J . Phys. Chem. 1987, 91, 2046-2050
2046
ARTICLES Matrix Isolation Investigation of the Vibrational and Electronic Spectroscopy and Photochemistry of Complexes of CIF and CI, with Sulfide Bases Nicholas P. Machara and Bruce S. Auk* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: July 7, 1986; In Final Form: October 6, 1986)
Matrix isolation has been coupled with twin jet deposition for the formation of 1:l molecular complexes of C1F and CI, with dimethyl sulfide and related bases. The infrared spectra of the complexes were dominated by an intense absorption which has been assigned to the perturbed halogen stretching mode. This mode was greatly intensified and red-shifted upon complex formation; the observed shifts of up to 300 cm-’ were much greater than shifts for the complexes of CIF with oxygen bases. The spectral differences have been rationalized in terms of hard/soft acid/base theory. Visible-ultraviolet spectra were also recorded for these complexes at high dilutions; for several an intense charge-transfer band was observed in the UV. Mercury arc irradiation into this charge-transfer absorption led to photochemically induced rearrangement reactions for several of the complexes studied.
Introduction
Charge-transfer complexes of the halogens have been of interest to chemists since the very beginning of the formulation of charge-transfer Only recently has there been any investigation of the complexes and chemistry of the more reactive halogens such as ClF.4 Matrix isolation5“ studies of ClF with oxygen-containing bases such as methanol and dimethyl ether have demonstrated that a complex can indeed be i ~ o l a t e d .Infrared ~ spectra of these complexes were all characterized by a perturbed halogen stretch shifted 50-100 cm-’ to lower energy from the parent. Bases in which the electron donor is a sulfur atom5bare known to form molecular complexes as well; previous studies of methyl sulfide complexes with I,, IBr, ICl, and Br, have shown that although the higher frequency vibrations of the donor are only slightly perturbed, the halogen acceptor vibration undergoes a considerable red shift and intensification.@ Also, Barnes et al.,I2 have briefly reported formation of 1:l complexes of C1, with (CH3)2Sand H2Sin argon matrices. The central sulfur atom is traditionally viewed as a “soft” atom and is expected to interact more favorably with “soft” or polarizable Lewis acids. The previously studied oxygen base compounds, on the other hand, are generally regarded as “hard” bases. Significantly different orbital interactons may occur between ClF and sulfur bases than was noted between ClF and oxygen bases; observation of such differences may lend support to the general hard/soft acid/base theory. 1,6,10,11 (1) Jensen, W. B. Chem. Rev. 1978, 78, 1 and references therein. (2) Mulliken, R. S . J . Am. Chem. Soc. 1952, 74, 811 and references therein. (3) Mulliken, R. S.; Person, W. B. Annu. Reu. Phys. Chem. 1962,13, 107 and references therein. (4) Machara, N. P.; Ault, B. S. Znorg. Chem. 1985, 24, 4251. (5) (a) Barnes, A. J. Molecular Interactions; Ratajczak, H., OrvilleThomas, W., Eds.; Wiley: New york, 1980; Vol. 1, p 286, and references therein. (b) Land, R. P. J . Am. Chem. SOC.1962, 84, 1185. (6) Jensen, W. B. The Lewis Acid-Base Concepts, An Overview; Wiley: New York, 1980. (7) Hayward, G. C.; Hendra, P. J. J. Chem. SOC.A 1969, 1760. (8) Tveter, T.; Klaeboe, P.; Nielsen, C. J. Soectrochim. Acta 1984, 40A. 351 and references therein. (9) Yamado, M.; Saruyama, H.; Aida, K. Spectrochim. Acta 1972, 28A, 439 and references therein.
