Thermochemistry of halomethanes CF_nBr_(4-n) (n ≥ 0 - 3) based on iPEPICO experiments and quantum chemical computations

摘要

Internal energy selected bromofluoromethane cations were prepared and their internal energy dependent fragmentation pathways were recorded by imaging photoelectron photoion coincidence spectroscopy (iPEPICO). The first dissociation reaction is bromine atom loss, which is followed by fluorine atom loss in CF_3Br and CF_2Br_2 at higher energies. Accurate 0 K appearance energies have been obtained for these processes, which are complemented by ab initio isodesmic reaction energy calculations. A thermochemical network is set up to obtain updated heats of formation of the samples and their dissociative photoionization products. Several computational methods have been benchmarked against the well-known interhalogen heats of formation. As a corollary, we stumbled upon an assignment issue for the ClF heat of formation leading to a 5.7 kJ mol~(-1) error, resolved some time ago, but still lacking closure because of outdated compilations. Our CF _3~+ appearance energy from CF3Br confirms the measurements of Asher and Ruscic (J. Chem. Phys. 1997, 106, 210) and Garcia et al. (J. Phys. Chem. A 2001, 105, 8296) as opposed to the most recent result of Clay et al. (J. Phys. Chem. A 2005, 109, 1541). The ionization energy of CF _3 is determined to be 9.02 - 9.08 eV on the basis of a previous CF_3 - Br neutral bond energy and the CF_3 heat of formation, respectively. We also show that the breakdown diagram of CFBr _3~+, a weakly bound parent ion, can be used to obtain the accurate adiabatic ionization energy of the neutral of 10.625 ± 0.010 eV. The updated 298 K enthalpies of formation Δ_fHo(g) for CF_3Br, CF_2Br_2, CFBr_3, and CBr_4 are reported to be - 647.0 ± 3.5, - 361.0 ± 7.4, - 111.6 ± 7.7, and 113.7 ± 4 kJ mol~(-1), respectively.