Average Molecular Dipole Moment Research Articles

A Fuzzy ARTMAP‐Based Quantitative Structure—Property Relationship (QSPR) for the Henry′s Law Constant of Organic Compounds.

Quantitative structure−property relationships (QSPRs) for estimating a dimensionless Henry's Law constant of organic compounds at 25 °C were developed based on a fuzzy ARTMAP and back-propagation neural networks using a heterogeneous set of 495 organic compounds. A set of molecular descriptors developed from PM3 semiempirical MO-theory and topological descriptors (second-order molecular connectivity index) were used as input parameters to the neural networks. Quantum chemical input descriptors included average molecular polarizability, dipole moments (total point charge, total hybridization, and total sum), ionization potential, and heat of formation. The fuzzy ARTMAP/QSPR correlated Henry's Law constant for −6.72 ≤ logH ≤ 2.87 with average absolute errors of 0.03 and 0.13 logH units for the overall data and the test set, respectively. The optimal 7−17−1 back-propagation/QSPR model was less accurate and exhibited larger average absolute errors of 0.28 and 0.27 logH units for the validation (recall) and te...

A fuzzy ARTMAP-based quantitative structure-property relationship (QSPR) for the Henry's law constant of organic compounds.

Quantitative structure-property relationships (QSPRs) for estimating a dimensionless Henry's Law constant of organic compounds at 25 degrees C were developed based on a fuzzy ARTMAP and back-propagation neural networks using a heterogeneous set of 495 organic compounds. A set of molecular descriptors developed from PM3 semiempirical MO-theory and topological descriptors (second-order molecular connectivity index) were used as input parameters to the neural networks. Quantum chemical input descriptors included average molecular polarizability, dipole moments (total point charge, total hybridization, and total sum), ionization potential, and heat of formation. The fuzzy ARTMAP/QSPR correlated Henry's Law constant for -6.72 </= logH </= 2.87 with average absolute errors of 0.03 and 0.13 logH units for the overall data and the test set, respectively. The optimal 7-17-1 back-propagation/QSPR model was less accurate and exhibited larger average absolute errors of 0.28 and 0.27 logH units for the validation (recall) and test sets, respectively. The fuzzy ARTMAP-based QSPR was superior to the back-propagation and multiple linear regression/QSPR models for Henry's Law constant of organic compounds.

Methane in Water: An Ab Initio Study

In preceding publications we discussed some properties of pure water in condensed phases using an ab initio approach. Here this study is used as a basis of comparison for analysing the behaviour of water as a solvent in the presence of an apolar molecule. Our analysis is focused on the process of organization of the hydrogen bonding network around the solute. For this purpose we perform some ab initio calculations for a system of 32 water molecules and one methane molecule at 300 K; in particular, the average molecular dipole moment of water is determined and the result is compared with that of pure water. Next the attention is switched to the methane molecule; related properties such as excluded volume and sphericity of its shape are illustrated and discussed. A comparison with results obtained using classical approaches suggests that some classical models of water can be considered to be still valid when they are used to analyse the water-methane system.

Two Algorithms for Defining Atoms and Molecules in Condensed Phases

Two algorithms for defining single entities in condensed phase based on the calculation of a zero flux surface of the gradient of the electron density are proposed. First the two approaches are applied to a water molecule in a liquid system; the different behaviour of the two algorithms is graphically shown in the planar section of the molecular plane (i.c., HOH plane of the molecule analyzed). Next, using the two algorithms for partitioning electron density, the average molecular dipole moment of the water molecule in liquid phase is calculated averaging over several configurations; analogies and differences with other methods are reported and their physical interpretation is discussed. There exist different approaches for calculating the zero flux surface (or equivalently the volume defined by such a surface); the two described in this work differ from the others because the calculation in both cases is straightforward (i.e., the surface is determined directly from the data corresponding to the electron density) and is characterized by a relatively short computational time.

Development of Polarizable Water Force Fields for Phase Equilibrium Calculations

Recent simulation studies for four dipole-polarizable and for two fluctuating-charge water force fields have demonstrated that none of the force fields studied is capable of yielding a satisfactory description of the vapor−liquid coexistence curve from room temperature to the critical region. The performance of these polarizable force fields can be improved dramatically by introducing an additional coupling between the Lennard−Jones interaction parameters for a pair of oxygen sites and their partial charges (electronic configuration). Two different types of water models are presented which are based on either the three-site simple point charge (SPC) or the four-site transferable intermolecular potential 4 point (TIP4P) water representations. Adiabatic nuclear and electronic sampling Monte Carlo (ANES-MC) simulations in the Gibbs, isobaric−isothermal, and canonical ensembles were performed to calculate vapor−liquid coexistence curves, to determine the temperatures of maximum liquid density, and to evaluate dielectric constants along the coexistence line, respectively. The new SPC-pol-1 force field yields significantly better agreement with the experiment for the saturated vapor and liquid densities, the heats of vaporization, and the liquid-phase dielectric constants than the fixed-charge SPC, SPC/E (simple point charge/extended), TIP4P, and Errington/Panagiotopoulos (EP) force fields or than any other polarizable force field previously tested. However, the representation of the liquid water structure at ambient conditions is less satisfactory for the SPC-pol force fields. In contrast, the TIP4P-pol force fields produce much better low-temperature liquid structures and, in particular, a density maximum close to T = 277 K, but their performance for the vapor−liquid equilibria in the near-critical region is less satisfactory. Finally, it is important to note that the SPC-pol-1 force field yields an average molecular dipole moment of 2.5 D for the liquid phase at ambient conditions that is substantially smaller than the value of 2.7 D obtained for its minimum-energy hexamer cluster.

