FT-IR Investigation of Polarizable, Strong Hydrogen Bonds in Sulfonic

Roland Langner and Georg Zundel. The Journal of ... Ho Bum Park , Benny D. Freeman , Zhong-Bio Zhang , Mehmet Sankir , James E. McGrath. Angewandte ...
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12214

J. Phys. Chem. 1995,99, 12214-12219

FT-IR Investigation of Polarizable, Strong Hydrogen Bonds in Sulfonic Acid-Sulfoxide, -Phosphine Oxide, and -Arsine Oxide Complexes in the Middle- and Far-Infrared Region Roland Langner and Georg Zundel* Institute of Physical Chemistry, University of Munich, Theresienstrasse 41, 0-80333 Munich, Germany Received: March 30, 1995; In Final Form: May 23, 1995@

Ten 1:1 methanesulfonic acid-oxide systems are studied in acetonitrile-chloroform (2:1) solutions in the middle-infrared (MIR) and far-infrared (FIR) regions at 20 and -40 "C as a function of the basicity of the oxide. An IR continuum demonstrates that first a proton potential with a relatively narrow single minimum at the acid is present. This minimum shifts with increasing basicity in the direction of the oxide and becomes much broader. The proton polarizability is largest with the most symmetrical systems. With a further increase of the basicity, the minimum approaches the base and becomes narrow again. This shift is studied considering the SO stretching vibration bands. Furthermore, it is concluded from the splitting of these bands with increasing polarity of the complexes that these complexes associate via dipole-dipole forces. This is confirmed by osmometric measurements. The observed hydrogen bond vibrations differ largely from hydrogen bond stretching vibrations, yo, and have very difficult vibration character. With the asymmetrical systems the bands of the hydrogen bond vibrations are relatively narrow. With the more or less symmetrical systems the continua, caused by the hydrogen bonds with large proton polarizabilities, extend in the FIR region and the hydrogen bond vibration broadens extremely. This broadening effect of the hydrogen bond vibration should be explained by theoretical studies.

TABLE 1: Abbreviations of the Investigated Systems

1. Introduction Hydrogen bonds in sulfonic acid-oxygen base systems were already studied in the MIR region as a function of the ApKa (pKa of the protonated oxygen base minus PKa of the acid).' The most important result of these investigationswas that broad flat single-minimum proton potentials are present in these hydrogen bonds, whereby the well of these potentials shifts from the acid to the oxygen base with increasing basicity of the oxide. In the mean time, systems with double minimum proton potentials were sudied in the MIR as well as in the Those systems are found in a region of much higher pKa values. The most important result was that in phenol-trimethylamine oxide complexes the hydrogen bond vibration shifts with increasing acidity of the phenol first toward higher and after a sharp maximum toward lower wavenumbers. The hydrogen bond vibration is always relatively sharp with exception of the most symmetrical system (3,4-dinitrophenol), in which it is slightly broadened. Furthermore, with these systems the position of this band is almost independent of a variation of the mass of the phenols. This result was explained by the assumption that these hydrogen bonds are strongly bent. This was recently justified by theoretical treatments: In contrast, with homoconjugated N+H* *N* N* .H+Nbonds in pyridinium-pyridine complexes the continuum is independent of the pKa of the pyridines and the change of the hydrogen bond vibration vo is caused only by'the change of their mass.5 In this study a larger number of methanesulfonic acidsulfoxide systems were studied in the MIR as well as in the FIR, i.e., systems in a much lower pKa region. This study is completed by two phosphine oxide and one arsine oxide systems. Of main interest was the behavior of the hydrogen bond vibration in the FIR region, compared with the corresponding behavior in systems with double minimum proton potentials. @

Abstract published in Advance ACS Abstracts, July 1, 1995.

system (abbreviation) 1. MSA + DPhSO 2. MSA + DpToSO 3. MSA TPhPO 4. MSA + MPhSO 5. MSA + DBzSO 6. MSA + DMSO 7. MSA + DBSO 8. MSA + DtBSO 9. MSA + TBPO