0022-3654/87/2091-2046$01.50/0
Solution charge-transfer complexes, in general, also show a characteristic charge-transfer absorption band in the visible or u l t r a ~ i o l e t . ~ These ~ ’ ~ ~ may ~ ~ well be relatively intense for the systems under investigation. Further, strong W absorptions would suggest the possibility of photochemical rearrangements within the matrix cage, leading to additional intermediate species. Processes such as these have been postulated in organic syntheses involving sulfide/halogen reactant^.'^-'^ Consequently, with the opportunity to explore further the bounds of hard/soft acid/base theory, as well as the possibility of observing charge-transfer bands and subsequent photochemistry, an investigation was undertaken into the interactions of the halogens ClF and Cl, with sulfurcontaining bases. Experimental Section
The experiments reported in this paper were performed on conventional matrix isolation equipmentSdescribed p r e v i o ~ s l y . ~ ~ Methanethiol (Eastman Kodak) and methyl sulfide (Eastman Kodak) were introduced into the vacuum system as the vapor above the liquid, after degassing through repeated freeze-thaw cycles. ClF (Pennwalt), C12 (Matheson), and H2S (Fisher) were obtained in lecture bottles and introduced as gases into the vacuum line. They were condensed at 77 K and purified by freeze-thaw cycles. D2S was prepared on a glass vacuum line by reaction of C a s (Fisher) and D2S04 (Merck, 99% D),dried over a CaCl, (MCB Reagents, anhydrous), and further purified by repeated freeze-thaw cycles and trapping at 196 K, after which it was (10) Ho, T.-L. Hard and Soft Acids and Bases Principle in Organic Chemistry; Academic: New York, 1977. (11) Pearson, R. G.; Songstad, J. J . Am. Chem. SOC.1967, 89, 1827. (12) Agarwal, U. P.; Barnes, A. J.; Orville-Thomas, W. J. Can J . Chem. 1985, 63, 1705. (13) (a) Tamres, M.; Strong, R.L. Mol. Assoc. 1979, 2, 331. (b) Cataliotti, R. S.; Dellepiane, G.; Paliani, G.; Santini, S.; Tubino, R.J . Mol. Strurr. 1984, 114, 157. (14) Voigt, E. M.; Meyer, B. J . Chem. Phys. 1968, 49, 852. (15) Ho, T.-L.; Wong, C. M. Synrh. Commun. 1975, 5, 423. (16) Ahern, T. P.; Kay, D. G.; Langler, R. F. Can. J . Chem. 1978, 56, 2422. (17) Chow, Y.L.; Bakker, B. H. Can J. Chem. 1982, 60, 2268. (18) Bordwell, F. G.; Rtt, B. M. J. Am. Chem. SOC.1955, 77, 572. (19) Ault, B. S . J. Am. Chem. SOC.1978, 100, 2426.
0 1987 American Chemical Society
Complexes of C1F and C12 with (CH3)2S
I
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2047
f l
A r l C I F a 25011
\
ArlClF. 67011
I
I
I
750
703
650
I
I
6W 550 ENERGY (CM-')
I
I
503
450
I
Figure 1. Infrared spectra of matrices formed through the codeposition of CIF with sulfur-containingbases, in the low energy region, compared to a blank experiment of C1F in argon (leftmost trace).
introduced as a gas. Argon (Union Carbide) and nitrogen (PuA I - / ( C H ~ ) ~ S500 = ritan) were used as matrix materials with no additional purifi/ ! ! ! ! ! ! I ! ! ! / cation. 560 520 480 440 400 360 320 During the infrared experiments, matrices were deposited at ENERGY (Cfi') roughly 2 mmol/h from each vacuum line onto a 14 K cold Figure 2. Infrared spectra of the reaction product of the codeposition of window, for typically 22 h, before final spectra were recorded on C1, with sulfur-containing bases into argon matrices, in the spectral a Beckman IR 12 infrared spectrophotometer. Resolution was region between 300 and 600 cm-I, compared to a blank experiment of on the order of 1 cm-I at 1000 cm-' and slightly poorer in other C12 in argon, at M/R = 500. regions. During the UV-vis experiments, matrices were deposited at roughly 1 mmol/h from each vacuum line, for 1-4 h before may well be due to the HF complex with the new product formed final spectra were recorded on an IBM 9420 UV-vis spectrobetween ClF and H2S. photometer. A bandwidth of 2 nm was used in these experiments, Spectra recorded after irradiation of the matrix with a mercury and a spectral range from 185 to 900 nm was available. Twin arc lamp showed a considerable decrease in the band at 631 cm-I, jet deposition was employed throughout these studies due to the and also the 600-cm-' band when higher concentrations were used. high reactivity of the C12 and ClF. Irradiation was carried out The bands previously recorded at 482 and 3655 cm-l were unwith a mercury arc lamp (Morgan Instruments, HBO 200 w/4) changed, but the band at 3567 c d showed comparable decrease. for periods of 10 min to 1 h and a source-to-matrix distance of Several new absorptions were noted after Hg arc irradiation; these about 6 in. Prior to any twin jet deposition experiments, blank are listed in Table 11. When the matrix was irradiated through spectra of each individual reactant were recorded; these were found glass filters, passing only light with the wavelength greater than to be in agreement with spectra found in the l i t e r a t ~ r e . 