On the usefulness of quantum chemical bond order as a local reaction path indicator: the case study of a model carbonyl addition reaction

Abstract

The idea of using quantum chemical bond order(BO) as a local reaction path indicator proposed earlier with reference to ABC↔BCA type of isomerization processes is explored further with reference to model carbonyl addition reactions, e.g. H₂CO+H-Cl→ H₂CCl-OH and H₂COH⁺ + H^-Cl→ClCH₂OH + H⁺. In each case the reaction is viewed as a superposition of elementary chemical events, some representing bond making and some bond breaking processes, respectively. Altogether four bonds are involved in the model. The active bond order profile in each case is marked by a sharp inflection signalling the onset of a critical process, viz. bond breaking or bond making. An extended bond energy bond order description of the addition of HCl to the >C=O moiety is suggested and tested numerically. The semilocal description is seen to model the approximate reaction path well and predict the active portion of the transition state structure fairly successfully. The local RP modelling is also seen to predict changeover in the mechanism of the reaction when conditions are altered by protonating the carbonyl group. The origin of the observed BO inflexions is sought to be correlated with Fermi correlation.