C—2H bond

C-2H bond performs rotational random jumps on the surface of a cone with a full opening angle X = 6° (from Ref. 93). (b) Experimental 2H NMR spectra of chlorobenzene-ds in a mixture with cis-decalin at various temperatures T < Tg and at a long solid-echo delay tp [429]. 

If a C-2H bond diffuses uniformly over the surface of a cone having semiangle 6C, we have 

If a C-2H bond diffuses freely with the diffusion coefficient Z)w in a cone of semiangle 9C, then T is given by84 

Fig. 14. Effects of small-amplitude reorientation on 2H NMR experiments, as calculated by means of RW simulations. In the model, C-2H bonds (<5 = 2n 125 kHz, rj = 0) perform rotational random jumps on the surface of a cone with a full opening angle % = 6°. (a) 2H NMR spectra for various solid-echo delays tp (tj = t = 30 pis), and (b) 2H NMR correlation functions Fcos(tm) for various evolution times tp (tj = t = 10ms). (Adapted from Ref. 76.)

The predominant interaction for a 2H spin system is the quadrupolar interaction, which couples the electric quadrupole moment of the 2H nucleus to its electronic surrounding. This interaction is a second-rank tensor Hq which lies approximately along the C-2H bond in organic molecules. Thus, in practice, 2H nuclei may be considered to be isolated. It shows that the 2H NMR formalism is similar to that of an isolated proton pair [8]  

In a solid system, the resonance spectrum spreads over a wide frequency range ( 250 kHz), with a structure related to the static orientational distribution of C-2H bonds in the system (powder spectrum). 

In case of a deuterated sample (spin 1 case), the spectra are usually dominated by the quadrupolar interaction, that is, the coupling of the nuclear quadrupole moment with the electric field gradient of the C-2H bond. For deuterons in C-2H bonds this can lead to a splitting of about 250 kHz. As in the case of dipolar interaction, a Pake spectrum is obtained for a powder sample. The z-principal axis of the quadrupolar interaction is oriented along the bond axis which makes deuteron NMR particularly useful for studies of segmental orientations and molecular dynamics (reorientation) [1], 

The authors also evidenced an increase of the percentages of a-talo-pyranose and a-ia/ofuranose upon 2H substitution at C(l) concomitantly with a decrease of the percentages of (l-conformers. This observed shift on the talopyranose anomeric equilibrium was attributed to the preference of the shorter C-2H bond (relative to the C l l bond) to assume an equatorial orientation. 

RW simulations were performed to demonstrate that the use of suited inter-pulse delays tp in 2H NMR solid- and stimulated-echo experiments allows one to resolve spatially highly restricted reorientations.76 For this purpose, it was assumed that C-2H bonds (t] — 0) show rotational random jumps (cf Fig. 25) where all possible 

H NMR spectra are profoundly influenced by motions.1 7 It is possible to calculate the powder spectra in the presence of any type of motion by taking into account the geometry, and amplitudes and rates of motions involved.80,81 Consider a C-2H bond which can jump between three equivalent sites, 

In addition to the large amplitude motion, the libration is expected to exist at higher temperatures as well as at lower temperatures, so that the libration was taken into account as small angle three-site jumps. As a result, the C-2H bond undergoes motions superimposed with two independent three-site jumps. In the calculations, a 0 value of 14° at room temperature was used as the polar angle of the libration, and its distribution was not 

By an order parameter S =Sqd thus the residual anisotropy, which is not averaged by fast local motions, is characterised. This is the main information obtained for partially ordered systems, and can be considered a characterisation of the mobility of the C-2H bond, which on the other hand describes the chain packing order in oriented aggregates. 

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