Very short O-H...O hydrogen bonds (O...O = 2.2-2.5 Angstrom) 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 O-acid-H...O-water 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) Angstrom at 20, 100, 200, and 293 K). The COOH donor is activated by pi-cooperative hydrogen bonding (O-acid-H...O=C-acid), and the water-acceptor ability is enhanced by polarization assistance (pi-cooperative, O-w-H...O-acid, O-w-H...N-pyrazine).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 O-acid-H...O-w, hydrogen bond through synergistic pi- and sigma-cooperativity in the finite, neutral array 1, 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) Angstrom at 120 K), and the short hydrogen bonds in pyrazine dicarboxylic acids 2-4 (2.5513(11), 2.5269(12), and 2.5148(13) Angstrom) validate the novel SAHB model. A comparison of the short O-acid-H...O-w hydrogen bond with short O-w-H...O-acid 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 O-acid-H...O-w 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 similar to16 kcal mol(-1). Some examples retrieved from the Cambridge Structural Database together with crystal structures from this study provide 21 cases of short O-acid-H...O-w hydrogen bonds in multicenter synthons.