Knowledge Bank

Infrared spectra of the weakly-bound van der Waals complex $CH4-paraH2$ have been observed and analyzed for the first time. Measurements were made using a long path $(160-180m)$ low temperature $(61-92K)$ absorption cell which was probed with a Bomem Fourier transform spectrometer (350 and $1311cm-1$) or a tunable diode laser $(1311cm-1)$. The partly resolved spectrum accompanying the $S0(0)$ pure rotational transition of $H2$ around $350cm-1$ was analyzed in terms of an approximate model to obtain the rotational and centrifugal distortion constants of the complex in its ground state. The high resolution spectrum accompanying the R(0) transition of the $v4$ fundamental band of $CH4$ near $1311cm-1$ was assigned in detail and analyzed using the Coriolis model used previously for analogous spectra of methane - rare gas $complexes.a$ The effective intermolecular separation and dissocation energy of the complex in its ground state were determined to be about 4.12 \AA and $28cm-1$, respectively. Absorption due to $CH4-paraH2$ has also been observed in the $1317cm-1$ region near the R(1) transition of $CH4$, but this is not yet analyzed.