Global Potential Energy Minima of SPC/E Water Clusters without and with Induced Polarization Using a Genetic Algorithm

The global minimum potential energy structures of water clusters, (H2O)n, n = 2-14, have been calculated for the SPC/E (Simple Point Charge/Extended) model and a recent fluctuating charge version of the latter using a simple genetic algorithm. The SPC/E cluster geometries are in good agreement with previous TIP3P (Transferable Intermolecular Potential-3 Point) and TIP4P (Transferable Intermolecular Potential-4 Point) calculations as well as the interpretation of experimental measurements. In contrast to this, the polarizable version of the SPC/E model, which is based on the fluctuating charge approach, deviates rather strongly for n=6 with few exceptions. However, comparing the polarizable model to ab initio results for identical cluster geometries we find reasonable agreement for the magnitude of the average molecular dipole moment, the corresponding energy per molecule, and the average oxygen-oxygen distance as functions of n.

Properties of supercritical water: an ab initio simulation

Using ab initio molecular dynamics we have simulated the properties of supercritical water at the density p=0.64 g/cm 3 and temperature T=730 K. We calculate the neutron diffraction structure factors for various isotopic concentrations and find excellent agreement with experiment. The real-space pair correlations are qualitatively similar and our calculations confirm that the conditions prevalent in supercritical water disrupt the hydrogen bond network which characterizes water at ambient temperatures and pressure. In a quantitative respect there are significant differences between the simulation results for the pair correlation functions and those obtained from the inversion of the experimental data. We also studied some electronic properties of the system such as the average molecular dipole moment.

Hydrogen bonding and the static dielectric constant in liquid water

The static dielectric constants of several models for water are compared. These models have in common that they all give a good description of the energetics and structure of the hydrogen bonding in the liquid. Despite this similarity, the average molecular dipole moments vary over a range of 50% and the dielectric constants can differ by as much as a factor of 6. This observation verifies that the average dipole moment is a crucial parameter for the dielectric properties. On the other hand, this quantity seems of less importance for the stability of the hydrogen bonded network, which is largely determined by the details of the electrostatic and atom–atom interactions at short range. Applying this analysis we construct a polarizable model for liquid water with an average dipole moment of 2.6 D and a dielectric constant of 85. The structure and self-diffusion coefficient are in good agreement with experiment, and the value of the dimer binding energy in the gas phase is also acceptable.

Equilibrium distribution functions for plasma-molecular layers

The equilibrium distribution functions of free charged particles and those bound in molecules are calculated for a plasma-molecular system restricted by two parallel planes, taking into account all terms of the multipole expansion of the molecular fields. These distributions are used to find the inhomogeneous densities of free charged and neutral particles, the average molecular dipole moments and the total dipole moment of the system. In the case where particles approach the boundary of the system, the expressions obtained can diverge. The removal of the divergences is discussed. For a molecular subsystem modulated by rigidity coupled pairs of charged particles or an ensemble of harmonic atomic oscillators, the equilibrium distributions are investigated in the dipole approximation. The molecules contained in a purely molecular system or a plasmamolecular system with a hydrogen-like plasma subsystem are shown to have zero average dipole moments. For such systems the non-zero average dipole moments of mole...

Dielectric properties of 1,12-dinitrododecane in dilute solution

Measurements of the dielectric constant of 1,12-dinitrododecane, dissolved in toluene, are reported for temperatures between 25 and 50 degrees C and concentrations up to 0.036 g cm-3. At low concentrations, the results are well represented by the usual linear concentration dependence. However, the higher concentration values are clearly non-linear. Also the average molecular dipole moment increases with increasing temperature. These results are explained in terms of bond rotations in the molecule allowing 'chain-bending' such that the angle between the terminal polar NO2 groups is approximately 100 degrees at 25 degrees C, and increases with increasing temperature. It is suggested that for concentrations greater than 0.015 g cm-3 molecules are associated in pairs, with a consequent reduction in polarisability, and an estimate of the enthalpy of this association is presented.

Dielectric behaviour of stiff polymers in solution when subjected to high voltage gradients

We report measurements of dielectric saturation on solutions of poly-γ-benzyl-L-glutamate and ethyl cellulose. Saturation effects of considerable magnitude were observed, particularly with the former polymer. Measurements were made by the simultaneous application of high static and low alternating fields to these macromolecules in solution. When poly-γ-benzyl-L-glutamate was the solute, this technique has enabled us to observe, for the first time field-induced changes in the dielectric relaxation times. The influence of such variables as field strength, concentration, average molecular weight and average dipole moment on both the dielectric saturation and critical frequency shifts are reported.