P.M.R. SPECTRA OF AROMATIC AXD ALIPHATIC THIOLS
33 1
Proton Magnetic Resonance Spectra of Aromatic and Aliphatic Thiols’
by Sheldon H. Marcus2and Sidney I. Miller Department of Chemistry, Illinois Institute of Technology, Chicago 16, Illinois
(Received September 6 , 1963)
The sulfhydryl proton resonance frequencies in C.P.S.from tetramethylsilane are shown to be represented by U S H = - 1 7 . 0 ~ - 194.4 for eleven thiophenols and by V S H = -47.2a* - 73.0 for ten aliphatic thiols; four more compounds can be added to the aliphatic correlation providing the term 0.29n is included where n is the number of a carbon-carbon bonds. Deviation of representative aryl thiols from the correlation with u* is discussed in terms of conjugation and the magnetic anisotropy of the aromatic ring. Spin-spin coupling constants are evaluated for the phenyl protons of para-substituted thiophenols (&B2 system) and generally fall into small ranges: 8.0-8.5, 2.2-2.5, and 0.2-0.6 C.P.S. for ortho, mela, and para couplings, respectively. Coupling constants between sulfhydryl and alkyl protons in the aliphatic series fall in the range 6.4-8.6 c.p.s. No correlation between coupling constants and substituent parameters is observed. An unusual case of a paradisubstituted benzene derivative in which all four phenylene protons are magnetically equivalent is found in p-chlorothiophenol.
Studies of rate,3!4equilibrium5 and spectral properties6 of the thiol family have been reported. Proton magnetic resonance (p.m.r.) spectra of isolated thiols have been reported’-’ but no systematic survey is available. In this paper the p.m.r. spectra of about thirty aromatic and aliphatic thiols are discussed with respect to substituent effects on the proton resonance frequency ( u ) and spin-spin coupling constant ( J ),
Experimental Proton magnetic resonance spectra were produced on a Varian A60 analytical n.m.r. spectrometer operating a t a frequency of 60.005 1Ic.p.s. and equipped with a 4-4013A 12411. electromagnet system and a 2100B regulated magnet power supply. Homogeneity adjustments were made using the Hewlett-Packard 405BR automatic d.c. digital voltmeter. All spectra mere calibrated by the side-band technique using the HewlettPackard 200CDR wide range oscillator and 521C electronic counter. In all measurements the sample temperature was 28 f 1O , the radiofrequency amplitude was 0.07 gauss, and the scan rate was 1-2 c.p.s./sec. The samples were contained in 5-mm. 0.d. n.m.1. tubes (high resolution type). Carbon tetrachloride (Fisher certified) mas the solvent except in two cases where it was necessary to use chloroform (Fisher certified) due to relative insolubility
in CCl,. Tetramethylsilane (K and K Laboratories) was the internal standard, eliminating the necessity for bulk magnetic susceptibility corrections. The thiols studied usually were commercial samples used directly from freshly opened bottles; purities are Taken from the Ph.D. research of S. H. M . ; supported in part by the U. S. Army Research Office (Durham) ; presented a t the 144th National Meeting of the American Chemical Society, Los Angeles, Calif., April, 1963, p. 3P. National Science Foundation Fellow, 1961-1963. G. S. Krishnamurthy and S. I. iMiller, J . Am. Chem. SOC.,83, 3961 (1961). J. P. Danehy and C. J. Noel, ibid., 82, 2511 (1960). M. M. Kreevoy, E. T. Harper, R. E. Duvall, H. S. Wilgres, and L. T. Ditsch, ibid., 8 2 , 4899 (1960). (a) S. I. Miller and G. S. Krishnamurthy, J . Oro. Chem., 27, 645 (1962); (b) J. Jan, D. Hadzi, and G. Modena, Ric. Sci., 30, 1065 (1960). L. H. Meyer, A. Saika, and H. S. Gutowsky, J . Am. Chem. SOL, 75, 4567 (1953). S. Forsen, Acta Chem. Scand., 13, 1472 (1959). L. D. Colebrook and D. S. Tarbell, Proc. Natl. Acad. Sei. U . S., 47, 993 (1961). (10) A. S. N. Murthy, C. N. R. Rao, B. D. N. Rao, and P.Venkateswarlu, Trans. Faraday SOC.,58, 855 (1962). (11) A. S. N. Murthy, C. N. R. Rao, B. D. N. Rao, and P. Venkateswarlu, Can. J . Chem., 40, 963 (1962). (12) R. Mathur, N. C. Li, E. D. Becker, and R. B. Bradley, Abstracts, 144th National Meeting of the American Chemical Society, Los Angeles, Calif., April, 1963, p. 1P. (13) W. Stacy and J. F. Harris, Jr., J . Am. Chem. S O C . ,8 5 , 963 (1963).
