Understanding Lone Pair-π Interactions from Electrostatic Viewpoint

Abstract

Over the last two decades, studies on lone pair-π interaction have attracted lot of attention of experimental as well as theoretical chemists due to its intriguing nature and its suspected presence in biological systems. The present Chapter begins with a brief overview of the earlier theoretical and experimental work done in this area. This is followed by exploration of the nuances of bonding in lone pair-π interaction, employing the tool of molecular electrostatic potential (MESP) since such weak interactions are mainly dominated by electrostatic features of host and guest molecules. The critical points associated with the scalar field of MESP are exploited for scrutinizing the directionality and bonding sites involved in the lone pair-π complexes. Furthermore, the electrostatic potential for intermolecular complexation (EPIC) model developed by Gadre et al., has been employed for finding out the electrostatically optimized structures and interaction energies of these complexes. The outcomes of EPIC model are compared with the results obtained from quantum chemical calculations of the complexes employing M06L/6-311++G(d,p) level of theory. The present study details out four different cases of lone pair-π complexes, which are currently in vogue. Hexafluorobenzene, one of the most explored π-deficient host in the present context, is initially taken up to demonstrate various facets of MESP for gaining insights into this interaction. This is followed by the scrutiny of special classes of recently synthesized highly π-deficient molecules, viz. tetraoxacalix [2]arene[2]triazine and naphthalenediimide, which are known to have specificity and large affinity, respectively, towards the electron rich species. The chapter ends with the description of lone pair-π interaction in the case of urate oxidase, an enzyme present in biological systems.