An MRCI investigation of the electronically excited states of difluorocarbene and its monovalent ions

The MRCI/CBS computational protocol was employed to describe the low-lying electronic states of CF₂ in excellent agreement with experiment. Also of interest for modeling the fluorocarbon plasmas are the difluorocarbene ions, and the geometries, adiabatic excitation energies and orbital characteristics of their electronically excited states were established. Most of the transitions leading to the excited states were found to be monoelectronic and, for the dipole-allowed ones, the oscillator strengths and radiative lifetimes were predicted to aid in the spectroscopic studies.     

Tautomerization and solvent effects on the absorption and emission properties of the Schiff base N,N′-bis(salicylidene)-p-phenylenediamine – A TDDFT study

Time-dependent density functional theory combined with a polarized continuum model has been applied to study solvent effects on the geometrical and energetic properties, as well as the absorption and emission properties, of three tautomeric forms of N,N′-bis(salicylidene)-p-phenylenediamine (BSP). The calculated properties are in agreement with the available experimental data. It was observed that the solvent environment does not affect the vertical excitation energies significantly, whereas tautomerization strongly affects both the absorption and emission spectra of BSP.     

First principles determination of structural,electronic and lattice dynamical properties of a model dipeptide molecular crystal

We present the results of ab initio calculations of the structural, electronic and lattice dynamical properties of the solid-state crystal of the glycyl-l-alanine dipeptide. Intramolecular bond lengths are found to be in good agreement with experimental values; lattice constants are in reasonable agreement, although it is found that discrepancies do exist. A hierarchy of hydrogen bond strengths is found, with those between (oppositely-charged) amine and carboxy functional groups being strongest. The crystal is found to be an indirect-bandgap material, with indirect bandgaps ≈4.95 eV, compared to a direct bandgap of 5.00 eV. Analysis of the electronic structure reveals that the electronic states in the near vicinity of the energy gap arise from carboxylate and amide oxygen atoms. The arrangement of both molecules and hydrogen bonds in the unit cell is found to manifest itself in increased bandwidth along specific reciprocal space directions, reflecting coupling brought about by hydrogen bonds. Determination of the zone-centre lattice dynamical behaviour permits the IR absorption spectrum to be explained. Intermolecular hydrogen bonds are found to couple intramolecular motions in adjacent moelcules, revealing the importance of an accurate treatment of intermolecular interactions, even for high-frequency vibronic modes.     

Prediction and characterisation of a chalcogen···π interaction with acetylene as a potential electron donor in XHS···HCCH and XHSe···HCCH (X = F,Cl, Br,CN, OH,OCH3, NH2, CH3) σ-hole complexes

It is well-known that many covalently bonded atoms of group VI have specific positive regions of electrostatic potential (σ-holes) through which they can interact with Lewis bases. This interaction is called ‘chalcogen bond’ by analogy with halogen bond and hydrogen bond. In this study, ab initio calculations are performed to predict and characterise chalcogen···π interactions in XHS···HCCH and XHSe···HCCH complexes, where X = F, Cl, Br, CN, OH, OCH₃, NH₂, CH₃. For the complexes studied here, XHS(Se) and HCCH are treated as a Lewis acid and a Lewis base, respectively. The CCSD(T)/aug-cc-pVTZ interaction energies of this type of σ-hole bonding range from ?1.18 to ?4.83 kcal/mol. The calculated interaction energies tend to increase in magnitude with increasing positive electrostatic potential on the extension of X–S(Se) bond. The stability of chalcogen···π complexes is attributed mainly to electrostatic and correlation effects. The nature of chalcogen···π interactions is unveiled by means of the atoms in molecules, natural bond orbital, and electron localisation function analyses.     

The Kirkwood correlation factor of dense methanol as a function of temperature and pressure under isochoric conditions and its statistical mechanical treatment

Experimental data of dielectric susceptibilities of methanol available in the literature over a wide range of temperature and pressure have been used for constructing a state function of the Kirkwood correlation factor g _K of methanol applicable in the homogeneous fluid region between 298 and 580?K and up to 350?MPa. Results of three isochores at 786.32, 820 and 850 kg m^?3 of this state function have been compared with predictions made by a statistical mechanical theory developed in previous work. The theory is based on an association model of alcohols accounting for the correlation of effective dipole moments of chains of all length as well as ringlike cluster formations. Most of the parameters such as association constants and association enthalpies have been obtained from quantum mechanical ab initio calculations of methanol clusters up to six members, reducing the numbers of adjustable parameters to three association constants and an averaged value of the angle between dipoles. The theory which has turned out to provide excellent results for alcohol + hydrocarbon and alcohol +?CCl₄ liquid mixtures at ambient pressure is also able to predict isochoric values of pure methanol up to a pressure of 350?MPa and temperatures of 570?K without adjusting further parameters in satisfying agreement with experimental data.     

Improved techniques for the calculation of ab initio ion-neutral interaction potentials: application to coinage metal ions interacting with rare gas atoms

Ab initio values for the potential energy functions for ion–neutral interactions can be tested by comparison with gaseous ion transport coefficients, but only if special care is taken to compute the interaction potentials accurately over wide ranges of internuclear separation. This is illustrated here by a reanalysis of the ab initio values for the coinage metal ions interacting with rare gas atoms, precise calculations of the transport cross sections over extremely wide ranges of energy, and similarly precise calculations of the zero-field ion mobilities as functions of gas temperature and the field-dependent ion mobilities at various fixed temperatures. The calculations indicate that the mobilities for Ag⁺(¹S) moving in Ne or Ar can distinguish between the existing, very similar ab initio potentials. They also show that substantial differences exist among the mobilities of the coinage metal anions and the ground and excited states of the cations. The techniques implemented are recommended for future ab initio calculations.     

First-Principles Investigations of Precursor Molecules for Microclusters

Details of the reactive processes by which TiC and TiN microclusters are formed from precursor molecules in the gas phase are not clearly understood. We have performed ab initio calculations based on density functional theory methods in order to get some insight into the chemical reactions of precursor molecules. Using microcanonical molecular-dynamics we have simulated scattering processes between molecules consisting of the elements Ti, Cl, N and H. The simulations show that fluctuations of the kinetic energy can be large enough to break up bonds at moderate temperatures.     

Mechanism of reversible photo-induced magnetization in prussian blue analogues

Photo-induced magnetic phase transitions observed in Co—Fe Prussian blue analogues are discussed from a theoretical point of view. In this material a magnetization is induced by visible light, and the induced magnetization can be completely diminished not only by thermal annealing but also by irradiation of infra red light. After a brief survey of relevant experiments, we present an overview of our theoretical approaches to clarify the underlying mechanism of this interesting phenomenon. The ligand field at the Co site is shown to be the most relevant variable controlling spin states. The presence of Fe vacancies and the resultant ligand substitution by water molecules turns to be an essential prerequisite for the photoinduced magnetization.     

First-principles study of phase transition and elastic properties of ScSb and YSb compounds

The phase transition of ScSb and YSb from the NaCl-type (B1) structure to the CsCl-type (B2) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the B1 structure to the B2 structure obtained from the equal enthalpies are 38.3 and 32.1 GPa for ScSb and YSb, respectively. From the variations of elastic constants with pressure, we find that the B1 phase of ScSb and YSb compounds are unstable when applied pressures are larger than 46.3 and 64.2 GPa, respectively. Moreover, the detailed volume changes during phase transition are analyzed.