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 C 6 H 5 O−C 6 H 5 → C 6 H 5 O • + C 6 H 5 • (1) obeys k 1 (s -1 ) = 10 15.50 exp(−75.7/ RT ). Two pathways can be distinguished for C 6 H 5 OC 2 H 3: C 6 H 5 • + C 2 H 3 O • (2) and C 6 H 5 O • + C 2 H 3 • (3). The overall rate constant follows k 2+3 (s -1 ) = 10 15.50 exp(−73.3/ RT ). The rate ratio, v 2 / v 3, amounts to 1.8 and appears to be temperature independent. These findings result in bond dissociation energies (BDE) at 298 K for C 6 H 5 O−C 6 H 5, C 6 H 5 −OC 2 H 3, and C 6 H 5 O−C 2 H 3 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.