependence on pH of decomposition of hydrogen peroxide in aerated solutions. 10 cm3 solutions containing 20 mmol dmP3 H,O, at the indicated pH were continuously aerated at a flow rate of 0.1 dm3 min-I, the peroxide content being measured as described in the experimental section. The solutions were buffered either with borate (pH 10.0, 10.5 and 11.0) or with phosphate (pH 11 .O, 1 1.5, 1 1.7, 12.0 and 12.5) at a final concentration of 0.6 mol dmW3. At pH 13.0 the solution was unbuffered but contained 0.1 mol dm-3 NaOH. In all cases the pH was continuously monitored throughout the experiments with a pH-meter and adjusted when necessary.201 15 m E a 0 - g 10 < ---. 3 E 5 0 20 40 60 time/min Fig. 2. Kinetics of decomposition of hydrogen peroxide at pH 12 under different gases. Nitrogen, oxygen, carbon dioxide or air either with or without CO, were passed through unbuffered solutions containing 20 mmol dm-3 H,O, in a final volume of 10 cm3. The pH was initially adjusted to 12 and maintained constant throughout each experiment. The H,O, content was measured at the indicated times.J. A. NAVARRO, M. A. DE LA ROSA, M. RONCEL AND F. F. DE LA ROSA 25 1 0 20 40 timelrnin Fig. 3. Requirement of carbon dioxide (but not of carbonate anions) for decomposition of hydrogen peroxide at pH 12. Either air (0) or C0,-free air (0) were passed through 10 cm3 solutions containing 20 mmol dm-3 H20, and 0.5 mol dm-3 carbonate buffer at pH 12.The arrow indicates the moment when the air stream, initially free of CO,, was passed directly through the peroxide solution without passing through the Ba(OH), solution. The pH was maintained at 12 throughout the experiments. The H202 content was determined at the indicated times. RESULTS AND DISCUSSION As shown in fig. 1, H,O, decomposed rapidly in aerated solutions containing the peroxide at a concentration of 20mmoldm-3, the maximum rate (0.7 mmol dm-3 min-l) being attained at pH values between 1 1.5 and 11.7. Significant decomposition of H,O, was not observed at pH < 10 or > 13. As the first pK, value of H20, is 11.7, coinciding with the pH region at which H,O, decomposition reached a maximum, it seems likely that the presence of both peroxide molecules and perhydroxyl anions is necessary for the process to take place, in accordance with the uncatalysed mechanism reported previo~sly.~~ However, when the H,O,-containing solutions, at pH 12, had either oxygen or nitrogen passed through them, no decomposition of H,02 was observed.In contrast, when carbon dioxide was used, the peroxide decomposed at a very high rate (see fig. 2). H,O, decomposition did not occur when the solutions had C0,-free air, obtained by passing the air stream through a saturated solution of barium hydroxide, passed through them (see also fig. 2). From this we deduced the effect of carbon dioxide on the decomposition of H,O, in alkaline solutions. Note, however, that in aqueous solutions CO, is diluted and hydrated to H2C03, quickly losing one or two protons, depending on the pH of the solution, as shown in the following simplified reactions CO, + H20 f HCO; + H+ Cog- + 2H+ K1 K2252 CO,-MEDIATED DECOMPOSITION OF H,O, where pKl and pK, are 6.3 and 10.3, respectively.ll As the C0,-dependent decom- position of H,O, was observed to be especially high at pH 11-12, where almost all the CO, is present as Cog- anions, it was investigated whether decomposition of H,O, occurs in the presence of such anions.The results obtained are shown in fig. 3, where it can be seen that peroxide decomposition at pH 12 does not take place in the presence of carbonate unless the solutions have air containing CO, passed through them. If the carbonated solutions of H,O, had C0,-free air passed through them, the decomposition reaction was much slower.These findings mean that CO,, but not carbonate anions, is the species responsible for catalysing the decomposition of H,O,. In view of these results, it appears that one possible explanation would be the oxidation of HOT by percarbonic acid, previously formed by reaction of CO, with H,O,: CO, + H,O, + H,C04 H,C04 + HO, + 0, + H,O + HCO,. The last reaction can be visualized as taking place as the result of a nucleophilic attack of perhydroxyl anions on percarbonic I L A acid, as shown in the following scheme: 1 A 1 I According to the proposed mechanism, and as we were working at pH 12, the decomposition reaction must involve the release of protons, since the bicarbonate anion, at that pH, will lose its proton to a form carbonate anion.The total reaction, at pH 12, would be as follows: CO, + H,O, + HO; -P 0, + H,O + C0:- + H+. In order to check this hypothesis, the set of experiments shown in fig. 4 was carried out. The pH of unbuffered solutions containing H,O,, initially adjusted