Surrounding dipoles

Further computations [98] confirmed that, just as point charges can accurately reproduce the effects of full ions, polar groups can be simulated satisfactorily by point dipoles. The only exception occurs when the polar group is in the proper position and orientation to form a hydrogen bond with the system in question. In such a case, the perturbation is somewhat larger than would be predicted solely on the basis of the electrostatic field propagated by a pure point dipole. 

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In the excited state, the redistribution of electrons can lead to localized states with distinct fluorescence spectra that are known as intramolecular charge transfer (ICT) states. This process is dynamic and coupled with dielectric relaxations in the environment [16]. This and other solvent-controlled adiabatic excited-state reactions are discussed in [17], As shown in Fig. 1, the locally excited (LE) state is populated initially upon excitation, and the ICT state appears with time in a process coupled with the reorientation of surrounding dipoles. 

Fig. 1 Simplified energy diagram showing the influence of molecular relaxations (with lifetime tr) on the energies of LE and ICT states. The ICT states can be strongly stabilized in polar media by orientation of surrounding dipoles resulting in substantial shifts of fluorescence spectra to lower energies (longer wavelengths)...

These data may be explained in terms of the above mechanism of the long-wavelength shift of fluorescence spectra for red-edge excitation. The properties of the environment of the tryptophan residues in the proteins studied are such that during the lifetime of the excited state, structural relaxation of the surrounding dipoles fails to proceed. Studies of the dependence of the... 

Further developments in the theory of the structure of polar liquids included estimates of the correlation of a given dipole to its neighbors. Important contributions were made in this direction by Kirkwood [22] and Frohlich [23]. In Kirkwood s model, the field Ej is calculated by considering all possible orientations of surrounding dipoles in a spherical cavity for a fixed orientation of the central dipole. By averaging over these orientations, Kirkwood obtained an improved estimate of the polarization of the medium. For the case of non-polarizable dipoles the result is... 

In addition, if again the adsorbed layer is composed exclusively of i species and Xj is the field due to the surrounding dipoles acting on... 

The vibrational frequencies of each of the adsorbed molecules are determined by the electric field of surrounding dipoles due to the vibrational Stark effect ... 

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