Nonlinear spectroscopic probes of intramolecular proton-transfer in model organic systems.

摘要

The intramolecular proton-transfer dynamics in the lowest-lying singlet excited states of gas-phase tropolone and malonaldehyde were investigated through use of high-resolution Degenerate Four-Wave Mixing (DFWM) spectroscopy. Unique polarization configurations for the incident and detected electromagnetic fields were developed and implemented for discriminating the nature of rovibronic features according to their changes in rotation angular momentum, Δ J. The origins of these branch suppression schemes were deduced from perturbative (weak-field) analyses of the resonant DFWM response evoked from an isotropic ensemble of target molecules.; Polarization-resolved DFWM spectra acquired for the origin region of the tropolone Ã1B₂– X˜1A₁ (π* ← π) absorption system permitted the extraction of refined spectroscopic constants describing the coupled rotational and tunneling degrees of freedom. The tunneling splitting for the vibrationless level in the à 1B₂ potential surface was found to be =19.846(25)cm−1 (one standard deviation uncertainty), representing a twenty-fold increase over the corresponding X˜ 1A₁ value. Large inertial defects of −0.80(14) amuÅ2 and −0.88(15)amuÅ2 were measured, respectively, for the 0+ and 0− levels of the excited state, thereby suggesting a loss of planarity in the tropolone molecular framework. This assertion is in keeping with predictions derived from high level quantum chemistry calculations; however, such ab initio analyses also suggest that an unexpectedly small change in proton-transfer barrier height accompanies the π* ← π electron promotion.; High-resolution DFWM investigations performed on the malonaldehyde Ã1B₁–X˜ 1A₁ (π* ← n) system, in conjunction with analogous linear absorption studies, revealed that substantial changes in proton-transfer dynamics accompany electronic excitation. The 21.58 cm−1 tunneling splitting measured for the vibrationless X˜1A₁ manifold was found to decrease to ≤1cm−1 in the analogous level of the à 1B₁ state. The polarization-based branch suppression schemes that were applied with great success for unraveling the congested spectra in tropolone did not prove successful in the malonaldehyde case, thereby suggested that an addition electronic potential surface might be involved in the four-wave mixing interaction. Ab initio calculations predict a moderate increase in proton-transfer barrier height upon π* ← n electron promotion as well as a substantial increase in the distance and concomitant weakening of the intramolecular hydrogen bond. These quantum chemistry results were in good accord with experimental findings.