THE EFFECT OF ORDERED WATER ON A SHORT, STRONG (SPEAKMAN-HADZI) HYDROGEN-BOND

We have determined the structures of the sodium, tetrabutylammonium (T BA) and bis(triphenylphosphoranylidene)ammonium (PNP) salts of the bis (4-nitrophenoxide) anion by X-ray crystallography. The sodium salt is a dihydrate, with the water oxygens coordinated to the sodium cations, and one hydogen from each water hydrogen bonded to one of the bridgin g oxygens of the anion. The TEA and PNP salts are anhydrous. Neverthel ess the oxygen-oxygen distance is shortest in the sodium salt; 246.5 p m in the sodium salt, 247.5 pm in the TEA salt, and 249 pm in the PNP salt; suggesting that the hydrogen bond is not weakened by the water, and may be strongest in the hydrated salt. (All three compounds show H adzi type ii IR spectra, and are called Speakman-Hadzi compounds in th is paper.) The H-2 chemical shifts of the bridging hydrogen in the thr ee solids are 16.8 ppm for the sodium salt, 16.8 ppm for the TEA salt, and 16.5 ppm for the PNP salt. Again there is no evidence that the wa ter weakens the hydrogen bond. These results can be understood by noti ng that the additional hydrogen bonds to the bridging oxygens decrease their proton affinity, but the mutual repulsion of the oxygens is als o decreased. The net effect is to leave the hydrogen bond at least as strong as it was without the waters. It is competition and selective s olvation by otherwise disordered water which weakens the hydrogen bond in aqueous solution. Enzymatic transition states contain little unori ented water, and, therefore, can contain Speakman-Hadzi bonds. Several suggested Speakman-Hadzi bonds in enzyme inhibitor complexes are desc ribed.