Ab initio Quantum-Chemical Calculations and Spectroscopic Studies of the Intramolecular π-Type Hydrogen Bonding in Small Cyclic Molecules

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.