Rotational Motion Of Molecules Research Articles

Rotational Dynamics of Discoid Colloidal Particles in Attractive Quasi-Two-Dimensional Plastic Crystals.

Plastic crystals formed from anisotropic molecules or particles are an important state of matter characterized by the presence of long-range positional order and the lack of long-range orientational order. The rotational motion of molecules or particles in plastic crystals is the most attractive characteristic of the system. Here the rotational dynamics of the discoid particles in quasi-two-dimensional colloidal plastic crystals stabilized via depletion interactions are quantitatively studied using time-resolved confocal microscopy. The measured probability distribution of particle orientation reveals the existence of a strong coupling between the lattice symmetry and particle rotation, resulting in anisotropic rotational dynamics modes resembling the underlying hexagonal crystalline symmetry. Furthermore, the orientational distribution function provides information about the potential surface of rotational dynamics. The observed slow rotational diffusion can be attributed to the presence of orientational minima and potential barriers on the potential surface. Our findings with a real experimental system provide important insights into the role of attraction in the phase behaviors of plastic crystals.

On the Question of the Relationship between the Rotational Motion of Molecules and the Alternation of Properties in Homological Series

In the paper the emphasis is placed on attempt to describe the effect of alternation of melting points in homologous series using the example of melting points of alkanes of normal structure from the frame of reference of the concept of rotational motion of molecules in a liquid. In the existing literature, the presence of separate sequences for the melting points of even and odd homologues is usually explained by packing effects, in which, due to different symmetry, even and odd representatives crystallize with different densities of the crystal lattice. However, this behavior can also be explained by the difference in the entropies of the rotational motion of molecules in the liquid phase, which is observed due to the different positions of the axis of rotation for even and odd homologues. The theoretically predicted difference in the moments of inertia for molecules with an even and an odd number of carbon atoms in the chain was confirmed by calculating the moments of inertia of n-alkane molecules, for which the atomic coordinates were optimized using density functional theory calculations

Hydrogen bonds and dynamics of liquid water and alcohols

Using modified Arrhenius approximations, the activation energies of water, alcohols, and hexane structure rearrangement reactions responsible for the temperature dependences of their dynamic and dielectric characteristics were determined. The interactions of van der Waals and charged centers of water and alcohol molecules regulate the translational and rotational motion of molecules, ensuring the coordination and balance of the thermal effects of exothermic and endothermic rearrangement reactions in local structure of hydrogen bonding network. The long-range action of fluctuating dipoles of hydrogen bonds and their resonant excitation by thermal energy underlies the anomalies in the temperature dependences of water properties and initiates its phase transitions at points 273 K and 298 K. The deviation of the molecular dynamics of water from the Arrhenius and Stokes-Einstein equations in the range of 273–298 K was associated with a high contribution of the collective dynamics of the ice-like phase of water consisting of a network of hydrogen bonds structured by hexagonal clusters of Ih-ice.

Using the molecular rotational motion concept to predict the volume expansion coefficients of liquids

ABSTRACT The rotational motion of molecules in the liquid phase has been shown to be one of the main factor that determines the liquid’s thermal expansion and can be used for its calculation based on an expression for the partition function of the rotational motion of ideal gas molecules, taking into account the contribution of intermolecular interactions. This study establishes that the thermal expansion temperature coefficients are grouped in sequences depending on the functional class of compounds. The general hydrocarbon sequence breaks down into separate sequences according to their functional groups, such as saturated or aromatic hydrocarbons. Thus, it is shown a correlation between the thermal expansion of liquids with an average size of molecules, at temperatures well below the boiling point, and the entropy of the rotational motion which is a function of the moments of inertia of molecules and temperature.

An approach for predicting intracrystalline diffusivities and adsorption entropies in nanoporous crystalline materials

AbstractConfinement of molecules in nanoporous crystalline materials results in the unique and diverse characteristics of intracrystalline diffusion and adsorption, which can significantly affect the efficiency of gas separation and/or catalysis. However, understanding the interplay between confinement and intracrystalline diffusion and adsorption remains elusive at the quantitative level. In this work, it is found that the intracrystalline diffusion could be related to the hopping rate, which might be further connected to the translational and rotational motion of molecules and quantified by corresponding partition functions. Based on this analysis, the correlations capable of predicting the intracrystalline diffusivity and the adsorption entropy are developed. It is shown that the correlations can well capture the experimental and simulation results of more than 20 frameworks, including zeolites and MOFs, for a wide range of guest molecules. This approach can potentially serve as rapid screening tool for nanoporous crystalline materials in gas separation and catalysis.

