## 3 Results
### Gas-Phase Enthalpies of Formation of Indanones  
The combustion results and the ones obtained from the vaporization/sublimation studies allowed for the calculation of the experimental gas-phase standard molar enthalpy of formation, \(\Delta_{\mathrm{f}}\,H_{\mathrm{m}}^{{}^{\circ}}(\mathrm{g})\), at \(T\) = 298.15 K, for each compound. The data used in these calculations are presented in Table 7. As can be seen, the two sets of isomers present identical values of enthalpies of vaporization and sublimation, and an identical trend is also verified for the crystalline enthalpy of formation of the methoxy-indanones. In the case of methyl-indanones, an unexpected difference of 13 kJ-mol\({}^{-1}\) was observed for the \(\Delta_{\mathrm{f}}\,H_{\mathrm{m}}^{{}^{\circ}}(\mathrm{cr})\) results, which will also affect the results \(\Delta_{\mathrm{f}}\,H_{\mathrm{m}}^{{}^{\circ}}(\mathrm{g})\). Figure 2: Plot of \(\mathrm{ln}\;p\) = \(f(1/T)\) for 5 MI: \(\square\), small effusion orifices; \(\square\), medium effusion orifices; \(\Delta\), large effusion orifices. The values of \(\Delta_{\rm f}\,H_{\rm m}^{\rm c}(\rm g)\) have been derived computationally using the G3 MP2B3 composite method, which has been widely tested in our research group, with reliable results for these types of polycyclic compounds [8; 10; 31]. Therefore, a conformational analysis at the B3 LYP/6-31G(d) level of theory was performed to evaluate the more energetically stable conformers. For 2 MI, 3 MI and 4 MI there is only one stable conformation (Figure S1) that corresponds to the global minimum energy conformation, while for 5 MI, two possible conformations exist associated with the geometrical changes of the methoxy group (more details in Table S8). Working reactions were selected considering a set of the molecules with an accurate energetic characterization already available in the literature [9; 11; 31; 32; 33; 34]. A group of six reactions were chosen for each indanone (Tables S9-S12), and the mean values of the \(\Delta_{\rm f}\,H_{\rm m}^{\rm c}(\rm g)\) calculated are presented in Table 8. For the methyl-indanones, practically the same value was obtained, \(-94.4\pm 4.2\) kJ\(\cdot\)mol\({}^{-1}\) for 2 MI and \(-94.3\pm 4.2\) kJ\(\cdot\)mol\({}^{-1}\) for 3 MI. The same occurred for the methoxy-indanones, \(-219.1\pm 3.3\) kJ\(\cdot\)mol\({}^{-1}\) for 4 MI and \(-220.3\pm 3.3\) kJ\(\cdot\)mol\({}^{-1}\) for 5 MI. This leads us to conclude that, theoretically, different positions do not affect the enthalpic value of the gas-phase enthalpy of formation of indanones. Table 8 compiles the experimental and calculated \(\Delta_{\rm f}\,H_{\rm m}^{\rm c}(\rm g)\) values, showing a very good agreement between them, except for 2 MI, in which the difference between the results is considerable (\(\sim\)10 kJ\(\cdot\)mol\({}^{-1}\)). In order to rationalize the energetic effects of the two substituents in the indanone structure, the enthalpic increments associated with the presence of a methyl- and methoxy- group in the indanone core were calculated, being the corresponding results reported in the next section. Entexitic Effects Associated with the Substitution of H in Indanone by CH\({}_{3}\) or by OCH\({}_{3}\)  
Considering the experimental \(\Delta_{\rm f}\,H_{\rm m}^{\rm c}(\rm g)\) values of the molecules studied (Table 8), a structural analysis was performed based on the presence of a methoxy and a methyl group in the 1-indanone as molecular core. It is intended to evaluate the effects of the different substituents on the thermodynamic properties of this class of cyclic compounds. Comparing these results with those for other cyclic systems, such as benzene, anthracene and cyclopentanone, trends in the enthalpic increments of cyclic molecules were identified, which may be useful for predicting the chemical behavior of these molecules in several chemical reactions and can support the design and optimization of chemical processes.