Microwave Spectrum and Determination of the Ring Bending Potential Function of Trimethylene Selenide

The microwave spectrum of trimethylene selenide (TMSe) was measured in the region 7.3–32 GHz. The rotational constants for seven vibrational states of the ring bending mode for the most abundant isotopic species were determined. From the variation of the rotational constants with vibrational state a potential function was obtained for the ring bending vibration, V(Q) = 8.7056[Q4 − 13.2669Q2]. This potential function is in good agreement with that obtained from an analysis of the far ir spectrum, V(Q) = 8.7056[Q4 − 13.19Q2], and predicts a barrier height to the planar configuration of 383.1 ± 4 cm−1 as compared with 378.1 ± 4 cm−1 obtained from the far infrared. The equilibrium dihedral angle was calculated to be 29.5 ± 1°.