Kinetics of the Reactions of OH‐Radicals with Benzene, Benzene‐d₆ and Naphthalene

Absolute rate constants for the reactions of hydroxyl radicals with benzene (1), benzene‐d 6 (2), and naphthalene (3) have been obtained using excimer laser photolysis to generate OH and time resolved resonance fluorescence as its monitor. Reaction (1) was studied at total pressures between 1.5 and 112 mbar at 298 K using Ar as a diluent gas. k 1 is found to increase with pressure up to –40 mbar, but to be essentially pressure independent at still higher pressures, k (298 K) = (7.0 ± 1.5) 10 11 cm 3 /mol · s. The temperature variation of k 1 as studied over the range 244 – 24 K. At temperatures below 330 K normal Arrhenius behaviour with k 1 ( T ) = (3.8 ± 1.0) 10 12 exp[‐(500 ± 50) K/ T ] cm 3 /mol · s is observed, corresponding to the addition reaction (1) OH + C 6 H 6 → C 6 H 6 OH. Between 330 and 420 K a sharp decrease of k 1 with increasing temperature is noted, which we attribute to reaction (1) becoming reversible. For the stabilization energy of C 6 H 6 OH we derive (79 ± 6) kJ/mol. Above 420 K a transition of k 1 to a positive temperature dependence, corresponding to the abstraction reaction (1') OH + C 6 H 6 → H 2 O + C 6 H 5 is noted. Rate constants for the reaction of OH with benzene‐d 6 (2) are essentially identical to those of reaction (1) except for temperatures above 420 K, where k 2 falls considerably below k 1 . This isotope effect is a confirmation of the (1') abstraction route. Reaction (3) was studied at total pressures between 6 and 128 mbar (He) where it is found to be essentially pressure independent. Its temperature variation was studied between 337 and 525 K. Over this region the rate coefficient decreases from (8.8 ± 3.5) 10 12 cm 3 /mol · s to (6.6 ± 2.6) 10 11 cm 3 /mol · s, with the decrease with temperature being relatively weak up to 410 K, k 1 ( T ≤ 410 K) = (2.2 ± 1.0) 10 12 exp[(440 ± 50) K/ T ] cm 3 /mol · s, and stronger at still higher temperatures. This behaviour is interpreted as to correspond to the addition reaction (3) OH + C 10 H 8 → C 10 H 8 OH and its equilibration. The stabilization energy of the OH‐naphthalene adduct is estimated to be (95 ± 6) kJ/mol.