Dynamics of OH and OD radical reactions with HI and GeH4 as studied by infrared chemiluminescence of the H2O and HDO products

Abstract

The infrared chemiluminescence of vibrationally excited H2O and HDO from the highly exothermic reactions of OH and OD radicals with HI and GeH4 was observed in the 2200–5500 cm−1 range. The experiments utilized a fast-flow reactor with 0.3–1 Torr of Ar carrier gas at 300 K; the OH(OD) radicals were produced via the H(D)+NO2 reaction and the H or D atoms were generated by a discharge in a H2(D2)/Ar mixture. The H2O and HOD vibrational distributions were determined by computer simulation of the emission spectra in the 2200–3900 cm−1 range. The total vibrational energy released to H2O and HOD molecules is, respectively, 〈fv〉=0.36 and 0.41 from HI and 〈fv〉=0.46 and 0.51 from GeH4. These values are significantly smaller than for the reactions of OH and OD with HBr, 〈fv〉=0.61 and 0.65. The populations of the O–H stretching vibration of HOD and the collisionally coupled ν1 and ν3 stretching modes of H2O decrease with increasing vibrational energy. In contrast, the vibrational distribution from the HBr reaction is inverted. The bending mode distributions in all stretching states of H2O and HOD extend to the thermodynamic limit of each reaction. A surprisal analysis was made for H2O(HOD) distributions from the title reactions and compared with that for OH(OD)+HBr. The surprisal analysis tends to confirm that the dynamics for the HI and GeH4 reactions differ from the HBr reaction. The HI reaction may proceed mainly via addition-migration, while the GeH4 reaction may involve both direct abstraction and addition-migration. A rate constant for the OH+GeH4→H2O+GeH3 reaction was evaluated by comparing the H2O emission intensities with that of the OH+HBr→H2O+Br reaction, kGeH4/kHBr=6.5±0.9. Secondary kinetic-isotope effects, kOH/kOD=1.4±0.1, 1.0±0.2, and 1.3±0.2, were determined for reactions of OH and OD with GeH4, HI, and HBr, respectively, by comparing the relative H2O and HOD emission intensities.