Carbon-oxygen bond strength in diphenyl ether and phenyl vinyl ether: An experimental and computational study

The thermal decomposition of gaseous diphenyl ether (DPE) and phenyl vinyl ether (PVE) has been studied, at atmospheric pressure in hydrogen and in a very low-pressure reactor, over a temperature range of 1050-1200 K. The high-pressure rate constant for homolytic bond cleavage C6H5O-C6H5 --> C6H5O. + C6H5. (1) obeys k(1) (s(-1)) = 10(15.50) exp(-75.7/RT). Two pathways can be distinguished for C6H5OC2H3: C6H5. + C2H3O. (2) and C6H5O. + C2H3. (3) The overall rate constant follows k(2+3) (s(-1)) = 10(15.50) exp(-73.3/RT). The rate ratio, upsilon(2)/upsilon(3), amounts to 1.8 and appears to be temperature independent. These findings result in bond dissociation energies (BDE) at 298 K for C6H5O-C6H5, C6H5-OC2H3, and C6H5O-C2H3 of 78.8, 75.9, and 76.0 kcal mol(-1), respectively. The enthalpies for reactions 1-3 have been also determined at 298 and 1130 K by ab-initio calculations using the density functional theory formalism on the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) level. Comparison between experiments and theoretical calculations reveals distinct variances (ca. 3-4 kcal mol(-1)) for the BDE(C-O) in aryl ethers and the BDE(O-H) in phenol and vinyl alcohol but a close agreement for the BDE(C-H) in the related hydrocarbons: toluene, benzene, and ethene.