) calculates as -9.9 and -9.7 kcal/mol HCN (Supporting Information, Table ). These energies are in fair agreement with estimates derived from differential scanning calorimetry thermograms of solid-state and melt-polymerization of 5. That study, by Ruiz-Bermejo and colleagues, estimated heat releases of 3-5 kcal/mol HCN at 423-463 K. We furthermore predict that the HCN dimer 3 have an energy content of +7.5 kcal/mol HCN, i.e., its formation from HCN is uphill by 15 kcal/mol. This value is 8.5 kcal/mol lower than the corresponding value in gas phase (Supporting Information, Table ), in turn comparable to other computational results. We note that 3 has a similar energy content as hydrogen isocyanide in solution. Nevertheless, 3 is high in energy, and can be ruled out as a major component of reaction mixtures.