Knowledge Bank

The three bands $\nu 3,\nu 5,\nu 6$ of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH3D$ contributing to the absorption at $6-10\mu m$ were analyzed as a triad, i.e., by accounting for the Coriolis couplings by a diagonalization method. High resolution spectra $(\delta \sigma =0.005cm-1)$ recorded by the vacuum FT spectrometer of our laboratory gave us the whole absorption without any interference with the strong water absorption (in the region of $\nu 5$). The 3240 assigned transitions $(J\prime \le 18)$, of which about one thousand perturbation allowed transitions, were used for a new refinement of the ground state energies ($\sigma =0.0003cm-1$ for 2641 GS differences), and for a fit of the triad upper state energies ($\sigma =0.007cm-1$ for the 3240 data). The line strengths were calculated according to the strict selection rules on J and symmetry species, but without any restriction on $△K$. The effective dipole moment matrix elements required for the transitions within the triad $\nu 3,\nu 5,\nu 6$ were previously $\mathrm{<mrow\; data-mjx-texclass="ORD">(1)</mrow>}$ calculated. Values for the dipole moment derivatives $\delta \mu (s=3,5,6)$ were deduced from recent values of the band strengths $\mathrm{<mrow\; data-mjx-texclass="ORD">(2,3)</mrow>}S3$ ands $S4$ of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH4$, assuming the $8\mu /5q8$, in terms of internal coordinates $\Gamma i$, to be unchanged by H/D substitution. The line strengths calculated using these values are in surprisingly good agreement with the experimental intensity data presently available for $\nu 6$ and $\nu 3$. Nevertheless, some systematic discrepancies at high J require an intensity analysis involving Herman-Wallis-type corrections. Such an analysis is in progress in collaboration with F, Cappellani and R. Restelli (Ispra, Italy). The energy and intensity parameters obtained have been used to produce a list of about 6000 transitions $(J⩽18)$ predicted with line strengths $⩾4.10-25cm.molecule-1$ (at 300 K) between 874 and $1695cm-1$.