We seek to determine the two-way transfer of chemical character due to the coupling occurring between hydrogen-bonds and covalent-bonds known to account for the unusual strength of hydrogen-bonds in water. We have provided a vector-based quantification of the chemical character of uncoupled hydrogen-bonds and covalent-bonds and then determined the effects of two-way coupling consistent with the total local energy density H(rb) < 0 for hydrogen-bonds. We have calculated the precessions Kʹ of the eigenvectors around the bond-path for the Ehrenfest Force F(r) and compared with the corresponding QTAIM Kʹ. In doing so we explain why the Ehrenfest Force F(r) provides insights into the coupling between the hydrogen and covalent bonds whilst QTAIM cannot. Conditions for favorable transfer of electron momentum from the hydrogen atom of a sigma bond to the hydrogen-bond are found, with excellent agreement with the hydrogen-bond BCP and covalent-bond BCP separations providing the theoretical bounds for coupling.

The effect of the presence of a deuterium (D) or tritium (T) isotope bonded to the alpha carbon of glycine is determined without the need to apply external forces e.g. electric fields or using normal mode analysis. Isotopic effects were accounted for using the mass-dependent diagonal Born-Oppenheimer energy correction (DBOC) at the CCSD level of theory. We calculated the stress tensor trajectories of the dominant C-N bond within next generation quantum theory of atoms in molecules (NG-QTAIM). S-character chirality was discovered using the stress tensor trajectories, instead of the Cahn–Ingold–Prelog (CIP) rules, for ordinary glycine. The S-character chirality was preserved after the substitution of the H on the alpha carbon for a D isotope but transformed to R-character chirality after replacement with the T isotope. This reversal of the chirality depending on the presence of a single D or T isotope bound to the alpha carbon adds to the debate on the nature of the extraterrestrial origins of chirality in simple amino acids. We demonstrate that NG-QTAIM is a promising tool for understanding isotopic induced electronic charge density changes, useful in analysis of infrared (IR) or circular dichroism (CD) spectra explaining changes in mode couplings and bands intensities or sign.

The effect of a directional electric-field on the bonding of the undoped and sulphur doped diarylethene (DTE) switch molecule is investigated using next generation QTAIM (NG-QTAIM). We introduce chemical bonding concepts in the form of the least and most preferred directions of charge density accumulation relative to the associated bond-path, namely the precessions K and Kʹ that are demonstrated to be much more responsive to the electric-field than the Laplacian ∇2ρ(rb). A concept of bond fatigue is presented in terms of the tendency for a bond-path to rupture that provides directional versions of familiar bonding QTAIM concepts. Examples are included where the applied directional electric-field reduces the tendency towards bond-path rupture and also the converse. A brief discussion is undertaken of applications of the precessions K and Kʹ including switches, ring opening reactions and molecular rotary motors in the presence of fields that cause a redistribution of ρ(r).