Microwave spectrum, dipole moment and internal rotation potential function of gauche-cyclopropanol

Abstract

The Microwave spectra of cyclopropanol and cyclopropan-[²H₁]-ol slow the gauche-conformation is the dominant species present in the vapour at 293K. No evidence for the existence of the trans-conformation has been obtained. The spectra of this rotamer show transitions associated with dipole components in all three inertial axes, the b-type transitions, which form an appreciable part of the spectra, being split into doublets separated by ≈ 8200 MHz in the normal form and by ≈ 326 MHz in the [²H₁] form an account of contributions of the torisonal frequence 0^–â†� 0⁺ to these absorptions. Many lines of the normal form depart from rigid rotor behaviour. An analysis has been made in terms of nine parameters for this form, and confirmed by numerous double reasonances. The 0^–â†� 0⁺ excitation energy for this species is 4115.25(42) MHz. Spectra of gauche-cyclopropan-[²H₁]-ol can be fitted to a single set of rotational constants common to the 0⁺ and 0^– states and a 0^–â†� 0⁺ excitation energy of 163.74(1.80) MHz. The dipole moment components of the [²H₁₂] from are determined from Stark effect measurements as : µ_a= 1.58(10)× 10^–30 C m, [0.48(3)D]′µ_b= 3.75(50)× 10^–30 C m, [1.12(15) D]; µ_c= 2.70(57)10^–30 C m, [0.81(17) D]; and µ_total= 4.88(67)× 10^–30 C m, [1.46(20) D];. Substitution coordinates of the hydroxyl hydrogen accord with an equilibrium dihedral angle of internal rotation, α, of 106°± 5° relative to a zero at the trans-conformation. The 0^–â†� 0⁺ energy differences are used in conjunction with this value of α and the assumption that no stable trans-from occurs, to yield an approximate potential function for internal rotation. The cis-barrier separating the gauche-minima is 7.9(1.5)kJ mol^–1, while the barrier via the trans-conformation is 17(6) kJ mol^–1, while the barrier via the trans-conformation is 17(6) kJ mol^–1.