Microwave spectrum, dipole moment, and conformation of 2,3-dihydro-p-dioxin(1,4-dioxene)

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Abstract
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 C2 symmetry axis has been established by analysis of the Stark effect yielding a single nonzero component of the dipole moment μb = 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 C2 rather than C2v is established by the value of (1/2)(Ia + Ib − Ic) which is clearly too large for the C2v 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.