Vibrational excitation of H2O and HOD molecules produced by reactions of OH and OD with cyclo-C6H12, n-C4H10, neo-C5H12, HCl, DCl and NH3 as studied by infrared chemiluminescence

Abstract

The room-temperature reactions of OH(OD) radicals with cyclo-C6H12, n-C4H10, and neo-C5H12 have been investigated by observing the infrared chemiluminescence from the H2O(HOD) molecules generated in a fast-flow reactor. These hydrocarbon molecules are representative for abstraction from secondary and primary C–H bonds. The total vibrational energy released to H2O(HOD) was in the range of 〈fv〉=0.55–0.65. The majority (80%–85%) of the vibrational energy is in the stretching modes and the main energy release is to the local mode associated with the new OH bond. The dynamics associated with the energy disposal to H2O(HOD) resemble the H+L−H dynamics for the analogous reactions of F atoms. The data from H2O and HOD are complementary because of the different collisional coupling between the energy levels of the ν1, ν2, and ν3 modes; however, no specific isotope effect was found for the energy disposal to H2O versus HOD for reactions with the hydrocarbon molecules. In contrast, a very unusual isotope effect was found between the OH+HCl and OD+HCl pairs. The latter reaction gave the expected stretching mode excitation of HOD; however, the OH reaction gave H2O molecules with virtually no vibrational energy. This anomalous situation is partly associated with an inverse secondary kinetic-isotope effect, but the main isotope effect is on the dynamics of the energy disposal process itself.