Theory of linear electric field shifts in carbon-13 nuclear magnetic resonance

The charge state of trimethoprim bound to Lactobacillus casei dihydrofolate reductase

“…In the crystal, the carboxylate of Asp-26 (L. casei numbering) is close to N1 of bound methotrexate [11,1 6] or trimethoprim [15]. This interaction is doubtless responsible for the increase in pK of these ligands on binding, but the proximity of the carboxylate group will also lead to an electric field shift of the 2-13C resonance, whose sign wilt depend on the relative polarisability of the bonds to this carbon [19][20][21]. In addition, the 2-amino group probably forms a hydrogen-bond to Thr 116 (L. casei numbering) [ 11,15,1 6] and this will lead to a shift of the resonance of the directly bonded carbon.…”

Saturated amine oxides: Part 4. Steric and electric field effects on the 17O NMR chemical shifts of saturated six‐membered azaheterocyclic amine oxides

“…Compared with the previous group, most of the amine oxides in Scheme 2 show considerable low-frequency shifts, due to the additional -gauche interactions of the N C -O oxygen with the 2-and 6-methyl groups (11)(12)(13)(14) or with the side ring methylene groups (15)(16)(17)(18)(19)(20). Depending on the steric orientation of the N C -O bond, two different types of -gauche interactions can be distinguished, whose shift effects also appear to be different (see Table 2): axial oxygens, oriented cis with respect to equatorial 2-and 6-alkyl groups, undergo a cis--gauche interaction (which we will refer to as ' cis '), whereas with equatorial oxygens a trans--gauche (' trans ') interaction occurs.…”

“…The theory of electric field effects has however, been, successfully applied to rationalize long-range effects on 13 C NMR chemical shifts of numerous compounds bearing a point charge or a dipole able to generate an electric field. 15 19 ), it seems reasonable to expect the sensitivity of 17 O nuclei resonances towards electric field effects also to be correspondingly higher than that of 13 C nuclei. The reason why electric field effects on 17 O resonances have, though, not been observed, may be that in most of the oxygen-containing functional groups investigated by 17 O NMR spectroscopy, the element-O bonds were too weakly polar (see the Conclusions section) to induce strong enough polarization of the skeleton carbons, and/or the arrays and structures of the carbon skeletons were inappropriate to allow such observation.…”

Substituent effects in 13C NMR spectra of 6-endo substituted 9-thiabicyclo[3.3.1]non-2-enes

“…Direct comparison with the literature is hampered by the lack of 6 -erufo -methoxy -~~ycl~3.3.1]~~ -9 -ones but shifts (Experimental and results). The chemical shifts of compouads1,3,4and1Owmmeasuredcaretunyintwo or three solvents at low cooccntntions: CCI, CD& and CHs0H.t These measurements revealed tbat rather uniform solvent shifts are obtai& when the low-fktd signal ofliscombiaedwiththeb~klsignalsof3,4amti# andviceversa.Thetinalassignmeotswentheomadcon the followin assumption. The G-X groups in 3,4 and 10 act as dipoks (or tripoks) with the negative centers on the "inner" atom of the group X (e.g.…”