Interaction potentials and free energies for ferroelectric liquid crystals in the model of polar non-uniaxial molecules

The possible forms of the model interaction potentials are proposed for rigid polar non-uniaxial molecules with the molecular dipole moment making an arbitrary angle with the molecule's long axis. The molecule orientation is described by the direction of two molecular axes: its dipole moment and the long axis. The intermolecular potentials dependent on both molecular axes orientations are considered. The simple model interaction potentials between chiral molecules are used. It is shown that the form of the interaction potential determines the set of the relevant order parameters of the system. The free energy is calculated in the Landau expansion form in terms of the relevant order parameters.     

Dielectric dispersion of polypeptide solutions. II. Helix-coil transition of poly(epsilon-carbobenzoxy-L-lysine) in m-cresol.

Dielectric dispersion measurements were made on dilute solutions of poly(epsilon-carbobenzoxy-L-lysine) in m-cresol; this system underwent a very sharp thermal helix-coil transition of inverse type around 30 degrees. Mean-square dipole moments "mu-2" and mean rotational relaxation times t were obtained as functions of molecular weight and helical fraction fN. It was found that "mu-2"-one-half varied almost linearly with fN-one-half over a substantial portion of the transition. The relaxation times corrected for solvent viscosity and temperature changed only slightly with fN except in the region of small fN, while, at fixed fN, they displayed molecular weight dependence characteristic of rod-like molecules. These results can be explained if the transition is assumed to proceed almost in all-or-none fashion. This assumption is consistent with the previous finding from statistical thermodynamic analyses that the transition of the system PCBL-m-cresol is highly cooperative, with standard deviation of population one-half being 0.0025 to 0.0027, where standard deviation of population is the cooperativity parameter. Application of Nagai's theory of "mu-2" for interrupted helices yielded the result that standard deviation of population-one-half was in the range 0.001 to 0.005 and the monomeric dipole moment in the helical conformation was 5.4 to 6.2 D.     

Interaction potentials and free energies for ferroelectric liquid crystals in the model of polar non-uniaxial molecules

Abstract

The possible forms of the model interaction potentials are proposed for rigid polar non-uniaxial molecules with the molecular dipole moment making an arbitrary angle with the molecule's long axis. The molecule orientation is described by the direction of two molecular axes: its dipole moment and the long axis. The intermolecular potentials dependent on both molecular axes orientations are considered. The simple model interaction potentials between chiral molecules are used. It is shown that the form of the interaction potential determines the set of the relevant order parameters of the system. The free energy is calculated in the Landau expansion form in terms of the relevant order parameters.     

Single-molecule nanoprobes explore defects in spin-grown crystals

Thin, platelike single crystals of p-terphenyl (PT) doped with terrylene impurity molecules can be prepared by spin-coating from solution. Strikingly, individual terrylene molecules can be observed traveling inside the crystal over distances of several micrometers by using single-molecule fluorescence imaging at room temperature. Analysis of the motion by single-particle tracking and correlation methods indicates that the molecules act as nanoprobes by exploring long, thin crack-like defects with correlated orientations, defects that can be difficult to observe by other means. Apparently, the regions accessible to the moving molecules are in the interior of the crystal and hence are partially protected from oxidation. In addition to the traveling molecules, which photobleach in times on the order of 32 s under continuous irradiation at 2 kW/cm2, two other spatially fixed populations are observed: one with transition dipole oriented along the c-axis of the crystal with a characteristic photobleaching time greater than 32 h, and one with a characteristic photobleaching time of 18 min.     

Excited state dipole moments and polarizabilities of centrosymmetric and dimeric molecules. II. Polyenes, polyynes and cumulenes

Electro-optical absorption spectra are measured for a series of polyenes, polyynes and cumulenes with centrosymmetric π-chromophores in cyclohexane solution at 298 K. For all molecules the long-axis component of the polarizability tensor is considerably larger in the first dipole-allowed singlet state compared to the ground state. The transition moments are found to be parallel to the long molecular axis. All polyenes and one cumulene show a linear Stark component indicating a long-axis excited state dipole moment. Both the dipole moments and the polarizabilities are corrected within the extended Onsager model for solvent cavity and reaction field effects. It is suggested that symmetry lowering solvent perturbations are the reason for the apparent excited state dipole moments.     

