Water molecular dipole moment

The SPC/E model approximates many-body effects m liquid water and corresponds to a molecular dipole moment of 2.35 Debye (D) compared to the actual dipole moment of 1.85 D for an isolated water molecule. The model reproduces the diflfiision coefficient and themiodynamics properties at ambient temperatures to within a few per cent, and the critical parameters (see below) are predicted to within 15%. The same model potential has been extended to include the interactions between ions and water by fitting the parameters to the hydration energies of small ion-water clusters. The parameters for the ion-water and water-water interactions in the SPC/E model are given in table A2.3.2. 

FIG. 3 Left density profile, p z), from a 500 ps simulation of a thin film consisting of 200 TIP4P water molecules at room temperature. Right orientational distribution, p cos d), with 3 the angle between the molecular dipole moment p and the surface normal z. The vertical lines in the left plot indicate the boundary z-ranges,... 

According to the Kirkwood theory of polar dielectrics, simple relations (23) between molecular dipole moment vectors and the mean-square total dipole moment of water clusters can be used to compute the static dielectric constant of water. As the normalized mean-square total dipole moment increases towards unity, theory predicts decreases in the static dielectric constant. Since MD results indicate that the mean-square total dipole moment of interfacial water is greater than that for bulk water (48), the static dielectric... 

An analysis of the Wannier functions in CPMD simulations of one dimethyl sulfoxide (DMSO) molecule dissolved in water was carried out by us in 2004 in order to gain more insight into the unusual properties of the DMSO-water mixture (72). In this special case, we have utilized MLWCs to calculate molecular dipole moments of the DMSO molecule in gas phase and aqueous solution. Comparing those two a large increase of the local dipole... 

For the description of a solution of alanine in water two models were compared and combined with one another (79), namely the continuum model approach and the cluster ansatz approach (148,149). In the cluster approach snapshots along a trajectory are harvested and subsequent quantum chemical analysis is carried out. In order to learn more about the structure and the effects of the solvent shell, the molecular dipole moments were computed. To harvest a trajectory and for comparison AIMD (here CPMD) simulations were carried out (79). The calculations contained one alanine molecule dissolved in 60 water molecules. The average dipole moments for alanine and water were derived by means of maximally localized Wannier functions (MLWF) (67-72). For the water molecules different solvent shells were selected according to the three radial pair distributions between water and the functional groups. An overview about the findings is given in Tables II and III. 

In addition to the energies, the values predicted by these SCF calculations for a number of the one-electron properties of the water molecule are also compared by Kern and Karplus.6 As anticipated, the values predicted for these properties by the near Hartree-Fock functions are in good agreement with the experimental values. The molecular dipole moment, for example is calculated to be 2.054 D by the best STO set compared with an experimental value of 1.884 D.57 (Hartree-Fock estimates of molecular dipole moments are generally too large by ca. 0.2 D.)... 

By analyzing the Wannier orbitals of a sample of 32 water molecules, Silvestrelli et ah [242] have determined the average molecular dipole moment of water molecules in the liquid phase. They have assigned all Wannier centers... 

Carbon dioxide, CO2, is a three-atom molecule in which each carbon-oxygen bond is polar because of the electronegativity difference between C and O. But the molecule as a whole is shown by experiment (dipole moment measurement) to be nonpolar. This tells us that the polar bonds are arranged in such a way that the bond polarities cancel. Water, H2O, on the other hand, is a very polar molecule this tells us that the H—O bond polarities do not cancel one another. Molecular shapes clearly play a crucial role in determining molecular dipole moments. We will develop a better understanding of molecular shapes in order to understand molecular polarities. 

Physical Properties Sulfur mustard (mustard gas) is a colorless oil with bp of 227°C, mp of 14°C, molecular dipole moment 1.78 D (hexane), and molecular mass of 159. It normally is encountered as an impure, pale yellow-brown, odoriferous liquid. The color generally deepens with increasing amounts of impurity. HD has a vapor density of 5.4 relative to air and a vapor pressure of 0.072 mm Hg at 20°C. As a liquid, it is slightly denser than water (1.27 g/mL at 20°C). It is miscible in typical organic solvents (e.g., carbon tetrachloride, acetone or chloroform) but has a lower solubility in water (0.092 g/100 g at 22°C) (Sidell et al., 1998 Somani, 1992). 

A classification scheme for molecular polarity can be based on the electric potential surrounding the molecule. Traditionally, schemes for ranking molecular polarity are based on electronegativity differences, on molecular dipole moments, or on solubility. The root-mean-square (rms) value of the molecular electric potential can be used for this purpose. Table 1 shows a ranking of 24 small molecules according to their rms electric potential. Note that all of the amides are extremely polar by this criterion, more polar than formaldehyde or water. Hydrogen fluoride is less polar than acetone, acetaldehyde, water, or the amides. At the low end of the polarity scale, the amines are surprisingly nonpolar. As expected acetylene is more polar than ethylene, which is more polar than methane. 

Dinur and Hagler propose a novel method to determine atomic point charges and point dipoles from derivatives of the molecular dipole moment and second moments. The method is limited to planar molecules and has been applied to hydrogen fluoride, water, formaldehyde, formamide, ethylene, benzene, and pyridine. As was also noted by Williams, they found that atomic dipoles do not necessarily point along the bond directions. Price proposed a distributed multipole model for several aromatic hydrocarbons using carbon sites only. 

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