Trends in shift rules in carbon-13 nuclear magnetic resonance spectroscopy and computer-aided shift prediction

A major problem in the use of ¹³C-NMR spectroscopy for structure identification is to estimate the ¹³C shifts of compounds known or suspected to be present in the spectrum. The substituent chemical shifts of different functional groups were studied and new, detailed empirical rules are reported. Trends in these shift parameters are noted. It appears that for functional groups containing more than one nucleus, the observed shift parameters (x=α, β, γ, δ shifts) can be approximated by the shift parameters of the component nuclei (x_comp) in the functional group, i.e., x_obs ≈ x_comp. The detailed shift behavior and shift additivity rules were computerized. The resulting program (CSPEC) has many user friendly features, e.g., ease of input, modification of structure, storage and retrieval of known shifts, and rapid computation.