GIAO-DFT calculated and experimentally derived complexation-induced chemical shifts of calix[4]arene–solvent inclusion complexes

Abstract

Various possible geometries of host–guest complexes of p-tert-butylcalix[4]arene with guest molecules such as toluene or THF are modeled using fast force field optimisations. Based on minima structures obtained by this approach quantum chemical gauge-independent atomic orbital (GIAO)-DFT NMR calculations [B3LYP/3-21G and B3LYP/6-31G(d)] were performed. By a comparison of the theoretically derived chemical shifts for the guest molecule in the host–guest complex with the chemical shifts calculated for the free guest using the same theoretical approach, calculation of complexation-induced chemical shifts (CISs) is possible. Spatial arrangements for which the theoretically derived complexation-induced shifts are in accordance with data stemming from CP-MAS ¹³C NMR spectroscopy also show reasonable agreement with structures obtained by single crystal structure determinations. Thus, it is possible for calix[4]arene complexes using a combination of force field geometry optimisation and GIAO-DFT NMR shift calculations to screen various starting geometries against experimental data yielding good structural models for these complexes in the solid state.