Ultraviolet Photodepletion Spectroscopy of Dibenzo-18-Crown-6-Ether Complexes with Alkaline Earth Metal Divalent Cations

Ultraviolet photodepletion spectra of dibenzo-18-crown-6-ether complexes with alkaline earth metal divalent cations (A(2+)-DB18C6, A = Ba, Sr, Ca, and Mg) were obtained in the gas phase using electrospray ionization quadrupole ion-trap reflectron time-of-flight mass spectrometry. Each spectrum exhibits the lowest energy absorption band in the wavenumber region of 35 400-37 800 cm(-1), which is tentatively assigned as the origin of the S(0)-S(1) transition of A(2+)-DB18C6. This origin band shows a red shift as the size of the metal dication increases from Mg(2+) to Ba(2+). The binding energies of the metal dications to DB18C6 at the S(0) state were calculated at the lowest energy structures optimized by the density functional theory and employed with the experimental energies of the origin bands to estimate the binding energies at the S(1) state. We suggest that the red shifts of the origin bands arise from the decrease in the binding energies of the metal dications at the S(1) state by nearly constant ratios with respect to the binding energies at the S(0) state, which decrease with increasing size of the metal dication. This unique relationship of the binding energies between the S(0) and S(1) states gives rise to a linear correlation between the relative shift of the origin band of A(2+)-DB18C6 and the binding energy of the metal dication at the S(0) state. The size effects of the metal cations on the properties of metal-DB18C6 complex ions are also manifested in the linear plot of the relative shift of the origin band as a function of the size to charge ratio of the metal cations, where the shifts of the origin bands for all DB18C6 complexes with alkali and alkaline earth metal cations are fit to the same line.