Conformational studies by infrared and Raman spectroscopy, supported by ab initio calculations, of some unsaturated organic molecules

The Raman spectra (3500 to 40 cm−1) and infrared spectra (3500 to 60 cm−1) in the fluid and solid phases of several unsaturated organic compounds with potential for more than one conformers have been recorded. For all the compounds, the spectra of the fluid phases have shown two stable conformers in equilibrium at ambient temperature. The midinfrared spectra of these unsaturated compounds dissolved in liquefied xenon and krypton as a function of a temperature (−55 to −100°C or −105 to −150°C) have been recorded. Utilizing several conformer pairs for each compound, the enthalpy differences have been determined.; Complete vibrational assignments have been proposed based on the group frequencies, infrared band contours for the gas, Raman depolarization ratio values, spectral changes from different phases and ab initio predicted frequencies. The ab initio frequencies have been predicted from MP2/6-31G(d) calculations with scaling factors of 0.88 for the CH stretches, 0.9 for heavy atom stretches and CH bends, 1.0 for heavy atom bends and torsions and 1.3 for C≡C in-plane and out-plane bends. For all the molecules the average percentage error between the scaled ab initio predicted frequencies and the observed frequencies is less than 1.5% which is excellent agreement.; Equilibrium geometries and energies of the two conformers of these unsaturated compounds have been determined by using ab initio molecular orbital calculations (HF and MP2) and hybrid DFT (B3LYP) methods using a number of basis sets. The results are compared with the available structural parameters obtained from microwave or electron diffraction data for some molecules. Structural parameters have been obtained from an ab initio & Microwave program utilizing ab initio MP2/6-311G(d,p) calculations and previously reported rotational constants.; From the far infrared spectra of the gas torsional transition bands have been observed for 1-fluoro-4-butyne, 3,3-difluorobutene and 2-hexen-4-yne. The potential parameters have been determined for the conformational interchange compared to those obtained from ab initio calculations with different basis sets at different levels of the theory.