π and σ-Phenylethynyl Radicals and Their Isomers o-, m-, and p-Ethynylphenyl:  Structures, Energetics, and Electron Affinities

Molecular structures, energetics, vibrational frequencies, and electron affinities are predicted for the phenylethynyl radical and its isomers. Electron affinities are computed using density functional theory, −namely, the BHLYP, BLYP, B3LYP, BP86, BPW91, and B3PW91 functionals−, employing the double-ζ plus polarization DZP++ basis set; this level of theory is known to perform well for the computation of electron affinities. Furthermore, ab initio computations employing perturbation theory, coupled cluster with single and double excitations [CCSD], and the inclusion of perturbative triples [CCSD(T)] are performed to determine the relative energies of the isomers. These higher level computations are performed with the correlation consistent family of basis sets cc-pVXZ (X = D, T, Q, 5). Three electronic states are probed for the phenylethynyl radical. In C_2v symmetry, the out-of-plane (²B₁) radical is predicted to lie about 10 kcal/mol below the in-plane (²B₂) radical by DFT methods, which becomes 9.4 kcal/mol with the consideration of the CCSD(T) method. The energy difference between the lowest π and σ electronic states of the phenylethynyl radical is also about 10 kcal/mol according to DFT; however, CCSD(T) with the cc-pVQZ basis set shows this energy separation to be just 1.8 kcal/mol. The theoretical electron affinities of the phenylethynyl radical are predicted to be 3.00 eV (B3LYP/DZP++) and 3.03 eV (CCSD(T)/DZP++//MP2/DZP++). The adiabatic electron affinities (EA_ad) of the three isomers of phenylethynyl, that is, the ortho-, meta-, and para-ethynylphenyl, are predicted to be 1.45, 1.40, and 1.43 eV, respectively. Hence, the phenylethynyl radical binds an electron far more effectively than the three other radicals studied. Thermochemical predictions, such as the bond dissociation energies of the aromatic and ethynyl C−H bonds and the proton affinities of the phenylethynyl and ethynylphenyl anions, are also reported.