Spherand complexes with Li+ and Na+ ions in the gas phase: encapsulation structure and characteristic unimolecular dissociation

Abstract

We investigated the complexes of Cram's hexa(p-anisole) spherands (SPR, 1) with Li⁺ and Na⁺ ions (1·Li⁺ and 1·Na⁺) isolated in the gas phase. Despite the small conformational difference between 1·Li⁺ and 1·Na⁺ owing to the rigid framework of 1, ultraviolet photodissociation (UVPD) spectroscopy under cryogenic (∼10 K) conditions yielded clearly distinguishable absorption edges: ∼34 000 and ∼34 500 cm⁻¹ for 1·Li⁺ and 1·Na⁺, respectively. The spectral assignment and the preorganization characteristics of the host molecule were compared with those of dibenzo-18-crown-6-ether (DB18C6) complexes, which have more flexible frameworks. Furthermore, we revealed the characteristic unimolecular dissociation of the 1·Li⁺ complex using UVPD and collision-induced dissociation (CID); the formation of fragment ions with dibenzofuran moieties was detected. This dissociation pattern was ascribed to the efficient release of dimethyl ether molecule(s) from the 1·Li⁺ complex, which is characteristic of the cyclic skeleton formed with six methoxy groups in the SPR.