J . Phys. Chem. 1990, 94, 6196-6201
6196
barriers are largely overestimated. The 7-MCSCF pyramidalization curves of H2X=XH2 were also performed, keeping the X-X and X-H bond lengths constant (C=C = 1.34 A; Si=Si = 2.236 A; G e = G e = 2.364 A; Sn = Sn = 2.65 A; C-H = 1.085 A; Si-H = 1.475 A; Ge-H = 1.557 A; Sn-H = 1.733 A). For the sake of geometrical consistency with the energy curves of the XH3 groups, the HXH and HXX valence angles in H2X=XM2 are taken at 120' in the starting planar geometry and are varied according to relation 2 as the molecule is bent. 3. u K CASSCF Calculations. The 1 I-configuration MCSCF calculations were performed with all bond lengths kept constant and HXH angles kept close to their MCSCF optimized values. Consequently, in Figure 15 the optimum bending angles are correct (6 = 36', 42', and 50' for disilene, digermene, and distannene, respectively) but the barriers to planarity (the depths of the wells) are overestimated. The geometries were selected as follows. For ethylene, experimental and accurately calculated geometries are available. For disilene, MCSCF-calculated geometries are a ~ a i l a b l e .For ~ digermene and distannene, we first determined a DZd-SCF geometry (given in Table IV), and then we reoptimized the X=X bond lengths at the MCSCF level. We selected these X=X bond lengths, slightly shortened in order to compensate the effects of nonadiabaticity along the bending curves. Finally, the following parameters (in angstroms and degrees) were used: CC = 1.34, C H = 1.085, HCH = 118.0, SiSi = 2.236, SiH = 1.475, HSiH = 111.8, GeGe = 2.364, GeH = 1.557, HGeH = 108.9, SnSn = 2.728, SnH = 1.733, HSnH = 102.9. One can find in the literature more refinedly calculated geometries and barriers to planarity. For disilene, the best calculations including correlation effects predict a wagging angle 0 of 33-36°.3*4and a barrier to Dlanaritv of 0.5-1.5 k ~ a l / m o l . ~For ~'~ digermene, taking the DZd-Lptimizk geometry for the DZhplanar and CZhtrans-bent forms, one gets a barrier to planarity of 3.6
+
kcal/mol at a CI level5 Using a DZP basis set changes very little this value (3.2 k ~ a l / m o l ) . ~Most calculations converge toward a value of 38-40' for the bending angle 0.s,8*9 For distannene, the SCF-DZd geometry has been determined for the planar form (SnSn = 2.536 A, SnH = 1.701 A, HSnH = 114.7'). Starting from the C,, and D2 geometries, the following energy differences were calculated: SCF, 9.7 kcal/mol; MCSCF, 10.9 kcal/mol; CI(CIPSI), 10.5 kcal/mol. The same adiabatic differences, using SCF-DZP geometries (i.e., further including p orbitals on hydrogen) were calculated at SCF, 9.2 kcal/mol; CI(CIPSI), 9.4 k ~ a l / m o l .In ~ ref 8 the adiabatic barrier to planarity is calculated at the S C F level at 6.2 kcal mol. Most calculations predict a bending angle of 46-49°.8*9. In summary, we would say that the present state of theoretical treatments predict the following bending parameters for the parent molecule (in degrees and kcal/ mol) :
l
disilene digermene distannene
trans-wagging angle
barrier to planarity
33-36 38-40 46-50
0.5-1.5 3-4 8-10
4 . Ouerlap Curves. The overlaps in Figure 6 were calculated between 2p, atomic orbitals and n, hybrids sxpY, built from Slater orbitals centered on carbon atoms distant by 1.34 A. We used the FMO analysis possibilities offered by the ICON version of the EHT program.s3 By setting the H, of the hydrogen Is orbital to an arbitrarily high value (999 eV), one gets an hybrid with 20% s character, which was used in Figure 6. The Slater exponents for the carbon atoms were taken at 1.625 for both the 2s and 2p orbitals. Other hybridizations were tried for the n, hybrid. This mainly results in a vertical translation of the S,, curve. (53) Hoffmann, R. J . Chem. Phys. 1963, 39, 1397.
