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 -13-degrees-C to +80-degrees-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-degrees-C the spectrum consists of ten almost symmetrical doublets due to the ten distinct deuterons per bullvalene molecule. Above 5-degrees-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, k(J) = 13.6 x 10(17) exp(-19.1/RT) and k(c) = 4.02 x 10(14) exp(-15.1/RT), where R is in kcal/mol.deg and the k's are in s-1. 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 C-13 NMR and structural crystallographic measurements.