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 the microwave spectrum of a fourth conformer (GG) of jet cooled l-chlorobutane. Its energy has been calculated to be 140(50) cm(-1) above the most stable conformer, TT. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. Using the relative conformational and vibrational energy spacings, and the shifts of second moments of inertia upon conformational. change and vibrational excitation, the two-dimensional potential-energy surface of the skeletal torsions has been evaluated by a two-dimensional flexible model analysis. The relative energy of the fifth stable conformer (GG') was found to be 475 cm(-1) from flexible model calculations, and 730 cm(-1) from the most accurate ab initio calculations. The high value of the energy probably prevented the observation of the rotational spectrum of this conformer.