High-Resolution Solid-State NMR Investigation of the Phase Transition in Decamethylferrocene-Acenaphthenequinone Charge-Transfer Complex

A charge-transfer complex composed of decamethylferrocene (D) and acenaphthenequinone (A) was prepared. The material was a 1:1 neutral complex with a mixed-stack structure and exhibited a phase transition at -16 degrees C. High-resolution C-13 and H-1 NMR spectroscopy revealed that an inclination of A with respect to D occurs below the phase-transition temperature. The H-1 spin-diffusion rates of the complex undergoing highspeed magic-angle spinning (MAS) were measured to determine the shortest H-1-H-1 distance r between D and A. To analyze the experimental results, we derived the analytical expression of the spin-diffusion rate W-z for a homonuclear multispin system undergoing MAS. It was found that W-z for the complex is proportional only to 1/r(6) under high-speed MAS conditions. On the basis of this relationship and the crystal structure at 20 degrees C, it was determined that the shortest H-1-H-1 distance r at -27.7 degrees C (below the phase transition temperature) is 0.4 angstrom shorter than that at 20 degrees C. Given this information, a plausible model of the low-temperature structure is discussed.