Microwave Spectrum of the Ethylmethyl Ether Molecule

We have observed rotational transitions of ethylmethyl ether (CH(3)CH(2)OCH(3)) in the 24-110 GHz frequency range. We newly assigned the transitions of four Q-branch series for J=1-38 with Ka=0-5 and six R-branch series of b-type transitions for J=7-37 with Ka=0-3. All these assigned transitions were observed to be split into two or four components due to the internal rotations of the methyl groups. We analyzed the averaged frequencies of the split components on the basis of the Watson A-reduced Hamiltonian, neglecting the effect of the internal rotations. A total of 122 transitions were fitted to eight molecular parameters to a 1σ standard deviation of 24 kHz. The parameters A, B, C and D(J) were improved, and D(JK), D(k), d(J) and d(K) were determined for the first time.