to 12, decreased, as the peroxide was decomposing, at a higher rate than that of similarly treated solutions without H,O, in which the pH decrease was only due to CO, contained in the air being passed through the solutions. These results support the above mechanism but contradict that described previously by Abel,12 in which the decomposition of the peroxide proceeds through the uncatalysed reaction of perhydroxyls with undissociated peroxide molecules H,O, + HO; 4 H,O + 0, +OH- and brings about the concomitant release of hydroxyl anions. We thank Prof.Losada (Seville) for helpful advice and criticism and Prof. Munuera (Seville) and Mulders (Brussels) for stimulating discussions. This work was aided byJ. A. NAVARRO, M. A. DE LA ROSA, M. RONCEL AND F. F. DE LA ROSA 253 121 I 11 E 10 9 0 \ ' \ '\ 0 "\ '\ "\ ' 0 \ \ \ \ .\ \ \ \ 20 40 60 24 16 I Q 0 E E E M 1 - s 8' 0 time/min Fig. 4. Decrease of pH during the C0,-mediated decomposition of hydrogen peroxide in unbuffered solutions. A 10 cm3 solution containing 22 mmol dm-3 H,O,, whose pH was initially adjusted to 12 by addition of NaOH, was continuously aerated at a flow rate of 0.1 dm3 min-l. Its H,O, content (.---a) and pH (0-0) were measured at the indicated times. The pH of another aerated solution containing only 10 cm3 of distilled water was adjusted to 12 and its decrease also plotted (e-e).grants from Centro de Estudios de la Energia (Spain), Comision Asesora de Investigacion (Spain) and Philips Research Laboratories (The Netherlands). M. R. was a fellow of the Ministerio de Industria y Energia (Spain). M. A. De la Rosa, J. A. Navarro, M. Hervas, F. F. De la Rosa and M. Losada, Proceedings of1 Congreso Ibkrico de Energia Solar ISES, in press. M. A. De la Rosa, J. A. Navarro, F. F. De la Rosa and M. Losada, Photobiochem. Photobiophys., 1983, 5, 93. D. K. Jaiswal, R. N. Ram and B. B. Prasad, 2. Phys. Chim., 1982, 263, 74. H. W. Richter and W. H. Waddell, J. Am. Chem. SOC., 1982,104,4630. 0. Spalek, J. Balej and I. Paseka, J. Chem. Soc., Faraday Trans. I , 1982, 78, 2349. L. Nagy, Z. M. Galbacs, L. J. Csanyi and L. Horvath, J. Chem. Soc., Dalton Trans., 1982, 859. I. Mochida, A. Yasutake, H. Fujitsu and K. Takeshita, J. Phys. Chem., 1982, 86, 3468. E. N. Rizkalla, 0. H. El-Shafey and N. M. Guindy, Inorg. Chim. Acta, 1982, 57, 199. F. R. Duke and T. W. Haas, J. Phys. Chem., 1961,65, 304. lo A. G. Fontes, F. F. De la Rosa and C. Gbmez-Moreno, Photobiochem. Photobiophys., 1981, 2, 3 5 5 . l1 M. G. Guerrero, C. Manzano, J. Cardenas and M. Losada, Manometria (University Press, Seville, l2 E. Abel, Monatsh. Chem., 1952, 83, 422. 1975), pp. 18-19. (PAPER 3/ 1087)

ISSN:0300-9599    

DOI:10.1039/F19848000249

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1984, 80, 255-265 Muon Spin Rotation Spectra for Muonium Isotopically Substituted Ethyl Radicals BY MARIA JOXO RAMOS,? DANIEL MCKENNA AND BRIAN c. WEBSTER* Chemistry Department, University of Glasgow, Glasgow G12 8QQ, Scotland AND EMIL RODUNER Physikalisch Chemisches Institut der Universitat, CH-8057 Zurich, Switzerland Received 24th June, 1983 The temperature dependence of the B-hyperfine coupling constant is reported for muonic radicals formed in ethene CH,=CH,, [,H,]ethene CH,=CHD, [,H,]ethene CHD=CD, and [ZHJethene CD,=CD,. These studies are complemented by e.p.r. observations upon the radical CH,D~H,. The barrier to internal rotation for the radical CHD,cD, is discussed. Muonium-substituted organic radicals are formed in liquid unsaturated hydrocar- bons upon which is incident a beam of p+-mesons.Although the primary events yet have to be elucidated, one of the possible precursors is the muonium atom Mu, created as the bound state of a p+-meson with an excess electron. Radicals are generated by the addition of muonium to the unsaturated molecule.' The theory for the analysis of muon spin rotation spectra @.s.r.) has been extended recently by Roduner and Fischer to muonium-substituted radicals.,. Elsewhere muon-electron hyperfine coupling constants have been reported for muonium- substituted alkyl and ally1 radical^.^ The temperature dependence of the /3-hyperfine coupling constant for the radical CD,MucD, produced in deuterated [,H,] ethene has revealed the existence of a substantial barrier to internal rotation of the CD,Mu group about the C,-C, single bond.The height of the barrier is of the order of 2710 J mol-l.5 Here we report p.s.r. spectra for radicals created in the deuterated ethenes CH,CHD and CHDCD,. These substrates, together with C2H4 and C,D,, have been selected in order that the interactions which could give rise to the barrier can be discerned. The analysis is given in the following paper.s FOURIER-TRANSFORM p.S.R. SPECTRA Observations upon the deuterated ethenes were made at the muon channel of the Swiss Institute for Nuclear Research (SIN), Villigen. The gases, having been obtained from MSD isotopes, were used without further purification. Following liquefaction, oxygen together with other dissolved gases was removed on a vacuum line by a sequence of freeze-pumpthaw cycles.The samples contained in spherical glass ampoules of 25 and 35 mm diameter were maintained at liquid-nitrogen temperature before insertion into the cryostat of the ,u.s.r. spectrometer. t Present address: Departamento de Quimica, Faculdade de Cizncias, 4000 Porto, Portugal. 