Knowledge Bank

The lowest lying vibrational fundamental $(v12-l-O)$ in $CH3CD3$ has been studied between 640 and $720cm-1$ using a modified Bomem spectrometer with an unapodized resolution of $0.0013cm-1$. The experimental conditions were: $path=2m,T=-145C,P=0.4Torr$; and $path=2m,T-22C,P=1.2Torr$. Over 1000 transitions have been identified obeying the selection rules $\Delta v6=1\Delta (k-1)=0$, and $\Delta J=0,\pm$ with $\Delta v6=\Delta \sigma 0$, where $v6$ is the principal torsional quantum number and $\sigma$ labels the torsional sublevels. The data clearly reveal that many of the $(v12=1,v6=0)$ levels are resonantly perturbed by $(v12=0,v6-3)$ levels of the ground vibrational state for several different $(k,\sigma )$. The perturbation can be accounted for using a Hamiltonian $model1$ explicitly including the effect of the Coriolis interaction and a higher order Coriolis-like term that arises from the off-diagonal part of the operator responsible for the l-type doubling. The overall data set consists of 4 microwave transitions and molecular beam data$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ in the ground vibrational state, in addition to the lines from the $\nu 12$ band up to $K=11$ and $J=15$. The model fits the observed frequencies to within experimental error except for transitions connected to the levels $(v12=1,K=2,\sigma 1,1=1)$ for $J>6$. These levels are severely perturbed and undergo a crossover.