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Molecular complexes of hydrogen halides with ethers and sulphides studied by matrix isolation vibrational spectroscopy
Journal of Molecular Structure (Theochem), 135 (1986) 21-30 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MOLECULAR COMPLEXES OF HYDROGEN HALIDES WITH ETHERS AND SULPHIDES STUDIED BY MATRIX ISOLATION VIBRATIONAL SPECTROSCOPY*
A. J. BARNES and M. P. WRIGHT Department of Chemistry and Applied Chemistry, (Great Britain)
University of Salford, Salford M5 4WT
(Received 27 March 1985)
ABSTRACT Infrared spectra are reported for mixtures of dimethyl ether, diethyl ether, hydrogen sulphide, dimethylsulphide and diethylsulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices. In addition to the bands due to the 1 :l complexes, absorptions due to 1:2 and 2:l complexes were identified in many of the mixtures. The position and band shape of the perturbed HX stretching absorptions of the 1:l complexes are discussed. INTRODUCTION
Molecular complexes between the hydrogen halides and a wide variety of bases have been studied by infrared spectroscopy in the gas phase or in lowtemperature matrices [l]. The most thoroughly studied complex is that between dimethyl ether and hydrogen chloride. The band contour of the perturbed HCl stretching mode in the gas phase has been the subject of numerous experimental and theoretical studies [2]. Schriver et al. [ 3-51 have carried out a detailed study of the interactions between dimethyl ether and hydrogen halides in low-temperature matrices, identifying bands due to l:l, 1:2 and 1:3 (bifid), 2:l (bifid) and 2:l (ion pair) complexes. Andrews et al. [6] have recently reported spectra of the dimethyl ether-hydrogen fluoride complex in an argon matrix. By contrast, the complexes of the hydrogen halides with other ethers or with sulphides have received comparatively little attention [ 11. Infrared spectra of the water-hydrogen halide complexes in matrices have been reported [7-lo]. The water-hydrogen fluoride [ 11-131, water-hydrogen chloride [ 141, hydrogen sulphidehydrogen fluoride [15, 161 and hydrogen sulphidehydrogen bromide [17] complexes have been studied by rotational spectroscopy. Photoelectron spectroscopy has been used [18-201 to investigate the complexes of dimethyl ether and dimethyl sulphide with hydrogen fluoride and hydrogen chloride. The present paper reports an infrared matrix isolation study of the interaction of hydrogen chloride and hydrogen bromide with RzO and R2S bases, where R = H, Me or Et. *Dedicated to Professor Robert S. Mulliken. 0166-1280/86/$03.50
0 1986 Elsevier Science Publishers B.V.
22 EXPERIMENTAL
Hydrogen chloride, hydrogen bromide, hydrogen sulphide and dimethyl ether (all supplied by Matheson) were purified by trap-to-trap distillation under vacuum. Diethyl ether (B.D.H.), dimethyl sulphide (Aldrich) and diethyl sulphide (B.D.H.) were degassed under vacuum and distilled before use. High purity argon and nitrogen matrix gases (B.O.C.) were used without further purification. The hydrogen halide, base and matrix gases were mixed in the desired proportions (e.g., 1:l: 200) using standard manometric procedures. The mixture was sprayed at a rate of ca. 4 mmol h-l (controlled by a needle valve) onto a caesium iodide window maintained at ca. 20 K by a CT1 Cryodyne model 21 cryocooler. Spectra were recorded on Perkin-Elmer 180 or 225 spectrometers, calibrated using standard gases. RESULTS
Dime thy1 ether Spectra were recorded of mixtures of dimethyl ether with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1:1:200, 1:2:400, 2:1:400 and 1:2:1000. The spectra of the dimethyl etherhydrogen chloride mixtures showed features essentially the same as had previously been reported [ 3, 51. The 1:l complex in an argon matrix exhibited a broad absorption at ca. 2310 cm-‘, due to the perturbed HCl stretching vibration, with a high frequency shoulder. Relatively sharp bands at 1167, 1088, 900 and 411 cm-’ are assigned to the perturbed CHJ rocking, COC antisymmetric stretching, COC symmetric stretching and COC bending modes of the dimethyl ether. Similar bands were observed for the 1:l complex in a nitrogen matrix. The concentration dependence of broad absorptions at ca. 2480 cm-’ (Ar matrix) and 1980 cm-’ (nitrogen matrix) suggested assignment to 1:2 and 2:l (Me*O:HCl) complexes, respectively. The spectra of the dimethyl ether-hydrogen bromide mixtures were rather more complicated (Fig. 1). The 1: 1 complex in an argon matrix exhibited a broad absorption at ca. 2050 cm-‘, due to the perturbed HBr stretching vibration, with a high frequency shoulder. Relatively sharp bands at 1165, 1088, 888 and 408 cm-’ may be assigned to perturbed dimethyl ether modes. Additionally, broad absorptions at ca. 1080 and 870 cm-’ appeared to be due to the 1: 1 complex and are tentatively assigned as the 0. * H-Br bending modes (the corresponding modes of the dimethyl ether hydrogen fluoride complex appeared [6] at 801 and 687 cm-‘). Similar bands were observed for the 1:l complex in a nitrogen matrix. The concentration dependence of a broad absorption at ca. 2240 cm-’ and a relatively sharp band at 840 cm-’ suggested assignment to a 1:2 (Me,O:HBr) complex, while broad absorptions at ca. 1740, 1020 and 760 cm-’ appeared to be due to a 2:l (MezO:HBr) complex. The various bands observed for the dimethyl l
23
Fig. 1. Infrared spectra of dimethyl ether + hydrogen bromide mixtures in argon and nitrogen matrices; (0) indicates a band due to complex.
