J. Phys. Chem. 1996, 100, 8139-8143
8139
Vapor Phase Electronic Absorption and MCD Spectra for Dimethyl Sulfide, Dimethyl Selenide, and Dimethyl Telluride in the UV Region W. Roy Mason Department of Chemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115 ReceiVed: October 3, 1995; In Final Form: January 22, 1996X
Electronic absorption and magnetic circular dichroism spectra for (CH3)2X, X ) S, Se, and Te, in the vapor phase were measured in the range 2.5-5.3 µm-1 with a spectral bandwidth of e0.003 µm-1. The spectra show well-resolved vibrational structure for the nX f (n + 1)p Rydberg system with positive MCD B terms for (CH3)2Se and (CH3)2Te, but a negative B term for (CH3)2S. The difference in the MCD spectra is interpreted in terms of the influence of magnetic coupling with the broad nX f σ* valence transition, which lies rather close in energy to the Rydberg system for X ) S, but is separated to lower energy for X ) Se or Te. The interpretation requires a nX f σ* transition assignment of 1A1 f 1B1 and provides support for the most recent 1A1 symmetry assignment of the nX f (n + 1)p Rydberg excited state. The assignment of several weak, previously unreported, bands for X ) Se and Te is also given.
Introduction
Experimental Section
The UV and vacuum UV electronic absorption spectra for dimethyl sulfide have been investigated over the past four decades,1-7 and the spectra for dimethyl selenide and dimethyl telluride were reported more than two decades ago.7 The bands observed for all three Me2X compounds can be classified into (1) those that can be described as members of a Rydberg progression on the X atom given by eq 1, where IP is an
Liquid samples of the Me2X compounds were purchased from Alfa Inorganics and were used without further purification (CAUTION! STENCH). The NMR spectrum for each compound showed only a single methyl resonance (CDCl3 solution, δ ) 2.120, 2.009, and 1.917 ppm vs TMS for X ) S, Se, and Te, respectively). Absorption spectra were first measured in a preliminary way by means of a Cary 1501 spectrophotometer. MCD and absorption spectra were then measured simultaneously along the same light path by means of a spectrometer described earlier.8 A 1 m focal length monochromator (JY HR 1000) provided a spectral bandwidth of e0.003 µm-1, and spectral resolution was determined to be sample rather than instrument limited. A magnetic field of 7.0 T was provided along the light path by a superconducting magnet (Oxford Instruments SM27, fitted with a room temperature bore tube). Vapor phase samples were prepared by placing a small amount of the volatile liquid compound in a sealed quartz cell of 1.00 cm path length that had been purged with N2; the temperature was ca 296 K (ambient) and the total gas pressure in the cell was atmospheric. For measuring in the region of intense bands, the sample vapor was diluted with N2. All spectra were corrected for an N2 blank using the same cell as for the sample. No changes in the measured spectra were observed in the time required for the measurements (typically 0.5-1 h).
