The structures of pyrimidines and purines. III. An X-ray investigation of hydrogen bonding in amino-pyrimidines

Abstract

The crystal structures of two aminopyrimidines, 4-amino-2,6-dichloropyrimidine and 5-bromo-4,6-diaminopyrimidine, have been determined by two- and three-dimensional Fourier methods. The electron distribution determined by three-dimensional Fourier synthesis is sufficiently accurate to fix the positions of the hydrogen atoms, and experimental evidence is obtained in support of the view that in hydrogen bonding the hydrogen is covalently bound to one atom but the proton interacts electrostatically with the unshared electron pair on the other. The hydrogen atom and the orbit of the unshared electrons are both directed along the line joining the hydrogen-bonded atoms. Other effects associated with hydrogen bonding in these compounds are pointed out. Bond lengths calculated on the assumption that `resonance structures' involving a positive formal charge on an amino group make an appreciable contribution to the molecular state are in good agreement with the observed values.