Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260260901pmid: N/A

Aljančić‐Šolaja, Ivana; Milosavljević, Slobodan; Djermanović, Miodrag; Stefanović, Milutin; Macura, Slobodan

doi: 10.1002/mrc.1260260902pmid: N/A

The application of phase‐sensitive 2D 1H NOE NMR spectroscopy (PS NOESY) in the stereochemical assignment of sesquiterpene lactones such as 4‐oxo‐3,4‐seco‐ambrosan‐6,12‐olide‐3‐oic acid methyl ester, 4‐oxo‐6‐hydroxypseudoguai‐11(13)‐en‐8,12‐olide and 8‐angeloyloxy‐10‐acetoxyguai‐3‐en‐6‐,12‐olide is reported.

Pinto, B. Mario; Leung, Ronald Y. N.; Sharma, R. Dev

doi: 10.1002/mrc.1260260903pmid: N/A

The rotational barriers in bis(2,4,6‐tri‐tert‐butlphenyl) diselenide and ditelluride have been investigated by total line‐shape analysis of variable‐temperature 1H NMR spectra in CFCI3CD2Cl2 (85:15). The free energies of activation, ΔG 204k‡, for the diselenide and ditelluride were determined to be 49.4 ± 0.6 and 39.2 ± 0.2 kJ mol−1, respectively. The difference is attributed to the dominance of steric effects in the transition state for rotation.

Shashkov, Alexander S.; Lipkind, Grigory M.; Knirel, Yuriy A.; Kochetkov, Nikolay K.

doi: 10.1002/mrc.1260260904pmid: N/A

Basic stereochemical factors have been revealed which affect the 13C chemical shifts in glycosides with a chiral cyclohexane‐type aglycone and in disaccharides with a pyranosidic aglycone. The effects of glycosylation depend on the configuration of the anomeric centre of the glycosylating sugar residue (glycone) and on the relative absolute configuration of the glycone and aglycone. When at least one of the carbons of the aglycone, adjacent to the glycosylated carbon, bears an equatorial proton, this dependence is well accounted for by a strengthening or weakening of the spatial interaction between the protons of the glycone and aglycone, caused by a change in the conformation around the glycosidic linkage. Regularities in the glycosylation effects for disaccharides are generalized for various modes of substitution and different general configurations of the pyranosidic aglycones. Simple rules have been formulated for the prediction of the magnitudes of the glycosylation effects which can be used for the determination of some unknown structural elements in glycosides and disaccharides, in particular the absolute configuration of one of the constituent sugars.

Crabb, Trevor A.; Trethewey, Andrew N.

doi: 10.1002/mrc.1260260905pmid: N/A

Protonation of perhydrooxazolo(3,4‐a)pyridine, perhydropyrido(1,2‐c)(1,3)oxazine and perhydropyrido(1,2‐c)(1,3)oxazepine by hydrogen chloride gas in diethyl ether gave mixtures of trans‐ and cis‐fused hydrochlorides containing 28, 75 and 79% of the trans‐fused salts, respectively, which compare with free base equilibria containing 68, 90 and 74% trans‐fused conformers. The cis‐fused salts from perhydrooxazolo(3,4‐a)pyridine existed in solution (CDCI3, 298 K) as an equilibrium containing 55% O‐inside cis‐ and 17% O‐outside cis‐fused conformers. Similar mixtures, but with smaller percentages of cis‐fused salts, were formed in D2ODCI.

Preston, Keith F.; Sandall, John P. B.; Sutcliffe, Leslie H.

doi: 10.1002/mrc.1260260906pmid: N/A

A derivative of the 1,3,2‐dithiazol‐2‐yl radical was labelled with 33S in order to evaluate the p spin densities on the sulphur atoms. This information is required to provide a full description of the SOMO which, in turn, aids the understanding of the structure of the radical dimers. An unexpected result is that the two sulphur atoms are non‐equivalent in the solid state. Calculations suggest that this is due to out‐of‐plane twisting of the substituent groups. Comparison of the ESR parameters of the bis(methoxycarbonyl) derivative with other members of the 1,3,2‐dithiazol‐2‐yl group of radicals (including the parent radical) shows that it is a good model for the whole group.

