453
NOTES A Proton Magnetic Resonance Solute-Solute Correlation. Solvent Interactions with 250.(
the Sulfhydryl Proton
by Sheldon H. Marcuslb and Sidney I. Millerlo
,-
E
Department of Chemistry,Illinois Institute of Technology, Chicago, Illinois 60616 (Received June $6, 1968)
I
t:
Substituent effects of,2 self-association and donor bonding to thiol@-8 have been examined by pmr studies of the SH function. I n this paper, we report a useful solute-solute correlation (T = 0.977) of the pmr chemical shifts (in hertz) a t infinite dilution (6O) of n-butanethiol with those of thiophenol in 18 solvents (Table I, Figure 1). The number of solvents is given by N .
60(CeHijSH) = 2.00S0(n-C~H~SH) - 58.5 ( N
= 18)
(1)
8 Bm
9 a
9
200.1
/
I
100.0
The effect of medium on pnir spectra is complex. If this is reflected in the behavior of a solute-solvent pair, it is shown even more forcibly for different solutes in several solvents, Since 1960, five solvent contributions to 6obsd, have been postulated and discussed, bulk &bed
=
ab
+ 6 , + 6 . + + 6,'
(2)
BE
susceptibility (ab), dispersion (aw), anisotropy (&), electric field (b),and specific interactions (&I, as2, etc.). Despite numerous attacks (we cite only a few) on the problem of identifying and assessing the component terms of eq 2, success has been limited at bestn9-14 It is of some interest therefore that regularities in the solvent effects on 6 do turn up occasionally, e.g., in the form of eq 1. Table I : Sulfhydryl Chemical Shifts at Infinite Dilution in Organic Solvents (in hertz) from TMS Code
1 2 3 4 5 6
7 8 9 10 11 12
13 14 15 16 17 18
Solvent
- P(CaHsSH)
- Go(n-C4H8SH)
195 4 188.7 239.4 207.7 213.1 250.2 256.7 247.9 229.4 226.5 178.4 179.7 196.1 192.2 186.4 198.7 189.0 174.6
64.7 62.1 92.4 79.6 81.3 95.7 98.9 89.1 80.9 84.9 59.9 62.5 71.9 68.3 64.4 70.7 65.9 57.7
I
(n-BuSH) (Ha from TMS)
Figure 1. Relation between 8ks values of thiophenol and n-butanethiol in various solvent,s.
There have been several reports of solute-solute relations. These are obtained when the spectral parameters of two solutes in a series of solvents can be correlated :12 the NH infrared stretching frequencies of pyrrole and aniline are linearly re1ated;lb the OH infrared stretching frequencies and pmr 6's of phenol-base adducts are likewise related.le There do appear to be (1) (a) Research supported in part by National Institutes of Health Grant GM07021 and abstracted in part from the Ph.D. thesis of S, H. M.; (b) National Science Foundation Fellow, 1961-1963; (e) author to whom inquiries should be addressed. (2) S. H. Marcus and S. I. Miller, J . Phys. Chem., 68, 331 (1964). (3) S. H. Marcus and S. I. Miller, J . Amer. Chem. SOC.,88, 3791 (1966). (4) C. R. Kanekar, G. Govil, C. L. Khetrapal, and M. M. Dhingra, Proc. Indian Acad. Sci., A65, 265 (1967). (5) M-M. Rousselot, Compt. Rend., 262, 26 (1966). (6) R. Mathur, S. M. Wang, andN. C. Li, J . Phys. Chem., 68, 2140 (1964); R. Mathur, E. D. Becker, R. B. Bradley, and N. C. Li, ibid., 67, 2190 (1963). (7) M-M. Rousselot and M. Martin, Compt. Rend., 262, 1445 (1966); M-M. Rousselot, ibid., 263, 649 (1966). (8) E. Goldberg and 8. I. Miller, unpublished data. (9) A. D. Buckingham, T. Schaeffer, and W. G. Schneider, J . Chem. Phys., 32, 1227 (1960). (10) H. M. Hutton and T. Schaeffer, Can. J. Chem., 45, 1111 (1967). (11) H. J. Friedrich, 2. Naturforsch., 20, 1021 (1965). (12) P. Jouve, Ann. Phys., 1, 1 (1966). (13) I. D. Kunts, Jr., and M. D. Johnston, Jr., J . Amer. Chem. Soc., 89, 6008 (1967). (14) R. C. Fort, Jr., and T. R. Lindstrom, Tetrahedron, 23, 3227 (1967). (15) L. J. Bellamy, H. E. Hallam, and R. L. Williams, Trans. Faraday Soc., 54, 1120 (1958). (16) D. P. Eyman and R. S. Drago, J. Amer. Chem. SOC.,88, 1617 (1966); K. F. Purcell and R. 8. Drago, ibid., 89, 2874 (1967). Volume 79, Number d
February 1960
454
NOTES
far fewer prnr solute-solute correlations. From literature data or plots we can derive the following 6°(CoH5CSCH) =
+
(N
+ constant
(N = 11) (4)"
~.O~I?~(C~CH&=CH) constant 6°(C&&OC=CH)
= 11)
(3)"
=
1.56°(C1CH2C=CH)
G 0 ( C 6 H ~ C d X )= 1.06°(CIC(CH3)2CeCH)
+
constant (N = 31) Go(CClsH) = 0.766°(C6H5CsCH)
(5)12
+ constant (N = 31) (6)12
(N 6°(C6&SH)
= 1.760(CeHsC=CH)
= 10)
(7)'s
+ constant (N
= 6)
(8)
