Bond cooperativity effects, which are typical of “resonant” chains or rings of p-conjugated hydrocarbons, can also occur in H-bonded systems in form of s-bond and p-bond cooperativity or anticooperativity. S-bond cooperativity is associated with the long chains of O-H...O bonds in water and alcohols, while s-bond anticooperativity occurs when the cooperative chain is interrupted by a local defect reversing the bond polarity. Both effects are known to play an important role in nature by controlling proton transmission in water and water flow without proton transmission in aquaporins. P-bond cooperativity is the driving force controlling resonance-assisted H-bonds (RAHBs). In typical intamolecular RAHBs enolones (...O=C-C=C-OH...) and enaminones (...O=C-C=C-NH...) form p-cooperative 6-membered rings closed by strong O-H...O or N-H...O bonds [1-4]. P-bond anticooperativity has never been considered so far and it is investigated here by studying couples of H-bonded b-enolone and/or b-enaminone 6-membered rings fused through a common C=O or C-C bond. The effect is studied by X-ray crystal structure determination of five compounds and by extensive CSD [5] search of related fragments. It is shown that fusion through the C=O bond is always anticooperative and such to weaken the symmetric O-H...O...H-O and N-H...O...H-N bonds formed but not the asymmetric O-H...O...H-N one, a fact that is interpreted in terms of equal or different proton affinities of the H-bond donor and acceptor atoms. Fusion through the C-C bond may produce either cooperative or anticooperative H-bonds, the former being more stable than the latter and giving rise to a unique resonance-assisted 10-membered ring running all around the two fused 6-membered ones that can be considered a type of prototropic tautomerism never described before. The possible applications of these fused rings as two-state centers in potentially ferro/ antiferroelectric systems are finally discussed.
Pi-Bond Cooperativity and Anticooperativity Effects in Resonance-Assisted Hydrogen Bonds (RAHBs)
GILLI, Paola;BERTOLASI, Valerio;FERRETTI, Valeria;PRETTO, Loretta
2006
Abstract
Bond cooperativity effects, which are typical of “resonant” chains or rings of p-conjugated hydrocarbons, can also occur in H-bonded systems in form of s-bond and p-bond cooperativity or anticooperativity. S-bond cooperativity is associated with the long chains of O-H...O bonds in water and alcohols, while s-bond anticooperativity occurs when the cooperative chain is interrupted by a local defect reversing the bond polarity. Both effects are known to play an important role in nature by controlling proton transmission in water and water flow without proton transmission in aquaporins. P-bond cooperativity is the driving force controlling resonance-assisted H-bonds (RAHBs). In typical intamolecular RAHBs enolones (...O=C-C=C-OH...) and enaminones (...O=C-C=C-NH...) form p-cooperative 6-membered rings closed by strong O-H...O or N-H...O bonds [1-4]. P-bond anticooperativity has never been considered so far and it is investigated here by studying couples of H-bonded b-enolone and/or b-enaminone 6-membered rings fused through a common C=O or C-C bond. The effect is studied by X-ray crystal structure determination of five compounds and by extensive CSD [5] search of related fragments. It is shown that fusion through the C=O bond is always anticooperative and such to weaken the symmetric O-H...O...H-O and N-H...O...H-N bonds formed but not the asymmetric O-H...O...H-N one, a fact that is interpreted in terms of equal or different proton affinities of the H-bond donor and acceptor atoms. Fusion through the C-C bond may produce either cooperative or anticooperative H-bonds, the former being more stable than the latter and giving rise to a unique resonance-assisted 10-membered ring running all around the two fused 6-membered ones that can be considered a type of prototropic tautomerism never described before. The possible applications of these fused rings as two-state centers in potentially ferro/ antiferroelectric systems are finally discussed.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
