Torsional analysis of 2-butynol

The microwave spectrum of 2-butynol, CH₃CCCH₂OH, has been investigated in the frequency range of 6-26.5 GHz. The spectra of the A and E torsional states were observed using Fourier transform microwave spectroscopy. Due to the presence of the cylindrically symmetric CC “spacer” between the methyl group and the rest of the molecule, the barrier to internal rotation, V₃, is only 7 cm⁻¹. One conformer of 2-butynol was observed and assigned. The spectrum was analyzed with the ρ-axis method using a very flexible Hamiltonian which gives a fit an order of magnitude better than that obtained with more standard code for internal rotation. The spectroscopic constants are: 23744.(18), 2093.429(1), 1966.358(1), and −400.34(2) MHz for A, B, C, and D_ab; 0.48(1) kHz, −30.3(4) kHz, and 4.5(5) MHz for Δ_J, Δ_JK, and Δ_K; and 0.1406(6), 6.93(9) cm⁻¹, −33.4(7) kHz, and 192.0(9) GHz for ρ, V₃, L_V, and F, respectively. The root mean squared error of the fit is 14 kHz. The angles between the internal rotor axis and principal axes are θ_a=12°, θ_b=78°, with θ_c held fixed at 90°.