Volume 68, A’umber 2
February, 1964
332
those given by the manufacturer: thiophenol(98%), omethylthiophenol (96%) , p-bromothiophenol(9793, pt-butylthiophenol (98%), and p-nonylthiophenol (95%) were from Pitt Consol; p-aminothiophenol, p-acetamidothiophenol, and cyclohexyl mercaptan were from Aldrich Chemical; p-chlorothiophenol (97%), a-mercaptoacetic acid (98%), P-mercaptopropionic acid (99%), isooctyl a-mercaptoacetate (98.5%) , isooctyl P-mercaptopropionate (99%) , and a-mercaptopropionic acid, (95%) were from Evans Chemetics ; n-propyl, isopropyl n-butyl, t-butyl, n-amyl, and n-hexyl mercaptans were from Pennsalt ; p-methylthiophenol, m-methylthiophenol, and P-mercaptoethanol were Eastman White Label; p-fluorothiophenol, b.p. 57-58’ (10 mm.), (lit.I4 b.p. 64-65’ (12 mm.)), and p-nitrothiophenol, m.p. 75’ (lit.I5m.p. 75’) were prepared by R. Wielesek. No p.m.r. spectrum showed any impurity signal of greater intensity than any sample peak. Chemical shifts were obtained by taking the average of three values a t each of three concentrations in the dilute range, below ca. 3 M , where thcre is little hydrogen bonding.8-”r16 Here the dependence of chemical shift on concentration approaches linearity and can be extrapolated to zero concentration. Thus, each infinite dilution value is the result of nine independent measurements. Spectra for the analysis of spin-spin coupling constants were obtained using the pure liquids or, in the case of solid compounds, concentrated solutions in carbon tetrachloride. The precision in the chemical shifts and coupling constants is h0.2 C.P.S. Calculations. The fine structure of the ring protons of the thiophenols which lend themselves to an -4~Bz analysisI7 has been evaluated, using the Freqint IV program18 for solving the secular equations for the spin state energies. Required input in this program are chemical shifts and coupling constants; output consists of transition energies (resonance frequencies) and transition probabilities.
