Temperature-dependent rate coefficients for removal of hydroxyl radical (v = 1) by several species

We present temperature-dependent rate coefficients for the removal of OH X2pi, (v = 1), k q, by HNO3, DNO3, H2O, D2O, acetone, benzene, O2, CH4, and N2O. Pulsed laser photolysis was used to generate OH (v = 1), which was detected using pulsed laser-induced fluorescence. For most of these systems the dominant loss process for OH (v = 1) is probably quenching to form OH (v = 0). Aside from O2, CH4, and N2O, the above species efficiently remove OH (v = 1) (kq > 10 -12 cm3 molecule-1 s -1), even though most of these quenching species do not have a vibrational frequency which is close to that of OH. For most of the quenching reactions, kq has a negative temperature dependence; for HNO3, DNO3, H2O, D2O, and acetone, the negative temperature dependence is fairly strong (E a/R of -450 to -750 K). These negative activation energies are probably too large to be due to long-range attractive forces between OH and the quenching species. Our results indicate that in most of these systems OH (v = 1) is removed by forming a complex with the quenching species. Once this complex is formed, dissociation of the complex to regenerate OH (v = 1) competes with randomization of the vibrational energy of OH into the other modes of the complex; this competition results in the negative temperature dependence. Quenching of OH (v = 1) by O2 also appears to occur via a complex, despite weak bonding of the OH-O2 complex. Only kq for the removal of OH (v = 1) by CH4 and benzene are temperature independent. For benzene, Ea/R is approximately zero because randomization of the vibrational energy of OH in the OH-benzene complex is much faster than dissociation of the complex. On the other hand, for CH4, Ea/R is approximately zero because CH4 removes OH (v = 1) without forming a complex with it. This work provides evidence that OH readily forms complexes with many species, and helps confirm that the reactions of ground-state OH with acetone and HNO3 proceed via mechanisms involving formation of a hydrogen-bonded complex.