Masahiko Koizumi

Double proton transfer in the dihydroxyethylene-glyoxal complex

Abstract The cis -dihydroxyethylene-glyoxal (DE-G) complex was analysed by use of the ab initio molecular orbital method which is the model for hemiquinone-type molecular devices including double proton transfer. The singlet and triplet geometries of equilibrium points were optimized at the HF/STO-3G level, and the potential-energy surface of this system was obtained at the Hartree-Fock and MP2/3–21G levels. The molecular adduct DE-G, singlet C 2v structure, can transfer two protons in parallel, through the D 2h singlet transition state (RMP2/3–21G, E a = 12.6 kcal mol −1 ). The C 2v state has a very large dipole moment. Therefore, this model may be applicable to a double proton transferring molecular device. Mechanisms of the double proton transfer were investigated using the frontier orbital theory.

A string model for the chemical reaction coordinate in static external fields

A theoretical method for estimating the effects of a static external field upon a reaction path is proposed. The reaction path is defined as the intrinsic reaction coordinate (IRC) which is treated as a “string”. The string is deformed by the external static force field. We project the external force onto the normal modes orthogonal and parallel to the string. The string is dragged along each direction of the normal mode. The cell structure [10b] attached to the string is also deformed; the transition state (TS) is shifted, and the internal structure of the reaction system is expanded or compressed in configuration space. A min-max relationship of the external effect on the string is found: the smaller the magnitude of the force constant of the normal mode orthogonal to the string, the larger the deformation of the string in the direction of the normal mode. As an example, the effects of one water molecule for hydrogen-migrating isomerization of formaldehyde and fluoroformaldehyde are considered.

Reaction ergodography for the hydrolysis of urea

Abstract An ab initio molecular orbital (MO) calculation was carried out for the reaction path of the interaction between a urea and a water molecule. The potential energy surface for the urea—water super-molecule reveals that the hydrolysis of urea is a much more energetically favoured decomposition mechanism than unimolecular decomposition of urea. The RHF/4-31G energy barrier for the hydrolysis agrees with the experimental values and the analysis for the reaction path of unimolecular decomposition explains the geometrical change through the reaction process.

Dynamic stability of carbonyl ylides

The non- and fluorine-substituted singlet carbonyl ylides are studied by using ab initio MCSCF calculations. The thermodynamic stability of the carbonyl ylides and the intramolecular stability to isomerization or fragmentation reaction coordinates is demonstrated in terms of the topological structure of the ab initio potential energy surfaces. The allylic resonance is found to be dynamically unstable, considering out-of-plane vibrational mode. The instability is studied by the symmetries of the low-lying excitations out of the MCSCF wave function.