Rotationally resolved microwave (MW) and ultraviolet (UV) spectra of jet-cooled tropolone have been obtained in S(0) and S(1) 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 (18)O and four (13)C isotopomers were observed in natural abundance. The OD isotopomer was obtained from isotopically enriched samples. The two lowest tunneling states of each isotopomer except (18)O have been assigned. The observed inversion splitting for the OD isotopomer is 1523.227(5) MHz. For the asymmetric (13)C structures, the magnitudes of tunneling-rotation interactions are found to diminish with decreasing distance between the heavy atom and the tunneling proton. In the limit of closest approach, the 0(+) state of (18)O was well fitted to an asymmetric rotor Hamiltonian, reflecting significant changes 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 0(+) and 0(-) wave functions of the heavier isotopes favor the C-OH and C=O forms of tropolone, respectively. The only exception occurs for the (13)C-OH and (13)C[Double Bond]O structures which correlate to the 0(-) and 0(+) states, respectively. These preferences reflect kinetic isotope effects as quantitatively verified by the calculated zero-point energy differences between members of the asymmetric atom pairs. From rotationally resolved data of the 0(+) <--0(+) and 0(-) <--0(-) bands in S(1), line-shape fits have yielded Lorentzian linewidths that differ by 12.2(16) MHz over the 19.88(4) cm(-1) interval in S(1). The fluorescence decay rates together with previously reported quantum yield data give nonradiative decay rates of 7.7(5) x 10(8) and 8.5(5) x 10(8) s(-1) for the 0(+) and 0(-) levels of the S(1) state of tropolone.
The microwave spectrum of the molecules silacyclobutane and silacyclobutane-1,1-d2 has been observed between 8 and 55 GHz and the first four vibrational states assigned. The ground state rotational constants of the former molecule are 8815.75, 6289.00, and 4245.32 MHz. The first two inversionlike intervals of the ring puckering mode were determined to be 75.75 and 7790 MHz in the normal isotopic species; the corresponding intervals in the deuterated species are 43.06 and 4430 MHz. The effect of these frequencies on the determination of the potential energy for the ring puckering vibration was investigated. The ring configuration was shown to be significantly puckered in the ground state. The a component of the dipole moment was found to be nearly constant in the four vibrational states of the normal molecule, μa = 0.4396 D, while the out-of-plane transition moment between inversion doublets dropped rapidly as the inversion levels approached the top of the barrier.
The far infrared spectrum of the ring puckering vibration of 3,3-difluoroxetane has been observed and the potential determined to be V(Z) = 0.644×104Z2 + 0.425×106Z4 cm−1, where Z is in angstroms. The reduced mass for the vibration is 151 amu. From the vibrational dependence of the rotational constants, the dynamics of the ring vibration are found to shift from 1.5 times more bending about the C–O axis in trimethylene oxide to 1.5 times more bending about the C–C axis in difluoroxetane. An analysis of the changes in the barrier to inversion in cyclobutane, 1,1-difluorocyclobutane, trimethylene oxide, and difluoroxetane is based on the assumption of a model potential. The model potential for the ring puckering vibration is approximated as the sum of angle deformations and torsions about the ring bonds with a ``torsional strain'' parameter included which is analogous to the angle strain contribution to the deformation. Using literature values for the unstrained threefold contribution to the potential, the torsional strain contribution is found to decrease by 1.9±0.1 kcal mole−1 with difluoro substitution on both cyclobutane and trimethylene oxide.
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