Vibrational Analysis of an Electronic Emission Spectrum of 32SF2and 34SF2 Robert J. Glinski,* C. Douglas Taylor, and Frank W. Kutzler Department of Chemistry, Tennessee Technological University, Cookeville, Tennessee 38505 (Received: October 4 , 1989: In Final Form: January 23, 1990)
An electronic emission spectrum of SF2 extending from 5500 to 8500 %, is observed during the reactions of F2 with CS2, OCS, and sulfur vapor. Long vibrational band progressions of 355 i 2 cm-l and short progressions of 838 f 2 cm-' are definitively assigned to v2/1 and vl", respectively. Isotopic substitution of CS2with sulfur-34 has allowed a tentative assignment of the third ground-state frequency, Y; = 817 f 6 cm-I, and the electronic origin, 18 123 f IO00 cm-I. Ground-state frequencies and their isotopic shifts compared very well with those obtained in infrared absorption experiments by other workers. Estimates of the anharmonicities of the ground-state frequencies are presented. Weak bands lying 243 6 cm-l to the blue of the strongest bands are ascribed to the v i frequency. Results of first principles electronic energy calculations are presented, showing that two low-lying singlet electronic states are in the energy range of this emission spectrum. A discussion is presented relating the possible activity of the asymmetric stretch and the nature of the excited electronic state. The true multiplicities of the electronic states are undetermined and unaddressed. Reaction of CS2 with F2 produces SF,* together with an unknown feature displaying overlapping vibrational bands, 310 f 30 cm-l apart, extending from 7000 A into the near-infrared region. Reaction of F2 with SO2 produced no detectable chemiluminescence between 3000 and 8800 A.
*
Introduction The spectroscopy of sulfur difluoride has been studied by microwave,] infrared?~~ REMPI? and photoelectron spectroscopic5 (1) Kirchoff, W. H.; Johnson. D. R.;Powell, F. X . J. Mol. Spectrosc. 1973, 48. 157. Endo, Y.; Saito, S . ; Hirota, E.; Chikaraishi, T. J . Mol. Spectrosc. 1979. 377, 222. (2) Haas, A.; Willner, H. Spectrochim. Acta 1978, 34A, 541. (3) Deroche, J.-,C.; Burger, H.; Schulz, P.; Willner, H. J . Mol. Spectrosc. 1981, 89. 269. Willner, H. 2. Anorg. Allg. Chem. 1981, 117. 481.
0022-3654/90/2094-6196$02.50/0
methods, but direct observation of its lower lying electronic excited states by optical methods has not been reported. Isotopic substitution of CS2 in its chemiluminescent reaction with F2 has been used in this laboratory to demonstrate that SF2* produces one of the chemiluminescence featurese6 In this paper, we present (4) Johnson, R. D. 111; Hudgens, J. W. J . Phys. Chem. 1990, 94, 3273. ( 5 ) De Leeuw, D. M.; Mooyman, R.; De Lange, C. A. Chem. Phys. 1978.
34, 281.
( 6 ) Glinski, R. J.; Taylor, C. D. Chem. Phys. Lett. 1989, 155, 511.
0 1990 American Chemical Society
Electronic Emission Spectrum of '%F2 and 34SF2
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6197
I
CS + F 2 / H ~
1
0
o2 4 T a 5 5 0 B
100
200
30
P R E S S U R E MTORR Figure 2. Dependence of the chemiluminescence intensity at the maxima of the SF2 feature (6627 A) and the unknown feature (8550 A) on addition of N2 and 02. 120 mTorr of CS2 and 750 mTorr of F,/He.
6300
7000 WAVELENGTH
8000 ti)
Figure 1. Chemiluminescence spectra obtained during the reactions of
OCS and CS2 with a 10%F2 in He mixture showing the effects of added 02:(a) 140 mTorr of OCS and 1.6 Torr of F2/He; (b) 140 mTorr of CS, and I .6 Torr of F2/He; (c) 120 mTorr of CS2,750 mTorr of F,/He, (d) 120 mTorr of CS,,750 mTorr of F2/He, and and 50 mTorr of 0,; 160 mTorr of 02.The vertical scale is not necessarily the same in all four spectra.
