Mode-selective photoisomerization in 5-hydroxytropolone. II. Theory

Abstract

Ab initio calculations are used to explore the ground-state potential energy surface for the syn–anti photoisomerization reaction of 5-hydroxytropolone (5-HOTrOH). Two reaction coordinates are identified, involving 2-OH tunneling and 5-OH torsion. Hartree–Fock (HF) and perturbation theory (at the MP2 level) have been used to calculate the stationary points on the two-dimensional surface associated with these coordinates. Similar calculations on the parent molecule tropolone are carried out for comparison. As observed in previous studies, the 2-OH tunneling barrier drops dramatically at the MP2 level which includes electron correlation. Vibrational frequency calculations are carried out for both tropolone and 5-HOTrOH at the HF/6-31G** and MP2/6-31G** levels in order to correlate the modes with those observed experimentally. A method is introduced for evaluating which normal coordinates should be most strongly coupled to a given reaction coordinate. Normalized, mass-weighted intrinsic and direct reaction coordinates similar in form to the normal coordinates are devised by projecting atomic displacements from the reactant structure toward a transition state (intrinsic) or product (direct) structure. These serve as limiting cases for the initial projections of the multidimensional reaction trajectories. The intrinsic and direct reaction coordinates are then expanded in the basis set of normal coordinates to obtain coefficients of expansion of the reaction coordinates in this basis set. This simple scheme highlights the subset of normal coordinates which are important in promoting reaction by H-atom tunneling or O–H torsion. In 5-HOTrOH, an in-plane mode calculated at 348 cm−1 has a large coefficient of expansion along both intrinsic and direct reaction coordinates. This mode is assigned as the ‘‘promoter mode’’ W observed in the experimental study of paper I.