CARBON-13 CHEMICAL SHIFTS O F
VINYL
1947
CARBONS
Carbon-13 Chemical Shifts of Vinyl Carbons
by Gary E. Maciel Department of Chemistry, University of California, Davis, California (Received December 14, 1964)
The C13 chemical shifts 6c, and 6ca were determined for 17 vinyl compounds C,H2= C,H-X with a wide variety of substituents X. The chemical shifts of the a-carbon atom were found to cover a range of 67.5 p.p.m., while the span of 6c0 was found to be 57.5 p.p.m. No simple relationships between these carbon shifts and the corresponding proton shifts were evident. Rough linear relationships were demonstrated between 6c, and the corresponding C13 shifts of the substituted carbon atom in phenyl compounds, and between 6ca and the ortho carbon shifts in phenyl compounds. These correlations are interpreted as due to similar inductive, resonance, and neighbor effects in both sets of compounds as a result of similar electronic structures and molecular geometries. A linear correlation was not found between 6c, and the carbon C13shifts in corresponding acetyl compounds.
Introduction The many investigations of C13 magnetic resonance spectroscopy of the past few have included systematic studies of the relationships between C13 chemical shifts and structural details in a l k a n e ~ l - ~ and substituted aromatic hydrocarbons,8-10 substituted benzene^,^^'^-^^ carbonyl comp 0 ~ n d ~ and ~ ~ alkyne^.^^^^^^^ ~ ~ 3 ~ ~ ~The - ~trends ~ observed in these studies have led to correlations of interest from both a theoretical and practical point of view, and have pointed the way to promising avenues of future research. Carbon-13 shifts have been reported for several olefinic ~ o m p o u n d s , ~ ~ but ~ ~data - 2 ~on an extensive and related series of similarly substituted ethylenes have not been available. The absence of such data has contributed to the difficulty of satisfactorily interpreting C13 chemical shifts in other classes of compounds in which the carbon hybridization is the sp2type. In this paper we report the Cl3 shifts of 17 vinyl compounds CaH2=C,H-X and compare the shifts wiOh those in analogously substituted phenyl and acetyl compounds.
Experimental Carbon-13 Magnetic Resonance Measurements. The n.m.r. spectra were obtained at a frequency of 15.085 Mc./sec. by measuring the resonance of C13 in natural abundance, using Varian equipment and the rapid passage, dispersion mode technique described previously by Lauterbur.8 For most of the spectra, the C13
sample container was similar to that described by Spiesecke and Schneider6 except that no provision was (1) E. G. Paul and D. M. Grant, J . Am. Chem. Soc., 86, 2977 (1964). (2) D. M. Grant and E. G. Paul, ibid., 86, 2984 (1964). (3) J. J. Burke and P. C. Lauterbur, ibid., 86, 1870 (1964). (4) R. A. Friedel and H. L. Retcofsky, ibid., 85, 1300 (1963). (5) G. B. Savitsky and K. Namikawa, J. Phys. Chem., 67, 2430 (1963). (6) H. Spiesecke and W. G. Schneider, J. Chem. Phys., 35,722 (1961). (7) P. C. Lauterbur, Ann. N . Y . Acad. Sci., 70, 841 (1958). (8) P. C. Lauterbur, J . Am. Chem. Soc., 83, 1838 (1961). (9) P. C. Lauterbur, Tetrahedron Letters, 8, 274 (1961). (10) H. Spiesecke and W. G. Schneider, ibid., 14, 468 (1961). (11) G. B. Savitsky, J . Phys. Chem., 67, 2723 (1963). (12) C. P. Nash and G. E. Maciel, ibid., 68, 832 (1964). (13) G. E. Maciel and J. J. Natterstad, J. Chem. Phys., in press. (14) P. C. Lauterbur, ibid., 38, 1406 (1963). (15) P. C. Lauterbur, ibid., 38, 1415 (1963). (16) P. C. Lauterbur, ibid., 38, 1432 (1963). (17) H. Spiesecke and W. G. Schneider, ibid., 35, 731 (1961). (18) G. E. Maciel and R. V. James, J . Am. Chem. Soc., 86, 3893 (1964). (19) P. C. Lauterbur, ibid., 83, 1846 (1961). (20) G. E. Maciel and G. C. Ruben, 8id., 85,3903 (1963). (21) G. E. Maciel and G. B. Savitsky, J. Phys. Chem., 68,437 (1964). (22) D. D. Trafioante and G. E. Maciel, ibid., 69, 1348 (1965). (23) J. B. Stothera and P. C. Lauterbur, Can. J. Chem., 42, 1563 (1964). (24) P. C. Lauterbur in “Determination of Organic Structures by Physical Methods,” F. C. Nachod and W. D. Phillips, Ed., Academic Press, New York, N. Y., 1962, Chapter 7. (25) G. B. Savitsky and K. Namikawa, J . Phys. Chem., 67, 2754 (1963).
