The total atomization energy and heat of formation of HCN(g)

An exhaustive ab initio convergence study, using coupled cluster methods and basis sets of up to [7s6p5d4f3g2h/5s4p3d2f1g] quality, of the total atomization energy of HCN(g), both directly and via the isogyric reaction 2 HCN → C2H2 + N2, leads to a best estimate of Σ D0=303.08±0.25 kcal/mol, in between thermochemical and spectroscopic experimental values. The best estimate for Δf0 = 31.04±0.25 kcal/mol, or 129.9±1 kJ/mol.