Structure and potential energy function of cyclopent-3-enone. Part 1.—Microwave spectrum, ring planarity, rs-structure, and dipole moment

Abstract

The microwave spectra of cyclopent-3-enone in the ground state, the first seven excited states of the ring bending vibration and with one quantum of the ring twisting mode excited have been assigned. A planar equilibrium configuration for the ring is established on the basis of several arguments.

The rotational spectra in the vibrational ground state of the six different singly substituted isotopic species, [2-D], [3-D], [¹⁸O], [1-¹³C], [2-¹³C] and [3-¹³C], of cyclopent-3-enone have also been assigned, the first three in isotopically enriched samples and the latter three in natural abundance. The rotational constants determined from the spectra are: [graphic omitted]

The r_s-coordinates of each atom have been obtained from these constants. Two of the coordinates are small and their determination by other methods is discussed. The essentially r_s-structure reported is: r(C₁—C₂)= 1.524 ± 0.008 Å, r(C₂—C₃)= 1.509 ± 0.008 Å, r(C₃—C_3′)= 1.338 ± 0.004 Å, r(CO)= 1.210 ± 0.002 Å, r(C₃—H₃)= 1.079 ± 0.003 Å, r(C₂—H₂)= 1.086 ± 0.004 Å, ∠C₂C₁C_2′= 109° 12′± 1°, ∠C₁C₂C₃= 103° 08′± 48′, ∠C₂C₃C_3′= 112° 26′± 18′, ∠H₃C₃C_3′= 124° 42′± 16′, ∠H₂C₂H₂= 107° 18′± 40′.

The value of the electric dipole moment measured using the Stark effect is 2.79 ± 0.03 D.