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 Δ 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. Copyright © 2006 John Wiley & Sons, Ltd.