νjobs ) IP - R/(n - δ)2
(1)
ionization potential of X, R the Rydberg constant, n an integer quantum number, and δ the quantum defect, and (2) those that are assigned to molecular valence transitions, for example nX f σ*. The Rydberg bands for these molecules in the gas phase in general appear narrow and exhibit vibrational structure, while the valence transitions tend to be broad and featureless. Among the low-energy members of the Rydberg bands, evidence has been presented suggesting mixed Rydberg-valence character, with the bands having characteristics of both types of transitions. Detailed spectral assignments developed mainly from the study of Me2S have been presented and discussed, yet these have not been free of disagreement. For example the intense band system observed at 5.119 µm-1 (1 µm-1 ) 104 cm-1) for Me2S was at first assigned as 1A1 f 1B1 (nS(1b1) f 4p(5a1) Rydberg transition)4 and later as 1A1 f 1A1 (nS(1b1) f 4p(2b1) Rydberg transition).5,6 An even greater lack of accord is found for the assignment of the broad band for Me2S observed at 4.94 µm-1. This band has been given the following assignments: (1) a nonRydberg transition,2 (2) a nonbonding sulfur 3p f 3d Rydberg transition,3 (3) a nonbonding sulfur 3p f σ* valence transition,7 and (4) most recently a sulfur 3s f σ* valence transition.6 The present study reports absorption and magnetic circular dichroism (MCD) spectra for vapor phase samples of Me2X, X ) S, Se, and Te in the UV region to 5.3 µm-1. The interpretation of the MCD spectra obtained here requires a revision of the most recent assignment6 of the low-energy valence transition for Me2S at 4.94 µm-1 and by analogy the underlying, unresolved absorption for similar low-energy systems in the Me2Se and Me2Te spectra. X
Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)02904-2 CCC: $12.00
Results and Discussion Absorption and MCD Spectra. Figures 1-3 present the absorption and MCD spectra for the Me2X in the vapor phase. The absorption spectra compare favorably with those reported earlier.4,7 For example, bands I, II, and III (all in µm-1) for Me2X are observed (reported) at 4.384 (4.397,4 4.3887), 4.94 (4.88,4, 4.947), and 5.114 (5.119,4 5.1167) for X ) S; 4.193 (4.1927), 4.248 (≈4.267), and 4.921 (4.9247) for X ) Se; and 3.880 (3.8827), 3.997 (≈4.007), and 4.621 (4.6247) for X ) Te. The MCD spectra are generally as well resolved as the absorption and consist entirely of B terms. For example the νj (in µm-1) of the prominent 0,0 line of band system III has absorption maxima (MCD minimum (S) or maxima (Se and Te)) at 5.114 (5.113), 4.921 (4.922), and 4.621 (4.621), for X ) S, Se, and Te, respectively. The Me2Se and Me2Te spectra contain broad weak absorptions at low energy that are not © 1996 American Chemical Society
8140 J. Phys. Chem., Vol. 100, No. 20, 1996
Figure 1. Absorption (lower curves) and 7.0 T MCD (upper curve) spectra for (CH3)2S vapor. νj in µm-1; 1 µm-1 ) 104 cm-1. Absorbance, A, and differential absorbance, ∆A, axes are arbitrarily scaled to the largest feature. The lower energy absorption curve is multiplied by a factor of 5 in order to show detail.
Figure 2. Absorption (lower curves) and 7.0 T MCD (upper curves) spectra for (CH3)2Se vapor. See Figure 1 for x- and y-axis units. The lower energy curves resulted from measurements on a vapor sample that was approximately 50 times more concentrated.
present in the spectrum for Me2S. In the latter case, no shoulders or maxima were observed below band I at 4.38 µm-1. The MCD for Me2Se and Me2Te show associated broad B terms in the low-energy region. Apart from the systematic shift of the spectra to lower energy in the order Te < Se < S, which parallels the X ionization energy, there is a general increase in complexity of both absorption and MCD spectra from S to Se to Te. The increase in complexity is consistent with the expected increase in spin-orbit coupling, which would allow transitions to spin-forbidden states to gain intensity by admixture with spin-allowed states. Spin-orbit coupling for np electrons increases in the order S < Se < Te (free atom ζnp ) 0.0397, 0.199, and 0.475 µm-1, respectively).9 Therefore, transitions to states of triplet parentage will gain intensity in the same order, and are thus more likely to be observed for Se and Te than for S. Further, a notable difference between the MCD spectra for Me2Se and Me2Te and the spectrum for Me2S is the observation of positiVe B terms for band system III in the former two cases
Mason
Figure 3. Absorption (lower curves) and 7.0 T MCD (upper curves) spectra for (CH3)2Te vapor. See Figure 1 for x- and y-axis units. The lower energy absorption and MCD curves resulted from measurements on a vapor sample that was approximately 10 times more concentrated; the results were multiplied by factors of 10 and 50, respectively.
