Knowledge Bank

Rotationally resolved electronic spectra of methyl cyclopentadienyl radical ($CH3-C5H4$ and $CD3-C3H4$) were recorded in a cold jet $(Trot=0.7K)$. The origin band of the $B¯⟷X¯$ clectronic transition consists of two rotational bands with comparable intensity, which are separated by $2cm-1$ for $CH3-C5H4$, and by $1cm-1$ for $CD3-C5H4$. The higher frequency band, which has a simple rigid rotor type-A structure, was assigned to a transition between the ground torsional levels $(B~,0a1⟷X~Oa1)$. The lower frequency band was assigned to a transition between the first excited torsional levels $(B~,le⟷-X~lc\prime \prime )$. The two bands were combined in a joint analysis, by diagonalizing the full torsional-rotational matrices, instead of using the more familiar effective Hamiltonian method. In this way, the rotational parameters $(Ar.B$ and C) and torsional parameters ($V6$ and Btop) were directly determined. Then joint fits for $CH3.C5H4$ and $CD3-C5H4$ were performed, realizing that $CH3-$ and $CD3-$ are attached to the same framework, hence, $AfH=AfD$. and using the isotope relationship $BtopH=2BtopD$. The torsional parameters in $cm-1$ are: [FIGURE] The dramatic change in the torsional barrier $V6$ upon electronic excitation and its strong isotope dependence will be discussed. Finally, from the direction of the transition moment, we determined the symmetry of the lowest electronic states to be: $X~2B2.A¯2A2$ and $B¯2B2$