Journal of Molecular Structure, 248 (1991) 79-88 Elsevier Science Publishers B.V., Amsterdam

79

FT-IR spectra of matrix isolated complexes between some alkanethiols and sulfur dioxide

San Li and Ying-Sing Li’

Department of Chemistry, Memphis State University, Memphis, TN 38152 (USA)

(Received 3 January 1991)

Abstract

Complexes formed between some alkanethiols and sulfur dioxide have been prepared by con- densing the reagents diluted in argon and in nitrogen at 12-14 K. The 1: 1 RSH* * *SO2 complexes were identified from the infrared spectra of the mixtures of ethanethiol, iso-propanethiol, and tert-butanethiol, respectively, with sulfur dioxide in argon and in nitrogen matrices. Vibrational assignments and the possible interaction sites were suggested.

INTRODUCTION

The matrix-isolation technique coupled with Fourier transform-infrared spectroscopy has been applied very effectively and proven valuable in the char- acterization of weakly bound complexes for a large range of molecules. Infor- mation about structure and bonding for complexes involving alkyl sulfides and alkanethiols with HCl and HF in both Ar and N, matrices has been obtained by IR spectroscopic methods in several laboratories [l-3]. The coexistence of (R,S);.*(HCl), withm:nequalto1:1,2:1and1:2, (R,S),,***(HF),,and (RSH),,...(HF),s with m’ : n’ equal to 1: 1 and 1: 2, where R is CH, or C2H5, have been identified by the concentration dependence and annealing behavior. On the other hand, several different SO, complexes have been reported [4-

12]. The SO, molecule can serve either as a base or an acid when forming a complex with an acid or with a base. In our recent studies, complexes of R,S with SO, and SO, dimer were identified in argon and nitrogen matrices [ 131. Evidence of the complex formation through the disulfide interaction was found. In the complex formation, the sulfides served as bases and SO, served as acid. Alkanethiols can form complexes either through the S atom when complexed with an acid such as hydrogen halides, or through the sulfhydryl hydrogen atom when complexed with a base. As a continuation of our studies of the

‘Author to whom correspondence should be addressed.

0022-2860/91/$03.50 0 1991- Elsevier Science Publishers B.V.

80

complexes between sulfur compounds, this paper reports an infrared matrix isolation study of the interaction between some alkanethiols and sulfur dioxide.

EXPERIMENTAL

The cryogenic, spectroscopic and vacuum techniques used in this study have been previously described in detail [ 131. In a typical experiment, the reactant gases were mixed, diluted with argon in a desired ratio in the vacuum manifold and deposited onto the cold window until a proper amount was achieved. The spectra were recorded on a Mattson Polaris FTIR spectrophotometer equipped with ICON FTIR Analytical Software in the 400-4000 cm-’ spectral region using a l-cm-’ resolution and 400 scans. The sample compartment was con- tinuously purged with compressed air which had been pretreated by a Type 75- 50 Balston Air Dryer (Balston) in order to eliminate H,O and CO, vapors. Usually the samples were annealed at 30 K for 10 min and cooled to about 12 K for another 10 min before an additional spectrum was recorded.

Ar and Nz gas (both research grade, Air Products) were used without puri- fication. The sulfur dioxide, SOZ (Matheson), ethanethiol, C&H,SH, 2-pro- panethiol, 2-C3H7SH, and 2-methyl-2-propanethiol, (CH,),CSH (all Ald- rich) were subjected to several freeze-thaw cycles at liquid nitrogen temperature, dried by passing the sample several times through a U-tube filled with activated molecular sieves (4A) in a high vacuum system, and then kept in sample tubes with activated molecular sieves in order to eliminate traces of water. A final freeze-thaw cycle at liquid Nz temperature was applied to each sample prior to mixing with Ar.

RESULTS

Prior to the thiol-sulfur dioxide experiments, blank experiments were car- ried out on each of the reactants in solid argon or nitrogen. All the matrix isolated IR spectra obtained were in good agreement with the available litera- ture spectra [ 14-181.

