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A vibronic analysis of the ã3A2 ← X̃1A1 , ∗ ← n electronic transition in thiophosgene
JOURNAL OF MOLECULAR
SPECTROSCOPY
48, 336345
(1973)
A Vibronic Analysis of the ti3A2 + %A,, Z* -+ n Electronic Transition in Thiophosgene D. C. MOULE Department
of Chemistry,
Brock Uniwrsity,
St Catlrarines,
Ontario, Canada
AND
C. R. SWRAMANLW Department
of Chemistry,
The visible absorption
spectrum
M&faster
University, Hamilton,
of thiophosgene
has been recorded
Onlario,
Canada
under conditions
of high
resolution and long path length and has been assigned as an ?r* + n, SAZ +- -%A, electronic transition. Progressions in VI’, Y*‘,VZ’,~4’~~1”) ~a”, and Y,” have been identified in the spectrum and have been analzyed in terms of vibronic transitions between a planar ground state and a nonplanar excited state. A barrier height of 726 cm-r and a nonplanar equilibrium angle of 32” were obtained for the upper state from a fit of the energy levels of a Lorentzian function to the observed levels in vp. The origin of the system was assigned to a band at 17 492 cm-r. The oscillator strength of the singlet-triplet system relative to the corresponding singletsinglet system was obtained by comparing the intensities of selected singlet and triplet bands in their region of spectral overlap. The high intensity, j = 2 X 10-6, observed for the transition results from a spin-orbit coupling between the SA 2(T*,II) state and the ‘AI (T*,T) state. This mechanism predicts that the major bands in the triplet system should be polarized in the direction of the thiocarbonyl bond. INTRODUCTION
Vibronic analyses that have been reported on triplet-singlet transitions of the r* +- n type are few in number. This is mainly because of the extreme weakness of these absorption systems. The T1 +--So transitions for the molecules containing the thiocarbonyl chromophore, however, can be observed with pressure paths ranging up to 2-3 meteratmospheres. The present study was designed to furnish information about the structure and dynamics of the 3(r*,n) state of Cl,CS, particularly with respect to the height of the barrier to molecular inversion and the nonplanar equilibrium angle. The visible spectrum of thiophosgene has been studied by several workers. It was originally observed by Henri and Duchesne (1) and later by Burnelle (2) who was able to assign the bands to a R* +- n electronic transition in the thiocarbonyl chromophore from the observed strong blue solvent shift. A vibrational analysis of this transition by Brand and coworkers (3) has demonstrated that the combining states are lv3A2(a*,n) +~‘AI and that the upper singlet state is nonplanar to the extent of 32’. Recently Lombardi (4) has been able to resolve the rotational fine structure in the singlet 2$30’4& band and has obtained an out-of-plane angle of 27.2” for the upper level from a rota336
Copyright@ 1973by AcademicPress.Inc. .4llrightsof reproductionin any formresewed.
U.V. ABSORPTION
FIG. 1. Bands of the 8?A, + W’A 1 transition the partially resolved rotational fine structure.
337
OF Cl&S
in thiophosgene,
showing
the chlorine
isotope
effect and
tional contour analysis. The change in spin multiplicity in the longer wave length system has been established by Lombardi (5) who employed the Zeeman effect and by Eberhardt (6) who observed a magnetic rotation effect. A preliminary account of the present analysis has been reported (7). The assignment of the ground state normal modes appears to be complete with the recent identification (7, 8) of the Coriolis coupled vibrations ~2” and ve” as separate bands in the condensed phase infrared spectrum at low temperatures. EXPERIMENTAL
METHODS
AND
RESULTS
Thiophosgene was obtained commercially from the Aldrich Chemical Company and was purified by trap-to-trap distillation. Survey spectra were recorded over the range 7000-2000 a with a Cary Model 14 spectrophotometer. Moderately high resolution spectra were photographed in the first order of a 20’ Ebert spectrograph (9) at a resolution of about 150 000. Path lengths up to 60 meters were obtained from a 2 meter White-type multiple reflection cell. The sample was maintained in the gas phase at pressures between 20-70 mm. Spectra were recorded on Ilford FP3 film. Enlargements of the vz’, va’, and vq’ fundamentals are shown in Fig. 1. Tracings of the absorption spectra of Cl&S recorded with a Joyce-Loebl microdensitometer are shown in Fig. 2. The spectral features were measured with a comparator and were fitted to the reference lines by means of a least-squares program. The wave numbers of the observed bands are given in Table I. These wave numbers refer to the unresolved intense single peaks, which most bands exhibit under low resolution. The usual designation is followed in which a v’ - v” transition in the normal mode K is described by K$f. In keeping with the other tetratomic carbonyl and thiocarbonyl halide analyses, the energy levels in the sulphur wagging mode vb’ are numbered by the C~V low barrier designation 0, 1,2,3, . . . so that 0 and 1 correlate to 0+ and O- in the alternative C, notation.
338
MOULE
AND
SUBRAMANIAM
FIG. 2. Microdensitometer tracings of the absorption spectra of Cl&S obtained with a 20 foot Ebret spectrograph. The spectra labeled (a) were recorded at 13.0 meters path length and a pressure of 10 mm Hg. That labeled (b) was taken under 26.1 meters path and a pressure of 20 mm Hg while (c) was recorded at 45.6 meters path length and 40 mm Hg pressure. ASSIGNMENT ~1
(CS Stretch)
Bands involving the activity of ~1” were identified in the spectrum at 16 827.4, 16 888.9, 17 058.9, 17 133.5, and 17 290.2 from the temperature effect and were assigned as 1r02J, ll”404, 1r04$, lP3&4$, and lr”202, respectively. The ground state combination differences average VI” to 1139.0 cm-‘, which is to be compared to the 1139 cm-’ infrared value (3). In spite of a thorough search, no trace of ~1” has been found in the much more intense singletsinglet transition (3). It is somewhat of an enigma, therefore, that VI” is easy to recognize among the hot bands of this transition. The identification of ~1’ in the spectrum posed considerable difficulties as a result of interference from the S-S absorption. The magnetic rotation spectra of C12CS (6), which was very kindly provided to us by Dr. W. H. Eberhardt, proved to be invaluable in the analyses of the triplet system and, in particular, the assignment of ~1’. In the A1A2 state, ~1’ takes a value 907.4 cm-‘, which represents a drop of 231.6 cm-’ over the ground state value. As it has been established for the carbonyl series that Y(CO) assumes a higher value in the s (“*,I$ state than it does in the ‘(?r*,n)state, for example, y2(@A2, H~CO)/V~(~~A~, H2CO) = 1251/1182 (IO), we would anticipate, therefore, that in a3A2, Cl&S ~1’ would fall in the range 9.50-1003 cm-‘. The M.R.S. effect shows a
339
U.V. ABSORPTION OF Cl,CS TABLE I BAND ~~REQUENCIES ANDASSIGNMENTS FOR THE @A, + zlA1 TFLANSITION IN Cl&S Assign.
