On the photoisomerization of 5-hydroxytropolone: An ab initio and nuclear wave function study

Abstract

In this paper we perform ab initio calculations for the stable conformations and the transition states for the isomerization processes in 5-hydroxytropolone in both the ground (S0) and first excited (S1) singlet electronic states. The Hartree–Fock self-consistent field (SCF) level and a complete active space SCF (CASSCF) level for S0 are considered, whereas the configuration interaction all single excitation method (CIS) and the CASSCF levels are used to deal with the S1 state. Energies are reevaluated at all levels through perturbation theory up to second order: Mo/ller–Plesset for the Hartree–Fock and CIS methods, and the CASPT2 method for CAS results. The ab initio results are then used to perform different monodimensional fits to the potential energy surfaces in order to analyze the wave functions for the nuclear motions in both electronic states. Our best results predict that for the S0 state two stable conformers, syn and anti, can exist in thermal equilibrium. In accordance with experimental expectations the syn isomer is the most stable. As for the S1 state, and again in accord with experimental spectroscopical data, the order of stability reverses, the anti being the most stable. A more interesting result is that analysis of the nuclear wave functions shows an important syn–anti mixing in the S1 state that does not appear in S0. This result explains the appearance of syn–anti and anti–syn crossover transitions observed in the electronic spectra of 5-hydroxytropolone so that syn–anti reaction may take place through photoisomerization.

Comparison of methods for calculating the properties of intramolecular hydrogen bonds. Excited state proton transfer

“…22͒ and 5-hydroxytropolone. 63 The transfer barrier in the 1 * state of salicylic acid 28 follows the pattern CASSCFϾCISϾCASPT2, again consistent with our own findings in the more general case. Similarly, the inclusion of correlation has been shown capable of strongly affecting the relative energies of a pair of tautomers.…”

Temperature Dependence of Proton NMR Chemical Shift As a Criterion To Identify Low-Barrier Hydrogen Bonds

“…By performing single-point DFT/b2 calculations at every point along such a path we obtained a monodimensional energy profile, whose minima and maxima are, respectively, the DFT/b2 minimum energy structures and transition state (Table ) for the proton transfer. This procedure is more expensive but more reliable than the simple energy recalculation at some higher level while keeping frozen the geometries of the stationary points located at a lower level of electronic calculations . By doing this, the DFT results yield a classical potential energy curve for proton transfer involving two minima, geometrically much closer that the two HF minima (the same scenario as the one previously described for system 3 ), separated by a transition state.…”

Competition between Rotamerization and Proton Transfer in o-Hydroxybenzaldehyde

“…The 6-31+G** basis set (five d functions) was used to optimize the geometries which were performed at the SCF level for the ground states, and with CIS for excited states. The effects of electron correlation upon the ground-state properties were examined by second-order Møller−Plesset perturbation theory (MP2). , As various means of incorporating electron correlation into ESPT provide inconsistent results, − no attempt has been made here to go beyond the CIS level for excited states. There is indeed some evidence that suggests CIS can provide useful treatment of certain excited states. − …”