255256 MUON SPIN ROTATION SPECTRA OF ETHYL RADICALS After removal of background events and the natural decay of muonium, histograms representative of 107-108 events were subjected to Fourier transformation, procedures for apodisation having been im~lemented.~ Fig. 1 and 2 show the Fourier-transform p.s.r. spectra for the species formed in ethene and perdeuterated ethene, together with the effect of temperature upon the line position and the line width.In diamagnetic environments bare muons precess at a Larmor frequency equal to 1.355 x 1 O8 Hz T-l. Under a transverse applied magnetic field of 0.16 T a diamagnetic fraction should appear in each spectrum at 21.68 MHz. Such signals have been observed but are excluded here for the purpose of clarity. Each spectrum consists of a pair of lines at all of the temperatures studied, although for ethene at 1 1 1 K broadening of the lines is evident. The signals are attributed in fig. 1 to the muonic ethyl radical CH,Mu CH,, and in fig. 2 to the perdeuterated radical CD,MuCD,. Fig. 3 and 4 in contrast show the presence of two radicals formed in monodeuterated ethene and trideuterated ethene, respectively. For reasons to be stated later, the signals denoted by A,, A, in fig.3 are ascribed to the radical CHDMucH,, with the radical CH,MucHD being represented by the pair of lines labelled B,, B,. Similarly, in fig. 4 the signals denoted by A,, A, are attributed to the radical CHDMucD,, with the pair of lines B,, B, being assigned to the radical CD,MuCHD. Note that in both liquids a radical is formed having a chiral centre created by the hydrogen isotopes, muonium, protium and deuterium. In all of the p.s.r. spectra the frequencies at which the signals for a particular muonic radical appear to decrease with increasing temperature ; consequently there is a decrease of the /I-hyperfine muon-electron coupling constant with increasing tem- perature. Values for these coupling constants are collated in table 1. To allow a comparison to be made with proton-electron hyperfine coupling constants for the analogous protium-substituted radical, the muon-electron coupling constants are cited in the reduced form A; defined by where p p and pp are the magnetic moments for the proton and the muon, respectively, the ratio pp/pp being equal to 0.3141.Fig. 5 shows the variation of the /I-hyperfine muon-electron coupling constants with temperature for six muonium-substituted ethenes. E.P.R. SPECTRA FOR THE RADICAL CH,DcH, The radical CH,DcH, was generated following the procedure outlined by Itzel and Fischer and Burkhard and Fischer.87g Ethyl oxirane solutions containing 0.8 vol % monodeuterated ethyl bromide, 17 vol % di-t-butylperoxide and 10 vol % triethylsilane were photolysed in the cavity of a Varian E-4 spectrometer.Reaction of silyl radicals with deuterated ethyl bromide results in production of the radical CH,DcH,. The hyperfine interaction was measured in the temperature range 163-273 K. The first-derivative e.p.r. spectrum observed at 180 K is shown in fig. 6. Only the outer lines have been used to measure the coupling constant. Of the lines numbered 1-27 in fig. 6, lines 1-4 and 24-27 were scanned accurately with a frequency counter at each temperature. The relationshipsM. J. RAMOS, D. MCKENNA, B. C. WEBSTER AND E. RODUNER 257 120 K 1 . . . 1 . . . 1 . , . . 1 . . . 1 . . . 1 . . . 1 . . . 1 . . . 1 . . . 1 I b O 120 140 160 I60 200 220 240 260 280 300 frequency /MHz Fig. 1. Fourier-transform p.s.r. spectra for the muonic ethyl radical at five temperatures and applied transverse field of 0.16 T.258 MUON SPIN ROTATION SPECTRA OF ETHYL RADICALS frequency /MHz Fig.2. Fourier-transform p.s.r. spectra for the muonic radical formed in tetradeuteroethene at five temperatures and applied transverse field of 0.16 T.M. J. RAMOS, D. MCKENNA, B. C. WEBSTER AND E. RODUNER 259 I I80 K L ~ . . . ~ l l . l . I l l l l . l l l . l l . I 1 . . . I 140 160 180 200 220 240 260 280 frequency /MHz Fig. 3. Fourier-transform p.s.r. spectra for the muonic radicals formed in monodeuteroethene at five temperatures and applied transverse field of 0.16 T. A,, A, are frequencies for CHDMucH, and B,, B, are frequencies for CH,MueHD. were adopted to specify values for the a-protium A g , #?