Infrared spectroscopy of entrance channel complexes

摘要

The entrance channel to the OH $PLU H$-2$/ $YLD H$-2$/O $PLU H hydrogen abstraction reaction has been probed with unprecedented detail through IR overtone spectroscopy of binary OH-H$-2$//D$-2$/ complexes. IR-UV double resonance techniques have been used to obtain rotationally resolved vibrational overtone spectra for ortho-H$-2$/-OH as well as ortho- and para-D$-2$/OH. Assignment of the spectra has been aided by bound state calculations based on the ab initio potential of Clary, Werner, and coworkers. The pure overtone stretch and combination bands involving intermolecular bending vibrations at energies up to the OH(v $EQ 2) $PLU H$-2$//D$-2$/ dissociation limit have been observed. The intermolecular excitations can drastically alter the relative orientation of the reactants within the complex, producing some that resemble the transition state structure. Direct time-domain measurements show that vibrationally activated OH-H$-2$/ complexes have a lifetime of 115(13) ns for the pure overtone stretch and an upper limit of 5 ns for OH-D$-2$/. The 20 times faster decay for OH-D$-2$/ arises form near-resonant vibrational energy transfer from OH to D$-2$/. Finally, IR optical pumping provides sufficient population transfer to enable the rotational distribution of the OH(v $EQ 1) products from inelastic scattering to be determined.