Spectroscopic and theoretical investigations of π-type intramolecular hydrogen bonding of 3-cyclopenten-1-ol and 3-cyclopenten-1-amine *

The infrared and Raman spectra of 3-cyclopenten-1-ol (3CPOL) and 3-cyclopenten-1-amine (3CPAM) have been recorded and analyzed.  Theoretical CCSD/cc-pVTZ computations have also been carried out.  Both molecules can exist in six different conformers, two pairs of which are mirror images of each other.  3CPOL has a conformer that possesses an OH group which forms a π-type hydrogen bond with the C=C double bond across the five-membered ring.   3CPAM has two NH₂ mirror image conformers with a π-type hydrogen bond with the C=C double bond across the ring. The theoretical computations indicate that for 3CPOL and 3CPAM these π bonding conformations lie about 300 cm^-1 (for 3CPOL) and 200 cm^-1 (for 3CPAM) lower in energy than the minima for the other conformers.  The vapor-phase infrared and Raman spectra of both molecules show the presence of all of the predicted conformers and their frequency differences correspond well to those predicted by the theoretical computations.