Analysis of O-H...O hydrogen bonds

Abstract

Some geometrical parameters associated with the donor and the acceptor end of O-H...O hydrogen bonds have been analysed. The data consist of 356 hydrogen bonds from reported crystal structures of amino acids, peptides and oligosaccharides. The following conclusions are arrived at: (a) The most probable range for the hydrogen bond length is between 2.7-2.8 å, the average being 2.76 å. When the donor is a water molecule the distribution is slightly different from the other cases. The average becomes 2.82 å, indicating that O-H...O hydrogen bonds, where a water molecule acts as a donor, tend to be slightly larger. (b) The maximum in the distribution for the hydrogen bond angle (H-O·· .O) lies in the range 5°-10°, indicating that the O-H...O hydrogen bonds tend to be slightly nonlinear, a feature similar to that for N-H...O hydrogen bonds. (c) For carbonyl type of acceptor groups, the distribution of angle between O··.O (or H...O) and C=O extended shows a maximum in the range 50°-60° indicating a directional property towards the lone pair electrons. In order to study the effect of lone pair orbitals on the direction of orientation of the O-H group the following parameters connected with the direction of lone pair orbitals are analysed and the main results are: (d) The elevation of O··.O (and H...O) direction from the acceptor plane (plane containing the lone pair orbitals), (ζ), shows a maximum in the range O°-10° indicating that the direction thus tends to lie close to the acceptor plane. For carbonyl acceptor groups, the distribution is uniform up to 30° and many of the larger values of ζ are found to occur for carbonyl acceptor groups only. (e) The distribution for the angle between the projection O_accep...O_donor (and O_accep...H) direction on the acceptor plane and the direction of the lone pair orbital (ζ) shows a maximum in the range O°-30°. The distribution is found to be dependent on the number of hydrogen bonds for which the same oxygen atom is the acceptor. The distribution for ζ shows a maximum between 20°-30° for those cases when the oxygen atom is recipient of only one hydrogen bond and the maximum occurs between O°-10° for those cases when the oxygen receives two hydrogen bonds. (f) The geometry in the limited number of examples where the oxygen is the recipient of three hydrogen bonds is also discussed.