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 O<sub>a</sub><sub>cid</sub>−H···O<sub>w</sub><sub>ater</sub> 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<sup>-</sup>, O<sup>+</sup>H···O) or by resonance assistance (···OCCCOH···). We have characterized a very short <i>inter</i>molecular O<sub>acid</sub>H···O<sub>water</sub> hydrogen bond in the title crystal structure, <b>1</b>, 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 (O<sub>acid</sub>H···OC<sub>acid</sub>), and the water-acceptor ability is enhanced by polarization assistance (σ-cooperative, O<sub>w</sub>H···O<sub>acid</sub>, O<sub>w</sub>H···N<sub>pyrazine</sub>). 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 <b>1</b>. The shortening of the O<sub>acid</sub>H···O<sub>w</sub> hydrogen bond through synergistic π- and σ-cooperativity in the finite, neutral array <b>I</b>, 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, <b>5</b> (O···O = 2.4726(12) Å at 120 K), and the short hydrogen bonds in pyrazine dicarboxylic acids <b>2</b>−<b>4</b> (2.5513(11), 2.5269(12), and 2.5148(13) Å) validate the novel SAHB model. A comparison of the short O<sub>acid</sub>H···O<sub>w</sub> hydrogen bond with short O<sub>w</sub>H···O<sub>acid</sub> distances in carboxylic acids with similar motifs, for example, <b>1</b> and <b>5</b> and <b>2</b> and <b>4</b>, shows that the activation of water oxygen by polarization assistance makes a significant contribution to the SAHB. The behavior of the short O<sub>acid</sub>H···O<sub>w</sub> bond in <b>1</b> 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<sup>-1</sup>, and (3) hydrogen-bond energy of ∼16 kcal mol<sup>-1</sup>. Some examples retrieved from the Cambridge Structural Database together with crystal structures from this study provide 21 cases of short O<sub>acid</sub>H···O<sub>w</sub> hydrogen bonds in multicenter synthons.