Dipole Moment Autocorrelation Function Research Articles

Probing the mechanism of salts destroying the cage structure of methane hydrate by molecular dynamics simulation

Much attention has been generated in the exploitation of natural gas hydrate by injecting brine due to its low cost and eco-friendliness. However, it is not clear how inorganic salts destroy the cage structure of methane hydrate. This work focuses on the mechanism of inorganic salts destroying the cage structure of methane hydrate by molecular simulation. And the influence of cation type on methane hydrate decomposition is studied in detail. The results show that inorganic salts can effectively reduce the temperature required for methane hydrate decomposition. And inorganic salts can weaken the interaction between water molecules by reducing the dipole moment of water molecules in methane hydrate, then the cage structure of methane hydrate is destroyed. Methane hydrate both can completely decompose without and with CaCl2 solution at 307 K and 20 MPa, but the decrease rate of the dipole moment autocorrelation function (DACF) of water molecules in the system with CaCl2 solution is 6.10 times than that without CaCl2 solution, and the decomposition rate of methane hydrate increased by 5.8 times because of adding CaCl2 solution. In addition, the increasing charge and radius of cations result in greater methane hydrate decomposition which is caused by the faster destruction speed of cage structure, the faster rate of methane molecules escaping from cage structure and the faster diffusion velocity of cations are demonstrated. This study provides a theoretical basis for the efficient exploitation of methane hydrates by brine injection.

Conformational diversity of the THF molecule in N2 matrix by means of FTIR matrix isolation experiment and Car-Parrinello molecular dynamics simulations.

Tetrahydrofuran (THF) is a widely used chemical compound, in particular as a solvent in organic and inorganic synthesis. The THF molecule has also an interesting property, namely, undergoes pseudorotation, similar to the case of the cyclopentane. Low energy difference between the envelope (Cs symmetry) and twisted (C2 symmetry) conformations of the THF molecule leads to the interconversion between the two conformers. We study the influence of the molecular environment (N2) on the Cs-C2 equilibrium of tetrahydrofuran in the THF@N2 system utilizing nitrogen matrix isolation infrared spectroscopy. We observe a different ratio between envelope (Cs) and twisted (C2) conformations with respect to a change of the temperature. FTIR experimental studies are supported by the results of the static density functional theory calculations and Car-Parrinello molecular dynamics simulations. We focus on the dynamics of the pseudorotation process, in particular, the lifetime of the THF conformations and their mutual rearrangements. On the basis of the THF@N2 matrix model, with explicit nitrogen molecules, the anharmonic infrared spectra are generated from the Fourier transformation of the dipole moment autocorrelation function.

Semiempirical Atom-centered Density Matrix Propagation Approach to Temperature-dependent Vibrational Spectroscopy of Irinotecan

In the present study, a molecular dynamics study of irinotecan molecule with the atom-centered density matrix propagation scheme was carried out at AM1 semiempirical level of theory, at series of different temperatures, ranging from 5 K to 300 K. Molecular dynamics simulations were performed within the NVE ensemble, initially injecting (and redistributing among the nuclei) various amounts of nuclear kinetic energies to achieve the desired target temperatures. Subsequently to initial equilibration phase of 2 ps, productive simulations were carried out for 8 ps. The accuracy of simulations and the closeness of the generated trajectory to those at the Born-Oppenheimer surface were carefully followed and analyzed. To compute the temperature-dependent rovibrational density of states spectra, the velocity-velocity autocorrelation functions were computed and Fourier-transformed. Fourier-transformed dipole moment autocorrelation functions were, on the other hand, used to calculate the temperature-dependent infrared absorption cross section spectra. The finite-temperature spectra were compared to those computed by a static approach, i.e. by diagonalization of mass-weighted Hessian matrices at the minima located on the potential energy surfaces. Thermally-induced spectral changes were analyzed and discussed. The advantages of finite-temperature statistical physics simulations based on semiempirical Hamiltonian over the static semiempirical ones in the case of complex, physiologically active molecular systems relevant to intermolecular interactions between drugs and drug carriers were pointed out and discussed.

Strategy for Modeling the Infrared Spectra of Ion-Containing Water Drops.

