Intermolecular quadrupolar relaxation

Quadrupolar relaxation is frequently used to obtain information about molecular motion and intermolecular interactions in liquids and solutions. 

The effect of density on the relaxation time for benzene and perfluo-robenzene is shown in Figure 2 for the two temperature extremes. At high densities/low temperatures the relaxation times for both and C6H6 are similar. At density values > 0.8 g/cm the relaxation times for CeFe over the temperature range studied (25-150°C) were very similar. Benzene has a large variation in Ti as a function of temperature and density. The Ti values for C Fe and CeHe in CO2 are similar to those of the pure liquids for the lower temperature (31,33). The relaxation time for CeDe was much faster than the other two solute molecules due to quadrupolar relaxation. Using CeDe allows one to separate the intermolecular from the intramolecular dipole-dipole relaxation contributions for this series of solute molecules. As the molecular reorientation correlation time in Eq. (5) is the same as the molecular reorientation correlation time in Eq. (2). For benzene the dipole-dipole intramolecular relaxation time... 

The application of relaxation time measurements to study segmental motion (in polymers) as well as diffusional chain motion is very well documented but is still a subject of study, particularly using the frequency dependence of relaxation times to test the detailed predictions of models (McBriety and Packer 1993). The anisotropy of reorientation can also be studied conveniently, and recent interest in motion of molecules on surfaces (e.g. water on porous silica) has been investigated with great sueeess (Gladden 1993). Since the dipolar interaction is usually both intermolecular and intramolecular, the relaxation of spin- /2 nuclei (e.g. H) in the same molecule as a quadrupolar nucleus (e.g. H) can permit a complete study of reorientation and translation at a microscopic level (Schmidt-Rohr and Spiess 1994). 

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