Knowledge Bank

The rotationally resolved $S1\leftarrow S0$ spectrum of the 1,4-difluorobenzene-$N2$ van der Waals complex has been measured by Kang and $Pratta$. Due to internal motion, the spectrum is split into two subbands, of which only the stronger one can be fit with a rigid rotor Hamiltonian. The analysis of the stronger subband has revealed that $N2$ is located above the ring plane of 1,4-difluorobenzene. In order to obtain information about the internal motion, both subbands were analyzed with a semirigid $C2v$ frame---$C2v$ top internal rotor $modelb$. Upper limits of $20cm-1(S0)$ and $6cm-1(S1)$ for the barriers hindering $N2$ internal rotation have been obtained. Details about the problems in determing the equilibrium orientation and about implication on the structure determination will be discussed.