4 ~ In ~ - ~ * ~ 300 nm, no photolytic activity was observed. the experiments for which infrared spectra were recorded, reactant When deuterium sulfide was the codeposition partner of ClF, concentrations ranged from M / R = 2000/ 1 to 200/ 1, to aid in the product bands appearing before photolysis were nearly identhe identification of aggregate species. Visible-UV experiments tical. The absorbance maximum of the major product band was were conducted at lower concentrations, around 10 OOO/ 1, due recorded at 631 cm-' with the 601-cm-I band appearing at high to the high intensity of the product absorptions. concentrations (see Figure 1). After irradiation with a mercury arc lamp, the product bands listed in Table I1 were noted. Results ClF CH,SH. When ClF was codeposited with CH3SH in Infrared Spectra. CIF + HP, D P . When ClF was deposited an argon matrix, a very strong product band appeared at 536 cm-' with H2Sin an argon matrix, an intense new product band domalong with a weaker overlapping band at 524 cm-I (see Figure inated the spectra at 631 cm-' (see Figure 1). At higher con1). In addition, product bands were recorded a t 966 and 1430 centrations of the reactants, a nearby band at 600 cm-' appeared cm-' near parent absorptions of CH3SH. At higher concentrations, also, although the intensity of this absorption was unrelated to medium to weak absorptions were observed at 429, 472,467,489, the major product band at 631 cm-I. Other product bands were 561,584, and 612 cm-I. No change in the spectra was recorded noted at 482,3655, and 3567 cm-', and are probably due to HF after irradiation of the matrix with the full light of the H g arc imp~rity.~'The 482- and 3655-cm-' bands have previously been lamp. reported as vibrational absorptions of the HF complex with H2S?' CIF (CHJ2S. When methyl sulfide and ClF were codepowhile the 3567-cm-' band is clearly a perturbed HF stretch and sited in an argon matrix, a very intense product absorption was recorded at 471 cm-I (see Figure 1). Other weaker product bands (20) Andrews, L.; Chi, F. K.; Arkell, A. J. Am. Chem. Soc. 1974,96,1997. appeared at 623, 1331, and 1035 and 1040 cm-I. At higher (21) Barnes, A. J.; Howells, J. D. R.J . Chem. SOC.,Faraday Trans. 2 concentrations additional absorptions were recorded at 538,99 1, 1972, 68, 729. and 1050 cm-l. Similar results were obtained in a nitrogen matrix. (22) Pacansky, J.; Calder, V. J . Chem. Phys. 1970, 53, 419. Irradiation of the matrix by Hg arc produced no discemible change (23) Barnes. A. J.: Hallam. H. E.: Howells, J. D. R. J . Chem. SOC.,Faraday Trans. 2 1972,68, 737. in the spectra. (24) Ypenburg, J. W. Red. Trau. Pays-Bas 1972, 91, 671. C12 H2S. As shown in Figure 2, the codeposition of C12 with (25) Ohsaku, M.; Shiro, Y.; Murata, H. Bull. Chem. SOC.Jpn. 1972.45, H2S into argon matrices led to the formation of a single product 113. absorption at 516 cm-I, with a shoulder on the low energy side (26) Geiseler, G.; Hanschmann, G.J. Mol. Srruct. 1972, 1I , 283. (27) Arlinghaus, R. T.; Andrews, L. Inorg. Chem. 1985, 24, 1523. near 510 cm-'. Hg arc irradiation produced little change in the (28) Prochaska, E. S.; Ault, B. S.; Andrews. K. Inorg. Chem. 1977, 16, infrared spectrum. 2021. CZ2 CH,SH. The codeposition of C12 and CH3SHled to the (29) (a) Barrett, J.; Hitch, M. J. Spectrochim. Acta 1968, 2 4 4 265. (b) formation of a single intense product absorption doublet a t 469 Seery, D. J.; Britton, D.J . Phys. Chem. 1964, 68, 2263. (30) HF arises in the CIF vacuum line as an impurity in the CIF cylinder, and 472 cm-' (see Figure 2). When irradiated by a mercury arc as well as from reaction with residual H20in the line. Once formed, it adsorbs source, this product band showed a marked decrease in intensity. strongly to the stainless steel line and is very difficult to remove completely. This was accompanied by growth of the series of product bands Its concentration is very low, but it is a strong absorber in its hydrogen-bonded listed in Table 11. complexes.
