Conformation of saturated seven-membered rings: free jet absorption millimeter wave spectrum of 1,3-dioxepane

The free jet millimeter wave spectrum of 1,3-dioxepane has been investigated in the 60–78 GHz frequency range. Rotational transitions belonging to the ground vibrational state have been observed and measured, yielding the rotational constantsA = 3565.780(2), B = 3193.448(1) and C = 1936.312(2) MHz. The absence of low lying vibrational satellites and the kind of transitions observed (onlyμa-type) demonstrate that, in contrast with previous reports, the molecule is rather rigid and possesses C2 symmetry.