Study of spatial correlations in a supercooled molecular system

Spatial heterogeneities have been investigated in a supercooled system composed of diatomic molecules with an associated dipole moment by using the molecular dynamics simulation technique. Pair distribution functions of molecules with different mobilities have been evaluated, and it has been found that molecules belonging to the same dynamic domain are spatially correlated. Molecules with extremely large mobilities form larger clusters than those resulting from random statistics. These clusters are stringlike shaped. The mean cluster size displays a maximum at times between the ballistic and the diffusive regime, approximately at the end of the beta-relaxation zone. The value of this maximum increases upon cooling the system. An analogous profile has been observed for the characteristic cluster length when plotted against time. Agreement with Adam-Gibbs predictions has been encountered when considering these clusters as the basic dynamic units of the theory. For the extremely slow molecules, a cluster distribution has also been encountered. These clusters are smaller than the ones composed by fast molecules; they do not have a quasilinear geometry and no maximum is observed for their mean cluster size.     

Role of specifically adsorbed electrochemically inert species in electrode kinetics

The problem of statistical mechanical averaging for the probability of the elementary act of an irreversible electron transfer reaction occurring at a metal/solution interface in the presence of specifically adsorbed electrochemically inert ions or dipole molecules is discussed in detail. Analytical expressions are derived for the polarization curve of the interface in the cases of localization of the ionic reactant centers both inside and outside the compact part of the double layer. Within the framework of the theory presented the available experimental data on electrochemical kinetics complicated by the specific adsorption of charged and neutral components of the solution are analyzed.     

A mathematical model of photochemical transformations: Analysis of the influence of basic characteristics of the model

Basic aspects of the choice of parameters entering a system of differential equations that simulate the sequence of chemical transformations in the case of initiation of a reaction by electromagnetic pulse excitation of reactant molecules are discussed. These parameters are the probabilities of spontaneous and stimulated dipole–dipole transitions and the frequency of quantum beats that lead to the reaction. A method for an a priori estimation of optimum values of these frequencies on the basis of fundamental characteristics of intramolecular processes has been specified. It has been noted that in theoretical prediction of the course of chemical reactions, the solution of the problem in the natural coordinate system (distance between the atoms), rather than Cartesian coordinates, can be more appropriate.     

The Effect of Position Replacement of Functional Groups on the Stepwise character of 1,3‐Dipolar Reaction of a Nitrile Oxide and an Alkene

It is a well‐known fact that by changing the 1,3‐dipolar cycloaddition (1,3‐DC) reaction mechanism from concerted to stepwise, the stereospecificity is lost; since in synthesizing the required heterocyclic molecules that reaction is a requisite, it is important to study the concertedness of that reaction. Several papers on this subject have already stated that the existence of electron withdrawing groups (EWG) or electron donor groups (EDG) on dipole or dipolarophile leads to a high‐energy differentiation between the dipole HOMO and dipolarophile LUMO (or vice versa) as well as the emergence of an intermediate in the reaction pathway. This paper seeks answering the question of when an EWG on dipole and an EDG on dipolarophile could be a factor in making the reaction mechanism stepwise, and does repositioning of functional groups in replacing dipole and dipolarophile switches the reaction mechanism from stepwise into concerted or vice versa?     

Quantum theory of atoms in molecules charge-charge flux-dipole flux models for the infrared intensities of X(2)CY (X = H, F, Cl; Y = O, S) molecules

The molecular dipole moments, their derivatives, and the fundamental IR intensities of the X2CY (X = H, F, Cl; Y = O, S) molecules are determined from QTAIM atomic charges and dipoles and their fluxes at the MP2/6-311++G(3d,3p) level. Root-mean-square errors of +/-0.03 D and +/-1.4 km mol(-1) are found for the molecular dipole moments and fundamental IR intensities calculated using quantum theory of atoms in molecules (QTAIM) parameters when compared with those obtained directly from the MP2/6-311++G(3d,3p) calculations and +/-0.05 D and 51.2 km mol(-1) when compared with the experimental values. Charge (C), charge flux (CF), and dipole flux (DF) contributions are reported for all the normal vibrations of these molecules. A large negative correlation coefficient of -0.83 is calculated between the charge flux and dipole flux contributions and indicates that electronic charge transfer from one side of the molecule to the other during vibrations is accompanied by a relaxation effect with electron density polarization in the opposite direction. The characteristic substituent effect that has been observed for experimental infrared intensity parameters and core electron ionization energies has been applied to the CCFDF/QTAIM parameters of F2CO, Cl2CO, F2CS, and Cl2CS. The individual atomic charge, atomic charge flux, and atomic dipole flux contributions are seen to obey the characteristic substituent effect equation just as accurately as the total dipole moment derivative. The CH, CF, and CCl stretching normal modes of these molecules are shown to have characteristic sets of charge, charge flux, and dipole flux contributions.     

