Infrared multiple photon dissociation spectroscopy and density functional theory (DFT) studies of protonated permethylated β-cyclodextrin–water non-covalent complexes

We present infrared multiple photon dissociation (IRMPD) spectroscopy and quantum chemical calculation results for the protonated permethylated β-cyclodextrin (CD)–water non-covalent complex, the simplest β-CD non-covalent complex, in the gas-phase. The IRMPD spectrum in the region 2700–3750 cm−1 consisted of three strong peaks at 3096, 3315, and 3490 cm−1. These spectral features in the experimental IRMPD spectrum were compared with a large set of infrared absorption spectra predicted using density functional theory (DFT) calculations for the protonated β-CD–water complex. Complex III (see Fig. 4c), in which the water molecule (at the primary rim) and the proton (at the secondary rim) were separated, was found to suitably reflect the main spectral characteristics found in the experimental IRMPD spectrum. The absence of the homodromic hydrogen bond ring, due to replacement of hydroxyl groups with methoxy groups in permethylated β-CD, rendered the primary rim open compared with the unmodified β-CD ‘one-gate-closed’ lowest energy conformer. This study demonstrates that IRMPD studies combined with DFT theoretical calculations can be a good method for studying molecular interactions of large host–guest pairs.