Vibrational relaxation rate constants for HF(v=1–4) by CO, CO2, and HCN with product identification by infrared emission

Abstract

Vibrational relaxation of HF(v=1–4) by CO, CO2, and HCN has been studied by infrared emission in a flow reactor. The vibrationally excited HF molecules were generated by the reactions of F atoms with CH4, CH3Cl, H2S, and HBr in 0.8 Torr of Ar carrier gas. By monitoring the rotational distribution in the lower v levels, the fraction of the relaxation proceeding by Δv=−1 and −2, V→R transfer was determined. Numerical integration of the rate equations and modified Stern–Volmer analysis of the HF(v) distribution vs reagent concentration were used to determine the relaxation rate constants and the mechanisms. The relaxation rate constants increase with v, but the magnitude of the rate constants depend upon the collision partner; the HF(v=3) values (in cm3 s−1) are 2.5×10−12 for CO, 1.1×10−11 for CO2, and 1.0×10−10 for HCN. The HF(v) relaxation mechanism by CO is dominated by V→R transfer (≥70%), giving HF(v−1, J=10–14). Relaxation by CO2 has significant Δv=−2 and −3, V→R (≊30%) and Δv=−1, V→R (∼20%) pathways, as well as a V–V pathway. Comparing the HCN(ν3) emission intensity to the loss in HF(v) emission intensity gave the efficiency of V→V transfer from HF(v) to HCN(ν3) as 55±15%; some Δv=−1, V→R transfer also was identified. The HF(v, J≥8) rotational relaxation rate constants increase in the CO, CO2, HCN series. R–V transfer from HF(J≥16) to the CO2 bending mode was identified as the fastest rotational relaxation process.

Quantum-mechanical study of vibrational relaxation of HF in collisions with Ar atoms

“…The distance between the curves for the relaxation of different v states to ͉vϪ1͘ is approximately the same decreasing slightly as v goes up and the total VR cross sections scale approximately as v 3.44 . This is in accord with the experimental observation 4,13,15 that the rate constant for one-quantum VR scales as v n where v is the number of the initial vibrational state and n is a constant ranging from 2.0 to 3.8 depending on the bath gas. Figure 4 shows plots of the ratios of the coupling matrix elements ͗ jϭ0,v͉V͉vϪ1, j ϭ13͘/͗ jϭ0,vϪ1͉V͉vϪ2, jϭ13͘ in the region of R values where the coupling matrix elements have the maximum absolute magnitude.…”

“…Many gasphase experiments involving HX molecules use Rg as carrier gases and it is necessary to know the effect of the carrier gas in order to interpret the experimental results. 3,4 A recent interest in VR in atom-diatom collisions, in general, and the RgϩHX collisions, in particular, has been stimulated by observation of quasiresonant vibration-rotation energy transfer. 5,6 For these reasons, VR of hydrogen halides, and HF in particular, has been a subject of many experimental studies [7][8][9][10][11][12][13][14][15][16][17][18][19] whose results are collected and reviewed in a comprehensive work by Leone.…”

Temperature dependence of the gas-phase reactions F+CHFO, CFO+F, and CFO+CFO

“…Principal drawbacks of the chemiluminescence method are: (1) Emission of the region is observed corresponding to a 2.3 -3.0 m multitude of vibrational-rotational states of HF and (2) the emission curve is fitted to a double exponential expression corresponding to formation and deactivation of vibrationally excited HF. This is an oversimplification since deactivation is different for each collision partner and also for each vibrational state; the higher vibrational states will preferably re-(v ϭ 2, 3) lax with to form the lower vibrational states ⌬v ϭ Ϫ1 and these still will give emission [12].…”

Infrared chemiluminescence studies of the reactions of H atoms with CCl3, CF2Cl, and CH2CH2Cl radicals at 300 and 475 K: recombination–elimination vs. abstraction mechanisms

“…The Ar pressure could be varied from 0.3 to 1.0 Torr without loss of pumping speed. The HF(u) and HCl(u) vibrational relaxation was fully arrested for these conditions and the rotational relaxation of HF was partly arrested (5,20). Increasing the reaction time to ~0 .…”