title: Computational prediction of alpha/beta selectivities in the pyrolysis of oxygen-substituted phenethyl phenyl ethers.
abstract: Phenethyl phenyl ether (PPE; PhCH 2CH 2OPh) is the simplest model for the most common beta-O-4 linkage in lignin. Previously, we developed a computational scheme to calculate the alpha/beta product selectivity in the pyrolysis of PPE by systematically exploiting error cancellation in the computation of relative rate constants. The alpha/beta selectivity is defined as the selectivity between the competitive hydrogen abstraction reaction paths on the alpha- and beta-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 alpha/beta selectivities. The total alpha/beta 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 alpha-pathway of the hydrogen abstraction by the phenoxyl chain carrying radical for which the rate increases. For the beta pathway 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.
authors.affiliation: Computer Science and Mathematics Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831-6367, USA. bestea@ornl.gov
authors.forename: Ariana
authors.initials: A
authors.lastname: Beste
authors.forename: A C
authors.initials: AC
authors.lastname: Buchanan
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Harrison
mesh_terms: D002627:Chemistry, Physical / Q000379:methods; D019295:Computational Biology / Q000379:methods*; D005609:Free Radicals; D006859:Hydrogen / Q000737:chemistry; D007700:Kinetics; D008031:Lignin / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D010647:Phenyl Ethers / Q000737:chemistry*; D010940:Plant Proteins / Q000737:chemistry; D011487:Protein Conformation; D012984:Software
chemical_list: D005609:Free Radicals; D010647:Phenyl Ethers; D010940:Plant Proteins; D006859:Hydrogen; D008031:Lignin; D010100:Oxygen