Ab initio studies of the gas-phase thermodynamic properties and bond dissociation energies for haloethanes and halomethyl radicals

Abstract

Ab initio molecular orbital calculations have been performed on fluorine- and chlorine-substituted ethanes and radicals. The molecular equilibrium characteristics (energies, structural parameters and vibrational frequencies) of all the species were estimated at the MP2/6-311G** level of theory. A rigid harmonic oscillator model was used for the development of a data base for the ideal gas thermodynamic properties: C_p^°, S°, − (G° − H₀^°)/T, Δ_fH°, Δ_fG° and log K_f, of the haloethanes and halomethyl radicals over a temperature range 0–1500 K. From the estimated enthalpies of formation at 298.15 K and 1 atm, bond dissociation energies for the homolytic cleavage of C–C bonds for the haloethanes were obtained. The results of the MP2/6-311G** calculations of the heats of formation at 298.15 K, Δ_fH_298.15K^°, of the halogenated methyl radicals have been compared with those obtained using the CBS-RAD procedure and a modification of this called CBS-RAD(B3-LYP,B3-LYP). Finally, the influence of halogen substituents on reactivity has been interpreted in terms of polar and hyperconjugation effects.