Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260331101pmid: N/A

Rozhenko, A. B.; Bzhezovsky, V. M.; Polovinko, V. V.; Makarenko, A. G.

doi: 10.1002/mrc.1260331102pmid: N/A

A series of halogen‐substituted tetrahydrothiophene 1,1‐dioxides (sulpholanes) were investigated by 13C, 17O and 33S NMR spectroscopy. It was shown that successive replacement of hydrogen atoms in positions 3 and 4 of the sulpholane ring led to an increase in 33S nuclei shielding. The 33S NMR chemical shift range exceeded 39 ppm. The variation of the 33S nuclei shielding with increasing number of chlorine or bromine atoms was caused by an electrostatic through‐space effect of the halogen atoms and the strongly polarized CHal bonds.

Lindon, J. C.; Farrant, R. D.; Sanderson, P. N.; Doyle, P. M.; Gough, S. L.; Spraul, M.; Hofmann, M.; Nicholson, J. K.

doi: 10.1002/mrc.1260331103pmid: N/A

The efficiency of directly coupled HPLC—NMR spectroscopy for the separation and characterization of synthetic peptide libraries was investigated using a model system based on a mixture of all 27 tripeptide combinations of alanine (A), methionine (M) and tyrosine (Y) as the C‐terminal amides. The 1H NMR spectra were obtained in real time using on‐flow HPLC—NMR spectroscopy at 600 MHz and 21 of the 27 peptides were identified on the basis of chemical shifts and coupling constants, including the use of diagnostic values derived from 1H NMR spectra and 1H13C and 1H15N HMQC studies on authentic AAAOH and YYYOH. The effects on 1H NMR chemical shifts of pH and of varying proportions of acetonitrile and water as occurred during the gradient elution HPLC run were evaluated. The HPLC—NMR approach could provide a facile method for investigating the relative efficiency of resin‐based peptide synthesis for different amino acid combinations and should rapidly provide data suitable for deriving structure‐retention time relationships for peptides.

Blakemore, D. C.; Gilbert, A.; De Bruyn, A.; De Keukeleire, D.; Van der Eycken, E.

doi: 10.1002/mrc.1260331104pmid: N/A

1H NMR data (400 and 500 MHz) are reported for the intramolecular meta photocycloaddition products from 4‐phenoxybut‐1‐enes and 3‐benzyloxyprop‐1‐enes. The structures of these photoproducts, and in particular the discrimination between the 1,6‐ and 7,8‐bridged isomers, are deduced from COSY 45 and 60 experiments and spin‐decoupling techniques.

Kövér, Katalin E.; Forgó, Péter; Batta, Gyula

doi: 10.1002/mrc.1260331105pmid: N/A

A polarization transfer enhanced inversion‐recovery experiment is proposed for the combined measurement of heteronuclear T1 and NOE of low‐γ nuclei. This method offers significant time savings compared with conventional relaxation measurements. The reliability and accuracy of the combined T1, and NOE determination and its dependence on the experimental set‐up is also demonstrated. The proposed approach may be preferred for multiple field/multiple temperature model‐free analysis of molecular dynamics.

Cerioni, Giovanni; Plumitallo, Antonio; Frey, Joseph; Rappoport, Zvi

doi: 10.1002/mrc.1260331106pmid: N/A

Natural abundance 17O NMR spectra of eleven diaryl α‐diketones are reported. An excellent correlation was found between 17O shifts and σp+. Conformations and π character of the intercarbonyl CC bond are discussed.

Bernard, Guy M.; Schurko, Robert W.

doi: 10.1002/mrc.1260331107pmid: N/A

Long‐range isotope shifts, Δδ, due to 37Cl/35Cl and 81Br/79Br, in the 19F NMR spectra of some substituted benzenes are reported. The shifts appear to be invariant to solvent and are additive when the isotopes are in chemically equivalent positions relative to the fluorine site. The three‐bond isotope shift attributed to chlorine is approximately −1.60 ppb and that from bromine varies from −0.25 to −0.28 ppb. These shifts appear to be invariant to the substituent pattern. However, there is a large difference in the four‐bond shift for 2,6‐dichloro‐4‐fluorophenol (−0.16 ppb) and 2‐chloro‐6‐fluorocyanobenzene, (−0.55 ppb), suggesting that this isotope shift is mediated by the intervening group. In all cases, substitution by the heavier chlorine isotope resulted in increased shielding. The nearly equal natural abundance of 81Br and 79Br resulted in double peaks of approximately equal intensity. However, in almost all cases, the high‐field peak is of slightly lower intensity, suggesting that the sign of this isotope effect is also negative.

Lamartina, Liliana; Spinelli, Domenico; Guerrera, Francesco; Sarvà, Maria Concetta

doi: 10.1002/mrc.1260331108pmid: N/A

The 13C NMR chemical shift values of (aryl)(2‐nitrobenzo[b]thiophen‐3‐yl)amines were measured in DMSO‐d6 solutions, suggesting the occurrence of an alternate charge polarization at C‐3, C‐2, C‐3a, C‐7a, C‐4 and C‐5. A dual substituent parameter analysis of the experimental data indicates a large or a low resonance contribution for aryl para or meta substituents, respectively, while the inductive component remains constant throughout.

Crouch, Ronald C.; Davis, Ann O.; Martin, Gary E.

doi: 10.1002/mrc.1260331109pmid: N/A

Pulsed field gradients (PFG) have the potential to revolutionize further potent, inverse‐detected structural NMR techniques by eliminating artifacts and thermal noise in data taken with very small numbers of transients. A phase‐sensitive GE (gradient‐enhanced)‐HMQC‐TOCSY experiment is reported with features which allow the discrimination (or elimination) of direct correlations relative to the relayed TOCSY responses. Sensitivity losses which arise from molecular diffusion during the course of a pulse sequence are potentially serious when field gradients are employed for solutions containing small molecules. By carefully combining phase cycling with relatively weak pulsed field gradients, it is possible both to maintain good sensitivity and to suppress completely t1 noise in GE‐HMQC‐TOCSY data acquired with only two transients per t1, increment.

Schaumburg, K.; Shabanova, E.; Eichner, P.

doi: 10.1002/mrc.1260331110pmid: N/A

Important new information about the crystal structure of dicalcium phosphate (DCP) powders was obtained using the 31P NMR technique. There were two types of powders under investigation. One type includes pure DA (CaHPO4) and DH (CaHPO4· 2H2O) powders, and their mixtures in different proportions. The other includes powders from an industrial production. They were obtained by DH heating. An effective and fast method of identification of the structure of the nuclear nearest surroundings is presented. Determination of the concentration of nearest surroundings corresponding to DA or DH types for the 31P nuclei was demonstrated on examples taken from production powders. Good agreement was found in comparison with data obtained by chemical analysis. Especially interesting information is obtained by using the CP MAS NMR technique. This method allows the following conclusions about the production sample structure to be drawn: it is a powder consisting of DH‐type microcrystals, but with numerous vacancies at the positions of water molecules which disappeared during heating.