ether-hydrogen in Table 1.
chloride and hydrogen bromide complexes are summarised
Diethyl ether Spectra were recorded of mixtures of diethyl ether with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1: 1:200. The complex with hydrogen chloride in an argon matrix had previously been studied by Kuzniarski [21], who observed a doublet for the HCl stretching vibration at 2435 and 2395 cm-‘. This splitting was postulated [l, 211 to be due to complexes with two different conformers of the ether. In the gas and liquid phases, diethyl ether is present as TT and TG conformers, the TT conformer being the more stable [ 22, 231. Thomas [ 241 has suggested
24 TABLE 1 Bands observed (cm-‘) for dimethyl ether-hydrogen chloride hydrogen bromide complexes in argon and nitrogen matrices Assignment
Ar matrix Me,0
HX stretch (Me,O),.HX CH, rock COC asym. stretch O**mH-X bend (Me,O),mHX COC sym. stretch O*.*H-X bend Me,O. (HX), (Me,O),*HX COC bend
ether-
N, matrix Me,OnHCl
= 2480 = 2450 sh -2310
Me,00 (HX),
and dimethyl
1173 1099
1167 1088
926
900
415
411
Me,O*HBr *2240 = 2200 sh =2050 ml740 1165 1088 = 1080 1020 888 =870 840 760 408
Me,0
1169 1098
Me,O.HCl
= 2400 sh = 2300 ml980 1163 1087
924
902
415
810 412
Me,O. HBr
w 2200 sh FJ2020 * 1735 1162 1083 = 1080 1020 889 =800 842 766 412
that formation of the diethyl ether-hydrogen fluoride complex causes the ether to rearrange to the less stable GG conformer. Andrews et al. [6] observed two different 1:l complexes of diethyl ether with hydrogen fluoride, which they attributed to TT (higher wavenumber HF stretching) and TG conformers. In a nitrogen matrix, the diethyl ether-hydrogen chloride complex gives a very broad absorption centred at ca. 2440 cm-‘. The diethyl ether-hydrogen bromide complex gives a broad absorption at ca. 2150 cm-’ in both argon and nitrogen matrices. Part of the breadth of these absorptions is presumably due to unresolved components due to the two conformers. A number of other absorptions appeared in the spectra, due to perturbed vibrations of the diethyl ether and higher order complexes. The bl C-O stretching mode of the ether shifts from 1130 cm-’ for the isolated molecule in an argon matrix to 1114 cm-’ for the HF complex [6], 1115 cm-’ for the HCl complex and 1116 cm-l for the HBr complex. The complexity of the ether spectrum makes it difficult to assign the other bands that were observed. Hydrogen
sulphide
Spectra were recorded of mixtures of hydrogen sulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1:1:200. The spectra of the complexes in argon matrices are illustrated in Fig. 2 (the nitrogen matrix spectra are similar). In each case, the spectrum is dominated by a strong and relatively sharp new band: 2676 or 2661 cm-’ for the HCI complex in argon or nitrogen, 2378 or 2363 cm-’
25
J
I
I
3000
2800
2600
II
I
I
I
I
2600
2500
2400
2300
wavenumberl cm-1 Fig. 2. Infrared spectra of hydrogen sulphide + hydrogen chloride and hydrogen sulphide + hydrogen bromide mixtures in argon matrices (1 :l:200).
for the HBr complex in argon or nitrogen. These absorptions are clearly due to the 1:l H$*HX complexes. The weaker absorptions in the spectra could all be assigned to monomeric or aggregated hydrogen halide or hydrogen sulphide. Dimethylsulphide Spectra were recorded of mixtures of dimethylsulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1:1:200 and 1:2:1000. Spectra of dimethylsulphide in argon and nitrogen matrices were also recorded for comparison and assigned following the gas and liquid phase data of Allkins and Hendra .[25]. The principal feature in the spectrum of the dimethylsulphide + hydrogen chloride mixture in an argon matrix was a relatively sharp band at 2272 cm-l, with a weaker band at 2430 cm-‘. A similar pair of bands appeared in a nitrogen matrix at 2220 and 2380 cm-‘, but with much greater bandwidths. The spectra of the dimethylsulphide + hydrogen bromide mixtures were rather more complex (Fig. 3). The strongest band in an argon matrix is at 1823 cm-‘, with a weaker band at 2004 cm-l, while in a nitrogen matrix a similar pair of bands appear at 1745 and 1940 cm-‘, again with increased bandwidths. These features are all assigned to the perturbed HX stretching vibration (the origin of the high frequency component will be discussed below). The higher concentration dimethylsulphide + hydrogen bromide spectra also exhibit a very broad and diffuse absorption, centred near 1030 cm-‘, with transmission windows at 1042 and 680 cm-‘. This may plausibly be assigned as the S* - lH* S stretching vibration of [Me& - H* * SMeJBr-, the transmission windows arising from resonance interaction with the al CH, rocking and CSC symmetric l
l
l
l
26
11:200
Fig. 3. Infrared spectra of dimethylsulphide + hydrogen bromide mixtures in argon and nitrogen matrices; (0) indicates a band due to complex.