+

10. MSA

+ TPhAsO

full name of the base diphenyl sulfoxide di-4-tolyl sulfoxide triphenylphosphine oxide methylphenyl sulfoxide dibenzyl sulfoxide dimethyl sulfoxide dibutyl sulfoxide di-tert-butyl sulfoxide tributylphosphine oxide triphenylarsine oxide

2. Results and Discussion 2.1. Middle-Infrared Region. In Table 1 all investigated systems with the corresponding abbreviations are given. In all cases 0.5 mol dm-3 1:l acid-base mixtures in an acetonitrilechloroform solution (2: 1) were studied. The spectra were taken from the solutions at 20 and at -40 "C. In Figure 1 six selected systems are shown in the whole MIR region. Only heteroconjugated 1:l complexes are formed.' The pK, value of the protonated oxygen increases within this series of systems. The degree of complex formation is, however, not complete with all systems. It can be estimated considering the band of the v(0H) vibration of the associated methanesulfonic acid molecules at about 2980 cm-I. With the first system the complex formation is incomplete at 20 "C, but much improved with decreasing temperature (Figure la). In the next systems the complex formation becomes more and more complete (Figure lb,c). In system 6 the complex formation is complete at -40 "C (Figure IC). In the following systems the complex formation is already complete at 20 "C. The proton polarizability6-8 of the hydrogen bonds is indicated by the IR continua observed in Figure 1. In system 1 already a weak continuum is found (Figure la). Its intensity increases with increasing complex formation and is observed only in the region 3000-700 cm-', not extending into the FIR region (Figure 7a). The proton fluctuates in a relatively narrow

0022-365419512099-12214$09.00/0 0 1995 American Chemical Society

Polarizable, Strong Hydrogen Bonds

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single minimum proton potential well, located near the acid molecule. In the following systems the intensity of the continuum increases strongly in the region 1500-500 cm-I, whereas its intensity decreases at higher wavenumbers (Figure lb-d). In addition, it extends more and more toward smaller wavenumbers (Figure 7). All these results taken together prove that the well of the proton potential shifts more and more to the middle of the hydrogen bond and becomes much broader with further increasing basicity of the oxygen base.' In system 9 the opposite change of the continuum becomes visible, the intensity of the continuum increases slightly at higher wavenumbers and decreases a little at lower ones (Figure le). The most symmetrical case is passed over. The proton potential is already narrower and shifted in the direction of the oxide. In the arsine oxide system (system 10) the continuum and hence the proton polarizability has vanished to a large extent (Figure 10. The proton is now present in a relatively narrow potential well at the 0 atom of the arsine oxide. In Figure 2 systems are shown in the region 1400-800 cm-' with extended wavenumber scale. Finally, in Figure 3 the region 600-400 cm-] is given. The most important data are summerized in Table 2. In the following we compare the complexes at -40 "C, since the complex formation is much more complete at lower temperatures. With system 1, due to hydrogen bond formation, the antisymmetrical SO2 stretching vibration is shifted, compared

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to the acid, from 1359 to 1304 cm-l and the symmetrical one from 1176 to 1167 cm-I. In this case -S020H groups are present since the proton is located at the acid. With increasing shift of the proton potential well to the oxide, these groups change continuously to -SO3- groups and the two bands become antisymmetrical stretching vibrations of these groups. Two vas(S03-) vibrations are observed, since degeneracy of the antisymmetrical stretching vibration of the -SOs- groups is removed. The C3" symmetry of these groups is strongly disturbed by the hydrogen bond formation and thus the degeneracy of this vibration is lifted. Of course, the transition from one to the other situation is continuous within this series of complexes. The fact that both bands show doublet character with increasing polarity indicates that dimerization of the complexes occurs. This result will be discussed later. The v(S-OH) band of the -SO*OH group in the hydrogen bonds is found with system 1 at about 894 cm-' (Figure 2a). It broadens strongly in the series of the following complexes and may couple with other transitions. It vanishes as soon as the most symmetrical cases are reached. If the proton potential well approaches the oxide the vs(S03-)vibration is observed. It is finally shifted in complex 10 to 1024 cm-' (Figure 20. In the most symmetrical cases no assignment is possible, since the continuum is very intense and the SO stretching vibration of the sulfoxide molecules is also observed in this region. In addition the v(S-0) band of the sulfoxides may couple with the corresponding SO stretching vibration band of the acid or

