Isotope shifts in polycyclic aromatic hydrocarbon anions

Abstract

Isotope shifts are a well established tool for structural analysis by NMR. The substitution of a proton with a deuterium is the most widely studied of these effects. We have synthesized all three monodeuterated anthracenes and shown that their ¹³C spectra provide the same information that can be obtained from perdeuterated anthracene. The isotope shifts change when the PAH is reduced by an alkali metal. For a planar molecule such as anthracene, the isotope shifts change in approximate proportion to the change in charge density. However, when there is steric hindrance, such as in chrysene, reduction weakens the framework, allowing conformational change that substantially alters the isotopic shifts.