Exploring Chimeric Calix[4]tetrolarene Molecular Scaffolds: Theoretical Investigations

The structure and spectral characteristics of the chimeric mixture of calixarene and pyrogallolarene (usually referred to as calix[4]tetrolarene) and its derivatives are studied employing the M06-2X-based density functional theory. Different conformers, viz., cone, partial cone, 1,2-alternate, and 1,3-alternate, were identified as the stationary point structures on their potential energy surfaces. Among these, the symmetric C_{4 v} cone conformer is found to be energetically favorable, which can be attributed to the cyclic array of hydrogen-bonding network in the calix[4]tetrolarene or its thia analogue. The substitution of methoxy groups at the upper rims of calix[4]tetrol- and thicalix[4]tetrol-arenes significantly influences the cooperative hydrogen-bonding network and conformational behavior of these hosts. The methoxy-substituted macrocycles show lowering in symmetry from C_{4 v} to C_{2 v}, engendering the pinched cone conformer to be the lowest energy structure. The enhanced solubility of the modified calix[4]tetrolarene macrocycles has been further explained from diminutive cooperative hydrogen bonding in its top rim compared to the pyrogallolarene, which is evidenced from the quantum theory of atoms in molecule and noncovalent interaction reduce density gradient method. Discernibly, the underlying cooperative hydrogen bonding emerges its signature in the characteristic vibrational patterns of the calixarene-based molecular scaffolds. The chemical shift parameters in their ¹H NMR spectra have further been characterized.