Infrared spectra of CH3F(ortho-H2)n clusters in solid parahydrogen

The formation of CH3F(ortho-H2)n clusters in rapid vapor deposited solid molecular hydrogen containing low concentrations of CH3F and ortho-H2 has been investigated using high-resolution Fourier transform infrared spectroscopy of the C–F stretching mode (ν3) of the CH3F chromophore. Distributions of CH3F(ortho-H2)n clusters ranging in size from n=0 to n=12 are synthesized in para-H2 crystals by systematically varying the ortho-H2 concentration between 100 to 20 000 parts per million. The rotational motion of CH3F is quenched in solid para-H2; this simplifies the spectrum such that a single sharp transition is observed for each cluster. In the fundamental ν3 region, the spectrum consists of a series of well-separated peaks shifted toward lower energy with increasing numbers of nearest neighbor orthohydrogen molecules. The CH3F(ortho-H2)n clusters are also investigated in the ν3 overtone region and for analogous transitions of the CD3F(ortho-H2)n. Theoretical calculations based on an electrostatic interaction potential qualitatively reproduce the measured vibrational and isotopomer dependence of the spectra.