compared to the negatiVe B terms for band system III in the latter case. Also the broad valence transition band II shows a prominent positiVe B term in the MCD which is only about 0.18 µm-1 lower in energy than the 0,0 line of band system III for Me2S. The location of the analogous band for Me2Se and Me2Te has been suggested7 to be near the broad band I, which is 0.44-0.67 µm-1 and 0.5-0.75 µm-1 to lower energy than band III, respectively. Energy Levels, Excited States, and MCD Terms. The Me2X molecules are assumed to be C2V with the z axis parallel to the C2 axis and the x axis perpendicular to the molecular plane (y, z). A schematic one-electron energy level diagram that will be useful for visualizing the low-energy excited configurations and states is shown in Figure 4. The highest occupied orbital 1b1 is nonbonding (≈npx) and perpendicular to the molecular plane.6 The lowest energy empty MO’s are expected to be 2b2 (σ*), 3a1 ((n + 1)s/σ*), and 4a1 (σ*/(n + 1)s). The (n + 1)px,y,z orbitals of X are labeled 2b1, 3b2, and 5a1, respectively. The diagram, of course, is only schematic because orbitals of the same symmetry will intermix through configuration interaction (CI). Thus, even though the orbital labels of Figure 4 are used for the sake of simplicity, it is understood that CI will mix configurations to some extent. The lowest energy excited configurations and associated excited states are listed in Table 1. Since the ground state is 1A1, electric dipole transitions to 1A (z-polarized), 1B (x-polarized), and 1B (y-polarized) are 1 1 2 fully allowed. Transitions to 1A2 states are symmetry forbidden. Because C2V has only twofold rotational symmetry, there are no true degenerate states. The absence of degenerate states excludes MCD A terms; the diamagnetic ground state requires that C terms be absent also.10 The MCD B terms arise from the magnetic field induced mixing of A1, B2, and B1 excited states and are given in terms of the B h 0 parameter for the space averaged case appropriate for anisotropic molecules in the gas h 0(J,K) is the B term parameter for phase by eq 2.10,11 In eq 2, B
B h 0(J,K) )
〈J||µR||K〉 1 〈 A1||mβ||J〉〈K||mγ||1A1〉 (2) ∑ 3µBK*J ∆WKJ
-4i
the transition 1A1 f J resulting from a summation over excited
Spectra of (CH3)2X (X ) S, Se, Te)
J. Phys. Chem., Vol. 100, No. 20, 1996 8141 TABLE 2: Contributions to B h 0 for State J from State K to Higher Energya J
K
contribution to B h0
3B1 4A1 2B1 2B1 7B2 7B2
State of Singlet Parentage 4A1 6B1 3B1 4A1 4A1 8A1
pos pos 0 pos 0 neg
1A1 1A1 2B2 3B2 4B1 5A1 5A1 6B2 7B1
States of Triplet Parentage 1B1 1B2 2A1 3A1 4B2 5B2 5B1 6A1 7A1
pos pos neg neg neg pos pos neg neg
a Terms B h 0(J,K) for states J in the sum over K in eq 2. The numbering scheme is as given in Table 1. The energy difference ∆WKJ in eq 2 is positive.
Figure 4. Schematic molecular orbital energy level diagram for (CH3)2X with C2V symmetry.
TABLE 1: Excited Configurations and Statesa excited confgn
zero-order states
spin-orbit states
(1b1)(2b2)
(1A2) 3A 2 1 B1 3B 1 1 B1 3B 1 1 A1 3A 1 (1A2) 3A 2 1 B1 3B 1 1B 2 3B 2 1A 1 3A 1
(1A2) 1A1, 1B2, 1B1 2B1 2B2, 2A1, (2A2) 3B1 3B2, 3A1, (3A2) 4A1 (4A2), 4B1, 4B2 (5A2) 5A1, 5B2, 5B1 6B1 6B2, 6A1, (6A2) 7B2 7B1, (7A2), 7A1 8A1 (8A2), 8B1, 8B2
(1b1)(3a1) (1b1)(4a1) (1b1)(2b1) (1b1)(3b2) (1b1)(5a1) (2a1)(2b2) (2a1)(3a1)
1
a Filled orbitals omitted. Ground state configuration: ...(2a )2(1b )2, 1 1 A1. Electric dipole forbidden A2 states in parentheses.
