Shielding cone

The NMR spectrum shows that the methyl groups are strongly shifted upfield. They appear at 6 0.5 ppm from TMS as reference. We can explain this by the stronly hindered polysulfide, which causes the methyl groups to be located In the shielding cone of the phenyl rings. 

Figure 1. Traditional NMR shielding cone for a carbon-carbon double bond. (Adapted with permission from reference 9. Copyright 1998 Plenum.)...

Fig. 3. Stereoviews of the central trinucleotide d(CA G ) d(CTG) of the A G -platinated duplex d(CTCA G CCTC)d(GAGGCTGAG) (2). Top energy-minimized model in which the thymine conserves its Watson-Crick imino hydrogen bond with A, whereas the amino hydrogen bond is disrupted. Bottom energy-minimized model in which the thymine retains its Watson-Crick amino hydrogen bond with A, whereas the imino hydrogen bond is disrupted. The H(2 ) proton of the cytidine of the platinated strand lies in the shielding cone of the five-membered ring of A in both models, which accounts for the strong upfield shift observed for its NMR signal (reproduced from [70] with permission).

The factor (1-tr) converts the general resonance condition for the nucleus P to the specific resonance condition for the individual phosphorus atom in the actual compound that we want to measure. The parameter phosphorus atom is subject to. These are normally limited to diamagnetic factors, such as electronegativity of substituents, and diamagnetic anisotropy due to participation in double bonds or shielding cones of adjacent functional groups. 

ApSimon and co-workers have modihed the McConnell equation to allow the calculation of the screening effects of C—C and C—H bonds at nuclei relatively close to them new values of the anisotropy constants for these bonds are given. A new treatment of the C=C bond by these workers takes account of the complete anisotropy of this bond (i.e., three magnetic susceptibilities must be used). The idea usually adopted for shielding cones of C=C bonds is rejected instead it is pointed out that deshielding of nuclei occurs only within a restricted region at the ends of the bonds, elsewhere nuclei either in or out of the plane of the double bond being shielded. Calculations based on these considerations are applied in the diterpene series. 

The 7c-clouds of anti-6 penetrate one another only partly, so that protons 4-H, 5-H and 6-H (12-H, 13-H and 14-H respectively) absorb in the same range as the arene protons in 158 (Figs. 39, 40), whereas the inner proton 8-H (16-H respectively) are affected by the shielding cone of the opposite benzene ring, resulting in an upheld shift by about AS x 3 (Fig. 40) [35, 89, 140]. Absorption of the arene protons in the syn-conformation (sy -6) is comparable to that in 8, due to similar conformation (Fig. 39) [41a]. 

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