Molecular Rotational Dynamics Research Articles

Molecular Structure and Rotational Dynamics in the Acetonitrile:Acetylene (1:2) Plastic Co-crystal at Titan Conditions

Molecular Structure and Rotational Dynamics in the Acetonitrile:Acetylene (1:2) Plastic Co-crystal at Titan Conditions

Electrorheological effect of suspensions containing mixed size of poly(ionic liquid) microspheres

In this paper, the electrorheological (ER) effect of suspensions containing mixed size of poly(ionic liquid) (PIL) microspheres were studied by combining experiment and molecular dynamics simulation. The PIL microspheres with two particle sizes were prepared via a cooling-assisted phase separation method. The PIL microspheres were dispersed in silicone oil to form five kinds of suspensions with different volume ratios of large and small at a fixed particle volume fraction. The ER effect of these suspensions was measured by a rheometer. We established ER suspension models for five suspensions and simulated the rheological curves of the ER suspensions using molecular dynamics-stochastic rotation dynamics method. Both experiment and simulation results showed that the ER effect of suspensions containing mixed size of PIL microspheres was lower than that of suspensions containing uniform size of PIL microspheres. The reasons for the influence of mixed particle size on ER effect were analyzed through the three-dimensional visualization simulation results.

Customized compatibilizer to improve the mechanical properties of polylactic acid/lignin composites via enhanced intermolecular interactions for 3D printing

Polylactic Acid (PLA)/lignin composite shows great potential as a 3D printing material due to its eco-friendliness and cost-effectiveness, but the poor compatibility seriously reduces the mechanical properties, which restricts its wide applications. Herein, a new strategy for concurrently enhancing tensile strength, elongation, and toughness of PLA/Lignin (P/L) composites through intermolecular interactions enhancement with a lignin-based compatibilizer was firstly reported. In detail, the lignin-g-maleic anhydride-g-PLA (LMP) compatibilizer was tailor-made by a two-step method. Mechanical testing results indicated that the tensile strength, elongation, and toughness were increased by 22%, 68%, and 114%, respectively in P/L20 (20% lignin) after adding 5 wt% of LMP, which is superior to the addition of commercial compatibilizer, polyethylene glycol. SEM observation, rotational rheometer testing, DSC analysis, and molecular dynamics (MD) simulation remarkably verified that LMP distinctly increased the intermolecular interaction. In particular, the van der Waals energy of lignin-LMP and LMP-PLA was raised by 15.7% and 60.8% and the electrostatic potential energy was boosted by 11.0% and 60.7%, respectively, based on lignin-PLA. Additionally, P/LMP5/L20 achieved excellent antioxidant properties (91% radical scavenging activity rate), which could be a potential candidate to replace traditional materials for medical applications of 3D printing. Overall, this study proposes a novel approach to develop economical biodegradable composites with great mechanical and extraordinary antioxidant performances for 3D printing.

Fast Dynamics of Difluprednate in Micelles or Swollen-Micelles Revealed by 19F Nuclear Magnetic Resonance Spin Relaxation Rates

Knowledge of molecular rotational dynamics is critical to interpret solution nuclear magnetic resonance (NMR) spectroscopy. The observation of sharp solute NMR signals in micelles contradicted the surfactant viscosity effects noted in the Stokes-Einstein-Debye (SED) equation. Herein, the 19F spin relaxation rates of difluprednate (DFPN) drug dissolved in polysorbate-80 (PS-80) micelles and castor oil swollen micelles (s-micelle) were measured and adequately fit using an isotropic diffusion model based spectral density function. Despite the high viscosity of PS-80 and castor oil, the fitting results revealed fast 4 and 12 ns dynamics of DFPN in both micelle globules. The observation of the fast ns motion in the viscous surfactant/oil micelle phase demonstrated motion decoupling between solute molecules inside micelles and the micelle itself in an aqueous solution. These observations support the role of intermolecular interaction in governing the rotational dynamics of small molecules, versus the viscosity of the solvent molecules as defined in the SED equation.

Density Scaling of Translational and Rotational Molecular Dynamics in a Simple Ellipsoidal Model near the Glass Transition.

In this paper, we show that a simple anisotropic model of supercooled liquid properly reflects some density scaling properties observed for experimental data, contrary to many previous results obtained from isotropic models. We employ a well-known Gay–Berne model earlier parametrized to achieve a supercooling and glass transition at zero pressure to find the point of glass transition and explore volumetric and dynamic properties in the supercooled liquid state at elevated pressure. We focus on dynamic scaling properties of the anisotropic model of supercooled liquid to gain a better insight into the grounds for the density scaling idea that bears hallmarks of universality, as follows from plenty of experimental data collected near the glass transition for different dynamic quantities. As a result, the most appropriate values of the scaling exponent γ are established as invariants for a given anisotropy aspect ratio to successfully scale both the translational and rotational relaxation times considered as single variable functions of densityγ/temperature. These scaling exponent values are determined based on the density scaling criterion and differ from those obtained in other ways, such as the virial–potential energy correlation and the equation of state derived from the effective short-range intermolecular potential, which is qualitatively in accordance with the results yielded from experimental data analyses. Our findings strongly suggest that there is a deep need to employ anisotropic models in the study of glass transition and supercooled liquids instead of the isotropic ones very commonly exploited in molecular dynamics simulations of supercooled liquids over the last decades.

