Natural chemical shielding analysis

Bohmaim JA, Weinhold F, Farrar TC (1997) Natural chemical shielding analysis or nuclear magnetic resonance shielding tensors from gauge-including atomic orbital calculations. J Chem Phys 107 1173-1184... 

HONDO = bond orbital - neglect of differential overlap (SCF-MO) GIAO = gauge-including atomic orbital L = Lewis-type (or localized) LC-BO = linear combination of bond orbitals LCNBO = linear combination of NBOs LMO = localized molecular orbital MSPNBO = maximum spin-paired NBO NBBP = natural bond-bond polarizability NBO = natural bond orbital NCS = natural chemical shielding NEDA = natural energy decomposition analysis NHO = natural hybrid orbital NL = non-Lewis-type (or... 

Also in 2-substituted ethanesulphonates,35 the 33S chemical shift has a reverse substituent effect and correlates with both Taft substituent constants and the chemical shift of the carboxylic carbon in related carboxylic acids. It seems that the substituent effect does not depend on conformation, but prevailingly on intramolecular electronic effects. Density functional theory (DFT) calculations of 33S nuclear shielding constants and natural bond orbital (NBO) analysis made it possible to conclude that substituents cause a variation in the polarization of the S-C and S-O bonds and of the oxygen lone pairs of the C — S03 moiety. This affects the electron density in the surroundings of the sulphur nucleus and consequently the expansion or contraction of 3p sulphur orbitals. 

If the spectra are too crowded with sidebands, 2D NMR spectroscopy can advantageously spread the overlapping CSA patterns across a second dimension. An array of these techniques is currently available (PASS, MAT, VACSY, FIREMAT, etc.), and they are described in excellent review articles. However, the analysis of powdered samples with the natural abundance of C and is not capable of providing information about the orientation of individual shielding-tensor components in the molecular frame. At present, the orientation of the principal components in the molecular frame is available from quantum chemical calculations. This topic will be discussed briefly in Section 5. 

Common features of most of the important HAHA contributions appear to be the following a) they arise from the core shells of the heavy atom rather than from core tails of the valence orbitals like the dominant HALA SO-I effects discussed in section 3 (this has been confirmed also by a localized MO analysis of the FC/SZ-KE contributions to heavy-atom shieldings in various small hydrides in [59]). b) the contributions are predominantly isotropic [23,27a,58,59,60]. In agreement with our above discussion, this suggests that HAHA effects are essentially atomic in nature and should cancel to a large extent when looking at relative chemical shifts. This appears to be bourne out by the available computational studies. 

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