Deuterium NMR measurements are reported for two types of onium salts:(CH3) 3E+ I-, where E= O (counterion is BF4-), S, Se, or Te, and C6H5M (CH3) 3+ I-, where M= N, P, or As. Within each class of compounds the activation energy for rotation of the trimethyl groups about the axis increases with increasing size of the central atom. In the first class of compounds the CEC bond angle decreases with the size of the atom E. In addition the ...