# Energetic and Structural Studies of Two Biomass-Derived Compounds: 6- and 7-hydroxy-1-indanones
## 3 Results and Discussion
### Experimental and Computational Enthalpies of Formation in the Gaseous Phase  
The experimental gas-phase standard molar enthalpy of formation, \(\Delta_{\rm f}\,H_{\rm m}^{{}^{\circ}}({\rm g})\), at \(T=298.15\) K, of each compound was derived from the combination of the standard molar enthalpy of formation in the crystalline phase with its standard molar enthalpy of sublimation. These data are summarized in Table 4.  
The computational gas-phase enthalpies of formation of the two indanone derivatives were calculated at the G3(MP2)/B3 LYP level of theory, using the atomization reaction and the isodesmic reactions (8) to (13) presented in Table 5. The working reaction (13) was applied to 7-hydroxy-\(\alpha\)-tetradone and also to three more isomers, namely, 5-hydroxy-\(\alpha\)-tetradone, 6-hydroxy-\(\alpha\)-tetralone, and 8-hydroxy-\(\alpha\)-tetralone [26]. In this study, we present results only for 7-hydroxy-\(\alpha\)-tetralone, since for the other isomers, there is a large discrepancy between the experimental value and the results obtained, higher than 8 kJ\(\cdot\)mol\({}^{-1}\) for **6 HI** or **7 HI** molecules. Despite this, as can be seen in Table 6, there is an excellent agreement between the values of the gaseous enthalpy of formation of both compounds derived from the experimental data and the mean value obtained by the composite method. Data related to the G3(MP2)/B3 LYP absolute enthalpies and literature values of \(\Delta_{\rm f}\,H_{\rm m}^{{}^{\circ}}({\rm g})\), at \(T=298.15\) K, for all the atoms and molecules involved herein are given in SI (Table S7). To the best of our knowledge, there are no experimental or computational data available to compare with those obtained in this study. However, the consistency of the results described here gives us the confidence to use an identical methodology to estimate the values of the corresponding properties for related compounds. \begin{table}
\begin{tabular}{c c c c} \hline
**Compound** & \(\Delta_{\rm f}\,H_{\rm m}^{{}^{\circ}}({\rm cr})\)/k\(\cdot\)mol\({}^{-1}\) & \(\Delta_{\rm cr}^{\rm g}\,H_{\rm m}^{{}^{\circ}}\)/k\(\cdot\)mol\({}^{-1}\) & \(\Delta_{\rm f}\,H_{\rm m}^{{}^{\circ}}({\rm g})\)/k\(\cdot\)mol\({}^{-1}\)\({}^{a}\) \\ \hline
**6 HI** & \(-354.6\pm 2.6\) & \(114.4\pm 1.7\) & \(-240.2\pm 3.1\) \\
**7 HI** & \(-357.3\pm 2.3\) & \(91.7\pm 2.0\) & \(-265.6\pm 3.0\) \\ \hline \end{tabular} \({}^{a}\) Uncertainties calculated through the RSS method.  
\end{table} 
Table 4: Experimental derived standard (\(p^{\circ}=0.1\) MPa) molar enthalpies of formation, in the gaseous phase, at \(T=298.15\) K, for the compounds studied. Calculated Enthalpic Increments (\(H\to OH\)) and Analysis of the Intramolecular H-Bond Interaction  
The energetic effect involving the presence of a hydroxyl group in position 6 or 7 of 1-indanone was also analyzed by calculating the respective enthalpic increment (Figure 4). The enthalpic increment related to the hydroxyl group in position 6 is \(-(176.2\pm 4.9)\) kJ-mol\({}^{-1}\), instead of \(-(201.6\pm 4.8)\) kJ\(\cdot\)mol\({}^{-1}\) obtained for position 7. As a comparison, the corresponding substitution in benzene gives \(-(179.0\pm 1.1)\) kJ\(\cdot\)mol\({}^{-1}\) (for this calculation, the \(\Delta_{\text{H}}H^{{}^{\ast}}_{\text{m}}(\text{g})\) literature values for benzene, \((82.6\pm 0.7)\) kJ\(\cdot\)mol\({}^{-1}\)[27] and for phenol, \(-(96.4\pm 0.9)\) kJ\(\cdot\)mol\({}^{-1}\)[27] were used). In the case of **7 HI**, there is a difference of \(-25\) kJ\(\cdot\)mol\({}^{-1}\) between the observed magnitude of the enthalpic increments, probably due to the existence of a strong intramolecular interaction O\(-\)H\(\cdots\)O. Following our results, we have studied the intramolecular H-bond in this molecule using the _cis-trans_ method [28], which compares energetically the H-bonded system and the homologous system with the O-H fragment rotated around the C-O bond toward the _trans_ conformation, so that the H atom points away from the acceptor oxygen from the carbonyl group (Equation (7)).