Journal of Molecular Structure-theochem最新文献

We present conformational preferences, energies and dipole moments for six-membered ring cyclic phosphine oxides with phenyl substitution on phosphorus. Conformers found by molecular dynamics were fully optimized at the RHF B3LYP/6-31G∗ and MP2/6-31G∗ levels of theory. We compare the results of calculations against some experimental results. We discuss the reasons for the perpendicular vs. parallel preference of the phenyl group either in axial or equatorial orientation.

The non-linear optical properties of Acenaphthene (ANP) and its derivative Acenaphthenequinone (ANPQ) have been evaluated and compared using the Hartree Fock (HF) and density functional theory (DFT). According to the calculations, the β value of ANP is more than five times and ANPQ is almost five times than that of urea. The molecular HOMO, LUMO composition, their respective energy gaps, MESP contours/surfaces have also been drawn and compared to explain the activity of ANPQ over ANP. For a complete description of molecular dynamics, vibrational wavenumber calculation along with normal mode analysis have been carried out at the DFT level.

In this paper, we calculated a series of halogen-bonded complexes. The halogen bond donor is CF_nH_3−nCl, and the halogen bond acceptors are NH₃, H₂O, H₂S, and Br⁻. Ten stable halogen-bonded complexes were obtained and the C–Cl bond length was contracted in all of these complexes. We carried out AIM and NBO analysis under the MP2/aug-cc-PVDZ optimized structures. The variation of the electron density at the bond critical point of C–Cl bond correlates well with Δr(C–Cl). A balance among intra- and inter-hyperconjugation and rehybridization determined the contracted C–Cl bond.

A simple formula for the standard enthalpy of perfect-gas molecules, $\Delta H_f^{\circ }={\sum }_{K}F\left(K\right)+\mathrm{ZPE}+H_T-H_0-{\sum }_{k<l}{\epsilon }_{\mathrm{kl}}-\left(\mathrm{CNE}-E_{\mathrm{nb}}^{\mathrm{KL}}\right)$, is illustrated by novel applications and over 100 examples and comparisons with experimental results. The propounded model views molecules as constructs of chemical groups, K, L, … , etc. (such as CH₃, COOH, for example) characterized once and for all, independently of their belonging to one or another host, by fixed numbers $F\left(K\right)\text{,}F\left(L\right)\text{,}\dots$, etc. ZPE + H_T − H₀ is the familiar sum of zero-point + heat-content energy, ε_kl is the intrinsic energy (at 0 K) of the bond connecting K and L, CNE (for Charge Neutralization Energy) takes care of the fact that K, L, … , are usually not electroneutral in the host molecule, and $E_{\mathrm{nb}}^{\mathrm{KL}}$ measures nonbonded interactions summed over all pairs of groups K and L. New parameters, $F\left[\mathrm{CH}\right(X\left)\right]$ with X = CH₃, NH₂, OH, CHO, COOH, Cl, Br and SH, are described and an amazingly simple formula for carbon–hydrogen bonds, giving ${\epsilon }_{\mathrm{CH}}^{\ast }={\epsilon }_{\mathrm{CH}}+\mathrm{CNE}$, turns out to be most useful any time a fragment CH(X) is bonded to one hydrogen atom at least.

In this investigation we discussed one possible mechanism of the photoinduced cyclodimerization of two malonaldehyde molecules to form the “face-to-back” cyclodimer. Linear interpolation in internal coordinates approach was applied to study the excited-state relaxation mechanisms at the TD DFT level with aug-cc-pVDZ basis functions. It was found that the formation of the cyclodimer occurs through the first ¹ππ∗ excited state which has a charge transfer character. We also found a radiationless relaxation path of the ¹ππ∗ excited state mediated by a S_o–S₁ conical intersection.

Self-dimerisation of the three isomeric semiquinone radicals is considered herein. Optimised geometries and thermochemical parameters, in terms of heat of formation, entropy, heat capacity and Gibbs free energy of formation are provided for all possible cross coupling products. It is found that self-dimerisation of the three semiquinone radicals is not as thermodynamically favoured as the self-dimerisation of the phenoxy radical. Accordingly, the three isomeric semiquinone radicals can be regarded as less active precursors for the formation of dioxin compounds than phenoxy radicals.

The anticoagulent drug warfarin exhibits chameleon-like isomerism, where the environment-dependent composition of the ensemble of structures greatly influences its bioavailability. Here, the mechanism of conversion between the major isomeric forms is studied. The dramatic differences in transition state energies, as determined by density functional calculations, highlight the necessity for the involvement of intermolecular interactions in the key proton transfer step. A viable model for the mechanism underlying the isomerization reactions is presented.

The gas phase enthalpies of formation of C₁–C₄ mononitroalkanes were calculated using different multilevel (G1, G2, G3, G3B3, CBS-QB3) and density functional theory (DFT)-based B3LYP techniques. The enthalpies of the C–N bond dissociation of these nitroalkanes were also calculated. The calculated values of the formation and reaction enthalpies were compared with available experimental data. It was found that the G3 and G3B3 procedures gave accurate results for the formation enthalpy of nitroalkanes and radical products. The agreement of the CBS-QB3 calculations with experiment is also satisfactory, whereas less accurate than G3 and G3B3 estimations. The G3 and G3B3 multilevel techniques showed good results for the reaction enthalpies of the C–N bond dissociation.

The interactions of the Br₂ with hydrogen halide have been investigated by performing calculations at the second-order perturbation theory based on the Møller–Plesset partition of the Hamiltonian with the Sadlej PVTZ basis set. The X–Br type geometry and hydrogen-bonded geometry are investigated in these interactions. The calculated interaction energies show that the X–Br type structures are more stable than the corresponding hydrogen-bonded structures. To study the nature of the intermolecular interactions, symmetry-adapted perturbation theory (SAPT) calculations were carried out and the results indicate that the X–Br interactions are dominantly electrostatic and dispersion energy in nature, while dispersion energy governs the hydrogen bonding interactions.

The dynamic reaction pathways after passing the initial barrier for the reaction of atomic oxygen radical anion (O⁻) with ethylene (CH₂CH₂) have been investigated with Born–Oppenheimer molecular dynamics (BOMD) simulations. The BOMD simulations initiated at this [O⋯H⋯CHCH₂]⁻ barrier on the exit-channel potential energy surface (PES) reveal several different types of dynamic reaction pathways leading to various anionic products. In particular, as the energy added on the transition vector of the [O⋯H⋯CHCH₂]⁻ transition state increases remarkably, the OH⁻ and CH₂CH become the dominant products instead of the CH₂CHO⁻ and H. As a result, animated images are displayed and more extensive reaction mechanisms are illuminated for the title reaction from the perspective of the dynamic reaction pathways.