Dipolar vector

Angular restraints are another important source of structural information. Several empirical relationships between scalar couplings and dihedral angles have been found during the last decades. The most important one is certainly the Karplus relation for -couplings. Another, relaxation-based angular restraint is the so-called CCR between two dipolar vectors or between a dipolar vector and a CSA tensor. 

Chou PT, Pu SC, Cheng YM et al (2005) Femtosecond dynamics on excited-state proton/ charge-transfer reaction in 4 -N, N-diethylamino-3-hydroxyflavone. The role of dipolar vectors in constructing a rational mechanism. J Phys Chem A 109 3777-3787... 

Fig. 8.1 Orientation of two dipolar coupling vectors in a protein segment. The vectors connect the amide Hn and 15N atoms. In this case the interaction vector coincides with the chemical bond. The axis system of the alignment tensor is designated as A, Aw Aa. The angles ( n, y>n, and 02, define the orientation of the two dipolar vectors with respect to the alignment tensor. (Reproduced with permission from N. Tjandra, Structure 1999, 7, R205-R211.)...

Dipolar couplings of side chains can provide valuable structural information. Dipolar couplings for methylene and methyl sites have been applied to protein structure refinement [49]. In a side chain with a C-CH3 moiety, rapid rotation of the methyl group results in an averaged C-H dipolar vector aligned in the direction of the C-C bond. This rapid rotation scales the real value of the C-H dipolar by a factor of —1/3. Therefore, when including methyl dipolar couplings in protein structure refinement, the orientation of the C-CH3 bond is the one that is restricted, rather than the C-H bond. 

Figure 11 The direction of the dipolar vector, BC-170, in the principal axis system of EFG tensors, defined by azimuth and polar angles, denoted as a and / , respectively.

While the procedure outlined above yields the principal components of the CS tensor, NMR spectra of isolated spins in powder samples generally do not contain information about the orientation of the CS tensor in the molecular framework. This may sometimes be deduced from those determined for similar compounds, or from the local molecular symmetry. In our lab, we have often relied on the dipolar CS method, " which uses the orientation dependence of the dipolar interaction between a spin pair to orient the principal components of the CS tensor relative to the dipolar vector. Remaining ambiguities in the tensor orientation may be resolved via first-principles ca Iculations. ... 

EFISH has usually been used to evaluate nonlinearities of neutral organometallic complexes, the presence of ionic species rendering it difficult to apply high electric fields to a solution. It is also not possible to utilize EFISH when the complex has no net dipole, and so the method is not useful for molecules of trigonal symmetry, for example, although they can have a strong second-order nonlinearity of octupolar type (i.e. leading to a non-zero septor part of P) with a zero dipolar (vector) part of P... 

---