UV and IR Spectroscopy of Transition Metal–Crown Ether Complexes in the Gas Phase: Mn2+(benzo-15-crown-5)(H2O)0–2

Ultraviolet photodissociation (UVPD) and IR–UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn2+(benzo-15-crown-5), Mn2+(B15C5)(H2O)n(n= 0–2), under cold (∼10 K) gas-phase conditions. Density functional theory (DFT) calculations are also carried out to derive information on the geometric and electronic structures of the complexes from the experimental results. The n= 0 complex shows broad features in the UVPD spectrum, whereas the UV spectra of the n= 1 and 2 complexes exhibit sharp vibronic bands. The IR–UV and DFT results suggest that there is only one isomer each for the n= 1 and 2 complexes in which H2O molecules are directly attached to the Mn2+ion through Mn2+···OH2bonds with no intermolecular bond between the water molecules. Time-dependent DFT calculations suggest that the π–π* transition of the B15C5 part is highly mixed with the “ligand to metal charge transfer” transition in the n= 0 complex, which can result in broad features in the UVPD spectrum. In contrast, attachment of H2O molecules to Mn2+(B15C5) suppresses the mixing, providing sharp vibronic bands assignable to the π–π* transition for the n= 1 and 2 complexes. These results indicate that the electronic structure and transition of benzo-crown ether complexes with transition metals are strongly affected by solvation.