Volume 69. Number 6
June 1965
GARYE. MACIEL
1948
~~~~~~
Table I: CIS Chemical Shifts in Vinyl, Phenyl, and Acetyl Compounds, in P.p.m. with Respect to Benzene Substituent, X
I Br c1 COzCaHs
r NCO(CHd3' Sic& CH2Br CHzCl B(CH=CH& SnCI&H=CH2 CHzOCzHs CH20CHaCH=CH2 Sn(CH=CH2)8 SOzCH=CH2 CH&O OCOCHs Pb(CH=CH2)3 C(CHa)a O-TL-C~H~ OCHi
43.3 13.1 2.6 -1.1
-1.8 6.6 11.3 -1.8
32.3 5.4 -6.4 -3.6"
-1.3 -3.1 -4.5' -5.0' -5.8 -6.5 -7.1 -7.1 -7.5 -9.1 -9.8 -13.0 -16.7 -20.5d -22.9 -24.2
34.4 -10.0 11.0' 11.2' -6.8 -11.9 14.0 12.6 -7.9 -2.7 -0.6 32.3 -6.4 19.5d 45.6 44.1
-11.16 -5.0"
'
-2.6 -2.2 -1.0 -1.3"
0.4 1.0 2.0 -5.1"
9.ge
-0.2#
5.6"
0.3"
0.3"
-9.9
-3.3 -0.2 -1.3"
-29.3 -40.9 -41.9 -64.1
(0.3)" -71.9
-10.8"
-9.3 -23.0 -22.4" -30.2
0.6"
-0.2 6.4 3.2" 14.7
0.6"
-0.2 -1.3
(0.6)"
-4.2 2.3
-83.1
0.2" -0.9
-69.8 -37.3
8.1
-42.0
Data from ref. 13 (except as noted otherwise). Data from present work (except as noted otherwise). Data from ref. 22 and Data kindly provided by G. B. Savitsky and K. Data for acetanilide rather than N-phenylpyrrolidone. Shifts from ref. 4. 23. Namikawa.
made for spinning the sample; it consisted of two concentric, thin-walled spherical bulbs about 0.2 and 1.4 cc. in volume, a geometry which obviates the need for bulk susceptibility corrections in these results. The small inner bulb contained a reference for calibration of the resonance lines. Most of the present data were obtained using a reference consisting of a saturated aqueous solution of sodium acetate enriched to about 28% CI3 at both carbon positions. A calibration experiment with benzene and with this reference using the side-band technique showed the center of the benzene doublet to be 53.1 p.p.m. to higher field than the carbonyl resonance, and the separation between the carbonyl resonance and the center of the methyl quartet of the reference to be 156.3 i 0.2 p.p.m. These separations were used to calibrate the spectra with respect to the chemical shift of benzene. The spectra of vinyl bromide (in cyclohexane solution), vinyl iodide, divinyltin dichloride, vinylsilicon trichloride, allyl ethyl ether and diallyl ether were obtained using a previously described reference in the center bulb.12 The Cla spectra of vinyl chloride and vinyl methyl ether were obtained on solutions in dimethyl carbonate, saturated and sealed at 0', using the dimethyl carbonate as both The Journal of Physical Chemistry
'
solvent and internal reference. The C13 chemical shifts of dimethyl carbonate have been found previously to be insensitive to most adulterant^'^; the shift of the carbonyl carbon is about 28.0 p.p.m. to lower field than that of benzene, and the separation between the carbonyl resonance line and the center of the methyl quartet is about 102.7 p.p.m. Considering the use of different references and calibration procedures, the shifts of the vinyl carbons presented in Table I with respect to benzene can be considered reproducible to about *0.8 p.p.m. The C13 shifts of the monosubstituted benzenes obtained in this study were measured with respect to an external reference previously described,12and can be considered precise to about *0.5 p.p.m. Materials. Vinyl iodide was prepared by the reaction of iodine with tetravinyltin according to a modification of Seyferth's pr~cedure.~'The C I 3 spectrum was obtained on a sample which had b.p. 55-57' [lit.2* 58-56.5'1. (26) G. B. Savitsky and K. Namikawa, J. Phys. Chem., 68, 1956 (1964). (27) D.Seyferth, J. Am. C h m . Soc., 79, 2133 (1957). (28) J. Spence, ibid., 55, 1290 (1933).
