H Chemical Shift Anisotropy

For example, over the range of H- --O distances from 1.66 A to 2.15 A, the span of the H chemical shift tensor [Q=6n-6y, where 6n is the H chemical shift tensor component normal to the H20 plane and 6y is the in-plane component parallel to the O- -O vector (Fig. 3)] changes by more than 20 ppm, and is nearly 

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The hydrogen bond length in Watson-Crick base pairs can be characterized using the recently developed method of measuring the cross-correlated relaxation [61] between H chemical shift anisotropy and dipole-dipole coupling of H and its hydrogen bond donor... 

Deuterium ) (/ =1). The H chemical shift anisotropies (CSAs) for polycrystalline samples were determined by H MAS NMR spectroscopy at high magnetic field strength (14.1 T). It was shown that the precision of the CSA parameters depends strongly on the asymmetry parameter (tiq) for the H... 

From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

As an example of the measurement of cross-correlated relaxation between CSA and dipolar couplings, we choose the J-resolved constant time experiment [30] (Fig. 7.26 a) that measures the cross-correlated relaxation of 1H,13C-dipolar coupling and 31P-chemical shift anisotropy to determine the phosphodiester backbone angles a and in RNA. Since 31P is not bound to NMR-active nuclei, NOE information for the backbone of RNA is sparse, and vicinal scalar coupling constants cannot be exploited. The cross-correlated relaxation rates can be obtained from the relative scaling (shown schematically in Fig. 7.19d) of the two submultiplet intensities derived from an H-coupled constant time spectrum of 13C,31P double- and zero-quantum coherence [DQC (double-quantum coherence) and ZQC (zero-quantum coherence), respectively]. These traces are shown in Fig. 7.26c. The desired cross-correlated relaxation rate can be extracted from the intensities of the cross peaks according to ... 

In general, multiple pulse techniques sufficiently average the dipolar interactions, compress the chemical shift scale, but they do not affect heteronuclear dipolar interactions and the chemical shift anisotropy. A combination of both multiple pulse techniques and magic angle spinning, so-called CRAMPS (Combined Rotational And Multiple Pulse Spectroscopy) is found to yield satisfactory results in the solid state H NMR of solids 186). The limitations of all these techniques, from the analytical point of view, arises from the relatively small chemical shift range (about 10 ppm) as compared with some other frequently studied nuclei. However, high resolution H NMR of solids is useful in studies of molecular dynamics. 

The carbon-proton dipolar interaction and the chemical shift anisotropies broaden the lines in solid state 13C NMR spectra. The major effect arises from the dipolar coupling of the carbon nuclei with neighboring protons homonuclear dipolar couplings between two adjacent 13C nuclei are neglegible because of their low natural abundance. The large magnitude of dipolar 13C— H coupling (up to 40 kHz) results in broad and structureless proton-coupled 13C NMR absorptions. 

Yang, D. W., Konrat, R., and Kay, L. E. (1997). A multidimensional NMR experiment for measurement of the protein dihedral angle psi based on cross-correlated relaxation between (H alpha-13C alpha) XH dipolar and 13C (carbonyl) chemical shift anisotropy mechanisms. J. Am. Chem. Soc. 119,11938-11940. 

Fig. 6. Two-dimensional [1H,15NJ HSQC-NMR spectrum from the reaction of [Pt([15N3]di-en)Cl]+ with 5 -GMP recorded 1.2 h after mixing. NH2 and NH peaks for [Pt([ 15N3]dien)Cl]+ are labelled as A, A and B, respectively, and for the GMP adduct labelled a-e (all five NH protons are non-equivalent). The large downfield shift of peak a is notable. 195Pt Satellites in both the H and 15N dimensions are evident for [Pt([15N3]dien)Cl]+ but not for the GMP adduct (satellites broaden with increase in molecular size and chemical-shift anisotropy).

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