Y(35,35)
v(35,37)
Assign.
v(35,35)
Y(35,371
16797.5 16827.4 16888.9 17021.1 17035.4 17058.9 17133.5 17203.7 17268.7 17290.2 17323.7 17333.3 17451.0 17491.8 17501.5 17516.8 17572.9 17621.7 17677.7 17739.2 17739.8
16796.7 16824.9 16887.6 17022.1 17037.8 17056.2 If 130.1 17206.9 17267.6 If 285.5 17323.5 17334.8 17451.7 17492.2 17503.9 17513.6 -
402 3,040s 201 3oz 40’ 30’4$ 406 20’3t,’ 3oa 30’40’ 30*4l? 2oa 30’406 20’30~
17789.9 17908.6 17966.1 17987.6 18027.9 18037.2 18197.9 18210.6 18234.5 18272.6 18286.9 18429.4 18444.8 18451.6 18479.6 18596.5 18665.9 18714.4 18888.5 18916.2
17789.3 17909.2 17963.6 17984.1 18.026.4 -
17620.4 17677.5 17736.3 17740.4
10’
203310 2oz3$ II+301 2k 20’30’40~
18195.6 18205.7 18229.1 18269.1 18281.9 18424.3 18439.3 -
18659.1
-
band of moderate intensity and small isotope shift at 18 479.6 cm-’ which we assign as l&, which gives ~1’ = 987.8 cm-‘. As only one other band 10’30~was found to “lock in” with this frequency, this assignment must be regarded as tentative. v2
(CCZ Stretch)
Progressions of two or three members in intervals ranging from 474 to 4.55 cm-r were observed to attach to the origin and a number of suborigin bands. The assignment to VP’ is straightforward as v(CC1) assumes a value of 480 cm-l in the corresponding ‘A 2 state. This vibrational mode appears to have greater Franck-Condon activity than would be anticipated from a consideration of the v2 frequency in the two combining electronic states. The intensity enhancement of the v(C-X) progressions where X = F, Cl, however, is a feature common to the r* + n transitions in the series C12C0 (11), FLS (1Z), FCICS (13) and has been attributed (3) to a mixing of the internal coordinates Sr (CO or CS stretch) and S2 (CX stretch) in the formation of the normal coordinate Q2 Hot bands arising from the activity of vz” have not been observed in the spectrum. v3 (CZCCZBand) A series of well defined bands were observed in this system, which could be fitted into progressions in a nearly constant interval of 247 cm-‘. From an analogy to the neighboring singlet-singlet system, this was taken to be va’. This value is to be compared with the
MOULE
340
AND SUBRAMANIAM TABLE
II
CALCULATEDAND OBSERVEDLEVELS OF THE Y( MANIFOLDS OF THE A”lA2 AND iFA2 STATES OF TlKS
0
co+) co--_) ha cl+) Y4
3*4 Cl-_) 4v4 c2+1 54
(2-I
6~4
(3+)
obsd
calcd
obsd
calcd
0.0 0.4 279.6 292.5 447.0 586.9
0.00 0.42 279.58 293.53 484.96 578.00
0.0 0.3 298.1 302.9 536.1 600.9 706.1
0.00 0.13 298.13 303.27 531.72 588.17 748.17
frequencies 288.5 and 245.0 cm-r, which have been observed for the zlA1 and A”lA2 electronic states. Quanta of ~3” were observed to attach to many of the upper state vibronic levels. The rotational envelopes of every band bearing the ~3” assignment contained an unusual but regular fine structure, illustrated in Fig. 1, which may be due to the strong Coriolis coupling of ~3 and ~6 modes in the ground state (8). v4
(Out-of-Plane
Wag)
The molecular-orbital analysis, presented in the discussion, shows that the bulk of the Z3A2 +- XIAI oscillator strength results from a spin-orbit coupling between the ‘Al(?r*, T) and E3Az(?r*, n)states. The polarization and the selection rules of the C3Az +xlA1 transition should resemble those of the lAl(?r*, T) +- ..ylA~ from which the intensity is borrowed. Transitions from v” = 0, therefore, will connect to the v’ = 0 level giving rise to type A bands. This is to be contrasted to the A’A2 t TIA1 transition, where the dominant pseudo-origin band results from a transition ~1~’= 1 +- 214”= 0, which is brought about by Herzberg-Teller vibronic coupling. Consequently, the inversion doubling pattern in the i2Az system would be reversed over what it is in the A’Az system. In the triplet system the inversion splitting A is given by 4r1 - 40~ + yd”, where A is defmed as the energy difference between an even-odd pair of levels in the manifold of Ye’. Absorption arising from the activity of ~4” was identified from the spectra recorded at 2OO’C and the isotope effect. From an analogy to the formaldehyde systems where @*AZ Barrier Height)/(ii3Az Barrier Height) = 356/775 (14), it is to be anticipated that the barrier to inversion be somewhat higher in the Z3Az state of CSCIZ than the 600 cm-l value reported for the A”lA2 state. Therefore, for equivalent levels in yd”, the A values observed for the triplet system are expected to be smaller than those established for the singlet system. From this point of view, the band at 17 789.9 cm-’ provided the key to the vibrational quantum numbering. It was observed to have a A equal to 4.8 cm-’ and a chlorine isotope effect ~(3.5, 3.5)~~(35, 37) of 0.6 cm-‘, which allowed it to be unequivocally assigned as 402. The other members of the y4’ manifold were assigned from the isotope effect and from the observed A values. Table II gives the observed ~4’ manifold of levels of the iZ3Az state and, for the sake of comparison, the corresponding levels of the a1Az state. The calculated sets of levels
U.V. ABSORPTION TABLE
341
OF Cl&S III
CALCULATED AND OBSERVED INTENSITIES IN THE Y, PROGFUSSIONS
i?AzczA~ Assign.
Calcds
ALA* +*A, Obsda
Assign.
Calcdb
Obsdo
400
1
1
40’
1
-
42 40’ 406
18 103 69
20 103 -
4oi 406 40’
7 13 10
7 -
-
412 41° 428 42’
41’ 41J 416
0.4 3 8
3.4 3
5 0.8 0.9 0.2
3.6 1.9 0.9 0.6
* Calcd and obsd intensities are normalized at 1 for the 40~ origin band. b calcd intensities normalized at 1 for the 401 false origin band. c obsd intensities normalized to f for the 40%band.
were obtained as solutions to a one dimensional harmonic oscillator function which was perturbed by a Lorentzian barrier term. The matrix elements were set up using 40 harmonic-oscillator wave functions as basis functions. The eigenvalues and eigenvectors were obtained by diagonalization of the resulting Hamiltonian. The best fit to the E3Az data was obtained from the function V(Z) = 74.8322 + [4.45 X 106/(54.2 + 22)], where Z is the reduced normal coordinate related to the displacement coordinate by
defined by Chan and Stelman
2 = (4#z”)*z.
