Knowledge Bank

The Femi-resonating bands $\nu 3(799cm-1)$ and $\nu 1+\nu 4(774cm-1)$ of $C35Cl4$ have been recorded using an FTIR spectrometer and static cell, and a tunable diode laser with a supersonically-cooled expansion. For $\nu 3$, quantum assignments were made for individual tetrahedrally-split transitions from P(20) to R(24) and for the partially-resolved Q branch. For $\nu 1+\nu 4$ a scalar fit was made to the P(38)-R(56) frequencies. In both cases the spectroscopic and molecular constants were derive. Some rather unexpected results from these analyses were clarified with FTIR spectra of the $\nu 4$ fundamental at $315cm-1$, The Coriolis constants $\zeta 3=0.622$ and $\zeta^{\prime}_{4} = -0.143 (obtained from $\nu 1$ + $\nu 4)$, though satisfying the sum rule, are perturbed by the Fermi resonance, and are inconsistent with intramolecular force fields previously reported based on isotope shifts in matrix spectra. The true Coriolis constant $\zeta 4=-0.38$, obtained from the unperturbed $\nu 4$ band, agrees well with these force fields, and with our own measurements of the $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl-37Cl$ isotope shifts in $\nu 4$ certain systematics in the Coriolis constants of tetrahedral molecules will be discussed.