A theoretical interpretation of 13C screening data in unsaturated molecules

Carbon-13 screening constants are calculated within the INDO/S level of approximation to Pople's model. Satisfactory agreement is obtained in most cases between the calculated and observed screening results. An analysis of the contributions of the π → σ*, σ → π* and σ → σ* transitions to the paramagnetic term shows that a linear relationship between ¹³C chemical shifts and the lowest energy transition is not present. The average excitation energies are found to vary appreciably among the molecules studied.