Knowledge Bank

It is confirmed by microwave spectroscopy that normal propyl fluoride consists of two rotational isomers. One of them, the trans form, has methyl and fluorine groups in trans position with respect to the central C-C axis, and the other, the gauche form, has the structure in which two groups are twisted by $63\circ$ from the cis position. The rotational constants derived from the analysis of rotational lines are [FIGURE] where the calculated values are obtained by the following assumption: C-F 1.379 {\AA}, C-C 1.534 {\AA}, C-H 1.094 {\AA}, , dihedral angle for the gauche form and . Four sets of vibrational satellites are assigned for the trans form and six sets for the gauche form. Some transitions of one of these sets are split into doublets, so that the set is assigned to the methyl torsion. The barrier height of the methyl torsion obtained from these splittings is 2.7 kcal/mol for the trans form and 2.8 kcal/mol for the gauche form. The strongest set of satellites is assigned to the central C-C torsion for both forms, the set due to its overtone state is also detected. The intensities of these satellites are measured relative to the corresponding lines of the ground state. The frequencies of the C-C torsion thus obtained are $90cm-1$ for the trans form and $150cm-1$ for the gauche form. The intensity of the $40,4-41,3$ transition of the trans form is compared with that of the $71,5--72,5$ transition of the gauche form at both dry ice and room temperatures. It is found from the temperature dependence of this intensity ratio that the gauche form is more stable than the trans form by 0.4 kcal/mol.