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

Abstract

A rather neglected family of heterocyclic compounds, the 3‐pyrazolines or 2,3‐dihydro‐1H‐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 1H and 13C chemical shifts are consistent with those reported in the literature. A combination of both results, geometries, and 13C chemical shifts, shed light on the conformation of N‐phenyl groups.

Nuclear Magnetic Resonance

“…Alkorta and Elguero 73 reported on theoretical study of inversion barriers and NMR chemical shifts of 3-pyrazolines (2,3-dihydro-1H-pyrazoles). Schindler 74 predicted GIAO-MP2 spectra of E-and Z-clothianidin which were experimentally confirmed by low temperature NMR studies.…”

3-Pyrazolines (2,3-dihydro-1H-pyrazoles): synthesis, reactivity, and physical and biological properties

“…In 3-pyrazolines the substituents on the nitrogen atoms adopt a trans disposition; the cis isomer is much less stable to the point that in some cases it is not stable and corresponds to a transition state. 142 When a substituent is present at position 5 (the other being an H atom) the minimum energy conformation corresponds to the transtrans isomer. The experimental barriers to the double nitrogen inversion were measured by Kostyanovsky et al in two cases.…”