Zdeněk Havlas

On the formation of C2H8+ in the reaction CH4+ + CH4 → CH3 + CH5+: A theoretical study

Abstract On the potential surface for the title process, eleven stationary points (UHF, 3-21G) have been located: six minima, three saddle points and two stationary points of higher order. The most stable C 2 H 8 + system has a linear CHHC bond.

Ab initio quantum chemical study of the SN2 reaction, CH3F+H− → CH4+F−, in the gas phase

Abstract The reaction profile of the title S N 2 reaction has been studied by the ab initio method at various levels. The geometries of the van der Waals complexes of reactants and products and of the transition state have been optimized at the MP2/6-311G** level. It has been found that the reaction path loses C 3v symmetry and the reaction path is bifurcated in the product region. All the stationary point energies have been evaluated at the MP2 and MCSCF CI levels with various basis sets. The vibrational frequencies of the stationary points have been calculated using the SCF and, for some modes, MP2 methods.

MBPT And coupled-cluster activation barriers. The model SN2 reaction: H−+CH3F*lrhar2;CH4+F−

Abstract For the SN2 reaction H−+CH3F⇋CH4+F− the energies of the ion-molecule complexes, the intrinsic barriers, the overall barrier and the reaction energy have been calculated within two approximations of many-body perturbation theory (SDQ-MBPT(4), SDTQ-MBPT(4)) and in their coupled-cluster counterparts (CCSD, CCSD+T(CCSD)). The corresponding MBPT and CC energies of the ion-molecule complexes agree within 1–2 kJ/mol and for the barrier within 4–5 kJ/mol. It is concluded that the convergence of MBPT(4) is comparable and satisfactory over the entire energy profile. The results further indicate that the inclusion of triple excitations is essential.

Theoretical studies of reaction mechanism in chemistry

Publisher Summary The purpose of this chapter is to present an outline of both the dynamic and static approaches to theoretical studies of reaction mechanisms. The dynamic approach may be regarded as more sophisticated of the two. The purpose of dynamical studies is to find the general features and characteristics of elementary processes. The main effort is focused on the most complete view of the elementary processes as obtained from the experimental as well as theoretical studies of relatively simple model systems, with the aim of obtaining results that allow one to draw general conclusions about the course of elementary processes. The static approach, on the other hand, may seem not to be so sophisticated, but at present it provides answers to everyday chemistry in a variety of fields at different levels of theory. Statistical methods of calculating the rate (or equilibrium) constants, based on the fragmentary reaction profiles and statistical methods are now routine and can be used for a broad spectrum of chemical reactions.

The effect of geometry optimization on the reaction barrier height: SCF and MP2 studies of the dissociation and isomerization reactions of formaldehyde

Abstract The structures of formaldehyde and activated complexes of the dissociation and isomerization reactions were optimized at the SCF and the second-order Moller—Plesset (MP2) levels with 6-31G, 6-311G, 6-31G ++ , and 6-31G ++ + P′ basis sets. The barrier heights calculated at the MP2 level for the geometry optimized at the SCF level differ from those optimized at the MP2 level only negligibly, while the geometries change significantly. The barrier heights are much more affected by quality of a basis set and inclusion of electron correlation. The high level geometry optimization is necessary only for calculations, where accuracy higher than 6 kJ mol −1 is required.