CRITICAL-BEHAVIOR OF STRONG HYDROGEN-BONDS AND THE ISOTOPIC EFFECTS

It is shown that strongly hydrogen bonded complexes reveal, in a critical donor-acceptor range, a number of anomalies such as very low values of the IR frequencies of the protonic bands, stepwise increase of the polarity expressed by the dipole moment (solution) or nuclear quadrupole resonance frequency (solid), maximum value of the proton magnetic resonance. Crystallographic data indicate that hydrogen bonds from such a critical region are the shortest ones. Based on the IR data for the XH complexes in argon matrices an attempt was undertaken to correlate the isotopic ratio ν(XH)/ν(XD) with the normalized proton affinity parameter. This correlation seems to be entirely different as compared with OHO and NHN hydrogen bonded systems in the solid state. The critical region is characterized by high values of the isotopic ratio suggesting a single minimum flat potential in this region. The critical behaviour of strong hydrogen bonds is also reflected by primary and secondary isotope effects in nuclear magnetic resonance spectra as well as in the isotopic effect upon the charge distribution observed in nuclear quadrupole resonance studies. © 1990.