On the use of explicitly correlated treatment methods for the generation of accurate polyatomic -He/H2 interaction potential energy surfaces: The case of C3-He complex and generalization.

Through the study of the C3(X1Σg (+)) (1)Σg (+)) + He((1)S) astrophysical relevant system using standard (CCSD(T)) and explicitly correlated (CCSD(T)-F12) coupled cluster approaches, we show that the CCSD(T)-F12/aug-cc-pVTZ level represents a good compromise between accuracy and low computational cost for the generation of multi-dimensional potential energy surfaces (PESs) over both intra- and inter-monomer degrees of freedom. Indeed, the CCSD(T)-F12/aug-cc-pVTZ 2D-PES for linear C3 and the CCSD(T)-F12/aug-cc-pVTZ 4D-PES for bent C3 configurations gently approach those mapped at the CCSD(T)/aug-cc-pVXZ (X = T,Q) + bond functions level, whereas a strong reduction of computational effort is observed. After exact dynamical computations, the pattern of the rovibrational levels of the intermediate C3-He complex and the rotational and rovibrational (de-) excitation of C3 by He derived using both sets of PESs agree quite well. Since C3 shows a floppy character, the interaction PES is defined in four dimensions to obtain realistic collisional parameters. The C-C-C bending mode, which fundamental lies at 63 cm(-1) and can be excited at very low temperatures is explicitly considered as independent coordinate. Our work suggests hence that CCSD(T)-F12/aug-cc-pVTZ methodology is the key method for the generation of accurate polyatomic - He/H2 multi-dimensional PESs.

A five-dimensional potential-energy surface for the rotational excitation of SO2 by H2 at low temperatures.

The SO(2) molecule is detected in a large variety of astronomical objects, notably molecular clouds and star-forming regions. An accurate modeling of the observations needs a very good knowledge of the collisional excitation rates with H(2) because of competition between collisional and radiative processes that excite and quench the different rotational levels of SO(2). We report here a five-dimensional, rigid-body, interaction potential for SO(2)-H(2). As a first application, we present rate constants for excitation/de-excitation of the 31 first levels of SO(2) by para-H(2) at low temperatures. Propensity rules are discussed.

Modeling of protonation processes in acetohydroxamic acid

This work presents a theoretical study of acetohydroxamic acid and its protonation processes using ab initio methodology at the MP2(FC)/cc-pdVZ level. We find the amide form more stable than the imidic tautomer by less than 1.0 kcal mol(-)(1). For comparison with the experimental data, a three-dimensional conformational study is performed on the most stable tautomer (amide). From this study, the different barriers to rotation and inversion are determined and the intramolecular hydrogen bond between the OH group and the carbonyl oxygen is characterized. The electrostatic potential distribution shows three possible sites for electrophilic attack, but it is shown that only two of them, the carbonyl oxygen and the nitrogen atoms, are actual protonation sites. The protonation energy (proton affinity) is obtained from the results of the neutral and charged species. Proton affinities for the species charged on the carbonyl oxygen and the nitrogen atoms are estimated to be 203.4 and 194.5 kcal mol(-)(1), respectively. The development of a statistical model permits the quantification of DeltaG (gas-phase basicity) for the two protonation processes. In this way, the carbonyl oxygen protonated form is found to be more stable than that of the nitrogen atoms by 8.3 kcal mol(-)(1) at 1 atm and 298.15 K, due to the enthalpic contribution. As temperature increases, the proportion of the nitrogen protonated form increases slightly.

Rotationally inelastic collisions of SO(X3Sigma-) with H2: potential energy surface and rate coefficients for excitation by para-H2 at low temperature.

Rotational excitation of the interstellar species SO(X3Sigma-) with H2 is investigated. The authors present a new four-dimensional potential energy surface for the SO-H2 system, calculated at an internuclear SO distance frozen at its experimental minimum energy distance. It was obtained at the RCCSD(T) level using the aug-cc-pVTZ basis sets for the four atoms. Bond functions were placed at mid-distance between the SO center of mass and the center of mass of H2 for a better description of the van der Waals interaction. Close coupling calculations of the collisional excitation cross sections between the fine structure levels of SO by collisions with para-H2 are calculated at low energies which yield, after Boltzmann thermal average, rate coefficients up to 50 K. The exact level splitting is taken into account. The propensity rules between fine structure levels are studied. It is shown that F-conserving cross sections are much larger, especially for high-N rotational levels, than F-changing cross sections, as found previously for SO-He collisions and expected from theoretical considerations. The new rate coefficients are compared with previous results obtained for this molecule and they find that important differences exist that can induce important consequences on astrophysical modeling. Comparison with excitation by collision with He shows that the rate coefficients differ by important factors that cannot be only explained by the reduced mass ratio in the thermal average. This may be due to differences between the potential energy surfaces as well as to the contribution of the different reduced masses in the scattering equations.

