Theoretical investigation of intramolecular <i>π</i>-type hydrogen bonding and internal rotation of 2-cyclopropen-1-ol, 2-cyclopropen-1-thiol and 2-cyclopropen-1-amine

Theoretical computations utilising both CCSD and MP2 methods and the cc-pVTZ basis set have been carried out to determine the structures of several conformations as well as the internal rotation potential energy functions for 2-cyclopropen-1-ol, 2-cyclopropen-1-thiol and 2-cyclopropen-1-amine. The energies and wavefunctions for these potential functions have also been computed. Each of these molecules has an energy minimum corresponding to a conformation with intramolecular <i>π</i>-type hydrogen bonding. The <i>π</i> bonding stabilisation is about 2.3 kcal/mole for the alcohol, 2.1 kcal/mole for the thiol, and about 2.5 kcal/mole for the amine. The results for the thiol demonstrate a rare example of intramolecular <i>π</i>-type hydrogen bonding. The calculated O–H, S–H, N–H, and C=C stretching frequencies have also been compared for the conformations with and without the <i>π</i>-type hydrogen bonding. The C=C stretching frequency is substantially lower in all cases for the hydrogen bonded conformers.