Hakima Abou-Rachid

DFT studies of the hydrogen abstraction from primary alcohols by O2 in relation with cetane number data

Abstract Theoretical kinetic studies of the reactions of several primarily alcohols with molecular oxygen are reported. Previously, by using Density Functional Theory (DFT) calculations, the hypothesis according to which the cetane number of a pure organic depends on the initiation rate of its homogeneous gas-phase reaction with molecular oxygen, was confirmed [See H. Abou-Rachid, L. Bonneviot, G. Xu, S. Kaliaguine, J. Mol. Struct. Theochem 621 (2003) 293]. A fair correlation was indeed obtained between the rate constant of the initiation step in the auto-ignition reaction and the value of the cetane number of the pure compound. In the present work, new kinetic calculations are reported for the series of primary alcohols: methanol, ethanol, propanol, butanol and pentanol. The DFT method appears to be satisfactory since the calculated rate constants are well correlated with both the experimental cetane number data, and the length of the (C+O) atom chain of these alcohols.

Dynamical quenching of laser-induced dissociations of heteronuclear diatomic molecules in intense infrared fields

This article explores the influence of permanent dipole moments, i.e., of direct vibrational excitations, on the dynamical dissociation quenching (DDQ) effect, a mechanism for laser-induced vibrational trapping in the infrared (IR) spectral range which was recently demonstrated for the homonuclear H2+ ion, and was shown to result from a proper synchronization of the molecular motions with the oscillations of the laser electric field [see F. Châteauneuf, T. Nguyen-Dang, N. Ouellet, and O. Atabek, J. Chem. Phys. 108, 3974 (1998)]. To this end, the wave packet dynamics of the HD+ and, to a lesser extent, the HCl+ molecular ions are considered in an intense IR laser field of variable frequency. Variations in the absolute phase of the laser electric field, a form of variations in the initial conditions, reveal new signatures of the DDQ effect due to the presence of nonzero permanent dipole moments in these molecules. The added permanent dipole/field interaction terms induce a discrimination between parallel an...

On the 1,3-dipolar cycloaddition reactions of indenone with N-N-C dipoles: density functional theory calculations

Density functional theory (DFT) calculations at the B3LYP/6-311G* theoretical level have been performed to study the 1,3-dipolar cycloaddition (1,3-DC) reactions between indenone (1) and different 1,3-dipoles (diazomethane and N-methyl C-methoxy carbonyl nitrilimine, compounds 2 and 3, respectively). The geometrical and energetic properties were analysed for the different reactives, transition states and cycloadducts formed (compounds 4-11). The reactions proceed in the gas-phase by an asynchronous concerted mechanism, yielding different regiochemistry dependent on the 1,3-dipole chosen, although with dipole 3 some degree of synchrony was found in the formation of cycloadduct 5. The 1,3-DC between 1 and 3 was regioselective, being the cycloadduct 11 favoured against 9. The NMR chemical shift parameters (GIAO method) were also calculated for the reactives and cycloadducts.

Nonperturbative wave packet dynamics of the photodissociation of H2+ in ultrashort laser pulses

The wave packet dynamics of the photodissociation of H2+ under excitation by laser pulses of short durations at 329.7 nm are studied. The photodissociation process involves essentially two coupled channels, and the detailed mechanism for the formation of fragment kinetic energy spectra is examined by following the evolution of structures in the coupled‐channel wave functions in momentum space. These structures appear in the channels’ momentum wave functions at P≠0, as the v=0 ground vibrational state is promoted to the dissociative channel then accelerated. The variations of these structures reflect the interplay between local laser‐induced transitions and the accelerating–decelerating action of intrinsic molecular forces. The wave packet dynamics are studied for rectangular and Gaussian pulses of varying durations and peak intensities. In addition, two forms of channel couplings were considered corresponding to two different choices of the gauge: the electric‐field (EF) gauge, in which the matter–field i...

Dynamical quenching of laser-induced dissociations of diatomic molecules in intense infrared fields: Effects of molecular rotations and misalignments

The dynamical dissociation quenching (DDQ) effect is a new mechanism for laser-induced vibrational trapping of molecules in the infrared (IR) spectral range. Previously demonstrated for one-dimensional, prealigned diatomic molecules [see F. Châteauneuf, T. Nguyen-Dang, N. Ouellet, and O. Atabek, J. Chem. Phys. 108, 3974 (1998)], the effect was shown to result from a proper synchronization of the molecular motions with the oscillations of the laser electric field. The present paper explores the influence of rotations and misalignment of the molecular system on the DDQ effect. To this end, the two-dimensional (radial and angular) wave-packet dynamics of the H2+ and HD+ molecular ions are considered in an intense IR laser field starting from two types of initial angular distributions: The first type of distributions is appropriate for a field-free, pure angular momentum eigenstate and denotes typically an initially nonaligned, nonoriented molecule. The second type denotes a more or less well aligned and/or o...

Radiative transitions induced by short laser pulses in atomic collisions : nonperturbative study of pulse shape effects

A time‐dependent adiabatic electronic representation is defined by solving the local N‐level electronic time‐dependent Schrodinger equations at each nuclear configuration of a general N‐channel, laser‐driven molecular system. These solutions are eigenstates of a time‐dependent effective Hamiltonian with respect to which the exact time‐evolution of the N‐state system is adiabatic. For a two‐channel system, the time‐dependent adiabatic electronic representation depends on an effective area of the laser pulse and geometrical phases that are also functionals of the laser pulse shape. This adiabatic representation is used in constructing an algorithm for the generation and propagation of wavepackets in a two‐channel system irradiated by a short laser pulse. The algorithm is applied to the study of the wavepacket dynamics in the Na–Ar collisional system excited by short laser pulses. The dynamics of the channel populations are analyzed as functions of the shape, duration, and intensity of the laser pulses.

Theoretical prediction of the IR spectra of nitrosamide and diazohydroxide

The IR spectra of nitrosamide (H2NNO) and diazohydroxide (HNNOH) were predicted using the force constants obtained from SCF wave functions in a 4-31G basis set. The vibrational analysis of these molecules, never observed experimentally, was compared to data obtained with their isoelectronic molecules formaldoxime H2CNOH and nitrosomethane H3CNO.

Molecular Excitations Created by an Arbitrarily Shaped Laser Pulse: An Adiabatic Wavepacket Propagation Study

In pump-probe experiments, femtosecond laser pulses permit excited states to be accessed on a time-scale shorter than typical times for molecular motions, and are being used to investigate the dynamics of transitory species1. The theoretical descriptions of these experiments using wavepacket propagation techniques are currently limited to a first-order perturbative treatment of the time-dependent Schrodinger equation and rely on the rotating-wave approximation (RWA)2. A non-perturbative, non-RWA wavepacket description of molecular excitations created by ultra-short and intense laser pulses is proposed in the present communication, where a time-dependent electronic representation is introduced by solving the local, nuclear-coordinate parameterized Schrodinger equation describing the laser-driven, multistate electronic system. As shown in previous works3, these solutions can be identified with the eigenstates of a time-dependent effective hamiltonian with respect to which the exact time evolution of the N-state electronic system is adiabatic. For a two-channel system, the time-dependent adiabatic representation depends on an effective area of the laser pulse and geometrical phases which are also functionals of the laser pulse-shape. Due to this explicit dependence on the characteristics of the laser pulse, the representation is appropriate for studying pulse-shape effects in the short-time dynamics of laser-driven molecular systems.