Electron affinities,molecular structures,and thermochemistry of the fluorine,chlorine and bromine substituted methyl radicals

Four independent density functional theory (DFT) methods have been employed to study the structures and electron affinities of the methyl and F-, CI- and Br-substituted methyl radicals and their anions. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities (Chemical Reviews, 2002, 102, 231). The first dissociation energies together with the vibrational frequencies of these species are also reported. The basis sets used in this work are of double-ζ plus polarization quality with additional s- and p-type diffuse functions, labelled as DZP++. Previously observed trends in the prediction of bond lengths by the DFT methods are also demonstrated for the F-, Cl- and Br-substituted methyl radicals and their anions. Generally, the Hartree-Fock/DFT hybrid methods predict shorter and more reliable bond lengths than the pure DFT methods. Neutral-anion energy differences reported in this work are the adiabatic electron affinity (EA_ad), the vertical electron affinity (EA_vert), and the vertical detachment energy (VDE). Compared with the available experimental electron affinities, the BHLYP method predicts much lower values, while the other methods predict values (EA_ad, EA_vert, VDE) close to each other and almost within the experimental range. For those systems without reliable experimental measurements, our best adiabatic EAs predicted by BLYP are 0.78 (CHF₂), 1.23 (CHFCl), 1.44 (CHFBr), 1.61 (CHClBr), 2.24 (CF₂Cl), 2.42 (CF₂Br), 2.56 (CFBr₂), 2.36 (CCl₂Br), 2.46 (CClBr₂), and 2.44 eV (CFClBr). The most striking feature of these predictions is that they display an inverse relationship between halogen electronegativity and EA. The DZP++B3LYP method determines the vibrational frequencies in best agreement with available experimental results for this series, with an average relative error of ～2%. The value of using a variety of DFT methods is observed in that BHLYP does best for geometries, BLYP for electron affinities, and B3LYP for vibrational frequencies. These theoretical results serve to resolve several disagreements between competing experiments. Several other experiments appear to have drawn incorrect conclusions. For example, CHCl₂ is significantly pyramidal, unlike the experimental inferences, and clearly the experimental CCl₂—Cl dissociation energy is too large.