Structure and spectral characteristics of diquat-cucurbituril complexes from density functional theory

Abstract

Electronic structure, ¹H NMR and infrared spectra of diquat (6,7-dihydrodipyrido[1,2-b:1′,2′-e] pyrazine-5,8-diium or DQ²⁺) encapsulated by cucurbit[n]uril (n = 7,8) hosts are obtained using the density functional theory. Theoretical calculations have shown that both CB[7] or CB[8] host possesses strong affinity toward DQ²⁺ compared to its reduced cation or neutral species. Calculated ¹H NMR spectra reveal that H_α protons on bi-pyridinium rings of DQ²⁺@CB[8] complex are de-shielded owing to C=O⋯H interactions. On the other hand aromatic (H_β and H_δ) of DQ²⁺ within the CB[8] cavity exhibit significant shielding. The complexation of CB[8] with DQ²⁺ splits the carbonyl stretching vibration (1788 cm⁻¹) into two distinct vibrations which correspond to 1765 cm⁻¹ arising from hydrogen bonded carbonyls and the 1792 cm⁻¹ band from non-interacting ones. Further, the CN stretching vibration in DQ²⁺ exhibits a frequency blue-shift of 6 cm⁻¹ on its encapsulation within the CB[8] cavity. The direction of frequency shift has been explained on the basis of natural bond orbital analyses.