1408
J. Phys. Chem. 1985,89, 1408-1413
Proton Potentials and Proton Polarizability of Hydrogen Bonds in Sulfonic Acid-Oxygen Base Systems as a Function of the ApK, Ulrich Bohner and Georg Zundel* Institut fur Physikalische Chemie der Universitat Miinchen. 0-8000Miinchen 2, West Germany (Received: August 1, 1984)
1:1 mixtures of sulfonic acids with oxygen bases as, for instance, sulfoxides, phosphinoxides, and N-oxides, are studied in acetonitrile solution by IR spectroscopy. Only a negligible amount of charged species occurs in the solutions. With the exception of one system nearly completely 1:l acid-base complex formation occurs. The SO stretching vibration bands demonstrate that single minimum proton potentials are present in the A-...H+-B bonds. In these bonds the minimum of the proton potential shifts with increasing ApK, (pK, of the protonated base minus pK, of the acid) from the acid to the base molecule. These proton potentials are with acetonitrile solutions on the average symmetrical at about ApK, = 0.8. The IR continua indicate that these hydrogen bonds still show considerable proton polarizability, hence the proton potential well is still so broad and flat that the protons can easily be shifted within these hydrogen bonds. This is the case if these proton potentials are not too asymmetrical.
Introduction
Oxygen bases as, for instance, sulfoxides, phosphinoxides, or N-oxides, form complexes with hydrogen bond donors. The hydrogen bonds formed are relatively strong although the basicity of these oxygen bases is weak. The stability of phenol-oxygen base complexes was studied by Gramstad et al.,'" whereas Hadii et al.'-13 investigated the IR spectroscopic behavior of carboxylic acid oxygen base systems. Studies of carboxylic acid N-oxide systems were performed in Szafran's group"17 with various methods, and Grundwald et al.'* investigated halogenated acetic acids with triphenylarsinoxide. In the trifluoroacetic acid + N-oxide systems broad flat single minimum proton potentials are present in the O--H+.-ON hydrogen bonds.17J9-20This result suggests that if the acid and the protonated base have relatively high pK, values, double minimum proton potentials are observed in the AH-B +A--H+B hydrogen bonds. If, however, the pK, values become lower the proton potentials of the A--H+-B bonds change to broad single minima. In systems with intermediate pK, values (various carboxylic acids trimethylaminoxide), the proton potentials observed in the case of acetonitrile solutions change with increasing ApKa (pK, of trimethylaminoxide minus the pK, of the carboxylic acids) from a broad single minimum to a double minimum.21
+
+
+
( I ) Gramstad, T. Acta Chem. Scand. 1961, 15, 1337. (2) Gramstad, T.; Fuglevik, W. J. Acta. Chem. Scand. 1962, 16, 1369. (3) Gramstad, T. Spectrochim. Acta 1963, 19, 491. (4) Gramstad, T. Spectrochim. Acta 1963, 19, 829. (5) Gramstad, T. Spectrochim. Acta 1963, 19, 1363. (6) Gramstad, T. Spectrochim. Acta 1966, 22, 1681. (7) Hadii, D.; Kobilarov, N. J . Chem. SOC.A 1966, 439. (8) Hadii, D. J . Chem. SOC.1962, 5128. ( 9 ) Hadii, D.; Rajnvajn, J. J . Chem. Soc., Faraday Trans. 1 1973,69, 151. (10) Hadii, D. J . Chem. SOC.A 1970, 418. (11) Hadii, D.; Ratajczak, H.; Sobczyk, L. J . Chem. SOC.A 1967, 48. (12) Hadii, D.; Klofutar, C.; Oblak, S. J . Chem. Sot. A 1968, 905. (13) Detoni, S.; Hadii, D.; Smerkolj, R.; Hawranek, J. P.; Sobczyk, L. J . Chem. SOC.A 1970, 28s 1. (14) Brycki, B.; Dega-Szafran, 2.;Szafran, M . Adv. Mol. Relaxation Interact. Processes 1979, 15, 71. (15) Brycki, B.; Dega-Szafran, 2.;Szafran, M. Pol. J . Chem. 1980, 54, 221. (16) Brycki, B.; Szafran, M. J . Chem. Soc., Perkin Trans. 2 1982, 1333. (17) Brycki, B.; Szafran, M. J . Chem. Soc., Perkin Trans 2 1984, 223. (1 8) Grundwald, M.;Szafran, M.;Rychlewski, J. Pol. J . Chem. 1979, 53, 829. (19) Kreevoy, M. M.; Chang, K. J . Phys. Chem. 1976, 80,259. (20) Bohner, U.; Zundel, G., J . Chem. Soc., Faraday Trans. I , in press.
