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_acid-H···O_water hydrogen bonds in a multicenter array

Abstract

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 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) Å at 20, 100, 200, and 293 K). The COOH donor is activated by π-cooperative hydrogen bonding (O_acid-H···O=C_acid), and the water-acceptor ability is enhanced by polarization assistance (σ-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 p- and s-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 2-4 (2.5513(11), 2.5269(12), and 2.5148(13) Å) 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_acidH···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 OH 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 O_acid-H···O_w hydrogen bonds in multicenter synthons.