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

Electronic structure, (1)H NMR and infrared spectra of diquat (6,7-dihydrodipyrido[1,2-b:1',2'-e] pyrazine-5,8-diium or DQ(2+)) 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 DQ(2+) compared to its reduced cation or neutral species. Calculated (1)H NMR spectra reveal that Hα protons on bi-pyridinium rings of DQ(2+)@CB[8] complex are de-shielded owing to C=O⋯H interactions. On the other hand aromatic (Hβ and Hδ) of DQ(2+) within the CB[8] cavity exhibit significant shielding. The complexation of CB[8] with DQ(2+) 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 DQ(2+) 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.