Rotational Isomerism and Microwave Spectroscopy. I. The Microwave Spectrum of Normal Propyl Fluoride

Abstract

The normal propyl fluoride molecule has been confirmed by microwave spectroscopy to exist in two rotational isomers, trans and gauche. Rotational constants in the ground vibrational state are A = 26 986.73, B = 3748.32, C = 3509.88 Mc/sec for the trans form and A = 14 503.69, B = 5085.71, C = 4295.28 Mc/sec for the gauche form. The dihedral angle of the gauche form is about 63° from the cis position. The barrier height hindering methyl rotation has been determined, using lines in the first excited state of methyl rotation, to be 2.69 kcal/mole and 2.87 kcal/mole for the trans and the gauche forms, respectively. The dipole moment and its components along the principal axes have been determined by the Stark effect: μ_a = 1.97₀±0.026, μ_b = 0.56₆±0.064, μ_total = 2.05₀±0.043D for the trans form and μ_a = 1.13₇±0.008, μ_b = 1.45₀±0.033, μ_c = 0.47₂±0.137, μ_total = 1.90₂±0.064 D for the gauche form. The total dipole moment is nearly parallel to the C–F bond for both forms; the angle between them is 10.6° in the trans form and 7.4° in the gauche form. The intensities of satellites due to the first and the second excited states of the C–C torsion have been measured relative to the ground‐state lines. The energies of the first and the second excited states are 98.0 cm⁻¹ and 211.₃ cm⁻¹ for the trans form and 160.₈ cm⁻¹ and 322.₆ cm⁻¹ for the gauche form. The same technique of the relative‐intensity measurement has been applied to the temperature dependence of the intensity ratio of trans and gauche lines, giving the result that the gauche form is more stable than the trans form by 0.47±0.31 kcal/mole. Using these data the first seven constants were determined in the Fourier expansion of the potential function around the central C–C bond. The barrier at the cis position seems to be higher than that separating the trans form from two equivalent gauche forms.