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The rotational transition state in the isomerization of the syn- to anti-conformer for the parent and substituted benzoic acids has been characterized and the decarboxylation mechanism for these acids has been determined. A previously unreported intermediate in the salicylic acid decarboxylation pathway has been observed, demonstrating that there is a change in the reaction mechanism when the ortho-group participates in the development of the transition state. The intermediate, in a shallow well, has a keto-like structure. While salicylic acid reacts through the keto-like intermediate, other substituted benzoic acids, including 2-aminobenzoic acid, follow the previously reported direct mechanism.

Besides the fact that in nitrosamines, considered as a family of potent carcinogenic compounds, the barrier to internal rotation about the N–N bond is a significant conformational factor, it is as well important for understanding the mechanism of their bioactivity and detoxification. In this work, a detailed study of the internal rotation of the NO group about the N–N bond in various six-membered cyclic nitrosamine compounds was performed using ab initio (HF and MP2) and DFT methods with 6-31G** basis set. Between the two resulting rotational transition states (TS1 and TS2), TS1 was found to be more stable than TS2 according to the all used methods. The activation energy barriers obtained by ab initio methods were also in good agreement with the experimental values. In the studied compound series, the ab initio (HF and MP2) calculated internal rotation activation Gibbs free energies through TS1, lies between 80.01 and 96.26 kJ/mol. The results showed as well that the planar structure of the N-atom in ground state becomes pyramidal in transition states. In addition, in order to explore the origin of the internal rotation energy barrier, the natural bond orbital analysis (NBO) revealed, specifically, that the lp(N4)→BD*(N O) donor–acceptor interaction is the most significant interaction in nitrosamine compounds. NBO analysis showed also that the s-character of the N4 lone pair orbital and its occupation number increases as approaching the top of the rotational barrier in these studied compounds.

Classical trajectory calculation has been performed for the H+H→H2/Si(100)-(2×1) reaction by the 3D Generalized Langevin Equation (GLE) approach. The implementation of the 3D GLE approach to the H+H→H2/Si(100)-(2×1) reaction is presented. Reaction probabilities are calculated for given surface temperatures and given collision energies. We also calculated vibrational and rotational distributions of product H2 molecules from the reaction. About 80% of the product hydrogen molecules are in the ground vibrational state and the remaining 20% of the products are in the excited states. The rotational state shows non-Boltzmann distribution which can be seen in the direct collision process. Vibrational and Rotational distributions are strongly related to the impact parameter. The vibrational distribution is correlated with the x-component of the impact parameter and reflects the dimer nature of the silicon (2×1) surface. Details of the dynamics involving vibrational and rotational transitions are discussed.

Based on the subgroup chain of symmetry U 1 (4)⊗ U 2 (4) and the associated Lie algebra, the commonly used formula of analyzing the rovibrational energy levels for linear triatomic molecules is derived. This algebraic method is shown to be simple and effective in giving an expression with any accuracy at will. By fitting the formula to observable rotational levels, the rotational constants and the related vibrational energy levels can be obtained. As examples, the formula is used to analyze the infrared spectrum of carbon disulfide (CS 2 ) in the overtone region near 5330 cm −1 and of OC 34 S in the ν 1 fundamental region near 848 cm −1 , which gives satisfactory results.

The origin of the rotational barrier around the partial C C double bond in substituted ethylenes is discussed with reference to the stabilization of the conformational minimum (GS) and of the rotational transition state (TS). Molecules with different polar character of the double bond were chosen, ranging from ethylene to olefins with strong push–pull character. The enthalpies of hydrogenation, ΔHhydr., and of formation, ΔHf0, of these molecules were employed to obtain the stability of GS; these thermochemical properties were calculated with MO ab-initio theory at HF/6-311G∗∗//HF/6-311G∗∗ and MP2/6-311G∗∗//HF/6-311G∗∗ levels and with CBS-4M model chemistry. The stabilization of TS was derived from the torsional potential for rotation around the C C bond. The lowering of the energy content of GS of substituted ethylenes, referring to ethylene, is accompanied by an even greater stabilization of TS, thus a lowering of the rotational barrier with respect to ethylene is generally found in these molecules.

