NONLOCAL DENSITY FUNCTIONAL THEORY AS A PRACTICAL TOOL IN CALCULATIONS ON TRANSITION-STATES AND ACTIVATION-ENERGIES - APPLICATIONS TO ELEMENTARY REACTION STEPS IN ORGANIC-CHEMISTRY

Nonlocal density functional theory (NL) has been evaluated as a practical tool for theoretical studies of organic reactions. Calculations on the two abstraction reactions A (.CH3 + CH4 --> CH4 + .CH3) and B (.CH3 + CH3Cl --> CH4 + .CH2Cl) afforded the activation energies A = 11.7 kcal/mol and B = 8.5 kcal/mol, which compare favorably with the experimental activation energies of 14.1 kcal/mol for A and 9.4 kcal/mol for B. The local density approximation (LDA), without nonlocal correction, afforded a qualitative incorrect description of the two processes with a double-well reaction profile and too small barriers of 1.9 kcal/mol for A and -2.1 kcal/mol for B. Nonlocal (NL) calculations on the dissociation process C (H2CO --> H-2 + CO) afforded barrier and transition-state structures in close agreement with the best estimate from ab initio calculations. The NL barrier of 75 kcal/mol differs only slightly from the LDA estimate of 74 kcal/mol, and nonlocal corrections are not important for this type of reaction. Nonlocal calculations on the isomerization reactions D (CH3CN --> CNCH3) gave an activation energy of 37.5 kcal/mol, which is close to the experimental value of 38.5 kcal/mol. Studies were also carried out on the isomerization processes D' (HCN --> CNH) and E (HOC+ --> HCO+) for which experimental estimates are unavailable. The calculated barriers were 27.6 kcal/mol for D' and 26.0 kcal/mol for E. The nonlocal corrections were of minor importance for the reactions D, D', and E. It is concluded that nonlocal density functional theory provides the same accuracy in kinetic studies as the highest level of ab initio theory applied to the reactions A to E.