Magnetic Resonance in Chemistry

Krivdin, Leonid B.

doi: 10.1002/mrc.1312pmid: 14745811

A full set of carbon–carbon coupling constants have been calculated at the SOPPA level in the series of six most representative propellanes. Special attention was focused on spin–spin couplings involving both bridgehead carbons, and these data were rationalized in terms of the multipath coupling mechanism and hybridization effects. Many unknown couplings in the propellane frameworks were predicted with high reliability. Copyright © 2003 John Wiley & Sons, Ltd.

Simpson, Andre J.; Lefebvre, Brent; Moser, Arvin; Williams, Antony; Larin, Nicolay; Kvasha, Mikhail; Kingery, William L.; Kelleher, Brian

doi: 10.1002/mrc.1308pmid: 14745812

This paper describes procedures for the generation of 2D NMR databases containing spectra predicted from chemical structures. These databases allow flexible searching via chemical structure, substructure or similarity of structure as well as spectral features. In this paper we use the biopolymer lignin as an example. Lignin is an important and relatively recalcitrant structural biopolymer present in the majority of plant biomass. We demonstrate how an accurate 2D NMR database of ∼600 2D spectra of lignin fragments can be easily constructed, in ∼2 days, and then subsequently show how some of these fragments can be identified in soil extracts through the use of various search tools and pattern recognition techniques. We demonstrate that once identified in one sample, similar residues are easily determined in other soil extracts. In theory, such an approach can be used for the analysis of any organic mixtures. Copyright © 2003 John Wiley & Sons, Ltd.

Malek, Kamilla; Vala, Martin; Kozłowski, Henryk; Proniewicz, Leonard M.

doi: 10.1002/mrc.1289pmid: 14745813

1H and 13C NMR spectra of the oxocarboxylic acid oximes 2‐hydroxyiminopropanoic acid (1), 2‐(4‐methylthiazol‐2‐yl)‐2‐(hydroxyimino)acetic acid (2) and 2‐cyano‐2‐(hydroxyimino)acetic acid (3) were measured in DMSO‐d6, D2O and acetone‐d6 solutions. The data indicate the presence of hydrogen bonding in 1 and 2 and a strong electron‐withdrawing effect due to the cyano group in 3. The effect of intra‐ and intermolecular hydrogen bonding on the hydrogen and carbon chemical shifts in these molecules was studied theoretically. Total energy calculations of the stability of various hydrogen‐bonded species, in addition to equilibrium parameters and chemical shifts, were calculated using ab initio methods (RHF, MP2) and density functional theory (B3LYP), implemented in the Gaussian 98 software package. The gauge‐including atomic orbital (GIAO) method was used to predict magnetic shielding constants. Chemical shift calculations for the most stable species agree fairly well with the observed data, especially for the hydroxyl protons. Substituents adjacent to the α‐carbon show some influence of the oximic and carboxyl groups on the 13C chemical shifts, as expected for groups with different polar and anisotropic character. Copyright © 2003 John Wiley & Sons, Ltd.

Gee, Becky A.

