Computational Details
An iterative process is employed to create the two isomers in the presence of an electric field. To create the Sa and Ra stereoisomers, a directed E -field is applied parallel (+E ) or anti-parallel (-E ) to each of the C1-H3 or C1-H10 BCP bond-paths (see Scheme 1 for atom labeling). We assign the label Sa in cases where the C1-H3 BCP bond-path length > C1-H10 BCPbond-path length and the label Ra if the C1-H10BCP bond-path length > C1-H3 BCP bond-path length. Each stereoisomer is subjected to a two-step iterative process consisting of (i) a molecule alignment step in which the alpha C1 atom is fixed at the origin of the coordinate frame: the selected C-H is aligned along a reference axis with the positive sense of the axis from C to H and the N atom consistently aligned in the same plane, followed by (ii) a constrained optimization step with the selected electric field applied along the reference axis: the default G09 sign convention for the field relative to the reference axis is used. This two-step process is repeated ten times, ensuring the consistency of the field application direction and the chosen bond (C1-H3 or C1-H10) direction. The resulting structures are then used in the subsequent torsion calculations, with the C1-H3 and C1-H10 bond lengths constrained to their field-optimized values.
The achiral glycine is subjected to E -fields = ±20×10-4 a.u., ±100×10-4 a.u. and ±200×10-4 a.u. before the resultant structure is twisted to construct the trajectories Tσ(s ) from the series of rotational isomers -180.0º ≤θ ≤+180.0º for the torsionalBCPs (the C1-N7 BCP and the C1-C2 BCP) of glycine. Note that these dihedral angle definitions traverse the C-C bond in opposite directions, i.e. C2→C1 and C1→C2; therefore the definitions of CCW and CW are inverted for the C1-N7 torsion and the C1-C2 torsion. We determine the direction of torsion as CCW or CW from an increase or a decrease in the dihedral angle, respectively.
Single-point calculations were then performed on each scan geometry, converged to < 10-10 RMS change in the density matrix and < 10-8 maximum change in the density matrix to yield the final wavefunctions for analysis. QTAIM and stress tensor analysis was performed with the AIMAll[56] suite on each wave function obtained in the previous step. All molecular graphs were additionally confirmed to be free of non-nuclear attractor critical points.