an extended analysis of the chemiluminescence spectrum of SF2 along with other aspects of the emission spectra obtained during the reactions of F2 with OCS and CS2. Results of electronic energy calculations are also presented which indicate that electronic transitions corresponding to visible wavelengths may be expected. Experimental Section
A 10% fluorine in helium mixture was reacted with the sulfur compounds in a 1.5-L pumped reaction chamber as described previ~usly.'.~ The steady-state chemiluminescence, which to the eye appeared diffusely throughout the chamber, was dispersed by a McPherson Model 207, 2/rm scanning monochromator. A 110 X 1 10 mm2 holographic grating of 1800 lines/" gave the spectrometer an aperture of f/5.8 and a dispersion of 8.3 A/". The instrument was calibrated against a low-pressure neon lamp and the rotational lines from H F overtone emission9spectra from the chemiluminescent reaction of CH3CH2SHwith F2.8 A dry ice cooled GaAs photomultiplier tube (PMT, Hamamatsu R943-02), bathed in dry N2,was used to detect the photons. The PMT signal was sent to a picoammeter and then to a chart recorder. As an emission spectrum was being recorded by the chart recorder, the output of the recorder's digitizer was sampled and saved on a disk by a laboratory PC-AT. The digitized files were then sent to the university VAX 8800 where they could be averaged and displayed in several graphical formats. The degree (7) Glinski, R. J.; Nelson Getty, J.; Birks, J. W. Chem. Phys. Lett. 1985,
of digitization was three data points per angstrom while the spectral slit width was 2 A. Chemical Materials. All reagents were used as obtained from the manufacturers. The following is a list of reagent, purity, and source: CS,, ACS reagent grade, Fisher Scientific; OCS, 97.5% minimum, Matheson; SO,,99.98%, MG Industries; N, (dry), 99.998%; and 02, 99.8%, Airco; 10% F2 in He, 99.7%, Air Products. Isotopes: CS2 (C-13), 99 atom %; CS2(S-34), 90 atom %; and 0, (0-16), 99 atom %, ICON Services. Sulfur vapor was produced by heating sublimed sulfur to -200 OC in a porcelain boat located just below the reaction chamber center and the fluorine source. The vapor should be predominately S2at these conditions.I0 Results
Figure 1 presents a series of spectra which are the primary features and effects of interest in this paper. The reaction of OCS with the F,/He mixture produced the spectrum in Figure la. This spectrum is discussed at length below where it is assigned to an emission spectrum of SF,. Figure 1b shows the spectrum resulting from the reaction between CS2 and F2/He. The effects of added O2 on the CS, reaction are seen in Figure lc,d. Inspection of Figure 1 shows that (b) contains the SF2 feature and a new feature which extends from 7000 to beyond 8800 A. This new feature, which appears alone in Figure Id, is also discussed separately below. The dependence of the chemiluminescence intensity a t 6627 and 8550 A on addition of 0, and N2is shown in Figure 2. These wavelengths represent the intensity maxima of the two different features but contribute very little intensity one to the other. The SF, feature (6627 A) is quenched efficiently by O2 but very inefficiently by N2. The new, unassigned emission feature (8550 A) is quenched with moderate efficiency by both N2 and 0,. No effort was made to distinguish between physical and chemical quenching. It is important to note that the intensities of both features decrease monotonically even with the addition of trace amounts of 0,.In addition, Figure I C was reproduced by using 0, (0-18) with no apparent differences from the spectrum obtained when 0, (0-16) was used. It is thus believed that air or an oxygen impurity does not contribute to the production of either emission feature, except as a quencher.
117. 359.
(8) Glinski, R. J.; Mishalanie, E. A.; Birks, J. W. J . Photochem. 1987, 37, 217. (9) Mann, D. E.; Thrush, B. A.; Lide, D. R., Jr.; Ball, J. J.; Aquista, N . J . Chem. Phys. 1961, 34, 420.
(IO) Meyer, B. Chem. Reu. 1976, 76, 367. Rau, H.; Kutty, T. R. N.; Guedes de Carvalho,J. R. F. J . Chem. Thermodyn. 1973, 5,833. Meyer, B. Sulfur, Energy, and Enuironment; Elsevier Science Publishers: Amsterdam, 1971.
6198 The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
Glinski et al.
TABLE I: Deslandres Table of Maior Bands in Fimre In Showion Three Different Vibrational FreauenciesO M OOO-OMO OOO-OMI 000-IMI 000-2M1 000-3M1 0
I 2 3
4
17771.3 355.0 17416.3 355.4 17060.8 351.1 16709.7
17307.8 353.6 16954.2 354.0 16600.2 353.2 16247.0 354.1 15 892.8 351.4 15541.5 35 1.5 15 190.0 351.1 14 838.8 350.8 14488.0
817.1 816.0 813.9 8 16.9
5 6
I 8
839.7 838.2 834.5 830.7 828.1 825.3 822.2 818.5 814.5
9
IO
16468.1 352.1 16116.0 350.3 15 765.7 349.5 15416.2 351.5 15064.7 348.6 14716.1 348.4 14367.7 347.4 14020.9 346.8 13673.5 348.2 13 325.4 346.9 12 978.5
832.9 832.6 830.1 826.5 820.8 819.6 815.1 812.0 809.4
15635.2 351.8 15283.4 347.8 14935.6 345.8 I4 589.7 345.8 14244.0 347.4 13896.5 343.9 13552.7 344.3 13208.4 344.3 12 864.1
821.7
13768.0 343.0 13425.1 339.9 13085.2 340.1 12745.1 341.3 12403.8 341.3 12062.5
818.9 811.4 807.6 804.6 801.7
000-4M1
812.1 812.9 817.5
12955.9 343.8 12612.2 344.5 12267.7
-
“Precision of position measurement is i2.0. Precision of differences is f3.0. Units are cm-’. The vibronic transitions in this emission spectrum are designated by u1’u2/uJ’ uI“u2/1u,” throughout this paper. RELATIVE
INTENSITIES
FOR
T H E 0 4 N D SYSTEM
b.
