The microwave spectrum of 3,6hyphen;dihydrohyphen;2Hhyphen;pyran has been studied in the Rhyphen;band frequency range (26.5ndash;40.0 GHz) with a Hewletthyphen;Packard Model 8400 Starkhyphen;modulated microwave spectrometer. The spectrum is characterized by rather strong perpendicular transitions of a near oblate symmetric top. Transitions obeyinga, b, andcdipole selection rules have been assigned and 24 lowhyphen;J Rhyphen;branch transitions have been used to derive the rigid rotor constants for the ground vibrational state: (in MHz)Aequals; 5085.152plusmn;0.012;Bequals; 4849.365plusmn;0.013;Cequals; 2712.487plusmn;0.013. Stark displacement measurements were made for several transitions and the dipole moment components determined by least squares fitting the displacements; (in Debye) verbar;mgr;averbar;equals;0.892plusmn;0.006; verbar;mgr;bverbar;equals;0.507plusmn;0.010; verbar;mgr;cverbar;equals;0.771plusmn;0.002 yielding a total dipole moment mgr;totequals; 1.283plusmn;0.005. Model calculations to reproduce the rotational constants and the observed dipole moment components indicate that the data are consistent with a twisted ring skeleton with the oxygen atom on one side and carbon 2 on the other side of a plane formed by the other four carbon atoms. This oxygen atom and carbon 2 lie in a plane which passes halfway between carbons 3 and 6 and makes an angle tgr;equals;31.5deg; with the C3minus; C4equals; C5minus; C6plane. The valence angles within the ring are very near the values found in analogous open chain molecules.
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