A Low-temperature Infrared Study of Self-association in Thiolic Acids

I A Low-temperature Infrared Study of Self-association in Thiolic Acids Can. J. Chem. Downloaded from www.nrcresearchpress.com by 37.44.192.59 on 12/30/17 I R. BICCA DE ALENCASTRO' AND C. SANDORFY Diportetnerit de Chitnir, Universiti de Montre'crl, Montrial, Qukbec Received November 16, 1972 The i.r. spectra of thioloacetic and thiolobenzoic acids were measured in both the fundamental and first overtone regions of the S-H stretching vibration, in a mixt~ire of CC13F and C2F4Br2, at temperatiires ranging from 20 to - 190 "C. S-H---0 type bonds are formed at low temperatures or at high concentrations of solute. Two different species can be detected. The first is probably an open dimer and the second a cyclic dimer. The S-H anharmonicity constants are of the order of 50-60 cm-i, for both free and associated species, and have about the same value as in aliphatic or aromatic mercaptans. Carbonyl groups have lower anharmonicities. Their value is about 10 cm-' for both free and associated species. Les spectres i.r. des acides thioloacetique et thiolobenzoi'que oi~t CtB mesurcs ail niveaii de la fondamentale et de la premiere harmonique de la vibration de valence S-H dans un melange de CC13F et C,F,Br, B des temperatures allant de 20 B - 190 "C. Sur I'effet de l'abaissement de la teinperature OLI de I'augmcntation de la concentration des liaisons du type S-H---0 sont formees. Deux especes differentes sont observees. La premiere est probablement Lln dimere ouvert et la deuxierne un dimere ferrne. Les constantes d'anharn~onicite po~~r les vibrate~~rs S-H libres et associes sont de I'ordre de 50-60 cn1-i, c'est-h-dire du m&ne ordre de grandeur que celles des mercaptans. Lei constantes d5anharrnonicitc po~lr les carbonyles libres et associes sent plus faibles et sont de I'ordre de 10 cm-'. Can. J. Chem.. 51, 1443 (1973) Introduction In previous publications (1, 2) we have studied the self-association in thiols and the association of thiols with organic nitrogen bases. We have been able to detect associated S-H---S type species, in the case of the self-association of thiols and, S-H---N type species, in the presence of the strong bases pyridine and triethylamine. In this paper we deal with the interesting case of the self-association of thiolic acids, where the possibility of S-H---0 type hydrogen-bonded species occurs. Some earlier studies of the association of thiolic acids have been made (3-6). Ginzburg and Loginova (3) studied the self-association of thioloacetic and thiolobenzoic acids, in solution in CCI, by i.r. spectrometry, and were able to detect changes in the S-H and C=O stretching vibration regions. They suggested that the predominant associated species is, at room temperature, an open dimer of the S-H---0 type. The shape of the associated band suggested, furthermore, that another species was present, probably the cyclic din~er. In the present work we have studied the selfassociation of thioloacet~c and thiolobenzoic 'Brazil-Canada exchange fellow. On leave of absence from Instituto de Quimica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil. acids over a wide temperature range and have measured both the fundamental and first overtone of the S-H and C==O stretching vibrations. The solvent was a 1 : I mixture of CC1,F (Freon 11) and C2Br2F, (Freon 114-B2), (FR), which becomes a rigid glass at liquid nitrogen temperature. We hoped to follow the changing pattern of self-association from room to - 190 "C temperature and to identify the associated species which appeared. Experimental The technique and instruments used were fully described elsewhere (1, 2). CC13F (Freon 11) and CLBr2F4 (Freon 114-B2) were obtained from E.I. Dupont de Nemours Co., and were distilled and kept over phosphorus pentoxide. Spectrograde quality carbon tetrachloride from Anachemia Chems. Ltd., Montreal, was dried on molecular sieves of the type 4A. The thiolic acids were from Aldrich Chem. Co. Inc., and were i.r. and refraction index controlled. The i.r. spectra of these compounds were practically identical with their previoi~sly pi~blished spectra. They were handled in inert atmosphere to avoid oxidation or nloisti~re adsorption. The solutions were prepared gravinletrically and the concentrations were corrected for solvent contraction. They were used imn~ediately after preparation. Results The results we obtained by concentration and temperature studies are very similar for thioloacetic and tl~iolobenzoic acids and we shall dis-

