110
R. T. HOBGOOD, JR.,G. S. REDDY, AND J. H. GOLDSTEIN
Vol. 67
NUCLEAR MAGNETIC RESONANCE SPECTRA OF SOLMEALKYL VINYL ETHERS AND METHYL VINYL SULFIDE BY R. T. HOBGOOD, JR.,G. S. REDDY, AND J. H. GOLDSTEIN Department of Chemistry, Emory University, Atlanta 22, Georgia Received J u n e 8, 1961 The n.m.r. spectra of methyl vinyl sulfide, methyl vinyl ether, ethyl vinyl ether, and isobutyl vinyl ether have been observed and analyzed in tetramethylsilane solution. From the chemical shift data, it appears that lonepair conjugation with the r-electron system is more important in the case of oxygen than sulfur. I n methyl vinyl ether a coupling constant between the methyl group and the a-proton of 0.3 C.P.S. is observed, with no detectable coupling between the methyl group and the P-protons. However, in methyl vinyl sulfide the methyl group couples with the @-protons, 0.4 C.P.S. (cis) and 0.2 C.P.S. (trans), with no coupling being observed between the methyl group and the or-proton. This long-range coupling is thought to reflect the participation of structures involving the d-orbitals of sulfur.
Introduction This communication reports the results of a nuclear magnetic resonance (n.m.r.) study of methyl vinyl sulfide, methyl vinyl ether, ethyl vinyl ether, and isobutyl vinyl ether under comparable conditions in the inert solvent tetramethylsilane. N.m.r. spectra, as well as many other physical techniques, have been used previously in efforts to discover the relative mesomeric and inductive effects of substituents on an ethylenic system.l-a Analyses for the ethers have been performed previou~ly,~~6 but due to differences in solvents and methods of referencing, it was necessary to reanalyze them as ABC systems under the conditions employed here. The long-range coupling constants through the hetero atom have been determined by firstorder perturbation methods. The chemical shifts and coupling constants have been examined in an effort to deduce the difference in the manner and extent of interaction of sulfur and oxygen with the n-electron system. Experimental All spectra were observed on a Varian Model A-60 high-resolution spectrometer operating a t 60 Mc./sec. The spectra of methyl vinyl ether and methyl vinyl sulfide were calibrated on a Varian Mode1 4300 B high-resolution spectrometer operating at 40 Mc./sec. by the usual side-band technique6 while the spectra of the remaining compounds were calibrated on the A 4 0 spectrometer by the same method. The spectra of the ethers and the sulfide were observed in 10% solution (by volume) in tetramethylsilane used as solvent and internal reference. The 40 Mc./sec. spectrum of methyl vinyl sulfide is shown in Fig. 1 and 2 . Figure 1 is the vinylic portion under medium resolution, i.e., the splittings due to the methyl protons are not resolved. Figure 2 is the spectrum of the methyl group under conditions of very high resolution. The splitting between each pair of consecutive lines is 0.2 c.P.s., which is produced by two coupling constants (0.4 and 0.2 c.P.s.) with two of the vinylic protons.
Results and Discussion I n all pertinent cases, the vinylic portion of the spectrum was analyzed as an ABC system with any further splitting due to alkyl substituents being treated by first-order perturbation theory. The values of the (1) L. Pauling, “The Nature of the Chemical Bond,” 3rd Ed., Cornell University Press, Ithaca, N. Y., 19fi0, pp. 288-290. (2) E. B. Whipple, W. E. Stewart, G . S. Reddy, and J. H. Goldstein, J. Chem. Phys., 84, 2136 (1961). (3) C. N. Banwell and N. Sheppard, J. Mol. Phys., 8, 351 (1960). (4) C. N. Banwell, N. Sheppard, and J. 3. Turner, Spectiochim. Acta, 16, 794 (1960). ( 5 ) W. Brugel, Th. Snkel, and F. Xruckebezg, Z. EZ&t?ochem., 64, 1121 (1960). (6) J. A. Pople, W. G . Schneider, and H. J. Bernstein, “High-resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Ino., New York, N Y. 1959, p. 74.