SO, + C,H,SH

Several experiments were conducted with sulfur dioxide and ethanethiol at different ratios. These two reactants were mixed, diluted in Ar, and then con- densed on the cold CsI window through a single nozzle. First, a spectrum was recorded of a matrix sample with SO#&H$H/Ar = l/1/1000. In addition to the parent species absorptions, several new infrared bands were observed near the fundamental vibrations of the SO, subunit and the S-H stretching, CH, rocking, and CSH bending modes of the C2HBSH. They were situated at 1340, 1337 and 1323 cm-‘, 1145 and 1142 cm-’ in the SO, antisymmetric and sym-

81

metric vibration regions, 522 cm-l in the SOa bending region, as well as a broad band at 2581 cm-’ between the ethanethiol monomer and dimer bands of SH stretching, plus weak bands at 863,725 and 653 cm-’ on the lower energy side of the CSH bending, CH, rocking, and CS stretching modes of ethanethiol, respectively. The spectrum collected after annealing the Ar matrix at 30 K revealed that the relative intensity of the metastable SO, monomer band at 1351 cm-’ decreased significantly while the SO, dimer band around 1342 cm-‘, as well as the new bands due to the formation of the complex, increased. When an excess amount of C&H,SH was used in the experiment with SO,/C,H,SH/ Ar= l/5/1000, the same set of new bands was observed but the relative inten- sities of the 1340,1337,1323,1145,1142 and 522 cm-’ bands grew more rap- idly. Figure 1 shows representative spectra in the SO, fundamental vibration regions. In addition, several experiments were performed by keeping the con- centration of ethanethiol constant while varying the SO, amounts in the mix- tures. It was found that the intensities of the bands at 2581,863,725 and 653 cm-’ increased as the concentration of SO, increased in the mixtures. All these

-

/ I I I 1 I ! I Al 1 1

l360 l340 l.?m 1300 1160 1140 540 520

Fig. 1. FT-IR spectra of (a) Ar/C,H,SH=500/1; (b) Ar/S02=1000/1; (c) Ar/SO,/ C,H,SH = 1000/l/5.

82

new bands must include the perturbed vibrations of the SOz as well as the perturbed SH stretching, CSH bending and C-S stretching modes of the etha- nethiol. Figure 2 shows representative spectra including the CSH bending, CH, rocking and CS stretching regions of ethanethiol in the mixtures with different SO, concentrations.

Similar experiments were conducted for the reactant mixtures in nitrogen matrices in order to compare the behaviors of the complexes in different en- vironments. All the new bands were observed in different matrix conditions except for the relative intensities and the splitting due to the matrix site effect. The new peak positions in nitrogen matrices differed only slightly from those in argon matrices.

Water is one of the most likely contaminating species in the present matrix

880 840 800 760 720 680 660

Wavenumbers (cm-‘)

Fig. 2. FT-IR spectra of argon matrix containing C&H,SH and S02/C2H,SH at different ratios.

83

isolation experiment, especially when a slow deposition rate is applied. It is necessary to check if there are any water-dependent bands observed in the experimental spectra. For this reason, we did several experiments with differ- ent amounts of water added to the sulfur dioxide/ethanethiol. A 1: 1 complex between water and sulfur dioxide was observed to show the bands at 1343,115O and 522 cm-l, of which the relative intensity increased when more water was used. This result was in good agreement with that reported by Schriver et al. [ 41. The intensities of the new bands assigned for the sulfur dioxide/ethane- thiol complexes did not show any dependence on water concentration except the band at 522 cm-’ which could overlap with the SO,/H,O complex band. Thus, the new bands observed in the matrix isolated SO,/C,H,SH must be due to the formation of the SOz* - .C,H,SH complex.

SO, + 2-C,H,SH

These two reactants were mixed in three different ratios and diluted in Ar gas. In the FT-IR spectra of the matrix-isolated mixture S0,/2-C!,H,SH = l/ 3, some new bands with medium intensities appeared near the SO, fundamen- tals and some weak bands appeared near the SH stretching, CSH bending and CS stretching modes of the thiol. They were situated as a multiplet at 1342, 1338 and 1325 cm-l, a doublet at 1144 and 1140 cm-‘, and a resolved band at 521 cm-’ as perturbed vibrations to the SO, subunit, as well as a broad band centered at 2578 cm-‘, and some weak bands at 856, 846 and 629 cm-’ as perturbed vibrations to the thiol subunit. After annealing of the matrix, the relative intensities of the bands at 1338, 1140 and 521 cm-l significantly in- creased while the intensities of bands near the thiol vibrational modes did not show noticeable change. Matrix-isolated IR spectra for the SOJ2-C3H$H mixture at the l/l and 3/l ratios were also investigated. The same set of new product bands was observed, but the bands at 2578, 856, 846 and 629 cm-’ grew more rapidly than the others at higher SO, concentrations.