Time-resolved study of recoil-induced rotation by X-ray pump - X-ray probe spectroscopy.

Modern stationary X-ray spectroscopy is unable to resolve rotational structure. In the present paper, we propose to use time-resolved two color X-ray pump–probe spectroscopy with picosecond resolution for real-time monitoring of the rotational dynamics induced by the recoil effect. The proposed technique consists of two steps. The first short pump X-ray pulse ionizes the valence electron, which transfers angular momentum to the molecule. The second time-delayed short probe X-ray pulse resonantly excites a 1s electron to the created valence hole. Due to the recoil-induced angular momentum the molecule rotates and changes the orientation of transition dipole moment of core-excitation with respect to the transition dipole moment of the valence ionization, which results in a temporal modulation of the probe X-ray absorption as a function of the delay time between the pulses. We developed an accurate theory of the X-ray pump–probe spectroscopy of the recoil-induced rotation and study how the energy of the photoelectron and thermal dephasing affect the structure of the time-dependent X-ray absorption using the CO molecule as a case-study. We also discuss the feasibility of experimental observation of our theoretical findings, opening new perspectives in studies of molecular rotational dynamics.

Visualizing coherent molecular rotation in a gaseous medium

Inducing and controlling the ultrafast molecular rotational dynamics using shaped laser fields is essential in numerous applications. Several approaches exist that allow following the coherent molecular motion in real-time, including Coulomb explosion-based techniques and recovering molecular orientation from the angular distribution of high harmonics. We theoretically consider a non-intrusive optical scheme for visualizing the rotational dynamics in an anisotropic molecular gas. The proposed method allows determining the instantaneous orientation of the principal optical axes of the gas. The method is based on probing the sample using ultra-short circularly polarized laser pulses and recording the transmission image through a vortex wave plate. We consider two example excitations: molecular alignment induced by an intense linearly polarized laser pulse and unidirectional molecular rotation induced by a polarization-shaped pulse. The proposed optical method is promising for visualizing the dynamics of complex symmetric- and asymmetric-top molecules.

Time-Dependent Unitary Transformation Method in the Strong-Field-Ionization Regime with the Kramers-Henneberger Picture.

Time evolution operators of a strongly ionizing medium are calculated by a time-dependent unitary transformation (TDUT) method. The TDUT method has been employed in a quantum mechanical system composed of discrete states. This method is especially helpful for solving molecular rotational dynamics in quasi-adiabatic regimes because the strict unitary nature of the propagation operator allows us to set the temporal step size to large; a tight limitation on the temporal step size () can be circumvented by the strict unitary nature. On the other hand, in a strongly ionizing system where the Hamiltonian is not Hermitian, the same approach cannot be directly applied because it is demanding to define a set of field-dressed eigenstates. In this study, the TDUT method was applied to the ionizing regime using the Kramers-Henneberger frame, in which the strong-field-dressed discrete eigenstates are given by the field-free discrete eigenstates in a moving frame. Although the present work verifies the method for a one-dimensional atom as a prototype, the method can be applied to three-dimensional atoms, and molecules exposed to strong laser fields.

High-resolution movies of molecular rotational dynamics captured with ultrafast electron diffraction

This work uses ultrafast electron diffraction to capture a movie of a rotational wavepacket in nitrogen molecules that were impulsively aligned by a femtosecond laser pulse.

Real-time observation of electronic, vibrational, and rotational dynamics in nitric oxide with attosecond soft x-ray pulses at 400 eV

Photoinduced quantum dynamics in molecules have hierarchical temporal structures with different energy scales that are associated with electron and nuclear motions. Femtosecond-to-attosecond transient absorption spectroscopy (TAS) using high-harmonic generation (HHG) with a photon energy below 300 eV has been a powerful tool to observe such electron and nuclear dynamics in a table-top manner. However, comprehensive measurements of the electronic, vibrational, and rotational molecular dynamics have not yet been achieved. Here, we demonstrate HHG-based TAS at the nitrogen K-edge (400 eV) for the first time, and observe all the electronic, vibrational, and rotational degrees of freedom in a nitric oxide molecule at attosecond to sub-picosecond time scales. This method of employing core-to-valence transitions offers an all-optical approach to reveal complete molecular dynamics in photochemical reactions with element and electronic state specificity.