(1) (14) and is
(2)
A similar fit to the A1A2 data gave V(Z) = 76.1822 + [4.38 X 106/(55.0 + 22)].
(3)
The barriers in the double well functions (1) and (3) were 726 and 598 cm-r, respectively, while the corresponding values for Z,, the minima in the functions, were 4.8 and 4.6, respectively. The method of Jones and Coon (15) was used to relate 2, to the out-ofplane &. A variable reduced mass was employed in which the ClCCl angle and CC1 bond lengths were maintained at their ground state values, 111.3” and 1.746 A and the CS bond length extended by 0.1 A to give 1.73 A. etn from this calculation was 32’ for the E3Az state and 31’ for the B1A2 state. DISCUSSION
From a comparison of the optical densities of the singlet and triplet bands which lie adjacent to each other in the spectrum it should be possible to obtain a measure of the relative oscillator strengths of the two transitions. This method, however, does require a complete knowledge of the Franck-Condon factors of the vibronic transitions of both systems. For the purpose of this discussion, we assume that the intensity factors
342
MOULE AND SUBRAMANIAM
corresponding to the modes Qr, Qf, and Q3 are the same in the two systems, since ~1, vZ, and ~3 in the Z3As and zlAz states are nearly identical. To evaluate the relative band intensities of 14 we employed, for the singlet system, the Herzberg-Teller (16) integrals (z~‘)QjD”) and for the triplet system, the normal Franck-Condon integrals (v’[v”). These were obtained from the eigenvector coefficients resulting from the solutions to the double well problem, Eqs. (1) and (3), and the integrals (X’ 1QI A”) and (X’/ A”) which describe the overlap of the ground and excited state vibrational basis functions fix? and $~tt. The result of this calculation is shown in Table III. The second column of this table tabulates the ratios for the T-S system.
‘(V’,W”) -=
440)
(0’ 1w”)2v(v’,v”) exp{ [v(v',v") - ~(v',O)]h1kT},
(0IQ2 VW0
(4)
while the fifth column gives the ratios for the S-S system
(~(w’p”) -=
41,0>
(v’ 1Q 1v”) v(v’,D”) exp{ [v(v’,v”)
(llQlO>dl,O)
-
v(w',O)]h1kT).
(5)
That is, the extinction coefficients were normalized to the origins and false origins of the two systems, ~(48) = 1.0 and ~(401) = 1.0. The band extinction coefficients summed over the members of the v4 progression up to v’ = v” = 8, which are required in the next section are 5 e(v’,o”)/e(O,O) = 231 for the T-S system and 0’1” t e(v’,v”)/~(l,O) o’s”
= 47.2 for the S-S system.
The third and sixth columns of Table III list the intensities of the two systems. These were recorded as the band peak heights and are believed to be accurate to &20$‘&. The S-S band 42a at 18 066.4 cm-’ and the T-S band 40~ at 18 027.9 cm-’ are of the proper height for a meaningful comparison of optical densities. The 42a band is observed to have a relative optical density of 0.18 which gives a value of 0.18 X 47.2/0.9 = 9.45 for the sum of the intensities of the bands labeled 4::~ in the S-S transition. A similar calculation for the T-S relative-optical density-gives 0.70 X 231/103 = 1.56. Therefore, the ratio f(ir3A 2 t Z’A 1)/f@‘A2 + %A 1) = 1.56/9.45 = l/6.0. As the oscillator strength of the S-S system has been observed (17) asf = 1.2 X 10W4,it follows for the T-S system that f = 2.0 X 1W5. It is interesting to compare these values with the values for formaldehyde (18, 19), where the observed oscillator strengths aref(Al:‘dz +%Al) = 2.4 X lo-4 and f(a3A2 t _%?AJ = 1.2 X lO+. That is, while the oscillator strengths of the S-S systems are quite similar in a&O and Cl&S, there is a 17-fold intensification of the T-S transition in Cl&S over that of H&O. It is of some interest to speculate on the sources which give rise to the intensification of the T-S transition in Cl&S. If it is assumed that the largest contribution to the oscil-
U.V. ABSORPTION
lator strength
results from a first order spin-orbit
OF Cl,CS
coupling
343 (20) then
(I x [(‘A.IHsoIiiaAz)/v(1A.+a”aA2)]2.
(6)
That is, if an estimate can be made of the spinorbit matrix elements (‘A a 1HSO1daA2), then a knowledge of the oscillator strengths f(‘A .+x*A 1) and the energies v (*A.+%A 1) of the transitions adjacent to the Za3A2 + _%?A1 transition will determine the oscillator strength, f(@A 2 * x”A 1). As a starting point for this discussion, we turn to the spectrum of formaldehyde which, for many purposes, may be considered to be a reasonable thiocarbonyl prototype. It is now well established, for H&O (21) that the major pathway for T-S intensification involves a mixing of the H3A2(r*,n) and ‘Al(?r*,?r) wavefunctions by a spin-orbit operator which transforms as I @so) = u2. For this case, the summation in Eq. (6) extends only over a single term, where ‘A, is identified with the ‘Ar(a*,?r) state. While the ‘Ar(**,r) + %A 1 transition has not been definitely identified (22) in the vacuum U.V. spectrum of H2C0, quite detailed M.O. calculations (23) have placed the vertical transition energy at 78 200 cm-’ and the oscillator strength at 0.40. In a pioneering study on more than 62 thiocarbonyl compounds, Fabian et al. (24) found that the spectra of these compounds were invariably characterized by an intense absorption ranging from log(e,& = 3.6 to 4.4 and a vmax from 49 500 to 30 000 cm-‘. On the basis of an empirical M.O. treatment these authors assigned the absorption to the intervalence transition, ‘A ~(?T*,?T)+ xlAr. Recently, King and Famworth (25) have made an analysis of the vibronic structure of this system and have reaffirmed this assignment. Their values for Cl#ZS are vmlur = 36 007 cm-’ and f = 0.12. Given such a mechanism for the intensification of the triplet-singlet transition in the thiocarbonyl chromophores it follows that the triplet system of Cl&S should display 2 polarization characteristics. Moreover, from a consideration of the perturbation gap u(*A. + d3A2)2, the triplet-singlet transition should be 8.3 fold more intense in the ClzCS than it is in H&O. From the foregoing data and Eq. (6), the values obtained for the spin-orbit matrix elements of H&O and C12CS, respectively, were 137 and 375 cm-‘. It is possible to evaluate the spin-orbit matrix integrals in a straightforward fashion by the CND0/2 molecular orbital method (26). Under the approximation (20) that all but one-center integrals are ignored, the integral (‘A 11Hs0 ]_?