Rotational reorientation dynamics of ionic dye solutes in polar solvents with the application of a general model for the solvation shell

The rotational reorientation times of two cationic dyes, nile blue A and oxazine 720 have been determined in various protic and aprotic polar solvents from the picosecond decay of their transient absorption. The results have shown a good agreement with those obtained from theoretical calculations based on Stokes–Einstein–Debye theory, using a simple model for the rotating species. In this model these large closely planar solutes are treated as oblate symmetric tops, their solvent shells are represented by layers with uniform thickness of γ · 2r_S, where 2r_S is the diameter of the solvent molecule, and γ is a common fitting parameter for all types of solvent.     

Rotational motions of biradical metal chelates in polar solvents

Models of motion continuously varying from random jumps to rotational diffusion have been tested to simulate the ESR lineshape of biradical metal chelates spin probes in the slow-motion region. The evidence of a diffusional regime for the molecular reorientation contrasts with the anomalous temperature dependence of the process.     

QM/MM non-adiabatic decay dynamics of formamide in polar and non-polar solvents

Non-adiabatic on-the-fly dynamics simulations of the photodynamics of formamide in water and n-hexane were performed using a QM/MM approach. It was shown that steric restrictions imposed by the solvent cage do not have an influence on the initial motion which leads to the lowest energy conical intersection seam. The initial deactivation in water is faster than in n-hexane and in the gas phase. However, most of the formamide molecules in water do not reach the ground state. The reason for the deactivation inefficiency in water is traced back to a decrease of close COHOH and NHOH(2) contacts which fall in the range of hydrogen bonds. The energy deposition into H-bond breaking events leaves molecules with less energy for surmounting the CN dissociation barrier. In both solvents, after hopping to the ground state, the solvent cage keeps the HCO and NH(2) fragments or CO and NH(3) products in close proximity. Consequently, the number of trajectories where fast recombination happens is augmented with delayed recombinations that start when the dissociation fragments hit the cage wall and return back. The hot ground state formamide is formed in an internal conversion process identical to the path leading to CN photodissociation. In the case of aqueous formamide, good agreement with experimental results is achieved by combining dynamics simulations starting from the S(1) and the S(2) excited states collecting high and low energy trajectories, respectively.     

Rotational relaxation of polar molecules

The method of obtaining rotational energy relaxation times from experimental thermal conductivities using the Wang Chang-Uhlenbeck theory of transport coefficient for polyatomic gases is considered. For polar gases the method turns out to be useful only if serious calculations of the inelastic collisions involved are performed. Using a semiclassical, partly statistical collision treatment the results for the rotational energy relaxation numbers for halogen hydrides taking dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions into account are presented. It is characteristic for the results that hardly any temperature dependence or isotope effects are observed, a behaviour different from earlier investigations.     

Dispersion polymerization in polar solvents

The effect of the nitrogen purge, monomer purification, type of agitation, and presence of costabilizer on the particle size distribution (PSD) was investigated in the dispersion po-lymerization of styrene in ethanol and in the dispersion copolymerization of styrene and butyl acrylate in a water–ethanol mixture. Purging with nitrogen and, to a lesser extent, monomer purification, were of paramount importance to achieve monodispersity. The type of agitation had a week effect on the PSD, whereas the presence of costabilizer had no effect on the PSD. © 1995 John Wiley & Sons, Inc.     