-protiurn A? and #?-deuterium A; hyperfine coupling constants.These values are listed in table 2. For the #?-deuterium coupling constant a reduced value A;, defined by A6 = IAFl(pp/PD) (3) where pD is the magnetic moment of the deuteron, is given also to facilitate comparison with the p.s.r. results. Fig. 7 contrasts the behaviour of the temperature dependence for the #?-proton4ectron coupling constant with the #?-deuteron coupling constant. It is to be seen that A; decreases with increasing temperature, ranging from 77.88 MHz at 163 K to 76.54 MHz at 273 K. The #?-deuteron reduced interaction A;, increases from 70.8 to 72.6 MHz over the same temperature range. The mean of the coupling constants equal to 1/3 (2AF+Ab) shows only a slight variation with temperature, decreasing by 0.29 MHz from a value of 75.52 MHz at 163 K to 75.23 MHz at 273 K.260 MUON SPIN ROTATION SPECTRA OF ETHYL RADICALS 129 K I 144 K # I 164 K I 184K 1 1 l .. ~ ~ 140 160 180 200 220 240 260 280 frequency /MHz Fig. 4. Fourier-transform ,u.s.r. spectra for the muonic radicals formed in trideuteroethene at hur temperatures and applied transverse field of 0.16 T. A,, A, are frequencies for CHDMueD, and B,, B, are frequencies for CD,MueHD. HYPERFINE INTERACTIONS AND RADICAL CONFORMATION The hyperfine interaction originates from electron spin density at the nuclear site under observation. For the isotopically substituted ethyl radicals the major component of the electron spin density drives from the 2pz orbital centred at the a-carbon nucleus. If 0 defines the dihedral angle for rotation of the C, - X axis involving the substituent X to eclipse this 2p, orbital centred at C,, an equilibrium conformation for the radical is specified by 0, where 0, is the value of 0 at the minimum of a two-fold barrier taken to replicate the potential hindering internzl rotation about the C,-C, axis.A potential barrier is of the form defined by This barrier can be discerned to be derived by truncation of a Fourier series for V(0) with only the two-fold term being retained. Furthermore, assuming that the internal rotation of a CH,X group about the CB-C, internuclear axis can be treated indepndently of vibrational motion, molecular rotation and solvent interaction, the torsional Hamiltonian H(0) given byM. J. RAMOS, D. MCKENNA, B. C. WEBSTER AND E.RODUNER 26 1 Table 1. Muon+lectron hyperfine coupling constants for muonic radicals formed in ethene, monodeuterated ethene, trideuterated ethene and tetradeuterated ethene measured at an applied transverse magnetic field of 0.16 T olefin radical T/Ka AJMHz ALjMHzf A,/MHz ref. ethene CH,MueH, CH,=CH, [,H,]ethene CH,MucHD CHD=CH, CHDMucH, [,H,]ethene CD,M&HD CHD=CD, CHDMU~D, E2H,]ethene CD,MueD, CD,=CD, 111 120 140 162 182 110 122 139 160 180 110 122 139 160 180 129 144 164 184 129 1 44 164 184 113 120 141 161 183 416.3b 409.6 395.5 381.9 371.4 420.4b 410.6 397.8 384.3 373.9 423.4b 414.0 401.7 388.3 377.9 425.4d 412.6 396.8 383.2 428Sd 416.1 401.0 386.7 438.5b 431.7 416.0 401 .O 387.8 130.8b 75.3" (1 1) and (12) 128.7 124.2 120.0 116.7 132.1b this work 129.0 125.0 120.7 117.4 130.0 126.2 122.0 118.7 133.6d 129.6 124.6 120.4 1 34.6d 130.7 126.0 121.5 135.6 130.7 126.0 121.8 133.0b B (11) and (12) 1 37.7b e (11) and (12) a +2 K; kO.1 MHz; nearly temperature independent; k0.2 MHz; temperature dependent; at 98 K, for example, its value is 83.4 MHz; f Ah = lAplpp/pp; g temperature dependent; at 97 K, for example, its value is 79.1 MHz.can be employed to calculate the torsional energy 1evels.lO In eqn ( 5 ) I, the reduced moment of inertia. is defined as in terms of the moments of inertia Il and I, for the two groups rotating about the CB-C, axis evaluated at the equilibrium molecular geometry. If there exists a Boltzman population of the torsional energy levels Ei, the temperature dependence of the /I-hyperfine coupling constant will follow262 MUON SPIN ROTATION SPECTRA OF ETHYL RADICALS 1 1 I 1 1 1 I 1 120 140 160 180 TIK Fig.5. Temperature dependence of the reduced muon hyperfine coupling constants for CH,MuCH, (@), CHDMucH, (0), CH,MucHD (O), CD,MuCHD (a), CHDMU~D, ( X ) and CD,MuCD, (0) at an applied transverse field of 0.16 T. 12 14 16 1 2 3 25 26 27 Fig. 6. First-derivative e.p.r. spectrum for the CH,DcH, radical at 180 K. where y = O+O, and is the expectation value of the P-hyperfine coupling constant for the molecule in the ith torsional level. Often the angu!ar dependence of the P-proton-electron hyperfine interaction and P-deuteron-electron interaction is (8) represented in the form of = + cos2 where the leading constant L represents a contribution to the hypefine interaction arising from a spin polarisation mechanism and the second constant A4 describes the variation with torsion angle of the electron spin density at the Q-nucleus. In the following paper we demonstrate that such a relationship can be extended toM.J. RAMOS, D. MCKENNA, B. C. WEBSTER AND E. RODUNER 263 Table 2. a-proton (A:), 8-proton (Ap), 8-deuteron (A?) and /?