Clovis Darrigan

A theoretical study of stability, electronic, and optical properties of GeC and SnC

We present the results of a first principles study on the ordered Ge 0.50 C 0.50 and Sn 0.50 C 0.50 cubic alloys. A linear combination of atomic orbitals approach in the framework of density functional theory is employed for total energy calculations in the zincblende phase. A fitting of the energy surface to the equation of state yields the lattice constant of 4.61 and 5.17 A and the bulk modulus of 181 and 119 GPa for GeC and SnC, respectively. Analysis of band structure suggests a crossover of the nature of the band gap from indirect to direct in going from SiC to GeC to SnC. Although both alloys predicted to be unstable with respect to their elemental components at zero pressure and temperature, GeC appears to become stable at higher pressure. It appears that both the lattice

New parallel software (P_Anhar) for anharmonic vibrational calculations: Application to (CH3Li)2

We present the development of a new parallel computer code (P_Anhar_v1.0) to calculate the vibrational spectrum of medium size molecules using a variational algorithm. The method is applied to the determination of a complete quartic anharmonic force field (B3LYP/cc‐pVTZ) for methyllithium, leading to a new interpretation of experimental data.

Implementation of the finite field perturbation method in the CRYSTAL program for calculating the dielectric constant of periodic systems

The finite field approach has been implemented in the periodic ab initio CRYSTAL program and been used for calculating the dielectric constants of crystalline LiF and MgO (FCC structure) and BeO (wurtzite structure). To maintain the periodicity along the applied field direction, a “sawtooth” potential is used in conjunction with a supercell scheme. Supercells four to five times longer than the primitive cell in the direction of the applied field provide well‐converged results. The influence of the computational parameters is discussed. An alternative scheme has also been implemented, for inner check, that consists of applying a static electric field to a slab of increasing thickness in the direction orthogonal to the surface; the dielectric response at the center of the slab is shown to converge rapidly to the bulk value evaluated with the sawtooth field. The method is accurate and permits the determination of nonlinear corrections to the dielectric constant. When used in conjunction with the local density approximation (LDA) scheme, it provides for the dielectric constant of the three above‐mentioned compounds values close to those recently obtained with a time‐dependent density functional theory approach.

First-principles study of structural, electronic and optical properties of BaF2 in its cubic, orthorhombic and hexagonal phases

We present the results of a first-principles study on BaF2 in its stable (cubic) and high-pressure phases. A linear combination of atomic orbitals approach in the framework of density functional theory is employed for total energy calculations in cubic, orthorhombic and hexagonal phases of BaF2. A fitting of the energy surface to the equation of state yields the lattice constant and the bulk modulus of these phases at zero pressure which are in good agreement with the corresponding experimental values. Analysis of band structure determines the high-pressure phases to be direct-gap materials and no metallization of BaF2 is predicted to occur for pressures up to 50 GPa. Furthermore, several peaks observed in the spectroscopic experiments have been identified with interband transitions in the cubic BaF2. The calculated mean value of the refractive index is found to increase in going from the cubic to orthorhombic to hexagonal phases of BaF2.

Brillouin Spectroscopy, Calculated Elastic and Bond Properties of GaAsO4

Experimental and theoretical studies have been performed to demonstrate the high performance of the novel piezoelectric material GaAsO(4). Hydrothermally grown single crystals of α-quartz phase GaAsO(4) were studied by Brillouin spectroscopy to determine elastic constants. Experimentally obtained values of C(11), C(66), C(33), C(44), C(14) and C(12) are 59.32, 19.12, 103.54, 30.70, 1.7, and 21.1 GPa, respectively. Elastic and piezoelectric tensors were also calculated by a first principles method in this work, leading to a very good agreement with experimental results and confirming the values of elastic components obtained indirectly such as C(14) and the negligible piezoelectric correction for C(11). The thermal behavior of the elastic constant corresponding to the [100] longitudinal L mode (C(11)) was studied up to 1137 K to estimate potential piezoelectric performance. It was found that the thermal behavior is linear up to 1273 K which is just below the thermal decomposition temperature of 1303 K. High thermal stability can be linked to the higher polarizability of large cations Ga and As because of neighboring oxygen atoms. On the basis of thermal behavior, GaAsO(4) is a promising material for high temperature piezoelectric applications.

