ISOTOPE EFFECTS IN WEAK HYDROGEN-BONDS - ALLOWANCE FOR 2 STRETCHING AND 2 BENDING MODES OF THE A-H...B FRAGMENT

Allowance for a coupling between vibrational degrees of freedom of the A-H...B fragment including bending permits to interprete the recent e xperimental findings on the isotope effects in weak hydrogen bonds. Th e theory clarifies the reason of the DELTAR = R(e)D - R(e)H negative s ign for weak H-bonds and of its change with hydrogen bond strengthenin g as well as that of the increase of the intermolecular force constant by D substitution for H. New experimental regularities for the isotop ic effects on the nu(s)(AH) frequency shift are interpreted in the sam e terms. It is shown that the relative frequency shift (DELTAnu(s)/nu0 ) upon the hydrogen bond formation contains generally two contribution s: Van der Waals one, which does not depend on the bridged atom mass ( mu), and another which is proportional to mu-1/2 and is provided by th e nu(s)(A-H) and nu(sigma)(A...B) mode coupling. These contributions c an be determined from spectroscopic data.