Computational and Theoretical Chemistry 

Abstract: Many covalently-bonded atoms of Groups IV–VII have regions of positive electrostatic potential (σ-holes) opposite to the bonds, along their extensions. Through these positive regions, the atoms can interact highly directionally with negative sites. (Halogen bonding, in which the σ-hole is on a Group VII atom, is an example of this, and we suggest that hydrogen bonding is as well.) The formation and observed properties of the resulting noncovalent complexes can be fully explained in terms of electrostatics/polarization plus dispersion as the driving forces; this straightforward interpretation is based largely upon physical observables – electrostatic potentials, geometries, interaction energies and electric fields. More elaborate interpretations, involving less physically-based methods and models, have also been advanced. In this paper, we try to reconcile some of these seemingly different approaches.

Abstract: We present a simplified time-dependent density functional theory approach (sTD-DFT) that allows fast computation of electronic ultraviolet (UV) or circular dichroism (CD) spectra of molecules with 500–1000 atoms. The matrix elements are treated in the same way as in the recently proposed simplified Tamm-Dancoff approach (sTDA, S. Grimme, J. Chem. Phys., 138 (2013), 244104) but instead of applying the Tamm-Dancoff approximation, the standard linear-response density functional theory problem is solved. Compared to sTDA, the method leads to an increase in computation time (typically a factor of 2–5 compared to the corresponding sTDA) which is justified since the resulting transition dipole moments are in general of higher quality. This becomes important if spectral intensities (e.g. single-photon oscillator and rotatory transition strengths) are of interest. Comparison of UV and CD spectra obtained from sTD-DFT and sTDA for some typical systems employing standard hybrid functionals shows that both yield very similar excitation energies but the advantage of using the former approach for transition moments. In order to show the applicability of sTD-DFT to systems which are far beyond the scope of conventional TD-DFT, we present the CD spectrum of a substituted, chiral fullerene over a range of almost 1200 excited states. We propose this method as a more reliable alternative for the prediction especially of the more challenging CD spectra.

Abstract: Recently, hydroxylated and fluorinated graphene-like titanium carbide TiCx layers have been solvothermally fabricated in large amounts from so-called MAX phase Ti3AlC2. We assume, that a wide family of novel planar and tubular forms of titanium carbides may exist and design the atomic models for monolayers and nanotubes with nominal stoichiometry Ti2C, Ti3C2 and for their hydroxylated forms Ti2C(OH)2, Ti3C2(OH)2. The stability and electronic properties of these nanostructures are examined by means of density-functional theory tight-binding method depending on the composition and the type of OH arrangement. We reveal that the type of OH termination plays a minor role in the variation of nanotubes’ strain energies, but causes a difference in the relative stability of their parent planar phases. The electronic structure for all nanotubes studied has metallic-like character, while their precursors (planar layers) demonstrate either metallic-like or semiconducting behavior depending on the arrangement of the surface OH groups.

Abstract: The stability of Lithium intercalated 2H- and 1T allotropes of Molybdenum disulfide (Li x MoS 2 ) is studied within a density-functional theory framework as function of the Li content ( x ) and the intercalation sites. Octahedral coordination of Li interstitials in the van der Waals gap is found as the most favorite for both allotropes. The critical content of Lithium, required for the initialization of a 2H → 1T phase transition is estimated to x ≈ 0.4. For smaller Li contents the hexagonal 2H crystal structure is not changed, while 1T-Li x MoS 2 compounds adopt a monoclinic lattice. All allotropic forms of Li x MoS 2 – excluding the monoclinic Li 1.0 MoS 2 structure – show metallic-like character. The monoclinic Li 1.0 MoS 2 is a semiconductor with a band gap of 1.1 eV. Finally, the influence of Li intercalation on the stability of multiwalled MoS 2 nanotubes is discussed within a phenomenological model.

Abstract: Molecular dynamics simulation method was adopted to investigate the absorption behavior, inhibition mechanisms on Fe (1 0 0) surface in aqueous solution and the diffusion behavior of H 3 O + , Cl − and H 2 O in three of 3,5-dibromo salicylaldehyde Schiff base inhibitor films, including 3,5-dibromo salicylaldehyde-2-pyridinecarboxylic acid hydrazide (L1), 3,5-dibromo salicylaldehyde-2-thiol-phenecarboxylic acid hydrazide (L2), 3,5-dibromo salicylaldehyde-2-aminobenzothiazole (L3). The effects of the interaction energy, radial distribution function and the self-diffusion coefficient were studied accompanying with density functional theory (DFT) study. The results demonstrated that the order of adsorption energy is E (L2) > E (L1) > E (L3), absorption manner is a multi-center chemical adsorption for three inhibitor films; for different inhibitor films, the diffusion coefficients followed the order of D (L3) > D (L1) > D (L2) for the Cl − corrosive particles, the diffusion coefficients followed the order of D (L3) > D (L1) > D (L2) for the H 3 O + corrosive particles. For the three inhibitor films, the diffusion coefficients of the three corrosive particles all followed the order of D (H 2 O) > D (H 3 O + ) > D (Cl − ). The inhibition efficiency order was obtained from the diffusion coefficient which is agreed well with the experimental results as EI (L2) > EI (L1) > EI (L3). Three kinds of inhibitor films have good corrosion inhibition efficiency in aqueous solution.