The rotational barrier for substituted ethylenes was calculated with MO ab initio methods, both in the Hartree–Fock (HF) single determinant scheme and with multiconfigurational self-consistent field (MCSCF) approaches. The HF model affords reliable results only when applied to molecules substituted with strong electron donor groups, assigning a push–pull character to the molecule. The MCSCF approach was employed for calculating rotational barriers in poorly polarized ethylenes not directly amenable to the HF methods; however, this method was found hard to handle for low symmetry molecules with substituents interacting with the double bond. A method is proposed based on the interpolation of the energy of the perpendicular conformation from a Fourier truncated function constructed with HF molecular energies calculated for frozen conformations twisted up to 60°. The application of HF theory for studying internal rotation in substituted ethylenes with poorly polarized character is discussed and an upper limit of 35–40 kcal/mol can be set up for having reliable barriers from the calculated energy of the rotational transition state at this level of theory.

The conformational analysis has been performed on propyl nitrate by using ab initio and density functional methods. Seven energy minima corresponding to seven different conformers have been found and fully optimized at the B3LYP/6-31G∗ and MP2/6-311G∗ levels. The rotational transition states have been optimized too at the B3LYP/6-31G∗ level. The geometries obtained by the B3LYP agree well with those obtained by the MP2, and both methods predict that the gauche conformer with the ∠CCCO (t3) being twisted by about 63° has the lowest energy and is 1.71 (MP2) and 0.53 (B3LYP) kJ mol−1 more stable than the planar conformer, but the stability order of different conformers obtained by them are somewhat different. The energy differences between various conformers and the barriers to internal rotation are all very low. Torsion around ∠CCON decreases while torsion around t3 increases the stability by about 3.2 and 1.5 kJ/mol, respectively, from MP2 calculations. Vibrational analyses have been performed on the stable conformers at the B3LYP/6-31G∗ level. The standard thermodynamic functions and the equilibrium mole fractions have been derived. The heats of formation of various conformers have been estimated too from the B3LYP total energies and an isodesmic reaction. The calculated thermodynamic properties and heat of formation of propyl nitrate are in reasonable agreement with the experimental results.

The energy of the rotational barriers and electronic structure of the transition state in substituted ethylenes are discussed in the light of the results obtained from different theoretical MO ab initio approaches. The 6-31G ∗ basis set at Hartree–Fock (HF) level and with second-order Moller–Plesset perturbation theory (MP2) was employed, critical points were localized through full geometry relaxation and characterized by vibrational analysis. A multiconfigurational approach (MCSCF) with different active spaces was also employed. For alkenes the correct rotational transition state is obtained only from the MCSCF approach, whereas for push–pull olefins the HF approach with correlation corrections at MP2 level provides correct answers for the internal rotation around C(sp 2 )–C(sp 2 ) bonds. The choice is more critical when only acceptor or donor groups are present, especially when change of hybridization occurs at the atoms at the edges of the C–C bond in the critical points.

Extensive theoretical quantum-mechanical calculations are reported for the cross-section and related dynamical quantities of the B+OH→BO+H reaction, on a previously developed potential-energy surface (PES) describing the ground electronic state. These calculations show, as an outstanding feature, the presence of marked structures in the shape of the excitation function as a consequence of the existence of a dense spectrum of rather long-lived resonance states. These are narrow enough to survive the angular momentum averaging, thanks to an important stabilization caused by an electronic minimum corresponding to a linear HOB configuration. The centrifugal barriers due to high values of the orbital angular momentum are found to lead to several orbiting shape resonances, as revealed by the opacity function plots and the dependence of the reaction probability on energy. Differential cross-sections corresponding to energies lying at either reactivity peaks or valleys in the integral cross-section show a highly symmetric dependence on scattering angle, indicating an important resonant contribution to reactivity in both cases. The possibility of directly observing resonances in experimental integral cross-section measurements is analysed in the light of the present results.

