Detailed analysis of coupling constants and isotope effects in NMR spectra of isotopomers of (12)C(68) (13)C(2)

A preliminary study of the long-range (i.e. two-bond or longer) (13)C--(13)C coupling constants in natural abundance C(70) shows, consistent with recent theoretical calculations by Peralta et al. that the largest long-range J(CC) values for the polar and equatorial sites are clearly smaller than the largest long-range J(CC) values for the other three sites. The unusually large size of the (2)J(CC) couplings between inequivalent carbons in a nonpolar pentagon in C(70) has no analog among (2)J(CC) data reported for planar aromatic compounds. No long-range J(CC) values appear to have been reported for any curved aromatic compounds. In addition, much more precise (1)J(CC) values were obtained for C(70) than was possible about 15 years ago. Comparing the chemical shifts for each of the five isotopomers of C(70) containing only one (13)C nucleus and the frequencies of the satellites for each of the four isotopomers containing two adjacent and inequivalent (13)C nuclei indicates that replacing (12)C with (13)C shields the adjacent (13)C nucleus by 15 to 23 ppb, consistent with the limited (1)Delta(13)C((13/12)C) isotope effect data available on a few small aromatic molecules. Such measurements become possible with natural abundance C(70) only by using a (13)C cryoprobe and a high-field spectrometer (700 MHz). The additional information that could be obtained from a spectrum obtained under ultrahigh resolution conditions is discussed. Secure identification of the singlets arising from the four (12)C(68) (13)C(2) isotopomers with equivalent adjacent (13)C nuclei is necessary to allow the largest long-range J(CC) values to be precisely determined. The presence of numerous isotopomers containing two or more (13)C nuclei would present a great challenge in interpreting the various signals in a spectrum obtained under ultrahigh resolution conditions.