The electron density in the NaHC2O4thinsp;sdot;thinsp;H2O crystal has been studied by quantum mechanicalabinitioMOhyphen;LCAOhyphen;SCF calculations. Calculations have been performed on one hand for the free HC2Ominus;4and H2O molecules/ions, and on the other hand, for these molecular units in the crystalline surrounding. The main effects of the intermolecular interactions on the electron density can be summarized as follows: (1) The polarity of the intramolecular bonds is increased in the crystal, as compared to the free state. (2) The deviation from spherical symmetry around the oxygen atoms is smaller in the crystal than in the free molecules/ions. In particular, this implies a decrease of lone pair density at these atoms. (3) The experimental observation that lone pairs accepting stronger hydrogen bonds, in general, show less intense peaks in the deformation density than those which accept weaker hydrogen bonds is explained as being primarily a superposition effect between donor and acceptor deformation densities.
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