HF infrared chemiluminescence: Energy disposal and the role of the radical fragment in the abstraction of hydrogen from polyatomic molecules by F atoms

HF infrared chemiluminescence has been utilized to study the energy disposal for the abstraction of hydrogen by fluorine atoms from polyatomic molecules which yield radical fragments with large stabilization energies. The prototype systems selected for study, methyl benzenes, phenol, and acetonitrile, are cases which yield resonance stabilized radicals as products. Comparison is made to the energy disposal from the reaction of F with the primary C–H bonds of aliphatic hydrocarbons, which have smaller radical stabilization energies. In general the radical stabilization energy, which is associated with major changes in geometry of the radical relative to the parent molecules, was not available to the HF product. The reactions of F + benzene and ethylene also were studied to provide reference data for different types of C–H bonds. The HF vibrational energy distributions have been interpreted using an extension of the information theory which previously has been applied to three body reactions. Vibrational surprisal analyses are developed and discussed for three models of the reference (prior) product distributions: (i) the polyatomic fragment product was treated as an atom, i.e., the three body case, (ii) the rotations of the radical fragment were added to the three body model, (iii) a complete model including all vibrational and rotational modes of the polyatomic radical fragment. For (iii) with the use of the full thermochemical exoergicity linear surprisal plots were found and these plots were used to assign relative populations to HF (v=0). The information-theoretic parameters from the three reference models are compared for a series of F+HR reactions in which R increases in complexity from Cl to CH2C6H5. For reactions with large product stabilization energies, calculations for (i) and (ii) were done with a reduced ’’effective available’’ energy corresponding to the assumption that the energy available to HF was less than the full exoergicity. Some insight is gained into the role of the R fragment in the energy disposal.