UV and IR spectroscopy of cold 1,2-dimethoxybenzene complexes with alkali metal ions

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of 1,2-dimethoxybenzene (DMB) complexes with alkali metal ions, M+•DMB (M = Li, Na, K, Rb, and Cs), in a cold 22-pole ion trap. The UVPD spectrum of the Li+ complex shows a strong origin band. For the K+•DMB, Rb+•DMB, and Cs+•DMB complexes, the origin band is very weak and low-frequency progressions are much more extensive than that of the Li+ ion. In the case of the Na+•DMB complex, the spectral features are similar to those of the K+, Rb+, and Cs+ complexes, but the vibronic bands are not resolved. Geometry optimization with density functional theory indicates that the metal ions are bonded to the oxygen atoms in all the M+•DMB complexes calculated. In the Li+ and Na+ complexes in the S0 state, the metal ions are located almost in the same plane as the benzene ring, while the K+, Rb+, and Cs+ ions are substantially off from the plane. In the S1 state, the Li+ complex has a structure similar to that in the S0 state. In contrast, the other complexes show a large structural change upon S1–S0 excitation to the out-of-plane direction of the benzene ring, which results in the extensive low-frequency progressions in the UVPD spectra. For the Na+•DMB complex, fast charge transfer occurs from Na+ to DMB after the UV excitation, making the bandwidth of the UVPD spectrum much broader than that of the other complexes and producing photofragment DMB+ ion.