Hydrated ions are ubiquitous in environmental and biological media. Understanding the perturbation exerted by an ion on the water hydrogen bond network is possible in the nanodrop regime by recording vibrational spectra in the O-H bond stretching region. This has been achieved experimentally in recent years by forming gaseous ions containing tens to hundreds of water molecules and recording their infrared photodissociation spectra. In this paper, we demonstrate the capabilities of a modeling strategy based on an extension of the AMOEBA polarizable force field to implement water atomic charge fluctuations along with those of intramolecular structure along the dynamics. This supplementary flexibility of nonbonded interactions improves the description of the hydrogen bond network and, therefore, the spectroscopic response. Finite temperature IR spectra are obtained from molecular dynamics simulations by computing the Fourier transform of the dipole moment autocorrelation function. Simulations of 1-2 ns are required for extensive sampling in order to reproduce the experimental spectra. Furthermore, bands are assigned with the driven molecular dynamics approach. This method package is shown to compare successfully with experimental spectra for 11 ions in water drops containing 36-100 water molecules. In particular, band frequency shifts of the free O-H stretching modes at the cluster surface are well reproduced as a function of both ion charge and drop size.

Structure, spectroscopy, and dynamics of the phenol-(water)2 cluster at low and high temperatures.

Aqueous solutions are complex due to hydrogen bonding (HBing). While gas-phase clusters could provide clues on the solution behavior, most neutral clusters were studied at cryogenic temperatures. Recent results of Shimamori and Fujii provide the first IR spectrum of warm phenol-(H2O)2 clusters. To understand the temperature (T) effect, we have revisited the structure and spectroscopy of phenol-(H2O)2 at all T. While older quantum chemistry work concluded that the cyclic isomers are the most stable, the inclusion of dispersion interactions reveals that they are nearly isoenergetic with isomers forming π-HBs with the phenyl ring. Whereas the OH-stretch bands were previously assigned to purely local modes, we show that at low T they involve a concerted component. We have calculated the (static) anharmonic IR spectra for all low-lying isomers, showing that at the MP2 level, one can single out one isomer (udu) as accounting for the low-T spectrum to 3 cm-1 accuracy. Yet no isomer can explain the substantial blueshift of the phenyl-OH band at elevated temperatures. We describe the temperature effect using ab initio molecular dynamics with a density functional and basis-set (B3LYP-D3/aug-cc-pVTZ) that provide a realistic description of OH⋯O vs. OH⋯π HBing. From the dipole moment autocorrelation function, we obtain good description for both low- and high-T spectra. Trajectory visualization suggests that the ring structure remains mostly intact even at high T, with intermittent switching between OH⋯O and OH⋯π HBing and lengthening of all 3 HBs. The phenyl-OH blueshift is thus attributed to strengthening of its OH bond. A model for three beads on a ring suggests that this shift is partly offset by the elimination of coupling to the other OH bonds in the ring, whereas for the two water molecules these two effects nearly cancel.

Molecular dynamics study of the role of water molecules in glass transition of amorphous sugars

ABSTRACTThe glass transition temperature (Tg) is an important property affecting the quality and shelf life of various food products and pharmaceuticals. For example, amorphous sugars, i.e., sugars in the glassy state, have an effect on the thermal stabilization of proteins (enzymes). This stabilizing effect is lost or strongly decreased if glass transition occurs and sugars crystallize. It is therefore important to know the characteristics of the glassy state of sugars, in particular, the requirements to maintain this state and the factors affecting this state and variations in Tg. Although Tg is dependent on the water content of amorphous sugars, the microscopic role of water molecules in glass transition is unclear. These microscopic features were assessed by atomistic molecular dynamics simulation. The rotational reorientation time, τ, a type of relaxation time derived from the dipole moment autocorrelation function, was calculated for water molecules. τ was found to predict Tg values, which were in good agreement with the corresponding experimental values.

Complete Assignment of the Infrared Spectrum of the Gas-Phase Protonated Ammonia Dimer.

The infrared (IR) spectrum of the ammoniated ammonium dimer is more complex than those of the larger protonated ammonia clusters due to close-lying fundamental and combination bands and possible Fermi resonances (FR). To date, the only theoretical analysis involved partial dimensionality quantum nuclear dynamic simulations, assuming a symmetric structure (D3d) with the proton midway between the two nitrogen atoms. Here we report an extensive study of the less symmetric (C3v) dimer, utilizing both second order vibrational perturbation theory (VPT2) and ab initio molecular dynamics (AIMD), from which we calculated the Fourier transform (FT) of the dipole-moment autocorrelation function (DACF). The resultant IR spectrum was assigned using FTed velocity autocorrelation functions (VACFs) of several interatomic distances and angles. At 50 K, we have been able to assign all 21 AIMD fundamentals, in reasonable agreement with MP2-based VPT2, about 30 AIMD combination bands, and a difference band. The combinations involve a wag or the NN stretch as one of the components, and appear to follow symmetry selection rules. On this basis, we suggest possible assignments of the experimental spectrum. The VACF-analysis revealed two possible FR bands, one of which is the strongest peak in the computed spectrum. Raising the temperature to 180 K eliminated the "proton transfer mode" (PTM) fundamental, and reduced the number of observed combination bands and FRs. With increasing temperature, fundamentals red-shift, and the doubly degenerate wags exhibit larger anharmonic splittings in their VACF bending spectra. We have repeated the analysis for the H3ND(+)NH3 isotopologue, finding that it has a simplified spectrum, with all the strong peaks being fundamentals. Experimental study of this isotopologue may thus provide a good starting point for disentangling the N2H7(+) spectrum.

