Thermochemistry of oxabicycloheptenes: enthalpy of formation, entropy and heat capacity

Abstract

AbstractCyclohexene isomers with an epoxide ring (oxabicycloheptenes) are reportedly formed in atmospheric photochemical oxidation of aromatics. They may also be formed in higher temperature oxidation and oxygen‐catalyzed pyrolysis of aromatics. Thermochemical properties, ΔH°f(298), S°298 and Cp(T) (10 K ≤ T ≤ 5000 K) of five target oxabicycloheptenes were calculated using density functional methods. Structures for 7‐oxabicyclo[4.1.0]hept‐2‐ene, 7‐oxabicyclo[4.1.0]hept‐3‐ene, 7‐oxabicyclo[3.1.1]hept‐1‐ene, 7‐oxabicyclo[3.1.1]hept‐2‐ene and 7‐ oxabicyclo[2.2.1]hept‐2‐ene were calculated at the B3LYP/6–31g(d,p) level of theory. Enthalpies of formation were determined at the B3LYP/6–31G(d,p) calculation level using isodesmic and homodesmic working reactions with the ΔH°rxn and known enthalpies of reference reactants in each of four different work reactions. Enthalpy values for the following bicyclic ether structures were also calculated for comparison of the calculation method with literature: 6‐oxabicyclo[3.1.0]hexane, 7‐oxabicyclo[4.1.0]heptane, 8‐oxabicyclo[5.1.0]octane, 7‐oxabicyclo[2.2.1]heptane, endo‐2‐methyl‐7‐oxabicyclo[2.2.1]heptane, exo‐2‐methyl‐7‐oxabicyclo[2.2.1]heptane and 7‐oxabicyclo[4.1.0]hept‐2,4‐ene. Entropy (298 K) and heat capacity [Cp(T)] values along with Benson group additivity parameters for each ring system are reported. Copyright © 2005 John Wiley & Sons, Ltd.