Structures of Molecules at the Atomic Level: Caffeine and Related   Compounds
  Recent rsearches have shown that the lengths of the chemical bonds, whether completely or partially covalent or ionic, are sums of the radii of the adjacent atoms and/or ions. On investigating the bond length data for the molecular components of nucleic acids, all were found (for the first time) to be effectively the sums of the covalent radii of the adjacent atoms. This work shows that the bond lengths in caffeine and related molecules are likewise sums of the covalent radii of C, N, O and H. This has enabled arriving at the atomic structures of these molecules, also for the first time. 
Raji Heyrovska and Saraswathi Narayan