Equation 7 is one of five rather similar plots given by Eliel and Martin for 3- and 4-t-butylcyclohexyl halides. l8 We have found that our thiol chemical shifts correlate with those of certain ethynyl proton^'^^'^ and have given eq 8 as an example. In addition, Suhr has obtained a fair correlation for the aryl proton chemical shifts (center of gravity) of 4-chloroaniline and 4-nitroaniline. l9 Undoubtedly inany other such correlations could be derived from published data. These derived linear relations provide a compact means ttostore and to predict 6 0 . Moreover, a relat,ion such as eq 1 can be used t,o estimate the normally inaccessible value &' of any solute in itself as solvent. For one can first measure 602 for a second solute in a series of solvents, among them the first solute, and then apply an equation such as eq 1. A few comments on the solvent effects should serve to illustrate the theoretical problems. With respect to the t,hiols, though they do self-a~sociate,~ this is not an issue in this work, since we are dealing with infinitedilution shifts of isolated molecules. At the sulfhydryl group, one or more solvent molecules presumably take up some average orientation. Now, the usual hydrogen bonding by dipolar sites deshields a proton, e.g., in thiol^.^ In the case of magnetically anisotropic solvent molecules of cylindrical electronic distribution, the anisotropy effect depends on whether the solute molecule interacts with the end or side of the cylinder.g For orientation to the end, as might be the case with the SH on nitrogen of acetonitrile or the R electrons of benzene, the thiol proton would be shielded; for orientation to the side of the cylinder, as to an electronegaThe Journal of Physical Chemiatiy
tive substituent, of benzene, the thiol proton would be deshielded. These solvent factors are best discussed with reference to cyclohexane (Table I, Figure I). In aromatic solvents with no electronegative atoms, e.g., benzene and toluene, the dominant solute-solvent interaction appears to be hydrogen bonding to the T system. In these cases, the magnetic anisotropy effect, shields the thiol proton, and SO (SH) is upfield from cyclohexane. The usual deshielding effect of hydrogen bonding is apparently overwhelmed here. The aliphatic chloro-, nitrogen-, and oxygen-containing solvents are known to be hydrogen bonding. All of them shift aO(SH)downfield from cyclohexane, more or less to the extent expected from their hydrogen-bonding power. I n the absence of a dominant factor, one must resort to an appropriate "mix" of the major factors to account for the observed 6O in solvents such as trifluoromethylbenzene, nitrobenzene, and acetone. The preceding notions about the trends in d'obsd are not really satisfying. Any view of donor to acceptor strength should, of course, be associated with an equilibrium constant or possibly an enthalpy.6 French workers have used 6 O as a measure of solvent donor power to several thiols but without justification.? Eyman and Drago have studied phenol-donor adducts in an "indifferent" medium and found that 6(complex) is proportional to the enthalpy of the reaction.'6 Finally, there is the interesting dependence of 60(C6H&=CH) or P ( R S H ) on the temperature, T.7n12 In an atteinpt to clarify these problems and perhaps provide a rationale for the correlations 1 and 3-8, we devised the following "interaction" model. We suppose that the chemical shift 6 ( M ) of a monomeric species is the same for a species 1 4 in the gas phase and in a solvent S. When i\/l has fully interacted with liz molecules of 8, the chemical shift of the species MS, is taken to be 6(RIS,). Incidentally, the fact that So for thiols changes only slightly from the gas phase to solution in cyclohexane5 suggests that the idea of a gaslike solute is not unrealistic. Following a standard approach,916we take
+
[n1]&(11) [-\Isn]s(lrsn)
[~O]&bad
(9)
and
IC
=
[:\Isn]/ [AI][SIn
(15)
or 8obsd
= {8(1/1)
f K[8]"6(1\18,)][ a r ] / [ f i 1 0 ]
For any given solvent, in the range [ill,] define
-+
(11) 0, we
sEd&b.a/d[SI] = {6(:\1) f M[S]"6(filS,)]/[~Io] (12) (17) J. V. Hatton and R. E. Richards, Trans. Faraday Soc., 57, 28 (1961). (18) E. L. Elisl and R. J. L. Martin, J. Amer. Chsm. Soc., 90, 689 (1968). (19) H.Suhr, M o l . Phys., 6 , 153 (1963).