states K, µB is the Bohr magneton, ∆WKJ ) WK - WJ, the energy difference between the states K and J, µ ) -µB(L + 2S), and m ) er (the magnetic and electric moment operators, respectively), and the superscript R, β, and γ indicate the irreducible representations to which the operators belong in C2V. The calculation of B terms is difficult unless the summation over the states K in eq 2 can be reduced to only a few terms. This is sometimes possible because of the inverse dependence on ∆WKJ which causes the largest contributions to arise from states closest in energy. With the assumption that the states closest in energy will make the largest contribution to B h 0, a likely prediction of the B term sign can be made by unreducing the reduced matrix elements and approximating the states in terms of one-electron orbitals of the X atom. The precise magnitude probably cannot be predicted with reliability because of the approximations involved and the lack of quantitative CI parameters. The approach taken here is to attempt to predict the B term sign by making use of the symmetry properties of the orbitals involved. The predicted signs of contributions to B h 0 determined from eq 2 for selected states J from individual
states K (assumed to be at higher energy) are collected in Table 2. Of course, the properties of B terms dictate that the contribution to B h 0 for the transition 1A1 f K (at higher energy) from the state J (at lower energy) will be of opposite sign compared to those of Table 2.10 Spectral Interpretation. The structured band III for all three Me2X compounds has been assigned as the first member of the nX f (n + 1)p Rydberg series.4,7 The vibrational structure has been assigned and discussed previously7 and will not be considered further here. The symmetry of the electronic state for band III was at first assigned4 as 1B1 (6B1, 1b1 f 5a1, see Figure 4 and Table 1 for transition and state notation), but electric dichroism studies5 for Me2S showed a polarization difference of near 90° between band III and the lowest energy band, band I. Since band I had been reasonably assigned4 on energetic grounds as the transition to 1B1 (2B1, 1b1 f 3a1), the state expected from the lowest energy allowed transition, the state for band III must have perpendicular polarization and therefore was reassigned5,6 as 1A1 (4A1, 1b1 f 2b1). If the A1 symmetry is correct for the excited state corresponding to band III, then a consideration of the present MCD results which shows B terms of opposite sign for Me2S and Me2Se or Me2Te raises the question of which state(s) are magnetically coupled to the predominantly singlet 4A1 state of band III. One obvious difference between the three absorption spectra is the presence of the broad band II close in energy to the origin of band III for Me2S. This band has a very strong broad positive B term associated with it in the MCD, which has no separately resolved counterparts in the MCD spectra of Me2Se or Me2Te. Thus the possibility that the excited state for band II is magnetically coupled to the 4A1 state for band III presents itself as an attractive explanation. A previous assignment of band II to the transition to a 1B2 state (7B2, 2a1 f 2b2)4 was reaffirmed recently.6 However, the B term contribution to 7B2 from coupling with 4A1 is zero (see Table 2) for reasons of orthogonality because the excited configurations of 7B2 and 4A1 do not share common orbitals. In order for the earlier assignment of band II to be correct, the magnetic coupling of the 7B2 excited state must be with some other state at higher energy. One possibility that was considered, but must be rejected, is 8A1 (2a1 f 3a1) which is expected to be at higher energy than band III (assigned4 in the vacuum UV near 5.56 µm-1, which is beyond the limit of the present measurements). Table 2 shows that the B term contribution to the 7B2 state from
