Microwave spectrum and internal rotation of isopropyl alcohol

Two rotational isomers, trans and gauche, were detected for the isopropyl alcohol molecule from the analysis of the microwave spectrum. The rotational constants of the trans form were obtained A = 8488.99, B = 8041.75, and C = 4765.19 MHz for the H-species, (CH ₃) ₂CHOH, and A = 8098.72, B = 7917.76, and C = 4680.75 MHz for the D-species, (CH ₃) ₂CHOD. Rotational constants averaged for the two lowest torsional states were estimated for the normal species of the gauche form to be A = 8566.03, B = 7992.73, and C = 4714.59 MHz. Dipole moment of the trans form of the H-species obtained was μ_b = 1.40 ± 0.03, μ_c = 0.73 ± 0.02, and μ_t = 1.58 ± 0.03 D. The a-type lines, characteristic of the gauche form, were observed for the H-species. From a preliminary analysis of these a-type lines, the energy difference between the ground and the first excited torsional state was estimated to be ∆ = 46 GHz. The barrier height was obtained to be V₃ = 1.68 kcal/mole by assuming a threefold symmetry for the internal rotation of the OH group.