13C Nuclear Spin Relaxation Study of 1-Bromo-2-phenylacetylene. Determination of Carbon–Bromine Scalar Coupling Constant Including Its Sign
✍ Scribed by Andrzej Ejchart; Adam Gryff-Keller
- Publisher
- Elsevier Science
- Year
- 1996
- Tongue
- English
- Weight
- 103 KB
- Volume
- 122
- Category
- Article
- ISSN
- 1064-1858
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✦ Synopsis
Determination of the spin-spin coupling constants be-In oriented phases, the parallel and the perpendicular components of the effective coupling constants become tween spin-1 2 and quadrupole nuclei is usually complicated by the rapid relaxation of the latter (1). The appropriate scalar coupling constants have been usually determined with
] the aid of the interpretation of spin-1 2 longitudinal relaxation J ef ⊥ Å J 0 (DJ/3 / D)s.
[3] data. This method relies on extraction of the mechanism of scalar relaxation of the second kind from the total longitudi-
The anisotropy of the indirect interaction, DJ Å J 0 J ⊥ , nal relaxation rate of the spin-1 2 nucleus (2). This mechanism contributes to the effective coupling, although a much arises from fluctuation of the magnetic field at the given stronger part comes from the direct interaction (6), nucleus owing to the orientation changes of the nuclear magnetic dipole of the quadrupolar nucleus. A rigorous explana-
] tion of this phenomenon within the frame of the Redfield theory has been given (3,4).
According to Abragam (1), the rate of scalar relaxation of where D represents the dipolar coupling constant which can the second kind of nucleus A coupled to the rapidly relaxing be easily calculated from molecular geometry; s Å »3 cos 2 u nucleus X is given by the equation 0 1…/2 is the order parameter; r AX is the effective, vibrationally averaged distance between interacting nuclei; u is the instantaneous angle between r AX and the static magnetic
field; and the remaining symbols have their usual meaning.
Longitudinal relaxation requires magnetic-field fluctua-The scalar coupling constant J ( in hertz ) appears in the tions perpendicular to the external magnetic field. In the above equation in the second power and, therefore, the anisotropic surrounding, such fluctuations generated by the relaxation rate R 1SC is independent of its sign. One may neighboring nucleus are proportional to J ⊥ (1, 5). Therefore, note, however, that in the oriented phase this mechanism in oriented phases the scalar coupling constant J in Eq. [1] does offer a possibility of sign determination of the scalar should be replaced by J ef ⊥ . It appears that, in convenient coupling constant. In oriented phases, the interaction besituations, the order-parameter dependence of the effective tween magnetic nuclei consists of two parts: scalar ( indicoupling constant should allow the determination of the magrect ) and dipolar ( direct ) . The latter is averaged to zero in nitude and sign of the scalar coupling constant. Similar forisotropic phases by the rapid molecular reorientation (1, 5 ). mulae have been given by Lopes Cardozo et al. (7). Their Solute molecules in liquid crystalline media still undergo formulation differs only by another definition of D and the rapid reorientation but the averaged value of the dipolar omission of DJ. These authors observed line broadening of coupling constant is generally nonzero owing to their parthe nitrile carbon signal of acetonitrile dissolved in Merc tial alignment ( 6 ).
phase V owing to transverse dipolar-scalar relaxation of the In the nonisotropic surroundings characterized by the axial second kind. Similar phenomena were treated by Imbardelli symmetry, both direct and indirect interactions are described et al. (8), who investigated pyridine in a nematic solvent by the Hamiltonian and observed remarkable line broadening of proton signals caused by the influence of the 14 N nucleus. However, the contribution of the dipolar relaxation of the