MICROWAVE SPECTRUM OF 2,3-DIHYDROTHIOPHENE

Author Institution:The microwave spectrum of 2,3-dihydrothiophene, CHCHS$CH_{2}CH_{2}$, has been recorded from 18.0 to 39.0 GHZ. Only a--type transitions were observed. The R-branch assignments have been made for the ground and first excited states of the ring puckering mode. The ground state rotational constants are: $A = 6863.14\pm0.07, B = 4707.75\pm0.01$ and $C = 2953.43\pm0.01$MHz. It is shown that the quantity $I_{a} + I_{b} - I_{c} = 9.87 U.$ {\AA}$^{2}$ cannot arise from the four hydrogens on the two $CH_{2}$ groups but that the heavy atom skeleton contributes to it; thus, the heavy atom skeleton is non-planar. The rotational constants are consistent with the determined heavy atom structural parameters. From relative intensity measurements, it is concluded that the second and third excited states are closely spaced with frequencies of $155\pm15 \mbox{cm}^{-1}$ whereas the first excited state is nearly degenerate with the ground state. These results are consistent with the following potential function: V(X) = $6.40\times10^{5}X^{4} - 2.89 \times10^{4}X^{2}$, with a barrier to inversion of $325 cm^{-1}$. These results will be compared to the corresponding quantities in similar molecules