Microwave spectra and conformational studies of ethylamine from temperature dependent Raman spectra of xenon solutions and ab initio calculations

• Vibrational data indicate the presence of two conformers.
• The enthalpy difference going from the trans form to the gauche form is 62 ± 6 cm−1.
• Structural parameters have been determined for both conformers.
• Ab initio calculations have been performed for both forms.

FT-microwave spectroscopy was carried out where the trans conformer was identified to be the most stable conformer. Variable temperature (−60 to −100 °C) studies of the Raman spectra (4000–50 cm−1) of ethylamine, CH3CH2NH2 dissolved in liquefied xenon have been carried out. From these data both conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 62 ± 6 cm−1 (0.746 ± 0.072 kJ mol−1) with the trans conformer the more stable form. The percentage of the gauche conformer is estimated to be 60% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations with the Møller–Plesset perturbation method to the second order (MP2(full)) and the fourth order (MP4(SDTQ)) as well as with density functional theory by the B3LYP method by utilizing a variety of basis sets. Vibrational assignments have been made for the observed bands which have been predicted by MP2(full)/6-31G(d) ab initio calculations which includes harmonic force fields, frequencies, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some similar molecule.

Figure optionsDownload as PowerPoint slide