000
+
LMN
111
a.
I
I
l
I””
l
Figure 3. Representation of the SF, spectrum in Figure la as separate vibrational progressions. Band intensities were estimated as peak heights above the continuum, were not corrected for spectrophotometer response, and should be accurate to within about 20%. Relative intensities in Figure 3a are for M = 4 but are different for larger and smaller values of M (see text). The intensities of the band set in the vicinity of the band labeled 3M1 are less certain due to overlap of bands in that region. Figure la,b has been reproduced over a range of total pressures from 400 mTorr to over 3 Torr with no significant change in form. This seems to indicate that the excited state is mostly vibrationally relaxed. The SF2 feature is about a factor of 4 more intense in the reaction with OCS than with CS2under the same conditions. The pure SF2 spectrum of Figure l a has also been produced in the reaction of the vapor over molten S8.No chemiluminescence was observed anywhere between 3000 and 8800 A when SO2 was reacted with F2. Vibrational Analysis of the OCS/F2 Spectrum. We have recorded the positions of over 120 vibrational bands in the spectrum obtained in the reaction of OCS with the F, in He mixture. All of the bands reported previously have been reproduced.” Figure la is one of the 16 spectra which were averaged for this analysis. The averaged spectrum represents a signal-tonoise improvement of a factor of 7 with a spectral slit width slightly narrower than that used previously.” The improved spectrum and the effects of sulfur-34 substitution have allowed us to make an assignment of this spectrum to definite vibrational transitions in SF2. Table I shows a mixed Deslandres table of representative bands in progressions of the form OOO LMN. Three active frequencies are manifest in Table I. One can clearly see that two distinct stretching frequencies of 835 and 816 cm-l are displayed in this spectrum. The most consistent assignment of these two stretching frequencies is to a vl” and a u3” of the ground electronic state.
-
( I I ) Glinski, R. J. Chem. Phys. Lett. 1986,129, 342
TABLE 11: Least-Squares Fits of Eq 1” Droaression w” 838.1 ul” OOO-L41 (5 bands) OOO-LSI (5 bands) 83 1.6 355.0 u2“ 000-OMI (9 bands) 351.9 000-1 MI (1 1 bands) 349.8 000-2M1 (9 bands) 819.0 u,” 000-1 IN (3 bands) 823.3 000-14 N (3 bands) 812.3 000-15N (3 bands) 805.9 000-16N (3 bands) 810.1 000-24N (3 bands)
w”x’t
ref6
3.0 2.3 0.28 0.27 0.41 2.0 7.4 5.3 4.5 5.7
838.53 357 813.04
“Units are cm-I. Values in parentheses are the number of bands in the fit. bReference for u,” and u,”: 3. Reference for UT:I , 2.
The spectrum in Figure l a can be broken down, according to this analysis, into the schematic spectrum of Figure 3. Figure 3a shows the intensity trends for the progressions in vi’’ and u3” when M = 4 while Figure 3b shows the intensity trends for the progression in u; when L = 1 and N = 1. The largest part of the spectrum in Figure la can be reconstructed by multiplying the bands in Figure 3b by each band of Figure 3a and laying the ;Y progressions out to the right. A prominent feature in Figure la, as indicated by Figure 3, is the two progressions in the bending frequency which lie only about 20 cm-I apart, giving the bands a “doublet” appearance. Not every band, however, shows this “doublet” character. Indeed, the bands assigned to 000 OM0 appeared as single bands; the first bands in the progressions 000-OM1 and 010-OM1 appear without the corresponding 1M1 bands nearby; and the heights of the pairs of bands become nearly the same going toward red in all the progressions. Additionally, there is no special structure to any of the bands with the exception that most of them are slightly blue shaded. Figure 3 ignores anharmonicity, but Table I clearly indicates that each active vibrational mode is somewhat anharmonic. The fundamental frequencies and anharmonicities can be estimated to the precision of the data by fitting the progressions to an energy term equation. Treating each mode as a pseudodiatomic molecule, we can use the expression
-
iji
= a.
