Average Molecular Dipole Moment Research Articles

Derivation of limiting ion mobility equation based on the application of solvation effect-incorporated Poisson-Boltzmann equation

Interaction potential energy between ion and solvent molecular dipole moments in solvation shell around ion was derived on basis of the application of the solution of linear SEI-PB (Solvation Effects-Incorporated Poisson-Boltzmann) equation. Based on application of the solution of linear SEI-PB equation, distribution of solvent molecules in solvation shell around ion in electrolyte solution at thermodynamic equilibrium was derived. In case approaching to limit of infinite dilution, analytical expression for evaluating limiting ion mobility is derived from the boundary condition that the average number of solvent molecules accompanying with translational motion of ion is equal to the mean solvation number of ion. The derived limiting ion mobility formula is different from the Stokes–Einstein relation and lies in relation to direct proportion with the cube of dynamic size of ion in solution, depending not only on the viscous coefficient, dielectric constant, the density and molecular weight of solvent but also on temperature.By virtue of the equation, it was possible to estimate dynamic sizes of ions in solution. It was found the estimated dynamic sizes of ions in aqueous solution to be compatible with the average ion–water distances, and to be always greater than crystallographic ionic radii in size.A new relation between the limiting ion mobility and the solvent molecular dipole moment in solvation shell was established based on the interaction potential energy between ion and solvent molecular dipole moments in solvation shell around ion. By virtue of the proposed relation, we could find out the average water molecular dipole moment in hydration shell to be different more or less in size from that in bulk solution according to ionic species in aqueous solution.

Determining Partial Atomic Charges for Liquid Water: Assessing Electronic Structure and Charge Models.

Partial atomic charges provide an intuitive and efficient way to describe the charge distribution and the resulting intermolecular electrostatic interactions in liquid water. Many charge models exist and it is unclear which model provides the best assignment of partial atomic charges in response to the local molecular environment. In this work, we systematically scrutinize various electronic structure methods and charge models (Mulliken, natural population analysis, CHelpG, RESP, Hirshfeld, Iterative Hirshfeld, and Bader) by evaluating their performance in predicting the dipole moments of isolated water, water clusters, and liquid water as well as charge transfer in the water dimer and liquid water. Although none of the seven charge models is capable of fully capturing the dipole moment increase from isolated water (1.85 D) to liquid water (about 2.9 D), the Iterative Hirshfeld method performs best for liquid water, reproducing its experimental average molecular dipole moment, yielding a reasonable amount of intermolecular charge transfer, and showing modest sensitivity to the local water environment. The performance of the charge model is dependent on the choice of the density functional and the quantum treatment of the environment. The computed molecular dipole moment of water generally increases with the percentage of the exact Hartree-Fock exchange in the functional, whereas the amount of charge transfer between molecules decreases. For liquid water, including two full solvation shells of surrounding water molecules (within about 5.5 Å of the central water) in the quantum chemical calculation converges the charges of the central water molecule. Our final pragmatic quantum chemical charge-assigning protocol for liquid water is the Iterative Hirshfeld method with M06-HF/aug-cc-pVDZ and a quantum region cutoff radius of 5.5 Å.

An examination of the relationship between molecular dipole moment and blood-gas partition for common anaesthetic gases.

The solubility of inhalational anaesthetics in the bloodstream is related to the minimum alveolar concentration (MAC), which is an indicator of anaesthetic gas potency. The blood-gas partition coefficient (Kbg) is a measure of how much anaesthetics bind to plasma proteins in the blood compared to air. Just like the octanol-water partition coefficient, the Kbg may be related to the molecular dipole moment (μ), which can be modulated by the molecular conformation. Our quantum-chemical calculations demonstrated that subtle stereoelectronic interactions, namely those responsible for the anomeric and gauche effects, control the conformational equilibrium of some widely used volatile fluorinated anaesthetics and, consequently, of their molecular dipole moments. Since a remarkable correlation between empirical Kbg and calculated μ was found for these anaesthetics, the average molecular dipole moments may be used to predict the anaesthetic gas potency and other properties, such as lipid solubility, of inhalational anaesthetic analogs.

