## 1 Introduction  
The Liquid Organic Hydrogen Carrier (LOHC) technology is a promising approach for the storage of hydrogen [1, 2]. A LOHC system consists of a hydrogen-lean form (most commonly an aromatic compound) and a corresponding hydrogenated hydrogen-rich form (typically an alicyclic compound). The hydrogen-lean component of the system can take up hydrogen through catalytic hydrogenation to produce the hydrogen-rich compound. Hydrogen is stored in such systems through covalent bonds in a very safe form. Since the dehydrogenation enthalpies for the aromatic LOHC systems are typically at a high level of about 66 to 68 kJ mol\({}^{-1}\)/(H\({}_{2}\)) at ambient temperatures [3, 4], accidental and spontaneous hydrogen release is very unlikely and only possible in the presence of catalyst and heat provision at elevated temperatures. The conditions for hydrogen release are determined by the Gibbs energy of reaction, which can be calculated from the enthalpy and entropy of reaction. The entropy change during the reaction is in the same order of magnitude for most LOHC carrier molecules. In contrast, the enthalpy of reaction for the different molecules can deviate significantly. Thus, studies of the reaction thermodynamics of potential LOHCs focus on the enthalpy of reaction as the first step. The standard molar reaction enthalpies, \(\Delta_{r}H_{\text{m}}^{\text{o}}\), are important fundamentals for process optimization. They are calculated to Hess's Law, using the liquid-phase standard molar enthalpies of formation, \(\Delta_{t}H_{\text{m}}^{\text{O}}(\text{Iiq})\), of the reactants and products. The enthalpies of the hydrogenation/dehydrogenation reactions are desired to be as low as possible to decrease the energy demand of hydrogen release (without compromising the stable interaction of hydrogen with the organic carrier) [5]. In our recent work [6], the influence of structure on the magnitude of hydrogenation/dehydrogenation enthalpies in the LOHC system based on alkylbenzene/alkyl-cyclohexane component pairs has been analyzed. It has been found that in the case of the sterically hindered ortho-substituted species (e.g., 1,2-dimethyl-benzene and 1-methyl-2-ethyl-benzene, or alkylbenzenes) with branched tert-butyl substituents, the heats of reaction are significantly lower than the usual level for aromatic homocycles (see Figure 1). Furthermore, the enthalpy of hydrogenation of the highly crowded hexa-methyl-benzene has surprisingly been found to be at the lowest level (\(-55.2\) kJ mol\({}^{-1}\)/(H\({}_{2}\)); see Figure 1) ever reported for alkylbenzenes. At the same time, the hydrogenation of 1,3,5-tri-methyl-benzene is nothing special in terms of energy (\(-65.2\) kJ mol\({}^{-1}\)/(H\({}_{2}\)); see Figure 1), which is basically the same value as reported for the common LOHC dibenyl toluene with a value of \(-65.4\) kJ mol\({}^{-1}\)/(H\({}_{2}\)) [2]. These observations have prompted a project for a more detailed study of these phenomena: We want to find out how the steric hindrance and congesting of a molecule with branched iso-propyl and tert-butyl substituents could decrease the hydrogenation enthalpies of di-, tri- and tetra-substituted alkylbenzenes. Moreover, we are interested how the bulky iso-propyl and tert-butyl substituents in ortho-substituted alkylbenzenes affect the energetics of the reactions. Figure 1: Liquid-phase reaction enthalpies of hydrogenation (per mole of hydrogen released) for the LOHC alkylbenzene-based system [6]. From a practical point of view, the di- and tri-substituted iso-propyl- and tert-butyl-benzenes can be prepared using known synthetic routes [7; 8], and both the hydrogen-lean and hydrogen-rich counterparts have acceptably low vapour pressures [6], which is one of the criteria for LOHC applications. Thus, if specific structural motifs that allow for a reduction in reaction enthalpy could be revealed, this could support the development of improved LOHC systems.