Magnetic Resonance in Chemistry

Nilsson, Mathias; Morris, Gareth A.

doi: 10.1002/mrc.1805pmid: 16602081

DOSY data for mixtures are commonly processed either by single channel methods (e.g. HR‐DOSY) or multichannel methods (e.g. CORE). Both aim to separate the signals from species of different molecular sizes by their diffusion coefficients; the result is displayed either as a 2D plot (as in HR‐DOSY) or as individual spectra (as in CORE). Both types of methods are sensitive to any systematic errors in the experimental data. The effects of, and remedies for, two such sources of error, spatially non‐uniform pulsed field gradients (PFGs) and instrument instability, are demonstrated for CORE processing, using a corrected form of the Stejskal–Tanner equation and reference deconvolution, respectively. Copyright © 2006 John Wiley & Sons, Ltd.

Bühl, Michael

doi: 10.1002/mrc.1807pmid: 16602076

53Cr chemical shifts of CrO42−, Cr2O72−, CrO3X−, CrO2X2(X = F, Cl), and Cr(CO)5L (L = CO, PF3, CHNH2, CMeNMe2) are computed, using geometries optimized with the gradient‐corrected BP86 density functional, at the gauge‐including atomic orbitals (GIAO)‐, BPW91‐, and B3LYP levels. For this set of compounds, substituent effects on δ(53Cr) are better described with the pure BPW91 functional than with B3LYP, in contrast to most other transition‐metal chemical shifts studied so far. For selected cases, 53Cr NMR line widths can be rationalized in terms of electric field gradients (EFGs) computed with the BPW91 functional, but in general other factors such as molecular correlation times appear to be dominating. 53Cr chemical shifts and EFGs are predicted for CrO3, Cr(C6H6)2, Cr(C6H6)CO3, and, with reduced reliability, for Cr2(µ2‐O2CH)4. Copyright © 2006 John Wiley & Sons, Ltd.

Sýkora, Jan; Blechta, Vratislav; Sychrovský, Vladimír; Hetflejš, Jiří; Šabata, Stanislav; Soukupová, Ludmila; Schraml, Jan

doi: 10.1002/mrc.1810pmid: 16602078

29Si–13C couplings were measured in para substituted silylated phenols, XC6H4OSiR1R2R3(X 2, CF3, Cl, F, H, CH3, CH3O). The SiR1R2R3 silyl groups included trimethylsilyl (Si(CH3)3, TMS), tert‐butyldimethylsilyl (Si(CH3)2C(CH3)3, TBDMS), dimethylsilyl (SiH(CH3)2, DMS), and tert‐ butyldiphenylsilyl (Si(C6H5)2C(CH3)3, TBDPS). Previously developed (Si,C,Si)gHMQC methods and narrow 29Si lines allowed the determination of coupling constants over up to five bonds. Besides the number of intervening bonds between the silicon and carbon atoms, all the measurable couplings depend also on the nature of the substituents on the silicon. The two‐ and three‐bond couplings are not affected by ring substitution in the para position. These properties render the 29Si–13C couplings suitable for line assignment in the spectra of silylated polyphenols. The experimental results are in reasonable agreement with theoretical calculations. The calculations show, in agreement with the data reported in the literature for couplings between other nuclei, that the two‐bond and three‐bond couplings, which are of similar magnitudes, are of opposite signs. If the signs of these geminal and vicinal couplings could be determined experimentally, they would greatly facilitate the line assignment. The four‐ and five‐bond couplings are affected by the substituent X in a nontrivial manner. Copyright © 2006 John Wiley & Sons, Ltd.

Miglietta, M. L.; Lamanna, R.

doi: 10.1002/mrc.1826pmid: 16645934

Carotenoids are linear C40 tetraterpenoid hydrocarbons and represent a wide category of natural pigments. They are components of the pigment system of chloroplasts and are involved in the primary light absorption and the photon canalization of photosynthesis. Moreover, they also behave as quenchers of singlet oxygen, protecting cells and organisms against lipid peroxidation.

Gevrenova, Reneta; Voutquenne‐Nazabadioko, Laurence; Harakat, Dominique; Prost, Elise; Henry, Max

doi: 10.1002/mrc.1827pmid: 16685746

The assignments of 1H and 13C NMR spectra of two new aminoacyl triterpene saponins from roots of Gypsophila trichotoma Wend. are reported. In addition to 1D NMR methods, 2D NMR techniques (COSY, TOCSY, ROESY, HSQC, HMBC, and HSQC–TOCSY) were used for the assignments. The structures were completed by analysis of HR‐ESI‐MS and ESI‐MSn. Copyright © 2006 John Wiley & Sons, Ltd.

