The microwave spectrum, conformation, and vibration-rotation interaction of N-cyanopyrrolidine

The microwave spectrum of N-cyanopyrrolidine was observed and assigned in the ground and nine excited states. In the lowest two states, split 3.9 cm ^-1 by a ring-puckering, nitrogen-inversion motion, the rotational constants are (for v = 0) A = 6585.05 ± 3.83, B = 1919.54 ± 0.05, C = 1583.84 ± 0.05, and (for v = 1) A = 6575.31 ± 6.01, B = 1922.37 ± 0.08, C = 1586.44 ± 0.08 MHz. Deviations from rigid rotor behavior in the lowest two states were described and analyzed by inclusion of a Hamiltonian term coupling the states via the internal vibrational angular momentum. The observed conformation of the five-membered ring system was found to be the envelope equatorial form. The tunneling motion which interconverts equivalent conformers has been discussed, and the qualitative nature of the potential energy surface has been described and compared to the parent unsubstituted molecule.