G. Berthier

Intramolecular charge transfer and trans-cis isomerization of the DCM styrene dye in polar solvents. A CS INDO MRCI study

Abstract The solvent-induced changes of trans-cis isomerization efficiency and electronic structure of the excited state of the DCM dye have been considered by means of CS INDO MRCI calculations. The potential energy curves, dipole moments and atomic charge densities as a function of two internal coordinates, namely the rotation angle about the central “double” bond and the twisting of the dimethylamino group, have been obtained for the ground state and the lowest excited states. The structural requirements for the existence of ICT (intramolecular charge transfer) excited states have been investigated by considering internal rotations about three single bonds. The reliability of the potential surfaces and of the solvation models has been discussed with reference to test-calculations on the DMABN molecule. In the first excited singlet state of DCM, the low-energy barrier for the trans-cis isomerization has been found unaffected by the solvent polarity. The only singlet excited state presenting a large ICT character has been found to be the S 2 state for a perpendicularly twisted conformation of the dimethylamino group (TICT state). The assumption of a deactivation of the trans-isomer in the locally excited state through the TICT funnel has been largely discussed with reference to the simplifications of the present theoretical approach.

HYDROGEN BONDING EFFECTS IN QUANTUM MECHANICAL FORE FIELDS OF PYRIMIDINE BASES : URACIL

Abstract Using dihydrate uracil as a model of H-bonded compound formed by a uracil and two water molecules in the vicinity of HN1C2O and HN3C4O groups, DFT molecular orbital calculations have been performed in order to evaluate the changes in its simulated IR spectrum. To estimate the effect of intermolecular interactions on the vibrational properties of pyrimidine bases we have used the B3LYP functional and polarised 6–31G basis sets. The normal mode analysis enables us to assign the 30 modes of uracil, the 6 modes of the two water molecules along with the 12 additional modes resulting from their mutual interactions.

Structure électronique comparée des isomères conjugués du benzène

The molecular orbitals and energy levels ofσ andπ electrons in benzene, fulvene and 3,4-dimethylenecyclobutene have been calculated by the LCAO-SCF method, using a basis set of Gaussian orbitals. According to the values of binding energy, the larger stability of benzene is due to a electrons rather than toπ electrons. The main part of dipole moments arises fromσ electrons.

On the electronic structure and spectroscopy of the ScN molecule

Abstract The potential energy curves are calculated for the 10 low-lying electronic states of the ScN molecule. The electronic structure and bonding have been elucidated through the CIPSI method. We also report detailed equilibrium data for these states. The states that correlate with the 3d14s2 configuration are found weakly bound, and those correlating with the 3d24s1 configuration are more strongly bound. The bonding in the Xu20081Σ+ ground state arises mainly from the 3d24s1 configuration with a little mixture of the 3d3 configuration of Sc. Our proposed potential curves and derived spectroscopic constants are analyzed and connected with the observed perturbations in the emission spectrum of ScN.

Molecular force fields of uracil and thymine, through neutron inelastic scattering experiments and scaled quantum mechanical calculations

Abstract Neutron inelastic scattering (NIS) data have been obtained from polycrystalline samples of pyrimidine nucleic bases, i.e. uracil (U) and thymine (T). These spectra and those previously recorded by optical techniques (Raman scattering and infrared absorption) led us to carry out ab initio quantum mechanical calculations (SCF + MP2) in using split-valence gaussian sets with different d-orbital polarization functions for carbon, nitrogen and oxygen. The F x matrices (interatomic force constant matrices on the basis of cartesian coordinates) derived from these ab initio calculations have been then described in terms of internal molecular coordinates, allowing vibrational normal modes to be assigned in the framework of the so-called Wilson GF-matrix method. In addition, atomic displacements estimated from the normal mode calculations have been used to simulate the first-order NIS intensities by only considering the fundamentals of the internal molecular vibrational motions. For both molecules (U and T), the internal vibrational modes are located at higher wavenumbers than the lattice vibrational modes (external vibrational motions). In order to scale the calculated vibrational modes the so-called Pulay method has been used. A good agreement between calculated and experimental data (vibrational wavenumbers as well as NIS intensities) has been obtained for most of the internal vibrational modes.

