Molecules polar, charge distribution

An important feature of QM/MM methods is that the polarization of the solute molecule s charge distribution by the solvent can be evaluated, since the wave function of the former is... 

In Fig. 7, S denotes a polar molecule whose charge distribution in the polarizing field of the cation M+ may be approximated as follows Zi, Z2, Z3, Z4 denote the centres of positive and negative charges, respectively. The potential electrostatic energy between M+ and S is determined by the arithmetic sum of the interactions between Zi, Zg (attraction) and Zi, Z3-I-Z1, Z4 (repulsion). If distance b is large compared with distance a, the potential electrostatic energy between M+ and S may as a first approximation be described by interaction of Zi,... 

Attractive forces are present in gases composed of polar molecules. Nonuniform charge distribution on polar molecules creates positive and negative regions, resulting in electrostatic attraction and deviation from ideality. 

Dipole (polar molecule) A molecule whose charge distribution can be represented by a center of positive charge and a center of negative charge, which do not coincide. 

Bond and Valence Indices. - Atomic charges, bond indices (two-centre and three-centre) and valences have been calculated for a number of closed-shell molecules using HF and KS orbitals. It was observed that, compared to the HF orbitals, the KS orbitals predicted a slightly enhanced covalency. Beside AIM the authors employed MuUiken, Lowdin and Natural Population Analysis and noted that the AIM scheme led to a more polar charge distribution than NPA. 

Now let us examine the molecular origin of Molecular polarity may be the result of either a permanent dipole moment p or an induced dipole moment ind here the latter arises from the distortion of the charge distribution in a molecule due to an electric field. We saw in Chap. 8 that each of these types of polarity are sources of intermolecular attraction. In the present discussion we assume that no permanent dipoles are present and note that the induced dipole moment is proportional to the net field strength at the molecule ... 

The and Oj terms always contribute, regardless of the specific electric charge distributions ia the adsorbate molecules, which is why they are called nonspecific. The third nonspecific Op term also always contributes, whether or not the adsorbate molecules have permanent dipoles or quadmpoles however, for adsorbent surfaces which are relatively nonpolar, the polarization energy Op is small. 

At the molecular level, electric dipole moments are important because they give information about the charge distribution in a molecule. Examination of the experimental data for a few simple compounds reveals that the electric dipole moment is also a property associated with chemical bonds and their polarity. The... 

The Self-Consistent Reaction Field (SCRF) model considers the solvent as a uniform polarizable medium with a dielectric constant of s, with the solute M placed in a suitable shaped hole in the medium. Creation of a cavity in the medium costs energy, i.e. this is a destabilization, while dispersion interactions between the solvent and solute add a stabilization (this is roughly the van der Waals energy between solvent and solute). The electric charge distribution of M will furthermore polarize the medium (induce charge moments), which in turn acts back on the molecule, thereby producing an electrostatic stabilization. The solvation (free) energy may thus be written as... 

In connection with electronic strucmre metlrods (i.e. a quantal description of M), the term SCRF is quite generic, and it does not by itself indicate a specific model. Typically, however, the term is used for models where the cavity is either spherical or ellipsoidal, the charge distribution is represented as a multipole expansion, often terminated at quite low orders (for example only including the charge and dipole terms), and the cavity/ dispersion contributions are neglected. Such a treatment can only be used for a qualitative estimate of the solvent effect, although relative values may be reasonably accurate if the molecules are fairly polar (dominance of the dipole electrostatic term) and sufficiently similar in size and shape (cancellation of the cavity/dispersion terms). 

The charge distribution of neutral polar molecules is characterized by a dipole moment which is defined classically by jx = E, , , where the molecular charge distribution is defined in terms of the residual charges (qt) at the position r,. The observed molecular dipole moment provides useful information about the charge distribution of the ground state and its ionic character. 

The calculations so far have been concerned with the permanent charge distribution which the molecule would have when isolated, and which would be observed in its dipole moment. The factor that actually determines the orientation of the entering group, however, is the charge distribution at the instant of attack, which differs from the permanent charge distribution in consequence of polarization by the entering group. In most cases it is not necessary to take this latter effect into account explicitly, since it represents only a small correction. However, in certain cases it is apparently of decisive importance.7... 

A common feature of many mesogenic molecules is the presence of polar substituents and aromatic cores [3]. The electrostatic interactions between such groups can be incorporated into a molecular potential with the addition of dipolar and quadrupolar terms, respectively. Rather than represent these permanent electrostatic interactions by using a model in which a charge distribution is scattered over the surface of the molecule, it is very common to use one (or more) point multipoles [2,29]. Thus for an electrostatic Gay-Berne model, the pair potential is given by the sum... 

When one places an electron into the donor molecule, the equilibrium fast polarization, which is purely electronic forms first. Being independent of the electron position, it is unimportant for the dynamics of electron transfer. Afterward the average slow polarization Pg, arises that corresponds to the initial (0 charge distribution (the electron in the donor). The interaction of the electron with this polarization stabilizes the electron state in the donor (with respect to that in the isolated donor molecule) (i.e., its energy level is lowered) (Fig. 34.1). At the same time, a given configuration of slow, inertial polarization destabilizes the electron state (vacant) in the acceptor (Fig. 34.1). Therefore, even for identical reactants, the electron energy levels in the donor and acceptor are different at the initial equilibrium value of slow polarization. 

---