Capsule-like assemblies in polar solvents

Calix[4]arene derivatives with four anionic groups at their upper rim form discrete 1:1 complexes with complementary calix[4]arene derivatives bearing four cationic groups at their upper rim. Each cation is bound by two anions, and vice versa, in a mutual chelate arrangement, reinforced by a network of ionic hydrogen bonds. These multiple electrostatic interactions lead to the formation of highly stable capsule-like assemblies even in polar protic solvents such as methanol and water. In the capsule interior a cavity is formed that is in principle large enough for the encapsulation of small aliphatic and aromatic guests (170-230 A(3)). Monte Carlo simulations in water reproducibly lead to the same regular opimized structures. These differ mainly by their inner volume and flexibility, as demonstrated by molecular dynamics calculations. Most half-spheres can be synthesized by way of the tetrakis(chloromethyl) or the tetrabromocalix[4]arene intermediate. Oppositely charged calix[6]arenes also form strong complexes, but no indication was found for a lock in the cone conformation. The formation of the ball-shaped complexes from calix[4]arene building blocks was studied with Job plots, NMR titrations, NOESY, and variable-temperature experiments, as well as ESI-MS measurements. Investigations aimed at the inclusion of various guest molecules were carried out with alcohols, sulfoxides, benzene derivatives, and ammonium, as well as pyrazinium guests. Although binding isotherms were generated with cationic guests, these must be considered to be loosely associated around the seam rather than included inside the capsule.     

Proton transfer in nanoconfined polar solvents. II. Adiabatic proton transfer dynamics

The reaction dynamics for a model phenol-amine proton transfer system in a confined methyl chloride solvent have been simulated by mixed quantum-classical molecular dynamics. In this approach, the proton vibration is treated quantum mechanically (and adiabatically), while the rest of the system is described classically. Nonequilibrium trajectories are used to determine the proton transfer reaction rate constant. The reaction complex and methyl chloride solvent are confined in a smooth, hydrophobic spherical cavity, and radii of 10, 12, and 15 A have been considered. The effects of the cavity radius and the heavy atom (hydrogen bond) distance on the reaction dynamics are considered, and the mechanism of the proton transfer is examined in detail by analysis of the trajectories.     

Self-assembling cyclic peptides: molecular dynamics studies of dimers in polar and nonpolar solvents

The self-assembly of cyclic D,L-alpha-peptides into hollow nanotubes is a crucial mechanistic step in their application as antibacterial and drug-delivery agents. To understand this process, molecular dynamics (MD) simulations were performed on dimers of cyclic peptides formed from cyclo [(-L-Trp-D-N-MeLeu-)4-]2 and cyclo [(-L-Trp-D-Leu-)4-]2 subunits in nonpolar (nonane) and polar (water) solvent. The dimers were observed to be stable only in nonpolar solvent over the full 10 ns length of the MD trajectory. The behavior of the dimers in different solvents is rationalized in terms of the intersubunit hydrogen bonding, hydrogen bonding with the solvent, and planarity of the rings. It is shown that the phi and psi dihedral angles of a single uncapped ring in nonane lie in the beta-sheet region of the Ramachandran plot, and the ring stays in a flat conformation. Steered MD (SMD) simulations based on Jarzynski's equality were performed to obtain the potential of mean force as a function of the distance between the two rings of the capped dimer in nonane. It is also shown that a single peptide subunit prefers to reside close to the nonane/water interface rather than in bulk solvent because of the amphiphilic character of the peptide ring. The present MD results build the foundation for using MD simulations to study the mechanism of the formation of cyclic peptide nanotubes in lipid bilayers.     

Amide I vibrational dynamics of N-methylacetamide in polar solvents: the role of electrostatic interactions

The vibrational frequency of the amide I transition of peptides is known to be sensitive to the strength of its hydrogen bonding interactions. In an effort to account for interactions with hydrogen bonding solvents in terms of electrostatics, we study the vibrational dynamics of the amide I coordinate of N-methylacetamide in prototypical polar solvents: D2O, CDCl3, and DMSO-d6. These three solvents have varying hydrogen bonding strengths, and provide three distinct solvent environments for the amide group. The frequency-frequency correlation function, the orientational correlation function, and the vibrational relaxation rate of the amide I vibration in each solvent are retrieved by using three-pulse vibrational photon echoes, two-dimensional infrared spectroscopy, and pump-probe spectroscopy. Direct comparisons are made to molecular dynamics simulations. We find good quantitative agreement between the experimentally retrieved and simulated correlation functions over all time scales when the solute-solvent interactions are determined from the electrostatic potential between the solvent and the atomic sites of the amide group.     

