The temperature dependence of the HF vibrational energy distribution in the insertion–elimination reactions between NH and fluoromethanes

Abstract

The photolysis of hydrazoic acid, HN3, in the presence of fluorinated methanes and argon produces hydrofluoric acid, HF, through imine (NH, probably a 1Δ) insertion–elimination reactions, but chemical laser emission has not been observed. Nevertheless, by the tandem laser, zero-gain temperature method, the HF vibrational population ratios have been measured as a function of temperature (225–400 °K) for CF4, CF3H, CF2H2, and CFH3. Part of the HF is evidently produced by NH(a 1Δ) insertion into the C–H bond with subsequent HF elimination. However, there is an unexpected, competing reaction that involves the C–F bonds, either NH insertion–elimination or hot H atom abstraction to form HF. The HF produced from CF4 shows a simple, Arrhenius type temperature dependence fitted by the expression N1/N0=0.115 exp(950 cal/RT). The N1/N0 ratios for CF3H and CF2H2 can be correlated with those of CF4 by assuming competing C–F and C–H bond reactions weighted by the relative number of each type of bond. With this interpretation, the C–H bond insertion–elimination produces HF with the temperature dependence N1/N0=1.52 exp(−1240 cal/RT). Consistent with all earlier chemical lasers based upon elimination reactions, both the N2/N1 and N1/N0 ratios are below unity. At room temperature, the NH insertion into the C–H bond gives N1/N0=0.2, the lowest population ratio yet measured for an elimination reaction and apparently HF(v=0) is produced with a lower activation energy than that for HF(v=1).