1389
NOTES
S-H.
-
S Type Hydrogen-Bonding Interaction
1
by Samaresh Mukherjee, Santi R. Palit,l Department of Physical Chemistry,Indian Association for the Cultivation of Science, Jadavpur, Calcutta-$$,India
t
and Sadhan K. De
w
Chemistry Department, Indhn Institute of Technology, Kharagpur, West Bengal, India (Received May $6, 1969)
%tx
Although there is evidence of the formation of 0, 0-H . as, and S-H * * N type hydrogen S-H earlier workers considered hydrogen bonding of the type S-H...S to be unlikelya2"tb However, recent evidence on the formation of S-Ha .S hydrogen bonds in thiophenols at concentrations exceeding ca. 1.0 M in carbon tetrachloride has been ~ b s e r v e d P - ~ So far no quantitative studies have been done on the S-H. .Stype hydrogen bonding. We report herein some results of our studies on such hydrogen-bonding interaction where the S atom in ethylene trithiocarbonate (ETTC) acts as proton acceptor and S-H in thiophenols acts as proton donor. I n our studies we have utilized Nagakura and Baba's suggestion'O that
-
-
H&-S\
PAr
H 2 L dc=s\\\H
transition of organic molecules with suitable chromophores undergoes a blue shift in proton donor solvents due to solute-solvent hydrogen bonding interaction and also quantitative estimation of such interaction can be made by spectral measurements at the shifted peak.'OI'l
Experimental Section ETTC and p-bromothiophenol were kindly provided by Evans Chemetics, Inc., while p-thiocresol and thioph .-.lo1 w4re obtained as gift samples from Fallek Chemic a1 Corp. The nonhydrogen-bonding solvent used wss Merck's carbon tetrachloride, purified by standard method.12 The solutions were made gravimetrically and all the spectral measurements were taken on freshly prepared solutions in a Hilger uv spectrophotometer. Optical density measurements were taken a t 35' (a temperature constant within =k0.5" being maintained by circulating water through the cell holder). Results and 'Discussion The n-n* transition of ETTC in CC14 a t 458 mp
0'40
u z
$ 0'25 m Q
I 0'10 440
Ly
i
I
I
I
I
460
'h,
I
I
mp-
I
I
480
Figure 1 Absorption spectra of E T T C in 0, (I); 1-90,(11); 2.72, (111); and 4.25 (IV) M thiophenol in carbon tetrachIoride. Concentration of E T T C was 4.53 X M for all absorption curves.
undergoes a blue shift in thiophenols. Figure 1 shows how the n-n* band in CCL undergoes progressive blue shift with increasing concentration of proton donor and it was found that a t 454 mp the ETTC exists completely in the hydrogen-bonded state. Optical density measurements were made on mixtures of ETTC and thiophenols at various concentrations of proton donors at the shifted peak (454mp) and the equilibrium constant K was calculated by using eq 1.'l When [A] >> [B] and the base B forms a 1: 1 (1) To whom all correspondence should be addressed. (2) (a) M. J. Copley, C. 5. Marvel, and E. H. Ginsberg, J . Amer. 61,316 (1939); (b) W.Gordy and 8. C. Standford, ibid., Chem. SOC., 62,497 (1940). (3) S.K.De and S. R. Palit, J . Phys. Chem., 71,444 (1967). (4) B.Ellis and P. J. F. Griffiths, Spectrochim. Acta, 22, 2005 (1966). (5) M. L.Josien, C. Castinel, and P. Saumagne, Bull. SOC. Chim. F?., 648 (1957). (6) R.A.Spurr and H. F. Byers, J . Phys. Chem., 62,425(1958). (7) J. G.David and H. E. Hallam, Trans. Faraday SOC.,60, 2013 (1964). (8) J. G.David and H. E. Hallam, Spectrochim. Acta, 21, 841 (1965). (9) 8. H.Marcus and S. I. Miller, J . Amer. Chem. Soc., 88, 3719 (1966). (10) S.Nagakura and H. Baba, {bid., 74,5693 (1952). (11) A. K. Chandra and 8. Basu, Trans. Faraday Soc., 56, 632 ( 1960). (12) "Techniques of Organic Chemistry," A. Weissberger, Ed., Vol. VII, Interscience Publishers, Inc., New York, N. Y.,1955. Volume 7.4, Number 6 March IO, 1970
1390
NOTES The Circular Dichroism of Some Aliphatic Amino Acid Derivatives. A Reexamination
by Claudio Toniolo Institute of Organic Chemistry, University of Padua, Padua, Italy (Received June 2, 1060)
ILL
I
8
4
0
[AI XI0
12
Figure 2 Plot of [ AJ /(; - eo) us. [A] for I, thiophenol; I1 p-bromothiophenol; and 111, p-thiocresol. ETTC concentration in all cases was 4.53 X 10-8 M . Spectral measurements were made a t 454 mM.