0022-3654/85/2089-1408$01 S O / O
cm-') at 298 K of the TABLE I: Specific Conductivities I , (lo4 Svstems 0.1 M MSA Various Oxveen Bases in CHKN'
+
MSA + Ph2SO MSA + Ph3PO MSA + Me2S0 MSA + Bu~SO MSA + 2-ClPyNO MSA + 4-CO2CHpPyNO MSA + 3-CHSPyNO MSA + Ph3AsO
-3.19' -2.10' -1.80'
-1.27
-1.47' -0.81d
-0.18 0.12 0.45
1.12 7.95
0.09 0.27
7.80
0.11 0.96
1.11
8.45 9.14
~ 0 . 4 1 ~ 1.51 0.921 2.84 0.99' 2.91
6.66 6.00 8.79
0.17
0.16 0.14 0.33
,ISpecific conductivity of pure CH$N is 0.5 1 X 10" s2-I cm-I. The specific conductivity 0.1 M MSA in CH3CN is 1.09 X lo4 Q-l cm-I. From ref 33. 'From ref 34. From ref 35. From ref 36. 'From ref 37.
Very large proton polarizabilities of the hydrogen bonds are observed which are caused by the proton motion in double minimum proton potential^.^^-^^ They are indicated by continua in the infrared ~ p e c t r a .If~the ~ ~proton ~ ~ fluctuates in a broad flat single minimum, large proton polarizabilities are still observed23 but these polarizabilities are slightly smaller compared with the double minimum cases;23 herewith, of course, systems with a similar degree of symmetry must be compared since the proton polarizability strongly depends on the degree of symmetry of the hydrogen bonds?3 With oxygen bases not only trifluoroacetic acid, but also other strong acids, should form hydrogen bonds with broad flat proton potentials. Hence, the following sulfonic acid oxygen base systems are studied in acetonitrile. In the following investigation, the pK, values of the sulfonic acids from ref 26 and 27 are used. They differ very much from earlier given values and are probably much more correct.
+
Results and Discussion The IR spectra of 1:l acid-base mixtures were taken. The following systems were studied: three sulfonic acids having pK, values of -1.92, -2.80, and -4.00 with the acceptor triphenylphosphinoxide (TPPO) (spectra in Figure l), and methanesulfonic acid (MSA, pK, = -1.92) with nine oxygen bases with pK, values (21) Bohner, U.; Zundel, G., in preparation. (22) Weidemann, E. G.; Zundel, G. 2.Naturforsch. A 1970, 95A, 627. (23) Janoschek, R.; Weidemann, E. G.; Pfeiffer, H.; Zundel, G. J . Am. Chem. SOC.1972, 94, 2387. (24) Zundel, G. In "The Hydrogen Bond Recent Developments in Theory
-
and Experiments"; Schuster, P., Zundel, G., Sandorfy, C., Eds.; NorthHolland: Amsterdam, 1976; Vol. 11, S.683 ff. (25) Lindernann, R.; Zundel, G. J . Chem. SOC.,Faraday Trans. 2 1977, 73, 788. (26) Covington, A. K.; Thompson, R. J . Solution Chem. 1974, 3, 603. (27) Guthrie, .I.P. Can. J . Chem. 1978, 56, 2342. (28) Burger, H.; Burczyk, K.; Blaschette, A. Monarsh. Chem. 1970, 101, 102.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1409
H-Bonding in Sulfonic Acid-Oxygen Bases
1.
