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

In this study, we worked at the CCSD/aug-cc-pVTZ 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. [<i>J. Chem. Phys.</i>, <b>2018</b>, <i>148</i>, 024305] and Caputo and Provasi [<i>Sci</i>, <b>2021</b>, <i>3</i>, 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 ¹<i>J</i>(O,C) = ¹<i>J</i>(O_cis,C_O) - ¹<i>J</i>(O_trans,C_O) is positive when the acid group is in the <i>trans</i> conformation and negative in the <i>cis</i> conformation; however, this trend is not well reproduced in water. From magnetic shielding calculations, we cannot distinguish between an O-H in the <i>cis</i> or <i>trans</i> conformation.