stretching modes (1032 and 694 cm-‘, respectively, in isolated dimethylsulphide). A broad band at 1107 cm-’ in an argon matrix (1097 cm-’ in nitrogen) which persists at high dilution is assigned as the Se * H-Br bending mode of the 1:l complex, while a weak, sharp band at 918 cm-’ in argon, which also persists at high dilution, is probably the perturbed b 1 CH, rocking mode (899 cm-’ in isolated dimethylsulphide). A broad absorption at ca. 1500 cm-’ in an argon matrix disappears in the more dilute matrix spectrum and is assigned to a 2:l (Me$J:HBr) complex. l
Diethylsulphide Spectra were recorded of mixtures of diethylsulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of 1:1:200. In the gas and liquid phases, at least three conformations (TT, TG and GG) of diethylsulphide are present, with the TT conformer being the
27
most stable in the solid phase [26]. The presence of these different conformations undoubtedly causes the comparatively broad bands observed for the HX stretching vibration of the diethylsulphide-hydrogen chloride and hydrogen bromide complexes, even in argon matrices (Fig. 4). As for the dimethylsulphide complexes, the HX stretching absorption appears as a doublet, the higher wavenumber band being less intense. For the HCl complex, the bands appear at 2225 and 2380 cm-’ in argon and 2220 and 2350 cm-’ in nitrogen, while for the HBr complex they are at 1740 and 1945 cm-’ in argon and 1720 and 1925 cm-l in nitrogen. A number of other new absorptions were observed in the spectra of diethylsulphide + hydrogen bromide mixtures. A broad absorption at ca. 1071 cm-’ in argon (1075 cm-’ in nitrogen) is assigned as the S* H-Br bending mode of the 1:l complex, while a broad absorption near 660 cm-’ is due to a higher order complex. Other bands were relatively sharp and thus presumably due to perturbed vibrations of the diethylsulphide; the complexity of the diethylsulphide spectrum made it difficult to assign these bands with certainty. l
l
DISCUSSION
The perturbed HX stretching frequencies and relative frequency shifts (Av/v) of the R30 and R2S base-hydrogen halide complexes are summarised in Table 2. It can be seen that while there is a general trend for the HX stretching frequency to decrease with increasing base strength (as measured by the gas phase proton affinity of the base), this falls down for the water hydrogen sulphide and dimethyl ether-diethyl ether pairs. Diethyl ether also gives a low shift of the HX stretching frequency compared with dimethylsulphide, which has a similar proton affinity. These apparent anomalies may
I
2500
I
2000
I
1500
I
1000 wavenumber
/cm-l
Fig. 4. Infrared spectra of diethylsulphide + hydrogen bromide mixtures in argon and nitrogen matrices (1 :l:200); (0) indicates a band due to compler:.
28 TABLE! 2 HX stretching frequencies (cm-‘) of 1:l R,O and R,S base-hydrogen Base
PAa
halide complexes
B"'HF
B***HCl
B*.-HBr
Ar
Ar
N,
Ar
N,
3584b (0.086)
2664O (0.072) 2676 (0.068) (2450) 2310 (0.195) 2435 (0.152) 2395 (0.166) (2430) 2272 (0.209) (2380) 2225 (0.225)
254od(o.l10) 2661 (0.068) (2400) 2300 (0.194) 2440 (0.145)
2395'=(0.064) 2378 (0.071) (2200) 2050 (0.199) 2155 (0.158)
23lodco.093) 2363 (0.072) (2200) 2020 (0.207) 2150 (0.156)
(2380) 2220 (0.222) (2350) 2220 (0.222)
(2004) 1323 (0.288) (1945) 1740 (0.320)
(1940) 1745 (0.315) (1925) 1720 (0.324)
w
697
H*S
712
Me,0 Et,0
3350f (0.145) 804 838 TT 3361f (0.143) TG 3322f (0.153)
Me,S
839
Et,S
858
aProton affinity/kJ mol-’ (S. G. Lias, J. F. Liebman and R. D. Levin, J. Phys. Chem. Ref. Data, 13 (1984)695). bRef. 10. CRef. 8. “Ref. 7. eRef. 9. fRef. 6.
be due to the use of proton affinity as a measure of base strength. Morokuma [27] has pointed out that the interaction of a base with a proton can be considered as a special case of strong, ionic hydrogen bonding but it is unique in that the proton has no electron, therefore there is no exchange repulsion between the base and the proton. The importance of intermolecular interactions between the base and the hydrogen halide in the diethyl ether hydrogen halide (and diethylsulphide-hydrogen halide) complexes is demonstrated by the different shifts of the HX stretching mode observed for different conformers of the base. There is considerable variation in the effect on the HX stretching frequencies of the complexes of changing the matrix from argon to nitrogen. The water-hydrogen halide and dimethylsulphide-hydrogen halide complexes exhibit a significantly greater shift in a nitrogen matrix, whereas there is little difference between the HX stretching frequencies in the two matrices for the complexes of the other bases. Usually the nitrogen matrix does increase the strength of the complex, as measured by the shift in the HX stretching frequency, the effect being particularly marked for nitrogen bases [ 11. The band shape of the HX stretching absorption also exhibits considerable variations. Figure 5 illustrates this mode for the R$.* *HX complexes in argon and nitrogen matrices. It can be seen that the band breadth increases: (a) as the matrix is changed from argon to nitrogen, (b) as the hydrogen halide changes from HCl to HBr, and (c) as the base changes from dimethylsulphide to diethylsulphide. The last effect may well be due to the formation of complexes with different conformers of the diethylsulphide. The absorptions for the complexes with dimethylsulphide are very much sharper than the corresponding absorptions of the complexes with dimethyl ether. the HX stretching absorption has a For all the R2S*** HX complexes, distinct high frequency component. Maes [28] has attributed this to a 1:2
29
I
HCI Me25 I Ar
MefS/Ar
A = 158 cm-’
A = 181 cm-’
Me25 / N2
Me25 / N2
A = 160 cm-’
A = 195 cm-’
Et2S / Ar
Et2S / Ar
A=l55cm-’
A = 205 cm-l 1945 1740
Et25 / N2
Et2S/N2
A = 130 cm-’
A = 205 cm-’
Fig. 5. Comparison of the HX stretching absorptions for the dimethylsulphide and diethylsulphide complexes in argon and nitrogen matrices.