35003oO0250020#)Mww)16001u)(l
1200
m
800
m200
600
wave number cm'l
Figure 1. IR spectra of different sulfonic acids (- -) and triphenylphosphinoxide (TPPO) (-- -) and the respective acid-base complex in CD,CN, C6H5S020H concentration 0.3 mol dm-', layer thickness 96 pm: (a) CH3S020H(---), TPPO (---),CH3S020H+ TPPO (-); (b) C6H5SO20H + TPPO (-); (C) 4-NO2C6H4SO2OH (-*-), 4-NO2C6H4SO2OH + TPPO (-). (-e-),
of the protonated bases between -3.19 and +4.65 (examples of spectra in Figure 2). Species in the Solutions. To obtain information on the hydrogen bonds in the acid-base complexes it is necessary to investigate the appearance of all species which could be present in the solutions and of their concentrations. The following equilibria must be taken into account:
~
OH
+0
2O-...H+...O
0-+ H+O
[OHwO- + -O-.HO]
+ OH+O
+ [O+H.-O
O***H+O]
The presence of charged species is estimated by conductivity and IR measurements. The results are given in Table I. It shows that with solutions of 1:1 donoracceptor mixtures the conductivity increases by about 1 order of magnitude compared with solutions of pure acceptors. The conductivity of the pure acid solution amounts to 1.09 X lo4 f2-l cm-'. This relatively high conductivity is caused by a certain degree of dissociation of the acid. To estimate the quantities of charged species, Figure 3 shows the IR spectra of acetonitrile-d3 solutions of Bu4N+CH3S03-(- --). For comparison, the spectrum of the pure acid is shown (-.-.). The salt shows characteristic bands at 1204 (doublet, if degeneracy is removed), 551, and 525 cm-'. These bands are not observed in the acid solutions. Thus, the concentration of free anions A- in the acid is below the IR-spectroscopically observable limit. To estimate whether homoconjugated species are present in the 1:l solutions, in Figure 3 (-) the spectrum of an acetonitrile-d3 solution of a 1:l mixture of Bu4N+CH3S03-and CH3S03His
shown. Characteristic bands of the charged sulfonic acidsulfonate groups are found a t 958 and at 737 cm-l. Also these bands are not observed in the spectrum of the solution of the pure acid (Figure 3, ---.). Furthermore, all these bands are also not observed in the spectra of the 1:l acid-oxide mixtures in Figures 1 and 2. Hence, IRspectroscopically observable amounts of homoconjugated species can be excluded. All these results taken together show that the increase of conductivity of the acid-base solutions compared with the pure base solutions, is caused by a concentration of charged species which is below the IR-spectroscopically detectable limit. The conductivities of the systems studied here are comparable with the conductivities of RCOOH trimethylaminoxide systems. With these systems it could be estimated that the charged species is below 3%.21 Thus, also with the systems studied here the charged species is below 2% since the specific conductivities of these compounds should be similar to the systems studied in ref 21. Thus charged species can be neglected and the considerations can be limited to K, and to the nature of the O---H+--O hydrogen bonds present in the acid-base complexes. Formation of A--.H+43 Complexes. To obtain information on the formation of the 1:l complexes we have to clarify whether monomeric or dimeric acid is still present in the solutions: Sulfonic acid dimers show a broad band with maximum in the 3000-2800-cm-' region and a second band a t about 2400 cm-l. The band at higher wavenumbers is the OH stretching vibration and the band at about 2400 cm-' is assigned to the 26(OH)
+
1410 The
and Zundel
Figure 2. IR spectra of methanesulfonic acid (MSA) (-- -) and various oxygen bases (- - -) and the respective acid-base complexes in CD,CN; concentration 0.3 mol dm--’, layer thickness 96 prn: (a) MSA (---), (C6H5),S0 (---),MSA (C6HS)2S0(-); (b) TPPO (-- -), MSA TPPO (-); (c) 2-CIPyNO (- - -), MSA 2-CIPyNO (-); (d) 3-CH3PyN0 (- - -), MSA 3-CH3PyN0 (-); (e) trimethylaminoxide (- - -), MSA trimethylaminoxide (-).
+
+
+
+ +
The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1411
H-Bonding in Sulfonic Acid-Oxygen Bases
Fisure 3.
IR spectra of CH3S020H (---), Bu4N+CH3SO lo4 dm3 Hence, only in the MSA (C6H5)$0 system must incomplete complex formation be taken into account. K, was estimated by comparison of the intensity of this band in the 1:l mixture with that in the pure acid. With this system K, = 3.6 dm3 mol-' is obtained for the 0.3 M MSA
.-.,
-e--).
+
+
+
+
-
(29) Zundel, G. "Hydration and Intermolecular Interaction Infrared Investigations with Polyelectrolyte Membranes"; Academic Press: New York, (1969); Mir Moscow, 1972. (30) Chackalackal, S. M.; Stafford, F. E. J . Am. Chem. SOC.1966, 88, 4815.