+ alui’ -+ a2uF2
where a. = T, + 1/2(w‘ - w”) - 1/4(w”x”- w’x?, al = w “ x “ w”, and a2 = w”x”. Therefore, a plot of position, 8 , versus quantum number, u:’, will yield the fundamental frequency, a”,and the pseudodiatomic anharmonicity, w””’. Table 11 lists the values of w” and w”x”
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6199
Electronic Emission Spectrum of 32SF2and 34SF2 TABLE 111: Deslandres Tables Showing the Assigned Freauency M 0 10-OM1 000-OM1 0 17 545.5 237.7 17 307.8 351.2 353.6 1 17 194.3 240. I 16 954.2 354.5 354.0 2 16 839.8 239.6 16600.2 352.9 353.2 3 16486.8 239.9 16 247.0 347.8 354.1 4 16 139.1 246.2 15 892.8 M 010-1M1 000-1 MI 16468.1 352.1 16 116.0 350.3 242.2 15 765.7 16007.9 349.5 345.8 245.8 15 416.2 15 662.0 351.5 356.6 240.7 15 064.7 15 305.4 348.6 337.8 251.5 I 4 716.1 14 967.8 349.9 348.4 250.0 14 367.7 14617.8 347.4 14020.4 346.8 13673.5
Y?’~
0
0
000-13N
0 0 0 - L41 (A)
000- 14N (A)
L
1
2
0
N
1
2
,
I
1
1
(0)
301 c.
0
Precision of position measurements is f2.0. Precision of differences is f 3 . 0 . Units are cm-’. (I
for various progressions in 000-LMN. No attempt has been made to separate out individual polyatomic anharmonicities, xi,, due to the modest quality of the data. Electronic Origin and Excited-Electronic-State Vibrational Frequencies of SF2. Absolute values have been assigned to LMN in Table I based on analysis of the isotopic shifts (see below). From this the electronic origin can be estimated and would correspond to the missing entry in the upper left of Table I if the quantum numbers assigned are absolute. That value would be 18 123 f 10 cm-l. Since the isotopic shift in the ground-state bending frequency is so small, however, this analysis may have positioned the origin low by several bending frequencies. Absorption spectroscopists seeking this spectrum should, therefore, allow that 18 123 cm-l might be as much as three bending quanta or about 1000 cm-I low. The molecule possesses low-frequency bending modes, so one would expect that at least the first level of the bending frequency in the excited electronic state might be significantly populated even in what appears to be a thermal, room-temperature distribution. Table 111 presents Deslandres tables showing positions of two sets of weak bands that appear regularly to the blue of the two strongest vibronic progressions. The 243 f 6 cm-I frequency is not unreasonable for the excited-state bend. But as the structural nature of the excited state is not known, assignments of any excited-state frequencies will require confirmation by an absorption spectroscopy experiment. In a room-temperature distribution, about 25% of the electronically excited molecules would be in a vibrational state of energy 243 cm-l. The intensity of a 010-LMN band in the SF2 emission spectrum ranged from 5% to 20% that of the corresponding 000-LMN band. This intensity ratio is a bit low, although Franck-Condon factors could be contributing to the difference. These bands, unfortunately, could not be observed clearly in the %F2 spectrum because they were obscured by noise. Isotope Effects on the SF2 Spectrum. With the small quantity of isotopic CS2 available, only two spectra of 34SF2were of sufficient quality to be averaged for comparison to the 32SF2spectrum. Absolute differences in the positions of 30 of the most wellmeasured bands representing progressions in all three frequencies were analyzed in effort to estimate the shifts in the fundamental frequencies and possibly locate the band origin. Ignoring dif-
, --
000 L 21 ( 0 ) 000 L 31 (m)
0-
u 3 ~ - u 3 4cm-’ /l,
average for
YI”
8.7 f 1.4
000-L2 1 000-L3 1 000-L4 1
Yi’
1.8 f 0.3
000-OM1
1.85
10.7 f 3.0
000-IMI 000- 13N 000- 14 N
9.38 f 0.05
vi‘
reP 8.92 f 0.05
* 0.05
‘Reference values are from argon matrix infrared measurements.2
-
ferences in anharmonicity caused by isotopic substitution, for LMN the difference in band progressions of the form 000 positions between the two isotopic molecules is AI = I 3 4 - 932 = [ A w -~ Aw/] - A w F L - AwM”M - AWN’” (2) where Awo is the isotopic shift in the zero points and Awr is the isotopic shift in each of the three ground-state fundamental frequencies (wjq/’ - ~ ~ 2 / ) ) Figure ~. 4 presents three plots of the difference in the positions of the same band in the 34SFzand 32SFz spectra versus the quantum number for each of the three vibrational modes. The slopes of the lines of Figure 4, which correspond to the frequency shifts of the fundamentals, are tabulated in Table IV. Averages were obtained for the slopes of each combination of each progression in UT since the slopes were the same within experimental error. Hence, only one line is drawn in Figure 4a,b. As can be seen by comparison with the literature values, the shifts are consistent with those for the three ground-state frequencies of SF2. The intercepts of the plots in Figure 4 are also consistent in this analysis. In Figure 4c, for instance, the nonzero intercepts are expected for two reasons: the plots neither represent pure progressions in the vibrational frequency nor should the shifts in the zero points in the two states be quite the same. The difference in the intercepts in the two plots in Figure 4c of 12 f 5 cm-l is consistent with the shift in the ul” frequency. Subtracting the shift in the u3” frequency from the intercept of the line for the
Glinski et al.
