CONFORMATIONAL EQUILIBRIUM AND POTENTIAL-ENERGY SURFACE OF 1-CHLOROBUTANE BY MICROWAVE SPECTROSCOPY AND AB-INITIO CALCULATIONS

Ab initio calculations proved to be very useful in the assignment of t he microwave spectrum of a fourth conformer (GG) of jet cooled l-chlor obutane. Its energy has been calculated to be 140(50) cm(-1) above the most stable conformer, TT. The rotational spectra of several vibratio nal satellites of the four conformers have been studied by conventiona l microwave spectroscopy. Using the relative conformational and vibrat ional energy spacings, and the shifts of second moments of inertia upo n conformational. change and vibrational excitation, the two-dimension al potential-energy surface of the skeletal torsions has been evaluate d by a two-dimensional flexible model analysis. The relative energy of the fifth stable conformer (GG') was found to be 475 cm(-1) from flex ible model calculations, and 730 cm(-1) from the most accurate ab init io calculations. The high value of the energy probably prevented the o bservation of the rotational spectrum of this conformer.