Spectroscopic and Theoretical Study of the Intramolecular π-Type Hydrogen Bonding and Conformations of 3-Cyclopentene-1-Amine.

The infrared and Raman spectra of 3-cyclopentene-1-amine (3CPAM) have been recorded and analyzed, and the experimental investigations have been complemented by theoretical calculations. Ab initio coupled cluster theory with single and double excitations (CCSD) was used with the cc-pVTZ (triple-zeta) basis set to calculate the conformational energy and geometrical parameters for each of the six conformers of this molecule. MP2/cc-pVTZ and B3LYP/cc-pVTZ computations were utilized to calculate the vibrational frequencies of the conformers. Both the spectra and theoretical calculations verify the presence of the conformers and show that the conformer at the lowest energy has intramolecular pi-type hydrogen bonding involving the NH2 group. The hydrogen bonded conformer is about 2 kJ/mol lower in energy than the other conformers. The potential energy topographical contour map for the ring-puckering and NH2 internal rotation coordinates has been calculated, and this shows how the different conformers can interconvert into each other. The far-infrared spectrum in the 190 to 280 cm(-1) region shows several NH2 internal rotation bands for each of the different conformers, and these are consistent with one-dimensional representations for their torsional motions.