The 3-D bonding morphology of the infra-red active normal modes of benzene

Scoring molecular wires subject to an ultrafast laser pulse for molecular electronic devices

A nonionizing ultrafast laser pulse of 20-fs duration with a peak amplitude electric-field ±E = 200 × 10^-4 a.u. was simulated. It was applied to the ethene molecule to consider its effect on the electron dynamics, both during the application of the laser pulse and for up to 100 fs after the pulse was switched off. Four laser pulse frequencies ω = 0.2692, 0.2808, 0.2830, and 0.2900 a.u. were chosen to correspond to excitation energies mid-way between the (S₁ ,S₂ ), (S₂ ,S₃ ), (S₃ ,S₄ ) and (S₄ ,S₅ ) electronic states, respectively. Scalar quantum theory of atoms in molecules (QTAIM) was used to quantify the shifts of the C1C2 bond critical points (BCPs). Depending on the frequencies ω selected, the C1C2 BCP shifts were up to 5.8 times higher after the pulse was switched off compared with a static E-field with the same magnitude. Next generation QTAIM (NG-QTAIM) was used to visualize and quantify the directional chemical character. In particular, polarization effects and bond strengths, in the form of bond-rigidity vs. bond-flexibility, were found, for some laser pulse frequencies, to increase after the laser pulse was switched off. Our analysis demonstrates that NG-QTAIM, in partnership with ultrafast laser irradiation, is useful as a tool in the emerging field of ultrafast electron dynamics, which will be essential for the design, and control of molecular electronic devices.

Next-generation QTAIM for scoring molecular wires in E-fields for molecular electronic devices

The effect of a varying, directional E _x , E _y , and E _z electric field on the ethene molecule was investigated using next-generation quantum theory of atoms in molecules (QTAIM). Despite using low E-field strengths that are within the realm of experiment and do not measurably alter the molecular geometry, significant changes to the QTAIM properties were observed. Using conventional QTAIM, the shifting of the C─C and C─H bond critical points (BCPs) demonstrates polarization through an interchange in the size of the atoms involved in a bond, since a BCP is located on the boundary between a pair of bonded atoms. Next-generation QTAIM, however, demonstrates the polarization effect more directly with a change in morphology of the 3-D envelope around the BCP. Modest increases of ≈ 2% in the ellipticity ε of the BCP were uncovered when the C─C bond was aligned parallel or anti-parallel to the applied E _x -field. Significant asymmetries were found in the response of the next-generation QTAIM 3-D paths of the C─H bonds to the applied E-field. When the E-field coincided with the C─C bond, the BCP moved in response and was accompanied by the envelope constructed from 3-D next-generation paths. The response displayed a polarization effect that increased with increasing magnitude of the E _x -field parallel and anti-parallel to the C─C bond. Our analysis demonstrates that next-generation QTAIM is a useful tool for understanding the response of molecules to E-fields, for example, for the screening of molecular wires for the design of molecular electronic devices. © 2019 Wiley Periodicals, Inc.

Intramolecular mode coupling of the isotopomers of water: a non-scalar charge density-derived perspective

Left: The BCP trajectories T(s) for H₂O for the bending (Q1) mode, the axes labels of the trajectory T(s). The green spheres correspond to the bond critical point (BCPs). Right: The corresponding T(s) for H₂O for the symmetric-stretch (Q2) mode.

The destabilization of hydrogen bonds in an external E-field for improved switch performance

The effect of an electric field on a recently proposed molecular switch based on a quinone analogue was investigated using next-generation quantum theory of atoms in molecules (QTAIM) methodology. The reversal of a homogenous external electric field was demonstrated to improve the "OFF" functioning of the switch. This was achieved by destabilization of the H atom participating in the tautomerization process along the hydrogen bond that defines the switch. The "ON" functioning of the switch, from the position of the tautomerization barrier, is also improved by the reversal of the homogenous external electric field: this result was previously inaccessible. The "ON" and "OFF" functioning of the switch was visualized in terms of the response of the most preferred directions of motion of the electronic charge density to the applied external field. All measures from QTAIM and the stress tensor provide consistent results for the factors affecting the "ON" and "OFF" switch performance. Our analysis therefore demonstrates use for future design of molecular electronic devices. © 2019 Wiley Periodicals, Inc.