6200 The Journal of Physical Chemistry, Vol. 94, No. 16. 1990 TABLE V Calculated Relative Energies (in eV) of MO's of SFz at a = 98.2' and Four Bond Distances Relative to R e = 1.592 A (1 eV = 8065.7 cm-') R. - 0.1 Re Re + 0.1 Re + 0.2 1 . 1 -1.8 -2.8 -4.0 9a 1 -3.4 -4.4 -1.9 -2.6 6bI -6.3 -6.8 -5.9 3b2 ( H O M O ) -5.3 HOMO-LUMO gap"
" Differences are presumed
3.4
3.3
2.4
to be accurate within 30%.
TABLE VI: Calculated MO Energies (in eV) at Four Bond Angles Relative to a = 98.2" 90' 98.2' -1.6 -1.8 9a 1 -3.1" -2.6" 6bl -6.0 -5.9 3b2 (HOMO) HOMO-LUMO gap
2.9
2.9
3.3
R e = 1.592 A and 120' -3.1" -1.5 -5.8
150' -5.1" -0.07 -5.9
2.7
0.8
" Denotes the LUMO 000-OMI progression leaves a small negative value for the shift in the zero point. Figure 4a,b also shows this small negative remainder when the shifts of the other frequencies in the combination are subtracted from the intercepts. We note the three separate progressions in Figure 4a and two separate progressions in Figure 4b would not yield significantly different intercepts if analyzed separately because the shift in the bend frequency of 1.8 cm-' is smaller than the uncertainty in the intercepts. Electronic Energy Calculationsfor SF,. The SF2 one-electron energies were calculated with the discrete variational method-a first principles LCAO-MO technique which uses the local density approximation to electron exchange and correlation.I2 In this work the exchange-correlation potential suggested by Vosko, Wilk, and Nusair was used." A numerical basis set consisting of both occupied and virtual states was used. The sulfur basis set included functions for the 1s, 2s, 2p, 3s, 3p, 3d, 4s, and 4p states, and the fluorine basis set included Is, 2s, 2p, 3s, and 3p atomic functions. Matrix elements were calculated numerically by using the Diophatine method,14 and the molecular potential was computed from a multipolar e x p a n ~ i o n of ' ~ the molecular charge density. The transition energies reported below were computed simply from differences in one-electron energies. A test calculation for SF2 with a bond angle of 1 0 5 O using a singlet-to-singlet Slater transition calculation16was found to agree with the energy difference method to within 0.1 eV. Calculations were performed on SF2 at the ground-state equilibrium geometry (Re = 1.592 A and a = 98.2') and at increased and decreased bond distance and angle. Tables V and VI present the relative singlet energies of the highest occupied molecular orbital (HOMO) and the next two lowest unoccupied molecular orbitals (LUMOs) as a function of bond distance and bond angle." Table V shows the effect of varying bond distance on the energies of the first two electronic excited states. It can be seen that, at the equilibrium bond angle, vertical transitions could occur at visible wavelengths for longer bond distances than the equilibrium ground-state bond distance.
TABLE VII: Positions and Approximate Spacing of Figure Id arbitrary position band no. (*5 cm-I) I I3 844 2 13 542 3 13 225 4 12 896 5 12583 6 12264 7 11 943 8 1 1 666 A B C D
diff 302 317 330 313 319 321 217
13 147 13 093 1 I518 1 1419
Table VI shows the variation of the relative energies of the HOMO and the two lowest unoccupied MO's as a function of bond angle. One can see in this table that the two lowest unoccupied MO's cross at a bond angle between 98.2' and 120'. Therefore, one would expect that vertical transitions could occur in the visible while the molecule is bent either more or less than the equilibrium ground-state bond angle involving two different excited states. The lowest energy vertical trans$ion a: the ground-state equilibrium geometry is expected to be AiA2-X'AI. (This energy, 27 OOO cm-l, would approximate the Franck-Condon maximum in absorption.) New Bands beyond 7000 A. Table VI1 lists the positions of the bands observed in Figure Id. The set of eight bands that are 3 10 cm-' apart are not likely single vibronic level bands but a set of overlapping progressions of about that frequency. The positions of several sharper bands are also listed in Table VII. The broadness of all the bands made quantitation of the true vibrational frequencies or any isotope shifts impossible. By overlaying the isotopic spectra on a light table some changes could be discerned. Use of oxygen-18 yielded no perceptible changes in any features. Carbon-13 substitution gave a spectrum showing the sharp bands noticeably shifted but no alteration in the broad bands. Use of sulfur-34 clearly shifted the broad bands. It should be possible to find a reaction system where the features in Figure Id are produced more intensely and by themselves.