(R,S:HX) complex, but we observed no variation in the relative intensity of the two components over the range of concentrations studied. The wavenumber separation of the two components can be accurately measured for the relatively sharp bands of Me@ lHCl (158 cm-‘) and Me$+ lHBr (181 cm-‘) in argon matrices. The broad absorptions of the other complexes make it impossible to otbain an accurate value for the separation, but it appears to show little variation as the matrix is changed from argon to nitrogen or the base from dimethylsulphide to diethylsulphide. In the gas phase, the dimethyl etherhydrogen chloride complex exhibits an HX stretching absorption with a complicated structure which has been attributed to sum and difference bands with the hydrogen bond stretching mode (v,,), although recently it has been suggested that combinations with the low frequency bending modes (VP>may also be important in determining the band profile [2]. The high frequency shoulder(s) observed for this complex in an argon matrix have been assigned [5] to combinations with u, and v@. For the R&l- *HX complexes, the increased separation observed as the hydrogen halide is changed from HCl to HBr precludes assignment to the combination with v, since this is essentially a heavy atom stretching motion, leaving the combination with vp as the only possibility. In the dimethyl ether-hydrogen chloride complex, vp has generally been assumed to lie at ca. 50 cm-’ [2]. However, Bauer et al. [29] have recently suggested that this value is much too low and proposed a value of 100 cm- l. If the assignment proposed here HX complexes one component of vp must lie is correct, then for the R@ above 150 cm-’ (the degeneracy of vp is lifted in complexes with ethers or sulphides). l
l
l
l
l
30 ACKNOWLEDGEMENT
We are grateful to Dr. G. Maes for communicating results prior to publication. REFERENCES 1 A. J. Barnes, J. Mol. Struct., 100 (1983) 259. 2 D. J. Millen and 0. Schrems, Chem. Phys. Lett., 101 (1983) 320, and references therein. 3 L. S&river, A. LouteIIier and A. Bumeau, Chem. Phys. Lett., 60 (1979) 471. 4A. Loutellier, L. Schriver, A. Bumeau and J. P. Perchard, J. Mol. Struct., 82 (1982) 165. 5 L. Schriver, A. Louteiher, A. Bumeau and J. P. Perchard, J. Mol. Struct., 95 (1982) 37. 6 L. Andrews, G. L. Johnson and S. R. Davis, J. Phys. Chem., 89 (1985) 1710. 7 B. S. Ault, E. Steinback and G. C. Pimentel, J. Phys. Chem., 79 (1975) 615. 8 G. P. Ayers and A. D. E. Puihn, Spectrochim. Acta, Part A, 32 (1976) 1641. 9A. Schriver, B. Siivi, D. Maillard and J. P. Perchard, J. Phys. Chem., 81 (1977) 2095. 10 L. Andrews and G. L. Johnson, J. Chem. Phys., 79 (1983) 3670. 11 Z. Kisiel, A. C. Legon and D. J. MiIIen, Proc. R. Sot. London, Ser. A, 381 (1982) 419. 12 A. C. Legon and L. C. Willoughby, Chem. Phys. Lett., 92 (1982) 333. 13 Z. Kisiel, A. C. Legon and D. J. Millen, J. Mol. Struct., 112 (1984) 1. 14 A. C. Legon and L. C. Willoughby, Chem. Phys. Lett., 95 (1983) 449. 15 R. Viswanathan and T. R. Dyke, J. Chem. Phys., 77 (1982) 1166. 16 L. C. Willoughby, A. J. Filler-y-Travis and A. C. Legon, J. Chem. Phys., 81 (1984) 20. 17 E. J. Goodwin and A. C. Legon, J. Chem. Sot., Faraday Trans. 2,80 (1984) 51. 18 F. CamovaIe, M. K. Livett and J. B. Peel, J. Am. Chem. Sot., 102 (1980) 569. 19F. CamovaIe, M. K. Livett and J. B. Peel, J. Am. Chem. Sot., 104 (1982) 5334. 20 F. Camovaie, M. K. Livett and J. B. Peel, J. Am. Chem. Sot., 105 (1983) 6788. 21 J. N. S. Kuzniarski, M.Sc. thesis, University of Salford, 1978. 22R. G. Snyder and G. Zerbi, Spectrochim. Acta, Part A, 23 (1967) 391. 23 H. Wieser, W. G. Laidlaw, P. J. Krueger and H. Fuhrer, Spectrochim. Acta, Part A, 24 (1968) 1055. 24R. K. Thomas, Proc. R. Sot. London, Ser. A, 322 (1971) 137. 25 J. R. Allkins and P. J. Hendra, Spectrochim. Acta, 22 (1966) 2075. 26M. Ohsaku, Y. Shiro and H. Murata, Bull. Chem. Sot. Jpn., 45 (1972) 956. 27 K. Morokuma, Act. Chem. Res., 10 (1977) 294. 28 G. Maes, personal communication. 29 S. H. Bauer, T. Yamazaki, K. I. Lazaar and N.-S. Chiu, J. Am. Chem. Sot., 107 (1985) 743.
