Antonio J. Hernández

Intramolecular coupling study of the resonances in the four-wave mixing signal

Abstract The effect of the intramolecular coupling on the resonances associated to the Four-wave mixing signal is studied in the present contribution. The transition and permanent dipole moments are critical quantities for this analysis of the local macroscopic polarization in the adiabatic representation when the rotating wave approximation is not considered. New characteristics of the resonance, emerging as a consequence of the vibronic coupling, are studied here in the framework of a local homogeneous-linewidth third-order perturbation model.

Pyridine interaction with a partially hydrogenated MoS2 modelled surface. A molecular orbital study

Chemisorption of pyridine on a partially hydrogenated surface of MoS2 was modelled by the interaction of a pyridine molecule with a Mo3S8H2 cluster. Calculation of total energies, bond orders, diatomic energies, charge transferences and interatomic orbital overlaps was performed by the CNDO/UHF method. Results show a net charge transfer from the pyridine molecule to the Mo adsorption center. The interaction of π-adsorbed pyridine with chemisorbed hydrogen bridged on MoMo positions leads to a partial hydrogenation of the nitrogen compound. A mechanism of pyridine hydrogenation is proposed based on the fact that Mo adsorption centers (vacancies) are pivotal for hydrogen atoms transfer. These results suggest a regioselective hydrogenation.

Cavity ring down absorption at low temperatures: C–H spectra (Δυ = 1–6) of CH3D and C–H overtones (Δυ = 5, 6) of CH2D2 and CH4

The spectra of the C–H stretch fundamental and overtones (Δυ = 1–6) of CH3D have been recorded. Absorptions in the visible were obtained with a phase shift cavity ring down technique where an optical cavity is inside a low temperature cryostat. Absorptions below 12,000 cm−1 were observed with a Fourier transform spectrophotometer. The local mode harmonic frequency and anharmonicity were obtained and used with the harmonically coupled anharmonic oscillator (HCAO) model to calculate energy levels and assign the absorption bands to particular transitions. Overtone absorptions (Δυ = 5 and 6) of CH4 and CH2D2 have also been obtained. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the band strength and the cross- section of the Δυ = 5 and 6 C–H transitions for each molecule. Cross–sections for CH4 agree within 10% with traditional absorption measurements with a multiple pass cell at high pressures. The importance of the new experimental technique is emphasized for laboratory studies of planetary atmospheres.

Theoretical explanation of the observed shift in density of states upon hydrogen chemisorption on nickel

Abstract We report a calculation, using the MINDO/SR method, of the difference in density of states for a 14-nickel atom cluster. It is observed that the theoretical curve matches very closely the experimental one. Based on these results, the nature of NiH bond is discussed.

A finite B-spline basis set for accurate diatomic molecule calculations

A finite basis set particularly adapted for solving the Hartree–Fock equation for diatomic molecules in prolate spheroidal coordinates has been constructed. These basis functions have been devised as products of B‐splines times associated Legendre polynomials. Due to the large number of B‐splines, the resulting set of eigenfunctions is amply distributed over excited states. This gives the possibility of using these basis sets to calculate sums over excited states, appearing in various orders of perturbation theory. As an illustration, the second‐order corrections to the ground‐state energy of some atoms and diatomic molecules with closed electron shells have been calculated.

Transition probabilities of molecular systems in the presence of time dependent electric fields

Abstract Analytical expressions for the transition probabilities of a molecular system interacting with a low intensity time dependent electric field are derived in the present contribution. The molecular system is modeled as a quartic anharmonic oscillator, where the interaction with the electric field is described by the electric-dipole approximation. Using Lie algebraic techniques and typical states of quantum optics, we are able to obtain an approximate analytical expression for the temporal evolution operator in the present model system.

Semiempirical study of electronic and bonding properties of iron silicide clusters

SummaryMolecular orbital calculations of iron, silicon, and iron silicide clusters have been carried out using the UHF-MINDO/SR method. The nature of the bonding in these compounds has been investigated by analyzing the importance of bonding indexes and diatomic components of the total energy. It has been found that in iron silicide the strongest bond is formed between Fe-Si and that it arises mainly as the result ofsp-sp type orbital interactions. Althoughd orbitals show very little overlap withs-p orbitals, they do contribute significantly to bonding through electrostatic type diatomic interactions. By means of a detailed analysis ofsp, andd orbitals and total density of states (DOS) of Fe7Si7, Si7Fe7, Fe15, and Si17 clusters, the present calculations have permitted us to explain the origin of the iron silicide UPS experimental peaks.

Simple parameter-dependent relations for the maximum value of the optical activity of push—pull molecules in solution

Based on the valence bond-charge transfer model for push-pull polyenes and reaction field theory, several parameter-dependent relations for selecting the molecule-solvent pair that produces a maximum value of the nonlinear optical response have been established. From these relations, we have determined a limit to the value of the dielectric constant of the solvent (above ε = 9) and effectiveness of the donor-acceptor pairs (between 5 and 6eV) beyond which the (hyper)polarizabilities of push-pull molecules in solution do not exhibit any significant change in magnitude.

Ab initio basis set study of the CO⋯He van der Waals interaction

Abstract The ab initio potential energy surface for the weakly bound van der Waals CO⋯He has been performed in the framework of the supermolecule approach, using fourth order Moller–Plesset perturbation theory with different basis sets, constructed to give accurate values for the electric moments and polarizabilities, supplemented by bond functions placed at the midpoint along the line joining He with the center of mass of CO. All atom-centered basis sets explored in the present study predict single-minimum near collinear C–O⋯He optimum structures. Near T-shaped optimum structures are predicted when these basis sets are augmented with a set of bond functions located in the middle of the van der Waals bond, in agreement with recent experimental and ab initio potentials.

Theoretical calculations for catalytic activation of N2 on a modelled iron adsorption site

Abstract MINDO/SR-UHF calculations were done on [FeN 2 ] q ( q = 1, 0 or −1 ) systems in end-on and side-on geometries in order to study the electronic and geometric factors that affect the adsorption of N 2 on iron adsorption sites. All systems studied in the end-on geometry were found to be stable. The adsorption energy of N 2 increases in the order. [FeN 2 ] − > [FeN 2 ] o > [FeN 2 ] + . The N-N bond is activated by the electronic transfer from Fe q to N 2 in all cases, this effect is particularly large for the [FeN 2 ] − system. Side-on interaction leads to thermodynamically unstable systems with a highly activated N 2 molecule. Distortion energy curves show high energy barriers in going from end-on to side-on geometries and very low barriers for the inverse process.