Microwave Spectrum, Structure, and Dipole Moment of Cyclohexene

The microwave spectra of cyclohexene, cyclohexene-d10, and 3,3,6,6-cyclohexene-d4 have been assigned. Subject to certain assumptions, the nine moments of inertia were least-squares fitted to obtain the structural parameters ∢C1–C2–C3 = 123.3°, ∢C2–C3–C4 = 111.6°, ∢C3–C4–C5 = 110.3°, ∢C–C–H (methylene) = 109.9°, ∢C–C–H (ethylene) = 119.5°, and θ=30.1°, where θ is the angle between the C1–us;C2 and C4–C5 bonds projected on the a–c principal axis plane. θ measures the deviation of the ring from planarity. The assumptions related to symmetry were (1) the molecular point group is C2; (2) the methylene groups are symmetric with equal C–C–H angles; and (3) the ethylene group and two adjacent carbons lie in a plane. The bond lengths were taken to be r(C1 –C2) = 1.34 Å, r(C2–C3) = 1.51 Å, r(C3–C4) = 1.53 Å, r(C4–C5) = 1.53 Å, r(C–H) (ethylene) = 1.09 Å, and r(C–H) (methylene) = 1.10 Å. Stark effect measurements yielded μb = 0.331 D for the common isotopic species and showed that μb is unchanged (within the experimental uncertainty) in the deuterated species. The Stark effect also confirmed the haclf-chair conformation and C2 symmetry by requiring μc to be less than 0.003 D.