Discussion In this paper we have made definitive the reassignment of the strongest emission feature in the reaction of CS2 with F2 to an electronic spectrum of SF2. The spectrum had been tentatively thought to be due to FCS.Ii Long progressions in vZI/ and shorter progressions in vi" are unambiguously seen in this spectrum. These progressions appear strongest when accompanied by a single quantum change in the UTfrequency but weaker when they appear alone. the vibrational and isotopic analysis have allowed the tentative assignment of the electronic origin and the excited-state bending frequency. Some geometric parameters can be deduced from the character of the emission spectrum. The long progressions in bending frequency imply that the equilibrium bond angle is quite different between the two states in the transition. The progressions in the symmetric stretch also suggest moderately different bond distances between the two states. Perham the two most interestinn amects of this emission spectrum of SF2 is the activity of the asymmetric vibration and the nature of the relatively low-lying electronic state from which the emission originates. Indeed, these two problems are related; but unfortunately, the experimental and computational results of this work will not lead to an indisputable description of this transition in SF2. The activity of the asymmetric stretch depends on the nature of the electronic transition. Usually, only the totally symmetric vibrations would be active for allowed electric dipole transition. According to Tables V and VI and the Walsh diagramis for a Y
(12) Ellis, D. E.; Painter, G. S.Phys. Reo. 1970, B2, 2887. Painter, G. S.; Ellis, D. E. Phys. Reo. 1970, B I , 4747. (13) Vosko, S. H.;Wilk, L.; Nusair, M.Can. J . Phys. 1980, 58, 1200. (14) Ellis, D. E. Int. J. Quantum Chem. 1968, 2, 35-43. ( I S ) Delley, B.; Ellis, D. E. J . Chem. Phys. 1982, 76, 1949. (16) Slater, J. C. Ado. Quantum Chem. 1972.6, 1. In The Self-Consistent Field for Molecules and Solids: Quantum Theory of Molecules and Solids; McGraw-Hill: New York, 1974. ( I 7) The following are the energies (in electronvolts) of the highest occupied MOs at the ground-state equilibrium geometry: Sal, -31.5; 3bl, -30.3; 6al, -18.0; 4b1,-13.4; 7a,, -12.6; 2b2, -12.3; la2, -10.7; 5bl, -10.3; 8a,, -10.1; 3b2, -5.9. These compare relatively well with those of Hay, P. J. J . Am. Chem. SOC.1977, 99, 1003. Von Niessen, W. J. Electron Spectrosc. Relat. Phenom. 1979. 17, 197, and ref 5 . These authors do not report the LUMO energy.
Bands in
'
(18) Walsh, A D. J . Chem. SOC.1953, 2266.
J . Phys. Chem. 1990, 94, 6201-6208
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20-valence-electron XY2 molecule, the lowest energy transition could be either the allowed B2 A, or the forbidden A2 AI. Vibronically allowed transitions such as A, AI which include a change in the asymmetric stretch quantum number should, however, be missing the 000-000 band as well as those transitions of the form 000 LMO. We are reasonably confident of our identification of both types of progressions 000 LMO and 000 L M I . Alternative assignments were not nearly as consistent or reasonable. One possibility that has been considered is that we are seeing transitions of the form 000-LMO and 100 LMO which would not involve the asymmetric stretch. This assignment could be made if the vl' frequency is coincidentally 817 cm-I. Several factors would seem to oppose this. Population of this vibration level in the excited electronic state would imply that the v i bend would also be excited; however, progressions of the form 010-LMO are rather weak. Also, no isotopic shifts in the opposite direction of those in Figure 4 are detected. We, therefore, lean toward an assignment of this transition to the forbidden A2-AI, where progressions built upon the asymmetric stretch frequency are expected. The existence of the progressions not involving the asymmetric stretch would be unusual, however, although these transitions could gain intensity through magnetic dipole selection rules or another coupling
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6201
mechanism as have been observed in the AIAz RIA2 transition in thi~formaldehyde.~~ Unfortunately, the results of the electronic energy calculations on SFz summarized in Tables V and VI do not resolve the issue of whether the spectrum is due to an electric dipole allowed or vibronically allowed transition. The character of the excited state should be relatively easy to determine by an absorption spectroscopy experiment. A wide scan of moderate resolution could determine the possible activity of the three excited-state frequencies. A high-resolution scan of several bands would determine their type and hence the symmetry of the electronic excited state. Acknowledgment. R.J.G. and F.W.K. separately acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. C.D.T. thanks the Chapter 606 Student Monies Allocation Committee of TTU for the purchase of a sample of the isotopic CS,. We are grateful to H. R. Martin for the precise calibration of the monochromator. We thank D. C. Moule for pointing out the unusual appearance of the asymmetric stretch. Registry No. SF2. 13814-25-0;F2,7782-41-4; CS2, 75-15-0; OCS, 463-58-1; S,7704-34-9; "S, 13981-57-2;34S, 13965-91-4. ~~~
(19) Judge, R. H.; King, G. W. J. Mol. Spectrosc. 1979, 74, 175.
