Non-optimized Geometry Research Articles

O-H · O Hydrogen bonds in molecular crystals a statistical and quantum-chemical analysis

196 hydrogen bonds taken from 45 crystal structures of polyalcohols, saccharides and related compounds have been submitted to a statistical analysis. The resulting histograms for some of the geometrical parameters are interpreted as Boltzmann distributions, and force constants are derived from them. The energy of different O-H ·· O configurations is obtained from ab initio SCF calculations on some 40 near-equilibrium geometries of the (H 2O) 2 dimer. The observed and calculated results led to the following conclusions. (1) Non-linear H bonds occur frequently because of a trivial geometrical factor together with an energetical preference for non-linearity in most nonoptimal geometries. (2) A distinct preference was neither observed nor calculated for hydrogen bonding in the direction of one of the acceptor lone pairs. (3) Nearly all observed O-H ·· O arrangements have a calculated energy that is less than about l kcal mol −1 above that of the calculated optimum geometry. (4) The range of accessible dimer geometries is largely determined by the classical Coulomb energy.