Upper limit of the O-H•••O hydrogen bond.: Ab initio study of the kaolinite structure

Ab initio density-functional (DFT) investigations of the O-H stretching frequencies are performed on the kaolinite structure. Four distinct OH groups form weak hydrogen bonds with O-H . . .O distances between 1.8 and 2.6 Angstrom. The O-H stretching frequencies, calculated as the Fourier transforms of the velocity autocorrelation function, show a quasi-linear dependence on the O-H . . .O distance. Because of the nonspecific character of extremely weak hydrogen bonds, which are at large distances averaged over several similar hydrogen-bond contacts, we anticipate general validity of the frequency vs bond length dependence for all structures with O-H . . .O hydrogen bonds. In hypothetical isolated layer, two O-H groups are free of hydrogen bonds. The extrapolation of their frequencies indicates an upper limit of similar to3.0 Angstrom for the O-H . . .O and similar to 4.0 A for the O . . .O distances. The limiting value of effective hydrogen bond is checked by a detailed study of the geometry parameters of relaxed structures with continuously increased interlayer distance. Discontinuities observed for both the O-H bond length and the O-H . . .o angles reasonably agree with the value extrapolated in the frequency vs bond length dependence. The geometrical parameters r(O-H . . .o) = similar to 3.0 Angstrom and r(O . . .o) = similar to 4.0 Angstrom can thus serve as criteria in assessing the incorporation of hydroxyl groups in effective hydrogen bonds.