Solid-state 13C Nuclear Magnetic Resonance Spectra Research Articles

Solid-state nuclear magnetic resonance (NMR) spectra of pharmaceutical dosage forms.

Solid-state 13C NMR spectra of tablets or capsules of prednisolone, enalapril maleate, lovastatin, simvastatin, ibuprofen, flurbiprofen, mefenamic acid, indomethacin, diflunisal, sulindac, and piroxicam were obtained in the CP/MAS mode at 50 MHz. These studies show that (1) solid-state NMR spectroscopy can detect the active ingredients in low-dose tablets and capsules; (2) the use of interrupted decoupling often results in suppression of resonances due to excipients, thereby allowing better detection of resonances from the drug; and (3) the technique permits discrimination between two prednisolone polymorphs present in tablets obtained from various manufacturers even though the tablets contain only approximately 5% (w/w) of the drug.

Solid-state 13 C NMR spectra of iron carbonyl complexes: shielding tensor and error analyses

The high-resolution solid-state 13 C NMR (nuclear magnetic resonance) spectra are reported of the iron carbonyl complexes Fe 3 (CO) 8 (PhCCPh) 2 (I) and Fe 3 (CO) 12 (II), 13 C-enriched at the carbonyl carbons. The spectra were recorded with slow magic-angle spinning, and the intensities of the spinning side bands analysed by a computerized adaptation of the method of Herzfeld & Berger to give values for the principal components of the carbonyl carbon shielding tensors. Complex (I) includes two asymmetric double-bridging carbonyl groups, and the 13 C shielding-tensor components assigned to the C—O bond axis (σ 33 ) are intermediate between the values obtained for symmetric double-bridging carbonyl groups and linear M—C≡0 terminal carbonyls. It is proposed that the σ 33 shielding component may therefore be used to monitor the degree of asymmetric bridging of a carbonyl group. All 13 C resonances of complex (II) exhibit a degree of asymmetry in the shielding tensor and possible reasons for this are discussed. Three methods of estimating errors in the derived shielding-tensor components are considered. One method use calculated variances based upon differences between the experimental and calculated side-band intensities; this gives unreasonably large errors in the cases of axial symmetry in the shielding tensor. A second method explores different shielding components for values of ɸ 2 (the sum of the squares of the differences between the experimental and calculated side-band intensity ratios) which exceed the best minimum by an arbitrary percentage. The third method considers errors due to the signal: noise ratios in the experimental spectra. The relative merits of these three approaches of error estimation are examined.

Solid-state 13C nuclear magnetic resonance studies of coalified gymnosperm xylem tissue from Australian brown coals

We report here on the use of solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to contrast the average chemical composition of modern degraded gymnosperm woods with fossil gymnosperm woods from Australian brown coals (Miocene). We first established the quantitative nature of the NMR techniques for these samples so that the conventional solid-state 13C NMR spectra and the dipolar dephasing NMR spectra could be used with a high degree of reliability to depict average chemical compositions. The NMR results provide some valuable insights about the early coalification of xylem tissue from gymnosperms. Though the cellulosic components of wood are degraded to varying degrees during peatification and ensuing coalification, it is unlikely that they play a major role in the formation of aromatic structures in coalified woods. The NMR data show that gynmosperm lignin, the primary aromatic contribution to the coal, is altered in part by demethylation of guaiacyl-units to catechol-like structures. The dipolar dephasing NMR data indicate that the lignin also becomes more cross-linked or condensed.