Ab initio molecular dynamics study for C–Br dissociation within BrH2C–C≡C(ads) adsorbed on an Ag(111) surface: a short-time Fourier transform approach

The reaction dynamics for C–Br dissociation within BrH2C–C≡CH(ads) adsorbed on an Ag(111) surface has been investigated by combining density functional theory-based molecular dynamics simulations with short-time Fourier transform (STFT) analysis of the dipole moment autocorrelation function. Two possible reaction pathways for C–Br scission within BrH2C–C≡CH(ads) have been proposed on the basis of different initial structural models. Firstly, the initial perpendicular orientation of adsorbed BrH2C–C≡CH(ads) with a stronger C–Br bond will undergo dynamic rotation leading to the final parallel orientation of BrH2C–C≡CH(ads) to cause the C–Br scission, namely, an indirect dissociation pathway. Secondly, the initial parallel orientation of adsorbed BrH2C–C≡C(ads) with a weaker C–Br bond will directly cause the C–Br scission within BrH2C–C≡CH(ads), namely, a direct dissociation pathway. To further investigate the evolution of different vibrational modes of BrH2C–C≡CH(ads) along these two reaction pathways, the STFT analysis is performed to illustrate that the infrared (IR) active peaks of BrH2C–C≡CH(ads) such as vCH2 [2956 cm−1(s) and 3020 cm−1(as)], v≡CH (3320 cm−1) and vC≡C (2150 cm−1) gradually vanish as the rupture of C–Br bond occurs and then the resulting IR active peaks such as C=C=C (1812 cm−1), ω-CH2 (780 cm−1) and δ-CH (894 cm−1) appear due to the formation of H2C=C=CH(ads) which are in a good agreement with experimental reflection adsorption infrared spectrum (RAIRS) at temperatures of 110 and 200 K, respectively. Finally, the total energy profiles indicate that the reaction barriers for the scission of C–Br within BrH2C–C≡CH(ads) along both direct and indirect dissociation pathways are very close due to a similar rupture of C–Br bond leading to a similar transition state.

Structural flexibility of the sulfur mustard molecule at finite temperature from Car–Parrinello molecular dynamics simulations

Sulfur mustard (SM) is one of the most dangerous chemical compounds used against humans, mostly at war conditions but also in terrorist attacks. Even though the sulfur mustard has been synthesized over a hundred years ago, some of its molecular properties are not yet resolved. We investigate the structural flexibility of the SM molecule in the gas phase by Car–Parrinello molecular dynamics simulations. Thorough conformation analysis of 81 different SM configurations using density functional theory is performed to analyze the behavior of the system at finite temperature. The conformational diversity is analyzed with respect to the formation of intramolecular blue-shifting CH⋯S and CH⋯Cl hydrogen bonds. Molecular dynamics simulations indicate that all structural rearrangements between SM local minima are realized either in direct or non-direct way, including the intermediate structure in the last case. We study the lifetime of the SM conformers and perform the population analysis. Additionally, we provide the anharmonic dynamical finite temperature IR spectrum from the Fourier Transform of the dipole moment autocorrelation function to mimic the missing experimental IR spectrum.

Thermal efficiency of the principal greenhouse gases

Atmospheric gases are ranked according to the efficiency with which they absorb and radiate longwave radiation. The open international HITRAN database of gaseous absorption lines of high resolution together with inverse Fourier transform were used. The autocorrelation functions of the total dipole moment of the basic greenhouse gases molecules such as H2O, CO2, O3, N2O, and CH4 were obtained. Absorption coefficient spectra and emission power spectra of infrared radiation of these gases were calculated. Analysis of the emissive ability of all gases under consideration was carried out. Compared to CO2, all the gases under investigation have more effective emission except ozone. An efficiency criterion of IR absorption and emission is defined and is calculated for each studied gas, and the gases are ranked accordingly as follows (from strong to weak): H2O, CH4, CO2, N2O, and O3.