NOTES
455
If s 5 0, K is negative, and the model is not applicable! If s > 0, K is positive, and we obtain the limiting expressions S(RI) (for small K )
(13)
K[S]”fi(JIS,) (for large K )
(14)
Sobad N fiobsd N
Experimentally, it is found that the sign of s may be either positive or negative. In dilution plots of S(C3H7SH), s is positive for carbon tetrachloride or cyclohexane, and ca. zero for chl~roform.~Dilution plots fi(CH2) of p-nitrobenzyl and benzyl chlorides in carbon tetrachloride20 have negative and positive s values, respectively. Consider the two limiting cases with s > 0. We find eq 13 interesting but unrealistic. Here, fi0 of a solute is solvent independent. If the model leading to eq 14 is correct, then it follows that the magnitude of So can reflect donor strength, since it is proportional to K . Note that this proportionality differs from those in which a pmr shift or an infrared frequency are linear in log K or in (free) energy.16 In the context of the solute-solute relation, e.g. eq 1, eq 14 requires a parallelism in K 1 and K2 or log K 1 0: log K z . One can now consider eq 1 in the same way that one deals with linear free energy correlations; in particular, the slope of eq 1 may be regarded as the reIative sensitivity of the solutes to interaction with the solvent. Heretofore, the parallel of 60 with K had not been demonstrated, and the proportionally of 601 to 602 had not been rationalized! We wish to emphasize that although our interaction model fits certain systems, there are a number of probing questions one can raise. Some have to do with the fact that the model is deficient in detail: the several terms or contributions in eq 2 are ignored; the microstructure (shape, size, and conformation) of solute and solvent are disregarded; the electronic environment of the interaction site is not described, etc. Finally, coror S(MS,J (Y relations reported by others, e.g., So (Y TT*12 AH,lB do not follow from eq 14 and even appear to be incompatible with it, in view of the nonlinear dependence of K upon T and AII upon K . The other kinds of criticism of eq 14 are more specific. Even though the interaction mechanism is unspecified, i t nevertheless seems strange that the linear correlations, eq 1 and 3-8, include a broad range of solvents from the ‘‘weak” (nonpolar) hexane to the “strong” (polar) methanol (Table I). Even if some sort of acceptor-donor interaction could be envisioned between solute and solvent, as in the system of eq 1, there are solute pairs, e.g., the alkyl halides of eq 7, whose protons can hardly be cast in the role of acceptors. There are, of course, many sets of 60 data which appear to be unrelated and do not give acceptable linear correlations, e.g., 6(C6H&3H) us. 6(4-t-butylchlorocyclohexane-l-H),18 ~(C~HSSH vs.) G(CH3CN),$6(CH4) us. fi(CH3CN),getc.
Of course, the model cannot accomodate those strong donors in which the gaslike monomer is a fiction, and the dilution shift reflects association equilibriums among several polymers. An infinite-dilution shift is a complex quantity, whose origins are poorly understood. It is surprising, therefore, that certain solute pairs, e.g., arylthiol-alkylthiol, propargyl chloride-phenylacetylene, and chloroformphenylacetylene, evoke a parallel response from a diverse group of solvents. These derived correlations could be useful on an empirical level. Beyond this, the interaction model which leads to eq 14 is admittedly flawed and limited, but it does provide a rationale for some of the observations.
Experimental Section Details of the experimental procedure with thiols have been given previously.2ga The thiol spectra were obtained on a Varian A-60 nmr spectrometer operating at 60 IlfHz. Tetramethylsilane (TMS) was used as an internal reference. Values of 60 were obtained by extrapolation from values at three concentrations in the range of low thiol concentration (
