Molecular dipole moments, interaction energies

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. 

Since the parameters used in molecular mechanics contain all of the electronic interaction information to cause a molecule to behave in the way that it does, proper parameters are important for accurate results. MM3(2000), with the included calculation for induced dipole interactions, should model more accurately the polarization of bonds in molecules. Since the polarization of a molecular bond does not abruptly stop at the end of the bond, induced polarization models the pull of electrons throughout the molecule. This changes the calculation of the molecular dipole moment, by including more polarization within the molecule and allowing the effects of polarization to take place in multiple bonds. This should increase the accuracy with which MM3(2000) can reproduce the structures and energies of large molecules where polarization plays a role in structural conformation. 

The principle behind this investigation is electrochromism or Stark-effect spectroscopy. The electronic transition energy of the adsorbed chromophore is perturbed by the electric field at the electric double layer. This is due to interactions of the molecular dipole moment, in the ground and excited states, with the interfacial electric field induced by the applied potential. The change in transition frequency Av, is related to the change in the interfacial electric field, AE, according to the following ... 

The polarity of a solid surface can be regarded as the strength of its average electrostatic field F. This field interacts with permanent or induced dipoles of adsorbed molecules, whereas its gradient interacts with permanent or induced quadrupoles. These interactions give rise to components of the adsorption energy such as EFfl (with permanent dipoles), Ef (polarization contribution) or PQ (with permanent quadrupoles). Therefore, the selection of the appropriate immersion systems, differing mainly in the molecular dipole moment n of the immersion liquid, can be expected to provide information on the value of EPfl since ... 

The effect of induced dipoles in the medium adds an extra term to the molecular Hamilton operator. = -r R (16.49) where r is the dipole moment operator (i.e. the position vector). R is proportional to the molecular dipole moment, with the proportional constant depending on the radius of the originally implemented for semi-empirical methods, but has recently also been used in connection with ab initio methods." Two other widely available method, the AMl-SMx and PM3-SMX models have atomic parameters for fitting the cavity/dispersion energy (eq. (16.43)), and are specifically parameterized in connection with AMI and PM3 (Section 3.10.2). The generalized Bom model has also been used in connection with force field methods in the Generalized Bom/Surface Area (GB/SA) model. In this case the Coulomb interactions between the partial charges (eq. (2.19)) are combined... 

In MM3 the option exists to compute charge-charge and charge-dipole interaction energies between ions and between ions and polar molecules. The values for the bond dipoles were chosen to reproduce the molecular dipole moments for test molecules. It has been demonstrated that bond dipoles may be derived to reproduce the quantum mechanical molecular electrostatic potential (ESP) and that a dipole model can perform as effectively as the more common partial atomic charge model derived from the same potential.The... 

This wavefunction has a lower energy than we would obtain by multiplying 1/ (1) and 1/ (2) because the two terms in the wavefunction are both stabilized by an effective dipole-dipole interaction energy. The first term, i/ a-(1) Ab-(2). has two molecular dipole moments pointing in the +z direction, while the second term has two dipole moments pointing in the —z direction. Both are attractive orientations, so the two molecules are more stable next to each other than separated. However, because both terms are equally likely, there is still no net molecular dipole moment on either molecule (Fig. 10.13). 

The molecular descriptors used are as follows CMR (Calculated molar refractivity), pZ (Z component of the dipole moment), HOMO (Energy of the highest occupied molecular orbital), FZ (9), FY (6) and FY (11) (Z and Y are the components of the electric field at specified grid points), VDWE (4) (The van der Waal s energy of the interaction of a carbon atom at a specified grid point), ALP (3) (The self atom polarizability of the specified atom). 

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