Glass Dynamics and Domain Size in a Solvent-Polymer Weak Gel Measured by Multidimensional Magnetic Resonance Relaxometry and Diffusometry.

Nuclear magnetic resonance measurements of rotational and translational molecular dynamics are applied to characterize the nanoscale dynamic heterogeneity of a physically cross-linked solvent-polymer system above and below the glass transition temperature. Measured rotational dynamics identify domains associated with regions of solidlike and liquidlike dynamics. Translational dynamics provide quantitative length and timescales of nanoscale heterogeneity due to polymer network cross-link density. Mean squared displacement measurements of the solvent provide microrheological characterization of the system and indicate glasslike caging dynamics both above and below the glass transition temperature.

Dynamics of ionic liquids in poly(vinyl alcohol) porous scaffold. Low field NMR study

In this study molecular dynamics of ionic liquids in poly(vinyl alcohol) scaffolds were investigated. The binary poly(vinyl alcohol) – ionic liquid (PVA-IL) compound was prepared from initial solutions of water, ionic liquid (IL) and poly(vinyl alcohol) (PVA) at different concentrations. Subsequently water was evaporated under open conditions, leaving the scaffold/IL system of interest. Low field nuclear magnetic resonance (NMR) relaxation and diffusion measurements, as well as 2D T1-T2 correlated NMR experiments were performed to determine specific local and translational dynamics properties at different time scales. Data suggest that during water evaporation, partial demixing of IL from the polymeric matrix leaves the remaining solvent confined in the porous structure formed by the PVA polymer. The results show that the translational diffusion, as well as the local rotational molecular dynamics is comparable to the bulk liquid state. Moreover, in partial saturation conditions, diffusion shows enhancements relative to the bulk.

Cultural Heritage Studies with Mobile NMR.

Nuclear magnetic resonance (NMR) provides in situ information about selected isotope densities in samples and objects, while also providing contrast through rotational and translational molecular dynamics. These parameters are probed not only in NMR spectroscopy and imaging but also in nondestructive materials testing by mobile stray-field NMR, the unique properties of which are valuable in cultural heritage studies. We present recent progress in the analysis of cultural heritage with mobile 1 H NMR stray-field sensors, for which the detection zone is outside of the NMR magnet. Prominent applications include the analysis of stratigraphies in paintings and frescoes, and the assessment of material states changing under the impact of aging, conservation, and restoration.

Microscopic relationship between colloid–colloid interactions and the rheological behaviour of suspensions: a molecular dynamics-stochastic rotation dynamics investigation

ABSTRACTWe investigate the dependence of the shear viscosity of suspensions of spherical colloids as a function of the volume fraction of the suspension, the colloid–colloid interactions and the shear rate. We couple molecular dynamics to describe the motion of the colloids with stochastic rotation dynamics (MD–SRD) for the fluid environment by means of stochastic collisions, in order to incorporate hydrodynamics effects leading to non-newtonian responses. The shear viscosity is computed using non-equilibrium simulations by imposing explicit velocity gradients. The impact of the colloid–colloid interactions is examined by modelling the inter-colloid pair potential with a repulsive power law, that allows interpolating different degrees of colloidal softness. The general rheological behaviour of our suspensions can be described with a Krieger–Dougherty like equation, which must be corrected to account for the variations in the maximum packing fraction and non-equilibrium effects arising from the flux of momentum imposed to the suspension, which appear when varying the softness of the inter-colloidal interactions. We further show evidence for non-newtonian behaviour at high Péclet numbers, characterised both by shear thinning and shear thickening, and thus demonstrate these effects can be successfully captured using MD–SRD methods.

A hybrid molecular dynamics study on the non-Newtonian rheological behaviors of shear thickening fluid.

To investigate the microstructural evolution dependency on the apparent viscosity in shear-thickening fluids (STFs), a hybrid mesoscale model combined with stochastic rotation dynamics (SRD) and molecular dynamics (MD) is used. Muller-Plathe reverse perturbation method is adopted to analyze the viscosities of STFs in a two-dimensional model. The characteristic of microstructural evolution of the colloidal suspensions under different shear rate is studied. The effect of diameter of colloidal particles and the phase volume fraction on the shear thickening behavior is investigated. Under low shear rate, the two-atom structure is formed, because of the strong particle attractions in adjacent layers. At higher shear rate, the synergetic pair structure extends to layered structure along flow direction because of the increasing hydrodynamics action. As the shear rate rises continuously, the layered structure rotates and collides with other particles, then turned to be individual particles under extension or curve string structure under compression. Finally, at the highest shear rate, the strings curve more severely and get into two-dimensional cluster. The apparent viscosity of the system changes from shear-thinning behavior to the shear-thickening behavior. This work presents valuable information for further understanding the shear thickening mechanism.