Ab initio molecular dynamics study of temperature and pressure-dependent infrared dielectric functions of liquid methanol

The temperature and pressure-dependent dielectric functions of liquids are of great importance to the thermal radiation transfer and the diagnosis and control of fuel combustion. In this work, we apply the state-of-the-art ab initio molecular dynamics (AIMD) method to calculate the infrared dielectric functions of liquid methanol at 183–573 K and 0.1–160 MPa in the spectral range 10−4000 cm−1, and study the temperature and pressure effects on the dielectric functions. The AIMD approach is validated by the Infrared Variable Angle Spectroscopic Ellipsometry (IR-VASE) experimental measurements at 298 K and 0.1 MPa, and the proposed IR-VASE method is verified by comparison with paper data of distilled water. The results of the AIMD approach agrees well with the experimental values of IR-VASE. The experimental and theoretical analyses indicate that the temperature and pressure exert a noticeable influence on the infrared dielectric functions of liquid methanol. As temperature increases, the average molecular dipole moment decreases. The amplitudes of dominant absorption peaks reduce to almost one half as temperature increases from 183 to 333 K at 0.1 MPa and from 273 to 573 K at 160 MPa. The absorption peaks below 1500 cm–1 show a redshift, while those centered around 3200 cm–1 show a blueshift. Moreover, larger average dipole moments are observed as pressure increases. The amplitudes of dominant absorption peaks increase to almost two times as pressure increases from 1 to 160 MPa at 373 K.

Electro-Optical Behavior of a High Birefringence Mesogenic Mixture

Liquid crystal mixtures tailored to possess high birefringence (Δn ≥ 0.30) are being currently used extensively in photonic devices. The present work entails the study of electro-optical properties of such a high birefringence mixture (code name: 1770-2). The multicomponent thermotropic mixture, composed of compounds belonging to families of alkylfluoroisothiocyanatotolanes and alkylphenylfluoroisothiocyanatotolanes, exhibits only the nematic mesophase. The thermal variation of electric permittivities (ϵ| and ϵ⊥) at different frequencies has been determined. Using the birefringence data, thermal variation of polarizability has been determined. The variation of the effective average molecular dipole moment μsol of the set-up molecules in solutions of varying concentrations (in%wt) at a particular temperature has been determined and the average molecular dipole moment μiso of the set-up molecules has been obtained. With a view to determine the nature of the molecular association in the mesophase, the molecular correlation factors “g” has been estimated from measurement of average dipole moment of the set-up molecules.

Self-aggregation mechanisms of N-alkyl derivatives of urea and thiourea

The mechanisms of self-aggregation of N-alkyl and N,N'-dialkyl derivatives of urea and thiourea in weakly polar solvents (chloroform and 1,2-dichloroethane) were examined. The C-H···O or C-H···S hydrogen bonds formed with these two acidic solvents compete with the N-H···O or N-H···S hydrogen bonds formed between solute molecules, influencing the self-aggregation of urea derivatives in a particular solvent. The peculiarities of the solvent interactions were discussed and the stronger interaction of chloroform was noted. Aggregation of the N-alkyl derivatives was followed using IR spectroscopy, with two gradual aggregation constants (K1 and K2) determined. The average molecular weight and dipole moments were shown to depend on the concentration, and the form of aggregation was analyzed through the study of the dipole moments. All of the urea derivatives demonstrated an increase in dipole moment with increased concentration, resulting in stronger NH2···O hydrogen bond interactions and leading to linear-type aggregation. Contrastingly, the dipole moments of the mono-N-alkyl-substituted thioureas decreased with concentration. Density-functional theory calculation of these processes showed that reliable results could only be obtained if solvent interactions were considered, with a specific combination of local and bulk effects. It was also shown that going from N,N'-disubstituted to N-monoalkyl derivatives the ability to aggregate increases, which is related to a diminished steric hindrance to hydrogen bonding. Finally, it was demonstrated that the mechanisms of self-aggregation depend on the acid-base properties of the solute, hydrogen bonding to the solvent molecules, and steric interactions of the aliphatic chains.

Liquid Methanol from DFT and DFT/MM Molecular Dynamics Simulations.