1444 CAN. J. CHEM. VOL. 51, 1973 25D/ i b) u E max 2530 30.0 lcm' ml8' FIG. 1. The i.r. spectra (2450-2650 cm-') of solutions of thioloacetic acid in FR (a) 0.32 M; (b) 1.55 M, at different temperatures. cuss both cases as a whole (Figs. 1-6). In the case of thiolobenzoic acid our temperature studies were somewhat limited by poor solubility. (a) The S-H Vibrator Concentration studies on the S-H stretching region give essentially the same results as those published by Ginzburg and Loginova (3) and, therefore, we shall not reproduce them here. The free band of thioloacetic acid is found to have a half-width of 15 cm-' in a 0.21 M solution in CC1,. Its maximum, located at 2587 cm-', has an intensity coefficient of about 23.1 cm-' mol-', which is several times stronger than those we found for the mercaptans (1). In the case of thiolobenzoic acid, the free band has a halfwidth of 13 cm-' in a 0.3 1 M solution in CCl,, and its maximum, which is located at 2589 cm-', has an intensity coefficient of about 28.1 cm-' mol-'. In a solvent less active than CCl, (in the present case our solvent FR) the free bands are less intense (about 11.1 cm-' mol-' for both thiolic acids) but have about the same half- widths. The frequency is only slightly changed. These systems seem to behave similarly to the dimethylamine NH stretching band, as described by Wolff and Gamer (7). (For a more elaborate discussion of the effect of "inert" solvents on X-H stretching bands (X = O,N), see ref. 8.) Upon lowering of the temperature (Figs. 1 and 3), the free band loses intensity and a broad band appears at lower frequencies. In the case of thioloacetic acid, only one large and broad band can be seen in dilute solution (Fig. la), which has a half-width of 68 cm-', with maximum near 2515 cm-', at - 183 "C. Temperature studies in a more concentrated solution (Fig. lb) clearly show that this band is actually due to two different species. Naturally, since increasing concentration should favor association, the apparent extinction coefficient of the associated bands is larger in Fig. lb than in Fig. la, at the same temperature. In dilute solution of thiolobenzoic acid, the lowest temperature we could reach was - 129 "C but two bands are also clearly seen (Fig. 3). The frequencies of the associated bands are about the same as for thioloacetic acid. Concentration and temperature studies on the first overtone of the S-H stretching vibration (Figs. 2 and 4) seem to confirm these observations. Although no new maxima are observable when the concentration of the thiolic acids in CCl, is increased, the half-width is greatly increased and the free band loses intensity. In the case of a 0.66 M solution of thioloacetic acid in FR, however, two bands are readily seen when temperature is lowered. The case of thiolobeilzoic acid in FR is less clear when temperature is lowered, but the associated bands can be seen to appear in the same region. Weaker bands are present near 4915 cm-', for thioloacetic acid, and 4980 cm-', for thiolobenzoic acid. Since these bands are found to persist even in very dilute solutions (where the E,,,, of the free band no longer increases), and the possibility of intramolecular hydrogen bonding is considered unlikely on sterical grounds, these bands are probably combination tones. (b) The C=O Vibrator We have studied the C=O stretching region of thioloacetic acid. Thiolobenzoic acid must behave similarly as shown by concentration studies (3). Figure 5 shows the effect of temperature

BlCCA DE ALENCASTRO AND SANDORFY: SELF-ASSOCIATION IN THIOLIC ACIDS 1445 Can. J. Chem. Downloaded from www.nrcresearchpress.com by 37.44.192.59 on 12/30/17 FIG. 2. The i.r. spectra (4800-5300 crn-') of solutions of thioloacetic acid (a) at different concentrations in CCI, at 20 "C; (6) 0.66 M in FR, at direrent temperatures. FIG. 3. The i.r. spectra (2450-2650 cm-') of a 0.59 M solution of thiolobenzoic acid in FR at different temperatures. variation on the frequency of the CO stretching vibration. At room temperature one band is seen in a 0.26 M solution of thioloacetic acid in FR, which has its maximum at 1715 cm-'. At - 185 "C the band is at 1686 cm-' with a shoulder on the high frequency side. At intermediate temperatures two bands are seen. Concentration studies (3) give, at room temperature, the free carbonyl stretching band at 1710 cm-', in CCI,, and the associated carbonyl at 1690 cm-l. The first overtone of the carbonyl stretching vibration seems to confirm the presence of at least two different carbonyls (Fig. 6), and the maxima shift with temderature in the same wav as the fundamental. concentration studies show no double maxima in CCI,, but the increase of the half-width and a pronounced asymmetry at C6H5COSH the lower frequency side suggests that more than A 25 C one species are present. Discussion Although no spectroscopic evidence is found against the existence of S-H---S hydrogenbonded species in thiolic acids (these are to be FIG. 4. The i.r. spectra (4800-5300 cm-') of sol~~tions of thiolobenzoic acid ((1) at difierent concentrations in CC14, at 20 "C; (b) 0.32 M in FR, at 20 and - 98 "C.

