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.

Intermolecular shielding from molecular magnetic susceptibility. A new view of intermolecular ring current effects

“…we recently studied 4,7-di-t-butylacenaphthene, 17 where we observed larger than usual errors between the calculated and some of the experimental 13 C chemical shift principal values.…”

“…Nonetheless, because these molecular susceptibility effects do not originate in any intermolecular electronic interactions, they show linear additivity for each PAH molecule acting as a source 17 ; the combined effects of several neighboring PAH molecules can be calculated by appropriate interpolations using the values of the NICS calculated on a grid for each of them.…”

“…It is noteworthy to mention that this is the case in 4,7-di-t-butylacenaphthene, where the observed effects for protons and 13 C are of the same order of magnitude; of course, this makes the relative effect on the proton shieldings much larger, reaching up to 30% of its normal chemical shift range. 17 These effects are not restricted to PAH; the resonance of any nuclei in a molecular crystal located in favorable positions, i.e. those within the region in which the induced fields are significant as depicted in Figs 2-5, with respect to aromatic moieties in the crystal that will be affected by this intermolecular shielding mechanism.…”

High resolution NMR of free radicals: 13C magic angle spinning of two solid organic free radicals derived from 4,5-dihydro-1H-imidazol-3-oxide-1-oxyl and theoretical calculation of their NMR properties

“…The quality of the spectra is not as good as those of diamagnetic molecules, the computational level should be improved to reproduce all the carbon chemical shifts with the same accuracy and, as crystal field effects must be take into account, all the molecules present in the unit cell should be calculated. [26][27][28] It is expected that this type of research will considerably develop in the future, both in what experimental measurements and theoretical calculations are concerned.…”

Metric spaces in NMR crystallography

“…[14] and [15], and m ¼ Z. Multiplying expressions and matrices in Eqs. [14] to [18] yields the metric of the icosahedral representation as follows: : [19] One should note that this icosahedral metric is not only orthogonal (i.e., diagonal) but also every term in P icos (V) is equally weighted and thus normalized. The metric provides symmetrical weighting of the 2 d icos,m vs.…”