On pseudo-rotation in (AsCH3)5 and (AsCF3)5

Two internal rearrangement processes can be deduced from the temperature dependences of the 1 H and 19 F nuclear magnetic resonance (n.m.r.) spectra of the asymmetric puckered five-membered ring compounds (AsCH 3 ) 5 , (AsCF 3 ) 5 , and (PCF 3 ) 5 . The first is a fast "butterfly" motion of the ring puckering which yields a time-averaged plane of symmetry perpendicular to the ring through the puckered atom. This motion could not be stopped in any of the three compounds by cooling, even to −130 °C in the case of (AsCF 3 ) 5 . The second is a slow pseudo-rotation, or a rotation of the perpendicular symmetry plane around the ring yielding a time-averaged fivefold rotation axis. There is a choice between two mechanisms for this second rearrangement, either a "clickstop" process in which the puckering progresses around the ring by one position at a time, or a "pancake" process in which the puckering drops randomly into any one of the five ring positions from a planar transition state. This choice can be resolved for this cyclic permutation process by appeal to the n.m.r. lineshapes. The present data for (AsCH 3 ) 5 favor the "clickstop" mechanism in spite of a competing intermolecular process at elevated temperatures with other components in the system which are in equilibrium with the ring compound.