+
+
+ +
2048
Machara and Ault
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
TABLE I: Correlation of the Halogen Stretching Frequency" with Base Strength in 1:l Molecular ComDlexes
base
YCIF
H2S CH3SH (CHg)zS CH30H (CH3)20
770 631 536 471 695d 682d
AyCIF/ YCIF
0.18 0.31 0.39 0.10 0.11
Avc1,l uCll
ucll
PAb
IPc
fie
555' 516 472 360
0.08 0.15 0.35
170.2 187.4 200.6 182 192
10.4 9.4 8.7 10.8 10.0
0.97 1.52 1.50 1.70 1.30
"Band positions in cm-I. bProton affinity of the base, in kcal/mol, from ref 35. 'Ionization potential of the base, in eV, from ref 41. dFrom ref 4. eDipole moment of the base. /From ref 53. TABLE II: Primary Infrared Absorptions after Hg Arc Irradiation of Matrices Containing Halogen-SutTide Molecular Complexes
*m-m& E NE RGY (c m-'
Figure 3. Product bands, in selected spectral regions, arising from the Hg arc irradiation of a matrix formed through the codeposition of C12 and (CH3),S, compared to the same matrix before irraidation (the band at 698 cm-' in the spectra of (CH,),S is due to the antisymmetric C-S-C
stretching mode).
+
C12 (CH3)2S. When C12 and (CH3)2Sm e d e p o s i t e d into an argon matrix, a very intense product band appeared at 360 cm-' (see Figure 2), along with two other sharper product bands, at 1035 and 1331 cm-I. When the matrix was irradiated with a mercury arc lamp (see Figure 3, middle trace), these bands showed a dramatic decrease, and the product bands listed in Table I1 appeared in the spectra. VisiblelWSpectra. H2S. No distinct product band was noted upon the codeposition of H2Swith either C12or ClF at either low or high concentrations, although the scattering background of the cold window and matrix was very steep below 230 nm. Irradiation of the matrix produced only slight spectral changes below 230 nm, which may be due to either photolytic activity or slight changes in matrix quality. CH3SH. In low concentration experiments with CH3SH and C12, up to total matrix/reactant ratios of about 500011, a reproducible product band was noted on the scattering background a t 235 nm. This product feature disappeared on irradiation by mercury arc. A high concentration experiment with a total matrix/reactant ratio of 1000/1 produced a very intense, more distinct absorption at 235 nm which changed very little on irradiation. In similar experiments with ClF and CH3SH, a product feature was observed on the scattering background with a maximum at 230 nm; this feature disappeared on irradiation by the mercury arc. (CH3)$. In codeposition experiments with ClF, a very intense product band gradually grew in over the deposition period centered at 295 nm. This band decreased on irradiation by the mercury arc. At higher concentrations, additional UV absorptions were noted at 230 and 267 nm. In experiments with C12and (CH3)2S, the lowest concentration experiments (lOOOO/l/l) yielded a single, intense product band centered at 300 nm, which was destroyed by irradiation (see Figure 4). At somewhat higher concentrations, weak, broad bands were noted on the scattering background at approximately 225 and 260 nm; these absorptions were destroyed by irradiation. Discussion The codeposition of either ClF or C12 with H2S, CH,SH, or (CH3)*Sgave rise to one or more absorptions before irradiation which cannot be attributed to either reactant species. Every system was characterized by a very intense product absorption below 650 cm-I, although some aggregates were noted at higher concentrations, and some site splitting occurred. In the ClF experiments, this dominant absorption was noted at 631, 536, and 471 cm-' with HIS, CH3SH, and (CH3)2S,respectively, while the corresponding band positions were 5 16, 472, and 360 cm-I when C1,
complex H2S.CIF
absorption 426, 517, 525 (sh), 2780
D,S*CIF
428, 517, 525, 2011, 2030 (sh), 2723,' 2780' CHySH.CI2 518, 2624," 2768' (CH3)2S*C12 646, 702, 758, 983, 1229, 2498,2550," 2666,' 2761"
'Center band of multiplet. text.