chemical shifts and coupling constants, shown in Table I, were obtained by numerically fitting the observed spectra. The maximum discrepancy between observed and calculated shifts, 0.2 c.P.s., occurred for only a few peaks, and the over-all agreement is within the mean deviation of the measurements. The observed ABC intensities were obtained numerically by a triangular approximation of the peak area. Considering approximations due to further splittings by the substituents, the observed ABC intensities agree quite well with the calculated ones. All the chemical shifts in the table have been converted to 60 Mc./sec. for comparison purposes. Coupling constants for methyl vinyl ether have been reported previously4 which do not differ significantly from those obtained here. Complete ABC analyses for the three ethers reported here have been published earlier.6 However, due to differences in solvents and references used, individual chemical shifts vary from 5-15 C.P.S. from those listed in Table I. The most striking feature of the values in Table I is the extreme high-field position of the 0-protons in the ethers (-75-85 c.P.s.) above ethylene.’ It is difficult to conceive of an anisotropy effect of oxygen which would produce an up-field shift this great. In acrylonitrile, where the anisotropy of the -C=N group is expected to be larger or certainly not smaller than that for oxygen, the calculated anisotropy shift of the 0protons8 is much smaller (11.5 C.P.S. for the cis and 15.0 C.P.S. for the trans position in the upfield direction) than the observed upfield shift in the case of the ethers. It is more likely that lone-pair conjugation of the oxygen with the unsaturated system with its resulting flow of charge into the 0-position is responsible for this observed upfield shift. I n methyl vinyl sulfide the @-protonsare 14-28 C.P.S. above ethylene, whichis in sharp contrast with the corresponding ether @-shifts -75-85 C.P.S.above ethylene). The relative electronegativities of sulfur and oxygen would suggest that any inductive effect operating on the @-positionshould be in the opposite direction. Indeed, the a-proton shift does follow the order of electronegativities and since any inductive effect would probably be short-range, one might assume that the inductive effect is the controlling factor on the a-shift. Conceivably, however, the a-proton shift in the two conipounds might be due to a difference in the anisot(7) G. S. Reddy and J. H. Goldstein, J. Am. Chem. Soc., 83, 2045 (1961). (8) G. S. Reddy, J. H. Goldstein, and L. Mandell, %bid., 83, 1300 (1931).
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S.M.R. SPECTRA OF ALKYLVINYL ETHERS AND METHYL VINYLSULFIDE
Jan., 1963
TABLE I" N.M.R.SPECTRAL PARAMETERS FOR METHYL VINYL XULFIDE AND SOME ALKYLVINYL ETHERS
(4
€I
€3(4
\ / c=c / \
ScH,(d) Wa Wb WC Wd
Jab JW
Jbo Jad
Jbd JCd a
H (4
H
OCH,(d) (c)H
( 4H
H (a)
\ / c=c / \
\ / C=C / \
(b) (C)
H
(b)H H (a)
/ C=C / \
(d) OCHzCH(C€I3)2 ( 4H
(4 OCHzCH3
@)H
-380.7 -386.0 -290.4 -241.8 -304.5 -232.7 -127.2 -189.8 16.4 14.1 10.3 7.0 -0.3 -2.0 0.0 0.3 0.4 0.0 0.2 0.0 All chemical shifts and coupling conntants are expressed In
-381.6 -242.7 -231.0 -219.2 14.5 7.0 -1.7
..*
... .
.
C.P.S.
I
CHd4
\ / c=c / \
\,
(b)H -383.0 -242.8 -230.4
... 14.4 6.9 -1.8 0.5
OCHa(d)
-104.0 -225.0 -225.0 -205.5 0.5 0.5
...
...
a t 60 Mc./sec. with respect to internal tetramethylsilane.
