Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260340801pmid: N/A

Tierney, John; Houghton, Gregory; Sanford, Kimberley; Mascavage, Linda; McCoy, Mary; Findeisen, Alfred; Kilburn, James

doi: 10.1002/(SICI)1097-458X(199608)34:8<573::AID-OMR928>3.0.CO;2-Dpmid: N/A

Substituents placed on the phenyl rings of 2,3‐diphenyl‐1,3‐thiazolidin‐4‐one affect the electron density surrounding both the methine proton and the C(2) carbon. These changes are reflected in the differing chemical shifts for these atoms relative to the parent compound. The other carbons in the heterocyclic ring appear to be similarly affected by substituents on the phenyl rings. Correlations for the effects of various substituents in both the 2‐ and 3‐phenyl rings with the 1H and 13C chemical shifts for the aforementioned sites are discussed using both Hammett and Swain–Lupton dual parameter methods. A conformational analysis using ab initio calculations is in agreement with the observed NMR data.

Bardet, M.; Cadet, J.; Wagner, J. R.

doi: 10.1002/(SICI)1097-458X(199608)34:8<577::AID-OMR940>3.0.CO;2-Npmid: N/A

The 13C NMR spectra of several diastereomers of radiation‐induced decomposition products of thymidine and thymine (its homologue without a sugar moiety at N‐1) were interpreted in terms of the conformational features of the pyrimidine ring. The study was based on the analysis of the changes in 13C chemical shifts of the pyrimidine ring due to through‐space interactions between the different substituents. First, γ effects between the substituents at C‐6 (hydroxyl, hydroperoxyl or 2,2,4,4‐tetramethylpiperidone‐N‐oxyl) and the methyl group at C‐5 were considered. Then these steric effects were combined with hydrogen bonding interactions between the hydroxyl group at C‐5 and the carbonyl at C‐4. This made it possible to account for the observed changes in the 13C chemical shifts of the pyrimidine ring. The 5,6‐saturated pyrimidine moiety is proposed to adopt a half‐chair conformation with the substituent at carbon C‐6 in a pseudo‐axial position.

Bhavani, N.; Natarajan, D.

doi: 10.1002/(SICI)1097-458X(199608)34:8<582::AID-OMR925>3.0.CO;2-Opmid: N/A

The assignments of the proton and carbon signals and conformations of substituted 2‐aryl‐trans‐decahydroquinolin‐4‐ones were determined using a combination of 1H, 13C, COSY and HETCOR spectral data. Analysis of the spectral data reveals that these compounds exist predominantly in twin‐chair conformations with the aryl and alkyl substituents in the equatorial orientation. Introduction of an alkyl group at C‐3 causes a flattening of the ring about the C‐2—C‐3 bond. From the proton–proton coupling constants (J9,10) the junction between the rings is found to be trans.

Kaerner, Andreas; Weaver, Kim H.; Rabenstein, Dallas L.

doi: 10.1002/(SICI)1097-458X(199608)34:8<587::AID-OMR936>3.0.CO;2-5pmid: N/A

The conformational properties of somatostatin, a tetradecapeptide hormone with a disulfide bond joining cysteine residues at positions 3 and 14, and of the reduced dithiol form of somatostatin, the two somatostatin–glutathione single mixed disulfides and somatostatin–glutathione double mixed disulfide were investigated in aqueous solution by one‐ and two‐dimensional1H NMR. Resonances were assigned using connectivities derived from 2D TOCSY and ROESY spectra.1H–1H ROEs together with the temperature coefficients for the chemical shifts of amide NH resonances for the native disulfide form of somatostatin are consistent with previous conclusions that somatostatin exists as an equilibrium of several rapidly interconverting conformations; however, the ROE results provide more information about the conformations than was obtained from previous studies. In particular, CαHi–NHi+2 ROEs together with NHi–NHi+1 ROEs identify specific β‐turns in the family of low‐energy conformations. The NMR results indicate that reduced somatostatin and the somatostatin–glutathione mixed disulfides have some of the same β‐turn conformational features as cyclic somatostatin, but that the populations of the conformations having the β‐turns are less for the acyclic peptides.

Zhuo, Jin‐Cong

doi: 10.1002/(SICI)1097-458X(199608)34:8<595::AID-OMR941>3.0.CO;2-Opmid: N/A

Natural abundance 17O NMR spectra for 42 aliphatic and 13 cyclic enaminones with tertiary amino groups are reported. The 17O NMR chemical shifts of enaminones depend upon the type, number and position of the substituents. Substituents at C‐1 cause shielding, and those at C‐2 or C‐3 cause deshielding. The 17O NMR data of 4‐(N,N‐dialkylamino)but‐3‐en‐2‐ones correlate well with the 1H–C‐2 and the 13C‐2 chemical shifts and with the pKa values of the corresponding dialkylamines. The 17O NMR chemical shifts of enaminones are shifted to lower field in non‐polar solvents.

