UV and IR Spectroscopy of Cold H₂O⁺–Benzo-Crown Ether Complexes

Abstract

The H₂O⁺ radical ion, produced in an electrospray ion source via charge transfer from Eu³⁺, is encapsulated in benzo-15-crown-5 (B15C5) or benzo-18-crown-6 (B18C6). We measure UV photodissociation (UVPD) spectra of the (H₂O·B15C5)⁺ and (H₂O·B18C6)⁺ complexes in a cold, 22-pole ion trap. These complexes show sharp vibronic bands in the 35 700–37 600 cm^–1 region, similar to the case of neutral B15C5 or B18C6. These results indicate that the positive charge in the complexes is localized on H₂O, giving the forms H₂O⁺·B15C5 and H₂O⁺·B18C6, in spite of the fact that the ionization energy of B15C5 and B18C6 is lower than that of H₂O. The formation of the H₂O⁺ complexes and the suppression of the H₃O⁺ production through the reaction of H₂O⁺ and H₂O can be attributed to the encapsulation of hydrated Eu³⁺ clusters by B15C5 and B18C6. On the contrary, the main fragment ions subsequent to the UV excitation of these complexes are B15C5⁺ and B18C6⁺ radical ions; the charge transfer occurs from H₂O⁺ to B15C5 and B18C6 after the UV excitation. The position of the band origin for the H₂O⁺·B18C6 complex (36323 cm^–1) is almost the same as that for Rb⁺·B18C6 (36315 cm^–1); the strength of the intermolecular interaction of H₂O⁺ with B18C6 is similar to that of Rb⁺. The spectral features of the H₂O⁺·B15C5 complex also resemble those of the Rb⁺·B15C5 ion. We measure IR–UV spectra of these complexes in the CH and OH stretching region. Four conformers are found for the H₂O⁺·B15C5 complex, but there is one dominant form for the H₂O⁺·B18C6 ion. This study demonstrates the production of radical ions by charge transfer from multivalent metal ions, their encapsulation by host molecules, and separate detection of their conformers by cold UV spectroscopy in the gas phase