Assessment in the competition between steric and electronic effects in the elimination kinetic of hydrogen in 1,4-cyclohexadienes in the gas phase. Quantum chemical theory calculations

Abstract

The mechanisms of gas-phase thermal decomposition of alkyl-substituted cyclohexadienes were studied by the means of quantum chemical calculations with theory levels Møller-Plesset pertubation theory (MP2) and density functional theory (DFT) (B3LYP, MPW1PW91, PBEPBE, ωB97XD, CAM-B3LYP, M06, and M062X) with 6-31G(d,p), 6-31++G(d,p) basis sets. The examination of the reaction pathways of each substrate demonstrated a molecular mechanism through six-membered cyclic boat-like transition state (TS) structure. An alkyl group substituent causes a detrimental effect on the reaction rate, compared to the parent compound 1,4-cyclohexadiene; however, the reaction was favoured in the case of 3,6-dimethyl substitution. The 3,6-dimethyl-1,4-cyclohexadiene compound has activation energy 11.2 kJ/mol lower than the reference compound, which overcomes the effect of the most negative entropy of activation in the series. The effects of alkyl substituents in these reactions suggest a complex combination of electronic and steric influence. These reactions are characterised as highly synchronous concerted, with small predominance of C–H bond breaking in the TS.

Keywords:

• alkyl-substituted cyclohexadienes
• gas-phase kinetics
• thermal decomposition
• mechanism
• ab initio and DFT calculations