Ab initio single-point energy computations at the CCSD(T)/aug-cc-pVTZ level have been carried out on nine molecules possessing intramolecular π-type hydrogen bonding. These computations were done using the calculated geometrical structures from CCSD/cc-pVTZ optimizations. The outcome has been analyzed together with previous spectroscopic and theoretical work. Results are presented for 3-cyclopenten-1-ol, 2-indanol, 2-cylopenten-1-ol, 2-cyclohexen-1-ol, 3-cyclohexen-1-ol, and 2-cyclopropen-1-ol with O–H∙∙∙π(C=C) bonding. Results for 3-cyclopenten-1-amine and 2-cyclopropen1-amine with N–H∙∙∙π(C=C) bonding and for 2-cyclopropen-1-thiol with S–H∙∙∙π(C=C) bonding are also discussed. For each molecule, the conformer with a weak intramolecular π-type hydrogen bond was found to be the global minimum, with the exception of 2-cyclopropen-1-thiol. In this case, the S– H∙∙∙π bonded conformer is higher in energy, indicating a weaker interaction. The hydrogen bonded conformers for the cyclic alcohols typically have conformational energies about 250 to 350 cm−1 lower than the other conformers. For the amines the lowering is about half of that. The infrared and Raman spectra for several molecules in the O-H stretching region show hydrogen bonded conformers to be at the lowest frequencies. The calculated potential energy surfaces for six of the molecules are also presented.
Laane et al. (Mon,) studied this question.