J. Pacansky

Symmetry breaking in molecular calculations and the reliable prediction of equilibrium geometries. The formyloxyl radical as an example

A systematic approach to symmetry breaking in molecular calculations, based on MCSCF and multireference CI (MRCI) wave functions, is presented. A series of MCSCF expansions is generated by successively incorporating resonance effects and size effects into the wave functions. The character of the potential surface obtained at each level is analyzed. As an example, the potential energy curves of the ground state (σ) and the first excited state (π) of the formyloxyl radical (HCO2) are characterized. The σ and π equilibrium structures are shown to be symmetric, with an adiabatic σ−π excitation energy of 9.2 kcal/mol. Unlike earlier theoretical studies, our MCSCF model produces a qualitatively correct potential surface. Therefore, we are able to extract reliable vibrational frequencies from the MRCI potential surface.

SCF ab‐initio ground state energy surfaces for CO2 and CO2−

SCF ab‐initio computations are performed for the ground states of CO2 and CO2−. The CO2 and CO2− potential surfaces have been obtained over a large region of space; in particular, the intersection of these two surfaces. Our results predict that the stability of CO2− depends strongly on whether it is formed near the equilibrium bond angle (135°), the most stable situation, or at significantly different angles. The calculations show that the 6a1 molecular orbital of CO2− is diffuse in character and that the computed equilibrium geometry (bond angle, 135.3°, bond length, 2.35 bohr) and electron affinity (−0.36 eV) are consistent with experiment.

Ab initio force constants for the HCN molecule: SCF and CI results

Force constants for HCN have been determined from both the one‐electron model, SCF, and correlated, CI, wavefunctions. Using SCF wavefunctions, the stretching force constant for the CH bond, K11, is 9% larger than experimentally determined values and the CN stretch, K22, is 23% larger. When electron correlation is included through the use of configuration interaction wavefunctions the agreement of computed and experimental force constants is greatly improved; K11 is 5% larger than the experimental value and K22 is 9% larger. The force constants are determined by making least square polynomial fits to computed points on the potential surface. Force constants for both stretch and bending modes are reported through quartic terms.

The structure of the ethyl radical

A consistent experimental and theoretical view is presented for the structure of the ethyl radical. These results show that the stable conformation is the staggered form and that the CH3 group in the radical is distorted.

Ab initio SCF calculations on molecular silicon dioxide

Ab initio SCF calculations are performed on molecular silicon monoxide and dioxide. These computations show that the electronic ground state for molecular SiO2 is 1Σ+g and has a D∞h geometry, and that molecular SiO2 is bound with respect to SiO(1Σ+) and O(1D). Vibrational frequencies are computed in order to aid in the identification of this enigmatic species.

Calculation of the frequencies and intensities in the infrared spectrum of the ethyl radical

The infrared spectra of matrix isolated ethyl radicals have been simulated by a normal coordinate calculation, fitting the vibrational frequencies of all, partially and fully deuterated species CH3CH2⋅, CH3CD2⋅, CD3CH2⋅, and CD3CD2⋅, together with the calculation of the IR intensities by modified CNDO and INDO open shell procedures. A good reproduction of the observed spectra is in agreement with the theoretical molecular geometry and the special structural features shown by deviations of the force constants for the ethyl radical compared to that of related saturated and unsaturated hydrocarbons.

Infrared matrix isolation studies on the t‐butyl radical

The infrared spectrum of the t‐butyl radical isolated in an argon matrix was obtained by gas phase pyrolysis of azoisobutane and 2‐nitrosoisobutane. The infrared spectrum is consistent with a C3v structure for the radical. The data obtained from the matrix isolation studies were used to reassess the ΔH°f,300 for the radical.

Ab initio study for the structure of propane and the n‐propyl radical

Ab initio calculations are presented for three possible conformations of the n‐propyl radical. The completely optimized geometry and total energy for each conformation is found by using the gradient method. The theoretical results show that the conformations are energetically very close to each other. This agrees with experimental studies on the n‐propyl radical in rare gas matrices but disagrees with experiments performed in solutions.

Hartree–Fock electron affinity of the CN radical

Self‐consistent‐field calculations on CN(X 2Σ+) and CN−(X 1Σ+) have been performed at several internuclear separations. The estimated Hartree–Fock limit value for the adiabatic electron affinity of CN is 3.29±0.05 eV as compared with the observed value of 3.82±0.02 eV. A qualitative explanation has been proposed for the large electron affinity of CN.

Theoretical studies on the structure of isobutane and the tertiary‐butyl radical

Ab initio SCF calculations are reported for the structure of isobutane and the tertiary‐butyl radical. The calculation provides valuable information for the structure of the methyl groups in isobutane. For the tertiary‐butyl radical, only one minimum is found on the ground state potential energy surface which corresponds to a nonplanar C3v geometry.