Knowledge Bank

A complete heavy-atom substitutional structure has been determined for cyclopentanone, confirming the C$2$ twisted structure for this five-membered ring. The rotational constants of seven isotopomers of the argon cyclopentanone van der Waals complex have been determined from the pulsed-jet Fourier transform microwave spectra. An 88-line fit of the all $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}$C isotopomer gave rotational constants \it A \rm = 2611.6695(3), \it B \rm = 1112.3028(1), and \it C \rm = 971.3195(1) MHz. The molecular structure was determined from the rotational constants of the normal, the five singly-substituted $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}$C, and the $\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O isotopomers. The coordinates of the argon in the principal axis system of C$5$H$8$O are \it a \rm = 0.95, \it b \rm = 0.80, and \it c \rm = 3.46 \AA, where the carbonyl bond lies along the \it a\rm -axis and the oxygen atom position is \it a \rm = 2.05 \AA.