Dynamics Of Molecules In Liquids Research Articles

Reorientational dynamics of molecules in liquid methane: A molecular dynamics simulation study

Rotational motion of molecules in liquids plays an important role in determining the liquids' Nuclear Magnetic Resonance (NMR) spin-relaxation properties. The traditional theory of NMR spin relaxation assumes that the reorientational dynamics of molecules in liquids can be described by a continuous-time rotational-diffusion random walk with a single rotational-diffusion coefficient. However, recent experimental and theoretical studies have demonstrated that the simple rotational-diffusion model does not fully capture the rotational dynamics of molecules in liquids: for example, the reorientation of molecules in liquid water occurs through a combination of continuous-time rotational diffusion and discrete, large angular jumps resulting from collective rearrangements of the hydrogen-bond network of water.In order to obtain further insights into the origin of large angular jumps in the rotational motion of liquid water, we studied the rotational dynamics of methane - an apolar, non‑hydrogen-bonding liquid with a spherically symmetric molecule. The reorientational propagator and the ensemble-averaged Legendre polynomial reorientational functions of liquid methane were analysed using molecular dynamics simulations. On the femtosecond timescale, the reorientational motion of methane molecules was found to be characterised by free-rotation gas-like Gaussian decay of the reorientational Legendre polynomials, a result consistent with the available experimental data for liquid methane. On the long timescale (picoseconds) the decay of the reorientational Legendre polynomials was exponential, suggestive of a Debye-like continuous-time rotational diffusion behaviour. However, the ratios of the exponential time constants for different orders of the Legendre polynomials failed to match the rotational-diffusion model. We discuss the implications of these findings for the understanding of rotational motion of molecules in hydrogen-bonding and non‑hydrogen-bonding liquids.

Cold and thermal neutron scattering in liquid water—II: Scattering laws and group constants for H [formula omitted]O and D [formula omitted

On the basis of the cross-section model of liquid H 2 O, which has been developed in the previous paper, cold and thermal neutron scattering in liquid D 2 O is described. The dynamics of liquid molecules, together with their static distribution, is fully taken into account so that various results of neutron experiments and computer simulations are well reproduced especially in terms of coherent scattering like the diffuse Bragg one. The model covers a wide range of neutron incident energies from 0.1 μ eV to 10 eV at many different temperatures between 0 and 100 ∘ C. Double-differential and total scattering cross sections are calculated by the use of a generalized frequency distribution and, equivalently, a velocity auto-correlation function. Furthermore, a set of scattering laws and group constants for liquid H 2 O and D 2 O is generated, which will be useful for research and development of an advanced neutron source and a nuclear power reactor.

Vibrational dynamics in condensed matter probed with linac based FELs

The super-conducting linac pumped free electron laser (FEL) at Stanford has been used to study the vibrational dynamics of molecules in liquids and glasses. A variety of experiments have been performed, including the world's first vibrational photon echo experiments in liquids and glasses and polarization selective pump-probe experiments. In this paper vibrational photon echo experiments are described and characteristics of the FEL important to these experiments are discuss. Vibrational photon echo experiments were performed on the asymmetric CO stretching mode of tungsten hexacarbonyl (1976 cm−1) in the organic glass dibutylphthalate as a function of temperature using sub-picosecond infrared pulses. The echo decays display pronounced beats and are bi-exponential. The beats and bi-exponential decays arise because the bandwidth of the transform limited FEL pulses exceeds the vibrational anharmonicity, leading to the excitation and dephasing of a multilevel coherence. From the beat frequency, the anharmonicity is determined to be 14.7 cm−1. From the bi-exponential decay components, the temperature-dependent homogeneous vibrational dephasing rates of both the v = 0 → 1 and v = 1 → 2 transitions are determined.

Fluctuations in Potential and Dynamics of Molecules in Liquids

Abstract Results from molecular dynamics of liquid argon and far-infrared spectroscopy of liquid carbon dioxide are compared with those based on a model of a harmonic oscillator undergoing Brownian motion with fluctuations in the spring constant. It is found that both data are fitted well when the friction constant is set zero, indicating that the irreversibility of dynamical behaviour of the liquids arises from fluctuations in potential rather than collisions.

Time evolution on non-adiabatic conformational change. The high-viscosity regime

The role of solvent viscosity on the dynamics of molecules in liquids has been studied when the molecules relax through a non-adiabatic conformational change. Although the formalism is valid for any physical situation, as a first step, only the strong overdamped case has been analyzed here. The approach is analogous to the one previously developed by us to evaluate the rate constant of isomerization.

