Adolfo Bastida

A theoretical study of the HgAr2(3P1←1S0) vibronic spectrum

A quantum mechanical calculation of the vibronic spectrum of the HgAr2 van der Waals cluster in the region of the Hg(3P1←1S0) electronic transition is presented and compared with experiments. The potential energy surfaces for the ground and excited states are obtained using available empirical Ar–Ar and Ar–Hg potentials. For the ground electronic state, the potential is written as a sum of pairwise Ar–Ar and Ar–Hg(1S0) interactions. On the contrary, for the electronically excited states correlating to Hg(3P1), an axis switching rotation has to be applied to each individual Ar–Hg(3P1; Ωe=0, ±1) interactions in order to define a common quantization axis. This results in a nonpairwise additive diabatic matrix Hamiltonian which after diagonalization provides the adiabatic excited potential energy surfaces. The vibrational wave functions associated to the ground and the first two (A and B+) adiabatic electronically excited states are obtained by variational techniques using basis sets along Jacobi coordinate...

Vibrational predissociation of the I2⋯Ne2 cluster: A molecular dynamics with quantum transitions study

The MDQT (molecular dynamics with quantum transitions) method of Tully is applied to the vibrational predissociation of a Van der Waals cluster containing a diatomic molecule and two rare gas atoms, I2⋯Ne2. The vibrational degree of freedom of the diatomic is treated quantum mechanically using DVR (discrete variable representation) while all the other degrees of freedom are treated classically. The results are in very good agreement with the experimentally measured lifetimes and product state distributions. In particular, the final vibrational state distribution of I2, which could not be satisfactorily reproduced in quasiclassical studies, is well described. Based on these results a different kinetic scheme for interpreting the vibrational predissociation in this system is proposed. In addition, this work shows that the method is very promising for the study of clusters containing more rare gas atoms.

The Ehrenfest method with quantum corrections to simulate the relaxation of molecules in solution: equilibrium and dynamics.

The use of the Ehrenfest method to simulate the relaxation of molecules in solution is explored. Using the cyanide ion dissolved in water as a test model, the independent trajectory (IT) and the bundle of trajectories (BT) approximations are shown to provide very different results for the time evolution of the vibrational populations of the solute. None of these approximations reproduce the Boltzmann equilibrium vibrational populations accurately. A modification of the Ehrenfest method based on the use of quantum correction factors is thus proposed to solve this problem. The simulations carried out using the modified Ehrenfest method provide IT and BT relaxation times which are closer to each other and which agree quite well with previous hybrid perturbative results.

Intramolecular vibrational redistribution and fragmentation dynamics of I2 ⋯ Nen (n=2–6) clusters

Intramolecular vibrational energy redistribution and fragmentation dynamics in I2(B,v=22) ⋯ Nen (n=2–6) and I2(B,v=21) ⋯ Nen (n=2–5) clusters is studied by hybrid quantum/classical techniques and the results are compared with experiments. A vibrational version of the molecular dynamics with quantum transitions (MDQT) treatment is used in which the vibrational degree of freedom of I2 is treated quantum mechanically while all the other degrees of freedom are treated classically. The potential energy surface is represented as a sum of pairwise interactions with parameters taken from the literature. The calculated product state distributions are in very good agreement with the experiments. Fragmentation lifetimes were also calculated and agree reasonably well with those measured in time-dependent experiments. Fragmentation proceeds via sequential ejection of Ne monomers through three different mechanisms: (i) sequential intramolecular vibrational redistribution plus vibrational predissociation (in which the I...

Instantaneous normal modes, resonances, and decay channels in the vibrational relaxation of the amide I mode of N-methylacetamide-D in liquid deuterated water

A nonequilibrium molecular dynamics (MD) study of the vibrational relaxation of the amide I mode of deuterated N-methylacetamide (NMAD) in aqueous (D(2)O) solution is carried out using instantaneous normal modes (INMs). The identification of the INMs as they evolve over time, which is necessary to analyze the energy fluxes, is made by using a novel algorithm which allows us to assign unequivocally each INM to an individual equilibrium normal mode (ENM) or to a group of ENMs during the MD simulations. The time evolution of the energy stored in each INM is monitored and the occurrence of resonances during the relaxation process is then investigated. The decay of the amide I mode, initially excited with one vibrational quantum, is confirmed to fit well to a biexponential function, implying that the relaxation process involves at least two mechanisms with different rate constants. By freezing the internal motions of the solvent, it is shown that the intermolecular vibration-vibration channel to the bending modes of the solvent is closed. The INM analysis reveals then the existence of a major and faster decay channel, which corresponds to an intramolecular vibrational redistribution process and a minor, and slower, decay channel which involves the participation of the librational motions of the solvent. The faster relaxation pathway can be rationalized in turn using a sequential kinetic mechanism of the type P-->M+L-->L, where P (parent) is the initially excited amide I mode, and M (medium) and L (low) are specific midrange and lower-frequency NMAD vibrational modes, respectively.

