Saulo A. Vázquez

Implementation of a fast analytic ground state potential energy surface for the N(2D)+H2 reaction

A new implementation is presented for the potential energy surface (PES) of the 1 2A″ state of the N(2D)+H2 system based on a set of 2715 ab initio points resulting from the multireference configuration interaction (MRCI) calculations. The implementation is carried out using the reproducing Kernel Hilbert Space interpolation method. Range parameters, via bond-order-like coordinates, are properly chosen to render a sufficiently short-range three-body interaction and a regularization procedure is invoked to yield a globally smooth PES. A fast algorithm, with the help of low-order spline reproducing kernels, is implemented for the computation of the PES and, particularly, its gradients, whose fast evaluation is essential for large scale quasi-classical trajectory calculations. It is found that the new PES can be evaluated more than ten times faster than that of an existing (old) PES based on a smaller number (1141) of data points resulting from the same MRCI calculations and a similar interpolation procedure...

Semiempirical Hamiltonian for Simulation of Azobenzene Photochemistry

We present a semiempirical Hamiltonian that provides an accurate description of the first singlet and triplet potential energy surfaces of azobenzene for use in direct simulations of the excited-state dynamics. The parameterization made use of spectroscopic and thermochemical data and the best ab initio results available to date. Two-dimensional potential energy surfaces based on constrained geometry optimizations are presented for the states that are most relevant for the photochemistry of azobenzene, namely, S(0), S(1), and S(2). In order to run simulations of the photodynamics of azobenzene in hydrocarbons or hydroxylic solvents, we determined the interactions of methane and methanol with the azo group by ab initio calculations and fitted the interactions with a QM/MM interaction Hamiltonian.

Quasi-classical trajectory calculations on a fast analytic potential energy surface for the C(1D)+H2 reaction

Abstract Quasi-classical trajectory (QCT) calculations have been carried out on a new implementation of the first singlet state a 1 A ′ potential energy surface (PES) of the C ( 1 D )+ H 2 system based on a set of 1748 ab initio points previously reported. The implementation is performed by using the Reproducing Kernel Hilbert Space (RKHS) interpolation method, which allows the fast evaluation of the PES values and, particularly, their gradients analytically. Although there is a general good correspondence between the present surface and the previous version, the new PES is free of spurious small scale features and permits a faster evaluation of the PES and its gradients. The QCT results on the present PES are in general good agreement with those obtain on the previous PES.

Quasiclassical dynamics simulation of the collision-induced dissociation of Cr(CO)6+ with Xe

Quasiclassical trajectory calculations are employed to investigate the dynamics of collision-induced dissociation (CID) of Cr(CO)6 + with Xe atoms at collision energies ranging from 1.3 to 5.0 eV. The trajectory simulations show that direct elimination of CO ligands, during the collision, becomes increasingly important as the collision energy increases. In a significant number of cases, this shattering mechanism is accompanied with a concomitant formation of a transient Xe-Cr(CO)x +(x<6) complex. The calculated results are in very good agreement with the experimental results presented previously [F. Muntean and P. B. Armentrout, J. Chem. Phys. 115, 1213 (2001)]. In particular, the computed cross sections and scattering maps for the product ions Cr(CO)x +(x=3-5) compare very favorably with the reported experimental data. However, in contrast with the conclusions of the previous study, the present calculations suggest that CID dynamics for this system exhibits a significant impulsive character rather than proceeding via a complex surviving more than a rotational period.

Dynamics of CO2 Scattering off a Perfluorinated Self-Assembled Monolayer. Influence of the Incident Collision Energy, Mass Effects, and Use of Different Surface Models †

The dynamics of collisions of CO2 with a perfluorinated alkanethiol self-assembled monolayer (F-SAM) on gold were investigated by classical trajectory calculations using explicit atom (EA) and united atom (UA) models to represent the F-SAM surface. The CO2 molecule was directed perpendicularly to the surface at initial collision energies of 1.6, 4.7, 7.7, and 10.6 kcal/mol. Rotational distributions of the scattered CO2 molecules are in agreement with experimental distributions determined for collisions of CO2 with liquid surfaces of perfluoropolyether. The agreement is especially good for the EA model. The role of the mass in the efficiency of the energy transfer was investigated in separate simulations in which the mass of the F atoms was replaced by either that of hydrogen or chlorine, while keeping the potential energy function unchanged. The calculations predict the observed trend that less energy is transferred to the surface as the mass of the alkyl chains increases. Significant discrepancies were found between results obtained with the EA and UA models. The UA surface leads to an enhancement of the energy transfer efficiency in comparison with the EA surface. The reason for this is in the softer structure of the UA surface, which facilitates transfer from translation to interchain vibrational modes.