8142 J. Phys. Chem., Vol. 100, No. 20, 1996 the 8A1 state would be negatiVe, which does not agree with experiment. All other states that could be magnetically coupled with 7B2 are expected at even higher energy with concomitant weaker coupling. Therefore, the assignment of band II was reconsidered. One plausible assignment for band II is to the nX f σ* valence transition to 3B1 (1b1 f 4a1). The B term contribution for this state from the nearby 4A1 state of band III is predicted to be positiVe. Therefore, if other contributions are less important, this assignment is consistent with the observed MCD in the case of Me2S. The question then arises as to the location of band II for Me2Se and Me2Te, and why the B terms for band III in these cases are observed to be positive. It has been suggested that band II red shifts relative to band III for Me2Se and Me2Te, so that it overlaps or is combined with band I.7 This suggestion is certainly reasonable in view of the nX f σ* character of the valence transition. The σ* 4a1 orbital is expected to be relatively more stable for Se and Te (less antibonding) than for S due to weaker covalent bonding. As noted above, the difference in energy between band II and the 0,0 line of band III is only 0.18 µm-1 for Me2S, whereas it is in the range of 0.44-0.75 µm-1 for Me2Se and Me2Te if band II is assumed to be present in the lower energy system with band I. The larger ∆WKJ for Se and Te compared to S would certainly reduce the importance of this contribution to the magnetic coupling. The positive contribution to the B term for band III could then be explained from a coupling with a state or states to higher energy. One very reasonable possibility is the 6B1 (1b1 f 5a1), the other allowed nX f (n + 1)p singlet excited state and one which would be expected to be slightly higher in energy than the 4A1 because of some σ* valence character of the 5a1 orbital. The location of this state is uncertain, but a prominent, though somewhat weaker negative B term observed at 4.795 µm-1 for Me2Te, may signal its presence. The rather broad absorption at this energy does not resemble the narrower lines of the progressions built on the origin of band III. Unfortunately, an analogous band for Me2Se is not observed below the wavelength limit of the measurement. Thus the MCD B term signs for band III can be explained if the term is dominated by the negative contribution from band II in the case of Me2S but reverts to a positive value when the negative contribution is reduced due to the inverse dependence on the larger ∆W terms for Me2Se and Me2Te. This interpretation assumes the transition assignment for band II as 1b1 f 4a1, 3B1. It may be remarked that the 1b1 f 4a1, 3B1 assignment was one of those suggested earlier.7 The MCD thus provides experimental support for this nX f σ* assignment of band II and also serves to strengthen the nX f (n + 1)p 4A1 Rydberg assignment for band III. Band system IV observed at 4.962 µm-1 for Me2Te has been assigned as the first member of the nX f nd Rydberg series.7 The MCD in this region is quite strong and mirrors the complexity of the absorption, but unfortunately little can be concluded from the present spectra. The complete band was not accessible in the present study due to the measurement limit, and the intermediate energy position of the states associated with this band between the several to lower energy and many more to higher energy7 precludes any satisfactory B term interpretation. More MCD data into the vacuum UV would be required in order to give greater detail. The weak bands in the range 3.2-4.0 µm-1 observed here for Me2Se and in the range 2.4-3.7 µm-1 for Me2Te do not appear to have been reported before. They are broad and featureless and an order of magnitude weaker than the higher energy band systems discussed above. Two possibilities occur as to their interpretation. First, one of the bands could be due
Mason to a vibronic transition to the forbidden 1A2 state, which is expected to be the lowest energy spin-allowed excitation 1b1 f 2b2. The difficulty with this interpretation lies in the absence of an analogous weak band for Me2S and the presence of two bands for Me2Te. Second, and probably a more likely possibility, the weak bands could be due to predominantly spinforbidden transitions to 1A1, 1B1, 1B2, 2B2, 2A1, 3B2, or 3A1. These states are all expected to be lower in energy than their predominantly spin-allowed counterparts. The intensity and the complexity of these weak band systems increase from Se to Te, which because of the increase in spin-orbit coupling also argues for transitions to states of triplet parentage. In order to test this possibility, B term signs were determined for the coupling between several of these states that are expected to be