Fig. 1.-The observed vinyl spectrum of methyl vinyl sulfide in tetramethylsilane a t 40 Mc./sec. Linearity should not be assumed in the observed spectrum. The direction of increasing field is from left to right.
ropy of the two hetero atoms. The p-shift differences in the two compounds probably can be explained best by the difference in lone-pair conjugation of oxygen and sulfur. Previous n.m.r. data for the vinyl halides nndicate that the smaller the halide, the greater the capability for lone-pair conjugation.2 It therefore seems quite reasonable to expect that oxygen, because of its smaller size, would show a greater tendency for lonepair conjugation then sulfur. Ethyl vinyl ether and isobutyl vinyl ether in general have the same chemical shifts as methyl vinyl ether. Since no dilution studies were made in the present work, it is difficult to ascertain the chemical shift differences with a great degree of certainty. However, it appears that the R group does not have any significant effect on the P-shifts, implying that the R groups studied here do not alter the capacity for lone-pair conjugation of oxygen significantly and that oxygen acts as a buffer between the two groups. It has previously been observed' that the total methyl substituent effect on the two P-protons in an unsaturated system is generally 30 C.P.S. However, in isopropenyl methyl ether the total a-meth,yl effect is only 16 c.P.s., perhaps implying;, since the methyl and the metlioxy groups are cross-conjugated, that the charge transfer from the methyl group is reduced.
Fig. 2.-The observed methyl group spectrum of methyl vinyl sulfide in tetramethylsilane at 40 Mc./sec. The separation of the left-most peak from its nearest neighbor is 0.23 C.P.S. The direction of increasing field is from left to right.
The gem coupling constant in methyl vinyl sulfide has a greater degree of uncertainty than any of the other parameters due to the coupling of the ,&protons with the methyl group. It may vary from -0.5 to 1D.O C.P.S. without changing the calculated spectrum significantly. The value of -0.3 C.P.S. was chosen in order that the pairs of lines which are primarily influenced by this coupling constant would coincide exactly. It
112
KORBERT MULLERASD DUANET. CARR
should be pointed out that a value of 3.0.3 C.P.S. will not produce exact coincidence of these lines. Since each of these peaks is furtber split by the methyl group, any value from -0.5 to 0.0 C.P.S.could he used without serious divergence from the observed spectrum. One of the most surprising features about the spectrum of methyl vinyl sulfide is that the p-protons couple with the methyl group (0.4 C.P.S. cis and 0.2 c.p.s. trans) while tbere is no evidence of coupling between the a-proton and the methyl group. The absence of the latter might be due to the angles i n ~ o l v e d . ~I n Fig. 1, it can be seen clearly that there is no coupling between the a-proton and the methyl group. The or-proton quartet shows no evidence of further splitting since the ringing produced by a fast sweep does not show the effect of two or more frequencies beating against one another.’O The long-range coupling to the /3-protons is reminiscent of similar long-range interactions through conjugated systems.” Since it is well known that the S atom interacts with unsaturated structures as if it were itself pseudo-ethylenic in charac(0) H.6 . Gutowsky, M. Karplus, and D. M. Grant, J . Chem. Phys., Si, 1278 (1959). (10) d. J Turner, J Mol. Phys., 8, 417 (1960). (11) H.J. Bernstein, J. A. Pople, and W. G. Schneider, Can. J. Chem., 86, 66 (1957); R. R. Fraser, h d . , 88, 540 (1960).
Vol. 67
ter,12 the transmission of spin-spin coupling across methyl vinyl sulfide appears to be not unreasonable. The cited behavior of S may involve the participation of its d-orbitals and the methyl group can be pictured as hyperconjugating with these orbitals to produce a -
+
structure of the form CH,=CH-S=CH,. From energetic considerations, structures of this type involving the d-orbitals are more reasonable for sulfur than for oxygen. Consequently, if long-range methyl coupling depends significantly on such a structure, it will be expected to be larger for the sulfide than for the ether. The greater upfield displacement of the pprotons in the ether, it should be remarked, results from a correspondingly larger mesomeric transfer of p-electrons from the hetero atom, which is not related to the above mechanism of coupling. It should be noted that the question of the relative signs of the long-range coupling constants in methyl vinyl sulfide cannot be resolved with our present procedure and data. Acknowledgment.-The research described in this report was supported in part by grants from the National Institutes of Health (A-2397(C-3)) and Schering Corporation, Bloomfield, N. J. (12) H. C. Longuet-Higgins, Trans. Faraday Soc., 46, 178 (1949); V. Schomaker and L. Pauling, J . A m . Chem. Sos.. 61, 177R (1939).