-deuteron (Ah) reduced hyperfine coupling constants for the CH,DcH, radical measured at 180 K. T/K A,H/MHz AF/MHz A'IMHz" AF/MHz AL/MHz 163 173 183 193 203 213 223 233 243 253 273 62.38 f 0.03 62.30 f 0.06 62.35 f 0.03 62.33 f 0.03 62.33 f 0.03 62.30 f 0.01 62.33 f 0.03 62.33 f 0.03 62.24 f 0.03 62.27 f 0.03 62.27 f 0.03 77.88 & 0.03 77.77 f 0.03 77.63 f 0.03 77.49 f 0.03 77.35 f 0.03 77.2 1 f 0.03 77.07 f 0.03 76.93 f 0.03 76.93 f 0.03 76.76 f 0.03 76.54 & 0.03 75.52 75.51 75.49 75.49 75.47 75.44 75.41 75.32 75.39 75.34 75.23 10.76 & 0.03 10.79 f 0.03 10.8 17 f 0.008 10.87 0.01 10.90 f 0.03 10.93 fO.01 10.958 4 0.008 10.96 f 0.03 10.98 & 0.03 1 1.01 f 0.03 1 1.04 0.06 70.8 f 0.2 71 .Of 0.2 71.2 f0.2 71.520.2 71.7f0.2 71.9f0.2 72.1 f 0.2 72.1 f 0.2 72.3 & 0.2 72.5 f0.2 72.6 f 0.2 a A' = &(2A,H+Ai).I I I I I I 160 180 200 220 240 260 280 TIK Fig. 7. Temperature dependence of the (a) 8-proton and (b) Bdeuteron reduced hyperfine coupling constants for CH,DcH radical. /3-muon+lectron hyperfine interactions. For occupancy of the ith torsional level the expectation value can be evaluated from <A&))i = L + M ( ilcos2 yli) (9) adopting the wavefunction defined by 1 v / .= ~ I= c , exp (ima ' ~ ' ( 2 ~ 1 m264 MUON SPIN ROTATION SPECTRA OF ETHYL RADICALS The result is expressed as ( ilcos2 y l i ) = 4 Z C,[C, +gC,,, (cos 28, - sin 20,) +$Cm-, (COS 28, + sin 28,)]. m (1 1) For example, if 0, = 0 then (i)cos2 yli) reduces to the simple expression The expansion coefficients C , are obtained by solution of the secular problem arising from adoption of the Hamiltonian in eqn (5). The secular determinant then has the - . n2 Hmm = -m2-/-- 21 2 elements defined in HmmI =- -v, m'=m+2,m-2 Hmml = 0, rn' # m+2, m-2. 4 , (13) Some years ago Fessenden and Schuler measured the B-proton-electron hyperfine coupling constant for the radical CHD,cD, to equal 83.43 MHz at 98 K. Fessenden suggested that the equilibrium conformation is defined by 8, = 0" with a barrier of height 384.9 J mol-l.By comparison both of the radicals CH3eH, and CD$D2 reveal no temperature dependence of the /?-hyperfine coupling, from which observation it is concluded there is no detectable barrier which hinders internal rotation in these radicals. For the radical CHD,cD, with values 8,, L = 0.0 MHz, M = 150.61 MHz, = 384.9 J mol-l and Z = 3.47 x lov4' kg m2, a solution of the secular problem in the manner outlined provides three torsional levels within the barrier. These levels are located at E, = 148.7 J mol-l, E2 = 182.1 J mol-1 and E3 = 371.6 J mol-l, as depicted in fig. 8. With such a barrier to internal rotation the /I-hyperfine constant at 98 K is calculated from eqn (7) as 83.47 MHz.The temperature dependence of the B-deuterium-electron hyperfine interaction displayed in fig. 7 is consistent with an equilibrium conformation for CDH,cH, such that 0,(D) = 90°. For the muonic ethyl and perdeuterated radicals CH2MucH2 and CMuD2cD2 the high values for the reduced B-hyperfine muon-electron interaction, t Fig. 8. Barrier Y(O))/J rno1-l to internal rotation for the deuterated ethyl radical CHD$D,; the equilibrium conformation is defined by 8, = 0".M. J . RAMOS, D. MCKENNA, B. C . WEBSTER AND E. RODUNER 265 130.8 MHz at 1 1 1 K for CH,Mu CH, and 132.1 MHz at 110 K for CD,MuCD,, are indicative of an equilibrium conformation O,(Mu) = 0' restrained by a substantial barrier. In the monodeuterated muonic radical CHDMucH, and the trideuterated radical CHDMu CD2 the C, symmetry of the CX, group is maintained.Accordingly the higher-frequency doublet of the p.s.r. spectrum has been assigned to these species in fig. 3 and 4, for it is anticipated that the barriers to internal rotation will be slightly higher than for the radicals CH,MutHD and CD,MuCHD. In the following paper we support such an assignment in a detailed study of the barriers to internal rotation for muonium-substituted ethyl radicals. We thank Dr Lung-min, who kindly assisted with the e.p.r. measurements. We also thank the Instituto Nacional de Investigaciio Cientifica, Lisbon, for the award of a research studentship to M. J. R. and the S.E.R.C. for a research studentship to D. McK. This work is supported by the Swiss Institute for Nuclear Research. E. Roduner and B. C. Webster, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 1939. P. W. Percival and H. Fischer, Chem. Phys., 1976, 16, 89. E. Roduner and H. Fischer, Chem. Phys., 1981, 54, 