Sensitivity of polarization fluctuations to the nature of protein-water interactions: study of biological water in four different protein-water systems.

Since the time of Kirkwood, observed deviations in magnitude of the dielectric constant of aqueous protein solution from that of neat water (∼80) and slower decay of polarization have been subjects of enormous interest, controversy, and debate. Most of the common proteins have large permanent dipole moments (often more than 100 D) that can influence structure and dynamics of even distant water molecules, thereby affecting collective polarization fluctuation of the solution, which in turn can significantly alter solution's dielectric constant. Therefore, distance dependence of polarization fluctuation can provide important insight into the nature of biological water. We explore these aspects by studying aqueous solutions of four different proteins of different characteristics and varying sizes, chicken villin headpiece subdomain (HP-36), immunoglobulin binding domain protein G (GB1), hen-egg white lysozyme (LYS), and Myoglobin (MYO). We simulate fairly large systems consisting of single protein molecule and 20000-30000 water molecules (varied according to the protein size), providing a concentration in the range of ∼2-3 mM. We find that the calculated dielectric constant of the system shows a noticeable increment in all the cases compared to that of neat water. Total dipole moment auto time correlation function of water ⟨δMW(0)δMW(t)⟩ is found to be sensitive to the nature of the protein. Surprisingly, dipole moment of the protein and total dipole moment of the water molecules are found to be only weakly coupled. Shellwise decomposition of water molecules around protein reveals higher density of first layer compared to the succeeding ones. We also calculate heuristic effective dielectric constant of successive layers and find that the layer adjacent to protein has much lower value (∼50). However, progressive layers exhibit successive increment of dielectric constant, finally reaching a value close to that of bulk 4-5 layers away. We also calculate shellwise orientational correlation function and tetrahedral order parameter to understand the local dynamics and structural re-arrangement of water. Theoretical analysis providing simple method for calculation of shellwise local dielectric constant and implication of these findings are elaborately discussed in the present work.

Temperature effects on adsorption and diffusion dynamics of CH3CH2(ads) and H3C–C≡C(ads) on Ag(111) surface and their self-coupling reactions: Ab initio molecular dynamics approach

Density functional theory (DFT)-based molecular dynamics (DFTMD) simulations in combination with a Fourier transform of dipole moment autocorrelation function are performed to investigate the adsorption dynamics and the reaction mechanisms of self-coupling reactions of both acetylide (H3C-C(β)≡C(α) (ads)) and ethyl (H3C(β)-C(α)H2(ads)) with I(ads) coadsorbed on the Ag(111) surface at various temperatures. In addition, the calculated infrared spectra of H3C-C(β)≡C(α)(ads) and I coadsorbed on the Ag(111) surface indicate that the active peaks of -C(β)≡C(α)- stretching are gradually merged into one peak as a result of the dominant motion of the stand-up -C-C(β)≡C(α)- axis as the temperature increases from 200 K to 400 K. However, the calculated infrared spectra of H3C(β)-C(α)H2(ads) and I coadsorbed on the Ag(111) surface indicate that all the active peaks are not altered as the temperature increases from 100 K to 150 K because only one orientation of H3C(β)-C(α)H2(ads) adsorbed on the Ag(111) surface has been observed. These calculated IR spectra are in a good agreement with experimental reflection absorption infrared spectroscopy results. Furthermore, the dynamics behaviors of H3C-C(β)≡C(α)(ads) and I coadsorbed on the Ag(111) surface point out the less diffusive ability of H3C-C(β)≡C(α)(ads) due to the increasing s-character of Cα leading to the stronger Ag-Cα bond in comparison with that of H3C(β)-C(α)H2(ads) and I coadsorbed on the same surface. Finally, these DFTMD simulation results allow us to predict the energetically more favourable reaction pathways for self-coupling of both H3C-C(β)≡C(α)(ads) and H3C(β)-C(α)H2(ads) adsorbed on the Ag(111) surface to form 2,4-hexadiyne (H3C-C≡C-C≡C-CH3(g)) and butane (CH3-CH2-CH2-CH3(g)), respectively. The calculated reaction energy barriers for both H3C-C≡C-C≡C-CH3(g) (1.34 eV) and CH3-CH2-CH2-CH3(g) (0.60 eV) are further employed with the Redhead analysis to estimate the desorption temperatures approximately at 510 K and 230 K, respectively, which are in a good agreement with the experimental low-coverage temperature programmed reaction spectroscopy measurements.