On the similarity of the self-diffusion and shear viscosity coefficients in low-molecular liquids

We discuss the similarity of the self-diffusion and kinematic shear viscosity coefficients for low-molecular liquids. The rotational motion of molecules is the main prerequisite of this similarity. Due to it, the interparticle potentials are selves-averaged and they become argon-like. The applicability of the van der Waals equation of state for low-molecular liquids is one of the important consequences of this fact. Taking into account that as rule the period of rotational motion is essentially smaller in comparison with the characteristic time needed for a change of molecular configuration formed by the nearest neighbors, we conclude that the similarity should also take place for mentioned kinetic coefficients. We illustrate the manifestation of the argon-like similarity for atomic liquids of argon type, dumbbell a disk-like liquid of nitrogen and benzene types, water, and other associated liquids. At that, liquid states of associated liquids are divided into two regions: one of which adjoins to the critical point and corresponds to argon-like translational thermal motion, and the second includes the triple point and the rotational motion of molecules has activation character in it. In this region, the similarity relations also take place although they have another character in comparison with that for argon-like liquids. The inapplicability of standard activation representations for low-molecular liquids is in details discussed. The careful comparison with experimental data is carried out.

Structure, polarity, dynamics, and vibrational spectral diffusion of liquid–vapour interface of a water–methanol mixture from first principles simulation using dispersion corrected density functional

The effects of dispersion interaction on the structural, dipolar and dynamical properties of liquid–vapour interface of an aqueous solution of methanol have been investigated through first principles simulations. A description of structural properties including the inhomogeneous density profiles of molecules, hydrogen bond distributions and orientational profiles are presented in detail. Among the dynamical properties, diffusion, orientational relaxation, hydrogen bond dynamics and vibrational spectral diffusion of molecules are determined. The variation of the polarity of the molecules from bulk to the interface are also calculated. The current first principles simulation results are compared with the results of dispersion uncorrected study. The dispersion corrected study predicts an increase in the number of molecules in their neighborhood, which are held together by non-hydrogen-bonded dispersion interactions. This results in the faster relaxation of hydrogen bonds, diffusion and rotational motion of molecules in comparison to the dispersion uncorrected study. Furthermore, the effects of dispersion correction on the dynamics of vibrational frequency fluctuations are emphasized, which clearly captures the dynamics of hydrogen bond fluctuations in the respective regions.

Directionally tunable and mechanically deformable ferroelectric crystals from rotating polar globular ionic molecules.

Ferroelectrics are used in a wide range of applications, including memory elements, capacitors and sensors. Recently, molecular ferroelectric crystals have attracted interest as viable alternatives to conventional ceramic ferroelectrics because of their solution processability and lack of toxicity. Here we show that a class of molecular compounds-known as plastic crystals-can exhibit ferroelectricity if the constituents are judiciously chosen from polar ionic molecules. The intrinsic features of plastic crystals, for example, the rotational motion of molecules and phase transitions with lattice-symmetry changes, provide the crystals with unique ferroelectric properties relative to those of conventional molecular crystals. This allows a flexible alteration of the polarization axis direction in a grown crystal by applying an electric field. Owing to the tunable nature of the crystal orientation, together with mechanical deformability, this type of molecular crystal represents an attractive functional material that could find use in a diverse range of applications.

Analysis of specific auxetic properties of fullerite С60

For the first time the negative values of the Poisson ratios have been calculated and the auxetic directions of fullerite have been determined. The values of the Poisson ratios are directed along the crystallographic directions of type, which is indicative of the non-axial auxeticity. It has been established that with a rise in temperature from 100 K to 170 K, the degree of auxeticity is reduced in proportion to reduction of elastic anisotropy factor . The auxeticity of fullerite disappears completely in the temperature range of , the anisotropy remaining practically constant (А = 2.2). At the degree of auxeticity again quickly grows in proportion to a rapid growth of anisotropy А. The revealed regularity is typical for crystals of all types of cubic crystal system: increase in the degree of auxeticity close to phase transition points. Such abnormal behaviour of and has been explained on the basis of analysis of thermodynamic properties of fullerite . In the high-temperature region , due to thermal excitation of the rotational degrees of freedom, the chaotic rotation of molecules is increased, providing for the energy efficiency and stability of a closely packed face-centered cubic structure. It has been established that orientation glass-ordered phase transition is caused by a jump-like change in crystal lattice period of fullerite , leading to increase in the degree of auxeticity close to . The mechanisms and regularities of auxeticity formation at points of phase transitions and have been revealed. It has been established that the Gruneisen parameter which is a degree of shift of crystal vibration spectrum frequencies due to a change in the volume is more responsive to crystal lattice deformations. The method proposed here for determination of abnormal deformations with a change in rotational motion of molecules makes it possible not only to determine the appearance of auxetic properties, but also to get their quantitative value – the Poisson ratio in certain crystallographic directions ( at ).

