Microwave rotational spectrum, ring-puckering vibrations, ring conformation and electric dipole moment of γ-crotonolactone

Abstract

The rotational spectrum of γ-crotonolactone has been observed and measured in the frequency range 17.0–40.0 GHz. R-Branch and Q-branch transitions allowed by µ_a and µ_b have been assigned for the vibrational ground state, the excited states v_B= 1, 2 and 3 of the ring-bending mode and the excited state v_T= 1 of the ring-twisting mode. Relative intensity measurements among vibrational satellites give the vibrational separations v_B= 1 â†� 0 and v_T= 1 â†� 0 as 199 and 349 cm^–1, respectively. The electric dipole moment components determined from Stark-effect measurements are µ_a= 4.726 ± 0.01 D, µ_b= 1.718 ± 0.004 D and µ_c= 0.0 D, leading to µ_TOTAL= 4.905 ± 0.011 D. Consideration of the quantity Δ₀=I⁰_c–I⁰_b–I⁰_a, the variation of the rotational constants with v_B, vibrational separations and the value µ_c= 0.0 D confirms that the heavy atoms of γ-crotonolactone are strictly coplanar at equilibrium and shows that the mode ν_B is governed by an essentially quadratic potential-energy function.