Scheme 1. The molecular graph of the singly deuterated and
triterated isotopomers substituted onto the (alpha carbon) C1 atom of
glycine: Sa (X3 = D3/T3, X10 = H10) (left panel) and
Ra (X3 = H3, X10 = D10/T10) (right panel). The notation
HH will be used to denote X3 = H3, X10 = H10. The following notation HD
is used for {Sa (X3 = D3, X10 = H10),
Ra (X3 = H3, X10 = D10)} and HT is denotes
{Sa (X3 = T3, X10 = H10), Ra (X3 = H3,
X10 = T10)}. The torsional C1-C2 BCP and torsional C1-N7BCP are indicated by the black and blue circles respectively,
where the undecorated green spheres indicated the locations of bond
critical points (BCPs ).
In this article, we thus explore the extent to which subtle differences
between the electronic structures of isotopomers can be captured by
directly analyzing the electron density rather than by referring to the
vibrational spectrum. Tritium is a spin-½ isotope of hydrogen, with
effectively the same chemical shifts but with slightly higher
sensitivity, dispersion and coupling constants30.
We consider the total electronic charge density because it is an
observable31–33, rather than the wavefunction which
is not. Total electronic charge density distributions are available even
when sharp spectral lines are not such as is the case for the solid
state. We will analyze density-dependent quantities of the deuterium and
tritium isotopomers of glycine using QTAIM (Quantum Theory of Atoms in
Molecules)34 and more specifically the directional
next generation QTAIM see Scheme 1 . Previously, the stress
tensor trajectories T(s ), within Next generation QTAIM, were used
to quantify the O-H bond-flexing, bond-torsion and bond-axiality
(formerly referred to as bond anharmonicity) contributions to be
compared for different isotopomers of normal modes of the water
molecule35. This earlier analysis enabled the coupling
of intramolecular vibrational modes to be assessed and compared,
particularly between the bending normal mode and the symmetric-stretch
normal modes. The glycine conformer selected for current investigation
is characterized by an intense IR band36 involving the
C2-O5-H6 bending coupled with the wagging vibration of methylene group
(C1X3X10)37, which is expected to be shifted by D/T
isotopic substitution at that alpha carbon (C1), see Scheme 1 .
The ability to capture the chiral character and its changes upon the D/T
substitution allows the direct analysis of differences in the
chiroptical spectra. Such changes include sign changes, without the need
to introduce approximations based on the normal modes
description38.