15N shielding tensors

To conclude, we point out that aniline and its numerous derivatives still offer challenges for both experimental and theoretical NMR studies. Especially in the area of solid state NMR, the determination of the components of anisotropic 15N shielding tensors and their quantum chemical calculations can give valuable information on the polymorphism and interactions of aniline derivatives in the solid state. 

Fig. 13. Plots of the observed principal values of 15N shielding tensors (a) cru, (b)

Table 6. Isotropic 15N shielding (triso) and 15N shielding tensor element (022) of solid polypeptides [Leu, X] containing 15N-labelled L-leucine residue in the a-helix...

The principal values of the 15N shielding tensors were obtained by fitting the... 

Table 4 summarizes the isotropic 15N shielding (principal values of the 15N shielding tensors of [Ala ] and [Ala, X] . The ctu, a22, and solid state. For [Ala ] , (1) only the an value of the a-helix form appears to high frequency with respect to that of the /3-sheet form, whereas the other values (a22 and u33) of the a-helix appear to low frequency with respect to those of the /3-sheet and (2) the differences between the a22 (and sheet forms are larger than those of criso. 

Why is the displacement of the an different for [Ala ] and [Leu ] The reason for this is not clear at present, but this is a very important question in reaching and understanding of the correlation between structures and the l5N shielding tensor. The 15N shielding tensor may be useful for conformational analysis of solid polypeptides, if the origin of the chemical shift displacements can be elucidated. 

According to recent work by Oas et a/.36,37 the alignment of the 15N shielding tensors of L-[l-13C]-alanyl-L-[15N]-alanine has been determined from the... 

Figure 6. Model fragment used in 15N shielding calculation for imidazole. Also shown is the calculated orientation of the principal axis system of the shielding tensor. (Adapted from reference 26. Copyright 1991 American Chemical Society.)...

Through the observation of proton-decoupled 15N NMR lineshapes of two crystalline phases of Boc-glycylglycyl[15N,2H]glycine benzyl ester, the 15N-2H bond length and the direction of the most deshielded component of the shielding tensor, an, have been determined, combined with X-ray diffraction data.32... 

The first four N- H systems, all peptide linkages, yield similar results to [l-13C]glycyl[15N]glycine HCl-H20 where the an principal axis of the shielding tensor is rotated approximately 20° from the unique principal axis... 

Fig. 5. (a) Two-dimensional 1SN- H dipolar/chemical shift spectrum obtained from [lSN]acetylvaline showing the dipolar and chemical shift projections. Linewidths are typically 50-150 Hz for the dipolar and 0.5-1.0 ppm for the chemical shift dimension. vR = 1.07 kHz. (b) Dipolar cross-sections taken from the 2D spectrum. Each trace runs parallel to ivh through a particular rotational sideband in u>2. (i) Experimental i5N-H spectra from [15N]acetylvaline, vR = 1.07 kHz. The two simulations (ii and iii) assume two different orientations of the dipolar and shielding tensors, (ftD = 22°, D = 0°) and (Ai = 17°, aD = 0°), respectively, and illustrate the subtle differences in orientation which can be detected in the spectra. 

Fig. 14. Variation of the calculated 15N shielding and its tensor components with the N O hydrogen bond length (a)

The development of high-resolution solid-state NMR techniques has made possible the study of shielding tensors. No doubt 15N will receive more attention in the near future for a number of reasons such as the increased availability of high-field spectrometers and the development of polarization transfer techniques for solids. Recently some interesting results on the relation between l5N shielding tensors and structures of solid polypeptides have been reported. Nitrogen NMR could, therefore, be a useful intrinsic probe in 15N NMR studies of polypeptides and proteins in the solid state, particularly in cases where selective 15N labelling is possible. 

Frequencies and intensities of bands in the IR spectra of 1,2,4-triazines have been calculated by the 4-31G method50 and by ab initio Hartree-Fock level with 6-31G, 6-31G, U-9 and 3-21G methods.51 The shifts for the protons in the parent 1,2,4-triazine have been predicted and are in reasonable agreement with the observed values.52 The shifts of the 13C NMR signals for 3,5,6-trichloro-l, 2,4-triazine have been calculated by the first and second order SCS method and compared with the experimental values.53 The nature of lone pair effects of heteroatoms on direct 13C — H spin coupling constants has been calculated by the AMI method54 and the nuclear shielding tensors of 15N and 1 C nuclei by the SOLO (second-order corrected localized orbital-local origin method) ab initio method.55 14N Shifts have been predicted.75 The electron distribution of 1,2,4-triazines has been estimated from the observed NMR shifts.76... 

Figure 11. 15N NMR MAS NMR spectra and tensor results for [15N4]-ring labeled metalloporphyrins. A, 8.45 Tesla 15N MAS NMR spectrum of Fe(tetraphenylporphyrinate)(PhNO)( 1 -methylimidazole), vr = 2.6 kHz. B, graph showing correlation between experimental and DFT computed I5N tensor elements for Fe(TPP)(PhNO)( 1 -Melm) ( ) and Fe(TPP)(CO)(l-MeIm) (O). The mean experimental and theoretical shieldings over the four non-equivalent sites in each molecule are shown since the solid state shifts are not specifically assigned. Slope = 0.847, R2 = 0.995.

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