ChemInform Abstract: MICROWAVE SPECTRUM, DIPOLE MOMENT, AND CONFORMATION OF 2,3-DIHYDRO-P-DIOXIN(1,4-DIOXENE)

The microwave spectrum of 2,3‐dihydro‐p‐dioxin(1,4‐dioxene) has been studied in the R‐band range (26.5–40.0 GHz) using a Hewlett‐Packard model 8400 Stark modulated spectrometer. The rotational constants for the ground state and four vibrationally excited states have been derived from fitting b‐type, low J R ‐branch transitions. The rotational constants for the ground state are (in megahertz) A = 5697.703±0.010; B = 4968.410±0.008; C = 2865.181±0.010. The presence of a C 2 symmetry axis has been established by analysis of the Stark effect yielding a single nonzero component of the dipole moment μ b = 0.939±0.008 D and by the effect of nuclear spin statistical weights on the relative intensities of the rotational transitions. That the symmetry point group is C 2 rather than C 2v is established by the value of (1/2)(Ia + Ib − Ic ) which is clearly too large for the C 2v structure. Model calculations to reproduce the rotational constants show that they are particularly sensitive to two structural parameters: the C=C–O angle (123.4±0.5°) and the twist angle, i.e., the angle between the C=C bond and C–C bond (29.9±1.5°). Consideration of the ring angles for a structure derived by diagnostic least squares shows little, if any, ring strain.