MOLECULAR COMPLEXES OF HYDROGEN HALIDES WITH ETHERS AND SULPHIDES STUDIED BY MATRIX ISOLATION VIBRATIONAL SPECTROSCOPY*
A. J. BARNES and M. P. WRIGHT Department of Chemistry and Applied Chemistry, (Great Britain)
University of Salford, Salford M5 4WT
(Received 27 March 1985)
ABSTRACT Infrared spectra are reported for mixtures of dimethyl ether, diethyl ether, hydrogen sulphide, dimethylsulphide and diethylsulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices. In addition to the bands due to the 1 :l complexes, absorptions due to 1:2 and 2:l complexes were identified in many of the mixtures. The position and band shape of the perturbed HX stretching absorptions of the 1:l complexes are discussed. INTRODUCTION
Molecular complexes between the hydrogen halides and a wide variety of bases have been studied by infrared spectroscopy in the gas phase or in lowtemperature matrices [l]. The most thoroughly studied complex is that between dimethyl ether and hydrogen chloride. The band contour of the perturbed HCl stretching mode in the gas phase has been the subject of numerous experimental and theoretical studies [2]. Schriver et al. [ 3-51 have carried out a detailed study of the interactions between dimethyl ether and hydrogen halides in low-temperature matrices, identifying bands due to l:l, 1:2 and 1:3 (bifid), 2:l (bifid) and 2:l (ion pair) complexes. Andrews et al. [6] have recently reported spectra of the dimethyl ether-hydrogen fluoride complex in an argon matrix. By contrast, the complexes of the hydrogen halides with other ethers or with sulphides have received comparatively little attention [ 11. Infrared spectra of the water-hydrogen halide complexes in matrices have been reported [7-lo]. The water-hydrogen fluoride [ 11-131, water-hydrogen chloride [ 141, hydrogen sulphidehydrogen fluoride [15, 161 and hydrogen sulphidehydrogen bromide [17] complexes have been studied by rotational spectroscopy. Photoelectron spectroscopy has been used [18-201 to investigate the complexes of dimethyl ether and dimethyl sulphide with hydrogen fluoride and hydrogen chloride. The present paper reports an infrared matrix isolation study of the interaction of hydrogen chloride and hydrogen bromide with RzO and R2S bases, where R = H, Me or Et. *Dedicated to Professor Robert S. Mulliken. 0166-1280/86/$03.50
0 1986 Elsevier Science Publishers B.V.
22 EXPERIMENTAL
Hydrogen chloride, hydrogen bromide, hydrogen sulphide and dimethyl ether (all supplied by Matheson) were purified by trap-to-trap distillation under vacuum. Diethyl ether (B.D.H.), dimethyl sulphide (Aldrich) and diethyl sulphide (B.D.H.) were degassed under vacuum and distilled before use. High purity argon and nitrogen matrix gases (B.O.C.) were used without further purification. The hydrogen halide, base and matrix gases were mixed in the desired proportions (e.g., 1:l: 200) using standard manometric procedures. The mixture was sprayed at a rate of ca. 4 mmol h-l (controlled by a needle valve) onto a caesium iodide window maintained at ca. 20 K by a CT1 Cryodyne model 21 cryocooler. Spectra were recorded on Perkin-Elmer 180 or 225 spectrometers, calibrated using standard gases. RESULTS
Dime thy1 ether Spectra were recorded of mixtures of dimethyl ether with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1:1:200, 1:2:400, 2:1:400 and 1:2:1000. The spectra of the dimethyl etherhydrogen chloride mixtures showed features essentially the same as had previously been reported [ 3, 51. The 1:l complex in an argon matrix exhibited a broad absorption at ca. 2310 cm-‘, due to the perturbed HCl stretching vibration, with a high frequency shoulder. Relatively sharp bands at 1167, 1088, 900 and 411 cm-’ are assigned to the perturbed CHJ rocking, COC antisymmetric stretching, COC symmetric stretching and COC bending modes of the dimethyl ether. Similar bands were observed for the 1:l complex in a nitrogen matrix. The concentration dependence of broad absorptions at ca. 2480 cm-’ (Ar matrix) and 1980 cm-’ (nitrogen matrix) suggested assignment to 1:2 and 2:l (Me*O:HCl) complexes, respectively. The spectra of the dimethyl ether-hydrogen bromide mixtures were rather more complicated (Fig. 1). The 1: 1 complex in an argon matrix exhibited a broad absorption at ca. 2050 cm-‘, due to the perturbed HBr stretching vibration, with a high frequency shoulder. Relatively sharp bands at 1165, 1088, 888 and 408 cm-’ may be assigned to perturbed dimethyl ether modes. Additionally, broad absorptions at ca. 1080 and 870 cm-’ appeared to be due to the 1: 1 complex and are tentatively assigned as the 0. * H-Br bending modes (the corresponding modes of the dimethyl ether hydrogen fluoride complex appeared [6] at 801 and 687 cm-‘). Similar bands were observed for the 1:l complex in a nitrogen matrix. The concentration dependence of a broad absorption at ca. 2240 cm-’ and a relatively sharp band at 840 cm-’ suggested assignment to a 1:2 (Me,O:HBr) complex, while broad absorptions at ca. 1740, 1020 and 760 cm-’ appeared to be due to a 2:l (MezO:HBr) complex. The various bands observed for the dimethyl l
23
Fig. 1. Infrared spectra of dimethyl ether + hydrogen bromide mixtures in argon and nitrogen matrices; (0) indicates a band due to complex.
