Kinetics and Thermochemistry of the Hydroxycyclohexadienyl Radical Reaction with O 2 : C 6 H 6 OH + O 2 h C 6 H 6 (OH)OO Sergey Y. Grebenkin and Lev N. Krasnoperov* Department of Chemistry and EnVironmental Science, New Jersey Institute of Technology, UniVersity Heights, Newark, New Jersey 07102 ReceiVed: July 30, 2003; In Final Form: December 18, 2003 The UV absorption spectrum along with the self-reaction and oxidation reaction kinetics of the hydroxycy- clohexadienyl radical, C 6 H 6 OH (which results from OH addition to benzene), were studied using excimer laser photolysis coupled to transient UV absorption. The radicals were generated by photolysis of N 2 O/H 2 O/ C 6 H 6 /He mixtures at 193 nm in a series of chemical reactions initiated by O( 1 D). The radical has continuous absorption in the range 260-340 nm with a maximum absorption cross-section of (8.1 ( 1.4) × 10 -18 cm 2 molecule -1 at 280 nm. Reaction of the radical with molecular oxygen, C 6 H 6 OH + O 2 h C 6 H 6 (OH)OO (1), and self-reaction C 6 H 6 OH + C 6 H 6 OH f products (2), were studied over the 252-285 K temperature range at 1.01 ( 0.02 bar (He). The radical temporal profiles were recorded via transient absorption at 315 nm. In reaction 1, two-time-domain “equilibration” kinetics were recorded in the temperature range 252-273 K. The rate constant of the addition reaction is k 1 ) (1.4 ( 0.8) × 10 -12 exp(-18.6 ( 1.7 kJ mol -1 /RT) cm 3 molecule -1 s -1 . The standard enthalpy of reaction 1 was determined from the measured equilibrium constants using the third law method: ΔH° 298 )-43.6 ( 2.0 kJ mol -1 . The measured rate constant of self-reaction 2 is k 2 ) (6 ( 3) × 10 -11 exp(-2.00 ( 1.6 kJ mol -1 /RT) cm 3 molecule -1 s -1 . Introduction Reaction of hydroxycyclohexadienyl radical, C 6 H 6 OH (which results from OH addition to benzene), with molecular oxygen is the major transformation route of these radicals in the atmosphere. 1-4 Due to the relatively weak chemical bonding of the oxygen molecule in the adduct (DH° C 6 H 6 (OH)-OO is estimated as 48 kJ mol -12 ), the formation of the hydroxycy- clohexadienyl peroxy radical, C 6 H 6 (OH)OO, is reversible near ambient conditions. At higher concentrations of free radicals, as exist in the laboratory measurements, self-reaction 2 of hydroxycyclohexadienyl radicals is another important transfor- mation route: The hydroxycyclohexadienyl radical, C 6 H 6 OH, is an important intermediate in the oxidation of atmospheric aromatic hydrocar- bons. 2-10 In the atmosphere, hydroxycyclohexadienyl radicals are formed via addition of hydroxyl radical to benzene: The H-atom abstraction route 3b is a minor process at atmospheric conditions. 10-12 Despite the importance of C 6 H 6 OH radicals in the oxidation of benzene, they are not well studied. There are several important issues relevant to the characterization of the spec- troscopy and reactivity of the hydroxycyclohexadienyl radical, which are still poorly resolved. These include the rate constant of reaction 1, the R-OO bond in the hydroxycyclohexadienyl peroxy radical, as well as the shape of the absorption spectrum and the absolute UV absorption cross-section of the hydroxy- cyclohexadienyl radical. The rate constant of reaction 1 has been measured in several experimental studies. 1,9,11 The measured rate constants of this reaction [k 1 ) (1.8 ( 0.5) × 10 -16 at 298 K, 9 (5.0 ( 1.0) × 10 -13 at 338 K, 11 and (2 ( 1) × 10 -15 at 297 K 1 (cm 3 molecule -1 s -1 units)] differ by a 3 orders of magnitude. The R-OO bond energy in the peroxy radical which is formed in reaction 1 was estimated as DH° 298 ) 48 kJ mol -1 based on the group additivity approach and semiempirical (PM3) method. 2 This bond energy combined with the estimated standard entropy of reaction 1 of ∆S° 298 )-136 J mol -1 K -12 implies reversibility of reaction 1 near ambient conditions. The higher level calculations of Ghigo and Tonachini (DFT-B3LYP/ 6-31G+G(d)) 13,14 resulted in a much smaller bond energy in the peroxy adduct, only ca. 5 kJ mol -1 , 13 which implies a complete shift of the equilibrium toward the reactants in reaction 1 and unimportance of the peroxy radical as an individual chemical species near ambient conditions. The standard Gibbs energy of reaction 1, ∆G° 298 ) 44 kJ mol -1 , calculated by Ghigo and Tonachini 14 corresponds to a very small thermody- namic equilibrium constant for reaction 1, K 1,298 ) 2 × 10 -8 (which corresponds to K c1,298 ) 8 × 10 -28 cm 3 molecule -1 ). Recent theoretical calculations of Johnson et al., 15 CCSD(T)/ 6-31G(d,p)), DH° 298 ) 51 kJ mol -1 , and Chen and Bozzelli, 16 G3MP2, DH° 298 ) 56 kJ mol -1 , are consistent with the initial estimates 2 and confirm the importance of the peroxy intermedi- ate. The recent theoretical calculations indicate the (E)-ortho- hydroxycyclohexadienyl peroxy radical as the most stable isomer among the possible adducts produced in reaction 1. 15,16 * To whom correspondence should be addressed. E-mail: krasnoperov@ adm.njit.edu. C 6 H 6 OH + O 2 h C 6 H 6 (OH)OO (1) C 6 H 6 OH + C 6 H 6 OH f products (2) C 6 H 6 + OH f C 6 H 6 OH (3a) C 6 H 6 + OH f C 6 H 5 + H 2 O (3b) 1953 J. Phys. Chem. A 2004, 108, 1953-1963 10.1021/jp030935c CCC: $27.50 © 2004 American Chemical Society Published on Web 02/21/2004