Rotational diffusion in aprotic and protic solvents

We present the orientational relaxation times in protic and aprotic solvents for rose bengal in its lowest excited singlet state. The method uses a mode locked dye laser for polarized excitation, and time correlated single photon counting for determination of the time resolved polarized fluorescence. The observed orientational decay for the dipolar aprotic solvents and the alcohols are in agreement with the values predicted by the Stokes-Einstein diffusion equation. In the latter solvents, volume and shape corrections must be made for attachment of the alcohol to the two anion sites of the dye molecule. The solvent N-methylformamide, however, shows rose bengal reorienting much faster than the alcohols. Our interpretation of this data suggests that agreement with the Stokes-Einstein equation (stick boundary conditions) is coincidental. We propose a solvent torque model in which the solvent interaction at each anion site of rose bengal controls the deviations from an expected slip boundary condition. This qualitative model is used to correlate our data as well as relevant data in the literature. The values in picoseconds for the observed orientational relaxation times are given in parenthesis; acetone (70), DMF (160), DMSO (420), MeOH (190), EtOH (450), isopropanol (840), NMF (500).     

Self-assembly of molecular capsules in polar solvents

[structure: see text] We present a novel type of molecular capsule formed by self-organization of calix[4]arenes with several oppositely charged functional groups located at their upper rims. In highly polar solvents, the complementary half-spheres form stable 1:1 complexes with association constants of up to 7 x 10(5) M(-)(1) in methanol. The cavity inside the capsules is large enough for the inclusion of small aliphatic or (hetero)aromatic guest molecules.     

Confinement of polar solvents within beta-cyclodextrins

Using molecular dynamics techniques, we examined equilibrium and dynamical characteristics pertaining to the solvation of a single beta-cyclodextrin (CD) in water and in dimethylsulfoxide (DMSO). Compared to its global minimum structure, the overall shape of the solute in solution is reasonably well preserved. While in aqueous solutions, the average number of solvent molecules retained within the central cavity of the oligosaccharide is close to 5, for DMSO, that number reduces to approximately 1. No evidence of significant orientational correlations of the trapped molecules were found in either solvent. The main contributions to the hydrogen-bond (HB) connectivity between the solute and the bulk phases are due to the more distal HO6-O6 hydroxyl groups, acting as HB donors and acceptors. The average residence time for retained DMSO was found to be in the nanosecond range, and it is, at least, 1 order of magnitude longer that the one observed for water. We also analyzed the characteristics of the solvation of the beta-CD in an equimolar water-DMSO mixture. In this environment, we found a preferential localization of a single DMSO molecule in the interior of the CD and a very minor retention of water. In the mixture, the characteristic time of residence of the trapped DMSO molecule increases by a factor of approximately 2. The observed difference was rationalized in terms of the fluctuations of the local concentrations of the two species in the vicinity of the CD top and bottom rims.     

Hydrogen-bonded capsules in polar,protic solvents

A kinetically stable, dimeric capsule is formed by tetrahydroxyresorcinarene in methanol; it encapsulates tropylium and tetramethylammonium cations.     

Energetics of electron transfer reactions in polar solvents

The free energy change of an electron transfer reaction in a polar solvent is rigorously analyzed within the framework of the dielectric continuum model. An appropriate expression for the electrostatic energy between the two product ions separated by R is derived. The present result does not support a recent claim by Suppan that, if R is close to the contact distance, the electrostatic energy should be much larger in magnitude than estimated from the usual expression −e²/ε_sR.     

