The theory of carbon chemical shifts in N.M.R.

Abstract

An independent electron molecular orbital theory of diamagnetism is used to calculate carbon chemical shifts for some simple molecules characteristic of important organic groups. The molecules considered are ethane, ethylene, acetylene, allene, carbon disulphide, carbonate and formaldehyde. The theory predicts (1) a low field shift for molecules with low electronic excitation energies, (2) a high field shift if the carbon atom has high electron density, and (3) a multiple bond effect which may be estimated from molecular orbital bond orders between neighbouring atoms. It is shown that reasonable quantitative assumptions about these three effects in the molecules considered lead to calculated chemical shifts in satisfactory agreement with experimental data.