Results and Discussion Aliphatic Thiols. The effect of substituents on the magnetic shielding of protons in aliphatic compounds is complex : electronegativity,19-z2 magnetic anisotropy,Zo orientation,2a and the number (n) of a-carboncarbon b0nds1932~have all been implicated. The chemical shifts (vS,H) for a series of o r g a n ~ s i l a n e shave ~ ~ been correlated with Taft’s CT*valuesz5and n. The effect of alkyl groups on the CI3 resonance in the linear alkanes appears to be additive.26 Our sulfhydryl p.m.r. frequencies ( VSH in C.P.S. from tetramethylsilane at infinite dilution in carbon tetrachloride) and coupling constants (JHCSH, in c.P.s.) between sulfhydryl and a-carbon protons are given in The Journal of Physical Chemistry
SHELDON H. MARCUS AND
SIDNEY
I. MILLER
Table I. An attempted correlation of VSH with u* was unsatisfactory (correlation coefficient r = 0.854) ; however, the equation VSH = -47.2u* -73.0 gives a more satisfactory correlation ( r = 0.980) for ten unbranched compounds. The negative slope reflects the typical shift to higher fields due to increased electron shielding caused by electron-releasing groups. This is illustrated in Fig. 1. As in the case of the organosilanes the correlation for both unbranched and branched compounds may be improved ( r = 0.973) by the inclusion of the n term, ie., VSH = -45.6(a* 0.29n) - 60.6. Unlike the organosilanes, however, the u* term is dominant over the n term. The n term reflects the dependence of V S H on presence of 01 carbon-carbon bonds. Such a dependence may be explained on the basis of the magnetic anisotropy of the carbon-carbon bond and/or the so-called carboncarbon bond shift due to factors other than anisotropy. The present data do not enable one to make a choice between these alternatives. In any case, it is clear that for V S H inductive effects predominate over carboncarbon bond effects, while for VS,H the reverse is true. The proton-proton coupling constants have been shown to be dependent on the electronegativity of substituents in the case of the hexachlorobicyclo[2.2.1l h e p t e n e ~ ,but ~ ~ no regular dependence on substituent is observed for the cyclopropane derivatives.z1 Substituent effects on C13 proton coupling have been found to be additive for both tetrahedrally and trigonally hybridized and approximately so in the
+
(14) G. Olah and A. Pavlath, Acta Chim. Acad. Sei. Hunq., 4, 111 (1954). (15) C. C. Price and G. W. Stacy, J . Am. Chem. Soe., 68, 499 (1946). (16) S. H. Marcus, unpublished results. (17) J. A. Pople, W. G. Schneider, and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N. Y., 1959, p. 142. (18) (a) A. A. Bothner-By and C. Naar-Colin, J . A m . Chem. Soc., 83, 231 (1961); (b) adapted to the IBM 7090 by H. Kriloff, Illinois Institute of Technology. (19) J. R. Cavanaugh and B. P.Dailey, J . Chem. Phys., 34, 1099 (1961). (20) H. Spiesecke and W. G. Schneider, ibid., 35, 722 (1961). (21) J. D. Graham and M. T. Rogers, J . Am. Chem. Soc., 84, 2249 (1962). (22) K. L. Williamson, ibid.,85, 516 (1963). (23) W. C. Neikam and B. P. Dailey, J . Chern. Phys., 38, 445 (1963). (24) 0. W. Steward, R. H. Baney, and E. B. Baker, Abstracts, 144th National Meeting of the American Chemical Society, Lou Angeles, Calif.. April, 1963, p. 3P. (25) R. W. Taft. J r . , “Steric Effects in Organic Chemistry.” M .S. Newman, Ed., John Wiley and Sons, Inc., New York, N. Y., 1956, p. 619. (26) E. G. Paul and D. M. Grant, J . Am. Chem. Soe., 85, 1701 (1963). (27) E. R. Malinowsky, ibid., 83, 4479 (1961): 84, 2649 (1962).
P.M.R.SPECTRAOF AROMATIC AND ALIPHATIC THIOLS
333
- 100.0
4
2 Lo
F
0
-200.0
‘gH5
0 I 1.00
0.00
Figure 1. Graplh of
VSH =
-45.6(u*
+ 0.29n) - 60.6:
(o*
2.00
i- 0.29n).
0, aliphatic thiols; 0 , aromatic thiols.
