UV and IR Spectroscopy of Transition Metal-Crown Ether Complexes in the Gas Phase: Mn 2+ (benzo-15-crown-5)(H 2 O) 0-2 .

Ultraviolet photodissociation (UVPD) and IR-UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn ²⁺ (benzo-15-crown-5), Mn ²⁺ (B15C5)(H ₂ O) _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 H ₂ O molecules are directly attached to the Mn ²⁺ ion through Mn ²⁺ ···OH ₂ bonds 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 H ₂ O molecules to Mn ²⁺ (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.