Interaction and Dimerization Energies in Methyl-Blocked α,γ-Peptide Nanotube Segments

The building blocks of a promising class of peptide nanotubes composed of alternating D-alpha-amino acids and (1R,3S)-3-aminocyclohexane (or cyclopentane) carboxylic acid (D-gamma-Ach or D-gamma-Acp) were explored by computational methods. Specifically, density functional theory (DFT) calculations on monomers and dimers of gamma-Ach-based and gamma-Acp-based alpha,gamma-cyclo-hexapeptides and cyclo-octapeptides were carried out to investigate the experimentally observed preference for alpha-alpha over gamma-gamma dimerization, associated with the two types of stacking patterns present in these peptide nanotubes, as well as the preference for heterodimerization versus homodimerization. Full geometry optimizations were performed at the B3LYP/6-31G(d) level, and single point calculations were subsequently carried out with the B3LYP and M05-2X functionals and the 6-31+G(d,p) basis set. The calculations predict that the interaction energies in the alpha-alpha species are quite similar to those in the gamma-gamma dimers. However, a comparison of dimerization energies (i.e., interaction energies plus deformation energies of monomers) shows that alpha-alpha dimerization is energetically favored over gamma-gamma dimerization. The calculations strongly suggest that the preference for alpha-alpha binding is governed by differences between the deformation energies in the alpha and gamma monomers, rather than by differences between the relative strengths of the alpha-alpha and gamma-gamma hydrogen-bonding patterns. Calculations based on local properties of the electron density support the previous suggestion that the H-N bonds of the alpha-amino acids are more polarized than those of the gamma-amino acids.

Quasiclassical trajectory study of the collision-induced dissociation of CH3SH++Ar

Quasiclassical trajectory calculations were carried out to study the dynamics of energy transfer and collision-induced dissociation (CID) of CH(3)SH(+) + Ar at collision energies ranging from 4.34 to 34.7 eV. The relative abundances calculated for the most relevant product ions are found to be in good agreement with experiment, except for the lowest energies investigated. In general, the dissociation to form CH(3)(+) + SH is the dominant channel, even though it is not among the energetically favored reaction pathways. The results corroborate that this selective dissociation observed upon collisional activation arises from a more efficient translational to vibrational energy transfer for the low-frequency C-S stretching mode than for the high-frequency C-H stretching modes, together with weak couplings between the low- and high-frequency modes of vibration. The calculations suggest that CID takes place preferentially by a direct CH(3)(+) + SH detachment, and more efficiently when the Ar atom collides with the methyl group-side of CH(3)SH(+).

Ab initio-gradient optimized molecular geometry and conformational analysis of 1,3-propanediol at the 4-21G level

Abstract The geometries of twenty five conformations of 1,3-propanediol were refined by the ab initio gradient method at the 4-21G level. The two most stable conformers are found to be stabilized by internal hydrogen bonding. Several conformationally dependent structural trends found previously in similar systems have been confirmed. The results are consistent with experimental data.

Direct and Indirect Hydrogen Abstraction in Cl + Alkene Reactions

Reactions between Cl atoms and propene can lead to HCl formation either by direct H abstraction or through a chloropropyl addition complex. Barring stabilizing collisions, the chloropropyl radical will either decompose to reactants or form HCl and allyl products. Using velocity-map imaging to measure the quantum state and velocity of the HCl products provides a view into the reaction dynamics, which show signs of both direct and indirect reaction mechanisms. Simulated trajectories of the reaction highlight the role of the direct H-abstraction pathways, and the resultant simulated scattering images show reasonable agreement with measurement. The simulations also show the importance of large excursions of the Cl atom far from equilibrium geometries within the chloropropyl complex, and these large-amplitude motions are the ultimate drivers toward HCl + allyl fragmentation. Gas-phase measurements of larger alkenes, 2-methylpropene and 2,3-dimethylbut-2-ene, show slightly different product distributions but still feature similar reaction dynamics. The current suite of experiments offers ready extensions to liquid-phase bimolecular reactions.

Ab Initio and RRKM Study of the HCN/HNC Elimination Channels from Vinyl Cyanide

Ab initio CCSD and CCSD(T) calculations with the 6-311+G(2d,2p) and the 6-311++G(3df,3pd) basis sets were carried out to characterize the vinyl cyanide (C(3)H(3)N) dissociation channels leading to hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC). Our computations predict three elimination channels giving rise to HCN and another four channels leading to HNC formation. The relative HCN/HNC branching ratios as a function of internal energy of vinyl cyanide were computed using RRKM theory and the kinetic Monte Carlo method. At low internal energies (120 kcal/mol), the total HCN/HNC ratio is about 14, but at 148 kcal/mol (193 nm) this ratio becomes 1.9, in contrast with the value 124 obtained in a previous ab initio/RRKM study at 193 nm (Derecskei-Kovacs, A.; North, S. W. J. Chem. Phys.1999, 110, 2862). Moreover, our theoretical results predict a ratio of rovibrationally excited acetylene over total acetylene of 3.3, in perfect agreement with very recent experimental measurements (Wilhelm, M. J.; Nikow, M.; Letendre, L.; Dai, H.-L. J. Chem. Phys.2009, 130, 044307).