‘A 1) becomes (~1 Hso 1n). The expansion of the n and ?r M.O.‘s as an L.C.A.O. further reduces the integrals into the form (i/2) Ci Ci”Ci={<, where Gin and Ci* are the P,(n) and p,(r) atomic orbital coefficients for atom i. {i is the one-center spin-orbit coupling factor which may be approximated from atomic spectral data (27, Z’S). For this calculation, l = 28, 152,382, and 587 cm-’ for the atomic centers C, 0, S, and Cl. For H&O, the integral takes the form : l/2(0.774) (0.761){0 + 3(0.296) (0.648)lo = 42 cm-’ and for H2CS : l/2 (-0.927) (-0.596){s + l/2 (0.142) (-0.726)ro = 106 cm-‘. The observed factor of 137/375 for the ratio of the formaldehyde to thiophosgene coupling constants may be said to be largely rationalized by the calculated ratio 42/106 for formaldehyde to thioformaldehyde. When the calculation was repeated for thiophosgene, however, it was found that a large spin-orbit term from the Cl atomic centers resulted in almost a complete cancellation of the spin coupling from the sulphur center: (-0.828) (0.568)5s + (0.213) (0.37O)lo + 2 (-0.326) (-0.489){o, = 4.6 cm-‘. We feel that the contribu-
MOULE
344
AND SUBRAMANIAM TABLE
IV
VIURATIONALFREQUENCIES or THIOPHOSGENE X-1 Ai (ground state) a~Cl*CS 39
(ad
~2b) ~3(al) &I), r = 1 v=2 &rz) ve(bS
3sc137c1cs
1139.0 503.5 288.5 471.0 941.9
1138.8 499.4 285.8 470.7 941.3
Al.4 3QCS 907.4 480.0 245.0 0.4 279.6
2 (7r*,n) ~cla’clcs 906.8 476.7 243.5 0.4 278.4
ii3A2 (T*,n) ‘sC12cs
“CPrclcs
987.8 474.2 247.3 0.2 298.1
246.7 0.2 297.0
818 288
tion to the ?r orbital in ClKS from the chlorine 3p A.O.‘s is greatly overestimated by the CND0/2 method and that, basically, it is the difference in the l’s of oxygen and sulphur which is responsible for the triplet-singlet enhancement of the thiocarbonyls. A similar calculation on the effect of halogen on the intramolecular spin-orbit coupling constants of the substituted carbonyl series has been made by Carol1 et al. (29) who showed that a reduction in triplet-singlet strength is to be expected for substitution of chlorine for hydrogen. CONCLUSION
From the vibrational data collected in Table IV for the xlA 1, A”‘A2, and @A 2 states, it is clear that the structure and bonding of the singlet and triplet (?r*,n) states are similar and that the differences in the magnitude in the barrier height and out-of-plane angles are small. The high intensity observed for the transition, compared to the corresponding transitions in the carbonyl compounds results, in part, from a spin-orbit coupling between the 3(~*,n) state at 17 492 cm-l and the ’ (?r*,r) state at 36 007 cm-‘. This mechanism predicts that the major bands in the triplet system should be polarized in the direction of the thiocarbonyl band. The high triplet-singlet strength in Cl&X appears to result from the small perturbation gap between the @3A2(a*,n) and ‘Ar(**,*) states and the higher { of S relative to 0. ACKNOWLEDGMENTS We wish to thank DI. W. H. Eberhardt for providing us with his unpublished magnetic rotation spectra; Dr. G. W. King for the use of the Ebert Spectrograph; and Dr. J. Tyrrell for recording some of the high resolution spectra. This work was financially supported by the National Research Council of Canada. RECEIVED:
February
7, 1973 REFERENCES
1. V. HENRI ANDJ. DUCHESNE,Natzlrz 143, 28 (1939). 2. L. BURNELL.E,J. Chat. Phys. 24,620 (1956). 3. J. C. D. BRAND,J. H. CALU)YON, D. C. MOULE, J. TYRRELL, ANDT. H. GOODWIN,Tras. Sot. 61, 2356 (1965).
Faraday
U.V. ABSORPTION
OF Cl&S
345
4. J. R. LOMBARDI, J. Chem. Phys. 52, 6126 (1970). 5. J. R. LO~~ARDI, J. Chem. Phys. 53, 560 (1970). 6. W. H. EBEBBARDT,unpublished results, quoted by S. P. MCGLYNN, T. AZUMI, ANDM. KINOSHITA, “Molecular Spectroscopy of the Triplet State,” Prentice-Hall, Englewood Cliffs, NJ, 1969. 7. D. C. MOULE AND C. R. SUBRA~~ANIAM, Chem. Comm. 1340 (1969). 8. C. A. FRENZEL,K. E. BLICK, C. R. BENNET, ANDK. NIEDENZU,J. Chem. Phys. 53, 198 (1970). 9. G. W. KING, J. Sci. Instrum. 35, 11 (1958). 10. G. HERZBERG,“Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules,” Van Nostrand, Princeton, NJ, 1966. 11. D. C. MOULE AND P. D. Foo, J. Chem. Phys. 55, 1262 (1971). 12. D. C. MOULE ANDA. K. MEHRA, J. Mol. Spectrosc. 35, 137 (1970). 13. C. R. SUBRAMANIAM ANDD. C. MOULE, unpublished results. 14. S. L. CHAN ANDD. STELMAN.J. Mol. Spectrosc. 10, 278 (1963). 15. V. T. JONESANDJ. B. COON,J. Mol. Spectrosc. 31, 137 (1969). 16. J. R. HENDERSON,J. C/tern. Phys. 44,3496 (1966). 17. L. BURNELLE,Acad. Roy. Be&e Cl. Sci. M&n. CoU. k-8” 30, (1958). 18. A. B. F. DUNCANANDE. H. HOUSE,unpublished results, quoted by J. W. S~DMAN,J. Chem. Phys. 27,
1270 (1957). 19. V. E. DIGIORGIOAND G. W. ROBINSON,J. Chem. Phys. 31, 1678 (1959). 20. S. P. MCGLYNN, T. AZUMI, AND M. KINOSHITA,“Molecular Spectroscopy of the Triplet State,” Prentice-Hall, Englewood Cliffs, NJ, 1969. 21. J. W. SIDYAN, J. Chem. Phys. 29,644 (1955). 22. J. E. MENTALL, E. P. GENTIEU,M. KRAUSS, ANDD. NEUMANN,J. Chem. Phys. 55,5471 (1971). 23. (a) J. L. WEIITTEN,J. Chem. Phys. 56, 5458 (1972). (b) J. L. WHITTEN AND M. HACKMEYER,J. Chem. Phys. 51, 5584 (1969). 24. T. FABIAN, H. VIOLA, AND R. MAYER, Tetrahedron 23,4323 (1967). 2.5. E. FARNWORTEAND G. W. KING, unpublished results. 26. J. A. POPLE AND D. L. BEVERIDGE,“Approximate Molecular Orbital Theory,” McGraw-Hill, New York, 1970. 27. D. S. MCCLURE, J. Ckem. Phys. 17, 905 (1949). 28. J. H. CALLOMON,Proc. Roy. Sot. Ser. A, 244, 220 (19.58). 29. D. G. CARROLL,L. G. VANQUICKEHBORNE, AND S. P. MCGLYNN, J. Chem. Phys. 45, 2777 (1966).
