Equilibrium structures have been determined for s-trans-1,3-butadiene and ethylene after adjusting the rotational constants obtained from rotational spectroscopy by vibration-rotation constants calculated from the results of quantum chemical calculations. For butadiene, the formal C=C bond length is 1.338 A, and the formal C-C bond length is 1.454 A. For ethylene, the C=C bond length is 1.3305 A. These values appear to be good to 0.001 A. It is shown for the first time that pi-electron delocalization has the structural consequences of increasing the length of the formal double bond by 0.007 A and decreasing the length of the formal single bond by 0.016 A. Comparisons are made with structures computed with several quantum chemical models. The MP2/cc-pVTZ results agree best with the new re structure.

Starting from a general Hamiltonian for a molecule with two periodic large-amplitude motions, an effective rotational Hamiltonian is developed for bound states of the internal motions. It contains two types of matrix elements: (i) The internal motion energy, expressed as two-dimensional Fourier series, and transformed to a reference axes system. (ii) Matrix elements of rotational operators in the reference axes system multiplied by two-dimensional Fourier series. The spectroscopic parameters are the coefficients of the Fourier series (integrals of localized functions), the internal motion parameters ρ, and the angles between the vectors ρ and the reference axes. Expressions are given for cases with higher symmetry. The effectiveness of the Hamiltonian is demonstrated by fitting the rotational spectrum of the ground state of dimethylether (356 frequencies involving levels up to J=25) to experimental precision. The data of five vibrational excited states can be fit to almost the same precision.

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Dimethyl etheris a well-known interstellar molecule with a large abundance in hot core (CH 3 OCH 3 ) regions. We have measured many rotational-torsional transition lines of this molecule in the 100È550 GHz frequency range using two experimental techniques : a klystron-based traditional millimeter-wave spectrometer and a new fast scan spectrometer (designated "" FASSST ÏÏ) with a voltage-tunable backward wave oscillator. The lines arising from the ground vibrational state have been combined with previous data, mainly at lower frequencies, to form a global data set consisting of over 1600 lines that have been assigned and Ðtted by an e †ective Hamiltonian method. The spectral constants obtained from the Ðt allow us to predict the frequencies of almost 6000 additional lines of dimethyl ether through 600 GHz.

The torsional far infrared and Raman spectra of gaseous CH3OCH3, CD3OCH3, and CD3OCD3 are presented. They are analyzed using a computer program which is based on the results of an extensive investigation of the isometric groups and of the symmetry groups of the rotation–internal rotation Hamiltonians of a series of semirigid two-top models. Four or more Fourier coefficients of the potential functions in two variables could be determined for each isotope. Strong evidence was found for Fermi-resonance-type interactions with the COC bending mode.