SO, + (CHJ,CSH

Several new product bands were observed after these two reactants were mixed in inert gas and deposited on the cold CsI window. A comparison of the spectrum of the SO,/ (CH,)&SH mixture with those of SO, and of (CH,),CSH has identified new intense bands at 1342 and 1336 cm-l, overlapping with the less intense bands due to the SO, dimer and possibly isotopic SO, monomer, a shoulder at 1142 cm-’ and a new band at 522 cm-l near the SO, vibration modes. Other new’ bands were also observed at 2577,856,815 and 582 cm-l near the corresponding 2-methyl-2-propanethiol fundamentals. After anneal- ing, the intensities of the new bands increased. For the mixtures with relatively higher SO, ratios, all the new bands were observed to have their intensities

TA

BL

E 1

Per

turb

atio

ns

and

assi

gnm

ents

of

new

vib

rati

onal

ban

ds a

ppea

rin

g in

th

e su

lfu

r co

mpl

exes

in

sol

id a

rgon

at

12 K

SO

, + C

,H,S

H

Un

pert

urb

ed

mon

omer

1355

(SO

*)

Per

turb

atio

ns

1340

13

37

1323

SO

P + 2

C,H

,SH

Un

pert

urb

ed

mon

omer

1355

(SO

,)

Per

turb

atio

ns

1342

13

38

1325

SO

,+

tert

-C,H

,SH

A

ssig

nm

ents

Un

pert

urb

ed

Per

turb

atio

ns

mon

omer

1355

(SO

Q)

1342

S

Oz

asym

. str

. 13

36

1152

(SO

,)

519(

SO

*)

1145

11

42

522

1152

(SO

x)

519(

SO

,)

1144

11

40

521

1152

(S02

) 11

42

SO

, sy

m.

str.

519(

SO

x)

522

SO

x be

nd

2597

(CxH

,SH

) 25

81

2597

(2

-C,H

,SH

) 25

78

2597

(t

ert-

C,H

aSH

) 25

77

SH

str

.

870

(th

iol)

86

3 85

9 (t

hio

l)

856

851

(th

iol)

84

6 86

2 (t

hio

l)

856

CS

H b

end

818(

thio

l)

815

CC

str

.

732

(thi

ol )

73

0(th

iol)

658(

thio

l)

725

653

CH

2 de

f.

635

(th

iol

) 62

9 58

8(th

iol)

58

2 C

S s

tr.

85

increased while the bands near the thiol modes intensified more clearly. The product bands of SOJ (CH,),CSH, along with the new bands in the other two SO,/thiol systems, are listed in Table 1.

DISCUSSION

Barnes et al. [ 141 have investigated the infrared spectrum of ethanethiol suspended in Ar and N2 matrices. They assigned a weak absorption at 2572 cm-’ to the dimer and a strong peak at 2551 cm-l to the multimer. A multimer band at 866 cm-’ on the lower energy side of the monomer band was also reported. Two doublets at 2597.0 and 2600.0 cm-l, and at 731.0 and 732.8 cm-l were suggested as evidence of both trans and gauche conformers existing in the matrices, with the trans conformer being the more stable. The IR spectrum of ethanethiol in solid Ar obtained in the present study is essentially in agree- ment with theirs. In addition, a weak doublet was observed at 784 and 786 cm-’ when the ethanethiol concentration was high. A corresponding band was also observed at 783 cm-* in the solid IR spectrum [ 171.

Vibrational and microwave spectroscopic studies have identified the exis- tence of two different molecular conformations for 2-propanethiol with the trans conformer being more stable in the gaseous phase [ 16,191. The matrix- isolated IR spectra obtained for the molecule in this study was essentially in good agreement with the IR spectra obtained in the gas-phase IR studies, and additionally, a doublet was well resolved at 859 and 851 cm-’ which could be assigned to the CSH bending mode arising from two different conformers.

