Structure and molecular chirality of the 2∶1 salt of quinine with biphenyl 2,2′-dicarboxylic acid (diphenic acid)

Abstract

The molecule of the salt consists of two quinine cations and a biphenyl 2,2′-dicarboxylic dianion. Both of the quinine cations are protonated at the nitrogen atoms of the quinuclidine fragments. Each carboxylic group of the diphenic dianion acts as a proton donor with respect to the quinuclidine N(1) atom of one of the quinine cations, and as a proton acceptor with respect to the carbinol O(12′)-H(12′) group of the other. Thus, the interlocking hydrogen-bond system forms a rigid structure. The molecular conformation around the C(8)-C(9) bond is synclinal. The conformational parameters of the diphenic dianion are considerably different from those of diphenic acid in solid state. The absolute configuration of the biphenyl 2,2′-dicarboxylic dianion (S) is defined by the positive torsion angle around the line which connects the centers of both phenyl rings. The molecular chirality of the salt arises from the coupling between the quinoline and the biphenyl chromophores, as well as from the coupling of the quinoline-quinoline long-axis polarizedβ-band transition moments.