Carbon−Oxygen Bond Strength in Diphenyl Ether and Phenyl Vinyl Ether: An Experimental and Computational Study

Abstract

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 k1 (s-1) = 1015.50 exp(−75.7/RT). Two pathways can be distinguished for C6H5OC2H3: C6H5• + C2H3O• (2) and C6H5O• + C2H3• (3). The overall rate constant follows k2+3 (s-1) = 1015.50 exp(−73.3/RT). The rate ratio, v2/v3, 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.