P. J. S. B. Caridade

Dynamics of X+CH4 (X=H,O,Cl) reactions: How reliable is transition state theory for fine-tuning potential energy surfaces?

Trajectory calculations run on global potential energy surfaces have shown that the topology of the entrance channel has strong implications on the dynamics of the title reactions. This may explain why huge differences are observed between the rate constants calculated from global dynamical methods and those obtained from local methods that employ the same potential energy surfaces but ignore such topological details. Local dynamics approaches such as transition state-based theories should then be used with caution for fine-tuning potential energy surfaces, especially for fast reactions with polyatomic species since the key statistical assumptions of the theory may not be valid for all degrees of freedom.

Energy switching approach to potential surfaces. III. Three-valued function for the water molecule

A recently proposed energy switching scheme is used to improve the two-valued many-body expansion potential energy surface of Murrell, Carter, Mills, and Guest [Mol. Phys. 42, 605 (1981)] for H2O by merging it with the spectroscopically accurate polynomial-type form of Polyanski, Jensen, and Tennyson [J. Chem. Phys. 105, 6490 (1996)]. An attempt is also made to improve its long range forces, and Coulombic behavior at the collapsed molecular limits. The resulting ES two-valued surface has almost spectroscopic accuracy up to 13 650 cm−1, and like the original many-body expansion form may be used for studies of reaction dynamics. A brief analysis of the Σ–Π locus of conical intersection is also presented.

CROSS SECTIONS AND RATE CONSTANTS FOR THE O(1D) + H2 REACTION USING A SINGLE-VALUED ENERGY-SWITCHING POTENTIAL ENERGY SURFACE

Abstract A realistic single-valued energy-switching potential energy surface recently proposed for the water molecule has been used to perform quasiclassical trajectory calculations for the title reaction. The calculated cross sections and rate constants are compared with theoretical and experimental data, particularly with a view to assess the temperature dependence of the rate constant.

Potential energy surface for ground-state H2S via scaling of the external correlation, comparison with extrapolation to complete basis set limit, and use in reaction dynamics.

A global double many-body expansion potential energy surface is reported for the electronic ground state of H2S by fitting accurate ab initio energies calculated at the multireference configuration interaction level with the aug-cc-pVQZ basis set, after slightly correcting semiempirically the dynamical correlation by the double many-body expansion-scaled external correlation method. The function so obtained has been compared in detail with a potential energy surface of the same type recently reported (Song , Y. Z. and Varandas , A. J. C. J. Chem. Phys. 2009 , 130 , 134317.) by extrapolating the calculated raw energies to the complete basis set limit, eschewing any use of information alien to ab initio theory. The new potential energy surface is also used for studying the dynamics and kinetics of the S(1D) + H2/D2/HD reactions.

Quasiclassical Trajectory Study of the C(1D) + H2 Reaction and Isotopomeric Variants: Kinetic Isotope Effect and CD/CH Branching Ratio

The recently proposed ab initio single-sheeted double many-body expansion potential energy for the methylene molecule has been used to perform quasiclassical trajectory (QCT) calculations for the title reaction. Thermal and initial state-specific (v = 0, j = 0) rate constants for the C((1)D) + H(2)/HD/D(2) reactions have been obtained over a wide range of temperatures. Cross sections for the reaction C((1)D) + H(2) and its deuterated isotopes have also been calculated, as well as the CD/CH branching ratios for the C((1)D) + HD reaction. It is found that the CD + H product channel in the C((1)D) + HD reaction is preferred relative to the CH + D channel. The estimated rate constants are predicted to be in the order k(H2) > k(HD) > k(D2) and the calculated cross sections and rate constants compared with available theoretical and experimental data.

Dynamics of OH + O2 vibrational relaxation processes

The vibrational relaxation processes occurring during collisions of O2 and OH are investigated for vibrationally excited O2 (OH) colliding with OH (O2) in its ground state. All calculations employed the quasiclassical trajectory method and the realistic double many-body expansion (DMBE I) potential energy surface for ground-state HO3. Although a multiquantum mechanism turns out to dominate, two regimes for deactivation of high and low excited molecules have been found. A comparison with available experimental data is also reported.

The OH(v') + O2(v) reaction : a new source of stratospheric ozone?

We report a comparative dynamics study of the title reaction using a realistic double many-body expansion potential energy surface (DMBE I) for ground-state HO3 [Mol. Phys. 91 (1997) 301], and a recently reported DMBE II [Chem. Phys. Lett. 334 (2001) 173] surface obtained by modifying DMBE I to fit extensive ab initio calculations corrected for the complete basis set limit. The comparison provides strong evidence that the title reaction can be a significant source of ozone under stratospheric conditions.

Is there a barrier for the C2v insertion reaction in O(1D)+H2? A test dynamics study based on two-valued energy-switching potential energy surfaces

Abstract We have calculated cross-sections and rate constants for the title reaction by using the quasiclassical trajectory method and a recently reported two-valued energy-switching potential energy surface for the water molecule. By varying the amplitude and rate of decay of a local Gaussian term which controls the appearance of a barrier along the C 2 v minimum energy profile, an attempt has been made to answer the title issue. A comparison of the calculated rate constants with the available experimental data suggests that the barrier, if existing, lies below the energy of the reactants, and separates the small van der Waals well from the deep chemical one at short distances.

Dynamics study of the atmospheric reaction involving vibrationally excited O3 with OH

We investigate the title atmospheric reaction of highly excited O(3) with vibrationally cold OH. Particular attention will be paid to initial vibrational energies of O(3) between 9 and 21 kcal mol(-1). The calculations employ the quasiclassical trajectory method and the realistic double many-body expansion potential energy surface for HO(4)((2)A). Many aspects of the title process are presented. It is found that it may not be possible to ignore this process when studying the stratospheric ozone budget.

Theoretical investigation of vibrational relaxation of highly excited O3 in collisions with HO2

We report calculations of the relaxation process involving highly excited O3 with vibrationally cold HO2. The initial vibrational energies of O3 between 9 and 21 kcal mol−1 are considered. All calculations employ the quasi-classical trajectory method and the realistic double many-body expansion potential energy surface for HO5(2A). Both the traditional histogram boxing and momentum Gaussian binning schemes are utilized in examining the energy transfer process, the latter for the first time in the case of triatomic fragments. The results indicate that it may notable in studying the stratospheric ozone budget.