doi: 10.1002/mrc.1303pmid: 14745814

The spatial ordering of aluminum atoms in CsAl(SiO3)2 and 3Al2O3·2SiO2 was probed by 27Al dipolar solid‐state NMR spectroscopy. The 27Al response to a Hahn spin‐echo pulse sequence in a series of aluminum‐containing model crystalline compounds demonstrates that quantitative 27Al homonuclear dipolar second moments can be obtained to within ±20% of the theoretical values, if evaluation of the spin‐echo response curve is limited to short evolution periods (2t1 ≤ 0.10 ms). Additionally, selective excitation of the central transition $m = {1 \over 2} \to - {1 \over 2}$ is necessary in order to ensure quantitative results. Restriction of spin exchange affecting the dephasing of the magnetization may decelerate the spin‐echo decay at longer evolution periods. Considering these restraints, the method was used to probe the spatial distribution of aluminum atoms among the tetrahedral sites in two aluminosilicate materials. Experimental 27Al spin‐echo response data for the aluminosilicates CsAl(SiO3)2 (synthetic pollucite) and 3Al2O3·2SiO2 (mullite) are compared with theoretical data based on (I) various degrees of aluminum–oxygen–aluminum bond formation among tetrahedrally coordinated aluminum atoms (Al T d—O—Al T d) and (II) the maximum avoidance of Al T d—O—Al T d bonding. Analysis of the second moment values and resulting echo decay responses suggests that partial suppression of spin exchange among aluminum atoms in crystallographically distinct sites may contribute to the 27Al spin echo decay in 3Al2O3·2SiO2, thus complicating quantitative analysis of the data. Silicon‐29 and aluminum‐27 magic angle spinning (MAS) NMR spectra of 3Al2O3·2SiO2 are consistent with those previously reported. The experimental 27Al spin‐echo response behavior of CsAl(SiO3)2 differs from the theoretical response behavior based on the maximum avoidance of Al—O—Al bonding between tetrahedral aluminum sites in CsAl(SiO3)2. A single unresolved resonance is observed in both the silicon‐29 and aluminum‐27 MAS spectra of CsAl(SiO3)2. Copyright © 2003 John Wiley & Sons, Ltd.

Akhmedov, Novruz G.; Myshakin, Evgeniy M.; Dennis Hall, C.

doi: 10.1002/mrc.1306pmid: 14745815

The 1H and 13C NMR spectra of methyl‐8‐(2‐furyl)‐5‐methyl‐1,3‐dioxo‐3,3a,4,6,8,9,9a,9b‐octahydrofuro[3,4‐f]isoquinoline‐7(1H)‐carboxylate (1) and trimethyl 8‐methyl‐3‐phenyl‐3,4,4a,7‐tetrahydro‐2,5,6(1H)‐isoquinolinetricarboxylate (2) at room temperature displayed doubling of the majority of signals, suggesting the presence of two rotational conformers (rotamers) in a ratio ∼1:1.2 (in a mixture of CDCl3 and C6D6), ∼1:1 (in CD2Cl2) and ∼1:1.4 (in C6D6). On the basis of the temperature‐dependent 1H NMR spectra of 1 and 2, the barrier to interconversion of the rotamers was calculated to be ∼16 kcal mol−1. The average chemical shifts and spin–spin coupling constants were analyzed for the resolution‐enhanced 300 MHz 1H NMR spectrum of 1 at 333 K in CDCl3 solution. From analysis of the spin–spin coupling constants, it is proposed that the nitrogen‐containing ring is in a chair conformation with C‐2—H equatorial. Low‐ and room‐temperature NOESY experiments in conjunction with theoretical ab initio calculations supported the hypothesis that the two rotamers interchange via rotation about the C—N bond. Spectral assignments of all proton and carbon resonances were made on the basis of one‐ and two‐dimensional NMR experiments (DEPT, DQCOSY, NOESY, HETCOR and gHMBC). Copyright © 2003 John Wiley & Sons, Ltd.

Watanabe, Eiji; Tamaki, Makoto; Yamakura, Fumiyuki; Akiyama, Minoru

doi: 10.1002/mrc.1310pmid: 14745816

The extended TORO technique was applied to the structural analysis of endo‐D‐Tyr‐gramicidin S, cyclo(‐Val‐Orn‐Leu‐D‐Phe‐D‐Tyr‐Pro‐Val‐Orn‐Leu‐D‐Phe‐Pro‐), which has a slightly distorted symmetry from C2, by the insertion of D‐Tyr and equivalent α‐proton chemical shifts in the 1H NMR spectrum. All NMR signals of the two dominant isomers of this antibiotic with trans–trans prolines were determined by using the extended TORO technique with TOCSY and ROESY spectra. This technique is generally applicable for distinguishing overlapped signals of α‐ and amide protons from the main chains of peptides. Copyright © 2003 John Wiley & Sons, Ltd.

