Solid-state 13C NMR investigations of 4,7-dihydro-1H-tricyclopenta[def,jkl,pqr]triphenylene (sumanene) and indeno[1,2,3-cd]fluoranthene: Buckminsterfullerene moieties.

4,7-Dihydro-1H-tricyclopenta[def,jkl,pqr]triphenylene (sumanene) and indeno[1,2,3-cd]fluoranthene (indenofluoranthene) are structural moieties related to Buckminsterfullerene (C(60)). As such, understanding their structural characteristics is of great interest because of the insight they shed upon C(60). Hence, solid-state NMR (ssNMR) and ab initio quantum mechanical calculations with Gaussian03 are used in order to understand and to better characterize the molecular conformation and properties of sumanene and indenofluoranthene. Sumanene has bowl shaped curvature in its natural conformation and indenofluoranthene is planar in its natural conformation, which led us to examine how altering the curvature affects the chemical shifts in relation to those of C(60). Using X-ray structures of both sumanene and indenofluoranthene as our starting model, we calculate the energy and chemical shielding tensors and compare these data with those collected utilizing the (13)C ssNMR FIREMAT experiment. We define curvature of sumanene and indenofluoranthene using the pi-orbital axis vector (POAV) pyramidalization angle (theta(p)). We calculate the energy of varying conformations of indenofluoranthene versus their theta(p) associated with each deformed conformation.

Ring current effects in crystals. Evidence from 13C chemical shift tensors for intermolecular shielding in 4,7-di-t-butylacenaphthene versus 4,7-di-t-butylacenaphthylene.

13C chemical shift tensor data from 2D FIREMAT spectra are reported for 4,7-di-t-butylacenaphthene and 4,7-di-t-butylacenaphthylene. In addition, calculations of the chemical shielding tensors were completed at the B3LYP/6-311G** level of theory. While the experimental tensor data on 4,7-di-t-butylacenaphthylene are in agreement with theory and with previous data on polycyclic aromatic hydrocarbons, the experimental and theoretical data on 4,7-di-t-butylacenaphthene lack agreement. Instead, larger than usual differences are observed between the experimental chemical shift components and the chemical shielding tensor components calculated on a single molecule of 4,7-di-t-butylacenaphthene, with a root mean square (rms) error of +/-7.0 ppm. The greatest deviation is concentrated in the component perpendicular to the aromatic plane, with the largest value being a 23 ppm difference between experiment and theory for the 13CH2 carbon delta11 component. These differences are attributed to an intermolecular chemical shift that arises from the graphitelike, stacked arrangement of molecules found in the crystal structure of 4,7-di-t-butylacenaphthene. This conclusion is supported by a calculation on a trimer of molecules, which improves the agreement between experiment and theory for this component by 14 ppm and reduces the overall rms error between experiment and theory to 4.0 ppm. This intermolecular effect may be modeled with the use of nuclei independent chemical shieldings (NICS) calculations and is also observed in the isotropic 1H chemical shift of the CH2 protons as a 4.2 ppm difference between the solution value and the solid-state chemical shift measured via a 13C-1H heteronuclear correlation experiment.

Solid-state 15N NMR studies of tobacco leaves.

Nitrogen-containing compounds are one important class of constituents in tobacco because of various pharmacological and biological properties. Three types of tobacco leaves (burley, bright, and oriental) were studied using solid-state (15)N NMR cross polarization with magic-angle spinning, dipolar dephasing and five pi replicated magic angle turning (FIREMAT) experiments. The results show that burley tobacco leaves contain significantly more pyridinic nitrogen than that of bright or oriental tobacco leaves. The principal values of (15)N chemical shift tensors of nitrogen functional groups were obtained from the FIREMAT data. Possible assignments of solid-state (15)N NMR resonances were made using nitrogen chemical shift tensors in some model compounds or isotropic chemical shift values from liquid NMR results. To the best of our knowledge, this is the first solid-state (15)N NMR study of tobacco plant material.

Ring-chain tautomerism in solid-phase erythromycin A: evidence by solid-state NMR.

Chemical shift modeling, utilizing the DFT B3LYP/D95** method, provides the spectral assignment of the 35 visible 13C resonances from the solid-phase erythromycin A dihydrate. A new resonance at 110.8ppm is observed in the high-resolution 13C CP/MAS spectrum upon the application of heat or sample desiccation. With the use of the dipolar-dephasing spectral editing technique, this resonance is identified as a hemiketal carbon and the alternative hypothesis, a conformational change to the anomeric carbon of the desosamine sugar, is ruled out. Hence, the formation of a cyclic hemiketal in erythromycin A while in the solid phase is proven by solid-state NMR. The principal components of the 13C chemical-shift tensor corresponding to this hemiketal are reported. This is the first measurement of hemiketal 13C principal values. The delta11 and delta22 components are unique compared to anomeric carbon values reported in the literature.