case of the substituted methanes.28 As in the case of the cyclopropyl derivatives our J-values show no regular substituent effects. Aromatic Effects. The aromatic thiols included in Fig. 1 show marked deviation from a plot of V S H us. ( D * 0.29%). The expression (u* 0.29%) contains the inductive substituent effect and the effect of neighboring carbon-carbon bonds. In the aromatic compounds there is a net deshielding due to other effects. Thiophenol, thiolbenzoic acid, and benzylmercaptnn all experience the deshielding effect of the magnetic field induced by the electron current through the aromatic ring.2g In the case of thiophenol there is‘a net deshielding of 67.0 C.P.S. A simple calculationzg of the ring current correct.ioii was performed using the following geometry3”: C- C (aromatic), C-S, and S--H bond distances, 1.39, 1.82, and 1.33 k., respectively; 6-S-€3 bond angle, 100.3’. The resulting correction is 48 c.P.s., somewhat smaller than
+
+
C6H5C0
observed. Though the simple calculation is known to underestimate the magnitude of the ring current c ~ r r e c t i o n it, ~is ~ not expected that this error be sufficient to account for the remaining deshielding. A further deshielding may result from conjugation of the sulfhydryl group with the aromatic ring placing a residual positive charge a t the sulfur atom. In the case of benzyl mercaptan, the ring current effect is decreased, falling with the cube of the distance of the sulfhydryl proton from the center of the ring.29 In addition, there is no conjugation between the sulfhydryl group and the ring. However, there may be an effect due to hyperconjugation between the methylene group and the ring placing a residual positive charge on thie (28) S. G. Frankiss, J . Phys. Chern., 67, 752 (1963). (29) See ref. 17, pp. 180-183. (30) “Tables of Interatomic Distances and Configuration in Molecules and Ions,” The Chemical Society, London, 1958, pp M114, M194-196. Values taken were for CHsSH and C6H,C1
Volume 68, S u m h e r 2
Fehruarv, 1964
SHELDON H. MARCUS A N D SIDNEY I. MILLER
3 34
netic field induced by electron currents in the system of the phenyl ring.”
Table I : Chemical Shifts and Coupling Constants in the Aliphatic Series” Substituent
USH, C.P.8.
JHCSH, C . P . S .
-64.7 -75.7 -116.1 -91.5 -112.2 -97.5 -69.2 -80.2 -65.4 -65.5 -127.1 -89.5 -84.9 -62.4 - 195.4 -117.4 -245.2
7.6 6.4 8.2 8.6
8.0
... 8.0
5.8 7.6 7.7 8.4 7.9
8.4 7.6
Table I1 : Chemical Shifts in Substituted Thiophenols
4-t-CaHg 4-CHZ 3-CH3
H 4-C1 4-CsHis
4-F 4-Br 4-CH3CONH” 4-SOz 4-”z
-190.8 - 191 .o -191.9 - 195.4 -197.4 -191,s -195.4 -197.2 -194.5 -217.0 -184.8
-430,l -423.0 -420.6 -430.0 -431 . O -427.9 -424.2 -434.2 .....
-462.3 -406.5
a I n CHC13; corrected to CC1, solution; solvent resonance swamped phenyl peak.
Representative aromatic thiols included for comparison’ CHC13. See ref. 8. See ref. 7. e See ref. 10. Corrected to C.P.S. from tetramethylsilane a t 60 Mc.p.s., zero concentration in carbon tetrachloride. a
* In
T-
’
The relationship of VSH to Hammett’s u-values,a* as shown graphically in Fig. 2, is given by U S H = - 2 1 . 8 ~ - 195.1 ( r = 0.952). The correlation is improved by the use of the u- value for the p-nitro group: V S H = -17.0a - 194.4 ( r = 0.993). I n the methylene protons. In the thiolbenzoic acid, the ring light of these data it is puzzling that the infrared current effect is decreased as in benzyl mercaptan , S-H stretching frequency in thiophenols shows only B there is no possibility of conjugation of the sulfhydryl rough trend with u wliile the corresponding 0-H group with the ring, or of any hyperconjugative effect. frequency in phenols gives a fair correlation. As However, there can be conjugation between the has been noted elsewhere, different measures of polarity sulfhydryl and carbonyl groups, placing residual posiare presumably involved in these different properties. tive and negative charges on the sulfur and oxygen %or the present, the following simplified