Temperature dependence of vibrational modes of CH3CC(ads) and I(ads) coadsorbed on Ag(111): Ab initio molecular dynamics approach

Ab initio molecular dynamics simulations accompanied by a Fourier transform of the dipole moment (aligned perpendicular to the surface) autocorrelation function are implemented to investigate the temperature-dependent infrared (IR) active vibrational modes of CH3C(β)C(α)(ads) and I(ads) when coadsorbed on an Ag(111) surface at 200 and 400 K, respectively. The analytic scheme of the Fourier transform of a structural coordinate autocorrelation function is used to identify two distinguishable IR active peaks of C(β)C(α) stretching, which are characterized by two types of dynamic motion of adsorbed CH3C(β)C(α)(ads) at 200 K, namely, the motion of the tilted CC(β)C(α) axis and the motion of the stand-up CC(β)C(α) axis. These two recognisable IR active peaks of C(β)C(α) stretching are gradually merged into one peak as a result of the dominant motion of the stand-up CC(β)C(α) axis as the temperature increases from 200 to 400 K. The calculated intensities of the IR active peaks of the asymmetrical deformation mode of CH3 and the asymmetrical stretching mode of CH3, with their dynamic dipole moments nearly perpendicular to the CC(β)C(α) axis, become relatively weak; however, the symmetrical deformation mode of CH3 and the symmetrical stretching mode of CH3, with their dynamic dipole moments randomly directed away from the CC(β)C(α) axis, will not have direct correspondence between the intensities of their IR active peaks and the angle between the Ag(111) surface and the CC(β)C(α) axis as the temperature increases from 200 to 400 K. Finally, the increased flipping from the motion of the tilted CC(β)C(α) axis to the motion of the stand-up CC(β)C(α) axis followed by its diffusion, resulting from the increasing temperature from 200 to 400 K or even higher, seems to be the initial event that initiates the alkyne self-coupling reaction that leads to the final production of H3CCCCCCH3.

Amplification of Anharmonicities in Multiphoton Vibrational Action Spectra

The influence of one or several infrared laser pulses on the stability of bare and argon-tagged sodium chloride clusters is investigated theoretically by a combination of computational methods involving explicit molecular dynamics and properly calibrated unimolecular rate theories. The fragmentation spectra obtained by varying the laser frequency in the far-IR range is compared to the linear absorption spectrum resulting from the dipole moment autocorrelation function. Under appropriate laser field parameters, the action spectra are found to resemble the absorption spectra quite accurately in terms of positions, line widths, and even relative intensities. However, the action spectra exhibit residual and systematic redshifts of a few percent, which are partly due to the finite spectral bandwidth but are amplified by the progressive heating by the laser. A quantitative analysis suggests that these anharmonicity effects should generally arise upon multiple photon absorption.

Dielectric Relaxation in Liquid Water: Two Fractions or Two Dynamics?

Dielectric relaxation in liquid water is studied using molecular dynamics (MD) simulations in the temperature range of 240 to 340K at atmospheric pressure. The main dielectric and fast relaxation mode are identified in the spectra of dipole moment autocorrelation functions. The microscopic origin of the fast dielectric relaxation process, which takes place on a time scale of subpicoseconds at room temperature, is discussed. A new hypothesis for the fast dielectric mode is presented. It is based on the assumption of the intrawell rotational relaxation taking place during the waiting period between thermally activated large angle jumps occurring in the course of changing H-bond partners.

Ion–ion and ion–water aggregations and dielectric response of aqueous solutions of Li2SO4: molecular dynamic simulations study

The dielectric properties of Li2SO4 aqueous solutions are studied using the method of molecular dynamic (MD) simulations. The dielectric response is computed for solutions with concentrations between 0.001 and 0.071 mole fraction of salt in the temperature range of 288 to 348 K. Computed values of the static dielectric constant and effective (weighted average) relaxation time are compared with available experimental data. The distributions of dielectric relaxation times are obtained by the decomposition of dipole moment autocorrelation functions (ACF) into a series of single exponentials in the time domain. The distributions of ion–ion and ion–water aggregations in concentrated solutions are computed and their connections to the low frequency modes observed in experimental dielectric spectra are examined. The nature of anomalous (different from single-exponential) dielectric relaxation in Li2SO4 aqueous solutions is discussed.