assignmentb HSF* and/or HSCI*, HF*, HCI* DSF* and/or DSCI*, DF*, DCI* CHjSCI, HCI CH3SCH2C1, HCI
* indicates tentative
assignment; see
was employed. It is also noteworthy that no significant shift was noted upon deuteriation of the hydrogen sulfide. The observation of a single product absorption in most of these systems suggests complex formation, rather than a more complete rearrangement or elimination reaction. In the latter reactions, a number of new absorptions would be anticipated (this was observed in the photochemical reaction of C12with (CH3),S; see below). Molecular acid-base complexes are generally characterized by substantial perturbation to the acid subunit and relatively minor perturbation to the base s u b ~ n i t . ~ ,In~ ~those - ~ ~systems where additional product absorptions were noted, each fell near an absorption of the base subunit. Moreover, the one intense product band that was observed fell to the red of the parent halogen vibration, and shifted systematically to lower energy as the base strength of the sulfide increased. Finally, it is well-known that molecular halogens do form complexes r e a d i l ~ , even ~ . ~ the reactive ClF species has been shown to form charge-transfer complexes with weak Lewis bases.4 All of these observations point toward the formation of a molecular complex upon initial deposition, rather than a rearrangement product. The fact that this dominant absorption persisted at total dilutions of up to 4000/1/1 suggests that the complex involves a single acid and base subunit, Le., a 1:l complex. At higher concentrations, additional weak features were observed, such as the 601-cm-I absorption in the C1F/H2S system. The concentration dependence of these bands were clearly different (being favored at high reactant concentrations) and mark them as due to higher aggregates or larger molecular complexes (the 601-cm-I absorption is most likely due to the complex of H2S with the ClF dimer, although the stoichiometry of larger aggregates is always difficult to pin down precisely). Complexes of the molecular halogens with a wide range of bases have been studied by a number of techniques and have been investigated theoretically as we11.2-437-9J2,33Since the halogens serve as Lewis acids by accepting electron density into an antibonding orbital (o*),a weakening of the halogen bond is anticipated, with concomitant reduction in stretching force constant and vibrational frequency. A red shift of 50-100 cm-l was observed in the complexes of ClF with oxygen bases; similar or larger (31) Ault, B. S. J. Am. Chem. SOC.1983,105, 5742. (32) Sass, C. S.;Auk, B. S.J . Phys. Chem. 1986, 90, 1547. (33) Saenger, K. L.; McClelland, G. M.; Herschbach, D. R. J . Phys. Chem. 1981,85, 3333. (34) La Grange, J.; Leroy, G.; Louterman-Leloup, G. Adu. Mol. Relaxation Processes 1976, 9, 15.
