Conformational equilibrium and potential energy surface of 1-fluorobutane by microwave spectroscopy and ab initio calculations

The rotational spectra of four (GT, TT, TG, and GG) of the five possible co nformers of 1-fluorobutane have been assigned by combining free jet and con ventional microwave spectroscopy. The geometry optimization was performed a t the MP2 (full) level of theory with the 6-31G (d) and 6-311G id, pi basis sets and by using the B3LYP (3df, 3pd) density functional method. The rela tive stability of the five rotamers is calculated at the QCISD (T)/6-311G ( d, p) level of theory. In spite of the fact that ab initio calculations ind icated the unobserved GG' conformer to be more stable than at least one of the observed conformers it was not possible to detect its rotational spectr um. GT and TC are the most and the least stable species, respectively. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. The overall conf ormational equilibrium is governed by the two-dimensional potential energy surface of the skeletal torsions MeC-CC and FC-CC, which have been evaluate d by a flexible model analysis, based on the experimental values of the rel ative conformational and vibrational energy spacings, and on the shifts of second moments of inertia upon conformational change and vibrational excita tion. The relative energy of the fifth stable conformer (GG') was determine d to be 333 cm(-1) from flexible model calculations, and to be 271 cm(-1) f rom the most accurate ab initio calculations.