PNNL

Abstract

The lifetime of the lignin radical model compound, 1-phenyl-2-phenoxyethanol-1-yl, PhC ?(OH)CH2-OPh is several orders of magnitude greater thansuggested by the previous experimental studies. The ketyl radical is competitivelytrapped by thiophenol to yield PhCH(OH)CH2-OPh in competition with ? -scission toyield phenoxyl radical and acetophenone. A basis rate expression, hydrogen atomabstraction by sec-phenethyl alcohol, PhC ?(OH)CH3, from thiophenol, log(kabs/M -1 s -1 ) =(8.88 ?0.24) ? (6.07 ?0.34)/? , ? = 2.303RT,was determined by competing reduction andradical termination pathways. The Smoluchowski equation was used to calculate thetemperature dependent rate of PhC ?(OH)CH3 termination. The hydrogen abstractionpathway was used as a basis reaction to determine the activation barrier for the ? -scissionof phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl: log(k ? -sci/s -1 ) = (12.85 ?0.22) ?(15.06 ?0.38)/? ), k ? -sci(298K) ca. (64.0s -1 in benzene), log(k ? -sci/s -1 ) = (12.50 ?0.18) ?(14.46 ?0.30)/? , k ? -sci(298K) ca. (78.7s -1 in benzene containing 0.8 M 2-propanol).B3LYP/cc-PVZT methods predict intramolecular hydrogen bonding between the ? -OHon the ketyl radical and the ?OPh leaving group stabilize both the ground state andtransition state structures. At this level of theory the activation barrier (14.9 kcal/mol) isin fair agreement with the experimentally determined activation barrier.