The Electronic Structure of Some Cyanohydrins—A Spectroscopically Under‐Investigated Family of Compounds

Cyanohydrins are usually formed by addition of hydrogen cyanide to aldehydes or ketones while the elimination of HCN from cyanohydrins is easily observed upon heating. The low thermal stability of these highly boiling compounds leads to difficult studies in the gas phase where partial or complete decomposition is usually observed. Consequently, the reported physicochemical properties of such compounds in the gas phase are still scarce. Keeping with this, four simple cyanohydrins, the glycolonitrile and methyl, vinyl and ethynyl derivatives, have been selected. These are possible candidates for the Interstellar Medium, where the corresponding aldehydes and HCN have been detected, and could have played an important role in prebiotic chemistry, as already proposed for some of them. Three well-suited spectroscopic techniques, namely, UV photoelectron, infrared, and Raman spectroscopies, in tandem with quantum calculations, have been chosen for the structure analysis. Photoelectron spectroscopy, successfully performed with gaseous compounds, provides the first comparative study on cyanohydrins in the gas phase.

SYNTHESIS, STRUCTURE, AND PROPERTIES OF LUMINESCENT DIAZABOROLE AND INDOLE SYSTEMS

Thiophenes and phenylacetylenes, decorated with 1-methylindol-2-yl, 1,3,2-benzodiazaborol-2-yl, 1,3,2-diazaborol-2-yl, or 1,3,2-diazaborolidinyl groups at one end and dimesitylborolyl or CN substituents at the opposite end of the molecules, were synthesized. Upon UV irradiation, these push-pull systems in THF solution gave rise to bright-blue emission with Stokes shifts ranging from 4100 to 9300 cm–1 and quantum efficiencies up to 0.99. Thereby intramolecular charge transfer took place from the HOMO of the indolyl or borolyl fluorophores to the LUMO, mainly centered on the electron-withdrawing cyano or dimesitylboranyl group.

Isoselenocyanates versus Isothiocyanates and Isocyanates

Alkyl and aryl isoselenocyanates are well known intermediates in the synthesis of various organoselenium compounds, but the knowledge of the physicochemical properties of simple unsaturated derivatives is still fragmentary. Vinyl-, 2-propenyl-, and cyclopropyl isoselenocyanates have been prepared by reaction of selenium in powder with the corresponding isocyanides. The isoselenocyanates of this series, with a variable distance between the N═C═Se group and the unsaturated or pseudounsaturated group, have been studied by UV-photoelectron spectroscopy and quantum-chemical calculations. For each of these three isoselenocyanates, the exploration of conformers and geometrical optimization always converge toward only one local minimum. The vinyl and cyclopropyl derivatives are characterized by similar order of magnitude of interactions between the NCSe group and the substituent, while for allylic compound two noninteracting moieties should be considered. The same conclusions were obtained for vinylic and cyclopropylic sulfur and oxygen derivatives. Thus the type and extent of interactions between the N═C═X (X = O, S, Se) group and an unsaturated (vinyl, allyl, or cyclopropyl) moiety are now clarified.

Chapter 4 – UV-Photoelectron Spectroscopy of Organosilicon Compounds

Abstract This chapter describes key work performed in the last 15 years, in which the use of the ultraviolet-photoelectron spectroscopy (UV-PES) has made significant progress in the understanding of various phenomena, such as hyperconjugation or other interactions due to high-energy σ bonds (Si–C, Ge–C, Si–Si, etc.), neutral cyclic cross hyperconjugation, influence of substituents on the stability of Si=C bond, or structure of the low-valent group 14 element analogues of carbenes and radicals. The knowledge of the electronic structure of compounds of interest allows not only comprehension and explanation of their reactivity or physical chemistry properties but provides an essential tool for the prediction and design of new compounds designed for specific properties. UV-PES is certainly a key tool to consider in the search for information on the electronic structure of molecules. Short descriptions of the UV-photoelectron spectrometer for gas phase characterization and theoretical methods for the evaluation of ionization energies are also given.