Journal of Theoretical and Computational Chemistry

The laser control of photodissociation branching in a diatomic molecule is demonstrated to be effectively achieved with use of the complete reflection phenomenon. The phenomenon and the control condition can be nicely formulated by the semiclassical (Zhu–Nakamura) theory. The method is applied to the branching between I(²P_3/2) (HI → H + I) and I*(²P_1/2) (HI → H + I*) formation, and nearly complete control is shown to be possible by appropriately choosing an initial vibrational state and laser frequency in spite of the fact that there are three electronically excited states involved. Numerical calculations of the corresponding wavepacket dynamics confirm the results.

Recently, we have introduced a method based on geometric considerations, termed geometric direct minimization (GDM), to achieve robust convergence of self consistent field calculations. GDM was limited to calculations involving either spin-unrestricted orbitals or closed shell systems. We report the extension of the GDM method to treat open shell systems involving spin-restricted orbitals. Open shell systems pose a challenge for achieving robust convergence of the calculation. We compare the convergence using the GDM method to the convergence achieved by the well known direct inversion in the iterative space (DIIS) technique. This comparison demonstrates the ability of the GDM method to achieve robust convergence. Additionally we assess the importance of geometric considerations by comparing against an alternative direct minimization method that is not geometrically correct.

New global ab initio potential energy surfaces (PES) are presented for the low-lying 1¹A′, 1¹A′′ and 2¹A′ electronic states which are correlated to O(¹D) + HCl. These potential energy surfaces are computed by using the multi-reference configuration interaction method with the Davidson correction (MRCI+Q). The reference functions are constructed by the complete active space self-consistent field (CASSCF) calculations using the quadruple zeta + polarization basis set augmented with diffuse functions. The computations are carried out at about 5000 molecular conformations on each three-dimensional potential energy surface. The high accuracy of the computations is confirmed by a comparison with the available most accurate data for the ground state 1¹A′; thus the present work is the first report of the accurate potential energy surfaces for the two excited states. Three low-lying transition states on the excited surfaces, two (TS2 and TS4) on 1¹A′′ and one (TS3) on 2¹A′, are found. Since TS2 and TS3 are as low as 0.07 eV and 0.28 eV, respectively, and correlate to the OH(²Π) + Cl(²P) product, these excited surfaces are expected to play quite important roles in the reaction dynamics. Possible effects of nonadiabatic couplings among the three PESs are also briefy discussed, although the nonadiabatic couplings have not yet been estimated. The quantum reaction dynamics on these three PESs are discussed in the second accompanying paper, Paper II.

Using the accurate global potential energy surfaces for the 1¹A′, 1¹A′′, and 2¹A′ states reported in the previous sister Paper I, detailed quantum dynamics calculations are performed for these three adiabatic surfaces separately for J = 0 (J: total angular momentum quantum number). Overall reaction probabilities for O + HCl → OH + Cl and H + ClO, the branching ratio between the two reactions, effects of the initial rovibrational excitation, and product rovibrational distributions are evaluated in the total energy region E_tot ≤ 0.9 eV. Significant contributions to the overall reaction dynamics are found from the two excited 1¹A′′ and 2¹A′ potential energy surfaces, clearly indicating the insufficiency of the dynamics only on the ground 1¹A′ surface. The detailed dynamics on the excited surfaces are reported in the third paper of this series.

Using the accurate global potential energy surfaces for the 1¹A′′ and 2¹A′ states reported in the previous sister Paper I, detailed quantum dynamics calculations are performed for these adiabatic surfaces separately for J = 0 (J: total angular momentum quantum number). In addition to the significant overall contributions of these states to the title reactions reported in the second Paper II of this series, quantum dynamics on these excited potential energy surfaces (PES) are clarified in terms of the PES topographies, which are quite different from that of the ground PES. The reaction mechanisms are found to be strongly selective and nicely explained as vibrationally nonadiabatic transitions in the vicinity of potential ridge.

