Deuterons, aliphatic

Fig. 13. Calculated 2H solid echo spectra for log-Gaussian distributions of correlation times of different widths. Note the differences of the line shapes for fully relaxed and partially relaxed spectra. The centre of the distribution of correlation times is given as a normalized exchange rate a0 = 1/3tc. For deuterons in aliphatic C—H bonds the conversion factor is approximately 4.10s sec-1...

Fig. 14. Experimental (symbols) and fitted (lines) spectral densities of 5CB-di5. J and J2 data are smoothed experimental values, (a), (b), (c), and (d) contain C1 C2, C3, and C4 data, respectively. In addition, the C0 (ring) data are also shown in (b). For the aliphatic deuterons, circles, uptriangles, and diamonds denote /o(0), and /2(2< ), while for the ring deuterons, downtriangles and squares denote J co) and J2(2a>), respectively. Closed symbols are for 46 MHz and open symbols are for 15.1 MHz. Solid and dashed curves are fitted spectral densities. (Adapted from Ref. 180.)...

The validity of (3.2.9) is restricted to the symmetries mentioned above, that is to cylindrical molecules, macroscopically uniaxial samples, and r] = 0. For many samples, these conditions are fulfilled when using NMR, because the quadrupole coupling tensor of aliphatic deuterons is often found to be axially symmetric. In wideline NMR, the anisotropy of the magnetic shielding is used. Here the angular resolution is lower, and the calculation has to be extended to include p > 0 [Henl]. In combination with MAS (cf. Section 3.3), the Legendre subspectral analysis has been used successfully for the determination of molecular order in partially ordered polymers [Harl]. 

The chemical shift range for deuterium, as well as its dipolar and scalar interactions, are of the order of only a few (<5) kHz at 11.7 T. Thus, they can be ignored in view of the dominant quadrupolar coupling constant, which is e Qlh — 167 kHz for aliphatic C— H bonds [19, 21]. Here, e is the elementary charge and eq the largest field gradient. Q = 2.875 x 10 cm is the scalar quadrupole moment for the deuteron, and h is Planck s constant. Since the deuterium quadrupolar frequency is much smaller than the Zeeman frequency difference, the H NMR spectrum can be described by treating the quadrupolar interaction as a first-order perturbation of the Zeeman interaction [1, 19, 21, 60]. 

Hydrogen in the stable bonds of aliphatic and aromatic compounds can be exchanged only under particularly energetic conditions. It is favored, for instance, by solid catalysts, by strong bases which ease the removal of protons in such cases, or by strong acids which act as effective deuteron-donors. An acid-base-catalysed ionic mechanism and an electrophilic substitution mechanism have both been discussed as explaining the formation of deuterated compounds.70,71... 

Deuterium NMR spectroscopy of the discotic phase of hexa-n-hexyloxy triphenylene has led to similar conclusions. Spectra of two selectively deuterated isotopic species, one in which all aromatic positions are substituted and the other in which only the a-carbon side chains are substituted, bring out the difference between the order parameters of the cores and the tails. Fig. 6.1.3 gives the quadrupole splittings of the aromatic and the a-aliphatic deuterons versus temperature in the meso-phase region. It is seen that the rigid core is highly ordered, the orientational order parameter s ranging from 0.95 to 0.90, whereas the a-aliphatic chains are in a disordered state. 

Fig. 6.1.3. Quadrupole splittings for the aromatic and a-aliphatic vj deuterons of deuterated hexa-n-hexyloxytriphenylene (THE6) as functions of temperature (r— 7 ) in the mesophase region, where 7 is the mesophase-isotropic transition point. The open circles correspond to measurements on neat THE6-ard, and THE6-adi2 separately, while the filled circles correspond to a 2 1 mixture of the two isotopic species. The scale on the upper right-hand side gives the orientational order parameter of the aromatic part. The curve at the bottom gives the ratio of the quadrupole splittings for the a-aliphatic and aromatic deuterons (Goldfarb, Luz...

Deuterium, whose spin is equal to 1, is a quadrupolar nucleus. Its NMR parameters are almost exclusively governed by the quadrupolar interaction with the electric field tensor (FGT) at the deuteron site. Although the intermolecular contributions to the electric field gradient can be significant, the field gradient usually originates mainly from the electrons in the C- H bond and is considered to a first approximation to be intramolecular. It is found to be axially symmetric about the C- H bond in aliphatic compounds, and to a good approximation in aromatic compounds as well. Thus, information obtained from H NMR on molecular order and dynamics mostly concerns individual C- H bond directions. 

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