SPECTROSCOPY
48, 336345
(1973)
A Vibronic Analysis of the ti3A2 + %A,, Z* -+ n Electronic Transition in Thiophosgene D. C. MOULE Department
of Chemistry,
Brock Uniwrsity,
St Catlrarines,
Ontario, Canada
AND
C. R. SWRAMANLW Department
of Chemistry,
The visible absorption
spectrum
M&faster
University, Hamilton,
of thiophosgene
has been recorded
Onlario,
Canada
under conditions
of high
resolution and long path length and has been assigned as an ?r* + n, SAZ +- -%A, electronic transition. Progressions in VI’, Y*‘,VZ’,~4’~~1”) ~a”, and Y,” have been identified in the spectrum and have been analzyed in terms of vibronic transitions between a planar ground state and a nonplanar excited state. A barrier height of 726 cm-r and a nonplanar equilibrium angle of 32” were obtained for the upper state from a fit of the energy levels of a Lorentzian function to the observed levels in vp. The origin of the system was assigned to a band at 17 492 cm-r. The oscillator strength of the singlet-triplet system relative to the corresponding singletsinglet system was obtained by comparing the intensities of selected singlet and triplet bands in their region of spectral overlap. The high intensity, j = 2 X 10-6, observed for the transition results from a spin-orbit coupling between the SA 2(T*,II) state and the ‘AI (T*,T) state. This mechanism predicts that the major bands in the triplet system should be polarized in the direction of the thiocarbonyl bond. INTRODUCTION
Vibronic analyses that have been reported on triplet-singlet transitions of the r* +- n type are few in number. This is mainly because of the extreme weakness of these absorption systems. The T1 +--So transitions for the molecules containing the thiocarbonyl chromophore, however, can be observed with pressure paths ranging up to 2-3 meteratmospheres. The present study was designed to furnish information about the structure and dynamics of the 3(r*,n) state of Cl,CS, particularly with respect to the height of the barrier to molecular inversion and the nonplanar equilibrium angle. The visible spectrum of thiophosgene has been studied by several workers. It was originally observed by Henri and Duchesne (1) and later by Burnelle (2) who was able to assign the bands to a R* +- n electronic transition in the thiocarbonyl chromophore from the observed strong blue solvent shift. A vibrational analysis of this transition by Brand and coworkers (3) has demonstrated that the combining states are lv3A2(a*,n) +~‘AI and that the upper singlet state is nonplanar to the extent of 32’. Recently Lombardi (4) has been able to resolve the rotational fine structure in the singlet 2$30’4& band and has obtained an out-of-plane angle of 27.2” for the upper level from a rota336
Copyright@ 1973by AcademicPress.Inc. .4llrightsof reproductionin any formresewed.
U.V. ABSORPTION
FIG. 1. Bands of the 8?A, + W’A 1 transition the partially resolved rotational fine structure.
337
OF Cl&S
in thiophosgene,
showing
the chlorine
isotope
effect and
tional contour analysis. The change in spin multiplicity in the longer wave length system has been established by Lombardi (5) who employed the Zeeman effect and by Eberhardt (6) who observed a magnetic rotation effect. A preliminary account of the present analysis has been reported (7). The assignment of the ground state normal modes appears to be complete with the recent identification (7, 8) of the Coriolis coupled vibrations ~2” and ve” as separate bands in the condensed phase infrared spectrum at low temperatures. EXPERIMENTAL
METHODS
AND
RESULTS
Thiophosgene was obtained commercially from the Aldrich Chemical Company and was purified by trap-to-trap distillation. Survey spectra were recorded over the range 7000-2000 a with a Cary Model 14 spectrophotometer. Moderately high resolution spectra were photographed in the first order of a 20’ Ebert spectrograph (9) at a resolution of about 150 000. Path lengths up to 60 meters were obtained from a 2 meter White-type multiple reflection cell. The sample was maintained in the gas phase at pressures between 20-70 mm. Spectra were recorded on Ilford FP3 film. Enlargements of the vz’, va’, and vq’ fundamentals are shown in Fig. 1. Tracings of the absorption spectra of Cl&S recorded with a Joyce-Loebl microdensitometer are shown in Fig. 2. The spectral features were measured with a comparator and were fitted to the reference lines by means of a least-squares program. The wave numbers of the observed bands are given in Table I. These wave numbers refer to the unresolved intense single peaks, which most bands exhibit under low resolution. The usual designation is followed in which a v’ - v” transition in the normal mode K is described by K$f. In keeping with the other tetratomic carbonyl and thiocarbonyl halide analyses, the energy levels in the sulphur wagging mode vb’ are numbered by the C~V low barrier designation 0, 1,2,3, . . . so that 0 and 1 correlate to 0+ and O- in the alternative C, notation.
338
MOULE
AND
SUBRAMANIAM
FIG. 2. Microdensitometer tracings of the absorption spectra of Cl&S obtained with a 20 foot Ebret spectrograph. The spectra labeled (a) were recorded at 13.0 meters path length and a pressure of 10 mm Hg. That labeled (b) was taken under 26.1 meters path and a pressure of 20 mm Hg while (c) was recorded at 45.6 meters path length and 40 mm Hg pressure. ASSIGNMENT ~1
(CS Stretch)
Bands involving the activity of ~1” were identified in the spectrum at 16 827.4, 16 888.9, 17 058.9, 17 133.5, and 17 290.2 from the temperature effect and were assigned as 1r02J, ll”404, 1r04$, lP3&4$, and lr”202, respectively. The ground state combination differences average VI” to 1139.0 cm-‘, which is to be compared to the 1139 cm-’ infrared value (3). In spite of a thorough search, no trace of ~1” has been found in the much more intense singletsinglet transition (3). It is somewhat of an enigma, therefore, that VI” is easy to recognize among the hot bands of this transition. The identification of ~1’ in the spectrum posed considerable difficulties as a result of interference from the S-S absorption. The magnetic rotation spectra of C12CS (6), which was very kindly provided to us by Dr. W. H. Eberhardt, proved to be invaluable in the analyses of the triplet system and, in particular, the assignment of ~1’. In the A1A2 state, ~1’ takes a value 907.4 cm-‘, which represents a drop of 231.6 cm-’ over the ground state value. As it has been established for the carbonyl series that Y(CO) assumes a higher value in the s (“*,I$ state than it does in the ‘(?r*,n)state, for example, y2(@A2, H~CO)/V~(~~A~, H2CO) = 1251/1182 (IO), we would anticipate, therefore, that in a3A2, Cl&S ~1’ would fall in the range 9.50-1003 cm-‘. The M.R.S. effect shows a
339
U.V. ABSORPTION OF Cl,CS TABLE I BAND ~~REQUENCIES ANDASSIGNMENTS FOR THE @A, + zlA1 TFLANSITION IN Cl&S Assign.