We present a comparative study of computational protocols for the description of liquid methanol from ab initio molecular dynamics simulations, in view of further applications directed at the modeling of chemical reactivity of organic and organometallic molecules in (explicit) methanol solution. We tested density functional theory molecular dynamics (DFT-MD) in its Car-Parrinello Molecular Dynamics (CPMD) and Quickstep/Born-Oppenheimer MD (CP2K) implementations, employing six popular density functionals with and without corrections for dispersion interactions (namely BLYP, BLYP-D2, BLYP-D3, BP86, BP86-D2, and B97-D2). Selected functionals were also tested within the two QM/MM frameworks implemented in CPMD and CP2K, considering one DFT molecule in a MM environment (described by the OPLS model of methanol). The accuracy of each of these methods at describing the bulk liquid phase under ambient conditions was evaluated by analyzing their ability to reproduce (i) the average structure of the liquid, (ii) the mean squared displacement of methanol molecules, (iii) the average molecular dipole moments, and (iv) the gas-to-liquid red-shift observed in their infrared spectra. We show that it is difficult to find a DFT functional that describes these four properties equally well within full DFT-MD simulations, despite a good overall performance of B97-D2. On the other hand, DFT/MM-MD provides a satisfactory description of the solvent-solute polarization effects with all functionals and thus represents a good alternative for the modeling of methanol solutions in the context of chemical reactivity in an explicit environment.

First principles Monte Carlo simulations of aggregation in the vapor phase of hydrogen fluoride

The aggregation of hydrogen fluoride vapor is explored through the use of Monte Carlo simulations employing Kohn-Sham density functional theory with the exchange/correlation functional of Becke-Lee-Yang-Parr to describe the molecular interactions. Canonical ensemble simulations sampling the classical phase space were carried out for a system consisting of ten molecules at constant density (2700 A(3)/molecule) and at three different temperatures (T = 310, 350, and 390 K). Aggregation-volume-bias and configurational-bias Monte Carlo approaches (along with pre-sampling with an approximate potential) were employed to increase the sampling efficiency of cluster formation and destruction. A hydrogen-bond analysis shows that about two thirds of the HF molecules are part of small aggregates at 310 K, whereas only about 10% of the molecules are clustered at 390 K. As for other hydrogen-bonding systems, the size distribution exhibits some sensitivity to the criteria used to define a hydrogen bond, but the qualitative features are not affected by these differences. From the temperature dependence of the equilibrium constants, the dimer and trimer aggregation energies (not corrected for nuclear quantum effects) are estimated using a simple distance-based hydrogen-bonding criterion as -13 +/- 3 and -65 +/- 16 kJ mol(-1), respectively, whereas these binding energies are found to be somewhat different for a combined distance-angular criterion with values of -17 +/- 6 and -63 +/- 11 kJ mol(-1), respectively. The strictness of the hydrogen-bonding criterion plays a significant role for the assignment of clusters to linear, cyclic, and branched architectures with the fraction of the latter being drastically reduced for the distance-angular criterion. The average molecular dipole moment increases from 1.85 Debye for isolated molecules to about 2.0 D for dimers to about 2.75 D for larger aggregates, and the H-F bond length shows a concomitant, but smaller increase from about 0.94 to 0.98 A.

Incorporating Phase-Dependent Polarizability in Non-Additive Electrostatic Models for Molecular Dynamics Simulations of the Aqueous Liquid-Vapor Interface.

