Intramolecular pi-type hydrogen bonding and conformations of 3-cyclopenten-1-ol. 1. Theoretical calculations.

The 3-cyclopenten-1-ol (3CPOL) molecule possesses two large-amplitude, low-frequency vibrations, namely, the ring-puckering and OH internal rotation, which can interconvert its four conformers into each other. Ab initio and density functional theory (DFT) calculations have been carried out to understand the energetics of these conformational changes. The lowest energy 3CPOL conformer possesses weak pi-type intramolecular hydrogen bonding between the hydroxyl hydrogen and the carbon-carbon double bond, and this lies 274 cm(-1) (0.78 kcal/mol) to 420 cm(-1) (1.20 kcal/mol) lower in energy than the other three conformations according to CCSD/6-311++G(d,p) computations. The two-dimensional potential energy surface for 3CPOL was computed as a function of the ring-puckering and OH internal rotation coordinates with the MP2/6-31+G(d,p) model.