1949
CARBON-13 CHEMICAL SHIFTS O F VINYL CARBONS
The spectra of the other compoundswere obtained on neat liquids as they are available from commercial sources. Allyl chloride, allyl ethyl ether, diallyl ether, ethyl benzoate, and benzyl ethyl ether were Eastman White Label reagents. Vinyl bromide, vinyltrichlorosilane, tetravinylsilane, divinylsulfone, and tetravinyllead were from Columbia Organic Chemicals. Vinyl chloride and vinyl ethyl ether were from the Matheson Co. Ethyl acrylate, allyl bromide, and vinyl acetate were Eastman Practical materials. N-Vinylpyrrolidoneand t-butyl benzene (Puriss) were from Aldrich Chemical Co. Divinyltin dichloride, tetravinyltin, l-butene-3one, and vinyl n-butyl ether were from K and K Laboratories.
Results and Discussion Carbon-13 magnetic resonance spectra were obtained on 17 vinyl compounds C,,HF=C,H-X with a wide variety of substituents X. In each case the spectrum consisted of several peaks: one or more lines due to the reference; a signal due to the a-carbon resonance, split into a doublet by the a-hydrogen atom; a resonance due to the @-carbon,split into a triplet by the @-hydrogens; and peaks due to carbon atoms (if any) in the substituent X. Illustrative examples are the spectra of vinyl iodide and tetravinyltin shown in Figures 1 and 2, respectively. I n some cases five distinct lines traceable to vinyl carbons were not observed, but consistent and reasonable assignments could be made on the assumption of peak overlap and C13-H splittirigs on the order of about 150 c.P.s.; Figure 3, which shows the spectrum of divinyl sulfone, is an example. The data are collected in Table I. The C13 chemical shifts of the a-carbon atom, (6c, in Table I) cover a range of 67.5 p.p.m. This is considerably smaller than the corresponding range of C13 shifts of the a-carbon in l-substituted-l-hexynes,22 and about the same as the range of a-carbon shifts in substituted ethanes.6 It is slightly larger than the range of shifts of the substituted carbon atom of monosubstituted benzenes13J7(6cx in Table I), and is somewhat larger than the corresponding range of carbonyl C13shifts in acetyl compounds ( ~ C Oin Table I).23s24 The shifts in Table I of the @-carbonatom, 6cg, cover a range of 57.5 p.p.m., which is considerably larger than the ranges of CY3 shifts for the @-carbonsin substituted ethanes6 or of l-substituted-l-hexynes,22or of the ortho carbons in monosubstituted b e n ~ e n e s ~(6c, ~ , ' ~in Table I). Comparison of the vinyl CI3 chemical shifts of Table I with the comprehensive proton chemical shift data published by Briigel, Ankel, and Kriickeberg29 for the vinyl group revealed no apparent correlations. This is
-50
0
50
-50
0
50
100
-50
0
50
100
Figure 1. Carbon-13 n.m.r. spectrum of vinyl iodide with CHaC'a02Na reference (Ref.) and side band (SB), increasing field sweep (upper), and image of decreasing field sweep (lower) in p.p.m. with respect to benzene. Figure 2. Carbon-13 n.m.r. spectrum of tetravinyltin with C13HsC02Naand CHaC'aOZNa reference, increasing field sweep (upper), and image of decreasing field sweep (lower) in p.p.m. with respect to benzene. Figure 3. Carbon-13 n.m.r. spectrum of divinylsulfone with C'aHaC02Na and CHaC'302Na reference, increasing field sweep (upper), and image of decreasing field sweep (lower) in p.p.m. with respect to benzene.
