Spectra and structure of small-ring molecules. XLI. Microwave spectrum of 3-phospholene

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Abstract
The microwave spectra of 3‐phospholene and 3‐phospholene‐1‐d1 were recorded from 12–40 GHz and 26.5–40 GHz, respectively. Only A‐type transitions were observed. The R branch assignments have been made for the ground state of the ‐d1 molecule, and the ground, first, and second excited states of the ring‐puckering mode for the ’’light’’ molecule. It is shown that the skeletal atoms of the five‐membered ring are noncoplanar and the ring is puckered by an angle of approximately 18°. Additionally, it is shown that the most stable form of the molecule is the ’’endo’’ form with the P–H bond directed toward the double bond. From relative intensity measurements it is concluded that the first excited state of the ring‐puckering mode has a frequency of 160±15 cm−1. From measurements of the Stark effect for the 3‐phospholene molecule the dipole moment components were determined to be ‖μa‖=1.10±0.02 D, ‖μb‖=0.0 D (assumed), and ‖μc‖=0.16±0.05 D with the total dipole moment being ‖μt‖=1.11±0.02 D. These results are compared to other similar molecules.