Knowledge Bank

The Raman spectrum of benzene was measured with the aid of the ionization-detected stimulated Raman $technique1$ in the region $1580-1625cm-1$ containing the $v16E2g$ fundamental band and the $\nu 2+\nu 18E2g$ combination band enhanced by close Fermi resonance with the fundamental. With the $\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$-$0.007cm-1$ resolution attained, the strong $\mathrm{<mrow\; data-mjx-texclass="ORD">\circ </mrow>}D$ and sS branch lines were completely resolved for the higher K values. Both bands have the regular structure of $\Delta K=\pm 2,\Delta 1=\pm 1$ transitions, but the lower frequency band exhibits a perturbation localized near K = 21, which was ascribed to a $Jx,y-$ Coriolis interaction with the $\nu 7+\nu 18E1g$ state. A Hamiltonian matrix containing the three states and their Fermi, Coriolis, and l-type interactions was used for the treatment of the upper states of the observed bands yielding a set of spectroscopic constants which reproduce the observed spectrum with an over-all standard deviation of $0.0025cm-1$. The unperturbed origins of the $\nu$16 and $\nu -2+\nu 18$ states are at 1600.966 and $1599.881cm-1$ respectively : the identity of the lower lying state as $\nu 2+\nu 18$ was established by its constant $C\zeta =+0.05498cm-1$ which is very close to the value reported in the literature for $\nu 18$.