Determination of precise relative energies of conformers of n-propanol by rotational spectroscopy

Abstract

The rotational spectrum of n-propanol (n-CH₃CH₂CH₂OH) was studied with several techniques of contemporary broadband rotational spectroscopy at frequencies from 8 to 550 GHz. Rotational transitions in all five conformers of the molecule, Gt, Gg, Gg′, Tt, and Tg, have been unambiguously assigned. Over 6700 lines of the Gt, Gg, and Gg′ species, for quantum number values reaching K_a = 33 and J = 67, were fitted in a joint analysis leading to the determination of ΔE(Gg–Gt) = 47.82425(25) cm⁻¹ and ΔE (Gg′–Gg) = 3.035047(11) cm⁻¹. Stark effect measurements in supersonic expansion were used to further confirm the assignment. The results are compared with those for the ethanol molecule and with ab initio calculations, allowing several inferences to be drawn concerning the differences in the large amplitude torsional potential of the hydroxyl group in the two molecules.