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Figure 2. FT-IR spectra of 1:1 MSA-oxide base mixtures in acetonitrile-chloroform (2:1) at 20 "C (- - -) and at -40 "C (-) and pure base in acetonitrile at room temp. (- -) in the region 1400-800 cm-I, concentration 0.5 mol dm-3, layer thickness 100 pm: (a) MSA DPhSO, DPhSO; (b) MSA + TPhPO, TPhPO (saturated solution); (c) MSA + DBzSO, DBzSO (saturated solution); (d) MSA + DMSO, DMSO; (e) MSA TBPO, TBPO; (f) MSA TPhAsO, TPhAsO (saturated solution).

+

+

anion, respectively, in this region. A further complication is caused by the w(CH3) vibration? which occurs also in this region. The rocking vibration of the >SO:! groupg is found in the methanesulfonic acid at 502 cm-I. Beginning with complex 4, a shoulder arises at slightly higher wavenumbers (Figure 3b). If the basicity of the oxygen base becomes larger, finally only the band at higher wavenumbers is observed. This second band which arises with increasing polarity of the complexes, is caused by dimerization of the complexes via dipole-dipole forces. This result is confirmed by the temperature dependence of this band pair in Figure 3. This association process can also be studied considering the region of the SO stretching vibration bands in the region 13101240 cm-' in Figure 2. Beginning with system 3 a noticeable shoulder arises at lower wavenumbers at the band at 1295 cm-I. Within the series of systems the intensity of this shoulder increases. In system 6 it is observed as a band at 1267 cm-I. With further increasing polarity this band becomes more intense and the band at higher wavenumbers vanishes. With all systems the band at smaller wavenumber becomes more intense with decreasing temperature. An analogous effect indicating the association of the complexes is observed with the SO stretching vibration band in the region 1180-1080 cm-' in Figure 2. The assumption that with increasing polarity of the complexes dimerization occurs is confirmed by osmometric measurements. The results are shown in Figure 4. The osmometric value is a

+

measure of the number of particles in the solution, Le., it increases in proportion to the number of particles. The curve with the dots in Figure 4 is the pure dimethyl sulfoxide (DMSO) solution in chloroform. As expected, a linear relation is observed. The curve with squares are 1:1 solutions of MSADMSO. The slope of this curve decreases with increasing concentraction, indicating that the number of particles is much less than in the pure DMSO solution. Thus, with increasing concentration the complexes associate more and more. Figure 5 shows the IR spectra of 1:1 MSA-DMSO solutions at different concentrations and temperatures in chloroform. A comparison of the dotted spectrum with the dashed and solid one shows that at low concentrations the band at about 1165 cm-' of the nonassociated complexes is still intense, whereas the band of the associated complex at about 1100 cm-' is very weak. The same is valid for the band at about 1300 cm-I of the nonassociated complex, whereas the band at lower wavenumbers is not yet visible, caused by the incompensation region of a chloroform band. At higher concentrations the bands of the associated complexes become stronger and at -40 "C (solid line) these absorptions will be dominant. An analogous course is shown in the region of the r(SO2) bands of the nonassociated (%SO0 cm-l) and associated ( ~ 5 2 cm-I) 0 complexes. These measurements as well as the osmometric measurements were performed in pure chloroform. The comparison of these results with those obtained with the acetonitrile-chloroform solutions shows almost the same association tendency.

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Polarizable, Strong Hydrogen Bonds

TABLE 2: Data of the Most Important SO Bands in MSA, TBAfMSA- (TetrabutylammoniumMethanesulfonate) and the Investigated Systems MSA Oxygen Bases (rt = Room Temperature, sh = Shoulder)

+

H3CS020H *OS(P,As)R2(3)

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0

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500 t50 Wavenumber cn-l

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to0

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0. MSA 1. MSA 2. MSA 3. MSA 4. MSA

+ DPhSO + DpToSO + TPhPO + MPhSO 5. MSA + DBzSO 6. MSA + DMSO 7. MSA + DBSO 8. MSA + DtBSO 9. MSA + TBPO 10. MSA + TPhAsO

