## 3 Results and Discussion
### Validation of Vaporisation Enthalpies Using Structure-Property Relationships
#### 3.3.5 Quantum Chemistry: Theoretical Enthalpies of Formation of Iso-Propyl- and Tert-Butyl-Benzenes
The algorithm of validation is given in Table 11.  
We have taken the _theoretical_ gas-phase enthalpies of formation of these three crowded tert-butylbenzenes from Table 10, column 6. Their vaporisation enthalpies were evaluated in Table 2. Their enthalpies of fusion were estimated using Walden's rule in Table S15. The sum of the latter values resulted in the required sublimation enthalpies (see Table 11, column 3). The difference between columns 2 and 3 in this table provides the theoretical \(\Delta_{\mathrm{f}}H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{cr})_{\mathrm{theor}}\)-values, which are compared to the available experimental \(\Delta_{\mathrm{f}}H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{cr})_{\mathrm{exp}}\) data in column 5. From Table 11 it can be seen that the agreement for 1,2-di-tert-butylbenzene and 1,2,4,5-tetra-tert-butylbenzene is good. The agreement for 1,2,4-tri-tert-butylbenzene is quite fair and could be considered acceptable as no details on combustion experiments and purity of the sample were reported [64]. This additional validation provides confidence that the QC results for the di-, tri- and tetra-tert-alkylbenzenes can be considered reliable. 3.6 Liquid-Phase Enthalpies of Formation and Thermodynamic Analysis of the Hydrogenation/Dehydrogenation of the Alkylbenzene-Based LOHC Systems  
Having established a consistent data sets for the gas-phase enthalpies of formation and vaporisation enthalpies for alkylbenzenes, reliable liquid-phase enthalpies of formation for di-, tri-, and tetra-substituted iso-propyl- and tert-butyl-benzenes can now be derived using Equation (2). These results can be utilized to calculate the enthalpies of hydrogenation for reactions given in Figure 1 and for other alkylbenzenes listed in Table 12.  
The liquid-phase enthalpies of formation, \(\Delta_{\mathrm{f}}H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{liq})\), of the hydrogen-lean (HL) compounds given in Table 12 were calculated as shown in Table S16. The gas-phase enthalpies of formation of the hydrogen-reach (HR) counterparts of the LOHC systems were calculated using the G3 MP2 method (for details see Ref. [6]) and the results are given in Table S17. The vaporisation enthalpies of the alkyl-cyclohexanes related to this work are also given in Table S16 and the resulting liquid-phase enthalpies of formation of the hydrogen-rich compounds are listed in Table S16, column 7. The final \(\Delta_{\mathrm{f}}H_{\mathrm{m}}^{\mathrm{o}}(\mathrm{liq})\)-values for the HL and HR compounds are summarized in Table 12 and were used to calculate the reaction enthalpies according to Hess's Law (see Table 12, column 4). The enthalpies of reaction per mole of hydrogen are given in Table 12, column 5 and help to analyse the influence of structure and degree of crowding on the hydrogenation enthalpies of alkylbenzenes. The results of this study on the enthalpies of hydrogenation of branched alkylbenzenes turned out to be less spectacular than expected. As can be seen from Table 12, the \(\Delta_{\mathrm{r}}H_{\mathrm{m}}^{\mathrm{o}}/\mathrm{H}_{2}\)-values for all iso-propyl-substituted benzenes are at the level of \(-64\) to \(-66\) kJ mol\({}^{-1}\)/(H\({}_{2}\)), which is hardly distinguishable within experimental uncertainties. The only exception is evident for 1,3,5-tri-iso-propyl-benzene with \(\Delta_{\mathrm{r}}H_{\mathrm{m}}^{\mathrm{o}}(298.15\) K) = \(-59.1\) kJ mol\({}^{-1}\)/(H\({}_{2}\)). An opposite trend than expected was observed for the tert-butyl-substituted alkylbenzes. Indeed, for the crowded 1,2-di-tert-butyl-, 1,2,4-tri-tert-butyl-, 1,2,3-tri-tert-butyl- and 1,2,4,5-tetra-tert-butyl-benzenes, the enthalpy of hydrogenation increased dramatically up to -85 kJ mol\({}^{-1}\)/(H\({}_{2}\)), as shown in Table 12. Interestingly, the reaction enthalpy of 1-iso-propyl-2-tert-butyl-benzene does not follow this trend and exhibits an anomalously low value of -61.4 kJ mol\({}^{-1}\)/(H\({}_{2}\)).