Host-Guest Interactions Accompanying the Encapsulation of 1,4-Diazabicyclo[2.2.2]octane within endo-Functionalized Macrocycles

The binding of novel endofunctionalized bis-urea/thiourea molecular receptors toward neutral 1,4-diazabicyclo[2.2.2]octane (DABCO) demonstrates stronger binding of the bis-thiourea macrocydes than for their urea analogues by employing M06-2X/6-31+G(d,p)-based density functional theory. The formation of such inclusion complexes is spontaneous, thermodynamically favorable, and facilitated via bifurcated N-H...N...H-N hydrogen bonding and C-H...pi, dipole-dipole, and other noncovalent interactions, which are reflected in the frequency shift of their characteristic N-H vibrations in the calculated vibrational spectra of these complexes. The underlying noncovalent interactions are analyzed using the molecular electrostatic potential topography and quantum theory of atoms in molecules in conjunction with the noncovalent interactions reduced density gradient method. It has also been shown that the encapsulation of DABCO within the pi-electron-rich cavity of such hosts brings about shielding of the guest protons confined within the host cavity whereas those facilitating hydrogen bonding engender the downfield signals in their calculated H-1 NMR spectra.