Cope rearrangement and molecular reorientation in solid bullvalene: a single crystal deuterium NMR study

Deuterium NMR spectra of single crystals of deuterated bullvalene were recorded in the temperature range -13 °C to +80 °C. The measurements were performed with the magnetic field parallel to the crystallographic b-axis where all four molecules in the unit cell are magnetically equivalent. Below about 5 °C the spectrum consists of ten almost symmetrical doublets due to the ten distinct deuterons per bullvalene molecule. Above 5 °C dynamic line broadening effects set in which were quantitatively interpreted in terms of two independent thermally activated processes: (i) symmetric 3-fold jumps about the molecular C3 axis and (ii) Cope rearrangement combined with molecular reorientation that preserves the crystal symmetry. The kinetic equations for the rates of the 3-fold jumps and the Cope rearrangement processes are respectively, kJ = 13.6 X loi7 exp(-19.1/RT) and k, = 4.02 X loi4 exp(-lS.l/RT), where R is in kcal/mol.deg and the k's are in s-'. Deuterium NMR measurements on a powder sample of deuterated bullvalene were also performed and found consistent with those obtained from the single crystals. The results are discussed in comparison with earlier solid state proton and 13C NMR and structural crystallographic measurements