Determination of molecular symmetry in crystalline naphthalene using solid-state NMR

Abstract

DIFFRACTION techniques have shown that the crystal structure of naphthalene has a unit cell with C_i symmetry^1–7. These studies were unable, however, to resolve any departure of the molecular structure from the D_2h, symmetry observed in the gaseous state. We found recently⁸ that the solid-state ¹³C-nuclear magnetic resonance (NMR) chemical shifts for naphthalene exhibit the C_i symmetry of the unit cell. If these chemical-shift data reflect structural distortions of the molecule, rather than simply intermolecular effects on the shifts owing to the C_i symmetry of the environment of each molecule, one could assert that the NMR data are able to reveal structural information beyond the limits of the diffraction methods. Here we show that this is the case by performing ab initio quantummechanical calculations of the ¹³C chemical shifts for naphthalene, and their derivatives, with respect to structural parameters. We find that intermolecular shift terms (which of necessity exhibit C_i symmetry) can account for only about 30% of the maximum deviations from D_2h symmetry; the remainder must therefore result from structural distortions of the molecules below D_2h symmetry. This sensitivity of NMR chemical shifts to very small changes in molecular structure opens up the possibility of using solid-state NMR along with quantum-chemical methods to refine structural parameters obtained from diffraction methods.