Molecular electrostatic potentials in Cucurbit[ n]uril ( n = 13–16) hosts

Abstract

Charge distribution and 1H NMR chemical shifts in different Cucurbit[ n]uril {CB[ n] ( n = 13–16)} conformers, generated by flipping methylene groups adjacent to glycouril units, have been obtained by employing the density functional theory. The lowest energy conformer of CB[ n] has been endowed with symmetric architecture of methylene groups which engender uniform cavity and yield shallow minima near carbonyl oxygen’s on the host portals along the series. The effective cavity diameter and cavity height from the molecular electrostatic potential topography reveal that cavity diameter of these hosts varies from 16.1 Å for CB[13] to 20.9 Å for CB[16]; the cavity height of these hosts turn out to be nearly constant and equals ∼8.6 Å. CB[ n] family of hosts exhibit three different types of proton signals in the NMR spectra which include: methylene protons directing outside and towards the cavity of the host are denoted by H1 and H3 whereas the methine protons pointing outside the cavity are labeled as H2. Calculated 1H NMR chemical shifts in CB[ n] hosts follow the order: H3 > H2 > H1 in the gas phase and with water as solvent in consonant with the experimental spectra of lower CB[ n] homologues (J. Kim, I. Jung, S. Kim, E. Lee, J. Kang, S. Sakamoto, K. Yamaguchi, K. Kim, J. Am. Chem. Soc. 122 (2000) 540).