The O-H vs O... O distance correlation, the geometric isotope effect in OHO bonds, and its application to symmetric bonds

On the basis of recent accurate crystal-structure data, the correlation between O-H and O...O distances and the geometric isotope effect in OHO bonds have been investigated. The average correlation curve of asymmetric bonds appears to coincide with that of symmetric bonds only at the shortest limit of O...O length, around 2.4 A. On deuteration an expansion can be seen in the O...O distance range 2.44-2.64 A, with a maximum value of 0.027 A. Factors smearing the correlation are discussed. The equilibrium position of H in symmetric bonds is considered using the correlation curve of O-H vs O...O distance and the geometric isotope effect as an empirical basis, under the assumption that the potential curve can be expressed as the superposition of two asymmetric bonds. The isotope effect on the distance between the two equilibrium positions is discussed in connexion with an anomalous isotope effect on the transition temperature of hydrogen-bonded crystals which undergo ferro-, antiferroelectric, and other structural phase transitions. (Auth.)