Conformational studies of cyclopropane carboxaldehydehyde from temperature-dependent FT-IR spectra of xenon solutions

Abstract

Variable temperature (−60 to −100°C) studies of the infrared spectra (3500–400 cm −1) of cyclopropane carboxaldehyde, c-C 3H 5CHO, dissolved in liquid xenon have been recorded. Utilizing several doublets due to the syn and anti conformers, the enthalpy difference has been determined to be 95±8 cm −1 (1.14±0.10 kJ/mol) with the anti conformer (oxygen atom trans to the three-member ring) the more stable rotamer. From this Δ H value, along with assigned torsional transitions for both the anti and syn conformers, the potential function governing the conformational interchange has been estimated. Using the new infrared data from the xenon solution, along with some additional Raman data, and ab initio predictions from MP2/6-31G(d) calculations some reassignments of the fundamentals have been made. Ab initio calculations have been carried out with several different basis sets upto 6-311++G(d,p) with full electron correlation by the perturbation method to second order from which structural parameters and conformational stabilities have been determined. From all of the ab initio calculations, the syn conformer is predicted to be the more stable which is at variance with the experimental results. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.