Polar solvent dipoles

Figure 6.22 Rearrangement of polar solvent dipoles (arrows) during the electron transfer process R + M — R + M. The initial stage is a vertical (Franck-Condon) electron transfer, and this is followed by reorganisation of the solvent dipoles...

If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. If the solvent is nonpolar, then the rough description of the interaction is dipole-induced dipole type. In polar solvents, dipole-dipole interactions also become important. The London forces are always present. For the calculation of dipole-dipole interaction energy, point dipole approximations are made which are poor description for large extended molecules. 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

According to its and NMR spectra, this vinylogous amide exists as the E-s-E form in polar solvents such as [D4]methanol, and as a mixture of Z-s-Z and E-s-E isomers in nonpolar solvents such as deuterio-trichloromethane (30 cmol/mol /36a) and 70 cmol/mol (36b)). As expected, the more dipolar E-s-E form is stabilized in polar solvents (dipole moment of the related E-s-E 3-dimethylaminopropenal 21 10 Cm). 

Surface potentials arise from electronic polarisation and (in a polar solvent) dipole orientation of the solvent molecules at the free surface of the solution. 

Coulomb forces are responsible for the stability of ionic crystals (e.g., NaCl). When such a compound is dissolved in a polar solvent (dipole moment //), dissociation and simultaneous solvation of the ions occur. The force of attraction between the ions is now inversely proportional to the dielectric constant of the solvent, and is thus reduced. New ion dipole forces are formed as a result of the attraction of the permanent dipoles of the solvent by the ions ... 

For a polar solute in polar solvents, dipole-dipole interactions will stabilize the ground state of the solute according to the polarity of the solvent. In this case the solvent molecules will be oriented around the ground-state dipole and, if the dipole moment is increased by the transition, the excited state will be stabilized more by more polar solvents (those with higher dielectric constants). The resulting red shift will again add to the general red shift compared to the gas phase. 

The solvation energy can be related to the dipole moment and the polarizabilities of the solute and solvent molecules. The most significant contribution to the solvatochromic shift comes from the dipole-dipole and solute polarization-solvent dipole interactions. For solvatochromism, the energy shift is given by... 

Fig. 6.19 Reorganization of polar solvent dipoles during PET process...

Van der Zwan G and Hynes J T 1984 A simple dipole isomerization model for non-equilibrium solvation dynamics in reactions in polar solvents Chem. Phys. 90 21-35... 

This h)rpothesis has, however, been supported. The o p-ratio in chlorobenzene was found to be lower when acetic anhydride was the solvent, than when nitric acid or mixed acids were used. The ratio was still further reduced by the introduction into the solution of an even less polar solvent such as carbon tetrachloride, and was increased by the addition of a polar solvent such as acetonitrile. The orientation of substitution in toluene in which the substituent does not posses a strong dipole was found to be independent of the conditions used. The author... 

Once again, frequency shifts provide a good way of examining the solvent effect. The highly polar solvent produces about the same shift for each compound with the exception of fortnamide, where it is quite a bit greater, the result of combining the polar solvent with formamide s rather large dipole moment. ... 

However, polar solvents shift the equilibrium toward the Z conformer 12a, which has a significantly larger dipole moment. 

If we had any means of reducing the electrostatic repulsion without, at the same time, affecting the quantum-mechanical attraction, we should have the possibility of forming such doubly charged molecular ions. Now a polar solvent has just the required properties the alignment of the solvent dipoles greatly reduces the electrostatic repulsion, but the quantum-mechanical forces of attraction arise from the rapid motion of... 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

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