## 2 Experimental Data and Computational Methods
### Computational Methods  
The most common and generally applicable method for deriving the heat of formation from electronic structure calculations is by way of computing the atomization energy. The atomization energy is simply the energy required to break the molecule into its constituent atoms, as illustrated for hydrocarbons by the following formula  
\[\text{C}_{\text{m}}\text{H}_{\text{n}}=\text{mC}+\text{nH} \tag{2}\]  
This is thus simply a calculation based on the standard definition of the heat of formation of a compound being the sum of the enthalpy change of the reaction by which the compound is formed from the elements. Following on from our previous work [4], also in the present study we applied ab initio type calculations based on the G4 method [16; 17; 18]. Employing Gaussian 16 (G16) [27] (including the Gaussview visualization tool) for geometry optimization, force constants were calculated analytically and tight convergence criteria were used (fopt = (calcfc, tight)). Structures were verified as minima on the potential energy surface via the calculation of second derivatives (frequency calculation). For structures with multiple low-energy conformations, conformational searching was performed, and subsequently Boltzmann averaged enthalpies were obtained. In cases with ~30 or fewer conformations, the conformational search was performed manually; in more complex cases, the GMMX conformational search tool in GaussView was utilized. In a small number of cases, the potential number of conformations was so large (more than a few hundred) that only the presumed lowest energy conformation was considered. In Table S1 (Supplementary Materials), we provide the optimized G4 structures (XYZ coordinates) of the lowest energy conformations of all the species studied. Along with the structures, we list the G4 energies, ZPEs, and thermodynamic corrections for 298.15 K. In two separate tables (also in Supplementary Materials S1), we summarize the G4 enthalpies at 298 K, which are the quantities used in our analysis. The G4 data were converted into the enthalpies of formation in the way described by [PERSON] and [PERSON] [21], which, in essence, follows the correction scheme proposed earlier by [PERSON] et al. [28]. This procedure aims to correct for systematic deviation in the calculation of the atomization energies of the elements. The enthalpy of formation is obtained using the formula  
\[\text{dHf}(\text{C}_{\text{m}}\text{H}_{\text{n}}\text{N}_{\text{p}}\text{O} _{\text{w}};\text{298 K})=\text{dH}_{\text{G4}}(\text{C}_{\text{m}}\text{H}_{ \text{n}}\text{N}_{\text{p}}\text{O}_{\text{w}};\text{298 K})-\text{mX}-\text{nY}-\text{pZ}-\text{qW} \tag{3}\]  
The term on the left is the computed enthalpy of formation, whereas the first term on the right hand side is the direct G4 result. X, Y, Z, and W are the empirically corrected per-atom G4 enthalpies of C, H, N, and O in their standard states. However, here we have used the numerical values of X, Y, Z, and W recommended by [PERSON] [19] rather than the ones developed by [PERSON] and [PERSON] (the differences are relatively small, but the former appear to yield somewhat better agreement with experiment). At the researcher's convenience, Equation (3) can be extended to include other atoms. This approach overcomes the problem of the systematic increase of the deviation in ab initio and DFT-based methods in terms of the number of atoms because the atomic energies, which are the reference for the evaluation of the heat of formation from the elements, are not perfect. As the contributions X, Y, and Z are determined empirically using a set of experimental data, we may observe some differences between G4 results from different publications. These are generally in the kJ/mol range, but can incidentally be higher. We adopted the atom-specific corrections, so the values for X, Y,... in Equation (3), from [PERSON] c.s.