Understanding non-covalent interactions by NMR in urea- and thiourea-substituted calixarene complexes

High level quantum mechemical gauge-independent atomic orbital (GIAO)-DFT NMR calculations (B3LYP/6-311++G**) were performed to address the binding interactions of functionalized thiourea and urea substituted calixarenes. ¹H NMR chemical shifts were evaluated for hydrogen-bonded protons of calixarenes (1,2) and their anionic-complexes with F⁻, Cl⁻, CN⁻, N₃⁻ and SCN⁻. Comparison of experimental vs. calculated anion binding geometries along with their chemical shift were made to decipher the structural features. The calculated chemical shift exhibit reasonable agreement with the experimental data. We showed that the substitution of C=O to C=S in calixarenes significantly affect the interaction with anions. In particular, urea substituted calixarenes-F⁻ possess most deshielded hydrogens among the modeled inclusion complexes. Our study showed that NMR calculation in combination with calixarene structural models can be helpful in characterizing the non-covalent interactions. Graphic abstract: