Computational prediction of α/β selectivities in the pyrolysis of oxygen-substituted phenethyl phenyl ethers

Phenethyl phenyl ether (PPE; PhCH₂CH₂OPh) is the simplest model for the most common β-O-4 linkage in lignin. Previously, we developed a computational scheme to calculate the α/β product selectivity in the pyrolysis of PPE by systematically exploiting error cancellation in the computation of relative rate constants. The α/β selectivity is defined as the selectivity between the competitive hydrogen abstraction reaction paths on the α-and β-carbons of PPE. We use density functional theory and employ transition state theory where we include diagonal anharmonic correction in the vibrational partition functions for low frequency modes for which a semiclassical expression is used. In this work we investigate the effect of oxygen substituents (hydroxy, methoxy) in the para position on the phenethyl ring of PPE on the α/β selectivities. The total α/β selectivity increases when substituents are introduced and is larger for the methoxy than the hydroxy substituent The strongest effect of the substituents is observed for the a-pathway of the hydrogen abstraction by the phenoxyl chain carrying radical for which the rate increases. For the βpathway and the abstraction by the R-benzyl radical (R = OH,OCH ₃) the rate decreases with the introduction of the substituents. These findings are compared with results from recent experimental studies.