A molecular propeller effect for chiral separation and analysis.

Enantiomers share nearly identical physical properties but have different chiral geometries, making their identification and separation difficult. Here we show that when exposed to a rotating electric field, the left- and right-handed chiral molecules rotate with the field and act as microscopic propellers; moreover, owing to their opposite handedness, they propel along the axis of field rotation in opposite directions. We introduce a new molecular parameter called hydrodynamic chirality to characterize the coupling of rotational motion of a chiral molecule into its translational motion and quantify the direction and velocity of such motion. We demonstrate >80% enrichment level of counterpart enantiomers in solution without using chiral selectors or circularly polarized light. We expect our results to have an impact on multiple applications in drug discovery, analytical and chiral chemistry, including determination of absolute configuration, as well as in influencing the understanding of artificial and natural molecular systems where rotational motion of the molecules is involved.

Isochoric thermal conductivity of the “plastic” phase of cyclic hydrocarbons. Thiophene

Isochoric thermal conductivity of solid thiophene is measured in “plastic” phase I, on three samples with different molar volumes at temperatures ranging from 170 K to the melting point. The thermal conductivity increased with temperature for all samples. We conducted a comparison with the isochoric thermal conductivity of other cyclic hydrocarbons in “plastic” phases. It is shown that the observed effect correlates to the changes in the magnitude of the NMR second moment in the “plastic” phases, and, respectively, with the degree of disinhibition in the rotational motion of molecules.

Time-domain spectroscopy in the mid-infrared.

When coupled to characteristic, fingerprint vibrational and rotational motions of molecules, an electromagnetic field with an appropriate frequency and waveform offers a highly sensitive, highly informative probe, enabling chemically specific studies on a broad class of systems in physics, chemistry, biology, geosciences, and medicine. The frequencies of these signature molecular modes, however, lie in a region where accurate spectroscopic measurements are extremely difficult because of the lack of efficient detectors and spectrometers. Here, we show that, with a combination of advanced ultrafast technologies and nonlinear-optical waveform characterization, time-domain techniques can be advantageously extended to the metrology of fundamental molecular motions in the mid-infrared. In our scheme, the spectral modulation of ultrashort mid-infrared pulses, induced by rovibrational motions of molecules, gives rise to interfering coherent dark waveforms in the time domain. These high-visibility interference patterns can be read out by cross-correlation frequency-resolved gating of the field in the visible generated through ultrabroadband four-wave mixing in a gas phase.

Buffer gas cooling of polyatomic ions in rf multi-electrode traps

Cooling all degrees of freedom of a molecule, a cluster, or even a nanoparticle which is suspended in a vacuum, is an experimental challenge. Without suitable schemes, cold or ultracold chemical reactions are not feasible. Methods such as laser based preparation of very slow atoms, decelerating molecules to low velocities with electric fields or freezing molecular ions into Coulomb crystals, are generally not suitable to cool the vibrational or rotational motion of molecules. This contribution describes a new method in which a beam of slow atoms or molecules (H, He, H2, or D2) is used for cooling charged particles confined in a multi-electrode rf trap. For reaching sub-K temperatures, the fast part of a cold effusive beam is removed with a shutter before the slow remaining neutrals interact with the ion cloud. The development of a pulsed cold beam source is discussed as well as suitable methods for determining the ion temperature. A challenging application is to prepare internally cold CH5+ for spectroscopy or chemistry. New experimental results for hydrogen abstraction in collisions with slow H atoms are reported at energies of a few meV. For evaluating these measurements and for predicting effective rate coefficients at lower energies, the kinematic conditions of the slow neutral beam-ion trap arrangement have been analyzed in detail. The potential of cooling ions such as protonated methane or H3+ with slow energy selected H atoms is briefly mentioned. An interesting process is the formation of weakly bound ions such as H4+ or CH6+ via radiative or ternary association. Such ions are ideal candidates for preparing the corresponding collision complexes very close (microeV) to the dissociation continuum using infrared transitions.