Complexes of ClF and C12 with (CH3)2S
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2049
9EFOM IIRADIATION
atom is viewed as soft as a consequence of its location in the periodic table (and its resultant ease of polarization) while oxygen is relatively hard. Coordination of ClF or C12to such a base m u r s through the chlorine atom, which can reasonably be viewed as a soft acceptor site. Hard/soft acid/base theory predicts substantially greater interaction between like units, i.e., softsoft or hard-hard. This leads to the prediction that C1F and C12 will interact more strongly with sulfur bases than with oxygen bases, as was observed. Similar results have been noted for the interaction of SO3 with first and second row bases3' as well as for the hard acid HF with a similar set of While HSAB theory is (at this level) only qualitative, the spectral observations are consistent with this approach. The magnitude of the shift Av or relative shift A v / v cannot be predicted even semiquantitatively without a more detailed knowledge of the orbital energies involved. The shifts for the C1F complexes are generally larger than those for the C12 complexes, which can be understood in terms of the presence of an electrostatic or dipolar interaction in addition to the charge-transfer interaction. The reason why relatively small shifts were observed for the complexes of C12 with H2S and CH3SH,while a much larger one was noted for C12.(CH3)2Sis less apparent. However, the HOMO-LUMO orbital match (or mismatch) plays a key role in determining the exact strength of interaction, and a knowledge of these orbitals energies would be needed to rationalize this point further. Assignment of the remaining product bands to perturbed modes of the base subunit is straightforward, since these bands are only slightly shifted from the parent vibration. In each case, the perturbed mode was either a methyl deformation mode or a methyl rocking mode. It is surprising that the S-C stretching modes were not detectably perturbed, since the counterpart C - 0 modes were weakly observed for the oxygen base complexes. However, careful scans were made in the appropriate regions, and no product bands were observed. The lack of shift of the C-S stretching modes may, in part, be rationalized by considering the geometry of the complex. The internuclear axis of the halogen is likely to be at a sharp angle (approaching 90°) to the C2axis of dimethyl sulfide. This is the geometry deduced40 for the complex of SO2 with NH3. In this case, there will be substantial steric interaction of the halogen with the methyl groups of the base and a blue shift may be expected. At the same time, the C-S bonds are nearly perpendicular to the halogen-sulfur bond and will not be significantly affected. Consequently, observation of perturbed methyl vibrations and not the C-S stretching modes is not entirely unreasonable.
,L AFTER
IRlUDIATlON
I
200
I 210
I
1
I
so0
350
WAVELENGTH
*oo
1
450
tnm)
Figure 4. Visible-ultraviolet spectra of a sample of Clz (M/R = 40000) and ( C H & 3 (M/R = 10000) in an argon matrix, before and after Hg arc irradiation. The blank spectra of each reagent alone in argon showed no absorptions in this spectral region at these concentrations.
shifts might be expected for the present complexes. In addition, complexation often results in the intensification of the halogen stretching of course, for the C12complexes the parent mode is infrared inactive, while it is activated in the complex. Since all of the spectra in the present study were dominated by an intense absorption to the red of the parent halogen mode, the assignment of this mode to the perturbed halogen stretch is straightforward (see Table I). The magnitude of the red shift of the halogen stretch is much larger than for the oxygen complexes, up to 300 cm-I, and the bands were consistently very intense (several were fully absorbing under our experimental conditions, including the C12 complex with (CH3),S). The present results are in disagreement with the previous reportI2 by Agarwal, Barnes, and Orville-Thomas of the infrared spectra of the complexes of C12 with H2S and (CH3)2S. Their band position for the former complex, 518 cm-', is in good agreement with the present results, but their reported band position for the (CH3)2Scomplex, 525 cm-I, is in substantial disagreement. Careful scans were carried out in the full region 300-550 cm-' in the current work; no absorptions were noted near 525 cm-'. Further, the absorption reported here at 360 cm-' was very distinct and intense (fully absorbing in some experiments), so that it is not clear how this band could have been overlooked by the previous workers. Moreover, it is unlikely that the (CH3)2SC12complex would have a smaller shift than that of H2S.C12. In any event, the prominent absorption at 360 cm-' is assigned to the C1-C1 stretching mode of C12 in a 1:l molecular complex with (CH3)2S. The magnitude of the red shift of the acid subunit upon complexation is usually ~ o r r e l a t e d ~ * ~with * ~ 'the - ' ~intrinsic basicity of the base subunit; typically, the gas-phase proton affinity of the base is taken to measure this quantity. Proton affinity alone, however, cannot rationalize the very large red shifts observed here compared to those observed for the oxygen bases. The proton affinities of the sulfur and oxygen bases differ35by only a few kcalories per mole (see Table I), yet the shifts were up to 4 times as large with the sulfur bases (with a much greater degree of intensification as well). These results can, however, be rationalized well in terms of hard/soft acid/base t h e ~ r y . ' ~ ~The " ~ sulfur ~~ ~
(35) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Re$ Data 1984, 13, 695.