ether-hydrogen in Table 1.
chloride and hydrogen bromide complexes are summarised
Diethyl ether Spectra were recorded of mixtures of diethyl ether with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1: 1:200. The complex with hydrogen chloride in an argon matrix had previously been studied by Kuzniarski [21], who observed a doublet for the HCl stretching vibration at 2435 and 2395 cm-‘. This splitting was postulated [l, 211 to be due to complexes with two different conformers of the ether. In the gas and liquid phases, diethyl ether is present as TT and TG conformers, the TT conformer being the more stable [ 22, 231. Thomas [ 241 has suggested
24 TABLE 1 Bands observed (cm-‘) for dimethyl ether-hydrogen chloride hydrogen bromide complexes in argon and nitrogen matrices Assignment
Ar matrix Me,0
HX stretch (Me,O),.HX CH, rock COC asym. stretch O**mH-X bend (Me,O),mHX COC sym. stretch O*.*H-X bend Me,O. (HX), (Me,O),*HX COC bend
ether-
N, matrix Me,OnHCl
= 2480 = 2450 sh -2310
Me,00 (HX),
and dimethyl
1173 1099
1167 1088
926
900
415
411
Me,O*HBr *2240 = 2200 sh =2050 ml740 1165 1088 = 1080 1020 888 =870 840 760 408
Me,0
1169 1098
Me,O.HCl
= 2400 sh = 2300 ml980 1163 1087
924
902
415
810 412
Me,O. HBr
w 2200 sh FJ2020 * 1735 1162 1083 = 1080 1020 889 =800 842 766 412
that formation of the diethyl ether-hydrogen fluoride complex causes the ether to rearrange to the less stable GG conformer. Andrews et al. [6] observed two different 1:l complexes of diethyl ether with hydrogen fluoride, which they attributed to TT (higher wavenumber HF stretching) and TG conformers. In a nitrogen matrix, the diethyl ether-hydrogen chloride complex gives a very broad absorption centred at ca. 2440 cm-‘. The diethyl ether-hydrogen bromide complex gives a broad absorption at ca. 2150 cm-’ in both argon and nitrogen matrices. Part of the breadth of these absorptions is presumably due to unresolved components due to the two conformers. A number of other absorptions appeared in the spectra, due to perturbed vibrations of the diethyl ether and higher order complexes. The bl C-O stretching mode of the ether shifts from 1130 cm-’ for the isolated molecule in an argon matrix to 1114 cm-’ for the HF complex [6], 1115 cm-’ for the HCl complex and 1116 cm-l for the HBr complex. The complexity of the ether spectrum makes it difficult to assign the other bands that were observed. Hydrogen
sulphide
Spectra were recorded of mixtures of hydrogen sulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1:1:200. The spectra of the complexes in argon matrices are illustrated in Fig. 2 (the nitrogen matrix spectra are similar). In each case, the spectrum is dominated by a strong and relatively sharp new band: 2676 or 2661 cm-’ for the HCI complex in argon or nitrogen, 2378 or 2363 cm-’
25
J
I
I
3000
2800
2600
II
I
I
I
I
2600
2500
2400
2300
wavenumberl cm-1 Fig. 2. Infrared spectra of hydrogen sulphide + hydrogen chloride and hydrogen sulphide + hydrogen bromide mixtures in argon matrices (1 :l:200).
for the HBr complex in argon or nitrogen. These absorptions are clearly due to the 1:l H$*HX complexes. The weaker absorptions in the spectra could all be assigned to monomeric or aggregated hydrogen halide or hydrogen sulphide. Dimethylsulphide Spectra were recorded of mixtures of dimethylsulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of ca. 1:1:200 and 1:2:1000. Spectra of dimethylsulphide in argon and nitrogen matrices were also recorded for comparison and assigned following the gas and liquid phase data of Allkins and Hendra .[25]. The principal feature in the spectrum of the dimethylsulphide + hydrogen chloride mixture in an argon matrix was a relatively sharp band at 2272 cm-l, with a weaker band at 2430 cm-‘. A similar pair of bands appeared in a nitrogen matrix at 2220 and 2380 cm-‘, but with much greater bandwidths. The spectra of the dimethylsulphide + hydrogen bromide mixtures were rather more complex (Fig. 3). The strongest band in an argon matrix is at 1823 cm-‘, with a weaker band at 2004 cm-l, while in a nitrogen matrix a similar pair of bands appear at 1745 and 1940 cm-‘, again with increased bandwidths. These features are all assigned to the perturbed HX stretching vibration (the origin of the high frequency component will be discussed below). The higher concentration dimethylsulphide + hydrogen bromide spectra also exhibit a very broad and diffuse absorption, centred near 1030 cm-‘, with transmission windows at 1042 and 680 cm-‘. This may plausibly be assigned as the S* - lH* S stretching vibration of [Me& - H* * SMeJBr-, the transmission windows arising from resonance interaction with the al CH, rocking and CSC symmetric l
l
l
l
26
11:200
Fig. 3. Infrared spectra of dimethylsulphide + hydrogen bromide mixtures in argon and nitrogen matrices; (0) indicates a band due to complex.
