Structures and conformations of cyclopentane, cyclopentene, and cyclopentadiene

Abstract

The structures and conformations of cyclopentane, cyclopentene, and cyclopentadiene have been investigated by SCF gradient ab initio computation with a double zeta basis set. For cyclopentane the half-chair ( C 2) and the envelope ( C s) forms were found to be isoenergetic, consistent with the experimentally observed free pseudorotation. The barrier to coplanarity of the ring is 4.3 kcal mol −1. Cyclopentene has a puckered C s equilibrium conformation with a very low barrier to planarity. Cyclopentadiene has a planar ring. The compounds with 5, 3 and 1 methylene groups, respectively, show a decreasing tendency to form puckered rings as interaction between adjacent methylene groups becomes of less significance compared with the influence of angle strain which favors ring planarity. A corrected geometrical optimization of the half-boat ( C s) form of cyclohexene is also presented.