Resonant four-wave mixing probes of vibrationally-mediated proton-transfer dynamics.

The mode specificity of proton-transfer dynamics in the ground electronic state (X˜1A1) of tropolone has been explored by implementing a coherent variant of the stimulated emission pumping (SEP) technique within the framework of two-color resonant four-wave mixing (TC-RFWM) spectroscopy. These studies have exploited rovibronically-resolved features of the A -- X˜ origin band as a "doorway" for selectively interrogating vibrationally-excited levels of the ground electronic manifold, with judicious selection of incident/detected polarization characteristics affording a means for discriminating rotational branches and alleviating spectral congestion. The lowest 1700 cm-1 portion of this potential surface has been interrogated under ambient bulk-gas conditions, enabling rotationless term energies (T v+) and tunneling-induced bifurcations ( DX&d5;n ) to be extracted for 43 assigned vibrational features of a1 and b2 symmetry. The resulting values of DX&d5;n reflect the state-specificity long attributed to the hydron-migration pathways of tropolone and range in magnitude from 0.0 cm -1 to 17.8 cm-1, where the former implies essentially complete quenching of unimolecular dynamics whilst the latter represents nearly a twenty-fold increase in reaction rate over that of the zero-point level. The dependence of tunneling rate on vibrational motion is discussed in terms of attendant corresponding atomic displacements and permutation-inversion symmetries for the tropolone skeleton.