stretching modes (1032 and 694 cm-‘, respectively, in isolated dimethylsulphide). A broad band at 1107 cm-’ in an argon matrix (1097 cm-’ in nitrogen) which persists at high dilution is assigned as the Se * H-Br bending mode of the 1:l complex, while a weak, sharp band at 918 cm-’ in argon, which also persists at high dilution, is probably the perturbed b 1 CH, rocking mode (899 cm-’ in isolated dimethylsulphide). A broad absorption at ca. 1500 cm-’ in an argon matrix disappears in the more dilute matrix spectrum and is assigned to a 2:l (Me$J:HBr) complex. l
Diethylsulphide Spectra were recorded of mixtures of diethylsulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices at concentrations of 1:1:200. In the gas and liquid phases, at least three conformations (TT, TG and GG) of diethylsulphide are present, with the TT conformer being the
27
most stable in the solid phase [26]. The presence of these different conformations undoubtedly causes the comparatively broad bands observed for the HX stretching vibration of the diethylsulphide-hydrogen chloride and hydrogen bromide complexes, even in argon matrices (Fig. 4). As for the dimethylsulphide complexes, the HX stretching absorption appears as a doublet, the higher wavenumber band being less intense. For the HCl complex, the bands appear at 2225 and 2380 cm-’ in argon and 2220 and 2350 cm-’ in nitrogen, while for the HBr complex they are at 1740 and 1945 cm-’ in argon and 1720 and 1925 cm-l in nitrogen. A number of other new absorptions were observed in the spectra of diethylsulphide + hydrogen bromide mixtures. A broad absorption at ca. 1071 cm-’ in argon (1075 cm-’ in nitrogen) is assigned as the S* H-Br bending mode of the 1:l complex, while a broad absorption near 660 cm-’ is due to a higher order complex. Other bands were relatively sharp and thus presumably due to perturbed vibrations of the diethylsulphide; the complexity of the diethylsulphide spectrum made it difficult to assign these bands with certainty. l
l
DISCUSSION
The perturbed HX stretching frequencies and relative frequency shifts (Av/v) of the R30 and R2S base-hydrogen halide complexes are summarised in Table 2. It can be seen that while there is a general trend for the HX stretching frequency to decrease with increasing base strength (as measured by the gas phase proton affinity of the base), this falls down for the water hydrogen sulphide and dimethyl ether-diethyl ether pairs. Diethyl ether also gives a low shift of the HX stretching frequency compared with dimethylsulphide, which has a similar proton affinity. These apparent anomalies may
I
2500
I
2000
I
1500
I
1000 wavenumber
/cm-l
Fig. 4. Infrared spectra of diethylsulphide + hydrogen bromide mixtures in argon and nitrogen matrices (1 :l:200); (0) indicates a band due to compler:.
28 TABLE! 2 HX stretching frequencies (cm-‘) of 1:l R,O and R,S base-hydrogen Base
PAa
halide complexes
B"'HF
B***HCl
B*.-HBr
Ar
Ar
N,
Ar
N,
3584b (0.086)
2664O (0.072) 2676 (0.068) (2450) 2310 (0.195) 2435 (0.152) 2395 (0.166) (2430) 2272 (0.209) (2380) 2225 (0.225)
254od(o.l10) 2661 (0.068) (2400) 2300 (0.194) 2440 (0.145)
2395'=(0.064) 2378 (0.071) (2200) 2050 (0.199) 2155 (0.158)
23lodco.093) 2363 (0.072) (2200) 2020 (0.207) 2150 (0.156)
(2380) 2220 (0.222) (2350) 2220 (0.222)
(2004) 1323 (0.288) (1945) 1740 (0.320)
(1940) 1745 (0.315) (1925) 1720 (0.324)
w
697
H*S
712
Me,0 Et,0
3350f (0.145) 804 838 TT 3361f (0.143) TG 3322f (0.153)
Me,S
839
Et,S
858
aProton affinity/kJ mol-’ (S. G. Lias, J. F. Liebman and R. D. Levin, J. Phys. Chem. Ref. Data, 13 (1984)695). bRef. 10. CRef. 8. “Ref. 7. eRef. 9. fRef. 6.
be due to the use of proton affinity as a measure of base strength. Morokuma [27] has pointed out that the interaction of a base with a proton can be considered as a special case of strong, ionic hydrogen bonding but it is unique in that the proton has no electron, therefore there is no exchange repulsion between the base and the proton. The importance of intermolecular interactions between the base and the hydrogen halide in the diethyl ether hydrogen halide (and diethylsulphide-hydrogen halide) complexes is demonstrated by the different shifts of the HX stretching mode observed for different conformers of the base. There is considerable variation in the effect on the HX stretching frequencies of the complexes of changing the matrix from argon to nitrogen. The water-hydrogen halide and dimethylsulphide-hydrogen halide complexes exhibit a significantly greater shift in a nitrogen matrix, whereas there is little difference between the HX stretching frequencies in the two matrices for the complexes of the other bases. Usually the nitrogen matrix does increase the strength of the complex, as measured by the shift in the HX stretching frequency, the effect being particularly marked for nitrogen bases [ 11. The band shape of the HX stretching absorption also exhibits considerable variations. Figure 5 illustrates this mode for the R$.* *HX complexes in argon and nitrogen matrices. It can be seen that the band breadth increases: (a) as the matrix is changed from argon to nitrogen, (b) as the hydrogen halide changes from HCl to HBr, and (c) as the base changes from dimethylsulphide to diethylsulphide. The last effect may well be due to the formation of complexes with different conformers of the diethylsulphide. The absorptions for the complexes with dimethylsulphide are very much sharper than the corresponding absorptions of the complexes with dimethyl ether. the HX stretching absorption has a For all the R2S*** HX complexes, distinct high frequency component. Maes [28] has attributed this to a 1:2
29
I
HCI Me25 I Ar
MefS/Ar
A = 158 cm-’
A = 181 cm-’
Me25 / N2
Me25 / N2
A = 160 cm-’
A = 195 cm-’
Et2S / Ar
Et2S / Ar
A=l55cm-’
A = 205 cm-l 1945 1740
Et25 / N2
Et2S/N2
A = 130 cm-’
A = 205 cm-’
Fig. 5. Comparison of the HX stretching absorptions for the dimethylsulphide and diethylsulphide complexes in argon and nitrogen matrices.
