Prediction Of Reaction Mechanism And Absolute Rate Constant Using Computer Algorithm: The Bromate Ion In Acidic Media Reaction

ABSTRACT

Computational and theoretical advances have progressed sufficiently so that it is now possible to calculate reaction rates directly from ab initio theory without the intervening use of any analytical method. The rate constants of chemical reactions are calculated using information about the structures, energies, and Vibrational frequencies of reactants and transition states. Computer algorithms help in determining the structures of the molecules and in understanding how molecules react with one another. In the present work, one of the elementary steps in the chemistry of bromate ion in acidic medium was investigated. The highlighted step is: The above reaction is not a simple binary reaction; three reactants are combining to generate two products. Derivation of a mechanism required combining the known experimental data with the various ways that these elementary reactions could happen to find the consistent path or paths. The one which was impossible was a single step which required three particles coming together concertedly. The Hartree-Fock (HF) and the Density Function Theory (DFT) at B3LYP were adopted to study the reaction mechanism of hypobromous acid and bromide ion in acidic medium in both gas phase and in solution phase. Moreover, this aimed at comparing the two levels of theory and various basis sets. Three different reaction pathways were derived and their potential energy surfaces (PES) computed. Reaction pathways were successfully verified using the Intrinsic Reaction Coordinate (IRC) approach. An outline of a general theoretical scheme for the various reaction mechanistic pathways is well illustrated. Reaction pathway 1 occurs in the atmosphere (gas phase), pathway 2 is a bromination reaction which occurs in dilute acid and pathway 3 is an oxidation reaction which occurs in strong acid. The optimized energies at minima and the first-saddle point (maxima) were used to estimate the rate constants using the transition state theory (TST). The rate constants ( ) computed using the B3LYP for reaction pathway 1, 2 and 3 are 6.1E-10 cm3 mol-1 s-1, 3.7E+12 M-2s-1 and 6.3E+8 M-2s-1, respectively. For the reaction pathway 1 and 3 the experimental data is at least 3.5E-15 cm3 mol-1 s-1 and 2.3E+9 - 1.6E+10 M-2s-1 which compared relatively well with the calculated kinetic parameters. In addition, and calculated were compared with a number of experimental values reported by earlier workers. HOBr Br- H+ H2O Br2 + + + k1 k-1 1 k 1 k K