Rate constants and vibrational energy disposal for reaction of H atoms with Br2, SF5Br, PBr3, SF5, and SF4

Abstract

Rate constants and initial HBr and HF product distributions for the title reactions were measured in a fast-flow apparatus using infrared chemiluminescence techniques. The spectra were interpreted using a new set of Einstein coefficients for HBr, which are listed in the Appendix. The rate constants for HBr(v⩾1) and HF(v⩾1) formation, relative to the H+Cl2 reaction, are 3.3, 0.39, 0.50, 3.4, and 0.003, for Br2, SF5Br, PBr3, SF5, and SF4, respectively. This directly measured Br2 rate constant supports the smaller values that have been estimated in the literature. The initial HBr vibrational distribution (v1:v2:v3:v4:v5=0.03:0.20:0.40:0.31:0.06) from H+Br2 corresponds to 〈fV〉=0.49. The observed HBr vibrational distributions (v1:v2:v3:v4) are 0.28:0.43:0.23:0.06 and 0.63:0.24:0.13 for SF5Br and PBr3, respectively. The SF5Br results are close to the initial distribution and give 〈fV〉=0.36. The low vapor pressure of PBr3 limited the [PBr3] and high [H] was required to observe HBr emission; correcting the observed distribution for vibrational relaxation gives 〈fV≅0.47. These 〈fV〉 values include estimates for HBr(v=0). Based upon the highest HBr level observed from SF5Br and PBr3, D0(Br–SF5)<55 and D0(Br–PBr2)<62 kcal mole−1. The HF vibrational distributions from SF5 and SF4 decline with increasing v, which suggests that these reactions proceed via a long-lived complex. For these cases the formation of HF(v=0) is important, and significant corrections must be made to the HF(v⩾1) formation constants to obtain the total HF formation rate constants. The rate constants and energy disposal data are used to discuss models and to compare the H+Br2 reaction to H+Cl2 and F2.

A novel method to decompose two potent greenhouse gases: Photoreduction of SF6 and SF5CF3 in the presence of propene

“…The existence of H 2 O (g) in the gas mixture turned out to enhance DRE by 6.3%. Both OH radical and H atom, generated from the direct photolysis of H 2 O (g) [52], might account for the enhanced DRE [52][53][54][55][56]: …”

Time-resolved IR chemiluminescence in gas-phase chemical kinetics

“…0 . According to formula (11), the rate of formation of HCl molecules should also decrease during this period. Thus, in this example the initial period of the chain reaction should manifest itself in the luminescence signal of HCl as a `kink' segment.…”

“…The initial region contains complete information on the rate constants for both stages of the chain growth. Using the formulas (11) or (14), it is possible to retrieve this information from the experimental signal. For approximation of the experimental curve, for example by formula (11), only three independent parameters have to be found, i.e., the signal amplitude, the reaction rate constant k 1 and one of the values y 1 or y 2 related by y 1 + y 2 = 1.…”

“…Using the formulas (11) or (14), it is possible to retrieve this information from the experimental signal. For approximation of the experimental curve, for example by formula (11), only three independent parameters have to be found, i.e., the signal amplitude, the reaction rate constant k 1 and one of the values y 1 or y 2 related by y 1 + y 2 = 1. These parameters are sufficient for finding the rate constants.…”

“…This spectrum contains information on the distribution of the reaction products over the vibrational and rotational states. Many reactions resulting in the formation of vibrationally excited molecules, such as HF, 4 ± 9 HCl, 10 HBr, 11 CO, 12 OH…”