Sensing cyclosarin (a chemical warfare agent) by Cucurbit[n]urils: A DFT/TD-DFT study

Abstract

The detection of toxic chemical warfare agents (CWAs) has been a prime aim of the scientific community. The non-toxic nature and capability to mimic the biological systems of the Cucurbit[n]uril (CB[n]), a macrocyclic molecular family has made them good candidates for capturing and sensing the CWAs. Here density functional theory (DFT) and time-dependent density functional theory (TD-DFT) has been exploited to study the sensing of an important nerve agent i.e., cyclosarin molecule with CB[n] (n = 5–8) host molecules. M06-2X (out of twelve functionals tested) offers good agreement with the experiment in terms of mean absolute deviations (MAD) of bond lengths and bond angles. The cyclosarin binding with CB[6] is thermodynamically favoured compared to other CB[n]s indicated by vander Waals interactions through Bader's quantum theory of “Atoms in Molecules” (QTAIM) calculations. This binding stability fizzles out on further addition of cyclosarin guest (total five) with the host molecules. The inclusion complexes having cyclosarin within the CB[6-8] host cavity are thermodynamically favoured over lateral or exterior host-guest interactions. The Infra-red (IR) spectroscopic vibrational signatures indicate the charge transfer which provide stability to the inclusion complexes. The smaller Egap values in CB[6-8]_NBD (where NBD is 7-nitrobenzo-2-oxa-1,3-diazole) compared to the simple CB[6-8] indicate kinetic control and reversible nature of inclusion complex formation in the former case. The sensing of cyclosarin is not possible through naked eye because the absorption and fluorescence spectra of CB[6-8]_cyclosarin complexes fall outside the visible region of wavelengths (380–750 nm). The NBD (7-nitrobenzo-2-oxa-1,3-diazole) fluorophore attached hosts CB[n]_NBD absorb and fluoresce in visible range (380–750 nm) on cyclosarin binding, thus making them important fluorescent sensors for cyclosarin and other CWAs.