We discuss a new classical water force field that explicitly accounts for differences in polarizability between liquid and vapor phases. The TIP4P-QDP (4-point transferable intermolecular potential with charge dependent-polarizability) force field is a modification of the original TIP4P-FQ fluctuating charge water force field of Rick et al.(1) that self-consistently adjusts its atomic hardness parameters via a scaling function dependent on the M-site charge. The electronegativity (χ) parameters are also scaled in order to reproduce condensed-phase dipole moments of comparable magnitude to TIP4P-FQ. TIP4P-QDP is parameterized to reproduce experimental gas-phase and select condensed-phase properties. The TIP4P-QDP water model possesses a gas phase polarizability of 1.40 Å(3) and gas-phase dipole moment of 1.85 Debye, in excellent agreement with experiment and high-level ab initio predictions. The liquid density of TIP4P-QDP is 0.9954(±0.0002) g/cm(3) at 298 K and 1 atmosphere, and the enthalpy of vaporization is 10.55(±0.12) kcal/mol. Other condensed-phase properties such as the isobaric heat capacity, isothermal compressibility, and diffusion constant are also calculated within reasonable accuracy of experiment and consistent with predictions of other current state-of-the-art water force fields. The average molecular dipole moment of TIP4P-QDP in the condensed phase is 2.641(±0.001) Debye, approximately 0.02 Debye higher than TIP4P-FQ and within the range of values currently surmised for the bulk liquid. The dielectric constant, ε = 85.8 ± 1.0, is 10% higher than experiment. This is reasoned to be due to the increase in the condensed phase dipole moment over TIP4P-FQ, which estimates ε remarkably well. Radial distribution functions for TIP4P-QDP and TIP4P-FQ show similar features, with TIP4P-QDP showing slightly reduced peak heights and subtle shifts towards larger distance interactions. Since the greatest effects of the phase-dependent polarizability are anticipated in regions with both liquid and vapor character, interfacial simulations of TIP4P-QDP were performed and compared to TIP4P-FQ, a static polarizability analog. Despite similar features in density profiles such as the position of the GDS and interfacial width, enhanced dipole moments are observed for the TIP4P-QDP interface and onset of the vapor phase. Water orientational profiles show an increased preference (over TIP4P-FQ) in the orientation of the permanent dipole vector of the molecule within the interface; an enhanced z-induced dipole moment directly results from this preference. Hydrogen bond formation is lower, on average, in the bulk for TIP4P-QDP than TIP4P-FQ. However, the average number of hydrogen bonds formed by TIP4P-QDP in the interface exceeds that of TIP4P-FQ, and observed hydrogen bond networks extend further into the gaseous region. The TIP4P-QDP interfacial potential, calculated to be -11.98(±0.08) kcal/mol, is less favorable than that for TIP4P-FQ by approximately 2% as a result of a diminished quadrupole contribution. Surface tension is calculated within a 1.3% reduction from the experimental value. Results reported demonstrate TIP4P-QDP as a model comparable to the popular TIP4P-FQ while accounting for a physical effect previously neglected by other water models. Further refinements to this model, as well as future applications are discussed.

Electrostatic Polarization Effects and Hydrophobic Hydration in Ethanol−Water Solutions from Molecular Dynamics Simulations

Nonadditive electrostatic force fields based on the charge equilibration formalism coupled with long time-scale molecular dynamics simulations are used to investigate the microscopic structural aspects of hydrophobic hydration in ethanol-water solutions. Employing a combination of polarizable ethanol and water force fields (developed independently), we find that solution properties are satisfactorily reproduced across the ethanol mole fraction range between 0.1 and 0.9. Solution densities are predicted within 3.6% of experimental measurements, while excess mixing enthalpies are overestimated as in earlier studies. The solvation free energy of ethanol in infinite dilution is determined via thermodynamic integration to be 5.70 +/- 0.23 kcal/mol, overestimating the free energetics of solvation relative to experiment (5.01 kcal/mol). Bulk solution dielectric constants and diffusion constants reproduce experimental trends and are in reasonable agreement across the ethanol concentration range studied. Because of explicit accounting of induction effects, ethanol and water exhibit varying molecular dipole moment distributions with concentration. The polarizable ethanol model, possessing higher condensed-phase polarizability relative to the TIP4P-FQ water model (4.54 A(3) versus 1.1 A(3), respectively), displays greater variation upon perturbation by the electric field of water. With regard to hydrophobic hydration, the current force fields indicate positive hydrogen bonding excess for water in the dilute ethanol concentration range, consistent with previous theoretical and experimental studies. Strikingly, we find that there are both positive and negative hydrogen bond excess contributions within the first hydration shell of both the ethanol hydroxyl oxygen and ethylene carbon atoms. The larger positive contributions dominate the overall hydrogen bonding patterns to yield overall net positive excesses. Moreover, we do not find evidence of excess hydrogen bonding vicinal to the nonpolar moieties as has been suggested based on the "iceberg"-like models proposed by Frank and Evans. The present results suggest negative excess in the regions surrounding the alkyl groups that vis-a-vis corresponds to a reduction in the average molecular dipole moment of resident water molecules due to smaller dipole induction in the weaker electrostatic fields of the nonpolar groups.

