Preparation of CHF (1A′) by infrared multiphoton dissociation and reactions with alkenes

Abstract

A study of the vibrational, and translational energies of the CHF (X¹ A′) radical prepared by infrared multiphoton dissociation, IRMPD, is presented. The vibrational and rotational temperatures measured near the CO₂ laser pulse peak depend on the delay, nature and pressure of the buffer gas. For pure precursor (20 mTorr) and at delays of 0.4 and 3 µs the measured rotational temperatures were 900 and 600 K, respectively. Vibrational temperatures at 3 µs delay for samples of pure precursor (20 m Torr) and of precursor with 1 Torr of Ar were 790 and 630 K.

The transient migration method was applied to measure the diffusion coefficient, and hence the collisional diameter, of CHF (X¹A′) in Ar. Attempts to measure the average translational energy of the CHF fragment by the same method at low pressures produced extremely low temperatures, and forced a re-examination of the validity of the technique. The average vibrational relaxation rate constant, as determined by the same method and confirmed by direct measurements, gives k_VT < 10^–15 cm³ molecule^–1 s^–1, while the rotational relaxation rate constant is k_RT > 10^–10 cm³ molecule^–1 s ^–1.

The bimolecular rate constants for removal of CHF (X¹A′) with several alkenes are reported to be (/10^–12 cm³ molecule^–1 s^–1) : k_ethene= 5.4 ± 0.3; k_propene= 13.0 ± 1.0; k_butene= 9.5 ± 2.0; k_isobutene= 17.0 ± 2; k_butadiene= 22.0 ± 3.0. Some of the reactions are CO₂ laser fluence dependent, and, in addition, butadiene gives a considerable luminescence. A method is suggested for extrapolation of the apparent kinetic constants to zero fluence to obtain meaningful results.