A Halomethane Thermochemical Network from iPEPICO Experiments and Quantum Chemical Calculations

Abstract

Internal energy selected halomethane cations CH(3)Cl(+), CH(2)Cl(2)(+), CHCl(3)(+), CH(3)F(+), CH(2)F(2)(+), CHClF(2)(+), and CBrClF(2)(+) were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH(3)F(+), CH(2)F(2)(+), and CH(3)Cl(+), chlorine atom loss for CH(2)Cl(2)(+), CHCl(3)(+), and CHClF(2)(+), and bromine atom loss for CBrClF(2)(+). Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl(3)(+), CHClF(2)(+), and CBrClF(2)(+) do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep-well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 ± 0.01 eV, 12.30 ± 0.02 eV, and 11.23 ± 0.03 eV for CHCl(3), CHClF(2), and CBrClF(2), respectively. The updated 0 K enthalpies of formation, Δ(f)H(o)(0K)(g) for the ions CH(2)F(+), CHF(2)(+), CHCl(2)(+), CCl(3)(+), CCl(2)F(+), and CClF(2)(+) have been derived to be 844.4 ± 2.1, 601.6 ± 2.7, 890.3 ± 2.2, 849.8 ± 3.2, 701.2 ± 3.3, and 552.2 ± 3.4 kJ mol(-1), respectively. The Δ(f)H(o)(0K)(g) values for the neutrals CCl(4), CBrClF(2), CClF(3), CCl(2)F(2), and CCl(3)F and have been determined to be -94.0 ± 3.2, -446.6 ± 2.7, -702.1 ± 3.5, -487.8 ± 3.4, and -285.2 ± 3.2 kJ mol(-1), respectively.