## 2 Materials and Methods
### Vaporization Thermodynamics of Alkyl-Quinolines and Alkyl-Pyridines
#### 3.3.1 Validation Using Retention Indices \(J_{x}\)  
The additional validation of the \(\Delta_{\mathrm{I}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\)(298.15 K)-values evaluated in Tables 2 and 3 was carried out in this work using the available chromatographic retention indices for alkyl-quinolines and alkyl-pyridines [26; 27; 28; 29; 30; 31]. We know that the \(\Delta_{\mathrm{I}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\)(298.15 K)-values and retention indices are usually linearly correlated in series of structurally similar compounds [32]. Indeed, we derived a high-quality linear correlation between vaporization enthalpies \(\Delta_{\mathrm{I}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\)(298.15 K) and a set of Kovats's indices, \(J_{x}\), of only alkyl-quinolines collected in Table 4:  
\[\Delta_{\mathrm{I}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\mathrm{(298.15 K)}/\left(\mathrm{kJ\cdot mol}^{-1}\right)=9.2+0.0406\times J_{x}\text{ with }\left(R^{2}=0.9947\right) \tag{3}\]  
In addition, a high-quality linear correlation between vaporization enthalpies \(\Delta_{\mathrm{I}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\) (298.15 K) and a combined set of [PERSON]'s indices, \(J_{x}\), of alkyl-pyridines and alkyl-quinolines collected in Table 5 was derived:  
\[\Delta_{\mathrm{I}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\mathrm{(298.15 K)}/\left(\mathrm{kJ\cdot mol}^{-1}\right)=15.3+0.0357\times J_{x}\text{ with }\left(R^{2}=0.9970\right) \tag{4}\]  
We deliberately performed the \(\Delta_{\mathrm{I}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\)(298.15 K) correlations with two significantly different sets of the retention indices. As can be seen from Tables 4 and 5 the \"theoretical\" vaporization enthalpies calculated with Equations (3) and (4) are very close. In addition, the differences between experimental vaporization enthalpies and values calculated according to Equations (3) and (4) are mostly below 1 kJ\(\cdot\)mol\({}^{-1}\). Hence, the uncertainties of enthalpies of vaporization which are estimated from the correlation of \(\Delta_{1}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\)(298.15 K) with [PERSON]'s indices are evaluated with an uncertainty of \(\pm 1.0\) kJ\(\cdot\)mol\({}^{-1}\). The \"theoretical\" results derived from these correlations are given in Tables 2 and 3 and labeled as \(J_{x}\). These results are valuable to support the level of enthalpy of vaporization derived from other methods, especially in cases where data are scarce (e.g., for the most of ethyl, propyl and butyl-substituted pyridines and quinolines). From these tables it can be seen that for every compound agreement among \(\Delta_{1}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\) (298.15 K-values, which were derived in different ways, all lie within the assigned error bars. To get more confidence and reliability, we calculated the weighted average (the uncertainty was used as a weighing factor) for alkyl-quinolines and alkyl-pyridines given in Tables 2 and 3. These values are highlighted in bold and are recommended for thermochemical calculations. Such good consistency of results obtained from the \(\Delta_{1}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\)(298.15 K)-\(J_{x}\) correlations can be seen as additional validation of the data evaluated in Tables 2 and 3. Moreover, the retention indices for dimethyl-quinolines also available on the OV-101 column [27] have been utilized according to Equation (3) to estimate the vaporization enthalpies for the series of different dimethyl-quinolines (see Table 4). The 2-phenyl-quinoline is of particular interest for hydrogen storage. It is obvious that according to the reaction R3 (Figure 1) this molecule releases 8 moles of hydrogen. However, only the sublimation enthalpy, \(\Delta_{\mathrm{cr}}^{\mathrm{g}}H_{\mathrm{m}}^{\mathrm{o}}\)(298.15 K), for this compound is avail