Magnetic Resonance in Chemistry

Kříž, Jaroslav; Dybal, Jiří; Budka, Jan; Makrlík, Emanuel

doi: 10.1002/mrc.2188pmid: 18273877

The interaction of 1,3‐alternate tetrapropoxycalix(4)arene (1) with n‐butylammonium ion (2) in CD2Cl2 was examined using 1H, 13C and 14N NMR spectroscopy and DFT (density functional theory) calculations. NMR shows that 1 forms with 2 an equimolecular hydrogen‐bonded complex with the equilibrium constant 5.91 × 103 l/mol at 296 K. The structure of the complex can be shown to be asymmetric at 203 K, with 2 interacting by hydrogen bonds with the two ethereal oxygen atoms of one half of 1 and with the π system of the other half, but is rapidly averaged to an apparent C4h symmetry by chemical exchange at higher temperatures. Using two related but independent techniques based on transverse and rotating‐frame proton relaxation, it is shown that only an intermolecular exchange of 2 between the bound and free states takes place, in contrast to previously studied interaction of 1 with H3O+. Its correlation time is 0.169 ms. It is shown by DFT calculations that such swift exchange is not possible without a cooperative interaction of both 2 and 1 with several molecules of water present. Similarities and contrasts between the exchange processes of 2 and H3O+ bound to 1 are discussed, in particular with respect to the apparent quantum tunneling of the latter inside the molecule of the complex. Copyright © 2008 John Wiley & Sons, Ltd.

Feng, Jian; Lee, Young J.; Kubicki, James D.; Reeder, Richard J.; Phillips, Brian L.

doi: 10.1002/mrc.2191pmid: 18306443

Solid‐state NMR spectroscopy is a potentially powerful method for obtaining molecular level structural information crucial for understanding the specific relationship between calcite crystals and occluded organic molecules that are important in biomineralization and biomimetic materials. In this work, a method is developed based on cross‐polarization/magic angle spinning (CP/MAS) NMR to measure the heteronuclear distances and obtain structural information for large intracrystalline citrate defects in a synthetic calcite/citrate composite. Using compounds with well‐characterized crystal structures, Mg(II) citrate and Sr(II) citrate, a correlation is established between TIS, the CP time, and M2IS, the van Vleck heteronuclear dipolar second moment, which contains distance and structural information. This correlation is supported by peak assignments obtained from calculations of the 13C chemical shifts for crystalline Mg(II) citrate. On the basis of TIS−1 versus M2IS correlation, measurement of TIS for carbonate ions associated with citrate defects in a calcite(13C‐enriched)/citrate coprecipitate yields an estimate for the distance between citrate and the nearest carbonate carbon that indicates close spatial proximity and provides useful constraints for future computational study. The applicability of TIS−1 versus M2IS correlations to other weakly coupled spin‐1/2 systems is discussed in terms of the effects of 1H homonuclear dipolar coupling, using the CP kinetics of Zn(II) dihydroxybenzoate and kaolinite for comparison. The results suggest a limited range of correlation constants and indicate that quantitative information can be obtained from CP/MAS kinetics obtained under similar experimental conditions. Copyright © 2008 John Wiley & Sons, Ltd.

Gonçalves, Biank T.; Esteves, Pierre M.; Pinto, Angelo C.; Kaiser, Carlos R.; da Silva, Fernanda L.; Miguez, Eduardo; da Silva, Joaquim F. M.

doi: 10.1002/mrc.2192pmid: 18327891

Interesting anisotropic effects were observed for phenylglyoxamides and their respective mandelamides. Such effects were observed in experimental 1H and 13C NMR (in CDCl3, CD3OD, and DMSO‐d6 solvents) and in some cases with good correlation to theoretical 1H and 13C NMR DFT–GIAO (B3LYP/6‐311 + + G**//B3LYP/6‐31G*) calculations. A systematic conformational analysis of these compounds was performed in a two‐step methodology, using PM3 and DFT (B3LYP/6‐31G*) calculations; with good accomplishment and computational time economy. It was observed that intramolecular hydrogen bonding plays a significant role in the conformation of such compounds. Finally, a geminal nonequivalence of an NCH2 moiety, in one of the alkyl side chain (R1 = R2), was found for the tertiary mandelamides studied. Copyright © 2008 John Wiley & Sons, Ltd.