Rusakov, Yury Yu.; Krivdin, Leonid B.; Schmidt, Elena Yu.; Mikhaleva, Albina I.; Trofimov, Boris A.

doi: 10.1002/mrc.1828pmid: 16607675

Conformational study of 2‐(2‐pyrrolyl)pyridine and 2,6‐di(2‐pyrrolyl)pyridine was performed on the basis of the experimental measurements and high‐level ab initio calculations of the one‐bond 13C–13C, 13C–1H and 15N–1H spin–spin coupling constants showing marked stereochemical behavior upon the internal rotation around the pyrrole–pyridine interheterocyclic bonds. Both compounds were established to adopt predominant s‐cis conformations with no noticeable out‐of‐plane deviations. Copyright © 2006 John Wiley & Sons, Ltd.

Jimeno, María‐Luisa; Alkorta, Ibon; Elguero, José; Del Bene, Janet E.

doi: 10.1002/mrc.1840pmid: 16671054

Seventy‐three unique spin–spin coupling constants have been analyzed for the ten species in the two series X(CH3)nH4–n, where the central atom X is 13C or 15N+. Thirty‐seven experimental values have been obtained from the literature, and several new coupling constants have been measured for the methyl‐substituted ammonium ions. Both DFT with the B3LYP functional and ab initio EOM‐CCSD calculations have been carried out on these same systems. Coupling constants computed by these two methods are in agreement with experimental values. Some problems related to coupling constants for the cationic ammonium systems have been resolved when these were recomputed at EOM‐CCSD for complexes in which NH4+ is hydrogen‐bonded to H2O molecules. Copyright © 2006 John Wiley & Sons, Ltd.

Du, Xingang; Bai, Yanjing; Liang, Hong; Wang, Zhiying; Zhao, Yuying; Zhang, Qingying; Huang, Luqi

doi: 10.1002/mrc.1806pmid: 16602082

Four 3′‐hydroxy‐4′‐methoxy‐isoflavonoids: calycosin‐7‐O‐β‐D‐glucopyranoside (1), calycosin (2), pratensein‐7‐O‐β‐D‐glucopyranoside (3), and pratensein (4) were isolated from Astragalus membranaceus var. mongholicus, among which compound 4 was obtained from this plant for the first time. The solvent effect obscuring 1H signal patterns of B ring of compounds 1–4 was reported to avoid mis‐assignments. Previously reported NMR data of compounds 1 and 2 were corrected based on 1D and 2D NMR experiments. Copyright © 2006 John Wiley & Sons, Ltd.

Mitaine‐Offer, Anne‐Claire; Miyamoto, Tomofumi; Semmar, Nabil; Jay, Maurice; Lacaille‐Dubois, Marie‐Aleth

doi: 10.1002/mrc.1809pmid: 16602077

A novel oleanane‐type triterpene saponin (1) together with two known molecules, soyasapogenol B and astragaloside VIII were isolated from the roots of Astragalus caprinus. Their structural elucidation was performed mainly by 2D NMR techniques (COSY, TOCSY, NOESY, HSQC, HMBC) and mass spectrometry. Compound 1 was determined as 3‐O‐[α‐L‐rhamnopyranosyl‐(1 → 2)‐β‐D‐glucuronopyranosyl]‐22‐O‐β‐D‐apiofuranosyl‐soyasapogenol B. Copyright © 2006 John Wiley & Sons, Ltd.

Ahmed, Zaheer; Ali, Dildar; Malik, Abdul

doi: 10.1002/mrc.1803pmid: 16607673

Two new ursene‐type triterpenes, nudicauline A and nudicauline B, have been isolated from Launaea nudicaulis. Their structures have been assigned as 3β‐hydroxy‐urs‐11‐ene (1) and 3β‐acetyl‐urs‐11‐ene (2), respectively, by extensive NMR studies. In addition, olean‐11,13(18)‐diene (3), 3β‐hydroxy‐13(28)‐epoxy‐urs‐11‐ene (4) and 3‐keto‐13(28)‐epoxy‐urs‐11‐ene (5) are also reported for the first time from this species. Copyright © 2006 John Wiley & Sons, Ltd.