Knowledge Bank

The pure rotational spectrum was measured and assigned for the normal and the $\mathrm{<mrow\; data-mjx-texclass="ORD">22</mrow>}$Ne isotopomers of the weakly bound complex, neon methylene cyclobutane. The rotational constants for the all $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}$C isotopomer of neon methylene cyclobutane, Ne C$5$H$8$, are $A$ = 3522.991(4), $B$ = 2069.181(2), and $C$ = 1653.733(1) MHz. The structure of Ne C$5$H$8$ has been determined, and the coordinates of the neon in the C$5$H$8$ principal axis systems are $a$ = 0.272, $b$ = 0.630, and $c$ = 3.480 \AA. The position of the neon atom is shifted 0.63 \AA from the plane of symmetry of methylene cyclobutane due to the large amplitude motion of neon across the ring.