Anti-cooperativity of the two water OH groups

This paper presents the results concerning anti-cooperativity effects between two H-bonds of a water molecule. The IR OH stretching band shifts Delta nu-as a measure of the H-bond energy-are compared for HOD with different bases B of 1:1 complexes B...HOD...CCl4 (Delta nu(11)) and 1:2 complexes B...HOD...B (Delta nu(12)) We found that Delta nu(12) of the 1:2 complexes for different bases B are 25% smaller than Delta nu(11) for the 1:1 complexes. Corrections for the solvent shifts are introduced. This effect is in line with different observations concerning cooperativity effects of OH H-bonds by polarization with neighbouring molecules. The reduction of Delta nu(12) in 1:2 complexes can be understood on the assumption of a negative polarization by the first H-bond to the second OH or OD group and is called anti-cooperativity. This anti-effect has been already detected by NMR observation on NH2. We already observe a decrease of the CCl4 solvent shift by van der Waals forces OH...CCl4 of 1:1 complexes, induced by the H-bond of the other OH or OD group. This decrease is measured by the CCl4 solvent effect for monomers. This indicates a real negative polarization by the H-bond in 1:1 complexes on the second OH/OD. This experiment establishes the real polarization and excludes the importance of repulsions of the bases as the cause. The dependence of intermolecular forces is known on the polarizability. Our method demonstrates directly the polarization by interactions. The anti-cooperativity of symmetric complexes B-1...HOD...B-1 by a strong base B-1 can be reduced in unsymmetric 1:2 complexes B-1...HOD...B-2 by weaker bases B-2. This weakening of the anti-cooperativity of the stronger base could be predicted quantitatively. Similarly, the anti-cooperativity of the weaker base B-2 is strengthened in unsymmetric 1.2 complexes by stronger bases B-1. It is known that H-bonds XH...B can be strengthened by cooperativity with a second H-bond XH...XH...B. They can be weakened for water by anti-cooperativity of two H-bonds B...HOH...B. The H-bond B (1)...HO of 1:2 complexes B-1...HOH...B-2 can be weakened if the base strength of B-2 is stronger than of B-1 or strengthened if B-2 is weaker than B-1. Nature may use these possibilities in biochemistry. (C) 1997 Elsevier Science B.V.