UV Spectra The codeposition of these reactants into argon matrices, followed by the recording of vis-UV spectra indicated product formation as well. The primary new band observed in each experiment in the UV was quite intense, persisting at dilutions up to 40 OOO/ 1/ 1. (At high sample concentrations, secondary UV absorptions were noted further to the blue, and these can be ascribed to aggregate species.) This is characteristic of a charge-transfer absorption,l which would be expected for the molecular complexes formed here. For (CH3)2S,the C T abospriton was observed at 300 nm with C12 and 295 nm with ClF, while the bands shifted to 235 and 230 nm for the CH3SH complexes. No CT bands were observed for the H2Scomplexes, which is in accord with the higher ionization potential4I for H2S. The position of the C T band is often related to the ionization energy of the donor, the electron affinity of the acceptor, and the charge separation. The latter two factors should be similar for the C12 and ClF complexes, suggesting only slight shifts going from C12 to C1F for a given base; this is precisely the ~
Douglas, J.; Kollman, P. J . Phys. Chem. 1981, 85, 2717. Sass, C. S.; Ault, B. S.J . Phys. Chem. 1987, 91, 551. Andrews, L. J . Phys. Chem. 1984.88, 2940. (a) Arlinghaus, R.T.;Andrews, L. J . Chem. Phys. 1984,81,4341. (b) Andrews, L. J . Mol. Struct. 1983, 100, 281. (40) (a) Lucchese, R. R.; Haber, K.; Schaefer, H. F. J . Am. Chem. SOC. 1976, 98, 7617. (b) Sass, C. S.; Auk, B. S. J . Phys. Chem. 1984, 88, 432. (41) Handbook of Chemistry and Physics, 62nd ed.; Weast, R. C., Ed.; Chemical Rubber Co.: Boca Raton, FL. (36) (37) (38) (39)
2050 The Journal of Physical Chemistry, Vol. 91, No. 8, 1987
observed result. The band position shifts further to the blue with increasing ionization energy of the base,as expected. The observed band positions for the (CH3)# and CH3SH complexes lead to a prediction that the HIS complex CT band positions will be on the steep scattering background, or beyond the instrumental cutoff. Consequently, it is not surprising that the C T bands of the HIS complexes were not observed.
Photochemistry Several of the product absorptions assigned to molecular complexes observed here were sensitive to H g arc irradiation; the C12-(CH3)2Scomplex was completely destroyed by irradiation. In this case, a number of product bands were observed after irradiation; they are readily assigned by comparison to the literature41 to chloromethyl methyl sulfide CH3-S-CH2Cl. For example, the 702-cm-l band is assigned to the C-S stretch of the species, while the 758-cm-I band is attributed to the C-Cl stretch (with the structure on the low energy side due to 37Clin natural abundance). This photoreaction is well-known in the organic literatureI5'* and is thought to involve complex formation, followed by HC1 elimination, and chlorine atom migration. Further support for this product species comes from the series of absorptions in the H-Cl stretching region. HCl is expected as the second product of this photochemical rearrangement. However, most of the HCl is likely to be trapped in the matrix cage with the CH3SCH2Cl product, as a consequence of matrix rigidity. Hydrogen bond formation is then possible, to either the sulfur atom or the chlorine atom of the chloromethyl group. The latter, by analogy to previously studied HCl complexes should absorb43between 2750 and 2800 cm-', while the former44 should absorb near 2500 cm-I. Groups of absorptions were noted in each of these regions; that a single absorption was not observed in each region is probably due to a variety of geometries of the hydrogen-bonded complex, since the matrix was already in place before irradiation, and relaxation to the preferred geometry cannot readily occur. Finally, the absorption near 2666 cm-I is attributed to the complex of HC14S with impurity HzO, which is always present in minor amounts in these experiments. The complex of C1, with CH3SH was also sensitive to irradiation, with the most intense absorptions of the photoproduct falling near 2700 cm-'. This indicates formation of HCl as a photoproduct, with the second product l i k e l F 8 to be methanesulfenyl chloride, CH3SCl. Most of the product absorptions of this species are nearly coincident with parent CH3SH; only the S-Cl stretch should be distinct and definitive. The product absorption at 5 18 cm-I matches well the literature value49for this absorption and is so assigned. Some photoactivity of the ClF complex with H2S was noted, leading to some decrease in intensity of the absorption of the complex and growth of weak new product absorptions. The (42) Pouchert, C. J. The Aldrich Library of Infrared Spectra, AIdrich Chemical Co.: Milwaukee, WI, 1970; Part A, p 125. (43) Andrews, L., private communication. (44) Barnes, A. J. J . Mol. Struct. 1983, 100, 259. (45) Ault, B. S.; Pimentel, G. C. J . Phys. Chem. 1973, 77, 57. (46) Capozzi, G.; Modena, G. The Chemistry of the Thiol Group; Patai, S., Ed.; Wiley: New York, 1974; Part 2, p 785. (47) Shum, L. G. S.f Benson, S. W. Int. J . Chem. Kinet. 1983, 15, 433. (48) Nesbitt, D. J.; Leone, S . R . J . Chem. Phys. 1981, 75, 4949. (49) Winther, F.; Guarnieri, A. Spectrochim. Acta 1975, 31A, 689.
