Simulated structure, dynamics, and vibrational spectra of liquid benzene (original) (raw)
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The fast dynamics of benzene in the liquid phase. Part II. A molecular dynamics simulation
Physical Chemistry Chemical Physics, 2001
A molecular dynamics simulation is performed for liquid benzene in the rigid body approximation. The results concerning the structural and dynamical properties of the system provide the basis for the interpretation of recent experimental data. In particular, it is shown that the system is characterised by a well deÐned cage structure, and that the average dynamics of the cages describe the main dynamical features of the bulk liquid. The calculated mean lifetime of the cages is in good agreement with the value of the Kubo correlation time derived from the experiments. In the picture emerging both from experiments and calculations, the fast intermolecular dynamics of liquid benzene is characterised by an inhomogeneously broadened distribution of intermolecular vibrational frequencies, whose dephasing is primarily due to the relaxation of the local structures. In particular, this mechanism is responsible for the dephasing of the low frequency librations giving rise to the intermediate quasi-exponential relaxation observed in the optical Kerr e †ect experiments.
Dynamics of liquid benzene: A cage analysis
The Journal of Chemical Physics, 2005
Dynamics of single molecules in liquids, inspected in the picosecond time scale by means of spectroscopic measurements or molecular-dynamics ͑MD͒ simulations, reveals a complex behavior which can be addressed as due to local confinement ͑cage͒. This work is devoted to the analysis of cage structures in liquid benzene, obtained from MD simulations. According to a paradigm proposed for previous analysis of atomic and molecular liquids ͓see, for example, A. Polimeno, G. J. Moro, and J. H. Freed, J. Chem. Phys. 102, 8094 ͑1995͔͒, the istantaneous cage structure is specified by the frame of axes which identifies the molecular configuration at the closest minimum on the potential-energy landscape. In addition, the modeling of the interaction potential between probe molecule and molecular environment, based on symmetry considerations, and its parametrization from the MD trajectories, allows the estimation of the structural parameters which quantify the strength of molecular confinement. Roto-translational dynamics of probe and related cage with respect to a laboratory frame, dynamics of the probe within the cage ͑vibrations, librations, re-orientational motions͒, and the restructuring processes of the cage itself are analyzed in terms of selected time self-correlation functions. A time-scale separation between the processes is established. Moreover, by exploiting the evidence of fast vibrational motions of the probe with respect to the cage center, an orientational effective potential is derived to describe the caging in the time scale longer than ϳ0.2 ps.
On the structure and dynamics of liquid benzene
Chemical Physics, 1982
The structure and dynamics of liquid benzene are analysed on the basis of a molecular-dynamics experiment involving 216 molecules and lasting 20 ps. Benzene molecules are modelled as rigid hexagons interacting by a six-centre Lennard-Jones potential. The rwulting spatial and time correlation functions are compared to X-ray, neutron. NMR and light scattering data. Selected expansion coefficients of g(R, fl ,, $, D,2) are used in a critical examination of the applied potential. 1. Motivation Although the field of polar liquids has been extensively investigated by computer simulation only a few papers exist [l-4]? which are at least qualitatively correct. This comes from the long-range interactions which must be carefully summed in order to avoid spurious artefacts [5,6]. On the contrary. mere trimcation of the pair potential turned out to be sufficient for non-polar liquids being governed by short-range interactions: As a consequence highly accurate results can be obtained from the simulation of "small-number systems". For this reason a large variety of systems has been published since the pioneering work of Singer et al. [7], who studied two-centre Lennard-Jones molecules. At present more complex molecules like benzene seem to be tractable. Apart from a preliminary paper of Kushick and Beme [S] representing benzene molecules by rotational ellipsoids the only computer-simulation study taking into account the six-fold symmetry of the benzene ring is due to Evans and Watts [9]. However, their Monte Carlo study is restricted to static properties. Even in this respect their analysis is not complete. For example the atom-atom correlation functions g
Journal of the American Chemical Society, 2004
Molecular dynamics atomistic simulations of solid and liquid benzene have been performed, employing a model intermolecular potential derived from quantum mechanical calculations. The ab initio database includes approximately 200 geometries of the benzene dimer with interaction energies computed at the MP2 level of theory. The accuracy of the modeled force field results is satisfactory. The thermodynamic and structural properties, calculated in the condensed phases, are compared with experimental data and previous simulation results. Single particle and collective dynamical properties are also investigated through the calculation of translational and rotational diffusion coefficients, reorientational dynamics, and viscosities. The agreement of these data with experimental measurements confirms the reliability of the proposed force field.