Alam, Todd M.; Assink, Roger A.; Prabakar, S.; Loy, Douglas A.

doi: 10.1002/(SICI)1097-458X(199608)34:8<603::AID-OMR917>3.0.CO;2-Lpmid: N/A

High‐resolution 29Si NMR was used to identify and characterize the hydrolysis products of tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS). The identification of the different hydrolysis species was accomplished using the spectral editing properties of the DEPT and INEPT heteronuclear polarization transfer experiments. Previous resonances assignments for the hydrolyzed monomers in TMOS were quickly confirmed using these techniques. Unambiguous assignment of the MTMS hydrolysis products is complicated by a small spectral dispersion and the presence of two different hydrogen–silicon J couplings in these species. Analytical expressions for the INEPT signal response under multiple heteronuclear J couplings allowed these assignments. It is shown that the optimal polarization transfer efficiency in TMOS, MTMS and corresponding hydrolyzed products can only be obtained if the pulse sequence parameters are optimized for each species. The temperature dependences of the 29Si NMR resonances for the hydrolysis products in TMOS and MTMS were investigated and found to be very similar, with the magnitude and sign of the temperature response being dependent on the number of hydroxyl groups attached to the silicon. For MTMS hydrolysis products the small 29Si chemical shift dispersion along with the variation of chemical shift with temperature and solvent requires that resonance assignments be made for each experimental condition.

Novak, P.; Vikić‐Topić, D.; Meić, Z.; Gacs‐Baitz, E.

doi: 10.1002/(SICI)1097-458X(199608)34:8<610::AID-OMR938>3.0.CO;2-Opmid: N/A

Deuterium‐induced isotope effects on 13C chemical shifts were measured in a series of benzene derivatives, viz. toluene, benzoic acid and benzophenone. The effects over two, three and four bonds reflect a dependence on the transmission pathway and the nature of the side‐chain. Steric and inductive influences of side‐chain lone pair electrons located at the β‐position to the phenyl ring account for a lower magnitude and negative sign of some effects in the deuteriated phenyl ring. In monodeuteriated benzophenones isotope effects are transmitted to the second phenyl ring, e.g. in p‐deuteriobenzophenone over up to nine bonds. Although the carbonyl group allows only a minor conjugation between the phenyl rings, the extent, the magnitude and the sign of isotope effects resemble those in fully conjugated molecules, indicating a similar transmission mechanism. In polydeuteriated isotopomers, the additivity of isotope effects holds for the majority of carbon atoms. However, in few cases significant deviations exist owing to perturbations in hydrogen‐bond dynamics and redistribution of rotamer populations caused by deuteriation. For monodeuteriated isotopomers, a number of 13C–2H coupling constants through one and three bonds are also reported.

Nogaj, Bolesaw; Mazurek, Paweł

doi: 10.1002/(SICI)1097-458X(199608)34:8<616::AID-OMR935>3.0.CO;2-Fpmid: N/A

35Cl‐NQR (77 K) spectra for some phenoxyl herbicides are reported. The frequencies are assigned to individual chlorine atoms in the molecules. The influence of substituents on the electron density distribution in the phenyl ring is discussed. Inductive, conjugative and hyperconjugative effects are analysed. The substituent constants σm and σp for OCH2COOH and OCHCH3COOH are estimated.

Stockfisch, Dorte; Kaaber, Marianne; Pedersen, Jens A.

doi: 10.1002/(SICI)1097-458X(199608)34:8<619::AID-OMR924>3.0.CO;2-Apmid: N/A

The hyperfine splitting constants of 11 mostly symmetric β‐substituted anthraquinones, with electron‐donating (methyl) and electron‐withdrawing (carboxyl) substituents, were calculated by means of additivity principles. The calculated hyperfine constants are compared with experimental values obtained from electron spin resonance spectra of the corresponding anthrasemiquinone radicals. A close match is observed for all constants studied, and the consistent assignment found agrees completely with one previously put forward for 31 asymmetric substituted anthraquinones. It is shown experimentally that electron‐donating substituents at C‐2 increase the spin densities at C‐1, C‐6 and C‐8 and decrease them at C‐3, C‐4, C‐5 and C‐7. The opposite effect is observed for electron‐withdrawing substituents.