Ultraviolet photodepletion spectra of dibenzo-18-crown-6-ether complexes with alkali metal cations (M+-DB18C6, M = Cs, Rb, K, Na, and Li) were obtained in the gas phase using electrospray ionization quadrupole ion-trap reflectron time-of-flight mass spectrometry. The spectra exhibited a few distinct absorption bands in the wavenumber region of 35 450−37 800 cm−1. The lowest-energy band was tentatively assigned to be the origin of the S0-S1 transition, and the second band to a vibronic transition arising from the “benzene breathing” mode in conjunction with symmetric or asymmetric stretching vibration of the bonds between the metal cation and the oxygen atoms in DB18C6. The red shifts of the origin bands were observed in the spectra as the size of the metal cation in M+-DB18C6 increased from Li+ to Cs+. We suggested that these red shifts arose mainly from the decrease in the binding energies of larger-sized metal cations to DB18C6 at the electronic ground state. These size effects of the metal cations on the geometric and electronic structures, and the binding properties of the complexes at the S0 and S1 states were further elucidated by theoretical calculations using density functional and time-dependent density functional theories.
We have constructed a quadrupole ion trap time-of-flight mass spectrometer to study photo-induced dissociation (PID) of biologically important molecules in the gas phase. The performance and capabilities of the mass spectrometer were investigated by measuring the mass spectra of protonated peptide ions produced by electrospray ionization. The typical mass resolution (m/Δm) was around 1000 and the collection efficiency measured for photo-fragment ions of protonated Tyr-Ala ions (YAH + ) was about 41%. The PID spectra of YAH + and protonated Gly-Trp ions (GWH + ) were compared to their respective spectra obtained by collisioninduced dissociation in the same instrument. The chromophore effect in photodissociation of protonated peptide ions was addressed with the PID spectrum of GWH + .
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