Encapsulation of alkyl and aryl derivatives of quaternary ammonium cations within cucurbit[n]uril (n=6,7) and their inverted diastereomers: density functional investigations

Electronic structure, vibrational frequencies, and H-1 chemical shifts of inclusion complexes between CB[n] (n=6,7) or their inverted iCB[n] diastereomer hosts and quaternary diammonium viz., 1,6-hexyldiammonium (HDA) or p-xylyldiammonium (XYL) cationic guests are obtained from the density functional calculations. The interaction of CB[n] or iCB[n] with HDA (guest) conduce inclusion complexes in which the guest attains gauche conformation within the host cavity. The lowest energy XYL complexes of CB[6] or iCB[6] are comprised of one ammonium group orienting parallel to aromatic ring. The CB[7] or iCB[7] complexes of XYL on the other hand, reveal ammonium group(s) perpendicular to aromatic ring of the guest. The ureido C=O and N-H stretching vibrations on complexation engender frequency down-shift in the calculated spectra. This can be attributed to C-H--O and N-H--O interactions in the complex. The inverting of glycouril unit in iCB[n] renders a frequency shift (12 cm(-1)) for the C=O stretching in the opposite direction. Molecular electron density topography and natural bond orbital analyses have been used to explain the direction of frequency shifts. Calculated H-1 NMR reveal that guest protons within the host cavity not participating in hydrogen bonding interactions, exhibit shielded signals compared to isolated XYL or HDA. Likewise the inverted protons in the iCB[6]-XYL complex led to up-field signals in calculated (HNMR)-H-1 as a result of C-H-pi interactions.