261. E. Roduner, W. Strub, P. Burkhard, J. Hochmann, P. W. Percival, H. Fischer, M. Ramos and B. C. Webster, Chem. Phys., 1982, 67, 275. B. C. Webster, M. J. Ramos and E. Roduner, Proc. 5th Tihany Symp. (Akademiai Kiado, Budapest, 1983), pp. 135-140. M. Ramos, D. McKenna, B. C. Webster and E. Roduner, J . Chem. Soc., Faraday Trans. I , 1984,80, 267. ' J. H. Brewer, D. G. Fleming and P. W. Percival, in Fourier, Hadamard and Hilbert Transforms in Chemistry, ed. A. G. Marshall (Plenum Press, New York, 1982), pp. 345-385. a H. Itzel and H. Fischer, Helv. Chim. Acta., 1976, 59, 880. P. Burkhard and H. Fischer, J . Magn. Reson., 1980, 40, 335. lo P. J. Krusic, P. Meakin and J. P. Jesson, J. Phys. Chem., 1971, 75, 3438. l1 R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 1963,39, 2147. lZ R. W. Fessenden, J. Chim. Phys., 1964,61, 1570. (PAPER 3/1089)

ISSN:0300-9599    

DOI:10.1039/F19848000255

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1984,80, 267-274 The Barriers to Internal Rotation for Muonic-substituted Ethyl Radicals BY MARIA Joio RAMOS,~ DANIEL MCKENNA AND BRIAN C. WEBSTER* Chemistry Department, University of Glasgow, Glasgow G12 8QQ, Scotland AND EMIL RODUNER Physikalisch-Chemisches Institut de Universitat, CH-8057 Zurich, Switzerland Received 24th June, 1983 By fitting of the observed temperature dependence of the b-hyperfine muon-electron interaction to a theoretical model for the muonic radicals CH,MueH,, CHDMucH,, CHDMucD, and CD,MucD, values for the barrier height & restricting internal rotation are calculated. A decomposition of the barrier into pair interactions indicates the isotope effect upon the barrier to be in the sequence VpD < VpH < VFH < VpuD < TuH.On this basis the barriers to internal rotation for the radicals CH,MuCD, and CD,M&H, are predicted to be of the order 3076 and 3 186 J mol-l, respectively. Muon spin rotation (,u.s.r.) and electron paramagnetic resonance (e.p.r.) studies of several isotopically substituted ethyl radicals in the liquid phase have shown that the radicals formed exhibit temperature-dependent B-hyperfine coupling constants.' The hyperfine coupling constants for protons in analogous B-positions for alkyl radicals in liquids are known to obey the relation given by where L and M are constants (M & L) and 8 is the dihedral angle between the axis of the p,orbital at C, and the Cp-H axis. 8, specifies the equilibrium conformation of the radical by the value of 8 at the minimum of the potential barrier to internal rotation.In a classic study of internal rotation in alkyl radicals Fessenden has shown that eqn (1) is equally valid for deuterons in B-positions.2 Therefore it might be anticipated that eqn (1) will be applicable to radicals with a muon located at the p-posi tion. By fitting the experimental temperature dependence of the /I-coupling constants to a theoretical model using the quantum-mechanical averaging of the /I-coupling constants described previously, the barrier hindering internal rotation can be assayed for the radi~al.l-~ INTERNAL-ROTATION STUDIES In order to calculate the barrier V(8) to internal rotation, the experimental curve for the /I-hyperfine coupling constants at differing temperatures has been fitted to the t Present address : Departamento de Quimica, Faculdade de Cihcias, 4000 Porto, Portugal.267268 INTERNAL ROTATION IN ETHYL RADICALS curve, calculated from eqn (1) and z <AB(Y))i exp ( - Ei/k T) Cexp(-E,/RT) i = i (3) v, V(0) = - (1 - cos 20) 2 with L, M and v, as adjustable parameters. Quantitative values have been determined for the barriers to internal rotation exhibited by the different muonic radicals reported Table 1. Reduced moments of inertia for isotopic ethyl radicalsa radical I / kg m2 CH2MucH2 1.65 CHDMU~H, 1.92 CH,MucHD 2.08 CHDMU~D, 2.85 CD,MueHD 2.75 CD,MueD, 3.26 CH,D~H, 2.08 CHD2cD, 3.54 a Geometry for the radicals: dis- tances/ 10-lo m CB-Mu (1.094), CB-H angles/' MuCBC, (109.5), HC,H (120). (1.094), CBxC, (1.335), C5-H (1.083); previously, and also for the monodeuterated ethyl radical CDH,cH, together with the tetradeuterated ethyl radical CHD,t]D,.'