Trifunović, Snežana; Milosavljević, Slobodan; Vajs, Vlatka; Macura, Slobodan; Todorović, Nina

doi: 10.1002/mrc.2193pmid: 18306444

From detailed study of 1D and 2D NMR spectra of ten natural 1,2‐epoxyguaianolides (bis‐1,2:3,4‐epoxyguaianolides and guaianolide‐1,2‐epoxychlorohydrins), we identified general spectral traits helpful for stereochemical assignment of such sesquiterpene lactones. We found that the chemical shifts of certain 1H and 13C nuclei are consistently dependent on the configuration of 1,2‐epoxy‐ring which could be used as a simple rule for establishing this configuration. Then, from 1D and 2D (COSY, NOESY, HMQC, HMBC) NMR data, applying the observed rule, the structure and stereochemistry of two new, diastereomeric guaianolide‐1,2‐epoxychlorohydrins, isolated from Achillea serbica, are determined. The NMR data, namely, nuclear overhauser enhancement (NOE) correlations, pointed out two conformations of guaianolide's cycloheptane ring. The semiempirical calculations (AM1 and PM3 methods), performed in order to gain additional information regarding conformations, resulted in three geometries of investigated lactones. Even so, the conformations derived from the NMR data agreed well with those calculated by semiempirical methods. Copyright © 2008 John Wiley & Sons, Ltd.

Xia, Youlin; Moran, Sean; Nikonowicz, Edward P.; Gao, Xiaolian

doi: 10.1002/mrc.2195pmid: 18288679

A pulse sequence of z‐restored spin echo, − π− β− τ− π− τ−, employing a π pulse in the middle of the delay (2τ) to form a spin echo and the two π pulses together to restore the residual longitudinal magnetization back to + z direction, is described. 13C spectra of organic compounds provide a wealth of structural information; however, 13C 1D spectra acquired using reverse geometry probes can have significant baseline humps or rolls because of pulse ring‐down within the coil. The baseline distortions are especially apparent in spectra acquired using cryogenically enhanced probes. The baseline problem may be alleviated by extending the delay between the last pulse and the starting point of acquisition. However, uses of long delay times introduce large negative first‐order phase corrections which themselves produce baseline roll. The prescribed experiment can be used to completely remove the hump, roll or dip in the baseline of the 13C spectrum and at the same time obtain sensitivity similar to the experiment of a single β pulse. We believe that this experiment will be of general applications in acquiring high‐quality 13C NMR data with reverse geometry probes and spectral interpretation. Copyright © 2008 John Wiley & Sons, Ltd.

Irvine, Nicholas M.; Cooper, David H.; Thornburgh, Scott

doi: 10.1002/mrc.2196pmid: 18318451

The structures of 4‐hydroxy‐3,5,6‐trichloropyridine‐2‐carboxylic acid (1a) and 6‐hydroxy‐3,4,5‐trichloro‐2‐carboxylic acid (1b) were verified by the NMR analysis of their corresponding methylated and decarboxylated derivatives 2,3,5‐trichloro‐4‐methoxypyridine (5) and 3,4,5‐trichloro‐2‐methoxypyridine (8), respectively. The 6‐hydroxy isomer (1a) was found to be in equilibrium with its pyridinone tautomer as evidenced by the formation of significant amounts of 3,4,5‐trichloro‐1‐methyl‐6‐oxo‐1,6‐dihydropyridine‐2‐carboxylic acid methyl ester (6b) on exhaustive methylation. The one‐bond chlorine‐isotope effect was used and shown to be an effective tool for the identification of chlorinated carbons in 13C NMR spectra providing an additional tool for solving structural problems in chlorinated compounds. Copyright © 2008 John Wiley & Sons, Ltd.

