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π and σ-Phenylethynyl Radicals and Their Isomers o-, m-, and p-Ethynylphenyl: Structures, Energetics, and Electron Affinities
journal contribution
posted on 2008-04-03, 00:00 authored by Raj K. Sreeruttun, Ponnadurai Ramasami, Chaitanya S. Wannere, Andrew C. Simmonett, Henry F. SchaeferMolecular 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 C2v 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 (EAad) 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.