Exploring the Structure and Complexation Dynamics of Azide Anion Recognition by Calix[4]pyrroles in Solution

The structure and anion recognition dynamics between calix[4]pyrroles and azide (N3-) anions in the form of its TBA+ and Na+ salts were investigated in dimethyl sulfoxide solutions by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. Vibrational energy redistribution of the N3- anion in the complex is accelerated through hydrogen bonding interactions with the N-H proton of the receptor. Rotational dynamics of the bound N3- is greatly restricted, demonstrating a distinct countercation effect. The detailed binding modes of N3- with the receptor were further evaluated by the density functional theoretical (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. All of these measurements support the notion that the calix[4]pyrroles are capable of capturing the azide anion in solution. However, the calix[4]pyrroles may not necessarily undergo a conformational change to a conelike geometry when they bind to the azide anion in the solution.

Automated summary

The study revealed that calix[4]pyrroles can accelerate the vibrational energy redistribution of azide ions through hydrogen bonding interactions and restrict the rotational dynamics of the bound ions. Furthermore, they are capable of capturing azide ions in solution, but may not necessarily undergo a conformational change to a conelike geometry.