ESR linewidths in solution: anomalous behaviour in hydrogen-bonded liquids, anisotropic rotational diffusion and non-diffusional motions

ESR linewidths have been used to study the dynamics of molecules in liquids. The correlation times characteristic of both the anisotropic g-values modulated by molecular tumbling (t0) and spin—rotational interactions modulated by collision-induced changes in molecular angular momentum (tSR) are considered If isotropic rotational diffusion is assumed, a minimum linewidth as a function of temperature is predicted. Furthermore, r can be set proportional to r3,, where depends upon intermolecular torques and forces and r is the translational diffusion radius, and in many cases is independent of temperature for a given solvent; similarly, in rotational diffusion theory, r =(J/6kT) where 1 is the moment of inertia. The predicted and measured minima in linewidths often differ from each other, and the discrepancy can be attributed either to anisotropic diffusion or, equally well, to the inadequacy of the expression above for tSR. Studies have been carried out for (n-C4H9)4N NiS4C4(CN)4 in n-butanol, and in CHC13. If anisotropic rotational diffusion is assumed, the discrepancy in the linewidth minimum can be explained if (D1/D1) 2.2 in both butanol and CHC13. To correct the relationship between z8 and TSR' one can show on the basis of Ivanov's jump model that_rQrSR = (I/6kT) c(1 — 2Y' where is a mean rotational jump and )L2 is a function of a and the spread in jump angles. The value (0.81) of (1 — )2) 1, required to correct the Iinewidth minimum, indicates small jumps with a large spread in jump angles. If (D1/D1) and c(l — are taken to be temperature independent, x appears to be temperature dependent in butanol but not in CHC13; however, if c(1 — 2) is temperature dependent in butanol, could be temperature independent.

Rotational and Translational Diffusion in Complex Liquids

In two earlier papers, a theoretical and experimental study of the dynamics of molecules in liquids or solids by the neutron-scattering technique was reported. Special attention was given to the quasi-elastically scattered neutron line, the shape and width of which contains considerable information regarding the details of the diffusive process, its origin and nature. It was shown that the origin and formation of the quasielastic peak may be understood if the molecular motion is divided into two partial motions: (1) the motion of the center of gravity, which might be diffusive or vibratory; (2) the motion relative to the center of gravity performed by the molecular constituents, of which the protons are the most important for the case of neutron scattering (this relative motion may be of rotational, hindered rotational, or vibrational nature). In the present work the basic ideas developed in the earlier papers are evolved further. A model for the relative motion of the molecule is defined, and a neutron cross section based on this model is worked out and applied to the neutron-scattering results on glycerol and $n$-propanol. It is assumed that the molecule performs a partially hindered rotation around an effective center of gravity so that each proton may be visualized as performing a vibrational motion around a center which diffusing on the surface of a sphere. The diffusive motion might occur in small steps or in large jumplike steps. The resulting cross section for quasielastic scattering may approximately be described as a Lorentzian line with a full width at half-maximum that is a function of various parameters, such as the true self-diffusion coefficient ${D}_{\mathrm{e}.\mathrm{g}.}$, describing the motion of the molecular center of gravity, and a relative diffusion coefficient ${D}_{\mathrm{rel}}$, describing the proton motion on the surface of the sphere. The relative diffusion coefficient has three components: one each due to small and to large displacements of the protons, corresponding to small and large changes of the molecular orientational angles, respectively; and one due to vibration. It is possible to fit the calculated widths to the observed ones over the entire temperature range investigated for the two liquids, and for the momentum ranges covered in the present studies. This leads to numerical values for the various parameters entering the theory. The values obtained for the various mean lifetimes for the motions are compared with relaxation times obtained from dielectric and ultrasonic data, and certain connections are established. The neutron experiments indicate that the true molecular diffusive motion is a mixed translational-rotational-vibrational displacement. The present work is limited to a study of such systems (complex liquids of not too low viscosity, or solids) in which the rotations are more or less strongly hindered. The consequence of the various neutron observations and other evidences is that the velocity autocorrelation for translational as well as rotational motion is strongly nonexponential for shorter times. The consequences of this for the interpretation of nuclear-magnetic-resonance, dielectric, and ultrasonic data are discussed.