Ab initio thermal rate coefficients for H + NH3 H2 + NH2

The reversible reaction NH3 + H H-2 + NH2, which plays an important role in NH3 fuel combustion, is studied with a theoretical approach that combines the high-accuracy extrapolated ab initio thermochemistry (HEAT) protocol with semiclassical transition state theory (SCTST). The calculated forward reaction is endothermic by 11.8 +/- 1 kJ/mol, in nearly perfect agreement with the active thermochemical tables (ATcT) value of 11.5 +/- 0.2 kJ/mol. Using this improved thermochemistry yields better rate constants, especially at low temperatures. Experimental rate constants available from 400 to 2000 K for the forward and reverse reaction pathways can be reproduced (within 20%) by the calculations from first principles.