The microwave spectra of cyclopropanol and cycl~propan-[~H~]-ol show the gauche-conformation is the dominant species present in the vapour at 293 K. No evidence for the existence of the transconformation 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 W 326 MHz in the [2Hl] form on account of contributions of the torsional frequency 0--+ O+ 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 resonances. The 0-+-O+ excitation energy for this species is 41 15.26( 42) MHz. Spectra of gauche-cy~lopropan-[~H~]-ol can be fitted to a single set of rotational constants common to the O+ and 0states and a 0--+ Of excitation energy of 163.74(1.80) MHz. The dipole moment components of the [2Hl] form are determined from Stark effect measurements as: pa = 1.58(10)x Cm, [0.48(3) D] ; pb = 3.75(50) x C m, [1.12(15) D] ; pc = 2.70(57) x C m, [0.81(17) D] ; and ptotai = 4.88(67) x C m, [1.46(20) D]. Substitution coordinates of the hydroxyl hydrogenaccord with an equilibrium dihedral angle of internal rotation, a, of 106" rf: 5" relative to a zero at the trans-conformation. The 0-+ O+ energy differences are used in conjunction with this value of a and the assumption that no stable trans-form occurs, to yield an approximate potential function for internal rotation. The cis-barrier separating the gauche-minima is 7.9(1.5) kJ mol-', while the barrier via the trans-conformation is 17(6) kJ mol-'.

Microwave spectra have been measured for the planar molecules 4H-pyran-4-one (I), 4H-pyran-4thione (11), 4H-thiapyran-4-one (111) and 4H-thiapyran-4-thione (IV). Extensive isotopic data for I, I1 and IV enable precise, complete structures to be calculated. There is little structural evidence for enhanced aromaticity in these molecules.The high, almost constant, values for the dipole moments near 4 D, previously indicated from solution measurements on I, I1 and IV, have been confirmed and the values more closely defined.Vibrational satellite spectra indicate the lowest frequency fundamentals in these molecules to be out-of-plane motions near 100 cm-'.

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