Simulation of reorientation dynamics in bipolar nematic droplets

A two-dimensional model composed of a synthesis of the Leslie-Ericksen continuumtheory of nematics and the Euler-Lagrange equation for surface director motion is used to study the magnetic-induced director reorientation dynamics confined in spherical bipolar droplets with viscoelastic surfaces. The magnetic field is restricted to the droplet axis of symmetry direction. The numerical results indicate that the surface viscosity and anchoring strength must be taken into account to describe accurately director reorientation dynamics in droplets. In addition, the numerical results replicate frequently reported experimental observations on the performance of polymer dispersed liquid crystal films. These observations include the familiar exponential increase followed by saturation in light transmittance as the external applied field increases, and the exponential increase (decrease) followed by saturation as time increases in the on (off) state. Furthermore, this model is able to predict precisely the relationships between the rise and decay times and the external applied field strength, and the fact that the switching field strength is inversely proportional to droplet size.     

Theory of migration-controlled electron-transfer reactions in polar solvents

A general solution of the problem of finding the rate constant of electron-transfer reactions in polar solvents without the restrictions of the diffusional approximation has been obtained. Expressions for the reaction rate constant at the limit of the random-jump mechanism, as well as a convenient equation describing the transition between the random-jump and diffusional reaction mechanisms, have been found. A test for identifying a random-jump electron-transfer mechanism has been proposed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 27–33, January–February, 1985.     

Dispersion of cellulose nanocrystals in polar organic solvents

In an attempt to prepare stable dispersions of cellulose nanocrystals in dipolar aprotic solvents, dilute aqueous suspensions of cellulose nanocrystals were prepared by sulfuric acid hydrolysis of cotton. The aqueous suspensions were freeze-dried, and then sonicated in the solvent of interest. Dispersions of 1 and 3% w/v concentration were prepared in polar organic solvents DMSO and DMF. The dispersions showed flow birefringence. The redispersion was incomplete, and there was some evidence for aggregation in the suspensions. A small amount of water appeared to be critical to suspension stability. Birefringent cellulose films were prepared from the dispersions by drying under vacuum and at ambient conditions.     

Solution properties of ion-containing polymers in polar solvents

The placement of ionic groups within the molecular structure of a polymer produces marked modification in physical properties. A large number of studies have been performed on these ion-containing polymers, but few have focused on the effects of anion–cation interactions (i.e., counterion binding or ionization) on hydrodynamic volume, especially as the molecular structure of the solvent and nature of counterion are varied. In this study changes in hydrodynamic volume are followed through reduced viscosity measurements as a function of the abovementioned molecular parameters. The dilute solution properties of various polyelectrolytes that contain sulfonate and carboxylate groups were investigated as a function of the counterion structure, charge density, molecular weight, and solvent structure. The polymeric materials were selected because of their specific chemical structure and physical properties. In the first instance a (2-acrylamide-2 methylpropanesulfonic acid)-acrylamide-sodium vinyl sulfonate terpolymer was synthesized and subsequently neutralized with a series of bases. Viscometric measurements on these materials indicate that the nature of the cation affects the ability of the polyelectrolyte to expand its hydrodynamic volume at low polymer levels. The magnitude of the molecular expansion is shown to be due in part to the ability of the counterion to dissociate from the backbone chain, which, in turn, is directly related to the solvent structure. The changes in solution behaviour of these inomers lend support for the existence of ion pairs (i.e., site binding) and ionized moieties on the polymer chains. Measurements performed in a variety of solvent systems further confirm this interpretation. In addition, and acrylamide-sodium vinyl sulfonate copolymer was partially hydrolyzed with sodium hydroxide to study the effect of varying the charge density at a constant degree of polymerization and counterion structure. The results show that the charge density has a significant effect on the magnitude of the reduced viscosity and dilute solution behaviour. These observations, made in aqueous and nonaqueous solvents, are related to the interrelation of hydrodynamic volume, counterion concentration, and site binding. Again the controlling factor is the degree of site binding of the counterion onto the polymer backbone. Finally, we observe that the increased hydrodynamic volume affects viscosity behavior beyond the polyelectrolyte effect regime. If the average charge density on the macromolecule is relative high and/or the molecular weight is large (≥ 10⁶) sufficient intermolecular interactions will occur to produce rapid changes in reduced viscosity.