close in energy; the results are included in Table 2. On the basis of the B term signs, together with the rather broad nature of these weak bands, the lowest energy band in each case is assigned to the unresolved transitions to 1A1 and 1B1, corresponding to the spin-forbidden counterparts to the dipoleforbidden transition to 1A2. This latter transition is believed to be too weak to be observed or is obscured by stronger bands to higher energy. The second lowest energy band for Me2Te, which is not resolved in the absorption for Me2Se, is ascribed to the unresolved transitions to 3B2 and 3A1. These transitions are the counterparts of band II, the nX f σ* valence transition discussed above for Me2S, and so would be expected to be rather broad as observed. The MCD spectra for these weak low-energy bands appear as pseudo-A terms10 which consist of B terms of opposite sign located lower and higher in energy than the absorption band maximum. This is most clearly seen in the Me2Te spectra, where the two weak systems are separately resolved. The lowest energy absorption maximum at 2.727 µm-1 is surrounded by the positive and negative B terms at 2.555 and 2.835 µm-1, respectively; similarly the shoulder at 3.48 µm-1 is flanked by a negative and a positive B term at 3.270 and 3.545 µm-1, respectively. The former gives the appearance of a negative and the latter a positive pseudo-A term; and both are consistent with the assignments proposed here. A similar pattern can be visualized in the Me2Se spectra, though not as well resolved. A close inspection of the Me2Te MCD for the weak band at 4.568 µm-1, just to the red of the origin of band system III reveals a complex structure. This is shown in detail in Figure 5 where the absorption and MCD have been expanded. The absorption band at 4.568 µm-1 is 530 ( 10 cm-1 to the red of the band III origin and has been assigned7 as a hot band with one quantum of the ν6 totally symmetric Te-C stretch excited in the ground state (ground state ν6 517 cm-1). The MCD shows that this band cannot be a single quantum hot band: the three B terms present at 4.564 µm-1 (pos), 4.568 µm-1 (neg), and 4.573 µm-1 (pos) can only be explained by the presence of three transitions in the region and unresolved in the absorption. One of the positive B terms certainly could be due to a hot band built on the origin of band III, which exhibits a positive B term, but the other positive term and the negative term cannot be easily explained by vibrations. In view of the narrow line widths, a plausible assignment might be to nTe f 6p Rydberg states of triplet parentage that are accidentally coincident with the hot band. However, these states are unlikely to be from 3A (4B or 4B from 1b f 2b ) because their location just 1 1 2 1 1 530 cm-1 below the origin of the corresponding singlet 4A1 would imply as very small singlet-triplet energy separation in zero order. A more reasonable assignment would be to transitions to 5A1 and 5B1 or 5B2 states of triplet origin from 1b1 f 3b2, or to transitions to 6B2 and 6A1 triplet states from
Spectra of (CH3)2X (X ) S, Se, Te)
Figure 5. Absorption (lower curve) and 7.0 T MCD (upper curve) for the hot band region of band III for (CH3)2Te vapor from Figure 3. Both x-axis and y-axis have been expanded in order to show detail.
1b1 f 5a1. The former of these two choices is preferred since the states are expected to be lower in energy. The contributions to B h 0 (Table 2) then suggest that the 5A1 state would have the positive B term at 4.564 µm-1 and the 5B1 or 5B2 state the negative B term at 4.568 µm-1 (negative pseudo-A term). The band III hot band would then be assigned to the positive B term at 4.573 µm-1. It must be admitted that these suggested assignments are somewhat speculative because the location of the corresponding predominantly singlet states is not known for certain, only that they are likely higher in energy than 4A1. The important finding here is that the MCD spectrum has revealed spectroscopic complexity that is not apparent in the absorption spectrum alone. Finally, the preceding discussion raises the question of the location of the 4B1 and 4B2 states of triplet parentage corresponding to the nTe f 6p 4A1 singlet (band III) for Me2Te. Attention is drawn to the complex band labeled “I,II” in Figure 3. The three separately resolved