Molecular dynamic heterogeneity in relation to free volume and relaxation dynamics in organic glass-formers: oligomeric cis-1,4-poly(isoprene).

Herein, a combined study of the molecular rotation dynamics and free volume in cis-1,4-poly(isoprene) using two external probing techniques via ESR and PALS together with relaxation dynamics of the host medium via BDS is presented. The spectral evolution of the spin probe TEMPO from simulations over a wide range from 100 K up to 300 K exhibits three different regions of its correlation time consisting of a slow regime at low temperatures followed by the molecular dynamic heterogeneity zone from T = T = 155 K = 0.82 × T up to Tc ≅ 236 K = 1.26 × T and ending with a fast regime at high temperatures with the further characteristic ESR temperatures, T = 186 K ≅ T and T = 260 K. These are in close coincidence with four characteristic PALS temperatures: T = 160 K, T = 190 K, T = 227 K, and T = 263 K. Finally, using BDS, we revealed that the high-frequency features of the structural relaxation of 1,4-PIP 0.8k were related to the observed effects in the ESR and PALS response of the liquid state.

Observation of coupling between vibrational and configurational entropy in a supercooled molecular chalcogenide glass-forming liquid

The temperature dependence of the line widths and frequencies of the vibrational modes of As4S3 molecules is studied in the Ge3As52S45 molecular glass and supercooled liquid at temperatures ranging from ~Tg−189K to up to ~Tg+111K, using Raman spectroscopy. The results reveal the presence of strong dynamical coupling between vibrational and configurational degrees of freedom of the molecules such that both the phonon lifetime and the density of states are affected by the rotational (and possibly translational) molecular dynamics in the supercooled liquid. The physical origin of such coupling is discussed within the framework of the potential energy landscape model of the glass transition.

Quantum unidirectional rotation directly imaged with molecules.

A gas-phase molecular ensemble coherently excited to have an oriented rotational angular momentum has recently emerged as an appropriate microscopic system to illustrate quantum mechanical behavior directly linked to classical rotational motion, which has a definite direction. To realize an intuitive visualization of such a unidirectional molecular rotation, we report high-resolution direct imaging of direction-controlled rotational wave packets in nitrogen molecules. The rotational direction was regulated by a pair of time-delayed, polarization-skewed laser pulses, introducing the dynamic chirality to the system. The subsequent spatiotemporal propagation was tracked by a newly developed Coulomb explosion imaging setup. From the observed molecular movie, time-dependent detailed nodal structures, instantaneous alignment, angular dispersion, and fractional revivals of the wave packet are fully characterized while the ensemble keeps rotating in one direction. The present approach, providing an accurate view on unidirectional rotation in quantum regime, will guide more sophisticated molecular manipulations by utilizing its capability in capturing highly structured spatiotemporal evolution of molecular wave packets.

Molecular diffusion and dc conductivity perfectly correlated with molecular rotational dynamics in a plastic crystalline electrolyte.

We probe the ionic conduction and the molecular dynamics in a pure and lithium-salt doped dinitrile molecular plastic crystal. While the diffusion of the Li(+) ions is decoupled from the molecular reorientational dynamics, in the undoped plastic crystal the temperature dependence of the mobility of dinitrile ions and thus of the conductivity is virtually identical to that of on-site molecular rotations. The undoped material is found to obey the Walden and Stokes-Einstein rules typical of ideal liquid electrolytes, implying that an effective viscosity against diffusion can be defined even for a plastic crystalline phase. These surprising results, never reported before in a translationally ordered solid, indicate that in this dinitrile plastic crystalline material the timescale of translational diffusion is perfectly correlated with that of the purely reorientational on-site dynamics.

Molecular Dynamics Studies for the Sugar Transportation Mechanism in Phosphotransferase Systems (PTSS)

All living organisms not only need saccharides as nutrition but also use them as recognition markers by forming glycoconjugates. Therefore, a set of processes for sugar transport into the cell through the membrane is necessary for living cells. In bacteria, it is well known that sugar is transported into cytosol through the cell membrane by phosphoenolpyruvate-dependent phosphotransferase systems (PTSs). However, the detailed mechanism of how PTSs transport sugar through the cell membrane is still elusive. In 2011, Cao et al. first determined the crystal structure of an occluded state of membrane protein EIIC, which is the most important component in PTSs, of chitobiose-specific PTS. Even though the crystal structure can be used to suggest potential sugar transportation mechanisms, structural information of outward-open or inward-open states are needed to identify the correct mechanism. In this study, we model the plausible structures of outward-open states of EIIC by performing three independent rigid-body rotation molecular dynamics simulations in explicit membranes. Our model structures provide several key points that stabilize the outward-open states and give insights into the saccharide transport mechanism. This study could be a base for finding the reasonable mechanism of the saccharide transport and further experiments.