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.

We investigate the presence of helical character and chirality using a vector-based charge density perspective instead of energetic or structural measures. The vector-based perspective of the chemical bonding, constructed using the most preferred direction of charge density accumulation, finds the presence of induced symmetry-breaking for α,ω-disubstituted [4]cumulenes as the end groups are torsioned. The stress tensor trajectories Tσ(s) are used to provide the additional symmetry-breaking required to quantify the degree and nature of the chirality and helical character. We find an absence of chirality for [4]cumulene but a very significant degree of axiality as demonstrated by the purely axial form of the Tσ(s) indicating a lack of helical character. The S-1,5-dimethyl-[4]cumulene contains a very low degree of chiral character but significant axiality(helicity) resulting in a weakly helical morphology of the corresponding Tσ(s). The (-)S(-), (+)S(-) and (+)S(+) conformations of S-1,5-diamino-[4]cumulene contain very significant degrees of both chirality and helical character resulting in helical morphology of the corresponding Tσ(s). The chirality assignments are in agreement with the Cahn–Ingold–Prelog (CIP) classifications for the (-)S(-), (+)S(-) and (+)S(+) conformations of S-1,5-diamino-[4]cumulene. We discuss the consequences for the Tσ(s) in locating chiral character in these molecules in future experiment investigations.

We located ‘hidden’ S-character chirality in formally achiral glycine using a vector-based interpretation of the total electronic charge density distribution. We induced the formation of stereoisomers in glycine by the application of an electric field. Control of chirality was indicated from the proportionate response to a non-structurally distorting electric field. The bond-flexing was determined to be a measure of bond strain, which could be a factor of three lower or higher, depending on the direction of the electric field, than in the absence of the electric field. The bond-anharmonicity was found to be approximately independent of the electric field. We also compared the formally achiral glycine with the chiral molecules alanine and lactic acid, quantifying the preferences for the S and R stereoisomers. The proportional response of the chiral discrimination to the magnitude and direction of the applied electric field indicated use of the chirality discrimination as a molecular similarity measure.

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.

In this investigation we have used NG-QTAIM to fully quantity the response to the four IR-active modes of all the bonding in benzene in terms of bond-flexing, bond-torsion and bond-anharmonicity that includes the tendencies towards IR-responsivity and IR-non-responsivity. Bond-anharmonicity is found to be lacking for the C-C bonds comprising the lowest frequency mode (721.57 cm-1) measured as the absence of bond critical point (BCP) sliding. Additionally, bond-flexing was absent for this mode harmonic-like variation of the profile of the variation of the wrapping (torsion) of the {p,p’} path-packet, referred to as the Precession K, along the conventional QTAIM bond-path, the remaining three IR-active mode possessed step-like variations in the K profiles. The presence of non-nuclear attractors was detected for the IR-active mode with frequency 1573.93 cm-1 with C-C K profiles that most closely resemble those of the relaxed benzene. We quantified the C-H bonds in terms of bond-flexing and bond-anharmonicity and IR-responsivity and IR-non-responsivity.

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).

We seek to explain why the hydrogen bond possesses unusual strength in small water clusters that account for many of the complex behaviors of water. We have investigated and visualized the donation of covalent character from covalent (sigma) to hydrogen-bonds, by calculating the eigenvector coupling properties of QTAIM, stress tensor σ(r) and Ehrenfest Force F(r) on the F(r) molecular graph. The next generation 3-D bond-path framework sets are presented and only the F(r) bond-path framework sets reproduce the earlier finding on the coupling between covalent (sigma) and hydrogen-bonds that possess a degree of covalent character. The directional character of the covalent (sigma) and hydrogen-bonds 3-D bond-path framework sets for the F(r) explains differences found in the earlier results from QTAIM and the stress tensor σ(r).

We present, for the first time the S0 → S1 (S01) and S0 → S2 (S02) transition densities for fulvene, using the 3-D next generation QTAIM constructed using the preferred direction of electronic charge density accumulation. There is a symmetrization of the position of the bond critical point (BCP) of the torsional C2-C6 BCP along the bond-path associated with the presence of a conical intersection (CI) between the ground and first excited state (S1). The corresponding transition density S0 → S1 (S01) displays hindered BCP motion that is associated with a large rearrangement of the total electronic charge density, made apparent by the form of a 3-D bond-path. The reaction pathway for the second excited state does not have an associated CI to an adjacent state along this path, or symmetrization of the BCP position for the S0 → S2 (S02) transition density, or hindered motion, or a large deviation in the 3-D bond-path. We hypothesize that the symmetrization of the position of the torsional C2-C6 BCP along a bond-path for an excited state pathway is associated with a CI, where the transition density BCP is hindered and as a consequence the electron density undergoes a large rearrangement, made apparent by the form of the 3-D bond-path.