Carsten Hennig

Rotational effects in complex-forming bimolecular substitution reactions: A quantum-mechanical approach.

The quantum dynamics of the complex-forming S(N)2 reaction Cl(-)+CH(3)Br-->ClCH(3)+Br(-) is studied with emphasis on rotational effects. The pseudotriatomic system Cl-Me-Br is treated with a corresponding three-dimensional (3D) potential energy surface as a function of the two scattering coordinates and the enclosed angle where the geometry of the methyl group Me is optimized at each point. The 3D space is divided into three different parts, the interaction region, an intermediate region, and the asymptotic region. In line with simple classical-mechanical arguments and previous classical trajectory calculations, initial rotational motion of CH(3)Br seemingly decreases the reaction probability. However, the dynamical inclusion of the rotational degree of freedom and the presence of the many rovibrational product states overall lead to a large increase in reactivity compared to our previous collinear study on this reaction. If the reactant is rotationally excited, the higher vibrational product states are depleted in favor of lower-lying levels. Starting the reaction with rotationless reactants may end up in significant rotational excitation in the product molecules (translation-to-rotation energy transfer). On the other hand, initial rotational energy in rotationally highly excited reactants is to a large amount converted into translational and vibrational energy. The average amount of rotational energy in the products shows a twofold vibrational excitation-independent saturation (i.e., memorylessness), with respect to both initial rotational excitation and translational energy. Since only about one-half of all reactant states end in rotationless products, the reaction probability should be increased by a factor of 2; the actually larger reactivity points to other dynamical effects that play an important role in the reaction.

Quantum dynamics of the complex-forming SN2 reaction Cl−+ CD3Cl′→ ClCD3+ Cl′− on a four-dimensional coupled-cluster potential surface

Time-independent quantum scattering calculations have been carried out on the gas-phase SN2 reaction Cl− + CD3Cl′ (υ1,υ2,υ3) → ClCD3 (υ′1,υ′2,υ′3) + Cl′−. The two C–Cl stretching modes (quantum numbers υ3 and υ′3) and the totally symmetric modes of the methyl group (C–D symmetric stretching vibration, υ1 and υ′1, and symmetric or umbrella bending vibration, υ2 and υ′2) are treated explicitly, making use of a four-dimensional coupled-cluster potential energy surface. Converged state-selected reaction probabilities and product distributions have been calculated up to 4380 cm−1 above the vibrational ground state of CD3Cl, i.e. up to initial vibrational excitation (2,0,0). In order to extract all scattering resonances, a fine energetic grid had to be chosen. Excitation of the umbrella bending mode leads to a significant enhancement of the reaction probability, which, owing to the absence of the ν2 ≈ 2ν3 near-degeneracy, is smaller than in the Cl− + CH3Cl system, however. Exciting the high–frequency symmetric C–D stretching vibration has a considerable influence that is much larger than in the non-deuterated system. For small translational energies, reactants excited with one quantum in ν1 are more reactive than those with one quantum in either ν2 or ν3. This leads to the conclusion that the C–D stretching mode should not be treated as a spectator. The calculated state-selected reaction probabilities nicely confirm the inverse kinetic isotope effect found experimentally and reproduced earlier through variational transition state computations.