1446 CAN J CHEM VOL 51. 1973 perature in solution, or upon decreasing the temperature of a dilute solution. It seems, therefore, that S-H---0 type bonds are formed, and that the best analogy for the understanding of self-association in thiolic acids is the carboxylic acids, as pointed out by Ginzburg and Loginova (3). On the other hand, since mercaptans are known to associate less than the corresponding alcohols, we must expect to observe the formation of only a few associated species of the S-H---0 type. The possible structures are small chain oligomers and small cyclic structures with probably no more than three molecules in each. Our evidence (the existence of two associated bands in the S-H stretching region and the virtual absence of free S-H and C=O bands at low temperatures) seems to show that only the cyclic structures remain at the lowest tempera- FIG. 5. The i.r. spectra (1600-1800 cm-') of a 0.26 M ture but the possibility of having open structures solution of thioloacetic acid in FR at d~fferent tempera- is likely at intermediate temperatures, ~~~~i~ tures. acid is known (9) to form cyclic dimers, both in the gas phase and in dilute solutions. In more 80 A 021M concentrated solutions or in the pure liquid, 6 185M open chain oligomers are found. It seems 60 C 322M natural to assign the less dispiaced associated D 575M bands to the open dimer 1 which at lower 4 0 /P R-C, r2570 cm-' - 20 S-H---0 ~ 1 6 8 6 cm-' 'w - >C-R FIG. 6. The i.r. spectra (3250-3500 cm-i) of solutions of thioloacetic acid ((I) at difrerent concentrations in CC14 at 20 "C; (b) 0.90 M in FR at different temperatures. found also in the region 2500-2550 cm-' where we have found the association bands), they should be considered ulllikely because the oxygen atom is a better acceptor than the sulfur atom. In fact one call see that the carbonyl stretching frequency undergoes changes paralleliilg those of the S-H band, both upon increases in concentration of the thiolic acids at the same tem- H-S L2588 cm-i 1 temperatures changes into a cyclic structure. Earlier data on thioloacetic acid add support for the existence of the cyclic dimer structure 2 (10). A CSH angle of 105" is s~ifficient to obtain a structure similar to the acetic acid dimer. IVO distortion in the obtained structure in the gas phase is required supposing a linear hydrogen bonding (compare with a HSH angle of 92" in H,S (I I) and a CSH angle of 96" 30' in simple mercaptans (12, 13)).

BlCCA DE ALENCASTRO AND SANDORFY: SELF-ASSOCIATION IN THIOLIC ACIDS 1447 The anharmonicity coefficients computed from wexe = v01 - ~0212 are of the order of 50-60 cm-', for both the free and associated S-H groups which is about the same value we found for mercaptans (1). The values for the C=O stretching vibration are of about 10 cm-' and are not very different from those of free or associated carbonyls. Conclusions The above results seem to indicate that thiolic acids are associated by S-H---0 bonds. Two species are found when temperature is lowered or concentration raised. The first is probably an open dimer and the second a cyclic dimer. The S-H stretching anharmonicity constants are low and are of the order of 50-60 cm-' for both free and associated species and have about the same values as in aliphatic or aromatic mercaptans. Carbonyl groups have lower anharmonicities. Their values are about 10 cm-' for both free and associated species. We would like to express our thanks to the National Research Council of Canada for financial help. One of us (RBA) would like also to express his gratitude to the National Research Council of Canada and the Conselho Nacional de Pesquisas, Rio de Janeiro, Brazil, for financial support under the Brazil-Canada Program for Scientific Exchange, and to the Institute de Quimica da Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil, for a special leave of absence. I. R. BICCA DE ALENCASTRO and C. SANDORFY. Can. J. Chem. 50, 3594 (1972). 2. R. BICCA DE ALENCASTRO and C. SANDORFY. Can. J. Chem. In press. 3. I. M. GINZBURG and L. A. LOGINOVA. Opt. Spectrom. 20, 130 (1966). 4. A. S. N. MURTHY, C. N. R. RAO, B. D. NAGESWARA RAO, and P. VENKATESWARHI. Trans. Faraday Soc. 58, 855 (1962). 5. R. MECKE and H. SPIESECKE. Chem. Ber. 89, 1110 (1956). 6. G. ALLEN and R. 0. COLCLOUGH. J. Chem. Soc. 3912 (1957). 7. H. WOLFF and G. GAMER. J. Phys. Chem. 76, 871 (1972). 8. S. N. VINOCRADOV and R. H. LINNELL. Hydrogen bonding. Van Nostrand Reinhold Co. N.Y. 1971. p. 64. 9. (a) M. HAURIE. Ph.D. Thesis. Universite de Bordeaux. France. 1966. (6) M. HAURIE and A. NOVAK. Spectrochim. Acta, 21, 1217 (1965). 10. W. GORDY. J. Chen~. Phys. 14, 560 (1946). 11. G. HERZBERC. Infrared and Raman spectra. Van Nostrand Co., Inc. London. 1945. p. 489. 12. M. HAYASHI, H. IMAISHI, K. OHNO, and H. MURATA. Bull. Chem. Soc. Japan, 43, 872 (1971). 13. T. KOJIMA. J. Phys. Soc. Jap. 15, 1248 (1960).