Spatial correlation of dipole fluctuations in liquid water

The molecular dipole moments and local environments of water in its liquid phase were examined for a series of first-principles Gibbs ensemble Monte Carlo simulations along the vapour–liquid coexistence curve using the Becke–Lee–Yang–Parr (BLYP) and Perdew–Burke–Ernzerhof (PBE) exchange/correlation density functionals. Molecular dipole moments were computed using maximally localized Wannier functions with the Berry phase scheme, while the structure was analysed with respect to tetrahedral order parameter and hydrogen bonding. Increasing the temperature results in a decrease of both the average molecular dipole moment and the local structure, although the width of the dipole distribution remains fairly constant. A correlation is found between the extent of the local structure and the magnitude of the molecular dipole moment, but this correlation is limited to the first solvation shell.

Self-assembly of 2-aminopyrimidines in nonpolar solvents

A combined experimental and theoretical study on self-assembly of 2-aminopyrimidine (ampm), 2-amino-4-chloro-6-methyl-pyrimidine (ampm1), 2-amino-4,6-dimethyl-pyrimidine (ampm2) and 2-amino-4,6-dichloro-pyrimidine (ampm3) was performed. The effects of concentration increase in low polar solvents C 6H 6 and CCl 4 on average molecular weight, dipole moments and IR spectra of these compounds were investigated. Strong association of all these molecules was found, through N–H⋯N hydrogen bonding. The association leads to formation of less polar species. DFT B3LYP/6-31G** calculations for monomers and dimers of these molecules were performed to model the structure of possible aggregates.

Estimation of gas-phase reaction rate constants of alkylnaphthalenes with chlorine, hydroxyl and nitrate radicals

Quantitative structure-property relationship/quantitative structure-activity relationship (QSPR/QSAR) models were developed for rate constants (k) of alkylnaphthalene reactions with chlorine (Cl), hydroxyl (*OH) and nitrate (NO(3)) radicals using partial least squares (PLS) regression. Quantum chemical descriptors computed by Parametric Method 3 (PM3) Hamiltonian were used as predictor variables. The cross-validated Q(cum)(2)values for the optimal QSPR/QSAR models of alkylnaphthalenes are 0.896, 0.728 and 0.774 for Cl, *OH and NO(3) radicals, respectively. Results from this study showed that rate constants with Cl, *OH and NO(3) are governed by different molecular structural descriptors. In the developed optimal QSPR/QSAR models, frontier molecular orbital energies and atomic charges are major descriptors that affect log k values. When the highest occupied molecular orbital (E(HOMO)) energy, the lowest unoccupied molecular orbital (E(LUMO)) energy, E(LUMO)+E(HOMO), and the average of net atomic charges on carbon atoms (Q(Cave)) are higher, the corresponding alkylnaphthalene reaction rate constants would be higher. In contrast, higher values of the most positive net atomic charges on hydrogen atoms (Q(H)(+)) could lead to the decrease of log k values. H-atom abstraction may occur on the hydrogen atom with the highest atomic charge (Q(H)). Radical addition reaction may also occur on the carbon (e.g. the carbon bonded to the alkyl group) with higher atomic charge (Q(C)) values. Other descriptors such as molecular weight (M(w)), standard heat of formation (DeltaH(f)), total energy (TE), electronic energy (EE), core-core repulsion energy (CCR), average molecular polarizability (a) and dipole moment (mu) were also important descriptors in the QSPR/QSAR models.

Density functional study of ion hydration for the alkali metal ions (Li+,Na+,K+) and the halide ions (F−,Br−,Cl−)

We performed first principles density functional calculations to study the effect of monovalent ions M+ (M = Li,Na,K) and A- (A = F,Cl,Br) in water with the aim of characterizing the local molecular properties of hydration. For this reason, several ion-water clusters, up to five or six water molecules were considered; such structures were optimized, and the Wannier analysis was then applied to determine the average molecular dipole moment of water. We found that with an increasing number of water molecules, the molecular polarization is determined by the water-water interaction rather than the water-ion interaction, as one would intuitively expect. These results are consistent with those obtained in previous density functional calculations and with other results obtained by employing classical polarizable water models. The main message of this work is that as one increases the number of molecules the average dipole moment of all water molecules and the ones in the first shell tends to the same value as the average of a similar sized cluster of pure water. This supports the use of nonpolarizable classical models of water in classical atomistic simulations.

