Encapsulation of sulfur, oxygen, and nitrogen mustards by cucurbiturils: a DFT study

A density functional theory analysis on the encapsulation of sulphur mustard (HD), oxygen mustard (OM), and nitrogen mustard (N1) by cucurbit[7]uril (CB[7]) molecule was carried out using the dispersion corrected B3LYP-D/6-311+G(d,p) method. The encapsulation of HD, OM and N1, have favorable interaction energy and are found to be functional dependent. Amongst the encapsulated molecules, N1@CB[7] has the least highest occupied molecular obitual- lowest unoccupied molecular orbital gap, which indicates that nitrogen mustard can be reversible stored inside CB[7]. Molecular electrostatic potential studies shows that charge transfer occurs between the host and guest molecule during the complexation process. Atoms-in-molecules analysis at the intermolecular bond critical points, have positive charge density, negative Laplacian and close to zero value of total energy density, indicating the depletion of electronic charge along the bond paths, which implies an electrostatic nature of bonding. The natural bond order analysis of N1@CB[7] complex with highest strongest second order perturbation energies are due to the donation of lone pair nitrogen of CB molecule to the guest molecule and the back donation of charge, leads to the high interaction energy. Non covalent index isosurface clearly show the existence of hydrogen bonding and van der Walls intermolecular interaction between the guest molecule and the host CB[7].