Theoretical investigations on vibrational spectra of pillar[5]arene-bis(pyridinium) complexes

Electronic structure and vibrational spectra of pillar[5]arene (P5) complexes with bis(pyridinium) derivatives viz., 1,2-ethylenedipyridine (edpy), 1,2-propylenedipyridine (3-pdpy), 1,2-butylenedipyridine (bdpy), 1,2-pentamethylenedipyridine (pdpy) and 1,2-hexamethylenedipyridine (hdpy) are investigated employing density functional theory. B3LYP based density functional calculations predicted that interaction energies for complexation decreases steadily with increasing alkyl chain of bis(pyridinium) guest. The calculations have shown that OH⋯O hydrogen bonded and non-bonded hydroxyls in the host led to distinct vibrations at the 3515 cm−1 and 3681 cm−1, respectively in the vibrational spectra. Complexation of bis(pyridinium) guest engenders frequency down-shift for aromatic CHα vibrations owing to CH⋯O interactions with P5 portals. Moreover, CH⋯π interactions are inferred in edpy@P5 and 3-pdpy@P5 complexes which results in frequency up-shift (blue shift) of nearly 22–15 cm−1 for the corresponding CHα vibration.

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► Stabilization of the complex attributed to the hydrogen bonded interactions.
► Host–guest binding governed by alkyl chain length in bis(pyridinium) derivative.
► A downshift for CHα vibrations participating in CH⋯O interactions was predicted.
► Encapsulated CHα vibrations of edpy and 3pdpy guests show frequency blue-shift.