Wensheng Bian

Global ab initio potential energy surfaces for the ClH2 reactive system

Two new global ab initio potential energy surfaces (called BW1 and BW2) for the ClH2 reactive system are presented. These are based on internally contracted multireference configuration interaction calculations using a very large basis set, performed at 1200 geometries. Accurate analytical fits have been generated using the functional form proposed by Aguado and Paniagua. The BW1 surface is based on the original ab initio points. This surface slightly underestimates the dissociation energies of the diatomic fragments and overestimates the barrier height. Therefore, a second surface (BW2) has been computed by scaling the correlation energies at all geometries with a constant factor, which was chosen such that the dissociation energies of HCl and H2 are reproduced more accurately. The barrier heights for the collinear transition state of the Cl+H2→HCl+H reaction are computed to be 8.14 kcal/mol and 7.61 kcal/mol for the BW1 and BW2 surfaces, respectively. To these values the spin–orbit correction of 0.84 kc...

Accurate quantum dynamics of electronically nonadiabatic chemical reactions in the DH2+ system

Three-dimensional accurate quantum dynamics calculations are carried out for the DH2+ system for J=0 (J, total angular momentum quantum number) by the hyperspherical coordinate approach with use of the new potential energy surfaces constructed based on the recent ab initio quantum chemical calculations. Not only electronically nonadiabatic reactions, i.e., reactive charge transfer processes, but also electronically adiabatic reactions and electronically nonadiabatic nonreactive processes are investigated. Because of the deep well on the electronically adiabatic ground surface, there appear a large number of resonances and the electronically adiabatic reactions are mostly statistical. Nonadiabatic transitions along the potential crossing seam cause deviations from the statistical behavior and some interesting dynamical features are found.

ACCURATE AND HIGHLY EFFICIENT CALCULATION OF THE O(1D)HCl VIBRATIONAL BOUND STATES, USING A COMBINATION OF METHODS

Hypochlorous acid, HOCl, is an important intermediate in the O(1D)HCl reactive system. Due in part to a large number of vibrational bound states (over 800), extremely large direct product basis sets (around 300,000) are required to compute the energy levels just below the dissociation threshold. This situation, combined with a very high density of states, results in difficult convergence for iterative methods — e.g. Lanczos requires 50,000 iterations, and filter diagonalization uses 60,000 iterations. In contrast, using new methodologies, we are able to compute the highest-lying bound states with only 271 iterations, although the CPU cost per iteration is substantially greater. Lower lying states are also computed, for a fraction of the CPU cost of the highest energy calculation.

Influence of Solvent Polarity and Hydrogen Bonding on the Electronic Transition of Coumarin 120: A TDDFT Study

The characteristics of the electronic transition energy of Coumarin 120 (C120) and its H-bonded complexes in various solvents have been examined by time-dependent density functional theory (TDDFT) in combination with a polarizable continuum solvent model (PCM). Molecular structures of C120 and its H-bonded complexes are optimized with the B3LYP method in PCM solution, and the dihedral angle H14-N13-C7-H15 is dependent on solvent polarity and the type of H-bond. A linear correlation of the absorption maximum of C120 with the solvent polarity function is revealed with the PCM model for all solvents except DMSO. The experimental absorption maximum of C120 in nine solvents is well described by a PCM-TDDFT scheme augmented with explicit inclusion of a few H-bonded solvent molecules, and quantitative agreement between our calculated results and experimental measurements is obtained with an average error of less than 2 nm. H-bonding at three different sites shifts the absorption wavelength of C120 either to the blue or to the red, that is, a significant role is played by solvent molecules in the first solvation shell in determining the electronic transition energy of C120. The dependence on the H-bonding site and solvent polarity is examined by using the Kamlet-Taft equation for solvatochromism.

Accurate and highly efficient calculation of the highly excited pure OH stretching resonances of O(1D)HCl, using a combination of methods

Accurate calculation of the energies and widths of the resonances of HOCl--an important intermediate in the O(1D)HCl reactive system--poses a challenging benchmark for computational methods. The need for very large direct product basis sets, combined with an extremely high density of states, results in difficult convergence for iterative methods. A recent calculation of the highly excited OH stretch mode resonances using the filter diagonalization method, for example, required 462,000 basis functions, and 180,000 iterations. In contrast, using a combination of new methods, we are able to compute the same resonance states to higher accuracy with a basis less than half the size, using only a few hundred iterations-although the CPU cost per iteration is substantially greater. Similar performance enhancements are observed for calculations of the high-lying bound states, as reported in a previous paper [J. Theo. Comput. Chem. 2, 583 (2003)].

Efficient quantum calculations of vibrational states of vinylidene in full dimensionality: a scheme with combination of methods.

