Lattice vibrations rotation interaction

For intramolecular vibrations, each site was considered independently. However, the reorganizations in the surrounding solvent are necessarily properties of both sites since some of the solvent molecules involved are shared between reactants. The critical motions in the solvent are reorientations of the solvent dipoles. These motions are closely related to rotations of molecules in the gas phase but are necessarily collective in nature because of molecule—molecule interactions in the condensed phase of the solution. They have been treated theoretically as vibrations by analogy with lattice vibrations of phonons which occur in the solid state.32,33... 

As for all the systems relegated to Section 2 the attenuation function for structural H2O in the microwave and far-infrared region, as well as that for free H2O, can be understood in terms of collision-broadened, equilibrium systems. While the average values of the relaxation times, distribution parameters, and the features of the far-infrared spectra for these systems clearly differ, the physical mechanisms descriptive of these interactions are consonant. The distribution of free and structural H2O molecules over molecular environments is different, and differs for the latter case with specific systems, as are the rotational dynamics which govern the relaxation responses and the quasi-lattice vibrational dynamics which determine the far-infrared spectrum. Evidence for resonant features in the attenuation function for structural H2O, which have sometimes been invoked (24-26,59) to play a role in the microwave and millimeter-wave region, is tenuous and unconvincing. 

To study the reaction of molecules on surfaces in detail, it is important to consider the different degrees of freedom the system has. The reacting molecule s rotational, vibrational and translational degrees of freedom will affect its interaction as it hits the surface. In addition, the surface has lattice vibrations (phonons) and electronic degrees of freedom. Both the lattice vibrations and the electrons can act as efficient energy absorbers or energy sources in a chemical reaction. 

Spectra of samples in the liquid state (Fig. 2.6-lB) are given by molecules which may have any orientation with respect to the beam of the spectrometer. Like in gases, flexible molecules in a liquid may assume any of the possible conformations. Some bands are broad, since they are the sum of spectra due to different complexes of interacting molecules. In the low frequency region spectra often show wings due to hindered translational and rotational motions of randomly oriented molecules in associates. These are analogous to the lattice vibrations in molecular crystals, which, however, give rise to sharp and well-defined bands. The depolarization ratio p of a Raman spectrum of molecules in the liquid state (Eqs. 2.4-11... 13) characterizes the symmetry of the vibrations, i.e., it allows to differ between totally symmetric and all other vibrations (see Sec. 2.7.3.4). 

One can also wonder, which is the original cause of the ferroelectric-para-electric phase transitions in KDP-type crystals the H-hopping or the vibrations (rotations) of the H-bonded molecules. It has been shown that the angular displacements and the H-sites are coupled, thus the vibrations of molecules destabilize and facilitate the H-hopping. This coupling is essential for understanding the interactions between the lattice-mode vibrations in crystals, and the transformations in hydrogen bonds. Also other features of the KDP crystals, like the existence of soft modes, can be explained in this way. 

However, in molecular crystals (see Sec. II.E), in addition to the lattice vibrations, one can observe rotational vibrations of the molecules about their principal axes of inertia these are the so-called librations (from libre in French, meaning free to rotate). Of course, the symmetry of the elementary cell of the crystal determines which vibrations and/or librations are manifested in the Raman spectra (see Sec. V.E). In the crystalline state, the interactions between atoms, ions or molecules are stronger, so that the observed lines are generally broader. Rotations of bonds such as —C—N or —O— H in nonmolecular crystals are also considered as librations. Likewise, any chemical radical free to rotate about its inertia axis can be said to librate (e.g., —CO3 or —SO4, as long as such motion is not blocked by additional bonds). 

An ideal gas has by definition no intermolecular structure. Also, real gases at ordinary pressure conditions have little to do with intermolecular interactions. In the gaseous state, molecules are to a good approximation isolated entities traveling in space at high speed with sparse and near elastic collisions. At the other extreme, a perfect crystal has a periodic and symmetric intermolecular structure, as shown in Section 5.1. The structure is dictated by intermolecular forces, and molecules can only perform small oscillations around their equilibrium positions. As discussed in Chapter 13, in between these two extremes matter has many more ways of aggregation the present chapter deals with proper liquids, defined here as bodies whose molecules are in permanent but dynamic contact, with extensive freedom of conformational rearrangement and of rotational and translational diffusion. This relatively unrestricted molecular motion has a macroscopic counterpart in viscous flow, a typical property of liquids. Molecular diffusion in liquids occurs approximately on the timescale of nanoseconds (10 to 10 s), to be compared with the timescale of molecular or lattice vibrations, to 10 s. 

The interaction of radiation with matter can take many forms. The photoelectric effect, the Compton effect, and pair generation-armihilation are processes that occur at wavelengths shorter than those encountered in the infrared. Infrared photons can excite rotational and vibrational modes of molecules, but they are insufficiently energetic to excite electronic transitions in atoms, which occur mostly in the visible and ultraviolet. Therefore, a discussion of the interaction of infrared radiation with matter in the gaseous phase needs to consider only rotational and vibrational transitions, while in the solid phase lattice vibrations in crystals must be included. 

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