Microwave spectrum and rotational isomerism of n-propyl isocyanide

The microwave spectrum of n-propyl isocyanide has revealed the existence of two rotational isomers, trans (methyl trans to the isocyanide substituent), and gauche. Plausible structures have been fitted to the data, giving the gauche dihedral angle as 119° ± 2° from the trans position. Stark effect measurements have yielded dipole moments for the two rotamers: μ_trans = 4.16 ± 0.02 D and μ_gauche = 4.10 ± 0.09 D. The rotational constants of the trans form are A = 23 700 ± 100, B = 2407.632 ± 0.020, and C = 2278.853 ± 0.030 MHz, and those of the gauche form are A = 10 208.983 ± 0.030, B = 3479.219 ± 0.015, and C = 2859.981 ± 0.015 MHz. It has been found from relative intensity measurements that the gauche ground state is slightly more stable than the trans ground state, with an energy difference of 99 ± 45 cm^?1. Several vibrationally excited states have been assigned to the torsional motion around the central carbon-carbon bond, the CNC bending motion, and the methyl internal rotation. The torsional vibration frequency is 114 ± 20 cm^?1 in the trans form and 123 ± 20 cm^?1 in the gauche form. A four-term potential function for internal rotation about the central CC bond has been determined.