.The muonic radicals studied here, it can be recalled, are the muonic ethyl radical, CH,MucH,, the two muonic mono- deuterated ethyl radicals, CH,MucHD and CHDMucH,, the two trideuterated ethyl radicals, CD,MucHD and CHDMucD,, and the tetradeuterated ethyl radical, CD,MucD,. Amongst these radicals two do not have a C2 symmetry rotating group, CH,MueHD and CD,MucHD. The calculation has proceeded on the assumption that the departure from symmetry for these species will not influence strongly the values obtained for the parameters. The reduced moments of inertia used in the calculation are collated in table 1 . A least-squares fit of eqn (2) to the experimental data has been performed using twenty-one wavefunctions in the calculation.These fits have been conducted in two different ways: the first allows for a simultaneous variation of the three parameters L, M and V,, the barrier to internal rotation; the second allows for the simultaneous variation of the two parameters M and V,, whilst L is equated to zero. The effect that a change in I, the reduced moment of inertia, would have upon the optimum values for V, has also been investigated. Simultaneous variation of the three parameters L, M and V , produces theoretical B-coupling constants whose values are identical to the experimental values. The resulting values for L, M and V, are listed in table 2 for all of the radicals. The radical CHD,cD, has been studied also.The resulting values for the parameters are in good agreement with those given by Fessenden.,M. J. RAMOS, D . McKENNA, B. C. WEBSTER AND E. RODUNER 269 Krusic et aL3 report that L is a contribution arising from spin polarisation while M is probably related to the hyperconjugative delocalisation of the unpaired electron onto the CX,X,X, group (Xl, X,, X, = Mu, H, D). It can be.observed from table 2 that the parameters L and'M for all the muonic radicals are larger than those for the radicals CDH,CH, and CHD,CD,. The values of the parameters suggest that both L and M are isotopically dependent. Table 2. Values for the parameters L, M and V, for muonic radicals, simultaneous variation of L, M and V, radical L/MHz M/MHz V,/J m o P - 2.8 - 0.5 - 16.3 - 17.6 - 13.1 - 20.0 - 20.0 - 23.4 156.4 151.5 19 1 .O 198.9 190.6 200.7 201.7 197.1 340 376 2845 2483 2704 2898 2927 3452 a 8, (Mu) assumed to be zero.130 120 120 140 160 180 T/K Fig. 1. Temperature dependence of the experimental three-parameter fit (0, solid line) and two- parameter fit (El, dashed line) reduced muon-electron hyperfine coupling constants for the CH,M&H, radical. The values of V , are much larger for the muonic radicals. Thus for CD,MucD, a barrier equal to 3452 J mol-l is indicated, in comparison with 376 J mol-l for the species CHD,cD,. This indicates a strong isotope effect and larger vibrational effect for muonium on the rotational barrier than is exerted by either protium or deuterium. Previously it had been suggested for these muonic radicals that the larger hyperfine interactions are associated with higher barriers to internal rotation in the radical.The V , values listed in table 2 could therefore seem anomalous for CH,MucHD and270 INTERNAL ROTATION IN ETHYL RADICALS 140 130 2! E - a P 1 120 1 - - - - a P 1 I 1 1 I I 1 1 1 1 120 140 160 180 TI K Fig. 2. Temperature dependence of the experimental three-parameter fit (0, solid line) and two-parameter fit (m, dashed line) reduced muon-electron hyperfine coupling constants for the CHDMU~H, radical. ~ ~~~~ 120 140 160 180 Tl K Fig. 3. Temperature dependence of the experimental three-parameter fit (0, solid line) and two-parameter fit (D, dashed line) reduced muon-electron hyperfine coupling constants for the CHDMucD, radical.CHDMucH,, respectively. This disparity could be attributed to the restriction of 8, (Mu) to zero in the fit. For all of the other radicals in table 2 the pattern conforms to the above stated hypothesis. Although the results are reasonable one must allow for a possible variation in the three parameters when treated simultaneously; therefore similar fits have been made equating L to zero. Fig. 1-4 show the experimental and theoretical dependences of the muon-electron /%coupling constant with this choice for the parameters. In each case the theoretical curve does not coincide exactly with the experimental curve. ThisM. J. RAMOS, D. McKENNA, B. C. WEBSTER AND E. RODUNER 27 1 12 14 16 18 TI K Fig. 4. Temperature dependence of the experimental three-parameter fit (0, solid line) and two-parameter fit (m, dashed line) reduced muon-electron hyperfine coupling constants for the CD,MucD, radical.Table 3. Values for M and 4, for muonic radicals, with L=OMHz radical M/MHz V,/J mo1-I CDH,cH, CHD,cD, CH,MueH, CH,MueHDa CHDMucH," CHDMuCD," CD,MucD, CD,MU~HD" 150.8 150.5 170.9 171.6 173.2 173.5 174.8 177.0 352 379 2710 2649 2665 2894 2906 2710 a 8, (Mu) assumed to be zero. discrepancy could be attributed to the truncation of the expansion of the hyperfine coupling constant or it reflects some distortion in the radicals at both the C , and Cp centre^.