Magnetic Resonance in Chemistry

doi: 10.1002/omr.1270210901pmid: N/A

Mbagwu, G. O.; Bass, R. G.; Glennon, R. A.

doi: 10.1002/omr.1270210902pmid: N/A

Natural abundance 13C NMR chemical shifts have been experimentally determined for a series of mesoionic thiazolo[3,2‐a]pyrimidine‐5,7‐diones. The spectral data are compared with those of related mesoionic dihydrothiazolo[3,2‐a]pyrimidine‐5,7‐diones and mesoionic 1,3,4‐thiadiazolo[3,2‐a]pyrimidine‐5,7‐diones. Resonable correlation between the observed 13C NMR chemical shifts and CNDO/2 total charge densities have been obtained for the different carbon atoms of 8‐methylthiazolo[3,2‐a]pyrimidine‐5,7‐dione.

Colebrook, Lawrence D.; Hall, Laurance D.

doi: 10.1002/omr.1270210903pmid: N/A

The spin‐lattice relaxation rates of the aromatic, alkene, hydroxyl, methine and methyl protons of 19 steroid derivatives have been measured using the null point method. A simple procedure is described whereby the R1 values of molecules which have different motional tumbling rates can be directly inter‐compared, and it is shown that such ‘normalized’ relaxation data can provide novel insight concerning both the geometry and the local molecular motion of these substances in solution.

Majchrzak, Michal W.; Kotełko, Antoni; Lambert, Joseph B.

doi: 10.1002/omr.1270210904pmid: N/A

A new bicyclic system, 3,7‐diazabicyclo[4.1.0]heptane, has been prepared from 3‐(ethoxycarbonyl)‐7,3‐oxazabicyclo[4.1.0]heptane by reaction with sodium azide and reduction of the resulting tosyloxy azide with lithium aluminum hydride. The molecule can exist in four stereoisomeric half‐chairs, depending on the configuration of the two nitrogen atoms. Half‐chair ring reversal and piperidine nitrogen inversion are fast on the NMR time scale at all observed temperatures. Inversion of the secondary aziridine nitrogen becomes slow as the temperature is lowered (Tc= −10°C). Complete analysis of the 1H spectrum was possible with the 1,5,5‐trideuteriated analog. At slow exchange, two aziridine invertomers are present with an exo/endo ratio of approximately 0.7 in toluene‐d8, 0.7 in CH2Cl2 and 1.7 in CHCl3/CH2Cl2. The free energy of activation for nitrogen inversion is 13.2 kcal mol−1 at −10°C in CHCl3/CH2Cl2.

Brownlee, R. T. C.; Hall, J. G.; Reiss, J. A.

doi: 10.1002/omr.1270210905pmid: N/A

The routine use of the INEPT pulse sequence is shown to be the method of choice for the assignment of multiplicity from 13C NMR spectra where no prior information about the 1J(CH) coupling constants is available, or where these are expected to vary over a large range. This technique is illustrated using spectra obtained from polyhalogenated monoterpenes isolated from the red alga Plocamium angustum, which have 1J(CH) values varying from 127 to 203 Hz.

Aksaç, Zihni; Pinar, Esat; Içli, Sîddîk

doi: 10.1002/omr.1270210906pmid: N/A

3‐Aryl‐2‐thioxo‐4‐oxazolidinones and 3‐arylrhodanines have been studied for magnetic non‐equivalence of diastereotopically related proton and 13C nuclei in rotational isomers, and for steric interactions between the aryl and heterocyclic moieties of these compounds. For the majority of rotational isomers the barriers to internal rotation about the aryl CN bond were >100 kJ mol−1, due to the steric bulk of the thiocarbonyl group. Chemical isolation of several of the diastereomers was achieved. The enhanced steric effect and the difference in the electronic effect of the sulphur atom in relation to the oxygen atom appeared to have no influence on the small chemical shift differences of the rotational isomers, detected for some 1H and some 13C nuclei.

Nikki, Kunio; Nakagawa, Naoya

doi: 10.1002/omr.1270210907pmid: N/A

Solvent shifts of a series of alkanes have been studied in perfluorohexane, carbon tetrachloride and diiodomethane. The methylene proton resonances for longer alkyl chains are shifted downfield in perfluorohexane, but upfield in diiodomethane. It is suggested that n‐alkanes have a tendency to adopt folded structures in solution, the degree of folding for n‐alkane chains in perfluorohexane being larger than that in diiodomethane.

Rico, M.; Nieto, J. L.; Santoro, J.; Bermejo, F. J.; Herranz, J.

doi: 10.1002/omr.1270210908pmid: N/A

The 1H NMR chemical shifts and the spin‐spin coupling constants of the non‐exchangeable protons of the N‐terminal 13‐residue C‐peptide of ribonuclease A, obtained by cleavage of the enzyme with cyanogen bromide, have been measured in a 5 mM solution in D2O (pH 3.0, 24°C) at 360 MHz. The titration parameters for end groups (Lys‐1 and homo‐Ser‐13) and side chains (Lys‐1, Glu‐2, Lys‐7, Glu‐9 and His‐12) have been determined. The chemical shifts, their temperature coefficients and the vicinal coupling constants, 3J(HNCH‐α), for the exchangeable NH protons have been measured in a 5 mM solution in D2O/H2O (1:9 v/v) at pH 3.0. An assignment of observed signals to individual residue protons based on characteristic shifts, standard double resonance experiments, spectral simulations and titration shifts is proposed. All experimental evidence indicates that under the conditions studied the C‐peptide is in a random coil form.

Naulet, N.; Tomé, D.; Martin, G. J.

doi: 10.1002/omr.1270210909pmid: N/A

Natural abundance 15N NMR spectroscopy has been used to investigate the effect of pH on the 15N chemical shifts of lysine and of ε‐hydroxymethyllysine. A computer calcualtion which fits the chemical shifts of both α‐and ε‐nitrogen atoms versus pH has been used to predict the pKa values. 15N chemical shifts and some 1J(15NH) values of some other amino acids and of their reaction products with formaldehyde are also reported.

Gale, D. J.; Evans, N. A.

doi: 10.1002/omr.1270210910pmid: N/A

29Si NMR spectra of the O‐trimethylsilyl (OTMS) derivatives of various methyl α‐ and β‐D‐galactopyranosides have been recorded. The effect of changes in the anomeric configuration provides a means of assigning the resonance of the 2‐OTMS substituent. Whereas the signal of the OTMS group attached at the 6‐position can be assigned readily, those of the OTMS group at the 3‐ or 4‐position cannot be assigned unequivocally.