Inductive effect: a quantum theory of atoms in molecules perspective

Substituent effects are ubiquitous in chemistry and the most fundamental is the inductive effect. In this study, the so-called inductive effect was probed in derivatives of bicyclo[1.1.1]pentane-1-carboxylic acid using the isodesmic reaction energy of the acid-base deprotonation, calculated at the PBE0/6-31++G(d,p) level of theory (used throughout). Although structure, molecular orbitals, and nuclear magnetic shielding parameters are discussed, the main focus of this study is the use of the quantum theory of atoms in molecules to analyze the electron density distribution. It was observed that the effect propagates via the manipulation of atomic dipole moments controlled by that of the substituent. As the dipole moment conforms to the principle of atomic transferability, it is found that the substituent dipole determined in simple systems (e.g., R-H) can be used to describe the effect upon the bicyclo[1.1.1]pentane-1-carboxylic acid system.     

The role of hydrogen bonding in supercooled methanol

The role of hydrogen bonding on the microscopic properties of supercooled methanol has been analyzed by means of molecular dynamics simulations. Thermodynamic, structural, and dynamical properties have been investigated in supercooled methanol. The results have been compared with those of an ideal methanol-like system whose molecules have the same dipole moment as the methanol but lack sites for hydrogen bonding. Upon cooling the methanol samples, translational relaxation times increase more rapidly than reorientational ones. This effect is much more important when hydrogen bonds are suppressed. Suppression of hydrogen bonds also results in lower critical temperatures for diffusion and for several characteristic relaxation time constants. The anisotropy of individual dynamics and the existence of dynamical heterogeneities have also been investigated.     

The influence of changes in the dipole moment of reagents on the rate of photoinduced electron transfer

The influence of spatial charge redistribution modeled by a change in the dipole moment of the reagent that experiences excitation on the dynamics of ultrafast photoinduced electron transfer was studied. A two-center model based on the geometry of real molecules was suggested. The model described photoexcitation and subsequent electron transfer in a donor-acceptor pair. The rate of electron transfer was shown to depend substantially on the dipole moment of the donor at the photoexcitation stage and the direction of subsequent electron transfer. These parameters also determined the most important characteristic of ultrafast photoinduced electron transfer, the angle ? between the reaction coordinates corresponding to these reaction stages. The regions of model parameters corresponding to the strongest influence of the carrier frequency of the exciting pulse on the rate of electron transfer were established.     

Ultrafast electron diffraction: oriented molecular structures in space and time.

The technique of ultrafast electron diffraction allows direct measurement of changes which occur in the molecular structures of isolated molecules upon excitation by femtosecond laser pulses. The vectorial nature of the molecule-radiation interaction also ensures that the orientation of the transient populations created by the laser excitation is not isotropic. Here, we examine the influence on electron diffraction measurements--on the femtosecond and picosecond timescales--of this induced initial anisotropy and subsequent inertial (collision-free) molecular reorientation, accounting for the geometry and dynamics of a laser-induced reaction (dissociation). The orientations of both the residual ground-state population and the excited- or product-state populations evolve in time, with different characteristic rotational dephasing and recurrence times due to differing moments of inertia. This purely orientational evolution imposes a corresponding evolution on the electron scattering pattern, which we show may be similar to evolution due to intrinsic structural changes in the molecule, and thus potentially subject to misinterpretation. The contribution of each internuclear separation is shown to depend on its orientation in the molecular frame relative to the transition dipole for the photoexcitation; thus not only bond lengths, but also bond angles leave a characteristic imprint on the diffraction. Of particular note is the fact that the influence of anisotropy persists at all times, producing distinct differences between the asymptotic "static" diffraction image and the predictions of isotropic diffraction theory.     

Interactions between molecules in the neighborhood of a long linear molecule

The third-order interaction between two nonpolar molecules due to the instantaneous induced dipole effect of an infinitely long linear molecule is assessed following arguments similar to earlier research on three-body effects. It is found that an attractive force favors small polar angles in approach to the linear molecule; such behavior could be of interest to processes such as those involved in replication of DNA.     

