The heat of formation of chlorine-isocyanate and the relative stability of isoelectronic molecules: an experimental and theoretical study

Accurate thermochemical data of small molecules are invaluable to the progress of every aspect of chemistry, especially in the atmosphere, combustion and industry. In this work, photofragmentation translational spectroscopy and 1st principles electronic structure theory reveal the literature value of the heat of formation of chlorine-isocyanate to be in error by more than 40 kcalmol. We report a revised experimental value for D0(Cl-NCO) = 51+/-3 kcal/mol which leads to a Delta Hf (ClNCO) = 8.5+/-3 kcal/mol. High level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit give D0(Cl-NCO) = 6.3 kcal/mol, in good agreement with experiment. In light of the present results, the destabilization of azides relative to isoelectronic isocyanates has been evaluated empirically for three pairs of related molecules. It is found to be 90-110 kcal/mol, and has been attributed mainly to the weakening of the N-NN bond relative to the N-CO bond. Electronic structure calculations employing decomposition analysis suggest that, compared to homopolar N2, the (+delta)CO(-delta) pi polarity provides better orbital interaction (charge transfer) and electrostatic attraction and results in a closer encounter and larger stabilization between the fragments and that this is the origin of isoelectronic destabilization of azides relative to the isocyanates.