An accurate and rapid quantum chemistry approach to predicting chemical shifts is applied to ¹⁵N natural abundance NMR spectra of benzothiazoles. This method is of interest in the study of toxic contaminants of biological media. A GIAO approach is used to calculate the nuclear shielding tensor in a perturbational scheme. Similar methods had previously been used for both ab initio and semi-empirical calculations programmed in Gaussian and MOPAC respectively but atomic orbitals specific to molecular sites are shown to improve accuracy in the present work. Some molecular sites are defined in order to polarize the molecular orbitals in the presence of the solvent interactions. For convenience, the extensions are carried out in an independent NMR module designed to work with the MOPAC package and the present application is generalized to the third row elements for the first time. This new theoretical work is described. The application to ¹⁵N chemical shifts for benzothiazoles led to results within 0.5 ppm of values measured in this work. Structural investigations were carried out using the Gaussian98 suite of programs at the DFT B3LYP level of calculation over an extended basis set. These studies show the single chemical shift for benzothiazoles could be the result of rapid tautomeric equilibria. Further investigations show that solvent interactions involving hydrogen bonds should also be taken into account.

The results of a series of ab initio molecular orbital calculations on thioglycolic acid and methylthioglycolate are presented. Full geometry optimizations at different levels of theory have been performed for the equilibrium and transition state configurations, in order to determine the height of the inversion barriers, and to obtain information on the relaxation of some geometrical parameters.

In spite of using extended basis sets and the inclusion of electron correlation, there is an important inconsistency between the relative position of the thiolic hydrogen derived from experimental data and the ab initio predictions.

As regards the barrier height, the results more consistent with the available experimental data are obtained when the Gaussian2 compound methods [G2 and G2(MP2)] are used.

The possible influence of an –S–H⋯O = hydrogen bond on the conformational behavior has also been investigated by analyzing the topology of the electron density.

Extensions of traditional molecular dynamics to excited electronic states and non-Born–Oppenheimer dynamics are reviewed focusing on applicability to chemical reactions of large molecules, possibly in condensed phases. The latter imposes restrictions on both the level of accuracy of the underlying electronic structure theory and the treatment of nonadiabaticity. This review, therefore, exclusively deals with ab initio "on the fly" molecular dynamics methods. For the same reason, mainly mixed quantum-classical approaches to nonadiabatic dynamics are considered.

This paper briefly reviewed the Heisenberg model and its improvement by including higher order corrections, and their applications to bond lengths, stability, and reactivity of non-benzenoids and the low-lying excitation spectra of conjugated systems. Two efficient computational methods, the Lanczos method and the density-matrix renormalization group (DMRG), for exactly and approximately solving various Heisenberg models, respectively, were briefly introduced.

An approach to carrying out accurate quantum dynamics simulations of chemical reactions, termed the real wave packet (RWP) method, is outlined. The method focuses on propagation of just the real part of a complex-valued wave packet, halving computational memory and effort requirements in comparison with comparable high accuracy quantum propagation methods. Applications to 3-atom and 4-atom chemical reactions are reviewed. Potential future directions are indicated.

We briefly review developments of theories for phase transitions of molecular fluids and mixtures, from semi-phenomenological approaches providing equations of state with adjustable parameters to first-principles microscopic methods qualitatively correct for a variety of molecular models with realistic interaction potentials. We further present the generalization of the van der Waals–Maxwell description of fluid phase diagrams to account for chemical specificities of polar molecular fluids, such as hydrogen bonding. Our theory uses the reference interaction site model (RISM) integral equation approach complemented with the new closure we have proposed (KH approximation), successful over a wide range of density from gas to liquid. The RISM/KH theory is applied to the known three-site models of water, methanol, and hydrogen fluoride. It qualitatively reproduces their vapor-liquid phase diagrams and the structure in the gas as well as liquid phases, including hydrogen bonding. Furthermore, phase transitions of water and methanol sorbed in nanoporous carbon aerogel are described by means of the replica generalization of the RISM approach we have developed. The changes as compared to the bulk fluids are in agreement with simulations and experiment. The RISM/KH theory is promising for description of phase transitions in various associating fluids, in particular, electrolyte as well as non-electrolyte solutions and ionic liquids.