Electronic structure of metal monocarbonyls

Abstract The donation and back-donation processes in a series of transition-metal complexes (M-CO, M=Ti, Fe (and Ru and Os) or Cu) were analysed at the SCF-CI level, using the variation-perturbation (configuration interaction by perturbation with multiconfigurational zeroth-order wavefunctions selected by iterative process) CIPSI method. Two rules are stated as regards the symmetry and multiplicity of a MCO molecule in its linear groundstate. The first for symmetry is a criterion of best donation back-donation possibilities, the second for spin is a criterion of best exchange stabilization in the d n+1 s 1 and d n+2 shells of the metal.

A scaled force field for tetrahydrofuran and its isotopomers from quantum mechanical calculations and infrared and Raman spectra

Abstract Based on a comprehensive examination of the infrared and Raman spectra of tetrahydrofuran and its perdeuterated isotopomer, and on reliable ab initio quantum mechanical calculations for the electronic potential, a realistic force field, useful also for vibrational studies of molecules containing the furanose ring, is determined. The complete infrared and Raman spectra of tetrahydrofuran- d 8 are recorded and the spectrum of the normal frequencies is computed at the (SCF + MP2) level with a non-standard 6–31 G ∗ basis. From analysis of the observed spectra and strict consideration of the computed spectrum, all the fundamentals are attributed; the assignment is accomplished in terms of 33 symmetry coordinates. By means of a newly-developed scaling procedure, the force-constant matrix is adapted to fit the spectra both of the normal molecule and of the fully deuterated derivative; the average differences between the observed and the corresponding scaled frequencies are 11.3 and 10.8 cm −1 , respectively.

Dipole moments and polarizabilities of some substituted pyridine-1-oxides for optoelectronics

Abstract The electronic properties of pyridine-oxide and some of its substituted derivatives have been studied at the ab initio LCGO-SCF level. The results for pyridine-1-oxide and nitro-pyridine-oxide are in good agreement with experiment. As regards 3-methyl-4-nitro-pyridine-l-oxide (POM), the dipole moment of 0.22 or 0.48 D obtained for the two C s , conformations considered in this work do not confirm the value suggested by the analysis of the electronic density maps obtained from cristallographic data (3.5 D ). The polarizabilities ( =81 a.u.) and the hyperpolarizabilities (β z = 1050 a.u.) are the first ones obtained theoretically. Although they cannot be compared with experimental results because of the difficulty of measuring them, they allow to understand why POM is an organic material interesting for optoelectronics.

Direct momentum-space calculations for molecules

Abstract A calculations method for momentum-space wave functions, using the Fourier transforms fo the Hartree-Fock equations, is presented. As shown by results on the test-systems H2 and H3, the momentum-space equations can be directly solved by numerical integration techniques, including configuration interaction effects.

Components of the interaction energy of benzene with Na+ and methylammonium cations

The results of ab initio computations including correlation (MP2) performed on the complexes of Na+, monomethylammonium (MMA) and tetramethylammonium (TMA) with a same ligand, benzene, are compared at the same theoretical level (double-zeta basis set, with polarization functions optimized on molecular polarizabilities). The binding energies, corrected for the basis set superposition error, are in satisfactory agreement with the values of the measured gas-phase enthalpies. The nature of the components of the binding are discussed in light of decomposition of the interaction energies in different adducts. The values of the electrostatic, polarization, charge transfer and correlation (dispersion) components, considered at the equilibrium and upon approach to equilibrium, indicate that the order of the Coulomb attraction of these ions for the same ligand is a good indicator of the order of the total energies, but the weight of the other components is far from negligible, and varies from one ion to another.