^ Final values for M and % are listed in table 3 for all of the radicals. Although there is a disparity in magnitude between the values obtained for & by the two approaches as shown in tables 2 and 3 the trend is in good accord in both cases, except for the radical CD,M&D,.For this radical & is lower than anticipated for the choice of L equal to zero. Finally, the effect of a change in I by 10% results in a change in V , of only 0.5%. This is a negligible effect. PARTITION OF THE POTENTIAL It has been assumed that the total potential V(8) is given by the truncated expression of eqn (3). This total potential term can be viewed as being representable by a sum272 INTERNAL ROTATION IN ETHYL RADICALS of terms each of which simulates the interaction of two substituents, one located at the C , nucleus and the other at the Cg nucleus: where i a n d j refer to substituents at the a-carbon and /?-carbon nuclei, respectively.2 Assuming that each of these terms Vij can be expressed in a Fourier series of even terms which can be truncated, as in v;i vij = -(1 -cos 2yu) ( 5 ) 2 Vij will represent the barrier to rotation of the pair ij noted in Therefore the total barrier to internal rotation V can be expressed by (7) v;j i j 2 v= x x -(1 -cos 2yu) Applying eqn ( 7 ) to the general case of the radical CXYZ CA, (X, Y, Z, A, z Mu, H, D) we obtain V = 2(4 VfA [ 1 - cos 2(8 + 8,X)l) + 2(8 VTA [ 1 -COS 2(8 + 8:)]) + 2(ie~ [ 1 - cos 2(8 + e?)]).(8) To calculate the maximum and the minimum of the potential V relative to the angle 8, we require dV/d8 = 0 as in = (2 VFA cos 28f + 2 eA cos 28: + 2 qA cos 2 8 3 sin 28 d V d8 - + ( 2 VfA sin 28f + 2 cA sin 28: + 2 qA sin 2 8 3 cos 28 = 0. (9) Lastly a generalised equation is obtained as v = vmax - vmin = "2 v ~ * COS 2ef + 2 GA cos 28: + 2 q~ cos 2832 + (2 VFA sin 28F + 2 cA sin 28: + 2 @A sin 28?)2]1/2 (10) where the quantity ( Vmax - Vmin) is the barrier height that has been calculated in the previous section for the muonic radicals noted by 2( VFH - VpH) = a 2( VpH - VpD) = b 2(VP"D- v y ) = c 2( VPuH - VF") = d where a, 6, c and d are the barrier heights for the radicals CH,DcH,, CD,HCD,, CD ,MuCD , and CH ,MUCH 2, respec tivel y .Table 4 presents the values taken by VpH, VFH, VpuD and VyuH as a function of VpD calculated using the barrier heights obtained by simultaneous variation of the three parameters L, M and G, and with the placement of L = 0. As can be seen from table 4 the values for the interaction between two substituents, although differing in magnitude according to the value taken by L, are placed in the order VpH < V y < VpJD < V p H .(12)M. J. RAMOS, D. McKENNA, B. C. WEBSTER AND E. RODUNER 273 Such a sequence provides support for the notion that the lighter isotope experiences a higher and the heavier isotope a lower degree of steric hindrance; the lighter isotope has in effect a higher interaction radius than the heavier isotope. The interaction VpD is anticipated to be the smallest interaction between two substituents, one atom at the a-carbon nucleus and the other at the /?-carbon nucleus, as in the sequence (13) vpD < VpH < v p < V p D < V y H . Table 4. Calculated values (J mol-l) of VpH, VFH, VFuD and VFuH as a function of VFD simultaneous simultaneous variation of variation of VFY- VpD L, M and V, MandV,;L=O 188 358 1726 1781 190 365 1355 1721 Table 5.Calculated values of the barrier heights V, for the radicals CH,MutHD, CHDMueH,, CD,MdHD, CHDMutD,, CH,MutD, and CD,MU~H, radical V,/J mol-I CH,MucHD 2963 CHDMU~H, 3030 CD,MueHD 3322 CHDMU~D, 3280 CH,MutD, 3076 CD,MU~H, 3186 Using the values for the parameters listed in table 4 for the simultaneous variation of L, M and K, the heights of the barriers hindering internal rotation have been calculated for the radicals CH,MueHD, CHDMueH,, CD,MucHD, CHDMueD,, CD,MueH, and CH,MucD,. For the first four radicals listed in table 5 the estimated barrier heights should be compared with the barrier heights noted in table 2. The trend is the same in each case except for the radical CD,MucHD, for which the value of K of 3322 J mol-l is greater than that anticipated from the placement relative to CHDMucD, in table 2. This procedure for partitioning the barrier into pair interactions permits the barriers for the radicals CH,MucD, and CD,MucH, to be predicted to be of the order of 3076 and 3 186 J mol-l, respectively. Observations upon muonic radical formation in liquid CH,=CD, are required to confirm these estimates. We thank the Instituto Nacional de Investigaciio, Cientifica, Lisbon, for a Research Studentship for M.J.R. and the S.R.C. for a Research Studentship for D. McK.274 INTERNAL ROTATION IN ETHYL RADICALS M. J. Ramos, D. McKenna, B. C. Webster and E. Roduner, J. Chem. SOC., Faraday Trans. 1, 1984, 80, 255. P. J. Krusic, P. Meakin and J. P. Jesson, J. Phys. Chem., 1971, 75, 3438. J. K. Kochi, Adu. Free-radical Chem., 1975, 5, 189. P. D. Sullivan and E. M. Menger, Adv. Magn. Reson., 1977, 9, 1. * R. W. Fessenden, J. Chim. Phyx, 1964, 61, 1570. (PAPER 3/ 1090)

ISSN:0300-9599    

DOI:10.1039/F19848000267

出版商:RSC    

年代:1984

数据来源: RSC