title: 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.
abstract: 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.
authors: Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
authors: Zhiru
authors: Z
authors: Ma
authors: Merrill D
authors: MD
authors: Halling
authors: Mark S
authors: MS
authors: Solum
authors: James K
authors: JK
authors: Harper
authors: Anita M
authors: AM
authors: Orendt
authors: Julio C
authors: JC
authors: Facelli
authors: Ronald J
authors: RJ
authors: Pugmire
authors: David M
authors: DM
authors: Grant
authors: Aaron W
authors: AW
authors: Amick
authors: Lawrence T
authors: LT
authors: Scott
mesh_terms: D000073:Acenaphthenes / Q000737:chemistry*; D002247:Carbon Isotopes; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy
chemical_list: D000073:Acenaphthenes; D002247:Carbon Isotopes; C042553:acenaphthylene