Microwave spectrum,structure, dipole moment and internal rotation of trans-ethylmethylether

Microwave spectra of ethylmethylether and its eleven isotopically substituted species were measured. The r_s structure of the trans isomer was determined from the observed moments of inertia. Structural parameters of this isomer were roughly equal to those of the reported r_s structure for dimethylether and diethylether. The CH₂-O bond length was definitely shorter by about 0.01 Å than the CH₃-O bond length and the C-C bond length was nearly equal to those of ethylchloride and bromide. The OCH₃ group tilted by about 2° 13' towards lone pair electrons of the oxygen atom while no significant tilt angle was found for the CH₃C group.Dipole moments of the trans isomer for the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment was μ_a = 0.146 ± 0.022 D,μ_b = 1.165 ± 0.020 D and μ_total=1.174 ± 0.022 D making an angle of 7° 5' ± 32' with the b inertial axis. Direction of the dipole moment in the molecule was discussed.From splittings of the observed spectra, barriers to internal rotations of two CH₃ groups were obtained in the one-top approximation. They were 2702 ± 7 and 3300 ± 25 cal mol^?1 for the OCH₃ and CH₃C groups, respectively, from the analysis of splittings in the first excited CH₃ torsional states. The coupling effects among two tops and the skeletal torsion were briefly discussed.