TABLE 111: SO Bands in the Systems Methanesulfonic Acid (MSA) t Various Oxygen Bases in CH,CNa -u, cm" system APK. not evaluated (incomplete (1) MSA + Ph,SO -1.27 complex formation) 894 (2) MSA + Ph,PO -0.18 1330 1177 95 6 0.1 2 1294 1128 (3) MSA t Me,SO 980 (4) MSA t Bu,SO 0.45 1287 1115 962 (5) MSA t 2-ClPyNO 1.11 1272 1094 (6) MSA t 4-C02CH,PyN0 1.51 1293 1112 masked (7) MSA t 3-CH3PyN0 2.84 1258 1122 994 2.91 1251 1136 1007 (8) MSA t Ph,AsO 6.57 1245 1154 1023 (9) MSA t (CH,),NO For assignment see text.
+ (C6H5)$0
(AH:B = 1:l) solution in acetonitrile a t 25 O C . Proton Potential in A; -p.B Bonds. If in the systems studied double minimum proton potentials would be realized, bands of both proton-limiting structures AH-B + A--H+B should be observed simultaneously in the spectra in all cases in which the systems are largely ~ y m m e t r i c a l . ~Thus ~ - ~ ~in one and the same system the bands caused by -S020H and by -SO3- should be observed. Examples of bands observed with such acids and salts are given in Table 11. Furthermore, in this table tht characteristic SO vibrations of the homoconjugated -S03;..H+---OS3- grouping are given. For methanesulfonic acid the -S(=O)20H group shows vas(SO,) a t 1367 cm-' and v,(SOz) at 1177 cm-' and finally v(S-(OH)), the stretching vibration of a bond with single bond character, at 883 cm-'. The -S(-O)3- ion of this acid shows a degenerate antisymmetrical stretchhg vibration v, at 1204 cm-' and a symmetrical stretching vibration v, at 1034 cm-I. If the -SO3- ion is in an asymmetrical environment (see Table 11), for instance, if at one of the 0 atoms of this group a cation is attached or if one of the 0 atoms is involved in a hydrogen bond, the
- -
(31) Albrecht, G.; Zundel, G. J. Chem. Soc., Faraday Trans. I 1984.80, 553.
1412 The Journal of Physical Chemistry, Vol. 89, No. 8, 1985
degenerated uaS(-SO3-)vibration a t 1204 cm-' is split into two components. Herewith the splitting is larger the stronger the hydrogen bond formed by the -SO3- group. Consideration of the spectra in Figures 1 and 2 and the data collected in Table I11 shows that in none of the systems the bands of the -SO3- group, for instance, vs at about 1030 cm-' as well as the bands of the SO,OH groups,for instance, v,(SOz) at about 1350 cm-' or u(S-(OH)) a t about 900 cm-', are present simultaneously. This result demonstrates that the hydrogen bonds are not of the AH-B -;+ A-.-H+B type, i.e., no double minimum proton potentials occur in these acid-base hydrogen bonds. Thus A--.H+-.B bonds with a single minimum potential are present in the acid-base complexes. The SO bands show, however, characteristic shifts and changes as a function of the ApK, (pK, of the protonated oxide minus pK, of the acid). These bands are collected in Table 111. A comparison of these data with those in Table I1 shows the following: With system 2 (ApK, = -0.18) vas- and vs(S02) are slightly lowered and v(S-(OH)) is slightly raised compared with the bands of the SOzOHgroups in the pure acid. These results demonstrate that in this complex the proton is only slightly loosened from the sulfonic acid group and thus, the proton potential well in the A-.-H+-.B bond is still present near this group. With system 3 (ApK, = 0.12) uas- and uS(SOz) are much more shifted toward smaller wavenumbers and u(S-(OH)) is much more toward higher wavenumbers, demonstrating that the proton is much more loosened from the sulfonic acid group. Thus, the proton potential well is shifted away from the sulfonic acid groups. For system 5 the positions of the three bands are nearly the same as in the homoconjugated SOpH+-.-03S anions (cf. Table 11). Thus in this system the well of the potential is, on the average, nearly in the middle of the bond. Let us now consider systems with a higher ApK,. For system 9 (ApK, = 6.57) vs(-SO
