Analysis of the p-tert-butylcalix[4]arene bis-crown Derivative (Dc3)-Acetonitrile Host−Guest Complexing Behavior by Nuclear Magnetic Resonance (NMR) Spectroscopy and Computational Methods

We report the analysis of the structure and dynamics of the calix[4]arene bis-crown derivative (Dc3) and the host−guest system formed by Dc3 and acetonitrile in a chloroform solution. NMR spectral measurements, molecular mechanics, and molecular dynamics calculations have been used. Results showed that the host−guest complex in solution has chemical shift values that are considerably different from those of the pure compounds and from the Dc3-acetonitrile mixture. Molecular mechanics simulations showed that Dc3 has a cavity with an almost circular cone shape and that the CN group of acetonitrile in the host−guest complex pointed toward the outside of the cavity. This orientation appeared energetically favored. The value of the generalized order parameter ( S 2 ) from 13 C-NMR relaxation rates also showed that the methyl group of acetonitrile behaved as a free rotor in the Dc3 cavity with reorientational motion of almost two orders of magnitude faster than the aromatic carbons.