Molecular Dynamics Simulation of Liquid Water: Hybrid Density Functionals

The structure, dynamical, and electronic properties of liquid water utilizing different hybrid density functionals were tested within the plane wave framework of first-principles molecular dynamics simulations. The computational approach, which employs modified functionals with short-ranged Hartree-Fock exchange, was first tested in calculations of the structural and bonding properties of the water dimer and cyclic water trimer. Liquid water simulations were performed at the state point of 350 K at the experimental density. Simulations included three different hybrid functionals, a meta-functional, four gradient-corrected functionals, and the local density and Hartree-Fock approximations. It is found that hybrid functionals are superior in reproducing the experimental structure and dynamical properties as measured by the radial distribution function and self-diffusion constant when compared to the pure density functionals. The local density and Hartree-Fock approximations show strongly over- and understructured liquids, respectively. Hydrogen bond analysis shows that the hybrid functionals give slightly smaller average numbers of hydrogen bonds than pure density functionals but similar hydrogen bond populations. The average molecular dipole moments in the liquid from the three hybrid functionals are lower than those of the corresponding pure density functionals.

Structure, thermodynamics, and liquid-vapor equilibrium of ethanol from molecular-dynamics simulations using nonadditive interactions

We present a molecular-dynamics simulation study of the bulk and liquid-vapor interfacial properties of ethanol using a polarizable force field based on the fluctuating charge (FQ) formalism, as well as the nonpolarizable CHARMM22 force field. Both models are competitive with respect to the prediction of ambient liquid properties such as liquid density, enthalpy of vaporization, dielectric constant, and self-diffusion constants. The polarizable model predicts an average condensed-phase dipole moment of 2.2 D associated with an induced liquid-phase dipole moment of 0.6 D; though qualitatively in agreement with earlier nonadditive models as well as recent Car-Parinello calculations, the current FQ model underestimates the condensed-phase dipole moment. In terms of liquid structure, both models are in agreement with recent neutron-diffraction results of liquid ethanol structure, although the polarizable model predicts the hydroxyl-hydrogen-hydroxyl-hydrogen structure factor in closer agreement with the experimental data. In terms of interfacial properties, both models predict ambient surface tension to within 4% of the experimental value of 22.8 dyncm, while overestimating the surface excess entropy by almost a factor of 2. Both models display the characteristic preferential orientation of interfacial molecules. The polarizable model allows for a monotonic variation of the average molecular dipole moment from the bulk value to that of the vapor phase. Consequently, there is a dramatic difference in the surface potential predicted by the polarizable and nonpolarizable models. The polarizable model estimates a surface potential of -209+/-3 mV, while the nonpolarizable model yields a value of -944+/-10 mV. Finally, based on the vapor-liquid equilibrium simulation data from several temperatures, we estimate the critical properties of both models. As observed with other FQ models for associating fluids (such as water and methanol), and counter to what one would anticipate by modeling more physically the electrostatic response to local environment, the current FQ model underestimates the critical temperature and overestimates the critical density of ethanol; moreover, the FQ model is, in this respect, equivalent to the underlying fixed-charge model. These results further suggest the need to revisit polarizable models in terms of quantitative vapor-liquid equilibrium prediction.

Self‐assembly of molecules containing the 2‐aminopyridine unit in non‐polar solvents

AbstractA combined experimental and theoretical study on the self‐assembly of three molecules, 2,2′‐dipyridylamine, 2‐(1‐phenylamino)pyridine and 2‐methylaminopyridine was performed. The effect of concentration increase in low‐polarity solvents was investigated. The average molecular weights, dipole moments and IR spectra of these compounds were measured in C6H6, CCl4 and CHCl3. A strong association of all these molecules was found, through NH···N hydrogen bonding. The form of associates is different; in molecules containing the N‐aromatic ring the aggregates have a dipole moment decreasing with concentration whereas in N‐methylaminopyridine a considerable increase in dipole moment with concentration was observed. DFT B3LYP/6–31G(d,p) calculations for monomers, dimers, trimers and tetramers of these three molecules were performed to model the structures of possible aggregates. Copyright © 2005 John Wiley & Sons, Ltd.