explanation atoms, respectively, as well as the effect of the anisotof our correlation of V S H with u seems to be adequate. ropy of the carbonyl group. An increase or decrease in the electron density or Aromatic Thiols. I n aryl systems, the Fl9resonance shielding of the sulfhydryl proton will require a higher of substituted fluorobenzenesS1 and other aromatic or lower applied magnetic field, respectively, in order fluorine derivatiues,32the C13and para proton resonance to achieve resonance a t a fixed frequency. Since u is of monosubstituted benzenes,33 and the proton resoa measure of electron releasing or withdrawing power nance of substituted benzaldehyde^^^ and a n i ~ o l e s ~ ~of a substituent, v S H may be expected to show a trend have been correlated with various U-values while an absence of such correlation has been noted for the (31) (a) H. S. Gutowsky, D. W. McCall, B. R. McGarvey, and L. H , Meyer, J . Am. Chem. Soc., 74, 4809 (1952): (b) L. H. Meyer p h e n ~ l s . ~ eThe OH stretching frequency in the inand H. S. Gutowsky, J . Phus. Chem., 5 7 , 481 (1953): (e) R. W. frared has been correlated with u for the phenolsJ3’ Taft, Jr., J . Am. Chem. Soc., 79, 1045 (1957); (d) R. W. Taft, Jr., E. Price, 1. R. Fox, I. C. Lewis, K. K. Anderson, and G. T . while only a trend has been noted for the sulfur analogs, Davis, ibid., 85, 709 (1963). the thiophenols.6 Our data for the sulfhydryl ( V S H ) (32) D. R. Eaton and W. A. Sheppard, ibid.,8 5 , 1310 (1963). and phenyl ( u c ~ H ~ taken , as the center of the multiplet (33) H. Spiesecke and W. G. Schneider, J . Chem. Phys., 3 5 , 731 structure) protons are given in Table I1 in c.p.s. from (1961). tetramethylsilane at infinite dilution in carbon tetra(34) R. E. Klinck and J. B. Stothers, Can. J. Chem., 40, 1071 (1962). chloride. The range in V ~ H ca. , -220 to -180 c.P.s., C. Heathcock, ibid.,40, 1865 (1962). (35) may be compared ,with that of the aliphatic thiols W. G. Paterson and N. R. Tipman, ibid.,40, 2122 (1962). (36) (Table I), ea. -130 to -60 C.P.S. The aromatic (37) C. N. R. Rao and R. Venkataraghavan, ibid.,39,1757 (1961). series shows resonance at lower values of the applied (38) D. H. McDaniel and H. C. Brown, J . Org. Chem., 2 3 , 420 magnetic field due to the deshielding effect of the mag(1958). The Journal of Physical Chemistru
335
P.M.R.SPECTRA OF AROMATIC AND ALIPHATIC THIOLS
- 190
4
(
-200
z
4 m
- 210
-
-0.5
1.0
0.5
0.0
n.
Figure 2.
Graph of
v g ~=
- 1 7 . 0 ~-
194.4 for meta- and para-substituted thiophenols: 0, a; 0 , U -
bound directly to the benzene ring is considerably more with a; and since u is related to an energy difference sensitive to substituent effects than that of protons ( A A F ) , and V S H is in fact an energy difference, a reguremoved from the ring by one or two atoms in funclarity in the trend is expected and is indeed observed. tional groups. On the basis of the higher p-value, The negative slope indicates that electron-releasing vSH for the thiophenols shows greater sensitivity substituents shift the resonance position to higher aptoward variation in substituent than do VCHO and v O C H , plied magnetic fields due to an increase in electron density a t the sulfhydryl proton. The necessity of for the benzaldehydes and anisoles, respectively. The ring protons in this series being chiefly ortho and meta, using the U - value16 for the para-nitro substituent V C , H , shows no correlation with u. Previous work has may be explained by an enhanced deshielding of the demonstrated correlation for para protons only. 33 sulfhydryl proton due to direct conjugation between The effect of substituent position on resonance frenitro and sulfhydryl groups in the para position in quency in aromatic compounds has been examined and which a residual positive charge is on the sulfur atom. Further justification for such conjugation is given in the discussion of positional effects (see below). Table 111: Chemical Shifts in Methylthiophenols Interpreting the slope p as the sensitivity of V ~ to H variation in substituent, the value for the thiophenols, Position vga, o.p.8. U C H ~c.p.a. , -17.0, may be compared with those of other proton ortho -186.3 -139.4 resonances, the anisoles, 35 - 13.2, the b e n ~ a l d e h y d e s , ~ ~ meta -191.9 -138.4 - 14.5, and the monosubstituted benzenesSa (para para -191 . o -138.3 proton), -41.4. The resonance position of protons Volume 68, Number 2
