Time-Dependent Density Functional Theory As a Tool for Isomer Assignments of Hydrogen-Bonded Solute·Solvent Clusters

Can isomer structures of hydrogen-bonded solute·solvent clusters be assigned by correlating gas-phase experimental S0 ↔ S1 transitions with vertical or adiabatic excitation energies calculated by time-dependent density functional theory (TD-DFT)? We study this question for 7-hydroxyquinoline (7HQ), for which an experimental database of 19 complexes and clusters is available. The main advantage of the adiabatic TD-B3LYP S0 ↔ S1 excitations is the small absolute error compared to experiment, while for the calculated vertical excitations, the average offset is +1810 cm−1. However, the empirically adjusted vertical excitations correlate more closely with the experimental transition energies, with a standard deviation of σ = 72 cm−1. For the analogous correlation with calculated adiabatic TD-DFT excitations, the standard deviation is σ = 157 cm−1. The vertical and adiabatic TD-DFT correlation methods are applied for the identification of isomers of the 7-hydroxyquinoline·(MeOH) n , n = 1−3 clusters [ Matsumoto Y. ; Ebata T. ; Mikami N. J. Phys. Chem. B 2002, 106, 5591]. These confirm that the vertical TD-DFT/experimental correlation yields more effective isomer assignments.