Vibrational excitons, resonant energy transfer, and local structure in liquid benzene
The Journal of Chemical Physics
The presence of vibrational excitons in liquid benzene has been tested by the method of isotopic dilution. A C6H6/C6D6 concentration study on the infrared and Raman fundamental modes reveals that the umbrella (A 2u ) vibrational exciton in solid benzene retains its basic identity upon melting and at room temperature. The total liqUid excIton bandwidth is about 40 cm -I, practically the same as in the solid. This indicates an instantaneous local liquid structure similar to that of the solid (the C; crystal site symmetry is also nearly prese~ed), In general agreement with indications from other methods. The fastest nearest neighbor vIbratIOnal resonant transfer takes about I psec. The residual Iinewidth at isotopic dilution is 3-4 cm-I , whIch IS due to Inhomogeneous and/or homogeneous broadening. The respective overall reorientational and/or translational relaxation takes about 2 psec or longer. The exciton linewidth is proportional to the square root of the isotopic concentration except for a sudden break at some critical concentration.
Structure and vibrational spectra of benzidine
Journal of Molecular Structure, 2003
The geometry and vibrational spectrum of benzidine have been computed by ab initio calculations using the DFT/B3LYP method with 6-31 þ G(d,p) basis set. In the most stable geometry, the dihedral angle between the two phenyl rings was found to be around 388. Calculated wavenumbers were scaled by a single factor 0.965 to approximately correct for vibrational anharmonicity as well as for overestimation of the force constants. Normal coordinate analysis of benzidine and some of its deuterated derivatives have also been performed in valance force field approximation in order to demonstrate the transferability of the force field of aniline. Good agreements between the two different calculation results (ab initio and force field refinement methods) and between the calculated and observed values are found. q
Quantum effects in the structure of liquid benzene at room temperature
Molecular Physics, 2001
Structural differences between liquid light (protonated) benzene and heavy (deuterated) benzene at room temperature have been measured using high energy electromagnetic radiation scattering techniques. Intra-and intermolecular effects have been examined, and the main quantum contribution is shown to be intramolecular. This is in contrast to the quantum effects measured in liquid water at room temperature, which are primarily intermolecular.
The Journal of Chemical Physics, 2004
Extensive ab initio calculations at the MP2/6-31G* level have been carried out to sample the energy surface for the interactions of the benzene dimers. This database has been used to parameterize two anisotropic single-site models, meant to be used as building blocks in hybrid models of complex, liquid crystal forming molecules. A quadrupolar Gay-Berne ͑GBQIII͒ and an S-function ͑SF͒ Corner potentials have been obtained in this way. Their ability to reproduce, qualitatively at least, the phase diagram as well as energetic and structural properties of benzene has been tested with Monte Carlo simulations and compared with previous literature potentials, GBQI ͓S. Gupta et al., Mol. Phys. 65, 961 ͑1988͔͒ and GBQII ͓T. R. Walsh, Mol. Phys. 100, 2867 ͑2002͔͒. It turned out that GBQI showed no melting transition in the temperature range explored ͑100-400 K͒, while GBQII underwent a phase transition from solid to gas, with no liquid phase. Conversely, both models parameterized on our database of ab initio interaction energies ͑GBQIII and SF͒ gave rise to a stable liquid phase. Melting has been observed between 100 and 150 K ͑GBQIII͒ and in the range 300-350 K ͑SF͒, i.e., substantially below and slightly above the experimental value at ambient pressure, 278 K. The description of the crystal structure of benzene at atmospheric pressure is also in better agreement with experimental data if the SF model is used, while positional correlations in the liquid are better described by the GBQIII potential. The S-function potential is also computationally more convenient. These results could be useful in the semirealistic modeling of more complex molecules.