Thermochemical parameters of caffeine, theophylline, and xanthine

Thermochemical parameters of caffeine 1, theophylline 2, xanthine 3, uracil, and imidazole derivatives are determined by quantum chemical calculations. Using the composite G3B3 method, the standard heat of formation of caffeine in the gaseous phase amounts to Delta H-f(g)degrees(1) = -243 +/- 8 kJ . mol(-1), which ends a support for the recent experimental value of -237.0 +/- 2.5 kcal . mol(-1). We also obtain Delta H-f(g)degrees(2) -232 +/- 8 kJ . mol(-1) and Delta H-f(g)degrees(3) = -209 +/- 8 kJ . mol(-1). The adiabatic ionization energies are IEa(1) = 7.9 eV, IEa (2) = 8.1 eV, and IEa (3) = 8.5 eV using B3LYP calculations. The enhanced ability of caffeine to eject electron, as compared to the parent compounds and cyclic components, is of interest with regard to its potential use as a corrosion inhibitor. (C) 2009 Elsevier Ltd. All rights reserved.