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.