Manuel Braga

Transfer of mobile electrons in organic molecules

Abstract Electronic coupling between π electrons of two phenyl groups connected by conjugated systems of increasing length is calculated and found to decrease slowly and non-exponentially with distance. Neutral, unsymmetrically para substituted molecules of this type are capable of second harmonic generation. A simple model is set up to describe the various aspects of electron mobility in organic molecules: electron transfer, delocalization, polarizability and first hyperpolarizability.

Through-bond interactions in the lone-pair ionization of p-benzoquinone

The photoelectron spectrum of p-benzoquinone for the most loosely bound electrons is calculated using the CASSCF and related methods with localization of the hole state. The results are in satisfactory agreement with the experiments and support a through-bond interaction model for the splitting of the lone-pair ionization energies where the two major contributions have different signs and tend to cancel.

Correlation effects in the electronic coupling between π electrons through a cyclohexane spacer

Abstract The electronic coupling between symmetrically placed π electrons through a cyclohexane spacer is calculated using ab initio methods. In particular we have performed a multiconfigurational configuration interaction study involving the broken symmetry Hartree—Fock solutions. The value of Δ, in a simplified model the splitting between the symmetric and antisymmetric π orbitals, is moderately changed compared to the Koopmans theorem value. The influence of the basis set on the calculated splittings is also analyzed.

Correlation and relaxation effects in the electronic coupling between acetylenic π and π* electrons through saturated and aromatic spacers

Abstract The electronic factor Δ for electron and hole transfer between two acetylene molecules connected by the saturated 1,3-bicy- clo [1,1,1]pentane and 1,4-bicyclo[2,2,2]octane bridges and the unsaturated phenyl bridge is calculated using different correlation-corrected ab initio methods, particularly the multiconfigurational second-order perturbation theory (CASPT2) of Roos et al. Correlation and relaxation effects improve the agreement between calculated and experimental results.

Electronic transitions in C60 and its ions

The electronic spectrum of C60 is calculated using quantum mechanical methods. The first allowed transition in C60 is calculated at an energy of 3.5 eV and with an oscillator strength of 0.09. Several transitions are found at higher energies with comparatively strong oscillator strengths, the strongest one being at 5.78 eV (λ=214 nm). The calculated energy level diagram of C60 is also used to predict wavelengths for transitions in C60+ and C60−. A comparison is made with some recently observed diffuse interstellar bands at 1180 nm and 1320 nm, which have been speculated to originate from C60+.

Molecular orbital study of the electron donor-acceptor properties of the P atom in Ni(PX3)4 (X=H, F, Cl) complexes

Abstract A comparative study of the Ni(PH3)4, Ni(PF3)4 and Ni(PCl3)4 complexes was carried out using the self-consistent field (SCF) multiple scattering method. Ab initio calculations were also performed for the model system L-Ni-PF3 (L=CO, N2) in order to study the σ-π donor-acceptor capabilities of PF3, CO and N2. The results show the importance of π bonding between the Ni and P atoms. However, it is found that it is the P 3p rather than the P 3d orbital which forms a bond with the Ni 3d orbital. The correlation between the σ-donor and π-acceptor capabilities and the electronegativity of the P ligands is also analysed in detail. Our present results give further support to the analogy between the transition metal-P bond and the hyperconjugation bond in organic compounds.

Theoretical study of the interaction of CuCO+ with Ar

Abstract Ab initio calculations at the Hartree-Fock level and including correlation effects using the modified coupled-pair functional method have been performed in order to investigate the bonding interactions in the ArCuCO+ and CuCOAr+ clusters. Only ArCuCO+ is bound, and the inclusion of correlation effects in this cluster considerably increases the binding energy.

Electronic structure and bonding in L-Ni-PH3 (LCO, N2) complexes

Abstract Ab initio MO calculations were carried out for the model systems OC-Ni-PH3 and N2-Ni-PH3. The magnitude of the ligand → metal σ donation and metal → ligand π back-donation was calculated and the contribution of both processes to the bonding in these systems analysed in detail.