Noureddine Issaoui

Theoretical investigation of the relative stability of Na+Hen (n = 2–24) clusters: Many-body versus delocalization effects

The solvation of the Na(+) ion in helium clusters has been studied theoretically using optimization methods. A many-body empirical potential was developed to account for Na(+)-He and polarization interactions, and the most stable structures of Na(+)He(n) clusters were determined using the basin-hopping method. Vibrational delocalization was accounted for using zero-point energy corrections at the harmonic or anharmonic levels, the latter being evaluated from quantum Monte Carlo simulations for spinless particles. From the static perspective, many-body effects are found to play a minor role, and the structures obtained reflect homogeneous covering up to n = 10, followed by polyicosahedral packing above this size, the cluster obtained at n = 12 appearing particularly stable. The cationic impurity binds the closest helium atoms sufficiently to negate vibrational delocalization at small sizes. However, this snowball effect is obliterated earlier than shell completion, the nuclear wavefunctions of (4)He(n)Na(+) with n = 5-7, and n > 10 already exhibiting multiple inherent structures. The decrease in the snowball size due to many-body effects is consistent with recent mass spectrometry measurements.

One and two-electron investigation of electronic structure for Ba(+)Xe and BaXe van der Waals molecules in a pseudopotential approach.

The potential energy curves, vibrational energy levels, spectroscopic constants, and dipole moment curves for the ground and excited states of BaXe and its ion Ba(+)Xe molecules are calculated with an ab initio method using pseudopotential techniques and core polarization potentials. The molecules are treated as two (BaXe) or one (Ba(+)Xe) active electrons systems taking benefit of the zero pseudopotential approach for Xe. The vibrational levels and their energy spacing have been also determined for Σ(+), Π, and Δ states. The permanent and transition dipole moment curves are investigated for the (1,3)Σ(+) states of the BaXe neutral molecule and (2)Σ(+) states of the Ba(+)Xe ion. The analysis of these numerous results shows interesting behavior in potential energy curves imprinted by the strong repulsive interactions between electron and Xe and also indicates an intense transition dipole moment for both Ba(+)Xe and BaXe.

Molecular structure, vibrational spectra, AIM, HOMO–LUMO, NBO, UV, first order hyperpolarizability, analysis of 3-thiophenecarboxylic acid monomer and dimer by Hartree–Fock and density functional theory

In this work, the molecular structure, harmonic vibrational frequencies, UV, NBO and AIM of 3-thiophenecarboxilic acid (abbreviated as 3-TCA) monomer and dimer has been investigated. The FT-IR and FT-Raman spectra were recorded. The ground-state molecular geometry and vibrational frequencies have been calculated by using the Hartree-Fock (HF) and density functional theory (DFT)/B3LYP methods and 6-311++G(d,p) as a basis set. The fundamental vibrations were assigned on the basis of the total energy distribution (TED) of the vibrational modes, calculated with VEDA program. Comparison of the observed fundamental vibrational frequencies of 3-TCA with calculated results by HF and DFT methods indicates that B3LYP is better to HF method for molecular vibrational problems. The difference between the observed and scaled wavenumber values is very small. The theoretically predicted FT-IR and FT-Raman spectra of the title compound have been constructed. A study on the Mulliken atomic charges, the electronic properties were performed by time-dependent DFT (TD-DFT) approach, frontier molecular orbitals (HOMO-LUMO), molecular electrostatic potential (MEP) and thermodynamic properties have been performed. The electric dipole moment (μ) and the first hyperpolarizability (β) values of the investigated molecule have been also computed.

