Thermochemistry of the benzyl and allyl radicals and ions

Abstract

We have studied the thermochemistry of the resonantly stabilized radicals, C 6H 5CH 2 and CH 2CHCH 2 and their corresponding cations and anions. A flowing afterglow/selected ion flow tube instrument has been used to measure the rates of reaction: C 6H 5CH 3 + CH 3O − ⇌C 6H 5CH 2 − + CH 3OH C 6H 5CH 3 + CD 3O − ⇌C 6H 5CH 2 − + CD 3OH C 6D 5CD 3 + CH 3O − ⇌C 6D 5CD 2 − + CH 3OD C 6D 5CD 3 + CD 3O − ⇌C 6D 5CD 2 − + CD 3OD CH 2CHCH 3 + HO − ⇌CH 2CHCH 2 − + H 2O The ratio of the rate constants gives a free energy change for each reaction and use of the established gas phase acidity of CH 3OH or H 2O provides values for the acidities. We calculate the entropy changes, Δ acid S 300(C 6H 5CH 3) and Δ acid S 300(CH 2CHCH 3), to extract values for Δ acid H 300(C 6H 5CH 3) and Δ acid H 300(CH 2CHCH 3). Earlier photoelectron experiments have provided ionization potentials and electron affinities for the benzyl and allyl radicals. Use of the IPs and EAs, together with the enthalpies of deprotonation, provides values for the C H bond enthalpies at 300 K and the C H bond energies at 0 K. These bond energies are used to compute the heats of formation of the radicals and ions as well as the cation hydride affinities, HA (all values in kcal mol −1): C 6H 5CH 2−H CH 2CHCH 2−H Δ acid G 300(R−H) 374.9 ± 0.2 383.8 ± 0.1 Δ acid H 300(R−H) 382.3 ± 0.5 391.1 ± 0.3 DH 300(R−H) 89.8 ± 0.6 88.8 ± 0.4 D 0(R−H) 88.1 ± 0.6 87.4 ± 0.4 Δ f H 0(R) 54.1 ± 0.6 44.4 ± 0.5 Δ f H 300(R) 49.7 ± 0.6 41.4 ± 0.4 Δ f H 0(R −) 33.1 ± 0.6 33.1 ± 0.5 Δ f H 300(R −) 28.7 ± 0.5 30.1 ± 0.4 Δ f H 0(R +) 221.3 ± 0.6 231.6 ± 0.7 Δ f H 300(R +) 216.8 ± 0.6 228.9 ± 0.6 HA 0(R +) 237.9 ± 0.7 254.7 ± 0.7 HA 300(R +) 239.5 ± 0.6 258.9 ± 0.7 In addition, we find Δ acid G 300(C 6D 5CD 3) = 377.0 ± 0.3 kcal mol −1.