Kolehmainen, E.

doi: 10.1002/mrc.1260260907pmid: N/A

Solubilization of aromatics in aqueous bile salts was studied by two‐dimensional (2D) nuclear Overhauser enhancement spectroscopy (NOESY) for three model systems: benzene, anisole and veratrole solubilized in aqueous sodium cholate. The observed intra‐ and inter‐molecular cross‐relaxation properties are discussed in terms of different solubilization sites available for different solubilizates. The cholate anions located in the micelles seem to maintain their intramolecular cross‐relaxation characteristics during the mixed micelle formation. A molecular mechanical calculation of the cholate anion was performed in order to determine the non‐bonded internuclear distances between the NOE‐connected nuclei.

Casy, A. F.; Mercer, A. D.

doi: 10.1002/mrc.1260260908pmid: N/A

The influence of inclusion complex formation between cyclodextrins (chiefly the β‐polymer) and a variety of chiral antihistaminic and analgesic agents on the 1H NMR features of both guest and host partners is reported. In many cases equivalent protons of antipodes give resonances which differ in chemical shift after inclusion, and the potential value of the data to chiral analysis and to elucidation of the structures of the inclusion complexes is demonstrated.

Saitô, Hazime; Tabeta, Ryoko; Yokoi, Motoko

doi: 10.1002/mrc.1260260909pmid: N/A

High‐resolution 13C NMR spectra of 15 samples of uncomplexed and metal‐complexed tetranactin and nonactin were recorded in the solid state, revealing characteristic displacements of peaks due to complex formation and the effect of crystalline packing on the 13C chemical shifts and spin–lattice relaxation times of the methyl groups. The C‐1 13C chemical shifts of uncomplexed and complexed tetranaction and nonactin are well related to the variation of nearby torsion angles characteristic of the macrocyclic conformation, as determined by x‐ray diffraction. The existence of short intermolecular contact of methyl groups (<3.8 Å) at the surface of the molecules results in either prolonged 13C spin–lattice relaxation times in the laboratory frame (T1C) or substantial upfield displacement of peaks (up to 6 ppm). In addition, significantly reduced T1C values in uncomplexed nonactin (one order of magnitude smaller than those of other compounds) was ascribed to the presence of a puckering motion of the tetrahydrofuran ring and fluctuation of the macrocyclic ring in the solid state (with a time scale of 10−8 s). Finally, how the conformations of these compounds in the solid are retained in chloroform solution was examined in view of the differences in the 13C chemical shifts between the solid and solution.

Morton, J. R.; Preston, K. F.; Darensbourg, M. Y.

doi: 10.1002/mrc.1260260910pmid: N/A

UV‐irradiated crystals of PPh4+HCr(CO)5− exhibit anisotropic EPR spectra at room temperature and below which are characteristic of the presence of an electronic triplet (S = 1). From measurements of the angular dependence of the spectrum it is established that the carrier has axial symmetry with g‖ = 2.0019, g⟂ = 2.0202, |D| = 163.0 MHz, and with the unique direction parallel to the c axis of the tetragonal (P4/n) crystal. The magnitudes of the zero‐field parameter D and the g‐tensor components show that the carrier is a biradical consisting of a pair of Cr(CO)5− radicals separated approximately by one unit cell length along c. A weak single‐line spectrum which accompanies the triplet spectrum has the identical g‐tensor, and is assigned to the Cr(CO)5− radical itself. A third spectrum with the same g‐tensor, which is present in γ‐irradiated crystals only, is ascribed to another electronic triplet with |D| = 84 and E ≈︁ 0 MHz. It is associated with a second pair of exchange‐coupled Cr(CO)5− radicals whose interionic axis lies skew in the crystal axis system. Measurements of the signal intensities as a function of temperature show that both biradicals have singlet ground states and small isotropic exchange parameters, J ≈︁ 35 and 11 cm−1, respectively.