Vacuum-Ultravlolet Absorption and Fluorescence Excitation Spectra of IC1 Kenneth P. Lawley, Elinor A. Kerr,+ Robert J. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K.
Andrew Hopkirk, David Shaw, SERC Daresbury Laboratory, Daresbury. Warrington WA4 4AD, U.K.
and Andrew J . Yencha* Department of Chemistry and Department of Physics, State University of New York at Albany, Albany, New York 12222 (Received: October 9, 1989; In Final Form: March 15, 1990)
Absorption and fluorescence excitation spectra for IC1 in the vacuum-ultraviolet (1 25-195 nm) have been recorded using tunable synchrotron radiation. Fluorescence occurs mainly from the E(0') ion-pair state, and pronounced resonance structure is observed. These resonances result from an unusual three-state interaction. Both homogeneous and heterogeneous coupling between the E(0') ion-pair state and degenerate Rydberg states takes place. Pronounced broadening of the Rydberg levels occurs through heavy predissociation by a repulsive valence state or states.
Introduction The amount of information available concerning the higher excited states of diatomic halogens and interhalogens was sparse until the advent of synchrotron radiation. The majority of the earlier work was on the absorption spectra in the vacuum-ultraviolet (VUV), recorded using photographic methods, which yielded a reasonably complete picture of the Rydberg-state manifolds of ICI,l I,? and BrP3 More recently, synchrotron radiation has been used to study the absorption and fluorescence properties of CI2$-' Br2,* BrCI,IOICI," and IBrI2J3 in the VUV spectral range. The earliest report on the VUV absorption spectrum of IC1 was a photographic study in the wavelength range of 167-191 nm, in which two band systems were identified and ana1y~ed.I~ These same two (Rydberg) systems were again observed photographically much later on with the additional discovery of two new systems between 139 and 191 nm.I5 An analysis of these 'Present address: Structural Materials Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames TW 16 7LN, U.K.
newly found Rydberg systems followed, and, in addition, preliminary assignments were made on numerous other Rydberg tran( I ) Venkateswarlu, P. Can. J . Phys. 1975, 53, 812. (2) Venkateswarlu, P. Can. J . Phys. 1970, 48, 1055. (3) Venkateswarlu, P. Can. J . Phys. 1969, 47, 2525. (4) Wormer, J.; Moller, T.; Stapelfeldt, J.; Zimmerer, G.; Haaks, D.; Kampf, S.; LeCalvi, J.; Castex, M. C. Z . Phys. D 1988, 7 , 383. ( 5 ) Lee, L. C.; Suto, M.; Tang, K. Y. J . Chem. Phys. 1986, 84, 5277. (6) Moller, T.; Jordan, B.; Zimmerer, G.; Haaks, D.; LeCalvE, J.; Castex, M.-C. Z . Phys. D 1986, 4 , 73. (7) Moeller, T.; Jordan, B.; Gfirth, P.; Zimmerer, G . ;Haaks, D.; LeCalvt, J.; Castex, M.-C. Chem. Phys. 1983, 76, 295. (8) Austin, D. I.; Donovan, R.J.; Hopkirk, A,; Lawley, K. P.; Shaw, D.; Yencha, A. J . Chem. Phys. 1987, 118, 91. (9) Donovan, R. J.; MacDonald, M. A.; Lawley, K. P.; Yencha, A. J.; Hopkirk, A. Chem. Phys. Lett. 1987, 138, 571.
(IO) Hopkirk, A.; Shaw, D.; Donovan, R. J.; Lawley, K. P.; Yencha, A. J. J . Phys. Chem. 1989, 93. ( I I ) Kerr, E.; MacDonald, M.; Donovan, R. J.; Wilkinson, J. P. T.; Shaw, D.; Munro, I. J . Photochem. 1985, 31, 149. (12) Yencha, A. J.; Donovan, R. J.; Hopkirk, A,; Shaw, D. J. Phys. Chem. 1988, 92, 5523.
OO22-3654/90/2094-620 1$02.5O/O 0 1990 American Chemical Society