absorptions in this band have not been individually assigned previously, but the first member of the nTe f (n + 1)s Rydberg, band I, is predicted7 near the energy of the lower energy absorption at 3.588 µm-1. The middle, most intense feature at 3.997 µm-1 presumably corresponds to band II, nTe f σ*, and the high-energy maximum at 4.110 µm-1 is perhaps due to vibrational components from the other two. This absorption band is complicated for each of the Me2X compounds. However, in contrast to the Me2S and Me2Se spectra where the MCD is positive through the entire band, the MCD spectrum for Me2Te shows a curious, weak, narrow, and negative B term at 3.894 µm-1 emanating from an otherwise positive MCD envelope. The bandwidth of this MCD term does not match that of the absorption maximum at 2.588 µm-1. The narrow bandwidth suggests that the B term arises from a
J. Phys. Chem., Vol. 100, No. 20, 1996 8143 Rydberg-like transition. It is proposed here that this negative B term is due to a transition to 4B1, one of the triplet states corresponding to 1b1 f 2b1. The large spin-orbit coupling for Te makes the energy separation of the triplet 4B1 state from the singlet 4A1 state of band III (≈0.73 µm-1) plausible. The other triplet state, 4B2, is expected to have a positive B term, but this term would certainly be obscured by band II, which has a large positive B term. Transitions to analogous states for Me2S and Me2Se would no doubt be obscured by bands I and II since they are expected to be less intense due to the lower spin-orbit coupling in these cases. Thus the MCD spectra in the region of these bands has also revealed a complexity for the Me2Te complex which is not apparent from the absorption spectra alone. In summary, the absorption and MCD spectra for the Me2X compounds in the UV region can be interpreted reasonably well in terms of transitions from 1b1, the highest energy filled nonbonding orbital. Due allowance for differences in σ bonding in the Me2X compounds must be taken into account when the excited configurations are formulated, and increased spin-orbit coupling for X ) Se and Te must be considered in order to explain energy shifts and the complex weaker bands observed. Since the lowest energy nX f (n + 1)p Rydberg transition (band III) is between filled and empty (nonbonding orbitals (1b1 and 2b1), a shift to higher energy for the other nX f (n + 1)p transitions (1b1 f 3b2 and 1b1 f 5a1), which was assumed for the assignments presented here, is consistent with some degree of antibonding σ* valence character for the 3b2 and 5a1 (n + 1)p orbitals of X. References and Notes (1) Price, W. C.; Teegan, J. P.; Walsh, A. D. Proc. R. Soc. A 1950, 201, 600. (2) Clark, L. B.; Simpson, W. T. J. Chem. Phys. 1965, 43, 3666. (3) Thompson, S. D.; Carroll, D. G.; Watson, F.; O’Donnell, M.; McGlynn, S. P. J. Chem. Phys. 1966, 45, 1367. (4) McDiarmid, R. J. Chem. Phys. 1974, 61, 274. (5) Altenloh, D. D.; Russell, B. R. Chem. Phys. Lett. 1981, 77, 217. (6) Tokue, I.; Hiraya, A.; Shobatake, K. Chem. Phys. 1989, 130, 401. (7) Scott, J. D.; Causley, G. C.; Russell, B. R. J. Chem. Phys. 1973, 59, 6577. (8) Mason, W. R. Anal. Chem. 1982, 54, 646. The spectrometer has been upgraded by replacing an analog lock-in amplifier with a DSP lockin (Stanford Research Systems SR850) and by replacing the PDP 11/03 computer by a faster PDP 11/73 computer. The optical components and measurement methodology remain essentially the same. (9) Moore, C. E. Natl. Bur Stand. Circ. (U.S.) No. 467, 1949, Vol I; 1952, Vol II; 1958, Vol III. (10) Piepho, S. B.; Schatz, P. N. Group Theory and Spectroscopy with Applications to Magnetic Circular Dichroism; Wiley-Interscience: New York, 1983. This reference describes the standard (Stephens) definitions and conventions that are used here. (11) Equation 2 results from an expansion of eq 17.4.13, Chapter 17 of ref 10, after inserting the relevant 6j coupling factors and performing a summation over alternating tensors, MCD factors, and the irreducible representations R, β, and γ in C2V symmetry. It was assumed that the magnetic interaction between the 1A1 ground state and the excited states J and K is negligible due to the large ∆W. It may be noted that the identity of R, β, and γ in eq 2 is dependent upon the symmetries of the ground state and the states J and K.
JP952904Q
