Magnetic Resonance in Chemistry

Sharaf, Maged H. M.; Schiff, Paul L.; Tackie, Albert N.; Phoebe, Charles H.; Howard, Layne; Meyers, Carol; Hadden, Chad E.; Wrenn, Susan K.; Davis, Ann O.; Andrews, C. Webster; Minick, Doug; Johnson, Robert L.; Shockcor, John P.; Crouch, Ronald C.; Martin, Gary E.

doi: 10.1002/mrc.1260331003pmid: N/A

Cryptolepis sanguinolenta (Lindl.) Schlechter (Asclepiadaceae), a shrub indigenous to West Africa, has been employed by traditional healers in the treatment of various fevers, including malaria. Column chromatography of the alkaloid fraction obtained from a defatted ethanolic extract of the roots of the plant afforded the previously characterized alkaloids cryptolepine, quindoline and cryptospirolepine. Continued extensive column chromatography, followed by preparative high‐performance liquid chromatography, resulted in the isolation of nine new alkaloids, including approximately 100 μg of a novel indoloquinoline‐β‐carboline dimeric alkaloid, cryptolepicarboline. The structure was elucidated through the use of a combination of spectroscopic techniques which included UV, Fourier transform, electron impact mass spectrometry, and 500 MHz NMR spectroscopy. NMR data included 1D 1H and 13C reference spectra, the latter recorded in 40 μl of DMSO‐d6 using a new Varian heteronuclear Nano‐probe and in 140 μl of DMSO‐d6 using a Nalorac carbon‐optimized microprobe. These 13C NMR spectra allow the first direct comparison of these two probe formats. Homonuclear 2D NMR data acquired included both COSY and ROESY spectra. The homonuclear 2D experiments were variously performed in both a homonuclear Nano‐probe and micro‐inverse detection probes. Heteronuclear shift correlation experiments included HMQC, HMBC and IDR (inverted direct response)‐HMQC‐TOCSY spectra, which were uniformly performed using a 500 MHz Nalorac micro inverse‐detection probe.

Perumal, Subbu; Chandrasekaran, Ramasubbu; Vijayabaskar, Veerappan; Wilson, David A.

doi: 10.1002/mrc.1260331004pmid: N/A

The proton and carbon NMR spectra of nine 2‐substituted diphenyl sulphides (S‐2‐X), seven 3‐substituted diphenyl sulphides (S‐3‐X), nine 2‐substituted diphenyl sulphones (SO2‐2‐X), nine 3‐substituted diphenyl sulphones (SO2‐3‐X) and nine 4‐substituted‐2′,6′‐dimethyldiphenyl sulphides (Me2‐S‐4‐X) were obtained. Correlations of the 1H and 13C chemical shifts were made with benzene substituent‐induced chemical shifts (Lynch plots) and Hammett and dual‐substituent parameters and the results were compared with those of 4‐substituted diphenyl sulphides (S‐4‐X) and sulphones (SO2‐4‐X). The main conclusions are as follows: (i) the transmission of the substituent effects in substituted diphenyl sulphides decreases in the order S‐4‐X ≈︁ S‐2‐X > Me2‐S‐4‐X > S‐3‐X; (ii) the inductive effects are transmitted to a larger extent than the resonance effects to the unsubstituted ring in 3‐substituted diphenyl sulphides, while the reverse trend is observed in other substituted diphenyl sulphides; (iii) in 2‐methoxy‐, 2‐chloro‐, 2‐bromo‐ and 2‐nitrodiphenyl sulphides, an increase in the size of the substituent causes an upfield shift for H‐6 ascribable to the repulsion between the lone pairs of electrons on the sulphur and the substituent and its influence on the conformation; (iv) the diminished transmission of substituent effects to the remote rings in 4‐substituted 2′,6′‐dimethyldiphenyl sulphides is probably due to the orthogonal orientation of the rings; and (v) the signal due to the H‐6 of 2‐substituted diphenyl sulphones suffers a downfield shift with an increase in the size of the substituent, this being ascribable to the increasing steric interaction between the 2‐substituent and the sulphonyl oxygen and consequent changes in the conformation.

van Gorkom, Leon C. M.; Hook, James M.; Logan, Michael B.; Hanna, John V.; Wasylishen, Roderick E.

doi: 10.1002/mrc.1260331005pmid: N/A

Solid‐state Pb‐207 NMR spectra of lead(II) nitrate were found to be highly sensitive to variations in temperature. The Pb‐207 chemical shift exhibits a linear dependence on temperature with a positive slope of 0.70 ± 0.02 ppm K−1, i.e. increases in temperature cause a change in chemical shift to higher frequency. This slope is within experimental error of the value obtained from parallel experiments on the static solid, 0.72 ± 0.06 ppm K−1. At higher magic angle spinning (MAS) speeds and ambient probe temperature, the Pb‐207 signal splits into several components owing to localized heating effects. These effects are more pronounced in rotors made from partially stabilized zirconia than those made from silicon nitride and are attributed to the poorer thermoconductive properties of the zirconia ceramic. In the zirconia rotors, discrete thermal regions within the lead(II) nitrate sample were located and differ by 5 K at 12 kHz MAS speeds. The response of the Pb‐207 lead(II) nitrate signal to temperature variation suggests that it could be used for calibrating solid‐state NMR probes.