Monte Carlo simulation of polarization reversal of ferroelectric polymer polyvinylidene fluoride

We consider a system composed of planar zigzag chain molecules of ferroelectric polymer PVDF. Taking the Lennard–Jones potential and the dipole–dipole interaction into consideration and assuming restrictions on molecular degrees of freedom, we have performed a Monte Carlo simulation which enables us to discuss the polarization reversal of PVDF under constant external electric field. Our simulation shows that the phenomenon is accompanied by nucleation and expansion of reversed domains. It also indicates that the dipole–dipole interaction between molecules causes growth anisotropy of the reversed domains.     

New approaches to chiral discrimination in coupling between molecules

Results in the coupling of chiral molecules are reviewed from elementary points of view and some new results are given. We show that interactions between chiral molecules can be treated by using molecular quantum electrodynamics in electric and magnetic dipole approximation in ways different from standard diagrammatic perturbation theory. The interactions are the dispersive coupling of ground-state chiral molecules and excitation transfer, with emphasis on chiral discrimination. For ground-state molecules the coupling is dealt with first by calculating the coupling, at all separation distances, of electric and magnetic dipoles induced in the two molecules by fluctuations in the vacuum radiation field. The second method is the response by one chiral molecule to the field generated by the other. Excitation transfer is treated as the response by the accepting ground-state molecule to the dipole field of the donor. A novel variant in finding the rate of excitation transfer is by using Poynting's theorem. Received: 17 June 1998 / Accepted: 6 October 1998 / Published online: 16 March 1999     

Universal ultracold collision rates for polar molecules of two alkali-metal atoms

Universal collision rate constants are calculated for ultracold collisions of two like bosonic or fermionic heteronuclear alkali-metal dimers involving the species Li, Na, K, Rb, or Cs. Universal collisions are those for which the short range probability of a reactive or quenching collision is unity such that a collision removes a pair of molecules from the sample. In this case, the collision rates are determined by universal quantum dynamics at very long range compared to the chemical bond length. We calculate the universal rate constants for reaction of the reactive dimers in their ground vibrational state v = 0 and for vibrational quenching of non-reactive dimers with v ≥ 1. Using the known dipole moments and estimated van der Waals coefficients of each species, we calculate electric field dependent loss rate constants for collisions of molecules tightly confined to quasi-two-dimensional geometry by a one-dimensional optical lattice. A simple scaling relation of the quasi-two-dimensional loss rate constants with dipole strength, trap frequency and collision energy is given for like bosons or like fermions. It should be possible to stabilize ultracold dimers of any of these species against destructive collisions by confining them in a lattice and orienting them with an electric field of less than 20 kV cm(-1).     

Specific Features of a Two-Phase Bimolecular Chemical Reaction in the Case of the Association of Both Reactants

The effect of the association of both reactants on the kinetics of their bimolecular reaction in the liquid phase is studied. The mathematical modeling of chemical reactions that are described by nonlinear differential equations is performed. The steady states, the conditions for the emergences of intermediates, and the nature of their concentration oscillations in the reaction system are described. It is found that the concentration of the intermediates has two types of oscillations (harmonic and relaxation oscillations) characterized by significantly different times. The relationship between the observed rate constant of the process, the rate constants for the elementary stages, and the reactant concentrations is found.     

Real-time dipole orientational imaging as a probe of ligand-protein interactions

Single-molecule orientational imaging using total internal reflection fluorescence microscopy has been employed to investigate the dynamics of a protein-ligand system. Emission patterns from single tetramethylrhodamine (TMR)-biocytin molecules bound to streptavidin show that the TMR dipole adopts a limited number of favored orientations. The angular trajectories of individual dipoles exhibit remarkably similar patterns that are characteristic of single TMR molecules interacting with a relatively homogeneous population of nanoenvironments. Analysis of the polar and azimuthal angle distributions reveals a tendency for the dipole to assume three primary and two secondary orientations. Autocorrelation analysis of the dipole trajectories shows a predominantly bimodal behavior in the reorientation rates with the slow and fast components corresponding to the primary and secondary orientations, respectively. A number of mechanisms by which the observed orientations might be stabilized have been considered, in particular specific interactions between the zwitterionic TMR probe and charged residues on the streptavidin surface. Variations in the reorientation rates have been discussed in terms of local thermal fluctuations in the protein.