Jesus Santamaria

A Monte Carlo study of liquid benzene

Abstract A Monte Carlo study for liquid benzene, where the pair potential is described as a sum of Lennard-Jones (12, 6) interactions acting between six equivalent centres of each molecule, has been carried out. The potential parameters have been chosen in such a way that the internal energy and virial pressure were in agreement with the experimental values. The liquid structure has been examined in detail by computing several distribution functions: the radial, the angular correlations and the radial-angular distribution between molecular planes.

The effect of intermolecular potential model on the structure and conformational equilibrium of liquid n‐butane

Several Monte Carlo simulations with a sample of 125 molecules of n‐butane with four Lennard‐Jones centers and internal rotation, have been carried out by varying the values of e and σ parameters of the intermolecular potential. In this way the effect of such changes on the thermodynamic, structural, and conformational properties have been studied. Although the structure remains practically unaffected by moderate variations in the Lennard‐Jones parameters, nevertheless the conformational equilibrium is very sensitive to them. Also specific properties of chain molecules in the n‐butane, such as the effect of the relative position of centers (external vs internal ones) and the possibility of orientational correlation have been explored. The difference between the effects of the external centers vs the internal ones can be quantified and it turns out to be significant; on the other hand the orientational correlation is very weak at room temperature and normal liquid density.

Intramolecular and reactive dynamics of OH-overtone excited HOOH (D): A classical trajectory study on a quasiseparable potential surface

Abstract Classical trajectories are used to study the intramolecular dynamics of the OH overtone (νOH= 5–9) induced unimolecular dissociation of HOOH (D). Detailed examination of the reaction dynamics sensitivity to variations of a simple quasiseparable potential energy surface, overall molecular rotation and initial condition selection are investigated. The results clearly show the importance of accurate potential energy representation and provide strong evidence toward which type(s) of interactions are most essential to describe the reaction dynamics. In particular, the stretch-bend coupling is found to play an important role in the rotation-vibration dynamics of HOOH (D).

Monte Carlo calculations for liquid carbon tetrachloride

Monte Carlo simulations of liquid carbon tetrachloride have been carried out for a system of 125 molecules described by five interaction centres of the (12-6) Lennard-Jones type. Radial distribution functions and structure factors for several pairs of interaction parameters are compared with experimental values and with the molecular dynamics results of other authors.

Quasi-classical trajectory study of the dynamics of the H+H2O reaction: differential cross-sections and product rotational polarization

Abstract Quasi-classical trajectory calculations for the H + H 2 O → OH + H 2 reaction have been performed on the Ochoa–Clary potential energy surface [J. Phys. Chem. A 102 (1998) 9631] at 1.43 and 2.20 eV collision energies. Differential cross-sections, recoil energy distributions and product rotational polarization (polarization parameters and polarization-dependent differential cross-sections) have been obtained and compared with the existing experimental and theoretical data obtained on different potential energy surfaces. A qualitative agreement between theory and experiment has been found.

Contributions of parent molecule fixed and excess energies to product energy partitioning in four-center elimination reactions

Abstract In four-center elimination reactions such as hydrogen halide elimination from halogenated hydrocarbons the energy barrier is higher than the difference in enthalpy of formation between the parent molecule and its fragments (HX and olefin). This determines that the energy available to products has two origins: the reverse reaction barrier (fixed energy), and the excess energy (energy above the barrier). Both types of energy are partitioned among products following different laws: more or less statistical for excess energy and non-statistical for fixed energy. In a study of CF 3 -CH 3 decomposition, we describe a practical method, based on the variation of product energy partitioning with excess energy, to determine the partitioning of the fixed energy among different types of product energy, thus defining the exact nature of the reverse reaction energy barrier. We applied this model to other types of reactions, such as three-center molecular eliminations.

Dynamical mechanism for the intermolecular relaxation of the excited NH stretching vibration of a pyrrole molecule in liquid solution

Abstract Nonlinear Dynamics is used to explain the intermolecular relaxation of the NH strechinng vibration of pyrrole in liquid phase. We present first a two mode model, which implies a 3:1 anharmonic resonance between the NH Morse stretch of an excited pyrrole molecule and the ring mode (harmonic stretch) of a neighboring pyrrole molecule coupled by a hydrogen bond interaction. Next, the model has been extended to three modes, in order to better mimic the real phenomenon. We present here the population decay and vibrational relaxation time of the NH stretch for both models.