Y(35,35)
v(35,37)
Assign.
v(35,35)
Y(35,371
16797.5 16827.4 16888.9 17021.1 17035.4 17058.9 17133.5 17203.7 17268.7 17290.2 17323.7 17333.3 17451.0 17491.8 17501.5 17516.8 17572.9 17621.7 17677.7 17739.2 17739.8
16796.7 16824.9 16887.6 17022.1 17037.8 17056.2 If 130.1 17206.9 17267.6 If 285.5 17323.5 17334.8 17451.7 17492.2 17503.9 17513.6 -
402 3,040s 201 3oz 40’ 30’4$ 406 20’3t,’ 3oa 30’40’ 30*4l? 2oa 30’406 20’30~
17789.9 17908.6 17966.1 17987.6 18027.9 18037.2 18197.9 18210.6 18234.5 18272.6 18286.9 18429.4 18444.8 18451.6 18479.6 18596.5 18665.9 18714.4 18888.5 18916.2
17789.3 17909.2 17963.6 17984.1 18.026.4 -
17620.4 17677.5 17736.3 17740.4
10’
203310 2oz3$ II+301 2k 20’30’40~
18195.6 18205.7 18229.1 18269.1 18281.9 18424.3 18439.3 -
18659.1
-
band of moderate intensity and small isotope shift at 18 479.6 cm-’ which we assign as l&, which gives ~1’ = 987.8 cm-‘. As only one other band 10’30~was found to “lock in” with this frequency, this assignment must be regarded as tentative. v2
(CCZ Stretch)
Progressions of two or three members in intervals ranging from 474 to 4.55 cm-r were observed to attach to the origin and a number of suborigin bands. The assignment to VP’ is straightforward as v(CC1) assumes a value of 480 cm-l in the corresponding ‘A 2 state. This vibrational mode appears to have greater Franck-Condon activity than would be anticipated from a consideration of the v2 frequency in the two combining electronic states. The intensity enhancement of the v(C-X) progressions where X = F, Cl, however, is a feature common to the r* + n transitions in the series C12C0 (11), FLS (1Z), FCICS (13) and has been attributed (3) to a mixing of the internal coordinates Sr (CO or CS stretch) and S2 (CX stretch) in the formation of the normal coordinate Q2 Hot bands arising from the activity of vz” have not been observed in the spectrum. v3 (CZCCZBand) A series of well defined bands were observed in this system, which could be fitted into progressions in a nearly constant interval of 247 cm-‘. From an analogy to the neighboring singlet-singlet system, this was taken to be va’. This value is to be compared with the
MOULE
340
AND SUBRAMANIAM TABLE
II
CALCULATEDAND OBSERVEDLEVELS OF THE Y( MANIFOLDS OF THE A”lA2 AND iFA2 STATES OF TlKS
0
co+) co--_) ha cl+) Y4
3*4 Cl-_) 4v4 c2+1 54
(2-I
6~4
(3+)
obsd
calcd
obsd
calcd
0.0 0.4 279.6 292.5 447.0 586.9
0.00 0.42 279.58 293.53 484.96 578.00
0.0 0.3 298.1 302.9 536.1 600.9 706.1
0.00 0.13 298.13 303.27 531.72 588.17 748.17
frequencies 288.5 and 245.0 cm-r, which have been observed for the zlA1 and A”lA2 electronic states. Quanta of ~3” were observed to attach to many of the upper state vibronic levels. The rotational envelopes of every band bearing the ~3” assignment contained an unusual but regular fine structure, illustrated in Fig. 1, which may be due to the strong Coriolis coupling of ~3 and ~6 modes in the ground state (8). v4
(Out-of-Plane
Wag)
The molecular-orbital analysis, presented in the discussion, shows that the bulk of the Z3A2 +- XIAI oscillator strength results from a spin-orbit coupling between the ‘Al(?r*, T) and E3Az(?r*, n)states. The polarization and the selection rules of the C3Az +xlA1 transition should resemble those of the lAl(?r*, T) +- ..ylA~ from which the intensity is borrowed. Transitions from v” = 0, therefore, will connect to the v’ = 0 level giving rise to type A bands. This is to be contrasted to the A’A2 t TIA1 transition, where the dominant pseudo-origin band results from a transition ~1~’= 1 +- 214”= 0, which is brought about by Herzberg-Teller vibronic coupling. Consequently, the inversion doubling pattern in the i2Az system would be reversed over what it is in the A’Az system. In the triplet system the inversion splitting A is given by 4r1 - 40~ + yd”, where A is defmed as the energy difference between an even-odd pair of levels in the manifold of Ye’. Absorption arising from the activity of ~4” was identified from the spectra recorded at 2OO’C and the isotope effect. From an analogy to the formaldehyde systems where @*AZ Barrier Height)/(ii3Az Barrier Height) = 356/775 (14), it is to be anticipated that the barrier to inversion be somewhat higher in the Z3Az state of CSCIZ than the 600 cm-l value reported for the A”lA2 state. Therefore, for equivalent levels in yd”, the A values observed for the triplet system are expected to be smaller than those established for the singlet system. From this point of view, the band at 17 789.9 cm-’ provided the key to the vibrational quantum numbering. It was observed to have a A equal to 4.8 cm-’ and a chlorine isotope effect ~(3.5, 3.5)~~(35, 37) of 0.6 cm-‘, which allowed it to be unequivocally assigned as 402. The other members of the y4’ manifold were assigned from the isotope effect and from the observed A values. Table II gives the observed ~4’ manifold of levels of the iZ3Az state and, for the sake of comparison, the corresponding levels of the a1Az state. The calculated sets of levels
U.V. ABSORPTION TABLE
341
OF Cl&S III
CALCULATED AND OBSERVED INTENSITIES IN THE Y, PROGFUSSIONS
i?AzczA~ Assign.
Calcds
ALA* +*A, Obsda
Assign.
Calcdb
Obsdo
400
1
1
40’
1
-
42 40’ 406
18 103 69
20 103 -
4oi 406 40’
7 13 10
7 -
-
412 41° 428 42’
41’ 41J 416
0.4 3 8
3.4 3
5 0.8 0.9 0.2
3.6 1.9 0.9 0.6
* Calcd and obsd intensities are normalized at 1 for the 40~ origin band. b calcd intensities normalized at 1 for the 401 false origin band. c obsd intensities normalized to f for the 40%band.
were obtained as solutions to a one dimensional harmonic oscillator function which was perturbed by a Lorentzian barrier term. The matrix elements were set up using 40 harmonic-oscillator wave functions as basis functions. The eigenvalues and eigenvectors were obtained by diagonalization of the resulting Hamiltonian. The best fit to the E3Az data was obtained from the function V(Z) = 74.8322 + [4.45 X 106/(54.2 + 22)], where Z is the reduced normal coordinate related to the displacement coordinate by
defined by Chan and Stelman
2 = (4#z”)*z.
(1) (14) and is
(2)
A similar fit to the A1A2 data gave V(Z) = 76.1822 + [4.38 X 106/(55.0 + 22)].