complex with a proton donor A, then
where [A] is the concentration of proton donor, [B] is that of the solute (Le., ETTC), eo and are the extinction coefficients of the solute and complex, respectively, and Z is the formal extinction coefficient given by Z = D/[B]&, where D is the measured optical density of the solution containing an initial concentration of [B], mol/l. and 1 is the path length in cm. The plots of [A]/: us. [A] were found to be linear in all cases (Figure 2). In order to study any effect due to solute concentration, the ETTC-thiophenol system was studied at two different concentrations of ETTC and no significant variation in the equilibrium constant was observed. The results are summarized in Table I. It is evident that thiophenols Table I : Equilibrium Constants of H-Bonded Complexes between ETTC and Thiophenols a t 35’ Thiophenols
Thiophenol Thiophenola p-Thiocresol p-Bromo thiophenol [ETTC] was 7.77 X 10-8 M . was4.53 X 10-SM.
Equilibrium constant, M - 1
0.64 0.65 0.75 0.87
f 0.08 f 0.10 I0.12 f 0.10
I n all other cases [ETTC]
In 1964, Closson and Haugl identified the low-intensity ultraviolet (uv) absorption band near 210 mp in the spectra of saturated carboxy derivatives as an n + ?r* transition of the chromophore. A number of papers concerning circular dichroism (CD) studies of saturated carboxylic acids and esters have recently appeared in the literature.2-6 It was found that S-lactic acid and its derivatives exhibit, in addition to the 210-220-mp CD Cotton effectj3s5-’a weak CD band of opposite sign centered in the 240-250-mp r e g i ~ n . Djerassi ~ ~ ~ and coworkerse interpreted their results assuming that both dichroic bands are associated with the n + ?r* transition of the carboxy-chromophore and possibly related to different rotamers, in conflict with Anand and Hargreaves,s who attributed the lower-energy Cotton effect to an n + ?r* transition and the stronger Cotton effect at about 210 mp to a T + ?r* transition. I n view of the importance of the n --+ ?r* band in the assignment of configuration and in the application of octant and related rules in carboxylic acid derivatives, it was of interest to us to reexamine2the optical rotatory properties of the configurationally related amino acids. To obtain further information on the transitions involved in the carboxy chromophore, we have also examined the CD spectra (by a Roussel-Jouan Dichrographe-185 CD model) of R-lactic acid in solvents of different polarity and at different pH’s in the 200-260mp region. Both the positive band nearest the visible and the negative band a t about 210 mp blueshift and decrease in intensity on changing the solvent from cyclohexanedioxane to trifluoroethanol (Table I and Figure l),possibly indicating a common origin, i.e., the n 4 T* transition of the carboxy chromophore (for the OH group the expected region of absorption is below 180 mp8). (1) W. D. Closson and P. Haug, J. Amer. Chem. Xoc., 86, 2384 (1964). (2) M. Legrand and R. Viennet, Bull. Chem. SOC.Fr., 679 (1965). (3) S. Uyeo, J. Okada, and S. Matsunaga, Tetrahedron, 24, 2859 (1968). (4) J. D. Renwick and P. M.Scopes, J. Chem. Xoc., C, 2674 (1968). (5) R. D. Anand and M. K. Hargreaves; Chem. Commun., 421 (1967) (6) G. Berth, W.Voelter, E. Bunnenberg, and C. Djerassi, ibid., 355 (1969). (7) c. Toniolo, v. Perciaccante, J. Falcetta, R.RUPP,and M. Goodman, J . Ow. Chem., in press. ( 8 ) D. W. Turner in “Determination of Organic Structures by Physical Methods,” F. C. Nachod and W. D. Phillips, Ed., Academic Press, New York, N. Y., 1902,p 339. I
form weak 1 : 1hydrogen-bonded complexes with ETTC and the stability constants follow the order, p-bromothiophenol > p-thiocresol > thiophenol. Such order is not in parallel with the expected acidity of proton donors in water. The Journal of Physical Chemistry