CARBON- 13 SPLITTINGS I N FLUORISE NUCLEAR MAGNETIC RESONANCE
SPECTRA1 BY NORBERT MULLERAND D ~ 4 m T. CARR Department of Chemistry, Purdue University, Lafayette, Indiana Received J u n e 8, 1961 Spin-spin coupling constants between FIQand directly bonded C18 nuclei have been measured for a wide variety of organofluorine compounds. The coupling constants tend to decrease with increasing fluorine chemical shifts. It is suggested that the degree of C-F double bond character is the most important single parameter influencing both of these observed quantities, ryith increased double bonding leading to stronger C-F coupling and decreased fluorine magnetic shielding. Increasing C +-F- ionic character apparently entails increased fluorine shielding and reduced C-F coupling, but this effect is quantitatively less important. Increasing the s-character of the carbon orbital used in the C-F bond also appears to reduce the magnitude of the C-F coupling constant. A number of long-range C-F, H-F, and F-F coupling constants also are reported.
Considerable effort has been devoted to discovering, both by theoretical and empirical methods, how the chemical shifts and spin-spin coupling constants which characterize nuclear magnetic resonance (n.m.r.) spectra are related to parameters describing the electronic wave functions of molecules. I n particular, spinspin coupling constants, J M F , between an Fig nucleus and the nucleus of a directly bonded atom, M, have been The results measured for a variety of were interpreted with the hypothesis that the coupling constants depend primarily on the fractional p-character of the atomic orbital of atom M that is used in the M-F bond. Although data for organic fluorine compounds are particularly well suited to test this hypothesis, only a few scattered observations of Ci3-F (1) Support of this work by the National Science Foundation is gratefully acknowledged. (2) H. 8. Gutowsky, D. W. McCall, and C. P. Sliohter, J . Chem. Phys., 21, 279 (1953). (3) 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. 196. (4) E. L. Muetterties and W. D. Phillips, J . A m . Chem. Soc., 81, 1084 (1959).
coupling constants (JcF)have been p u b l i ~ h e d . ~ - ~ The aim of the present research was to make a systematic study of J C F values for a variety of organofluorine compounds and to attempt to determine how these values depend on changes in the nature of the C-F bond. Experimental The experimental procedures were analogous to those used to study C18-H coupling constants,lo that is, they involved measurements of the spacings between “C%atellites” in the F19 n.m.r. spectra recorded at 56.4 Mc.p.8. The compounds were either commercial samples or were prepared in our Laboratory using standard methods.’’ Most of the observations were made with liquid samples a t room temperature, but several of the fluoromethanes were studied as gases, and liquid formyl fluoride was (5) P. C. Lauterbur, J . Chem. Phys., 26, 217 (1957). (6) G.V. D. Tiers, J . Phye. SOC.Japan, 16, 354 (1960). (7) G. V. D. Tiers and P. C. Lauterbur, J . Chem. Phye., 86, 1110 (1962). (8) P. C.Lauterbur, in “Determination of Organic Structures b y Physical Methods,” F. C. Nrtchod and W. D. Phillips, Ed., Academic Press, New York, N. Y.,1962,p. 505. (9) A survey including eighteen compounds has appeared since this study was completed: R. K. Harris, J . Phye. Chem., 66,768 (1962). (10) N. Muller and D. E Pritchard, J . Chem. Phys., 81,768, 1471 (1059). (11) D. T. Carr, Ph.D. Thesis, Purdue University, 1962.
