Synthesis and characterization of a novel cyclic aluminophosphinate: structure and solid-state NMR study

We present the structure and a multinuclear solid-state NMR study of a new cyclic aluminophosphinate. The crystallographic structure of [Al(2)(HC(6)H(5)PO(2))(2)(C(4)H(9)OH)(8)]Cl(4) (compound 1) was obtained at low temperature (a = 11.830(7) A, b = 14.216(6) A, c = 17.790(6) A, beta = 91.25(4) degrees, monoclinic, P21/c, Z = 2). (13)C IRCP (inversion recovery cross polarization) and NQS (non quaternary suppression) NMR experiments allowed the complete assignment of the quaternary carbon atom of the phenyl ring and the precise determination of the isotropic /(1)J(P-C)/ coupling constant. (31)P CP MAS dynamics was carefully studied by varying the contact time. Dipolar oscillations even at slow MAS were observed. Up to 11 kHz, these oscillations were more pronounced, and the P-H distance was easily extracted. (27)Al NMR quadrupolar parameters for 1 were obtained with very good accuracy, and unusual satellite transition splitting was observed. Furthermore, the isotropic lines of the inner and outer transitions were clearly observable, leading to the unambiguous determination of the quadrupolar parameters.     

NMR assignments of a di-pentacyclo-undecane cyclic ether

The X-ray structure of a di-pentacyclo-undecane cyclic ether was recently reported. As part of a programme to use NMR spectroscopy for the structure elucidation of cage compounds, the complete NMR assignments of the cyclic ether was attempted. Major overlap of proton and carbon signals of the two cages is observed. It was required to elucidate the fragment analogues that represent similar structural features of the cyclic ether in order to get an approximate but reasonable insight into the complex overlapping signals. Normal 2D NMR techniques were utilized to assign the various NMR signals.     

Synthesis,structure, and NMR spectra of a new ferrocenylbenzene tricarbonylchromium complex

Complex 1 (1,2,4-triferrocenylbenzene tricarbonylchromium), possessing four heterometals, has been synthesized efficiently by a new method in one pot. Through detailed study on the NMR chemical shifts of 1 and the ligand 2, the NMR chemical shifts of 1 shift upfield from those of 2. The NMR spectra of 1 have reduced magnetic anisotropy of benzene after Cr(CO)₃ coordination with 2. The ferrocenyl groups have electron donating effect on the central benzene ring of 1.     

NMR structure elucidation of cyclic sialyl 6-sulfo Lewis x, a biologically dormant form of L-selectin ligand

The structure of cyclic sialyl 6-sulfo Lewis x, a new biologically dormant form of L-selectin ligand, was determined unambiguously by the accurate NMR analysis to have the lactamized ^{[5.], (a), (b), [2.], (a) and (b)}B conformation of its sialic acid. For the NMR structure elucidation were used various informations, such as chemical shifts values, appearance of amide proton, and NOE.     

Identification of degradants of a complex alkaloid using NMR cryoprobe technology and ACD/structure elucidator

Identification of degradants of pharmaceuticals is a necessary challenge of the drug development process following the subjection of candidate molecules to a variety of physico‐chemical stresses. It would be desirable to be able to conduct such studies on a minimal amount of material. As a prototypical study, the isolation and identification of degradants of a sample of the complex indoloquinoline alkaloid, cryptospirolepine, was undertaken after prolonged storage in DMSO solution using a combination of cryogenic NMR probe technology and CASE (Computer‐Assisted Structure Elucidation) programs. None of the starting alkaloid remained after storage; a chromatogram of the DMSO solution demonstrated the presence of >25 components in the mixture. The two most abundant degradation products were identified as the known alkaloid cryptolepinone (～35%) and an unprecedented rearrangement product, DP‐2, (～16%).     

NMR relaxation study of a hydrogen bonded complex

Spin-lattice relaxation times T₁ were measured over a large frequency region for H and D nuclei in the hydrogen bonded complex of trichloroacetic acid and dimethylsulphoxide. The measured relaxation times T₁ can be ascribed to the motion of the complex as a whole with a negligible contribution from the association—dissociation process.     

A simple molecule with a complex crystal structure: interplay of 31P solid-state NMR spectroscopy and single-crystal x-ray diffraction in the structure determination of a ruthenium diphosphine diamine complex

A comprehensive 31P solid-state NMR study of Ru(eta1-Ph2PCH2CH2OCH3)2(eta2-en)Cl2 (en = ethylenediamine) (1), by 1D (contact time variation, inversion-recovery, SPARTAN) and 2D techniques (homonuclear J-resolved, SECSY) indicated that the crystal structure of 1 should be complex. The single-crystal x-ray structure determination confirmed the presence of eight independent molecules in the asymmetric unit, with 31P isotropic chemical shifts in the range 27.3-40.1 ppm, while the spans of the phosphorus chemical shift tensors are of the order of 170 ppm. Based on unique structural features and NMR data, one molecule has been tentatively assigned.     