(3)
The barriers in the double well functions (1) and (3) were 726 and 598 cm-r, respectively, while the corresponding values for Z,, the minima in the functions, were 4.8 and 4.6, respectively. The method of Jones and Coon (15) was used to relate 2, to the out-ofplane &. A variable reduced mass was employed in which the ClCCl angle and CC1 bond lengths were maintained at their ground state values, 111.3” and 1.746 A and the CS bond length extended by 0.1 A to give 1.73 A. etn from this calculation was 32’ for the E3Az state and 31’ for the B1A2 state. DISCUSSION
From a comparison of the optical densities of the singlet and triplet bands which lie adjacent to each other in the spectrum it should be possible to obtain a measure of the relative oscillator strengths of the two transitions. This method, however, does require a complete knowledge of the Franck-Condon factors of the vibronic transitions of both systems. For the purpose of this discussion, we assume that the intensity factors
342
MOULE AND SUBRAMANIAM
corresponding to the modes Qr, Qf, and Q3 are the same in the two systems, since ~1, vZ, and ~3 in the Z3As and zlAz states are nearly identical. To evaluate the relative band intensities of 14 we employed, for the singlet system, the Herzberg-Teller (16) integrals (z~‘)QjD”) and for the triplet system, the normal Franck-Condon integrals (v’[v”). These were obtained from the eigenvector coefficients resulting from the solutions to the double well problem, Eqs. (1) and (3), and the integrals (X’ 1QI A”) and (X’/ A”) which describe the overlap of the ground and excited state vibrational basis functions fix? and $~tt. The result of this calculation is shown in Table III. The second column of this table tabulates the ratios for the T-S system.
‘(V’,W”) -=
440)
(0’ 1w”)2v(v’,v”) exp{ [v(v',v") - ~(v',O)]h1kT},
(0IQ2 VW0
(4)
while the fifth column gives the ratios for the S-S system
(~(w’p”) -=
41,0>
(v’ 1Q 1v”) v(v’,D”) exp{ [v(v’,v”)
(llQlO>dl,O)
-
v(w',O)]h1kT).
(5)
That is, the extinction coefficients were normalized to the origins and false origins of the two systems, ~(48) = 1.0 and ~(401) = 1.0. The band extinction coefficients summed over the members of the v4 progression up to v’ = v” = 8, which are required in the next section are 5 e(v’,o”)/e(O,O) = 231 for the T-S system and 0’1” t e(v’,v”)/~(l,O) o’s”
= 47.2 for the S-S system.
The third and sixth columns of Table III list the intensities of the two systems. These were recorded as the band peak heights and are believed to be accurate to &20$‘&. The S-S band 42a at 18 066.4 cm-’ and the T-S band 40~ at 18 027.9 cm-’ are of the proper height for a meaningful comparison of optical densities. The 42a band is observed to have a relative optical density of 0.18 which gives a value of 0.18 X 47.2/0.9 = 9.45 for the sum of the intensities of the bands labeled 4::~ in the S-S transition. A similar calculation for the T-S relative-optical density-gives 0.70 X 231/103 = 1.56. Therefore, the ratio f(ir3A 2 t Z’A 1)/f@‘A2 + %A 1) = 1.56/9.45 = l/6.0. As the oscillator strength of the S-S system has been observed (17) asf = 1.2 X 10W4,it follows for the T-S system that f = 2.0 X 1W5. It is interesting to compare these values with the values for formaldehyde (18, 19), where the observed oscillator strengths aref(Al:‘dz +%Al) = 2.4 X lo-4 and f(a3A2 t _%?AJ = 1.2 X lO+. That is, while the oscillator strengths of the S-S systems are quite similar in a&O and Cl&S, there is a 17-fold intensification of the T-S transition in Cl&S over that of H&O. It is of some interest to speculate on the sources which give rise to the intensification of the T-S transition in Cl&S. If it is assumed that the largest contribution to the oscil-
U.V. ABSORPTION
lator strength
results from a first order spin-orbit
OF Cl,CS
coupling
343 (20) then
(I x [(‘A.IHsoIiiaAz)/v(1A.+a”aA2)]2.
(6)
That is, if an estimate can be made of the spinorbit matrix elements (‘A a 1HSO1daA2), then a knowledge of the oscillator strengths f(‘A .+x*A 1) and the energies v (*A.+%A 1) of the transitions adjacent to the Za3A2 + _%?A1 transition will determine the oscillator strength, f(@A 2 * x”A 1). As a starting point for this discussion, we turn to the spectrum of formaldehyde which, for many purposes, may be considered to be a reasonable thiocarbonyl prototype. It is now well established, for H&O (21) that the major pathway for T-S intensification involves a mixing of the H3A2(r*,n) and ‘Al(?r*,?r) wavefunctions by a spin-orbit operator which transforms as I @so) = u2. For this case, the summation in Eq. (6) extends only over a single term, where ‘A, is identified with the ‘Ar(a*,?r) state. While the ‘Ar(**,r) + %A 1 transition has not been definitely identified (22) in the vacuum U.V. spectrum of H2C0, quite detailed M.O. calculations (23) have placed the vertical transition energy at 78 200 cm-’ and the oscillator strength at 0.40. In a pioneering study on more than 62 thiocarbonyl compounds, Fabian et al. (24) found that the spectra of these compounds were invariably characterized by an intense absorption ranging from log(e,& = 3.6 to 4.4 and a vmax from 49 500 to 30 000 cm-‘. On the basis of an empirical M.O. treatment these authors assigned the absorption to the intervalence transition, ‘A ~(?T*,?T)+ xlAr. Recently, King and Famworth (25) have made an analysis of the vibronic structure of this system and have reaffirmed this assignment. Their values for Cl#ZS are vmlur = 36 007 cm-’ and f = 0.12. Given such a mechanism for the intensification of the triplet-singlet transition in the thiocarbonyl chromophores it follows that the triplet system of Cl&S should display 2 polarization characteristics. Moreover, from a consideration of the perturbation gap u(*A. + d3A2)2, the triplet-singlet transition should be 8.3 fold more intense in the ClzCS than it is in H&O. From the foregoing data and Eq. (6), the values obtained for the spin-orbit matrix elements of H&O and C12CS, respectively, were 137 and 375 cm-‘. It is possible to evaluate the spin-orbit matrix integrals in a straightforward fashion by the CND0/2 molecular orbital method (26). Under the approximation (20) that all but one-center integrals are ignored, the integral (‘A 11Hs0 ]_?