8073
.
C6
D4
3
S/
c21
C0,COCl I
I
250
225
200
175
125
I50
100
75
50
25
0
ppm
Figure 1. 67.9-MHz I3C N M R spectrum of I1 at 1 1 5 "C. Cyclohexenyl cation peaks are marked x.
IIIa IVa VIP Va I1 VIC a
6 per D, ppm
T , "C
46.9 22.1
109 1I O 111 1 I9 1 I5 1I O
Reference 2.
11
6.4 1.18
0.33
Acknowledgment. We should like to thank the NSF for the support of this work through grants to M.S. and K.B.W. (CHE-7519380). The N M R spectra were obtained using the Southern New England High Field N M R facility supported by a grant from the Biotechnology Resources Program of the National Institutes of Health (RR0798). References and Notes
Table I. Relative IsotoDic Solittings ( & / A ) Compd
deuteration is reduced relative to methylene deuteration. The still smaller 8/A obtained for V is explained similarly, but here delocalization'~11~12 probably decreases A as well. The relative splitting for V, though the smallest value observed for an unequivocal equilibrium isotope effect, is still a factor of 10 larger than that for VI where the splitting reflects perturbation of resonance. That 6/A for I1 is also an order of magnitude smaller than the values for 111-V and VI1 is strong evidence that we are observing, not an equilibrium isotope effect, but isotopic perturbation of resonance in the I3C N M R spectrum of II.I3 In other words, the relative splitting indicates that the structure of I1 is extensively u-delocalized. Even if our estimate of A is incorrect, the relative splitting for I1 remains considerably smaller than that of VI, unless one sets A to less