(R,S:HX) complex, but we observed no variation in the relative intensity of the two components over the range of concentrations studied. The wavenumber separation of the two components can be accurately measured for the relatively sharp bands of Me@ lHCl (158 cm-‘) and Me$+ lHBr (181 cm-‘) in argon matrices. The broad absorptions of the other complexes make it impossible to otbain an accurate value for the separation, but it appears to show little variation as the matrix is changed from argon to nitrogen or the base from dimethylsulphide to diethylsulphide. In the gas phase, the dimethyl etherhydrogen chloride complex exhibits an HX stretching absorption with a complicated structure which has been attributed to sum and difference bands with the hydrogen bond stretching mode (v,,), although recently it has been suggested that combinations with the low frequency bending modes (VP>may also be important in determining the band profile [2]. The high frequency shoulder(s) observed for this complex in an argon matrix have been assigned [5] to combinations with u, and v@. For the R&l- *HX complexes, the increased separation observed as the hydrogen halide is changed from HCl to HBr precludes assignment to the combination with v, since this is essentially a heavy atom stretching motion, leaving the combination with vp as the only possibility. In the dimethyl ether-hydrogen chloride complex, vp has generally been assumed to lie at ca. 50 cm-’ [2]. However, Bauer et al. [29] have recently suggested that this value is much too low and proposed a value of 100 cm- l. If the assignment proposed here HX complexes one component of vp must lie is correct, then for the R@ above 150 cm-’ (the degeneracy of vp is lifted in complexes with ethers or sulphides). l
l
l
l
l
30 ACKNOWLEDGEMENT
We are grateful to Dr. G. Maes for communicating results prior to publication. REFERENCES 1 A. J. Barnes, J. Mol. Struct., 100 (1983) 259. 2 D. J. Millen and 0. Schrems, Chem. Phys. Lett., 101 (1983) 320, and references therein. 3 L. S&river, A. LouteIIier and A. Bumeau, Chem. Phys. Lett., 60 (1979) 471. 4A. Loutellier, L. Schriver, A. Bumeau and J. P. Perchard, J. Mol. Struct., 82 (1982) 165. 5 L. Schriver, A. Louteiher, A. Bumeau and J. P. Perchard, J. Mol. Struct., 95 (1982) 37. 6 L. Andrews, G. L. Johnson and S. R. Davis, J. Phys. Chem., 89 (1985) 1710. 7 B. S. Ault, E. Steinback and G. C. Pimentel, J. Phys. Chem., 79 (1975) 615. 8 G. P. Ayers and A. D. E. Puihn, Spectrochim. Acta, Part A, 32 (1976) 1641. 9A. Schriver, B. Siivi, D. Maillard and J. P. Perchard, J. Phys. Chem., 81 (1977) 2095. 10 L. Andrews and G. L. Johnson, J. Chem. Phys., 79 (1983) 3670. 11 Z. Kisiel, A. C. Legon and D. J. MiIIen, Proc. R. Sot. London, Ser. A, 381 (1982) 419. 12 A. C. Legon and L. C. Willoughby, Chem. Phys. Lett., 92 (1982) 333. 13 Z. Kisiel, A. C. Legon and D. J. Millen, J. Mol. Struct., 112 (1984) 1. 14 A. C. Legon and L. C. Willoughby, Chem. Phys. Lett., 95 (1983) 449. 15 R. Viswanathan and T. R. Dyke, J. Chem. Phys., 77 (1982) 1166. 16 L. C. Willoughby, A. J. Filler-y-Travis and A. C. Legon, J. Chem. Phys., 81 (1984) 20. 17 E. J. Goodwin and A. C. Legon, J. Chem. Sot., Faraday Trans. 2,80 (1984) 51. 18 F. CamovaIe, M. K. Livett and J. B. Peel, J. Am. Chem. Sot., 102 (1980) 569. 19F. CamovaIe, M. K. Livett and J. B. Peel, J. Am. Chem. Sot., 104 (1982) 5334. 20 F. Camovaie, M. K. Livett and J. B. Peel, J. Am. Chem. Sot., 105 (1983) 6788. 21 J. N. S. Kuzniarski, M.Sc. thesis, University of Salford, 1978. 22R. G. Snyder and G. Zerbi, Spectrochim. Acta, Part A, 23 (1967) 391. 23 H. Wieser, W. G. Laidlaw, P. J. Krueger and H. Fuhrer, Spectrochim. Acta, Part A, 24 (1968) 1055. 24R. K. Thomas, Proc. R. Sot. London, Ser. A, 322 (1971) 137. 25 J. R. Allkins and P. J. Hendra, Spectrochim. Acta, 22 (1966) 2075. 26M. Ohsaku, Y. Shiro and H. Murata, Bull. Chem. Sot. Jpn., 45 (1972) 956. 27 K. Morokuma, Act. Chem. Res., 10 (1977) 294. 28 G. Maes, personal communication. 29 S. H. Bauer, T. Yamazaki, K. I. Lazaar and N.-S. Chiu, J. Am. Chem. Sot., 107 (1985) 743.