This thesis focuses on tautomerization reactions described quantum mechanically. We study the geometry and kinetics of proton tautomerization reactions, and demonstrate quantum control of enantiomeric tautomerization.
In the first part, double proton transfer in porphycene is considered as an example of proton tautomerization. The potential energy surface for the concerted proton transfer reaction in porphycene (using atomic Cartesian coordinates) is calculated and the vibrational ground state eigenfunctions are evaluated for the normal and double deuterated species. The tunneling splitting for both species are calculated by diagonalizing the corresponding Hamiltonian using a Lanczos algorithm. The obtained value (1.2 cm−1) is found to be in relatively good agreement with the experimental one (4.4 cm−1). The strongest coupled mode which modifies the distance between the nitrogen atoms is also calculated. The reaction coordinates as well as the coupled modes are correlated to the so-called Cartesian reaction plane coordinates. It is demonstrated that the compact Cartesian reaction plane coordinates cover the atomic reaction coordinates as well as some of the modes that are strongly coupled to the reaction coordinates. In addition, we investigate the primary, secondary, and vicinal hydrogen/
deuterium (H/D) isotope effect on the geometry of the two intramolecular hydrogen bonds in porphycene. Multidimensional potential energy surfaces describing the anharmonic motion in the vicinity of the trans isomer are calculated for the different symmetric (HH/DD) and asymmetric (HD) isotopomers. From the solution of the nuclear Schr¨odinger equation the ground state wave function is obtained which is further used to determine the quantum corrections to the classical equilibrium geometries of the hydrogen bonds and thus the geometric H/D isotope effects. In particular it is found that the hydrogen bonds are cooperative, that is, both expand simultaneously even in the case of an asymmetric isotopic substitution. Furthermore, the primary H/D isotope effects on the chemical shifts give information about the primary, secondary and vicinal geometric H/D isotope effects of the two inner hydrogen bonds of porphycene. The vicinal effects
indicate a cooperative coupling of the two hydrogen bonds. These theoretical predictions compare favourably with NMR chemical shift data. In order to determine whether the tautomerization of porphycene is a concerted or a stepwise process, we calculate the potential energy surface using density functional theory as well as Møller-Plesset perturbation theory. Z. Smedarchina used the calculated potential energy surface and evaluated proton-transfer rates with the help of the approximate instanton method. The results are compared with experimental rate constants and activation energies. Fitting these data requires empirical adjustment of the potential, the adjustment being disproportionally large for the stepwise but modest for the concerted mechanism, which is therefore favoured.
In the second part, we demonstrate quantum mechanically how to resolve enantiomers from an oriented racemic mixture taking advantage of photodissociation. Our approach employs a femtosecond ultraviolet (UV) laser pulse with specific linear polarization achieving selective photodissociation of one enantiomer from a mixture of L and D enantiomers. As a result, the selected enantiomer is destroyed in the electronically excited state while the opposite enantiomer is left intact in the ground state. As an example we use H2POSD which presents axial chirality. A UV pulse excites the lowest singlet excited state which has nσ* character and is, therefore, strongly repulsive along the P-S bond. The model simulations are performed using wavepackets which propagate on two dimensional potential energy surfaces, calculated along the chirality and dissociation reaction coordinates using CASSCF level of theory.
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