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 0C to +80 0 C. The measurements were performed with the magnetic field parallel to the crystallographic 6-axis where all four molecules in the unit cell are magnetically equivalent. Below about 5 0C the spectrum consists of ten almost symmetrical doublets due to the ten distinct deuterons per bullvalene molecule. Above 5 0C 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, k3 = 13.6 X 10" exp(-19.1//?r> and £c = 4.02 X 1014 exp(~\5.\/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 13C NMR and structural crystallographic measurements. We also show that the bullvalene molecules undergo an inde­ pendent 3-fold jump process, and we determine the rate of this process as well. It thus turns out that solid bullvalene is at the same time a highly order crystalline system and also highly fluxional. It exhibits, both whole-molecule jumps and valence-bond tautomerism but using pathways that leave the site symmetry in the crystal completely unchanged. The experimental details including the mounting and NMR measurements of the bullvalene single crystals are described in the Experimental Section. The single crystal measurements at low temperatures and in the temperature range where dynamic effects are observed, are then presented and compared with theoretically calculated lineshapes from which kinetic parameters are derived. These parameters are subsequently used to simulate and analyze the spectra obtained from a powder sample of bullvalene. We close with a short summary and conclusions which