PNNL

Abstract

Rotationally resolved microwave (MW) and ultraviolet (UV) spectra of jet-cooled tropolone have been obtained in S0 and S1 electronic states using Fourier-transform microwave and UV laser/molecular beam spectrometers. In the ground electronic state, the MW spectra of all heavy atom isotopomers including one 18O and four 13C isotopomers were observed in natural abundance. The OD isotopomer was obtained from isotopically enriched samples. The two lowest tunneling states of each isotopomer except 18O have been assigned. For the asymmetric 13C structures, the magnitudes of tunneling-rotation interactions diminished with decreasing distance between the heavy atom and the tunneling proton. In the limit of closest approach, the v=0 state of 18O was well-fit to asymmetric rotor Hamiltonians, signifying a drastic change in the tautomerization dynamics. Comparisons of the substituted atom coordinates with theoretical predictions at the MP2/aug-cc-pVTZ level of theory suggest the localized v=0 and v=1 wavefunctions of the heavier isotopes favor the C-OH and C=O forms of tropolone, respectively. The only exception occurs for 13C-OH and 13C=O structures which correlate to the v=1 and v=0 states, respectively. These preferences reflect kinetic isotope effects as quantitatively verified by the calculated zero point energy differences (?ZPE's) between members of the asymmetric atom pairs. The ?ZPE's are principally defined by contributions from the out-of-plane zero-point motions and aid in elucidating the mode-specific nature of the tunneling splittings. From rotationally resolved data of the 0+-0+/0+-0- and 0--0-/0--0+ bands in S1, lineshape fits have yielded different Lorentzian linewidths, increasing from 131.1(8) MHz to 143.3(8) MHz, respectively. This 12.2(8) MHz increase over the 18.04(2) cm-1 tunneling splitting interval in S1 correlates with a nearly monotonic increase in the intersystem crossing rate when extrapolated to higher vibrational excess energies in S1, suggesting a dependence of the non-radiative decay dynamics on tunneling state.