NMR evidence of a supramolecular structure of water

The hypothesis advanced in this paper is based on the formation of dissipative structures, in the form of supramolecular structures of water, by virtue of its hydrogen bonds, structurally similar to a liquid crystal. The supramolecular structure could be a macromolecule typical of a living organism (plant or animal), which as we know is approximately 80% water, with a complex conformation induced initially by an active principle and which remains, organising itself as a result of an input of energy (dynamisation, electromagnetic wave or other). Water is the most abundant substance on earth and has been very well studied with a number of model structures having been proposed and refined. Notwithstanding this, it remains an anomalous liquid where no single model is able to explain all of its properties. Recently our research group in Siena carried out NMR studies on the water molecule measuring nuclear spin relaxation times T1 and T2. The phenomena observed lead to the conclusion that water and aqueous solutions should be regarded as continuous polymorphous self-organizing systems. At the macro-level the water behaviour is related with biodiversity, the core of biological evolution.     

Molecular structure from a single NMR experiment

A procedure is described for determining the structure of a small molecule from a single NMR experiment. Several standard NMR sequences are combined so that the essential structural information is obtained in just one pass. Two-dimensional (13)C-(13)C correlations ("INADEQUATE"), single- and multiple-bond (13)C-(1)H correlations, and the conventional (13)C spectrum are recorded in parallel, making use of separate receiver channels for acquisition of (13)C and (1)H signals. The natural-abundance (13)C-(13)C correlation measurements employ a high-sensitivity cryogenically cooled probe, optimized for (13)C detection. An extension of this "all-in-one" sequence with three parallel receivers permits the corresponding natural-abundance (15)N spectra to be included.     

Chiral anion-based NMR enantiodiscrimination of a dinuclear, cationic Ir(I) NHC complex with a figure-of-eight loop structure

A dinuclear Ir(I) complex with a bis-NHC ligand was formed in situ from 1,1'-((4,6-dimethyl-1,3-phenylene)bis(methylene))bis(3-methyl-1H-imidazol-3-ium) in the presence of Cs(2)CO(3) and [Ir(μ-Cl)(cod)](2). Its solid-state structure, determined by X-ray diffraction, shows a figure-of-eight loop with both the P and M enantiomers being present. Solution NMR studies were performed to examine the enantiodiscrimination between them by applying the enantiopure anions tris(tetrachlorobenzenediolato)phosphate(v) (Δ-TRISPHAT) and bis(tetrachlorobenzenediolato)mono([1,10]binaphthalenyl-2,20-diolato)phosphate(v) (Λ-BINPHAT).     

Prediction of the crystal structure of a cyclic tetrapeptide

Prediction of the known crystal structure of cyclic-L-Ser(O-t-Bu)-β-Ala-Gly-L-β-Asp(OMe) has been attempted by establishing the low-energy conformations of the isolated molecule by conformational analysis, and then regarding each of these as a rigid molecule, by establishing the favorable crystal packing arrangements by molecular packing analysis. The theoretical model closest to the observed structure was one of the lowest-energy solutions and was recognized as essentially correct by reference to the x-ray data. The limitations of the model follow from the fact that the side chain conformations are somewhat affected by packing interactions.     

The structure of the enzyme-substrate complex of the phosphodiesterase catalytic domain with cyclic diguanosine monophosphate

A three-dimensional, all-atom structure of the enzyme-substrate complex of the phosphodiesterase catalytic domain with diguanosine monophosphate was constructed based on the results of hybrid quantum mechanics/molecular mechanics (QM/MM) calculations.     

Dynamic NMR study of cyclic derivatives of pyridoxine

A series of pyridoxine derivatives was investigated by ¹H and 2D nuclear overhauser enhancement spectroscopy (NOESY) NMR. The free energies of activation for the pyridyl‐oxygen rotation of the 2,4‐dinitrophenyl ether of the seven‐membered acetals of pyridoxine were measured by dynamic NMR. A conformational exchange between the chair and twist forms of the seven‐membered acetal ring was confirmed by dynamic NMR and STO3G computations. Copyright © 2014 John Wiley & Sons, Ltd.     

17O NMR Investigation of cyclic aromatic ethers

A linear relationship between the C-O-C angle and the molecular dihedral angle in a series of phenoxathi-ins and azaphenoxathiins is reported. ¹⁷O nmr spectroscopic data (natural abundance in acetonitrile at 75°C) were obtained on eight cyclic aromatic ethers 1-8 , including phenoxathiins, and two model compounds, acyclic aromatic ethers 9 and 10. The chemical shifts of the cyclic aromatic ethers were very sensitive to structural variations and were dependent upon electonic and conformational effects; however, no quantitative relationship between ¹⁷O chemical shift and geometric parameters was found.