Electron diffraction and infrared spectroscopic study of the molecular structure of furan-2-aldehyde and 2-furanmethanethiol

Abstract Electron diffraction and IR spectroscopic studies of furan-2-aldehyde C 4 H 4 OCHO and 2-furanmethanethiol C 4 H 3 OCH 2 SH, have been carried out to obtain information on the conformational properties and geometry of these molecules. The electron diffraction data for both molecules were consistent with the assumption that the geometry of the ring skeleton and the hydrogen atoms attached to it were based on the furan structure as determined by microwave spectroscopy. The electron diffraction study ot furan-2-aldehyde confirmed the existence of two conformers with trans and cis oxygen atoms in the ring and aldehyde group. In the gas phase the trans conformer was found to be more stable, having less energy by an amount 0.5(4) kcal mol -1 . It was shown that the 2-furanmethanethiol molecule takes a non-planar conformation with dihedral angle O 1 C 2 C 6 S 7 = 39(4)° (0° value of the dihedral angle corresponds to the trans position of the O 1 -C 2 and C 6 -S 7 bonds, c.f. Fig. 4). The following values were obtained for bond lengths and bond angles characterizing the -CHO and -CH 2 SH groups in furan-2-aldehyde and 2-furanmethanethiol molecules, respectively (cf. Figs. 3 and 4) C 4 H 3 OCHO C 4 H 3 OCH 2 SH r a (C 2 -C 6 ) = 1.453(7) A r a (C 2 -C 6 ) = 1.481(15) A r a (C 6 O 7 ) = 1.212(4)A r a (C 2 -S 7 ) = 1.834(5) A ∠O 1 C 2 C 5 =117.6(4)° r a (C 6 -H) = 1.106(17) A ∠C 2 C 6 O 7 = 122.7(8)° ∠O 1 C 2 C 6 = 114.4(11)° ∠C 2 C 6 S 4 = 112.5(8)° The IR spectra of fuyan-2-aldehyde were measured in liquid and solid phases as well as in non-polar solvents at different temperatures between 308 and 210 K. Eight band pairs were identified which are dependent on the conformational equilibrium ; the band components are assigned to the trans or cis form. The energy difference determined from the 1470 cm −1 band pair is 0.51(4) kcal mol −1 . Evidence is given concerning the role of Fermi resonance in the splitting of the carbonyl stretching band. The IR spectra of 2-furanmethanethiol in the vapour and liquid phases measured between 233 and 323 K show the existence of only one conformer of the free molecule. The frequency of the symmetric methylene stretching vibration indicates a smaller hyperconjugative effect in 2-furanmethanethiol than in benzenemethanethiol. CNDO/2 and INDO calculations were performed for both rotamers of furan-2-aldehyde. The geometry was optimized in the CNDO/2 calculation starting with both microwave and electron diffraction data. The calculated energy difference of the conformers and their dipole moments are in good agreement with the experimental data.