Screwlike order, macroscopic chirality, and elastic distortions in high-density DNA mesophases

We investigate a new screwlike liquid-crystalline ordering in solutions of helical biopolymers and its influence on the state of individual molecules. In the resulting mesophase translational and rotational motions of molecules are coupled in screw fluctuations. We show that in contrast to the case of conventional chiral liquid crystals the elastic distortion does not twist the screw order but leads to overwinding of individual helical molecules. This explains the peculiarities of high-density DNA mesophases.

Concentration dependences of the transport coefficients of rod-like molecules in narrow slit-shaped pores

The concentration dependences of the label transport and shear viscosity coefficients for rod-like molecules in slit-shaped pores were studied. The calculations were carried out using the lattice gas model, which describes a broad range of fluid concentrations (from the gaseous to the liquid state) and temperatures (including the critical region). In the calculation of the local distributions of mixture components in the equilibrium states, lateral interactions were taken into account. The translational and rotational motions of molecules were described in terms of the transition state theory for nonideal reaction systems, which took into account the influence of neighboring molecules on the height of the activation barrier. The model equations reflect the pronounced anisotropy of the distribution of system components along the normal to the pore wall surface and ordering effects of molecules along various directions.

Approximation of contacts for calculating the velocity of the thermal motion of rodlike molecules in narrow slitlike pores

Expressions for calculating the thermal velocities of the motion of rodlike molecules in slitlike pores were obtained within the framework of a lattice gas model valid over a wide range of fluid densities (from rarefied gases to liquids) and temperatures, including the critical region. The translational and rotational motion of molecules was described using the transition-state theory for nonideal reaction systems, which takes into account the effect of the neighboring molecules on the activation barrier height. The local distributions of the components of the mixture under equilibrium conditions were calculated by describing lateral interactions in the approximation of isolated contacts. The equations of the model reflect the fact that the distributions of the components in the direction perpendicular to the pore walls (due to the effect of adsorption forces) and along the pore axis (if capillary condensation occurs) exhibit a strong anisotropy.

Rho-axis-system Hamiltonian for molecules with one large amplitude internal motion

The rho-axis system and the rho-axis-system Hamiltonian were devised and formulated to describe the rotational–internal–rotational motion of molecules possessing certain local geometrical symmetries. Nevertheless, they were employed to molecules without the required symmetries and apparently lying outside the range of their applicability. To justify such applications and to facilitate first-principles calculation of the corresponding spectroscopic parameters, the rho-axis-system and the rho-axis system Hamiltonian must be derived without assuming or imposing symmetry constraints. A derivation satisfying this requirement is described, and the rho-axis method is extended to molecules having a large amplitude internal motion of any kind.

Smoluchowski's problem for molecular gases taking the accommodation coefficients of the translational and rotational energy of the molecules into account

The effect of the accommodation coefficients of the energy of translational and rotational motion of molecules on the temperature jump in the classical Smoluchowski problem is investigated. Using the extension of the well-known kinetic BKW (Boltzmann-Krook-Welander) equation to the case of molecular gases, an analytical solution of this problem is obtained for diatomic and polyatomic gases taking the rotational degrees of freedom into account. A numerical analysis of the results obtained is carried out.

Manifestation of the collective effects in the rotational motion of molecules in liquids

The Lagrange approach to the theory of thermal hydrodynamic fluctuations is developed. The rotational motion of a Lagrange particle is studied in detail. The apparent expression for the angular velocity autocorrelation function (AVACF) at arbitrary times is derived. The obtained results are used to explain the specificity of the behaviour of the AVACF for a molecule. The collective contribution to the rotational self-diffusion coefficient as well as the manifestation of collective effects in the frequency dispersion of the dielectric permittivity are discussed. The peculiarities of the AVACF for two-dimensional liquids are considered.

HREELS study of C 70 molecules adsorbed on a Si(1 1 1)-(7×7) surface

We have measured the coverage dependence of vibrational excitation spectra of C 70 molecules adsorbed on a Si(1 1 1)-(7×7) surface using high-resolution electron-energy-loss spectroscopy. At the monolayer coverage, the intensity of the 57 meV peak increases, and those of the 83 and 178 meV peaks decrease. Taking into account the dipole selection rule, the change in intensity of the 57 meV peak indicates that the average angle between the long axes of C 70 molecules and surface normal is about 40°. The decreases in intensities of the 83 and 178 meV peaks suggest that the rotational motion of molecules is quenched upon adsorption. We will discuss the Coriolis interaction between the accidentally degenerate A 2″ and E 1′ modes.