Haire, D. Lawrence; Janzen, Edward G.; Robinson, Valerie J.; Zhang, Yong‐Kang

doi: 10.1002/mrc.1260331006pmid: N/A

Complete 13C NMR assignments for ten five‐membered ring nitrone (pyrroline‐N‐oxide) spin traps in CHCl3 are presented. A good correlation between the nitronyl 13C NMR chemical shift (δ) and the substituent constant (σ) for the substituted phenyl DMPOs was found. The 13C NMR chemical shift of the nitronyl carbon of these 5,5‐dimethyl‐2‐phenylpyrroline‐N‐oxides (or phenyl‐DMPOs) exhibited a large solvent effect (14 ppm from cyclohexane to water) which could be correlated with the solvent polarity parameter ET(30). Using a paramagnetic relaxation agent, the weighted time‐averaged fast exchange of the nitrone between the interior of a sodium dodecyl sulfate micelle and the bulk aqueous phase could be determined. It was found that the 2‐phenylpyrroline‐N‐oxides reside in microenvironments with polarities close to that of ethanol, methanol and between methanol and water. The fraction of nitrone completely inside the micelle at any given time was found to be around 77 ± 0.05%. This is the first time that the paramagnetic relaxation enhancement method has been used with 13C NMR spin‐lattice relaxation time measurements.

Orendt, Anita M.; Dunkel, Reinhard; Horton, W. James; Pugmire, Ronald J.; Grant, David M.

doi: 10.1002/mrc.1260331007pmid: N/A

A computerized analysis of a single 2D INADEQUATE spectra assigns all of the 13C chemical shifts in fairly complex aromatic systems. Without the aid of the computer this information is often difficult, if not impossible, to retrieve owing to overlap of the signals in the double quantum dimension. In addition, the use of the computerized analysis allows for the interpretation of INADEQUATE spectra with much lower signal‐to‐noise ratios than are practical if the analysis were to be done visually. Finally, narrowing the spectral window to a subset of the entire spectrum and the companion advantages are demonstrated. The assignments of the aromatic chemical shifts in 2‐methoxydibenzofuran, 6‐methylchrysene, 1‐decylpyrene, and 7‐methylbenzo[a]pyrene illustrate the power of automated pattern recognition methods while supplying previously unavailable chemical shift assignments on these complex fused polycyclic aromatic systems.

Wrackmeyer, Bernd; Kupče, ĒRiks; Köster, Roland; Seidel, Günter

doi: 10.1002/mrc.1260331008pmid: N/A

15N and 29Si NMR data of 2,5‐hydro‐1,2,5‐azasilaborole derivatives and related compounds were determined with emphasis on coupling constants 1J(29Si 15N) which cannot be measured from 15N NMR spectra of most boron‐nitrogen compounds. In the case of compounds with an SiNH fragment ψ‐Bird pulse sequences, combined with HMQC or HSQC, served for 1H detection of the 29Si resonance with preselection of the 15N‐1H spin pair. This permitted the measurement of 1J(29Si 15N) and the determination of its absolute sign (>0). In the absence of an NH group, the coupling constants 1J(29Si 15N) were measured from the 15N satellites in 29Si NMR spectra recorded by using the Hahn‐echo extended (HEED) refocused INEPT pulse sequence. Isotope induced chemical shifts 1Δ 15/14N(29Si) were also measured by this technique for all the compounds. The 1J(29Si 15N) values cover the range 8.4–17.0 Hz, depending on the substituents at boron, silicon and nitrogen and on the complexation of the heterocycle.

Aubke, F.; Hägele, G.; Willner, H.

doi: 10.1002/mrc.1260331009pmid: N/A

The decomposition of 15N3F at −20°C leads to cis‐ and trans‐15N2F2. The corresponding 19F NMR spectra were analysed as [AX]2 systems. 19F, 15N and 14N NMR spectra were simulated using the novel PC program systems DSYMPCD, DCYMPCD and WIN‐DAISY running under WIN‐NMR. Stereospecific coupling constants were obtained for both isomers. Comparison of the 19F NMR spectra from both the [1/2A1/2X]2 and the [1A1/2X]2 systems in 15N2F2 and 14N2F2 overcomes the inherent ambiguities of the [AX]2 analysis.

Danielsen, Knut

doi: 10.1002/mrc.1260331010pmid: N/A

The 13C and 1H NMR spectra of 1‐OH‐, 2‐OH‐, 1,2‐diOH‐, 1‐OAc‐, 2‐OAc‐ and 1,2‐diOAc‐9,10‐anthraquinone were assigned. The hypothesis that substituent‐effect additivity could be used as a chemical shift assignment tool was tested, with satisfactory results. For all compounds, errors were found in previous assignments. The NOE technique combined with SEFT, heteronuclear decoupling and 2D heteronuclear one‐bond correlation experiments were used for the assignment of the different resonances.