Conformational studies of monosubstituted three-membered rings by variable temperature FT-IR spectra of rare gas solutions

Variable temperature (-55 to - 100 degreesC) studies of the infrared spectra (3500-300 cm(-1)) of cyclopropane carboxaldehyde. c-C3H5CHO, and fluoromethyl cyclopropane, c-C3H5CH2F, dissolved in liquid xenon have been recorded. Utilizing the conformer pair at 959 cm(-1) (cis with the oxygen atom over the three-membered ring) and 925 cm(-1) (trans) for the aldehyde, the enthalpy difference has been determined to be 105 +/- 18 cm(-1) (1.26 +/- 0.22 kJ/mol) with the trans conformer the more stable rotamer. For fluoromethyl cyclopropane the conformer pair at 927 cm(-1) (gauche) and 961 cm(-1) (cis with fluorine atom over the ring) was utilized to obtain the enthalpy difference of 250 +/- 25 cm(-1) (2.99 +/- 0.30 kJ/mol) with the gauche conformer the more stable form. Ab initio calculations have been carried out with several different basis sets with full electron correlation for c-C3H5CHO, c-C3H5CH2F and c-C3H5CH2NH2, from which conformational stabilities have been determined and compared to the experimental results. (C) 2001 Elsevier Science B.V. All rights reserved.