Structure and normal vibrations in xanthine and its methyl derivatives from first principle calculations

Structure and vibrational characteristics of biologically important 3,7-dihydro-purine-2,6-dione (xanthine) and its 7- or 9-substituted methyl derivatives derived from the second order MØller–Plesset perturbation theory have been analyzed using the molecular electron density (MED) topography. Successive substitution of methyl group at 9 position engender carbonyl stretching vibrations at higher wavenumber compared to those in X₇ tautomer along the series. The C–O bond distance in 7-substituted xanthine tautomers correlate well to electron density at the bond critical point (bcp) in the MED topography.