February, 1864
SHELDON H. MARCUS AND SIDNEY I. MILLER
336
Table IV : Spin-Spin Coupling Analysis of para-Substituted Thiophenols
Case‘
(VAB
< JAB)
Number of lines
Groups appearing as single lines
18
( 6 , 7 , 8, 9, 10, 11, 12, 13)
-Relations*
+
dz,g = JAB JAB’ da,? = JAB JAB’ d4,6 = f ( J m e t a )
-
ds,o I11 (VAB =
Examplea
t-CaH9 O S H
AB analysis‘
A& analysisc---
VAB
JAB
Jmrta
JAB’
-3.6
8.0
3.8
0.6
~ A B
>-0.2
u
9.3
= B(VAB)
1
0.0
0)
+
a Y A B E YA - V B ; HA is ortho to SH. di,j is distance between lines i and j in 0.p.s.; J,,,, = ~ / ~ ( J A AJBB’); ’ JAB’= subscript refers to center of phenyl band. All values in c.P.s.; J-values are absolute values.
explained on the basis of induction and c~njugation.~g The sulfhydryl and methyl proton resonance frequencies for the tolyl thiols are given in Table 111. A shift of V S H to higher fields by the electron-releasing methyl group occurs in the order ortho > para > meta. The smallest effect is in the rneta position which experiences minimal con jugation; the largest effect is in the ortho position which experiences both conjugation and induction, the latter to a greater degree than does the para position. This presence of conjugation is added justification for the use of u- for the para-nitro group. The shift of vcHs td lower fields by the sulfhydryl group occurs in the order ortho > meta 2: para. The sulfhydryl group, being electron-withdrawing by induction and electron-releasing by conjugation, appears to be close to a crossover region between net electron withdrawal and release where the ortho, meta, and para resonances tend to become unresolved. Indeed this is the case for the meta and para methyl resonances. Spin-Spin Coupling. Coupling constants and chemfcal shifts have been evaluated for the ring protons of para-disubstituted benzenes by analysis of the p.m.r. spectra using AB and AzBz model^.^^-^* The results of our AzBz analysis are given in Table IV. All values are for Dure liauids or (in the case of solids) concentrated solutiois in carbon tetrachloride, and are given in C.P.S. Lines are numbered in order from low field to high (the spectra are symmetric about the center). The data fall into four categories distinguished by the
JA’B;
... zero
magnitude of the chemical shift ( v A B ) between nonequivalent ring protons relative to the ortho coupling constant ( J A B ) . The number of lines in the theoretical spectrum is generally greater than that observed due to the presence of closely spaced lines which are not resolved experimentally but which result in a broadened signal. An extreme case of this is the p-t-butylthiophenol spectrum in which V A B is very small and the central lines are of relatively high intensity and very closely spaced. In these cases the resultant broadened signal is taken to be a weighted average of the theoretical lines. A number of difference relations are observed between chemical shifts, coupling constants, and line spacings. Some of these are nonspecific functional dependences, e.g., d1,6(c.P.s.) = f ( J m e c aas ) is found in cases Ia and Ib. dl,5is chosen as the line spacing most sensitive to changes in J,,,,. Several calculations are then made keeping all other parameters constant at. their determined values and varying J,,,, in the region of its actual value, The calculated line spacings d1,6 are then plotted vs. the corresponding J,,,, values to obtain the functional dependence graphically. From the experimental value of d1,6 the value of Jmetamay See ref. 171 p. 260.
‘
The Journal of Physical Chemistry