The vibrational assignments of 2-methyl-2-propanethiol have been pro- posed by McCullough et al. [ 181. Some self-association features were observed in the matrix-isolated IR spectra from this study, namely, a dimer band at 2574 cm-l, a broad multimer band centered at 2543 cm-’ located at the lower fre- quency side of the monomer SH stretching mode at 2597 cm-‘, and a new band at 863 cm-’ on the higher energy side of the CSH bending mode at 862 cm-l. All these new features were pretty similar to the results reported for ethane- thiol in argon and nitrogen matrices [ 141 and, therefore, were considered as the absorptions due to the 2-methyl-2-propanethiol self-association.

The co-deposition of a series of alkanethiols with SO, in this study gave rise to new infrared absorption bands which could not be accounted for as those of the isolated parent species, and therefore must be assigned to a product species. Attempts were made to assign these new bands observed in the matrix isolated SOJalkanethiol mixtures. All these perturbation bands in the SO,/thiol mix- tures were only shifted slightly from the parent species vibration modes, which is a typical character for the formation of a weakly bound molecular complex. Furthermore, only one set of new absorptions was observed in several different concentration experiments, suggesting that the complex in the SO,/alkane- thiol mixtures in the inert gas matrices is most likely to have 1: 1 stoichiometry.

86

Consequently, the new bands were attributed to the 1: 1 SOJalkanethiol com- plex. This conclusion is in agreement with the results for the SO, complexes with benzene and related conjugated n-electron donor compounds reported by Ault [ 111. The assignment of these product bands is straightforward by com- parison with the parent species absorptions.

Table 1 summarizes the results of vibrational assignments for the new prod- uct bands. The bands located at the slightly lower frequency sides of the SO, antisymmetric and symmetric stretching modes were assigned to these modes of the SO, subunit in each of the SO,/alkanethiol complexes. The bands around 522 cm-’ on the higher energy side of the SOz bending vibration were assigned to the bending mode of the SO, subunit in the complexes. The bands appearing on the lower frequency sides of the alkanethiol SH stretching, CSH bending and CS stretching modes were assigned to the corresponding modes of the alkanethiol subunit in the molecular complexes. In the SOJ2-C$H,SH mix- ture, two new bands could be assigned to the CSH bending mode because of the existence of two conformers in the thiol. From these assignments, one may therefore suggest the presence of two conformers in the complex. A comparison of the relative intensities of these new weak bands in different SOJalkanethiol complexes might indicate that the SO,/C,H,SH mixture gives the highest yield of the 1: 1 complex but the SO,/ (CH,),CSH mixture gives the lowest yield. This is reasonable since C,H,SH is the least sterically hindered base while ( CH3) ,CSH is the most sterically hindered base of all.

From Table 1, it is of interest to note that the C-S stretching modes of the thiols decrease consistently from ethanethiol (658 cm-l ) to isopropanethiol (635 cm-l) and then to tert-butanethiol (588 cm-‘). Additionally, the corre- sponding vibration observed in the Ar matrix-isolated methanethiol is located at 704 cm-’ [ 141. If the vibration is not significantly coupling with other modes in these molecules, this variation of frequencies may indicate that the methyl substitution(s) of methyl hydrogen(s) in methanethiol decreases the C-S bonding order, and thus, the corresponding force constants. This result agrees with the “elongation effect” due to methyl substitution obtained from the mi- crowave spectroscopic studies of organogermanes [ 20-221.

In the complex formation, there are two possible bonding positions in an alkanethiol. The bonding sites depend on the relative acidity or basicity of the other component. It is generally expected that the S atom will complex with a strong acid and the sulfbydryl hydrogen atom will complex with a strong base. However, when the reactant with the thiol is a weak acid or a weak base, it is not so obvious which site in the thiol will be used. In the present study, since SO, can act either as a base or as an acid, it may form complexes through the oxygen atom as a Lewis base or through the sulfur atom as a Lewis acid. Several facts made us believe that the formation of a molecular complex through the disulfide interaction is most likely in this study. Firstly, recent quantum chem-