Full-dimensional quantum calculations of vibrational states of C(2)H(2) and C(2)D(2) are performed in the high-energy region (above 20,400 cm(-1) relative to the acetylene minimum). The theoretical scheme is a combination of several methods. To exploit the full parity and permutation symmetry, the CC-HH diatom-diatom Jacobi coordinates are chosen; phase space optimization in combination with physical considerations is used to obtain an efficient radial discrete variable representation, whereas a basis contraction scheme is applied for angular coordinates. The preconditioned inexact spectral transform method combined with an efficient preconditioner is employed to compute eigenstates within a desired spectral window. The computation is efficient. More definite assignments on vinylidene states than previous studies are acquired using the normal mode projection; in particular, a consistent analysis of the nu(1) (symmetric CH stretch) state is provided. The computed vinylidene vibrational energy levels are in general good agreement with experiment, and several vinylidene states are reported for the first time.

Quasiclassical trajectory study of H+SiH4 reactions in full-dimensionality reveals atomic-level mechanisms

This work elucidates new atomic-level mechanisms that may be common in a range of chemical reactions, and our findings are important for the understanding of the nature of polyatomic abstraction and exchange reactions. A global 12-dimensional ab initio potential energy surface (PES), which describes both H+SiH4 abstraction and exchange reactions is constructed, based on the modified Shepard interpolation method and UCCSD(T)/cc-pVQZ energy calculations at 4,015 geometries. This PES has a classical barrier height of 5.35 kcal/mol for abstraction (our best estimate is 5.35 ± 0.15 kcal/mol from extensive ab initio calculations), and an exothermicity of −13.12 kcal/mol, in excellent agreement with experiment. Quasiclassical trajectory calculations on this new PES reveal interesting features of detailed dynamical quantities and underlying new mechanisms. Our calculated product angular distributions for exchange are in the forward hemisphere with a tail sideways, and are attributed to the combination of three mechanisms: inversion, torsion-tilt, and side-inversion. With increase of collision energy our calculated angular distributions for abstraction first peak at backward scattering and then shift toward smaller scattering angles, which is explained by a competition between rebound and stripping mechanisms; here stripping is seen at much lower energies, but is conceptually similar to what was observed in the reaction of H+CD4 by Zare and coworkers [Camden JP, et al. (2005) J Am Chem Soc 127:11898–11899]. Each of these atomic-level mechanisms is confirmed by direct examination of trajectories, and two of them (torsion-tilt and side-inversion) are proposed and designated in this work.

Dynamical importance of van der Waals saddle and excited potential surface in C( 1 D )+D 2 complex-forming reaction

Encouraged by recent advances in revealing significant effects of van der Waals wells on reaction dynamics, many people assume that van der Waals wells are inevitable in chemical reactions. Here we find that the weak long-range forces cause van der Waals saddles in the prototypical C(1D)+D2 complex-forming reaction that have very different dynamical effects from van der Waals wells at low collision energies. Accurate quantum dynamics calculations on our highly accurate ab initio potential energy surfaces with van der Waals saddles yield cross-sections in close agreement with crossed-beam experiments, whereas the same calculations on an earlier surface with van der Waals wells produce much smaller cross-sections at low energies. Further trajectory calculations reveal that the van der Waals saddle leads to a torsion then sideways insertion reaction mechanism, whereas the well suppresses reactivity. Quantum diffraction oscillations and sharp resonances are also predicted based on our ground- and excited-state potential energy surfaces.

Study of electronic and spectroscopic properties on a newly synthesized red fluorescent material

The ground state (S(0)) and the lowest singlet excited state (S(1)) of a newly synthesized red fluorescent material, 2-[3-(2-{4-[(2-Hydroxy-ethyl)-methyl-amino]-phenyl}-vinyl)-5,5-dimethyl-cyclohex-2-enylidene]-malononitrile (A31), are investigated. The S(0) and S(1) geometries are optimized at the ab initio Hartree-Fock and the singles configuration interaction (CIS) levels of theory, respectively. The CIS and semiempirical Zerners Intermediate Neglect of Differential Overlap (ZINDO) methods provide the results for the absorption (S(0)-->S(1)) and emission (S(1)-->S(0)) transition energies. The Stokes shifts calculated at the CIS and ZINDO levels of theory are obtained. The absorption spectra in various solvents are calculated using the time-dependent density-functional theory method in combination with the polarized continuum model, which are in very good agreement with our experimental measurements. The solvent effects are discussed.

PARTIAL POTENTIAL ENERGY SURFACE AND ITS APPLICATIONS IN REACTIVE RESONANCES

The conception of partial potential energy surface (PPES) is presented in this paper. PPES can be abstracted from complete potential energy surface (CPES), therefore, it can be constructed with ab initio quantum chemical method. For the systems of H+H2→H2+H, I+HI→IH+I and I-+HI→IH+I-, the construction and applications of PPES are proposed as typical examples. It can be seen that the applications of PPES demonstrate remarkable virtues in the analysis of reaction mechanism and the formation of scattering resonance states.