‘A 1) becomes (~1 Hso 1n). The expansion of the n and ?r M.O.‘s as an L.C.A.O. further reduces the integrals into the form (i/2) Ci Ci”Ci={<, where Gin and Ci* are the P,(n) and p,(r) atomic orbital coefficients for atom i. {i is the one-center spin-orbit coupling factor which may be approximated from atomic spectral data (27, Z’S). For this calculation, l = 28, 152,382, and 587 cm-’ for the atomic centers C, 0, S, and Cl. For H&O, the integral takes the form : l/2(0.774) (0.761){0 + 3(0.296) (0.648)lo = 42 cm-’ and for H2CS : l/2 (-0.927) (-0.596){s + l/2 (0.142) (-0.726)ro = 106 cm-‘. The observed factor of 137/375 for the ratio of the formaldehyde to thiophosgene coupling constants may be said to be largely rationalized by the calculated ratio 42/106 for formaldehyde to thioformaldehyde. When the calculation was repeated for thiophosgene, however, it was found that a large spin-orbit term from the Cl atomic centers resulted in almost a complete cancellation of the spin coupling from the sulphur center: (-0.828) (0.568)5s + (0.213) (0.37O)lo + 2 (-0.326) (-0.489){o, = 4.6 cm-‘. We feel that the contribu-
MOULE
344
AND SUBRAMANIAM TABLE
IV
VIURATIONALFREQUENCIES or THIOPHOSGENE X-1 Ai (ground state) a~Cl*CS 39
(ad
~2b) ~3(al) &I), r = 1 v=2 &rz) ve(bS
3sc137c1cs
1139.0 503.5 288.5 471.0 941.9
1138.8 499.4 285.8 470.7 941.3
Al.4 3QCS 907.4 480.0 245.0 0.4 279.6
2 (7r*,n) ~cla’clcs 906.8 476.7 243.5 0.4 278.4
ii3A2 (T*,n) ‘sC12cs
“CPrclcs
987.8 474.2 247.3 0.2 298.1
246.7 0.2 297.0
818 288
tion to the ?r orbital in ClKS from the chlorine 3p A.O.‘s is greatly overestimated by the CND0/2 method and that, basically, it is the difference in the l’s of oxygen and sulphur which is responsible for the triplet-singlet enhancement of the thiocarbonyls. A similar calculation on the effect of halogen on the intramolecular spin-orbit coupling constants of the substituted carbonyl series has been made by Carol1 et al. (29) who showed that a reduction in triplet-singlet strength is to be expected for substitution of chlorine for hydrogen. CONCLUSION
From the vibrational data collected in Table IV for the xlA 1, A”‘A2, and @A 2 states, it is clear that the structure and bonding of the singlet and triplet (?r*,n) states are similar and that the differences in the magnitude in the barrier height and out-of-plane angles are small. The high intensity observed for the transition, compared to the corresponding transitions in the carbonyl compounds results, in part, from a spin-orbit coupling between the 3(~*,n) state at 17 492 cm-l and the ’ (?r*,r) state at 36 007 cm-‘. This mechanism predicts that the major bands in the triplet system should be polarized in the direction of the thiocarbonyl band. The high triplet-singlet strength in Cl&X appears to result from the small perturbation gap between the @3A2(a*,n) and ‘Ar(**,*) states and the higher { of S relative to 0. ACKNOWLEDGMENTS We wish to thank DI. W. H. Eberhardt for providing us with his unpublished magnetic rotation spectra; Dr. G. W. King for the use of the Ebert Spectrograph; and Dr. J. Tyrrell for recording some of the high resolution spectra. This work was financially supported by the National Research Council of Canada. RECEIVED:
February
7, 1973 REFERENCES
1. V. HENRI ANDJ. DUCHESNE,Natzlrz 143, 28 (1939). 2. L. BURNELL.E,J. Chat. Phys. 24,620 (1956). 3. J. C. D. BRAND,J. H. CALU)YON, D. C. MOULE, J. TYRRELL, ANDT. H. GOODWIN,Tras. Sot. 61, 2356 (1965).
Faraday
U.V. ABSORPTION
OF Cl&S
345
4. J. R. LOMBARDI, J. Chem. Phys. 52, 6126 (1970). 5. J. R. LO~~ARDI, J. Chem. Phys. 53, 560 (1970). 6. W. H. EBEBBARDT,unpublished results, quoted by S. P. MCGLYNN, T. AZUMI, ANDM. KINOSHITA, “Molecular Spectroscopy of the Triplet State,” Prentice-Hall, Englewood Cliffs, NJ, 1969. 7. D. C. MOULE AND C. R. SUBRA~~ANIAM, Chem. Comm. 1340 (1969). 8. C. A. FRENZEL,K. E. BLICK, C. R. BENNET, ANDK. NIEDENZU,J. Chem. Phys. 53, 198 (1970). 9. G. W. KING, J. Sci. Instrum. 35, 11 (1958). 10. G. HERZBERG,“Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules,” Van Nostrand, Princeton, NJ, 1966. 11. D. C. MOULE AND P. D. Foo, J. Chem. Phys. 55, 1262 (1971). 12. D. C. MOULE ANDA. K. MEHRA, J. Mol. Spectrosc. 35, 137 (1970). 13. C. R. SUBRAMANIAM ANDD. C. MOULE, unpublished results. 14. S. L. CHAN ANDD. STELMAN.J. Mol. Spectrosc. 10, 278 (1963). 15. V. T. JONESANDJ. B. COON,J. Mol. Spectrosc. 31, 137 (1969). 16. J. R. HENDERSON,J. C/tern. Phys. 44,3496 (1966). 17. L. BURNELLE,Acad. Roy. Be&e Cl. Sci. M&n. CoU. k-8” 30, (1958). 18. A. B. F. DUNCANANDE. H. HOUSE,unpublished results, quoted by J. W. S~DMAN,J. Chem. Phys. 27,
1270 (1957). 19. V. E. DIGIORGIOAND G. W. ROBINSON,J. Chem. Phys. 31, 1678 (1959). 20. S. P. MCGLYNN, T. AZUMI, AND M. KINOSHITA,“Molecular Spectroscopy of the Triplet State,” Prentice-Hall, Englewood Cliffs, NJ, 1969. 21. J. W. SIDYAN, J. Chem. Phys. 29,644 (1955). 22. J. E. MENTALL, E. P. GENTIEU,M. KRAUSS, ANDD. NEUMANN,J. Chem. Phys. 55,5471 (1971). 23. (a) J. L. WEIITTEN,J. Chem. Phys. 56, 5458 (1972). (b) J. L. WHITTEN AND M. HACKMEYER,J. Chem. Phys. 51, 5584 (1969). 24. T. FABIAN, H. VIOLA, AND R. MAYER, Tetrahedron 23,4323 (1967). 2.5. E. FARNWORTEAND G. W. KING, unpublished results. 26. J. A. POPLE AND D. L. BEVERIDGE,“Approximate Molecular Orbital Theory,” McGraw-Hill, New York, 1970. 27. D. S. MCCLURE, J. Ckem. Phys. 17, 905 (1949). 28. J. H. CALLOMON,Proc. Roy. Sot. Ser. A, 244, 220 (19.58). 29. D. G. CARROLL,L. G. VANQUICKEHBORNE, AND S. P. MCGLYNN, J. Chem. Phys. 45, 2777 (1966).