87

istry calculations for the intermolecular interaction between CH$H and SO, indicated that the RHS. * *SO, bonding complex has lower energy than the complex bonded through an RSH. * * OS0 arrangement [23]. Secondly, in a previous study on the CH,SH/CH,SCH, system in argon and nitrogen matri- ces [13],aSH * * .S complex was obtained and a 75 cm-l red-shift for the meth- anethiol SH stretching was measured in the complex. In the present study, however, only about 20 cm-’ red-shifts were observed on the alkanethiol SH stretching vibrations upon complex formation. A perturbation to the stretch- ing mode due to the SH* * - 0 bonding in the complex is expected to be much greater than that due to the S **.S interaction. Thirdly, if the complexation were through the sulfhydryl hydrogen, the CSH bending in the complex would have shifted to the high frequency side of the corresponding vibration of the thiol subunit. In the present study, red-shifts, instead of blue-shifts, were ob- served in all three complexes. From these arguments, a bisulfide linkage was favored in the SOJalkanethiol complex. This should not exclude the possible coexistence of the interaction between the oxygen and the sulfhydryl hydrogen as in the case of the H,S/SO, complex [ 5,10,24]. This result is consistent with what we expected from the gas phase basicity values of these compounds (153.6 kcal mol-’ for SOz, and 181.8, 185.5 and 188.2 kcal mol-’ for CH,CH,SH, (CH,)&HSH and (CH,),CSH, respectively) [25].

Listed in Table 1 are the measured new bands for SO, complexed with dif- ferent alkanethiols in Ar matrices. It is seen that the shifts from the corre- sponding vibrations of the isolated SO, are much the same for different alka- nethiols. If the magnitude of the shift can be used as an indication of the strength of interaction, one may suggest that the interactions of SO, with the various alkanethiols are of nearly equal strength.

ACKNOWLEDGEMENT

S.L. wishes to acknowledge the Van Vleet Memorial Graduate Fellowship Program at Memphis State University for fellowships.

REFERENCES

1 G. Maes, J. Mol. Spectrosc., 114 (1985) 289. 2 G. Maes and M. Graindourze, J. Mol. Spectrosc., 113 (1985) 410. 3 L. Andrews, R.T. Arlinghaus and R.D. Hunt, Inorg. Chem., 25 (1986) 3205. 4 A. Schriver, L. Schriver and J.P. Perchard, J. Mol. Spectrosc., 127 (1988) 125. 5 K. Matsumura, F.J. Lovas and R.D. Suenram, J. Chem. Phys., 91 (1989) 5887. 6 L. Nord, J. Mol. Struct., 96 (1982) 27. 7 C.S. Sass and B.S. Ault, J. Phys. Chem., 88 (1984) 432. 8 L. Andrews, R. Withnal and R.D. Hunt, J. Phys. Chem., 92 (1988) 78. 9 A.J. Fillery and A.C. Legon, J. Chem. Phys., 85 (1986) 3180.

10 D.J. Pauley, R.E. Bumgarner and S.G. Kukolich, Chem. Phys. Lett., 132 (1986) 67.

88

11 12 13 14

15 16 17 18

19

20 21 22

23 24 25

B.S. Ault, J. Mol. Struct., 238 (1990) 111. H. Bai and B.S. Ault, J. Mol. Struct., 238 (1990) 223. S. Li and Y.-S. Li, Spectrochim. Acta, Part A, 47 (1991) 201. A.J. Barnes, H.E. Hallam and J.D.R. Howells, J. Chem. Sot., Faraday Trans. 2,68 (1972) 737. D. Maillard, M. Allavena and J.P. Perchard, Spectrochim. Acta, Part A, 31 (1975) 1523. J.R. Durig, G.A. Guirgis and D.A.C. Compton, J. Phys. Chem., 84 (1980) 3547. D. Smith, J.P. Devlin and D.W. Scott, J. Mol. Spectrosc., 25 (1968) 174. J.P. McCullough, D.W. Scott, H.L. Finke, W.N. Hubbard, M.E. Gross, C. Katz, R.E. Pennington, J.F. Messerly and G. Waddington, J. Am. Chem. Sot., ‘75 (1953) 1818. J.H. Griffith9 and J.E. Boggs, J. Mol. Spectrosc., 56 (1975) 257. J.R. Durig, A.B. Mohamod, G.M. Attia andY.S. Li, J. Chem. Phys., 83 (1985) 10. Y.S. Li and J.R. Durig, Inorg. Chem., 12 (1973) 306. J.R. Durig, J.F. Sullivan, A.B. Mohamad, S. Cradock and Y.S. Li, J. Chem. Phys., 84 (1986) 5796. H. Kurtz, personal communication, 1990. R.E. Bumgarner, D.J. Pauley and S.G. Kukolich, J. Chem. Phys., 87 (1987) 3749. S.G. Lias, J.F. Liebman and R.D. Leivin, J. Phys. Chem. Ref. Data, 13 (1984) 695.