The Vinyl Radical and Fluorinated Vinyl Radicals, C 2 H 3-n F n (n ) 0-3), and Corresponding Anions: Comparison with the Isoelectronic Complexes [X‚‚‚YC≡CZ] - Andrew C. Simmonett, Steven E. Wheeler, and Henry F. Schaefer III* Center for Computational Chemistry, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: NoVember 12, 2003 Density functional theory (DFT) has been utilized to study the vinyl radical and fluorinated vinyl radical series, C 2 H 3-n F n, n ) 0-3. Six different functionalssB3LYP, B3P86, BHLYP, BLYP, BP86, and LSDAs were used. A double- basis set, augmented with additional s- and p-type diffuse functions as well as additional polarization functions (DZP++), was employed for all of the computations. Extensive calibrative studies have demonstrated that the DZP++ B3LYP, BLYP, and BP86 methods do a good job in the prediction of electron affinities. Neutral-anion energy separations were used to calculate the adiabatic electron affinities (EA ad ), the vertical electron affinities (EA vert ), and the vertical detachment energies (VDE). These electron affinities were found to get progressively larger as the number of fluorines is increased; ZPVE-corrected values predicted by the reliable BLYP method are 0.66 eV (C 2 H 3 ), 1.54 eV (C 2 H 2 F), 1.96 eV (C 2 HF 2 ), and 2.40 eV (C 2 F 3 ). This trend can be attributed to increasing anion stability, which can be rationalized in terms of inductive and negative hyperconjugative effects. Optimized geometries for all of the neutral and anionic species, which are indicative of the aforementioned effects, are presented. The 1-fluorovinyl radical is found to lie lowest in energy of the mono-fluorinated species, whereas the most stable anion of the same stoichiometry is a fluoride‚‚‚acetylene complex, which is found to lie 19.5 kcal mol -1 lower than the 1-fluorovinyl anion using BLYP. The most stable configuration for the difluorinated species is 2,2-difluorovinyl for both the neutral radical and the anion. I. Introduction The vinyl radical and its fluorinated derivatives have been studied over recent decades, both theoretically 1-5 and experi- mentally. 1,6-14 The 70’s and 80’s saw the publication 3,5 of studies of both the vinyl and trifluorovinyl radicals, carried out using semiempirical methods or Hartree-Fock theory with minimal basis sets and focused on calculating the radicals’ geometries and harmonic vibrational frequencies. The evolution of computers has enabled more elaborate levels of theory to be employed in more recent years, such as the recent paper by Goldschleger, Akimov, Misochko, and Wight, 1 which found excellent agreement between experimental and B3LYP predicted hyperfine coupling constants. A detailed study of the electronic transitions of the vinyl radical, computed at the MRCISD and CASSCF levels of theory was published recently by Zhang and Morokuma. 4 However, despite this interest, no systematic study of the effects of halogen substitution on the electronic properties of the vinyl radical has been published. Electron affinities (EAs) are extremely useful in thermochemistry 15-17 and have many roles in semiconductor technology. 18-21 An earlier study, 22 somewhat surprisingly, showed that the electron affinities of halide-substituted methyl radicals do not necessarily correlate with the electronegativities of the different halide substituents. However, the authors found that substituting the hydrogen atoms with fluorine atoms increases the electron affinity of the species, as expected on simple electronegativity grounds. Given these results, one of the principal aims of this research is to extend this study to fluorinated vinyl radicals and to ascertain whether similar trends are observed. In this discussion, the quantity EA ad will be used to represent the adiabatic electron affinity, i.e., the electron affinity associated with a slow neutral-to-anion transition that allows geometric rearrangement to occur. In polyatomic systems, attachment of an electron is usually accompanied by a change in geometry, thus making adiabatic EAs difficult to measure experimentally. For this reason, both the vertical electron affinity (EA vert ) and the vertical detachment energy (VDE) are reported here, where these quantities correspond to the Franck-Condon transitions from the optimized anion to the neutral and from the optimized neutral to the anion, respectively. Assuming that the geometry of the species does not change drastically upon addition of an electron, the VDE and EA vert provide lower and upper bounds, respectively, for the experimentally observed EAs. The experi- mental electron affinity should be close to the adiabatic value, should this change in geometry be small. II. Theoretical Methods The properties of the molecules were computed using six different DFT or hybrid HF/DFT functionals, defined below, with a DZP++ basis set. This basis set is comprised of the standard first row Huzinaga-Dunning double- basis sets, 23,24 augmented with additional pure spherical harmonic polarization functions with R d (C) ) 0.75, R d (F) ) 1.00, and R p (H) ) 0.75 and diffuse functions with R s (C) ) 0.04302, R p (C) ) 0.03629, R s (F) ) 0.1049, R p (F) ) 0.0826, and R s (H) ) 0.04415. Spin unrestricted formulations of the DFT functionals were employed for the computation of the radicals’ properties, as recommended by Pople, Gill, and Handy. 25 The functionals used in this study are the following: (1) Becke’s hybrid three-parameter exchange functional 26 (B3) with * Corresponding author. E-mail: hfs@uga.edu. 1608 J. Phys. Chem. A 2004, 108, 1608-1615 10.1021/jp031240e CCC: $27.50 © 2004 American Chemical Society Published on Web 02/10/2004