Low-frequency vibrational spectra and ring-puckering potential energy function of 3-phospholene and 3-phospholene-1-d₁

3-phospholene, CH₂CH=CHCH₂PH, and 3-phospholene-1-d₁, CH₂CH=CHCH₂PD, have been synthesized and their vapor-phase infrared and Raman spectra recorded. Seven far-infrared transitions associated with the ring-puckering vibration were observed for each molecule; several low-frequency Raman lines coincident with the infrared bands were also detected. The kinetic energy functions with no artificial modifications were calculated for the two molecules. Using these and the spectroscopic data the same potential energy function was determined for both isotopic species. The asymmetric function has a single minimum corresponding to the endo configuration with the double-bond and the P-H group pointing in the same direction. The planar ring structure lies 331 cm ^-1 (0.95 kcal/mole) higher in energy and the similiar exo configuration is 785 cm^-1 (2.24 kcal/mole) less stable. In addition to the pure ring-puckering transitions two weaker puckering sequences, originating from ring-twisting and P-H inversion excited states, and two sequences of hot bands and difference bands involving the P-H inversion excited states were also observed. From these a detailed energy map of the low frequency energy states was determined. The frequency shifts in the excited state of the P-H inversion demonstrate that this mode has a relatively strong interaction with the ring-puckering.