not surprising in view of the considerably different geometrical orientation of carbon and hydrogen atoms a t the a- and @-positionswith respect to the substituent, thus giving rise to profound differences in neighbor effects due to "neighbor ani~otropy"~ or intramolecular dispersion forces.a0 It is also consistent with the apparent lack of a direct correlation between the corresponding proton and carbon shifts in substituted ethanes and a t the ortho position in substituted benzenes in the work of Spiesecke and Schneider.6," The very geometrical factors which appear to preclude a direct correlation between proton and carbon chemical shifts in vinyl as well as other groups would seem to set the stage for simple relationships between 6c, in vinyl compounds and 6cx in phenyl compounds or 6co in carbonyl compounds, and between 6cs in vinyl compounds and 6c, in phenyl compounds. Each of the carbon atoms in these categories employs sp2 hybrid orbitals in u-bonds to three adjacent atoms and contributes a p-orbital to a ?r-bond. Furthermore, in each of these three pairs the geometrical relationship to the substituent should be nearly the same. Thus, within each pair of categories the neighbor anisotropy and intramolecular dispersion effects exerted by the substituents should be comparable and will not be ex(29) W. Bmgel, T. Ankel, and F. Krtickeberg, Z. Elektrochem., 64, 1211 (1960). (30) T. Schaefer, W. F. Reynolds, and T. Yonemoto, Can. J . Chem., 41, 2969 (1963).
Volume 69, Number 6 June 1966
GARYE. MACIEL
1950
-
e e 0
-80
-70
-60
-50
-40
-30
6CO.
8Cx.
Figure 4. Plot of 6c, for vinyl compounds us. 6c, for the corresponding phenyl compounds.
pected to obscure any correlations of C13 shifts due to similar inductive and resonance influences of the substituents on the molecular electronic configurations. This type of consideration previously has been used successfully to relate the C13shifts of carbonyl carbons and of the ethylenic carbons of symmetrical disubstituted ethylenes to carbonyl a-bond p01arity.l~ It also may be responsible for the relatively consistent success of the constitutive relations developed by Savitsky and Namikawa. 26 I n order to test these ideas, we have plotted 6c, for vinyl compounds us. 6cX in phenyl compounds and us. ac0 in acetyl compounds in Figures 4 and 5 , respectively, and 6ca for vinyl compounds us. 6c, for phenyl compounds in Figure 6 . Rough linear relationships of the expected type are apparent in Figures 4 and 6. While the scatter in Figure 4 is quite appreciable, the general trend lends support to the view expressed above that substituent effects should be similar in these closely related cases. The fact that the slope is nearly 1 indicates further that each type of electronic influence may be exerted to about the same degree in these two similar structural situations. The rough linear correlation displayed in Figure 6 is further evidence for the validity of the above assumptions, However, this plot differs from Figure 4 in two fundamental ways: the slope is nearly 2, rather than 1; and two points, those corresponding to the acetyl and carboethoxy substituents, deviate widely from the The Journal of Physic& Chemistry
Figure 5. Plot of 6c, for vinyl compounds vs. 6 ~ 0 for the corresponding acetyl compounds.
40
-
30 -
&20
-
‘e
10 -
0-10
-5
0
5
10
15
6%
Figure 6. Plot of 6cp for vinyl compounds va. 6c0 for the corresponding phenyl compounds.
correlation line. These departures may be related to differences in the importance of resonance interaction between the substituent and the p-carbon in a vinyl compound in one case, and between the substituent and
CARBON-13 CHEMICAL SHIFTS O F VINYL C l R B O N S
the ortho carbon in a phenyl compound in the other case. In vinyl compounds, the excess positive or negative charge due to contributions of canonical structures I11 or I1 must reside entirely on the 6-carbon, whereas in phenyl compounds it may be distributed to both ortho carbons as well as to the para position. This situation might tend to make a 6-carbon more sensitive than an ortho carbon to the resonance effect of a substituent, and might account for the twofold greater sensitivity of 6ca than 6c, to variations of the substituent. Furthermore, the conspicuous deviation of the acetyl and carboethoxy point,s in Figure 6 may be due to the fact that these substituents are the only -R groups on the graph and, hence, the only cases where structure I11 might be of importance. In stark contrast to the rough linear relations dis-
1951
played in Figures 4 and 6, Figure 5 demonstrates that 6c, is not so related to the carbonyl C13 chemical shifts of corresponding acetyl compounds. Since both classes of carbon atoms in question are the sp2type, this difference in behavior is most likely not due to differences in C-X bond lengths between these classes of compounds. More likely it is related to the different inductive and resonance effects of the =O fragment as compared to =CH2, and the influence of the carbonyl bond polarity. Figures 4 and 6 may also reflect a closer relationship between the excited electronic states of vinyl and phenyl compounds than between vinyl and acetyl compounds, as these states are responsible for the large paramagnetic contributions to Clashifts. Acknowledgment. The author is grateful to Professor George B. Savitsky for helpful discussions.
Volume 69, Number 6 June 1966