1308 1306 1304 1302

1359 (rt) 1304 1302 sh 1295 sh 1298

+

+

1298+sh

+ +

1294 sh 1291 sh 1288 sh sh 1267 1250

+

+

11. TBA+MSA-

400

1176 (rt) 1167 1165 masked 1163 ~1090 1294+sh 1166+X1100 1165+ ~1090 sh 1267 ~ 1 1 6 5 1106 1101 s h + 1267 ~ 1 1 6 5 1111 1110 1093 sh 1267 1096 masked sh 1261 masked 1138 1139 1248 1205 (rt)

+

+

1171 1169 sh masked sh 1167

+

+ +

+ +

+

vas(SO3-) [cm-'I (in the complex degeneracy lifted) H3CS03-a .H+OS(P,As)R2(3)

.

H3CS020H GS(P,As)R2(3, v,(S-OH) [cm-'1 20°C

system

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Figure 3. FT-IR spectra of 1:l MSA-oxide base mixtures in acetonitrile-chloroform (2:l) at 20 "C (- - -) and at -40 "C (-) and pure base in acetonitrile at room temp. (- -) in the region 600-400 cm-', concentration 0.5 mol dm-3, layer thickness 100 pm: (a) MSA DPhSO, DPhSO; (b) MSA MPhSO, MPhSO; (c) MSA DMSO, DMSO; (d) MSA DtBSO, DtBSO; (e) MSA TBPO, TBPO; (f) MSA TPhAsO, TPhAsO (saturated solution).

+

+

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+

2.2. Far-Infrared Region: The Hydrogen Bond Vibration. The wavenumbers of the hydrogen bond vibration was calculated and assigned with the semiempirical MNDO/H method. This method is particularly suitable for hydrogenbonded systems and the comparison with the spectra shows that this procedure is very helpful for the assignment of the FIR bands. One example for the nature of the hydrogen bond vibration of these complexes is shown in Figure 6 for the MSADMSO system. Figure 6 demonstrates that with these systems the hydrogen bond vibration deviates strongly from a pure hydrogen bond stretching vibration v m Often more than one hydrogen bond vibration is observed. In the case of system 6 the respective calculation was also performed ab initio with a 3-21G* basis set which yielded similar results. Therefore, the situation is much more complex as in the phenol-amine and in the pyridinium-pyridine system^.^-^ Furthermore, it must be mentioned that with regard to the discussion above the transition moments of the hydrogen bond vibration deviate more or less from the direction of the transition moment of the proton transition. Besides the assignment of the hydrogen bond vibration via MNDO/H calculations, the temperature effect is particularly conclusive. With decreasing temperature the hydrogen bond vibrations shift toward higher wavenumbers and become more intense since with decreasing temperature the hydrogen bonds

0. MSA 1. MSA DPhSO 2. MSA DpToSO 3. MSA TPhPO 4. MSA MPhSO 5. MSA DBzSO 6. MSA DMSO 7. MSA DBSO 8. MSA DtBSO 9. MSA TBPO 10. MSA TPhAsO 11. TBA+MSA-

+ + + + + + + + + +

-40°C

882 (rt) 890 894 890 894 90 1 902 889 890 886 89 1 880 sh sh masked sh masked masked sh ( ~ 9 8 5 ) 1022 1024 1040 (rt)

r(S02) [cm-'1

20°C

-40°C

502 (rt) 502 502 502 502 masked masked 502 sh 502 502 sh 502 sh 502 516 503 sh 502 514 503 511 sh 515 sh 515 sh 518 sh 522 522 526 (rt)

+ + + + + +

+ + + +

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.-2

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E

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0 0.0

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concentration [mol dm-3)

+

Figure 4. Osmometric value of system 6 (MSA DMSO) in chloroform at 40 "C, plotted as a function of the concentration. become stronger. These effects are particularly pronounced for all systems below the most symmetrical systems. In Figure 7 the far infrared spectra of all systems are shown. The spectra at 20 "C are drawn with dashed lines and those at -40 "C are drawn with solid lines. For comparison the spectra of the pure base are given additionally as dotted lines. In Table 3 the hydrogen bond vibrations and especially their band shapes are summarized. In the most asymmetrical systems 1, 2, 3, and 10 the continuum does not extend to the FIR region and the proton

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Figure 5. FT-IR spectra of 1:1 MSA-DMSO mixtures in chloroform at 20 "C and 0.1 mol dm-' (- -), at 20-"Cand 0.5 mol dm-3 (- - -) and at -40 "C and 0.5 mol dm-3 (-), layer thickness 100 pm: (a) spectral region 1400-800 cm-I; (b) spectral region 600-400 cm-I.

