Formamide electron density distribution

Hydrogen bonding must have an effect on the electron density distribution of a molecule. In principle, this should be observed in the deformation density distributions discussed in Chapter 3. There are, in fact, two methods available. One is purely theoretical, in which the calculated deformation density for a hydrogen-bond dimer or trimer is compared with that of the isolated molecule. The other method compares the experimental deformation density of a hydrogen-bonded molecule in a crystal structure with the theoretical deformation density of the isolated molecule. Formamide has been studied by both methods [298, 380], and there appear to be significant differences in the results which are not well accounted for. Theoretical difference (dimer vs. monomer) deformation density maps have been calculated for the water dimer and the formaldehyde-water complex [312]. When those for the water dimer are decomposed into the components described in Chapter 4, a small increase in the charges on the atoms in the O-H -O bond due to the charge-transfer component is predicted [312]. 

Stevens ED, Rys J, Coppens P (1978) Quantitative comparison of theoretical calculations with the experimentally determined electron density distribution of formamide. J Am Chem Soc 100 2324- 2328... 

Stevens, E. D., Low-temperature experimental electron density distribution of formamide, Acta Cryst, B34, 544-551 (1978). 

The electron density distribution of individual molecular orbitals may also be determined and plotted The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are often of particular interest as these are the orbitals most commonly involved in chemical reactions. As an illustration, the HOMO and LUMO for formamide are displayed in Figures 2.12 and 2.13 (colour plate section) as surface pictures. 

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