Dipolar molecule isotropic solution

Nuclear spins can be considered as dipoles that interact with each other via dipolar couplings. While this interaction leads to strongly broadened lines in soUd-state NMR spectroscopy, it is averaged out in isotropic solution due to the fast tumbUng of the solute molecules. In Uquid-state NMR spectroscopy, the dipolar interaction can only be observed indirectly by relaxation processes, where they represent the main source of longitudinal and transverse relaxation. 

When a dipolar molecule is dissolved in a magnetically anisotropic solvent consisting of disc-shaped molecules, e.g. benzene, the NMR signals of the solute H-atoms are usually shifted upheld with respect to their positions in an isotropic solvent such as 2,2-dimethylpropane (neopentane) or tetraehloromethane cf. the preceding discussion of (Ta in Eq. (6-21). The specific solvent-induced H NMR chemical shift of a solute H-atom signal when the solvent is changed from a reference aliphatic solvent to an aromatic solvent is called aromatic solvent-induced shift (ASIS) and is defined according to Eq. (6-24) [278, 279],... 

The work reported here has shown that inclusion complexation of organic and organometallic chromophores by thiourea, TOT and cyclodextrins can induce second harmonic generation capability in the polar crystals which result, even when the original bulk materials are themselves incapable of SHG. Structural evidence has been presented to show tht the solid state inclusion structures are acentric, and a simple electronic picture t0 the polarization response of these materials within the two-state modeP ° has been discussed. In an earlier section we remarked that of the many complexes we have made, only one has NOT been acentric. This result was not anticipated. We postulate that it is a natural tendancy in such materials, rather that an exception. If we consider a dipolar molecule in isotropic solution, we can imagine that if it were to aggregate, it would do so in a head to tail fashion in order to minimize electrostatic repulsion. The situation is illustrated in Scheme 3. The arrangement that would result is centrosymmetric. 

NMR spectroscopy in isotropic solution is characterised by the fact that free and random molecular reorientation averages many parameters that have a directional dependence, e.g. dipolar-coupling constants, chemical shifts, etc. Some parameters, such as dipolar-coupling constants, average to zero in isotropic solution so once the ability of a molecule to rotate and move freely and isotropically is removed, its NMR spectra immediately become more complex. 

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

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