Sprang, Thomas; Bigler, Peter

doi: 10.1002/mrc.1311pmid: 14745817

We present a new pulse sequence that detects simultaneously nJ(C,H) and 2J(C,H) connectivities. The corresponding coherences are created along independent pathways and therefore can be separated into two different subspectra. One spectrum is to show all nJ(C,H) connectivities and the other is to show exclusively 2J(C,H) connectivities. In contrast to the previously published 2J/nJ experiment, this sequence detects the 2J(C,H) connectivities via a C,H,H‐RELAY pathway leading to an intensification of the 2J(C,H) signals. Strictly, the 2J(C,H) spectrum does not show 2J(C,H) but 3J(H,H) coupling interactions within 13CHk–12CHl fragments. Therefore, 2J(C,H) signals can appear even if the corresponding 2J(C,H) coupling constant is zero. Copyright © 2003 John Wiley & Sons, Ltd.

Koskela, Harri; Kilpeläinen, Ilkka; Heikkinen, Sami

doi: 10.1002/mrc.1309pmid: 14745818

Relaxation times (T1, T2, T1ρ) are usually evaluated from exponential decay data by least‐squares fitting methods. For this procedure, the integrals or amplitudes of signals must be determined, which can be laborious with large data sets. Moreover, the fitting requires a priori knowledge of the number of exponential components responsible for the decay. We have adapted inverse Laplace transformation (ILT) for the analysis of relaxation data. Exponential components are resolved with ILT to reciprocal space on their corresponding relaxation rate values. The ILT approach was applied to 3D linewidth‐resolved 15N HSQC experiments to evaluate 15N T1 and T2 relaxation times of ubiquitin. The resulting spectrum is a true 3D spectrum, where the signals are separated by their 1H and 15N chemical shifts (HSQC correlations) and by their relaxation rate values (R1 or R2). From this spectrum, the relaxation times can be obtained directly with a simple peak‐picking procedure. Copyright © 2003 John Wiley & Sons, Ltd.

Bakhmutov, Vladimir I.

doi: 10.1002/mrc.1313pmid: 14745819

The deuterium spin–lattice relaxation times in (D2) ligands of W, Ru and Os complexes are reviewed and analyzed in terms of the fast internal (D2) motions: free rotation, librations and 180° jumps. The analysis was performed using quadrupolar coupling constant (DQCC) parameters taken from the solid‐state 2H NMR spectra and density function theory calculations. It is shown that the calculated DQCC values can be corrected for further use in interpretations of deuterium relaxation times for Ru and Os dihydrogen complexes. The resulting data led to a criterion for using the relaxation data to distinguish fast‐spinning dihydrogen ligands. It is shown that the principal components of electric field gradient tensors at D in the dihydrogen ligands are oriented closer to M–D directions Copyright © 2003 John Wiley & Sons, Ltd.

Heimer, Norman E.; Del Sesto, Rico E.; Carper, W. Robert

doi: 10.1002/mrc.1318pmid: 14745820

The ionic liquids 1‐ethyl‐3‐methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1‐methyl‐3‐propylimidazolium tetrafluoroborate ([PMIM][BF4]) were studied by H,H‐NOESY NMR using a cross‐relaxation matrix analysis. Cross‐peak intensities are seen to increase with increasing mixing time. Experimental and theoretical hydrogen–hydrogen distances are in agreement at short mixing times (50 ms). Mixing times longer than 50 ms result in an increasing contribution of spin diffusion that produces unrealistically short hydrogen–hydrogen distances. Gas‐phase ab initio molecular structures are obtained using Hartree–Fock (HF) and density functional theory (B3LYP) methods at the 6311 + G(2d,p) basis set level. The hydrogen–hydrogen distances obtained from the theoretical structures are in reasonable agreement with those calculated from the cross‐relaxation matrices. Published in 2003 by John Wiley & Sons, Ltd.