Ab Initio Determination of the Roto-Torsional Energy Levels of trans-1,3-Butadiene.

In this paper, the flexible model based on relaxed ab initio calculations, which has been several times employed for vibrational calculations, is extended to the analysis of the rotational structures starting by the roto-torsional bands of trans-1,3-butadiene. For this purpose, the potential energy surface and the kinetic energy parameters of the nu13 vibrational mode of butadiene are obtained with the Möller-Plesset perturbation theory up to the second order and the 6-31G(d, p), 6-31G(df, p), 6-311G(d, p), 6-311G(df, p), and 6-311G(df, pd) basis sets. The torsional levels of the -h6, -d4, and -d6 isotopic species are calculated variationally and are compared with experimental data. It may be concluded that the one-dimensional model appears sufficiently accurate for butadiene-h6 and -d4, whereas a large kinetic interaction with the lowest wagging mode is observed for butadiene-d6. The rotational levels corresponding to the first vibrational states of the -h6 and -d4 species are determined variationally up to J = 17 and J = 11 from the ab initio spectroscopic parameters which have been expanded as functions of the torsional coordinate using symmetry adapted series. The torsional wavefunction is contracted to reduce the size of the Hamiltonian matrix. A good agreement with the observed transitions is obtained for the first states v = 0 and v = 1. As is expected, the K doubling obtained is relatively small. For this reason, the quartic and sextic centrifugal distortion constants are obtained from the least-square fit of the variational levels to the perturbation theory equations for the symmetric top. Copyright 1998 Academic Press.

Ab Initio Potential Energy Surface and Internal Torsional-Wagging States of Hydroxylamine

The two-dimensional potential energy surface describing the interaction of the large-amplitude torsional and wagging motions in hydroxylamine has been determined from ab initio calculations. This surface has been sampled by a large set of grid points from a two-dimensional configuration space spanned by the torsional and wagging coordinates. At each grid point, the geometry optimization has been performed using the second-order Moller-Plesset perturbation theory with the basis set 6-311 + G(2d, p). At the optimized geometry, the single-point calculation of the electronic energy has been carried out using a larger basis set 6-311 + G(3df, 2p). This method was verified to yield the results comparable to those obtained by a direct optimization of the geometry with the basis set 6-311 + G(3df, 2p) which had been used by A. Chung-Phillips and K. A. Jebber (1995. J. Chem. Phys. 102, 7080-7087) to calculate the energies of only three points in the potential energy surface of hydroxylamine. The trans and cis local minima have been found on the determined potential energy surface. The localization features of the torsional-wagging states have been studied by solving the two-dimensional Schrodinger equation for the coupled torsional and wagging motions. Copyright 1997 Academic Press. Copyright 1997Academic Press

Variational Evaluation of the HSSH FIR Transitions: The Anomalous K-Doubling.

The roto-torsional energy levels of HSSH and DSSD up to J = 20 are evaluated variationally with a Hamiltonian expressed in terms of internal coordinates. The kinetic and potential parameters are derived from ab initio calculations with full optimization of the geometry. The calculated levels are employed for the determination of the centrifugal distortion constants. HSSH is a near-prolate symmetric rotor. The most stable C(2) conformer, calculated with MP4(SDQ)/cc-pVQZ, exhibits a 90.55 degrees dihedral angle. For J = 0, the lowest energies of HSSH and DSSD are 413.4876 cm(-1) (n = 1), 798.0304 cm(-1) (n = 2) and 1151.5773 cm(-1) (n = 3), and 304.3185 cm(-1) (n = 1), 594.2919 cm(-1) (n = 2), and 869.3508 cm(-1) (n = 3), respectively. For J = 60, the ab initio calculations allow the reproduction of the anomalous type-K doubling predicted with perturbation theory. Copyright 2000 Academic Press.