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e ( Figure 6. Graphical representation of a hyrogen bond vibration of system 6 (MSA method.

+ DMSO), calculated with the semiempirical MNDO/H

polarizability of the hydrogen bonds is relatively small. With all these asymmetrical systems the hydrogen bond vibration is a relatively narrow band. With the more symmetrical systems, the IR continuum extends toward smaller wavenumbers in the FIR region and the proton polarizability of the hydrogen bonds is larger. The hydrogen bond vibrations are strongly broadened and with the most symmetrical systems, 7-9, the hydrogen bond vibration merges almost completely in the continuum at 20 "C. Only with the systems at -40 "C can the hydrogen bond vibration

$50 $00 350 300 250 200 150 Wavenumber cmd

Figure 7. FT-IR spectra of 1:l MSA-oxide base mixtures in acetonitrile-chloroform (2:l)at 20 OC (- - -) and at -40 "C (-) and pure base in acetonitrile-chloroform (2: 1)at room temperature (- -) in the region 450-150 cm-', concentration 0.5 mol dm-3, layer thickness 100 pm: (a) MSA DPhSO, DPhSO; (b) MSA DpToSO, DpToSO; (c) MSA TPhPO, TPhPO; (d) MSA MPhSO, MPhSO; (e) MSA +DBzSO, DBzSO; (f) MSA DMSO, DMSO; (8) MSA DBSO, DBSO; (h) MSA DtBSO, DtBSO; (i) MSA TBPO, TBPO; (i)MSA TPhAsO, TPhAsO.

+

+

+

+

+

+

+

+

+

be slightly noticed. The result that in some systems association equilibria are present may contribute to this broadening but cannot explain this extreme effect. It is, however, of main importance that with all systems in which the hydrogen bonds

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Polarizable, Strong Hydrogen Bonds

J. Phys. Chem., Vol. 99, No. 32, 1995 12219

TABLE 3: Data of the Hydrogen Bond Vibration (Yhb) of the Investigated Systems MSA Oxygen Bases (sp = Sharp, br = Broad, v br = Very Broad, ex br = Extreme Broad, n vis = Not Visible)

+

H3CSOzOH * OS(P,As)R2(3, Vhb

system 1. MSA 2. MSA 3. MSA 4.MSA 5 . MSA 6. MSA 7 . MSA 8. MSA 9. MSA 10. MSA

+ DPhSO + DpToSO + TPhPO + MPhSO + DBzSO

+ DMSO + DBSO + DtBSO + TBPO + TPhAsO

20°C

["I'

band shape -40°C

2OoC

-4OOC

245 228 215 243 masked s210 =200 n vis Z200 200

250 sp SP 234 sp SP 218 sp SP 25 1 br br ~ 2 3 5 masked vbr ~ 2 3 0 vbr br ~ 2 3 0 ex br v br n vis n vis n vis ~ 2 0 5 vbr br 201 sp SP H,CSO3-* * .H+OS(P,As)R2(3,

show particularly large proton polarizability, this broadening is strongest. This has to be taken into account in a theory explaining this extreme broadening effect.'" In the phenoltrimethylamine oxide system^,^ with the most symmetrical system (3,4-dinitrophenol) the continuum is also observed in the FIR region and the hydrogen bond vibration is slightly broadened. This effect may have the same reason.