Afonin, Andrei V.; Vashchenko, Alexander V.; Ushakov, Igor A.; Zorina, Nadezhda V.; Schmidt, Elena Yu.

doi: 10.1002/mrc.2197pmid: 18318452

The NHċbondX (X = N,O,S) intramolecular hydrogen bond in the series of 2(2′‐heteroaryl)pyrroles and their trifluoroacetyl derivatives is examined by the 1H, 13C, 15N spectroscopy and density functional theory (DFT) calculations. The influence of the hydrogen bond on coupling and shielding constants is considered. It is shown that the NHċbondN intramolecular hydrogen bond causes a larger increase in the absolute size of the 1J(N,H) coupling constant and a larger deshielding of the bridge proton than the NHċbondO hydrogen bond. The effect of the NHċbondS interaction on the 1J(N,H) coupling constant and the shielding of the bridge proton is small. The NMR parameter changes in the series of the 2(2′‐heteroaryl)pyrroles due to NHċbondX hydrogen bond and the series of the 1‐vinyl‐2‐(2′‐heteroaryl)‐pyrroles due to CHċbondX hydrogen bond have the same order. The proximity of the nitrogen, oxygen or sulfur lone pair to the FċbondH hydrogen bridge quenches the trans‐hydrogen bond spin–spin couplings 1hJ(F,H‐1) and 2hJ(F,N). Copyright © 2008 John Wiley & Sons, Ltd.

Ishizu, Takashi; Kajitani, Shinya; Tsutsumi, Hiroyuki; Yamamoto, Hideji; Harano, Kazunobu

doi: 10.1002/mrc.2198pmid: 18318450

Inclusion complexes of (−)‐epicatechin gallate (ECg) as well as (+)‐gallocatechin gallate (GCg) and β‐cyclodextrin (β‐CD) in an aqueous solution were investigated using several NMR techniques and a computational method. ECg and EGCg formed a 1:1 complex with β‐CD, in which the A ring and a portion of the C ring were included from the wide secondary hydroxyl group side of the β‐CD cavity, and the B and B′ rings were left outside the cavity. GCg formed a 1:2 complex with β‐CD, in which the A and B rings of GCg were included by two molecules of β‐CD. The difference between the two modes of inclusion of the 1:1 complex of ECg, EGCg·β‐CD and the 1:2 complex of GCg·β‐CD might have resulted from the size of the space between the B and B′ rings in aqueous solution. As a result of nueclear Overhauser effect (NOE) experiments, GCg was considered to have a large enough space between the B and B′ rings to include the B ring in the β‐CD cavity; on the other hand, ECg and EGCg have no such large space. Copyright © 2008 John Wiley & Sons, Ltd.

Del Bene, Janet E.; Alkorta, Ibon; Elguero, José

doi: 10.1002/mrc.2199pmid: 18297741

Ab initio calculations have been performed to obtain structures and coupling constants 1J(NH), 1hJ(HN), and 2hJ(NN) for models of proton sponges with symmetric and asymmetric NH+N intramolecular hydrogen bonds (IMHBs). For a given model, the asymmetric structure has a lower energy, a longer NN distance, and a hydrogen bond which has a greater deviation from linearity. The computed values of 2hJ(NN) for the models are significantly less than predicted values based on the distance dependence of 2hJ(NN) for complexes with intermolecular NH+N hydrogen bonds. However, the reduced values of 2hJ(NN) cannot be attributed solely to the distortion of the hydrogen bond in the models, but also reflect differences in s electron populations at the nitrogens in both the ground state and the excited states which couple to it through the Fermi‐contact (FC) operator. Values of 2hJ(NN) for IMHBs can be related quadratically to the NN distances in the models, and demonstrate that there is no discrepancy between computed values of 2hJ(NN) at the short NN distances found in these systems and experimental data for proton sponges. Copyright © 2008 John Wiley & Sons, Ltd.

Parella, Teodor; Espinosa, Juan Félix

doi: 10.1002/mrc.2200pmid: 18288678

An out‐and‐stay 2D proton–proton NMR correlation experiment is proposed to detect long‐range proton–proton connectivities up to six and seven bonds away. The magnetization flow pathway is based on a consecutive, dual‐step J(CH)transfer mechanism and it allows one to trace out 1H1H connectivities between protons belonging to different spin systems. This novel experimental scheme will be particularly useful in cases when carbon resonances overlap, providing connectivity information that could not be obtained in a HMBC experiment. The success of the experiment is demonstrated in the structural studies of a wide variety of chemical compounds. Copyright © 2008 John Wiley & Sons, Ltd.