Analysis of the NMR Parameters Shielding and Spin–Spin Coupling Constants of Glycine Conformers

Abstract

In this study, we worked at the CCSD/aug-ccpVTZ level to obtain the conformers of glycine in its neutral and zwitterionic forms in the gas and water phases. We then computed the NMR properties (spin−spin coupling constants and nuclear magnetic shieldings) at the SOPPA/aug-cc-pVTZ-J level. We attempt to elucidate the apparent discrepancy arising from two previous works by Valverde et al. [J. Chem. Phys., 2018, 148,024305] and Caputo and Provasi [Sci, 2021, 3, 41]. Our optimized structures align with previous theoretical predictions, although some geometries were not found. Additionally, our results suggest that in the gas phase the Δ 1J(O,C) = 1J(Ocis,CO) − 1J(Otrans,CO) is positive when the acid group is in the trans conformation and negative in the cis conformation; however, this trend is not well reproduced in water. From magnetic shielding calculations, we cannot distinguish between an O−H in the cis or trans conformation.