Density functional investigations on binding patterns, electronic structure, electrostatic potential, and NMR spectral variations in the inclusion complexes of cyclodextrin and cucurbit[n]uril

Abstract

The thesis presents systematic investigation on electronic structure of α-, β- and γ- cyclodestrines (CD) and cucurbit[n]uril hosts and their complexes with sulfur hexafluoride (SF6) and Ferrocene (FC). Hydrogen bonding patterns in CD have been analyzed using molecular electrostatic potential (MESP) and molecular electron density (MED) as a tool. Consequent NMR chemical shifts obtained from the gauge independent atomic orbital (GIAO) method in gas phase as well as in water have been discussed. The thesis has been organized as follows. Chapter 1 provides brief account of different quantum chemical methods. An outline of hybrid density methods including the Becke’s three-parameter exchange-correlation functional as given by Lee, Yang and Parr have been discussed. Chapter 2. The work demonstrates how topography of the molecular electrostatic potential (MESP) yield cavity dimensions and insights for the hydrogen-bonded interactions. The interactions between primary hydroxyl groups engender ‘cone-like’ structure, while those from the primary -OH with ether oxygen in glucose ring facilitate ‘barrel-like’ cavity. Chapter 3 deals with the host-guest complexes of α-, β- and γ CD and FC. Parallel and perpendicular orientations of FC relative to the host and the spectral features thereof have been presented in depth. The penetration of guest in these complexes is quantified from the separation of center of masses of host and guest, which correlates well with the interaction energies of CD-FC complexes in water. Chapter 4. It has been shown that the height of host cavity increases from 7.25 A to 7.70 A along CB[n] homologue whereas the diameter of the CB[8] (8.57 A) cavity turn out to be twice as large as compared to that of CB[5] (3.91 A). Chapter 5 presents investigations on lateral interaction of the guest FC with hydrophilic exterior of the CB[n] portal and its encapsulation within hydrophobic cavity. Theoretical calculations suggested CB[5] does not yield stable complexes in either case. Self consistent reaction field (SCRF) studies reveal that in the presence of water as a solvent, Chapter 6 Inverted- cucurbit[n]uril (ixCB[n], x=1,2; n=6-8), the enantiomers of CB[n] comprising of one or more inverted glycouril units, show distinct selectivity in recognition toward the guest by virtue of= the shape and dimensions of its cavity. It has been shown that the inversion of glycouril unit of CB[6] and CB[7] engenders destabilization of 4.2 kJ mol–1 and 5.7 kJ mol–1, respectively. As opposed to this, the iCB[8] is favored (by 18.6 kJ mol–1) over the corresponding CB[8] host. Chapter 7 the inclusion of p xylylenediammonium (XYL) and 1,6- hexyldiammonium (HDA) cations within the i-CB[n], n = 6,7 are studied, which have shown high affinity toward these cationic guests. The cavity-size of iCB[n] hosts governs the guest conformation when encapsulated within the cavity.