Computer simulation study of rotational diffusion in polar liquids of different types

Rotational diffusion in liquid acetonitrile, dimethylsulphoxide (DMSO), water, and methanol is studied with molecular dynamics simulations. The effects of hydrogen bonding and local dipole-dipole correlations (Kirkwood g-factor) on the relationship between the single molecule and collective relaxation are examined. The first rank single molecule dipole moment autocorrelation functions (ACFs) are constructed in the molecule-fixed coordinate frame and the principal components of rotation diffusion tensor are reported. Higher rank orientational ACFs are computed. These ACFs, as a rule, are strongly nonexponential (at least not single exponential) at longer times and the decomposition of these functions into a series of single exponentials results in broad distributions of relaxation times, with the broadening being particularly prominent in the case of higher rank ACFs. The rank dependence of characteristic times calculated as weighted averages over the relaxation time distributions does not follow the pattern of small angle (Debye) diffusion model for all liquids studied in this work except methanol. In contradiction, the same rank dependence computed by direct integration of ACFs leads to good agreement with the Debye diffusion model in the case of acetonitrile, DMSO, and water (but not methanol). The linear-angular momentum cross correlation functions are also computed and the effect of rototranslational coupling on reorientaional relaxation at longer times (>1.0 ps) is found to be small.

Finite-temperature infrared spectroscopy of polycyclic aromatic hydrocarbon molecules. II. Principal mode analysis and self-consistent phonons

Following previous work [F. Calvo et al. J. Chem. Phys. 132, 124308 (2010)], infrared spectra of several polycyclic aromatic hydrocarbon molecules are simulated with classical and quantum molecular dynamics trajectories. The interactions are modeled using a tight-binding potential energy surface and quantum delocalization is accounted for using the partially adiabatic centroid and ring-polymer molecular dynamics frameworks, both built upon the path-integral representation. The spectra obtained directly by Fourier transformation of the dipole moment autocorrelation function are here compared with several quasiharmonic approximations that provide additional information about the vibrational modes. A principal mode analysis (PMA) is carried out from the covariance matrix of atomic displacements in classical and quantum trajectories. The method systematically overestimates the line shifts due to anharmonicities, except in the power spectra of atomic displacements, and is not robust in predicting IR intensities for such large molecules. Alternatively, effective normal modes have also been determined by adapting the self-consistent phonon (SCP) theory of condensed matter physics to the present tight-binding model, in both classical and quantum mechanical descriptions. The SCP approximation turns out as semiquantitative in estimating the redshift of tight stretching modes, and performs better for classical systems. More problematic, it predicts that many low- or medium-frequency modes should be blueshifted, in contradiction with the molecular dynamics results. The sets of anharmonic normal modes extracted from the PMA and SCP approaches reveal important mixings within the tightest C-H and C-C stretching modes, which are also manifested on the corresponding power spectra.

Evolution from Surface-Influenced to Bulk-Like Dynamics in Nanoscopically Confined Water

We use molecular dynamics simulations to study the influence of confinement on the dynamics of a nanoscopic water film at T = 300 K and rho = 1.0 g cm(-3). We consider two infinite hydrophilic (beta-cristobalite) silica surfaces separated by distances between 0.6 and 5.0 nm. The width of the region characterized by surface-dominated slowing down of water rotational dynamics is approximately 0.5 nm, while the corresponding width for translational dynamics is approximately 1.0 nm. The different extent of perturbation undergone by the in-plane dynamic properties is evidence of rotational-translational decoupling. The local in-plane rotational relaxation time and translational diffusion coefficient collapse onto confinement-independent "master" profiles as long as the separation d >or= 1.0 nm. Long-time tails in the perpendicular component of the dipole moment autocorrelation function are indicative of anisotropic behavior in the rotational relaxation.

Molecular Dynamics Simulation Study of the Liquid Crystal Phase in a Small Mesogene Cluster (5CB)<sub>22</sub>

The molecular dynamics (MD) technique was used to investigate the nano droplet composed of twenty mesogene molecules 4-cyano-4-n-pentylbiphenyl (5CB). The 5CB molecules were treated as rigid bodies, the intermolecular interaction was taken to be the full site-site pairwise additive Lennard-Jones (LJ) potential plus a Coulomb interaction. The radial distribution functions in the temperature range from 150 to 400 K, were calculated as well as the linear and angular velocity autocorrelation functions. In addition the total dipole moment autocorrelation function and dielectric loss of (5CB)22 mesogene cluster were calculated and the liquid crystal ordering in the nanoscale system was studied up to its vaporization temperature.