A theoretical study of inversion barriers and NMR chemical shifts of 3‐pyrazolines (2,3 ‐dihydro‐1 H ‐pyrazoles)

A rather neglected family of heterocyclic compounds, the 3‐pyrazolines or 2,3‐dihydro‐1 H ‐pyrazoles, has been studied theoretically at the B3LYP/6–311++G(d,p) level to obtain geometries and energies and at the GIAO/B3LYP/6–311++G(d,p) level for NMR chemical shifts. The calculated barriers of inversion of the N ‐substituents reproduce adequately the scarce experimental data. The calculated 1 H and 13 C chemical shifts are consistent with those reported in the literature. A combination of both results, geometries, and 13 C chemical shifts, shed light on the conformation of N ‐phenyl groups.