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···Owater Hydrogen Bonds in a Multicenter Array

Very short OH···O hydrogen bonds (O···O = 2.2−2.5 Å) usually occur when the H-bond is stabilized by a negative or positive charge (OH···O-, O+H···O) or by resonance assistance (···OCCCOH···). We have characterized a very short intermolecular OacidH···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) Å at 20, 100, 200, and 293 K). The COOH donor is activated by π-cooperative hydrogen bonding (OacidH···OCacid), and the water-acceptor ability is enhanced by polarization assistance (σ-cooperative, OwH···Oacid, OwH···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 OacidH···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 current classification of very short hydrogen bonds in three main types:  negative charge-assisted H-bonds ((−)CAHBs), positive charge-assisted H-bonds ((+)CAHBs), and resonance-assisted H-bonds (RAHBs). The very short hydrogen bond in the X-ray crystal structure of 2,3,5-pyrazinetricarboxylic acid dihydrate, 5 (O···O = 2.4726(12) Å at 120 K), and the short hydrogen bonds in pyrazine dicarboxylic acids 24 (2.5513(11), 2.5269(12), and 2.5148(13) Å) validate the novel SAHB model. A comparison of the short OacidH···Ow hydrogen bond with short OwH···Oacid distances in carboxylic acids with similar motifs, for example, 1 and 5 and 2 and 4, shows that the activation of water oxygen by polarization assistance makes a significant contribution to the SAHB. The behavior of the short OacidH···Ow bond in 1 is similar to short-strong CAHB/RAHB systems: (1) quasi-covalent character of 0.27−0.30 valence units, (2) red shift in the OH stretching frequency to 1200−1400 cm-1, and (3) hydrogen-bond energy of ∼16 kcal mol-1. Some examples retrieved from the Cambridge Structural Database together with crystal structures from this study provide 21 cases of short OacidH···Ow hydrogen bonds in multicenter synthons.