## 1 Introduction
Note that additional alkylation of the aromatic core of a hydrogen-lean LOHC molecule reduces its gravimetric hydrogen capacity. However, this negative effect could be outweighed if such multi-alkylated systems were to exhibit significantly reduced standard molar reaction enthalpies in the hydrogenation and dehydrogenation processes that establish the corresponding hydrogen storage cycle. In fact, the thermochemical properties of mono-, di- and tri-substituted iso-propyl- and tert-butyl-benzenes as the hydrogen-lean counterparts to the LOHC systems of interest in this paper have already been intensively investigated in our laboratory in the past [9; 10; 11; 12]. In the intervening period, some new data have appeared in the literature and it is important to ensure the consistency of all available data before recommending them for thermochemical calculations leading to the enthalpies of hydrogenation for the alkylbenzenes. Therefore, two priorities were set in this work. The first is to collect, evaluate and validate the available experimental data. Second, the reliable liquid-phase formation enthalpies for the hydrogen-lean and hydrogen-rich counterparts of the LOHC systems based on the branched alkylbenzenes will be provided in order to calculate their hydrogenation enthalpies and to meaningfully discuss structure-property relationships for them. The evaluation of the experimental data was based on a textbook equation, which relates the thermochemical properties relevant to this work:  
\[\Delta_{\mathrm{f}}H^{\mathrm{o}}_{\mathrm{m}}(\mathrm{g})=\Delta_{\mathrm{f} }^{\mathrm{g}}H^{\mathrm{o}}_{\mathrm{m}}+\Delta_{\mathrm{f}}H^{\mathrm{o}}_{ \mathrm{m}}(\mathrm{liq}) \tag{1}\]  
where \(\Delta_{\mathrm{f}}H^{\mathrm{o}}_{\mathrm{m}}(\mathrm{g})\) is the gas-phase standard molar enthalpy of formation and \(\Delta_{\mathrm{f}}^{\mathrm{g}}H^{\mathrm{o}}_{\mathrm{m}}\) is the standard molar enthalpy of vaporisation. The \(\Delta_{\mathrm{f}}^{\mathrm{g}}H^{\mathrm{o}}_{\mathrm{m}}\)-values are most frequently derived from the vapour-pressure-temperature dependences [13]. The \(\Delta_{\mathrm{f}}H^{\mathrm{o}}_{\mathrm{m}}(\mathrm{liq})\)-values are usually derived from combustion experiments, from solution calorimetry or from chemical equilibrium studies [14]. The data set for the vaporisation enthalpies and the data set for the enthalpies of formation were collected and evaluated separately. The set of vaporisation enthalpies was validated using a complementary transpiration experiment, and empirical and structure-property correlations. The set of enthalpies of formation was validated with quantum chemical calculations. The targeted thermochemical property that is relevant to the energetics of hydrogen storage is the liquid-phase enthalpy of formation, \(\Delta_{\mathrm{f}}H^{\mathrm{o}}_{\mathrm{m}}(\mathrm{liq})\). It was derived using the following equation:  
\[\Delta_{\mathrm{f}}H^{\mathrm{o}}_{\mathrm{m}}(\mathrm{liq},\;298.15\;\mathrm{ K})=\Delta_{\mathrm{f}}H^{\mathrm{o}}_{\mathrm{m}}(\mathrm{g},\;298.15\;\mathrm{K})- \Delta_{\mathrm{f}}^{\mathrm{g}}H^{\mathrm{o}}_{\mathrm{m}}(298.15\;\mathrm{K}) \tag{2}\]  
We have extended the symbols for enthalpies to include temperatures, since it is common in thermochemistry to refer all enthalpies given in Equations (1) and (2) to the reference temperature \(T=298.15\;\mathrm{K}\). The cross-validation of the two contributors to Equation (2) allows a reasonably accurate determination of the liquid-phase standard molar enthalpies of formation of the hydrogen-lean compounds of given LOHC systems. Along with the carefully evaluated and validated liquid-phase enthalpies of formation of the corresponding alkyl-cyclohexanes (as the hydrogen-rich compounds) [6], the interpretation of the structural effects on the energetics of the hydrogenation/dehydrogenation reactions relies upon the validated data sets and not on individual, possibly inaccurate, data.