Nd. Sokolov et Va. Savelev, ISOTOPE EFFECTS IN WEAK HYDROGEN-BONDS - ALLOWANCE FOR 2 STRETCHING AND 2 BENDING MODES OF THE A-H...B FRAGMENT, Chemical physics, 181(3), 1994, pp. 305-317
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.