Infrared induced rotamerization of halogenoalkanes in rare gas matrices and vibrational spectra of the gauche and trans species

Mixtures of the gauche- and trans-1-halogenopropanes are trapped in rare gas solids at 4 K and interconverted by infrared irradiation. Spectral subtraction techniques are used to generate and analyze the pure gauche and trans infrared absorption spectra. The infrared induced rotamerization process is studied as a function of the halogen substituent and as a function of matrix. The isomerization rates are found to be governed by the local, microscopic trapping site symmetry and properties, rather than by the polarizability, mass, or other bulk properties of the host matrix. The highly efficient isomerizations of halogenopropanes as well as of several longer chain 1-fluoroalkanes are indicative of nonstatistical, mode-specific reactions. In all the compounds studied the isomerization in the gauche → trans direction is found to be more efficient than the reverse process, regardless of which conformer is lower in energy. This trend is consistent with previous observations in a variety of other species with intramolecular hydrogen bonding.