13C NMR chemical shifts (delta(C)) were analysed via MO theory, together with the origin, using sigma d(C), sigma p(C) and sigma t(C), where C4- was selected as the standard for the analysis since sigma p(C: C4-) = 0 ppm. An excellent relationship was observed between sigma d(C) and the charges on C for (C4+, C2+, C0, C2- and C4-) and (C4-, CH22-, CH3- and CH4). However, such a relationship was not observed for the carbon species other than those above. The occupied-to-unoccupied orbital (psi i ->psi a) transitions were mainly employed for the analysis. The origin was explained by the pre-alpha, alpha, beta, alpha-X, beta-X and ipso-X effects. The pre-alpha effect of an approximately 20 ppm downfield shift is theoretically predicted, and the observed alpha and beta effects of approximately 10-15 ppm downfield shifts are well reproduced by the calculations, as are the variations in the alpha-X, beta-X and ipso-X effects. Large downfield shifts caused by the formation of ethene (similar to 120 ppm), ethyne (similar to 60 ppm) and benzene (similar to 126 ppm) from ethane and carbonyl (similar to 146 ppm) and carboxyl (similar to 110 ppm) groups from CH3OH are also reproduced well by the calculations. The analysis and illustration of sigma p(C) through the psi i ->psi a transitions enables us to visualize the effects and to understand the delta(C) values for the C atoms in the specific positions of the species. The occupied-to-occupied orbital (psi i ->psi j) transitions are also examined. The theoretical investigations reproduce the observed results of delta(C). The origin for delta(C) and the mechanism are visualized, which allows us to image the process in principle. The role of C in the specific position of a compound in question can be more easily understood, which will aid in the development of highly functional compounds based on NMR.