Machara and Ault weakness of the product absorptions makes definitive assignment of the photoproduct quite difficult. The most prominent product absorption was at 2780 cm-I, and shifted to 2011 cm-I upon deuteriation. This could be assigned to an H-Cl stretch, or possibly to an S-H stretch. The former seems much more likely; typical S-H stretches occur between 2550 and 2600 cm-I, and shift below 2000 cm-' upon deuteriation. The remaining atoms would constitute thiohypofluorous acid, HSF, analogous to hypofluorous acid, studied by Noble and Pimentel in argon matrices.50 Whether HSF is a stable species is unknown; the S-F stretch should be quite intense while the S-H stretch and the HSF angle bending vibrations should be quite weak. The remaining photoabsorptions were noted at 517 and 426 cm-I; these are quite low for an S-F stretch,51 although the molecule may be only weakly bound, leading to an unexpectedly low force constant and vibrational frequency. Of course, reactions to form H S F + HCl and HSCl H F might both occur; some evidence of a perturbed D F stretch was noted between 2700 and 2800 cm-' in the ClF and D2S experiments. (The H F region was always somewhat cluttered due to impurity HF30 and its complexes with HzO and HIS.) One of the low-frequency bands, then, could be assigned to the S-CI stretch of the HSCl product. Since the evidence is not clear, and the product intensities were low, the above discussion must be regarded as tentative. The remaining 1:1 complexes studied here were not sensitive (in the infrared experiments) to Hg arc irradiation. It is somewhat puzzling, however, that some of these complexes (notably the ClF.(CH3)2Scomplex) were photolytically destroyed in the UV experiments, but not in the infrared experiments. The best explanation for this unusual phenomenon is that deposition conditions needed for the infrared experiments were such that quite thick, highly scattering matrices were produced. These, then, scatter the ultraviolet radiation from the Hg arc lamp effectively so that very little light actually penetrates the matrix. On the other hand, the vis-UV spectroscopic experiments were conducted at high dilution, with short deposition times due to the very high absorption coefficients of the charge-transfer bands. These matrices, then, did not scatter the W radiation significantly, and photochemistry did occur. The photochemical results involving the complex of HIS with ClF are somewhat puzzling as well. Some photolytic activity was noted in the infrared experiments, although no charge-transfer band was observed in the UV experiments. This band should lie below 230 nm and may have been obscured by the steep scattering background. Yet, the Hg arc lamp has some output below 230 nm, and absorption and photochemistry could occur, even though the C T band was not distinct from the background. Alternatively, absorption could also occur into the weak ClF absorption near 290 nm, which is known,52but is too broad and diffuse to be seen at the low concentrations used here.
+
Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation through Grant C H E 84-00450. (50) Noble, P. N.; Pimentel, G. C. Spectrochim. Acta 1968, H A , 797. (51) Nakamoto, K. Infrared and Roman Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley-Interscience: New York, 1978. (52) Ault, B. S.;Andrews, L. J. Chem. Phys. 1976, 65, 4192. (53) Ault, B. S.; Howard, W. R., Jr., Andrews, L. J. Mol. Spectrosc. 1975, 55, 217.
