Soderquist, A
1992
Carbon-13 Chemical Shift Tensors in Aromatic Compounds. 3. Phenanthrene and Triphenylene
Soderquist, A.; Hughes, C.D.; Horton, W.J.; Facelli, J.C. and Grant, D.M.
Journal of American Chemistry Society, 114(8):2826-2832, 1992. Funded by National Institutes of Health and US Department of Energy.
Measurements of the principal values of the C-13 chemical shift tensor are presented for the three carbons in triphenylene and for three different alpha-carbons in phenanthrene. The measurements in triphenylene were made in natural abundance samples at room temperature, while the phenanthrene tensors were obtained from selectively labeled compounds (99% C-13) at low temperatures (~25 K). The principal values of the shift tensors were oriented in the molecular fram using ab initio LORG calculations. The steric compression at C4 in phenanthrene and in corresponding positions in triphenylene is manifested in a sizable upfield shift in the alpha33 component relative to the corresponding alpha33 values at C1 and C9 in phenanthrene. The upfield shift in alpha33 is mainly responsible for the well-known upfield shift of the isotropic chemical shifts of such sterically perturbed carbons. In phenanthrene C9 exhibits a unique a22 value reflecting the greater localization of pi-electrons in the C9-C10 bond. This localization of the pi-electrons at the C9-C10 bond in the central ring of phenanthrene also corresponds with the most likely ordering of electrons described by the various Kekulé structures in phenanthrene. The analysis of the C-13 chemical shieldings of the bridgehead carbons in triphenylene provides significant experimental information on bonding between rings in polycyclic aromatic compounds. The results confirm that the electronic structure of triphenylene is best described by three fairly isolated benzene rings linked by C-C bonds of essentially single bond character. Similarly in phenanthrene, the bonding structure that correlates the shielding information may be characterized by the dominance of two benzene rings comprising the biphenyl moiety. A strong C9-C10 pi-bond with only limited pi-electron character n the C8a-C9 and C10-C10a bonds is indicated by both the experimental and theoretical results.
1987
Structural Variations and Evidence of Segmental Motion in the Aliphatic Region in Coals Observed with Dipolar-Dephasing NMR
Soderquist, A.; Burton, D.J.; Pugmire, R.J.; Beeler, A.J.; Grant, D.M.; Durand, B. and Huk, A.Y.
Energy and Fuels, 1, (1), 50-55, 1987. 6 pgs. Funded by US Department of Energy and Standard Oil Co.
We report the structural variations of a number of coals and coal macerals. Through the time constants associated with dipolar-dephasing techniques, CP/MAS spectral data reveal the presence of segmental motion in certain low-rank coal samples. The motion detailed is in the aliphatic region and is thought to be due to CH2 groups associated with hydroaromatic and/or polymethylene structural units.
Soderquist, A
1992
Carbon-13 Chemical Shift Tensors in Aromatic Compounds. 3. Phenanthrene and Triphenylene
Soderquist, A.; Hughes, C.D.; Horton, W.J.; Facelli, J.C. and Grant, D.M.
Journal of American Chemistry Society, 114(8):2826-2832, 1992. Funded by National Institutes of Health and US Department of Energy.
Measurements of the principal values of the C-13 chemical shift tensor are presented for the three carbons in triphenylene and for three different alpha-carbons in phenanthrene. The measurements in triphenylene were made in natural abundance samples at room temperature, while the phenanthrene tensors were obtained from selectively labeled compounds (99% C-13) at low temperatures (~25 K). The principal values of the shift tensors were oriented in the molecular fram using ab initio LORG calculations. The steric compression at C4 in phenanthrene and in corresponding positions in triphenylene is manifested in a sizable upfield shift in the alpha33 component relative to the corresponding alpha33 values at C1 and C9 in phenanthrene. The upfield shift in alpha33 is mainly responsible for the well-known upfield shift of the isotropic chemical shifts of such sterically perturbed carbons. In phenanthrene C9 exhibits a unique a22 value reflecting the greater localization of pi-electrons in the C9-C10 bond. This localization of the pi-electrons at the C9-C10 bond in the central ring of phenanthrene also corresponds with the most likely ordering of electrons described by the various Kekulé structures in phenanthrene. The analysis of the C-13 chemical shieldings of the bridgehead carbons in triphenylene provides significant experimental information on bonding between rings in polycyclic aromatic compounds. The results confirm that the electronic structure of triphenylene is best described by three fairly isolated benzene rings linked by C-C bonds of essentially single bond character. Similarly in phenanthrene, the bonding structure that correlates the shielding information may be characterized by the dominance of two benzene rings comprising the biphenyl moiety. A strong C9-C10 pi-bond with only limited pi-electron character n the C8a-C9 and C10-C10a bonds is indicated by both the experimental and theoretical results.
1987
Structural Variations and Evidence of Segmental Motion in the Aliphatic Region in Coals Observed with Dipolar-Dephasing NMR
Soderquist, A.; Burton, D.J.; Pugmire, R.J.; Beeler, A.J.; Grant, D.M.; Durand, B. and Huk, A.Y.
Energy and Fuels, 1, (1), 50-55, 1987. 6 pgs. Funded by US Department of Energy and Standard Oil Co.
We report the structural variations of a number of coals and coal macerals. Through the time constants associated with dipolar-dephasing techniques, CP/MAS spectral data reveal the presence of segmental motion in certain low-rank coal samples. The motion detailed is in the aliphatic region and is thought to be due to CH2 groups associated with hydroaromatic and/or polymethylene structural units.