Isotropic H Chemical Shifts

Simple correlations have been established between isotropic H chemical shifts and 0 H and 0---0 distances in 0-H---0 hydrogen bonds for a variety of organic and inorganic solids. Correlations between isotropic H chemical shift and O--0 distance, as well as between 2H quadrupole coupling constant and O O distance, have also been reported [75]. 

A linear relationship between isotropic 2H chemical shift (6H) and 0---0 distance (r0...o) has also been established [77] for several metal phosphates and minerals. Similarly, for carboxylic acid protons, SH has been shown [78] to depend linearly on r0...0, and for several trihydrogen selenites, SH was shown [79] to correlate linearly with r0...0 and rH...0 distances. 

Using structural data obtained from neutron diffraction studies for 41 different crystalline solids, the following linear relationship was reported [70]  

As in the case of 2H quadrupole coupling constants discussed above, this relationship is supported by the bond polarisation theory. Furthermore, a linear relationship between SH and the 2H quadrupole coupling constant was reported [70]  

In contrast, however, a quadratic relationship between SH and e2qQlh was used in a recent report [71 ] based on the earlier correlation of Berglund and Vaughan [75]. 

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In addition, basic quantum mechanical calculations have shown that the change in isotropic H chemical shift (SH) due to hydrogen bond formation can be attributed primarily to O-H bond polarisation [80]. Similarly, the change in 2H quadrupole coupling constant is also expected to be caused by O-H bond polarisation. It would therefore be interesting to explore correlations between SH and the O-H bond length (r0 H) and correlations between e2qQlhand r0 H> as r0. H... 

Isotropic H chemical shifts for weakly hydrogen bonded hydrates have recently been compared [81] with previous data on carboxylic acids with O-H 0 hydrogen bonds of strong and medium strengths. The values of SH for the hydrogen bonded protons in this work varied from 4.8 ppm in NaCl04-H20 to 20.5 ppm in potassium hydrogen malonate. 

A clear correlation between isotropic H chemical shift and the frequency of the O-H stretching vibration has been reported [61] for surface hydroxyl groups in zeolites and related materials, as well as for water molecules in solid hydrates and strongly hydrogen bonded protons in inorganic solids. 

Interaction of water with inorganic materials at high pressure can result in sub-microscopic fluid water inclusions. In mineralogy it is important to know the pressure of the water to be able to determine its equation of state. If it is assumed that the molar magnetic susceptibility is a constant, the susceptibility is a function of the density (p) and hence the isotropic H chemical shift of the fluid in the inclusions can be used as an extremely accurate measure of its density (Withers et al. 2000). The relationship between these 2 parameters has been determined as... 

Fig. 10. The pulse sequence for the WISE experiment.21 This two-dimensional experiment separates H wideline spectra according to the isotropic l3C chemical shift of the 13C each H is bonded to. An initial H 90° pulse creates transverse H magnetization that is allowed to evolve in t. A short cross-polarization step then transfers the remaining H magnetization to the nearest 13C spin, i.e. the bonded one. The resulting 13C transverse magnetization is then allowed to evolve in ti under magic-angle spinning, where an FID is recorded.

Similar to the situation for 13C, isotropic 15N chemical shifts and the principal components of 15N chemical shift tensors have been used to study N-H- -0=C hydrogen bonds in peptides. It has been shown that isotropic 15N chemical shifts of proton donors (such as N-H) are displaced downfield by ca. 15 ppm, whereas those of proton acceptors are shifted upfield by ca. 20 ppm [110-112]. Amongst the CSA components, S33 (parallel to the C-N bond) has been shown to be most sensitive to the hydrogen bond strength, as reflected by the N- -O distance [113]. Detailed studies of the principal components and orientations of 15N chemical shift tensors for amide nitrogens in simple peptides have been reported recently [114]. This work confirmed that S33 and Siso are the 15N chemical shift parameters that are the most sensitive to details of the hydrogen bonding. It was also found that N-H... 

For these complexes, the isotropic and 15N chemical shifts and the 15N chemical shift tensor elements were measured as a function of the hydrogen bond geometry. Lineshape simulations of the static powder 15N NMR spectra revealed the dipolar 2H-15N couplings and hence the corresponding distances. The results revealed several correlations between hydrogen bond geometry and NMR parameters which were analysed in terms of the valence bond order model. It was shown that the isotropic 15N chemical shifts of collidine and other pyridines depend in a characteristic way on the N-H distance. A correlation of the and 15N... 

Fig. 8. PISA wheel patterns for the helix tilt (r) varying from 15 to 90° simulated using Eq. (8) in the text. The chemical shift tensor values of 533 = 64, 522 = 77, and 5 =2I7 ppm, an N-H bond length of 1.07 A, and the relative orientations of the dipolar and chemical shift tensors of a = Q°, fi= T were used in the simulations. Variation of the PISA wheel with respect to the tilt angle of the helix is shown for one peak of the dipolar-coupling doublet as the spectrum is symmetric with respect to the zero frequency. The centers (shown in dashed lines) of the wheels as a function of the helix tilt angle for both dipolar transitions are linear and intersect at the isotropic N chemical shift frequency (119.3 ppm) and 0 Hz H °N dipolar coupling [Eq. (9)].

The hnal row of Table 29.3 lists the change in the isotropic NMR chemical shift calculated for the bridging proton as a result of H-bond formation [177]. The values for all CH- - -O H-bonds are negative, consistent with the same sign for the OH- - -O bond, another indicator of the similarity between these different interactions. 

The principal motivation in recording a H— C HETCOR experiment is the determination and identification of the H chemical shifts of the protons bound to particular carbons, then short contact times must be used during CP (from 100 to 500 is). In that way, a correlation between the isotropic signals of a pair of hetero-nuclei is established through the observation of cross peaks, the occurrence of which reveals special proximity between the corresponding nuclei. 

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