A systematic study of the abilities of a broad family of density functional methods and basis sets has been carried out on hydrogen peroxide. Calculations have been performed using different basis sets from 3-21G to 6-311+G(3d, 2p). Various local and non-local exchange, exchange-correlation and hybrid functionals have been applied in order to study the structure, energetic and vibrational properties of hydrogen peroxide. Comparison with experimental data and also with results from ab initio methods (HF and correlated levels) shows a good performance of non-local density functional methods for the description of the hydrogen peroxide geometry, especially the torsion angle τ, when we use basis sets of sufficient quality (at the least basis sets with a polarization function on oxygen). Different density functionals have been used in order to study cis and trans rotational transition states, using 6-31G(2d,p) and 6-31+G(2d,p) basis sets. The calculated torsional barriers and zero-point vibrational energies (ZPEs) are in very good agreement with experimental values.

The four lowest lying electronic states of the BeH 2+ dication are described theoretically using the multireference single and double excitations configuration interaction approach. Only the ground state shows a potential energy well; this well is very shallow and supports at least eight vibrational levels. Although the ground state is thermodynamically unstable, this potential well arises as the interaction between the potential energy curves representing the structures BeH + H + and Be 2+ H. Dipole and transition moment functions are also calculated for all states and radiative transition probabilities and lifetimes are computed for rovibrational transitions in the ground electronic state; vibrational and rotational constants are also reported for this state.

The investigation of the role of electrostatic contributions in solute-solvent interactions as described by the continuous model of the solvent is systematically performed on a few solutes with rotational degrees of freedom whose preferential stabilization upon solvation has already been considered with various methods, ranging from free energy perturbation methods to the continuous solvent at several levels of approximation. The electrostatic effect produced by neutral molecules when immersed in a continuous dielectric medium is considered (a) for several conformers of 1,2-ethanediol, obtained via both rigid rotation about the torsional degrees of freedom and thorough geometry optimization in vacuo, to account for the effect of small geometry deformations on the solvation energy, and (b) for cis and trans N-methyl formamide and methylformate and their rotational transition states. A comparison with the purely electrostatic contribution deriving from a point charge model (placed in the position of the nuclei) is carried out. The point charge models considered are the Mulliken net charges and the best fit to the ab initio electrostatic molecular potential produced by the STO-3G, 4-31G and 6-31G∗ basis sets. The performance of simplified models, obtained by averaging and balancing the charges on a given atom in the different rotamers, is also evaluated. The possibility of using a geometry optimized in a different basis set is checked. The results confirm the noticeable incidence of electrostatics in this kind of interaction and the basic importance of reproducing the solute electrostatic potential rather than the solvent reaction potential for improving the quality of the representation.

In this paper we present a quasi classical study of the collision between He and Iodine in the state B at very low energies (

An outline is given of a systematic and automated computational procedure based upon our successful ab initio approach to the theoretical study of centrifugal distortions of rotating molecules and their associated energy stabilizations. Such a generalized procedure permits the ready evaluation of the centrifugal distortion spectroscopic constants for moderate-sized molecules of low or no symmetry; these molecules represent cases for which the older version of our computational procedure is unwieldy. More broadly, we explore the energies and geometries of molecules in those regions of nuclear coordinate hyperspace which are accessible by such distortions from the equilibrium geometry. A method is outlined for optimizing the geometry of a molecule in the presence of a centrifugal field which is parametrically dependent upon the rotational angular momentum. The field is not fixed for a given angular momentum, but is coupled with the geometry via the moment of inertia tensor. This development should prove to be of major benefit to both computational chemists and molecular spectroscopists, as it will advance the interplay of theoretical and experimental studies of molecular structure.