Hybrid quantum/classical simulation and kinetic study of the vibrational predissociation of Cl2⋯Nen (n=2, 3)

A hybrid quantum/classical method is applied to the vibrational predissociation of van der Waals clusters containing a diatomic molecule and several rare gas atoms, Cl2⋯Nen (n=2, 3). The vibrational degree of freedom of the diatomic is treated quantum mechanically while all the other degrees of freedom are treated classically. A kinetic mechanism is proposed in order to interpret the dynamics in terms of the following elementary steps; vibrational predissociation (VP), intramolecular vibrational redistribution (IVR), and evaporative cooling (EC). The resulting lifetimes are in very good agreement with the experimental linewidth measurements of Janda and co-workers, and with the quantum mechanical reduced-dimension results of Le Quere and Gray on Cl2⋯Ne2. The final rotational state distributions agree very well with the experimental results and exhibit a quasistatistical behavior. The final vibrational distributions reproduce the main experimental features.

Molecular dynamics simulation of the I2(X)⋯Ar isomers population in a free-jet expansion: Thermodynamics versus kinetic control

A molecular dynamics simulation addressing the problem of thermodynamic versus kinetic control of the isomers population of van der Waals complexes in a supersonic expansion is presented. The populations of the linear and T-shaped isomers of I2(X)⋯Ar in a supersonic beam expansion were determined by molecular dynamics simulation as a function of the distance to the nozzle and compared to the prediction of thermodynamics. The surprising conclusion is that although there is a barrier equal to half the well depth between the two isomers, their populations are consistent with the existence of thermodynamic equilibrium. This result is rationalized by examining the cooling mechanisms in the Ar+I2(X)⋯Ar collisions. In addition to the direct isomerization, a new mechanism (swap cooling), which induces isomerization even for complexes with barriers above the dissociation limit, is evidenced.

Competition between electronic and vibrational predissociation in Ar–I2(B): a molecular dynamics with quantum transitions study

Abstract A theoretical study of the competition between vibrational and electronic predissociation of the Van der Waals complex Ar⋯I 2 ( B ) using the Molecular Dynamics with Quantum Transitions (MDQT) method of Tully, is presented. The electronic predissociation is modeled by surface hopping from the B to the a state with probabilities given by the Franck–Condon factors between them. The vibration of I 2 is treated quantum mechanically using a Discrete Variable Representation (DVR) while the relative motion of the Ar atom with respect to I 2 is treated classically. The agreement between calculated and measured predissociation rates is very good. Analysis of the results indicate that first order rate equations are not adequate to describe the time dependent signals. This is due to the dependance of the electronic predissociation rates on the vibrational quantum number of the intermediate states of the process.

A Density Functional Theory Study of the Structure and Vibrational Spectra of β-Carotene, Capsanthin, and Capsorubin

A theoretical study of the structure and the vibrational spectra of the beta-carotene molecule and its derivatives capsanthin and capsorubin is carried out. We first investigate systematically the theoretical method which provides the best results for beta-carotene by performing ab initio calculations at the HF/6-31G(d), SVWN/6-31G(d), PBE0/6-31G(d), BLYP/6-31G(d), B3LYP/6-31G(d), B3LYP/6-31G(d,p), B3LYP/6-311G(d), and B3LYP/6-311G(d,p) levels and by using previous theoretical results available in the literature obtained at the AM1 and BPW91/6-31G(d) levels. The influence of both the level of calculation and the size of the basis set used in the geometry optimization and in the determination of the IR and Raman spectra of this molecule is thus analyzed. It is confirmed that the hybrid functional B3LYP with the basis 6-31G(d) is the method that gives the best results as a whole. By use of this level of calculation, we next optimize the molecular geometries of related molecules of capsanthin and capsorubin, which to the best of our knowledge have only been studied at the semiempirical AM1 level. In addition we calculate the IR and Raman spectra of these molecules at the B3LYP/6-31G(d) level of theory. The results obtained for capsanthin show on the one hand that the double bond of the beta-ionone ring is outside the polyene chain plane, due to the repulsion between the hydrogen atoms of the ring methyl groups and the hydrogen atoms of the polyene chain, and on the other hand that the carbonyl double bond in the other headgroup is very close to planarity with the polyene chain, since in this case such a repulsion does not exist. For the molecule of capsorubin the two carbonyl groups also take the same coplanar orientation relative to the polyene chain. The IR and Raman spectra theoretically computed for these two molecules are finally compared with their experimental spectra and the vibrational normal modes of the main signals are interpreted.

Determination of highly excited rovibrational states for N2O using generalized internal coordinates

Generalized internal vibrational coordinates are optimized and used to describe highly excited vibrational motions in the N2O molecule. These coordinates are defined as the magnitudes of two vectors, which are expressed as linear combinations of the internal displacement vectors and the angle formed between them. They depend on two parameters and contain, as particular cases, valence and orthogonal (Jacobi, Radau, etc.) coordinate systems. The coordinates are optimized by minimizing unconverged variationally computed vibrational energies with respect to the external parameters. A comparison of the optimal internal coordinates derived for N2O with valence and hyperspherical normal coordinates is made. The optimal internal coordinates are also used to determine a new potential energy function for N2O from the observed vibrational frequencies up to 15 000 cm−1 using fully variational calculations. The quality of the adjusted potential energy function is checked by computing vibrational-rotation energy levels...