Isotopic dependence of excited-state proton-tunneling dynamics in tropolone probed by polarization-resolved degenerate four-wave mixing spectroscopy.

ABSTRACT

The origin band of the Ã1B2-X1A1 (π* ← π) absorption system in monodeuterated tropolone (TrOD) has been probed with near-rotational resolution by applying the frequency-domain techniques of polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy under ambient, bulk-gas conditions. Judicious selection of polarization geometries for the incident and detected electromagnetic waves alleviated intrinsic spectral congestion and facilitated dissection of overlapping transitions, thereby enabling refined rotational-tunneling parameters to be extracted for the Ã1B2(π*π) manifold. A tunneling-induced bifurcation of Δ0à = 2.241(14) cm(-1) was measured for the zero-point level of electronically excited TrOD, reflecting the presence of a substantial barrier along the O-D···O ↔ O···D-O reaction coordinate and representing nearly a 10-fold decrease in magnitude over the analogous quantity in the parent (TrOH) isotopologue. Observed trends in hydron-migration rates are discussed in light of the changes in the potential-surface topology sustained from the π* ← π electron promotion and the dynamical effects incurred by selective isotopic modification of the nuclear framework, with similar considerations being applied to interpret rotational constants and inertial defects. Simultaneous analyses performed on an interloping sequence band built upon ν38(b1) gave an excited-state tunneling splitting of Δ(ν38)à = 1.217(61) cm(-1), highlighting the ability of this symmetric, out-of-plane normal mode to inhibit the unimolecular tautomerization process.