Isomer identification for fullerene C-84 by C-13 NMR spectrum: A density-functional theory study

Optimized geometries of ail 24 isolated pentagon rule (IPR) abiding isomers of fullerene C-84 have been calculated using density-functional theory (DF T) at the B3LYP/6-31G* level. C-13 NMR chemical shieldings are obtained emp loying the gauge-independent atomic orbital method. The calculated chemical shifts are in good agreement with experimental values for isomers 4, 22, a nd 23, all of which have been experimentally assigned without ambiguity. Th e calculated NMR spectra allow us to confirm earlier assignment and validat e the DFT approach. The previously temporarily assigned isomers D-2(II), C- 2, C-s(a), and C-s(b) are isomers 5, 11, 16, and 14, respectively. Discrepa ncies exist between the experimental and theoretical NMR spectra for isomer s 19 and 24. The predicted NMR spectra for other isomers are also presented . The local geometry is determined largely by connectivity. The relationshi p between the chemical shift and the,pi -orbital axis vector (POAV) angle i s far from linear, although the chemical shift generally increases when the POAV angle increases. The pyrene-type carbons form five distinct groups in the chemical shift vs POAV angle graph according to the local connectivity , providing a usable tool for their identification. Similarly, two and thre e groups can be identified for corannulene and pyracylene types of carbons, but these are not sufficiently distinct to be useful.