Proton and carbon-13 chemical shifts: Comparison between theory and experiment

A series of ab initio ¹H and ¹³C NMR chemical shifts are presented for all molecules for which gas-phase experimental measurements exist. Quantitative agreement with this large set of data is achieved by the use of gauge-invariant atomic orbitals in an SCF perturbation theory approach. The effect of basis set completeness on these ¹H and ¹³C chemical shifts is also examined. The 4-31G basis set is found to provide internally consistent results and give satisfactory agreement with gas-phase experimental data. Errors within 6% for ¹H shifts and 3% for ¹³C shifts result. Increasing the basis set to the 6-31G^* level does not significantly improve the agreement. For ¹H shifts only, the 3-21G basis set is adequate. The validity of the particular computational approach employed here is further substantiated by comparison to another ab initio magnetic shielding method.