Intramolecular pi-type hydrogen bonding and conformations of 3-cyclopenten-1-ol. 2. Infrared and Raman spectral studies at high temperatures.

The vapor-phase infrared and Raman spectra of 3-cyclopenten-1-ol (3CPOL) have been collected at temperatures ranging from 25 to 267 degrees C. These clearly show the presence of four conformations of 3CPOL with the one with intramolecular pi-type hydrogen bonding being most abundant. The spectra of all four conformations have been assigned, and these agree well with the computed values from the DFT calculation. The frequency shifts observed for the different conformations are in accord with the predicted values. In the O-H stretching region the conformer A with the pi-type intramolecular hydrogen bond has the lowest stretching frequency at 3623.4 cm(-1) while the three higher energy conformers have frequencies 14.2, 32.0, and 36 cm(-1) higher. In the C=C stretching region conformer A again has the lowest frequency at 1607.3 cm(-1) while the other conformers have bands 2.1, 8.0, and 13.4 cm(-1) lower. Both the O-H stretching and the C=C stretching force constants are decreased about 2% by the hydrogen bonding. Five of the other vibrations show significant predicted frequency shifts up to 193 cm(-1). Analysis of intensity data at different temperatures was used to calculate the energy difference between the two most stable conformers. This was found to be 435 +/- 160 cm(-1), and the result agrees reasonably well with the high level ab initio results which range from 274 to 401 cm(-1).