Association of model peptides and dehydropeptides: N-acetyl- l-alanine- and dehydroalanine N′, N′-dimethylamides

The comparative studies on the association of Ac-ΔAla-NMe 2 and Ac- l-Ala-NMe 2 in carbon tetrachloride were performed by the analysis of their average molecular weight, dipole moments and FTIR spectra. To aid spectroscopic interpretation and gain some deeper insight into the nature of associates, the geometries of the minimum energy of the dimers of Ac-ΔAla-NMe 2 and Ac- l-Ala-NMe 2 were calculated by the B3LYP/6-31+G** method. The average molecular weight in the studied concentration range, for the ΔAla and l-Ala peptide, as determined by the osmometric method, did not exceed 1.5 and 1.2 of the monomeric mass, respectively. Accordingly, the percentage of the monomeric form (α) decreased as concentration was increased more significantly for the ΔAla analogue than for its saturated counterpart. In the studied concentrations, the dipole moment of the unsaturated compound decreases and that of its counterpart is almost constant. We identified a wider range of dimeric forms of Ac-ΔAla-NMe 2 than those of Ac- l-Ala-NMe 2. While Ac-ΔAla-NMe 2 mainly forms cyclic dimers, built of open conformers H/F, specific for α,β-dehydroamino acids, Ac- l-Ala-NMe 2 forms cyclic and linear dimers, characteristic for the usual amino acids. Ac-ΔAla-NMe 2 has a greater tendency to associate than its saturated variant, which is the result of stronger hydrogen bonds.

Many-body interactions in liquid methanol and its liquid/vapor interface: A molecular dynamics study

Many-body interactions in liquid methanol and its liquid/vapor interface are evaluated using classical molecular dynamics techniques. The methanol molecule carries a molecular polarizability to account for induction energies and forces. The computed dipole moment for the methanol molecule changed from 1.7 to 2.8 D, respectively, from the vapor to the liquid phases. This result indicated that there are significant many-body interactions in this complex molecular system. The computed average molecular dipole moment in liquid methanol at room temperature is in good agreement with experimental measurements. The computed average dipole moments of methanol molecules near the interface are close to their gas phase values, while methanol molecules far from the interface have dipole moments corresponding to their bulk values. The structural and thermodynamic properties of the liquid methanol as well as the surface tension of its liquid/vapor interface are in good agreement with the experiments, demonstrating the high quality of our potential model and simulation approaches. A constrained molecular dynamics technique was used to investigate the transport mechanism of a methanol molecule across the methanol liquid/vapor interface. The computed transfer free energy changed gradually as the methanol molecule approached the Gibbs dividing surface, and it crossed the interface with no substantial minimum free energy. The computed solvation free energy of the methanol molecule in liquid methanol estimated from the free energy profile (4.25 kcal/mol) is in good agreement with the corresponding experimental measurement (4.89 kcal/mol).

Hydrogen fluoride phase behavior and molecular structure: A QM/MM potential model approach

A molecular-orbital derived polarizable potential function is developed to model liquid and supercritical fluid hydrogen fluoride. The model is based on a novel application of a combined quantum-mechanical and molecular-mechanical (QM/MM) approach, which treats molecular polarization by a semiempirical method. Two geometrical models are examined, differing in the intramolecular bond length for hydrogen fluoride to match values commonly seen in other empirical models. One QM/MM parameter is fit for each model to reproduce the experimental density at one liquid-phase state condition. The models are examined at this state and at one supercritical state condition. Results for the density, radial distribution function, and average molecular dipole moment are considered in comparison to experiment. Also vapor-liquid coexistence data are evaluated, including saturation densities, heat of vaporization, and vapor pressure. Both models perform well in describing the densities, but are no better than other molecular models in characterizing the vapor-liquid critical point, the heat of vaporization, and the vapor pressure. The QM/MM models are slightly better than others in describing the radial distribution functions, although it is clear that this QM polarization model can be further improved. The present study further demonstrates that a QM-based polarization model is a viable alternative to model polar fluids with strong intermolecular interactions.