Knowledge Bank

The microwave spectrum of $\beta$-propiolactone has been studied in the vibrational ground state and in the first four excited states of the ring-puckering vibrational mode. The rotational constants are ""[FIGURE]"" $$\begin{array}{llll}&\multicolumn{1}{c}{ A}&\multicolumn{1}{c}{B}&\multicolumn{1}{c}{C}\v=0&12,406.01,,{ Mc}&5.244.53,,{ Mc}&3,869.48,,{Mc}\v=1&12,359.20&5.247.00&3,876.36\ v=2&12,312.38&5,249.98&3,883.41\ v=3&12,265.45&5,252.52&3,890.44\ v=4&12,218.28&5,255.33&3,897.54\end{array}$$ There is no detectable barrier at the planar configuration and the vibrational potential is essentially harmonic.