Variable Temperature Neutron Diffraction Analysis of a Very Short O−H···O Hydrogen Bond in 2,3,5,6-Pyrazinetetracarboxylic Acid Dihydrate: Synthon-Assisted Short Oacid−H···OwaterHydrogen Bonds in a Multicenter Array

Abstract

Very short OH···O hydrogen bonds (O···O = 2.2−2.5 A) usually occur when the H-bond is stabilized by a negative or positive charge (OH···O-, O+H···O) or by resonance assistance (···OCCCOH···). We have characterized a very short intermolecular OacidH···Owater hydrogen bond in the title crystal structure, 1, by variable temperature neutron diffraction (O···O = 2.4751(11), 2.4765(10), 2.4807(12), and 2.4906(16) A at 20, 100, 200, and 293 K). The COOH donor is activated by π-cooperative hydrogen bonding (OacidH···OCacid), and the water-acceptor ability is enhanced by polarization assistance (σ-cooperative, OwH···Oacid, OwH···Npyrazine). The absence of proton migration in the temperature range 20−293 K to give ionic or tautomer forms rules out contribution from the charge- or resonance-assisted H-bonds in 1. The shortening of the OacidH···Ow hydrogen bond through synergistic π- and σ-cooperativity in the finite, neutral array I, named as a synthon-assisted hydrogen bond (SAHB), adds a new category to the curren...