Intrinsic properties of α-cyclodextrin complexes with benzoate derivatives in the gas phase: an experimental and theoretical study.

The noncovalent interactions in host-guest complexes of α-cyclodextrin (α-CD) with a series of benzoic acid derivatives (RBA) were investigated by electrospray ionization tandem mass spectrometry and density functional theory (DFT) calculations. The 1:1 stoichiometry of the anionic host-guest complexes was unequivocally confirmed by their mass-to-charge ratios (m/z) and isotope patterns. Collision-induced dissociation experiments revealed exclusive fragmentation into [α-CD](-) and neutral RBA and afforded the gas-phase kinetic stability trend [α-CD·3,5-diMeBA](-) < [α-CD·3-MeBA](-) < [α-CD·BA](-) < [α-CD·3-OHBA](-) < [α-CD·3,5-diOHBA](-). This trend follows that of the gas-phase basicities of the guest anions used, indicating that host-guest pairs with more comparable basicities form more stable complexes. DFT calculations at the M06-L/6-31+G(d,p) level of theory provided detailed structural assignments and further elucidated the experimental observations, suggesting that the anionic [α-CD·RBA](-) inclusion complexes are favored over the nonspecific complexes in the gas phase and that hydrogen bonding constitutes the primary host-guest interaction. Additionally, the results provide an estimated gas-phase basicity ΔG(0) = 325-327 kcal mol(-1) for [α-CD](-).