Ab initio study of Ba+Arn (n = 1–4) clusters: spectroscopic constants and vibrational energy levels

ABSTRACT This study is interested in the illustration of ab initio potential energy curves for Ba+Arn (n = 1–4) clusters. The electronic structures of these molecules are calculated using [Ba2+] and [Ar] non-empirical core pseudo-potentials complemented by the core polarisation operators for both atoms, which allow the consideration of core valence correlation effects. The structure and stabilities of Ba+Arn (n = 1–4) clusters are investigated. These molecules are treated as one-electron active system. Spectroscopic constants and vibrational energy levels have been derived from their potentials. The analysis of the geometric forms, basing on the potential energy curves and the spectroscopic constants, clearly shows the importance of rare gas induced dipole. We also show that the dipolar interactions can influence the coupling between atoms.

Molecular structure and vibrational spectroscopic studies on 2-furanacetic acid monomer and dimer.

In this work, molecular geometries and fundamental vibrational frequencies of 2-furanacetic acid (2FAA) and its hydrogen bonded dimer were investigated using DFT/B3LYP method with 6-311++G(d,p) as basis set. The FT-infrared and FT-Raman spectra of the 2FAA compound were recorded in the region 4000-400 cm(-1). The theoretical wavenumbers were scaled and compared with experimental FT-IR and FT-Raman spectra. Complete vibrational assignments and analysis of the fundamental modes of monomer and dimer structures were performed on the basis of the potential energy distribution (PED) calculations. A study on the electronic properties, such as excitation energies, oscillator strength, wavelengths, HOMO and LUMO energies, are performed by time-dependent DFT (TD-DFT) approach. Molecular stability arising from hyperconjugative interactions and charge delocalization has been analyzed using Natural Bond Orbital (NBO) analysis. Topological parameters such an electron density and its Laplacian at bond critical points (BCP) of O-H and O⋯H contact bonds were analyzed in details with the help of the atoms in molecules (AIM) approach in order to study the intermolecular hydrogen bonding. The nonlinear optical properties of the title molecule have been investigated. Moreover, molecular electrostatic potential (MEP) surface was plotted for predicting sites and relative reactivities towards electrophilic and nucleophilic attack. The nonlinear optical properties were reported and compared with that of the urea. The thermodynamic properties like heat capacity, entropy, and enthalpy have been calculated for the molecule at different temperatures.

A complete assignment of the vibrational spectra of 2-furoic acid based on the structures of the more stable monomer and dimer

The structural and vibrational properties of cyclic dimer of 2-furoic acid (2FA) were predicted by combining the available experimental infrared and Raman spectra in the solid phase and ab initio calculations based on density functional theory (DFT) with Poples basis sets. The calculations show that there are two cyclic dimers for the title molecule that have been theoretically determined in the gas phase, and that only one of them, cis conformer, is present in the solid phase. The complete assignment of the 66 normal vibrational modes for the cis cyclic dimer was performed using the Pulays Scaled Quantum Mechanics Force Field (SQMFF) methodology. Four strong bands in the infrared spectrum at 1583, 1427, 1126 and 887 cm(-1) and the group of bands in the Raman spectrum at 1464, 1452, 1147, 1030, 885, 873, 848, 715 and 590 cm(-1) are characteristic of the dimeric form of 2FA in the solid phase. In this work, the calculated structural and vibrational properties of both dimeric species were analyzed and compared between them. In addition, three types of atomic charges, bond orders, possible charge transfer, topological properties of the furan rings, Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) theory calculations were employed to study the stabilities and intermolecular interactions of the both dimers of 2FA.

Spectroscopic and electric dipole properties of the Van der Waals interaction between barium and krypton atoms

ABSTRACT Van der Waals interactions between the valence electrons of the barium atom and the closed-shell cores have been studied using ab initio methods with a combination of pseudopotentials and core polarisation potential. We have reported the potential energy curves, the spectroscopic constants, the vibrational levels as well as the electric dipole moments for the ground and several excited states of Ba+Kr and BaKr complexes. Interesting behaviour has been observed in the potential energy curves, particularly for Σ+ states. This can show the strong repulsive interactions between Rydberg electrons and the Krypton atom. GRAPHICAL ABSTRACT