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.