3. Experimental Section The substances were purchased from Fluka, Aldrich, and Merck. In each case, substances with the highest available purity were used. The methanesulfonic acid was dried with a well-defined excess of its anhydride. The solid oxides were dissolved several times in dry ethanol, and the solvent was removed. Dimethyl sulfoxide and the solvents were dried with 3 8, molecular sieve. Substances with minor impurities were recrystallized or sublimed at Torr. Di-tert-butyl sulfoxide was synthesized by oxidation of ditert-butyl sulfide with hydrogen peroxide at room temperature. It was purified by extraction with a sulfuric acid (50%)-ether mixture. The sulfoxide remains in the phase of the sulfuric acid and was after neutralization reextracted with ether. This procedure was repeated several times. All preparation and transfers of the solutions were performed in a water-free glovebox. The concentration of the donors and acceptors in the solution was 0.5 mol dm-3. For the IR investigations, cells with silicon windows were used. Because of the high reflectivity of this material, wedgeshaped layers were applied to avoid interference pattems superposed on the spectra. The mean layer thickness was 100 pm. The spectra of the samples were taken with a FT-IR spectrometer (Bruker IFS 113v). In the M R region a DTGS detector was used (resolution 4 cm-I). In the FIR region a Hecooled bolometer was used (resolution 1 cm-I). The solvent bands were subtracted. In the regions of the strongest solvent bands, the energy loss was too high to obtain any information. For the temperature-dependentmeasurements an ultracryomat (Lauda K 120 W) was used. The osmometric measurements were performed with a Knauer vapor pressure osmometer.

4. Conclusions In the series of the MSA-oxide systems with increasing basicity of the oxide, first the IR continuum becomes more intense in the region 1500-500 cm-', whereas at higher wavenumbers its intensity decreases. Furthermore, it extends more and more toward smaller wavenumbers, Le., in the FIR region. This behavior is most pronounced in the most symmetrical systems. Passing over these systems, the shape of the continuum undergoes the opposite change. This demonstrates that the proton is first located in a relatively narrow singleminimum proton potential near the acid. Then the well of this potential shifts in the direction of the oxide and becomes much broader. In the most symmetrical systems the proton polarizability is largest. With further increasing basicity of the oxide the proton potential well shifts to the oxide and becomes narrower. The proton polarizability decreases again. The SO stretching vibration bands of the acid demonstrate that in the series of systems the -SO;?OH groups change continuously to -SO3- anions. A splitting of the SO bands with increasing polarity of the complexes shows an increasing association of the complexes via dipole-dipole forces. This result is confirmed by observations on the r(SO2) bands and especially by osmometric measurements. It is shown that the hydrogen bond vibrations in the FIR region are not pure hydrogen bond stretching vibrations. The assignment of these bands is realized by MNDO/H calculations and is confirmed by their shift and intensity increase with decreasing temperature. With the asymmetrical systems 1-3 and 10 relatively sharp bands of the hydrogen bond vibration are observed. In the case of the more symmetrical systems the continuum extends toward smaller wavenumbers in the FIR region. In these systems the bands of the hydrogen bond vibration broaden extremely. A theoretical treatment of this broadening mechanism must take into account that this extreme broadening occurs with those systems showing the largest proton polarizability of the hydrogen bond. Acknowledgment. Our thanks are due to the Deutsche Forschungsgemeinschaft and the Fonds der Deutschen Chemischen Industrie for their support of this work. I am grateful to Mr. S . Geppert for yielding Figure 6. References and Notes (1) Bohner, U.;Zundel, G. J. Phys. Chem. 1985, 89, 1408. (2) Brzezinski, B.; Brycki, B.; Zundel, G.; Keil, T. J. Phys. Chem. 1991, 95, 8598. (3) Keil, T.; Brzezinski, B.; Zundel, G. J. Phys. Chem. 1992, 96,4421. (4) Rabold, A.; Zundel, G. J. Phys. Chem., in press. (5) Rabold, A,; Bauer, R.; Zundel, G. J. Phys. Chem. 1995, 99, 1889. (6) Eckert, M.; Zundel, G. J. Phys. Chem. 1987, 91, 5170. (7) Zundel, G. In Trends in Physical Chemistry;Menon, J., Ed.; Publ. Research Trends: Trivandrum, 1992; Vol. 3, p 129. (8) Zundel, G.; Brzezinski, B.; Olejnik, J. J. Mol. Srrucr. 1992, 300, 575. (9) Chakalackal, S.; Stafford, F. J. Amer. Chem. SOC. 1966, 88, 4815. (10) Geppert, S.; Rabold, A,; Eckert, M.; Zundel, G. J. Phys. Chem. 1995, 99, 12220. Jp9.509222