## 3 Results
### Substituted Benzenes
#### 3.3.6 Methoxybenzaldehyde  
Experimental and computed data for methoxybenzaldehyde are collected in Table 13. Again, the G4 results compare very favourably with the available experimental data. Our GC model reveals good (within chemical accuracy) to reasonably good agreement with the available experimental data for the majority of species considered. GC model values for 4-methoxybenzaldehyde and 2,5-dimethoxybenzaldehyde are within chemical accuracy of the experimental values considering the experimental errors of 2.0 and 2.2 kJ/mol, respectively. The larger differences include 2,3-dimethoxybenzaldehyde and 2,6-dimethoxybenzaldehyde for which we have no reliable experimental data and therefore compare to the G4 values. The conclusions are very much like those for the methoxyphenols: a few GC results do not agree with available experimental or G4 results, and this discrepancy is also observed for those species when invoking other GC methods such as the Marrero-Gani approach [10] as implemented in ICAS23 [41], viz, the last column in Table 13. The introduction of the methoxy-methoxy interaction parameter (see previous section) leads to the differences shown in column 6 in Table 13. The 2,3- and 2,6-dimethoxybenzaldehyde are, as before, revealing significant differences which are attributed to interaction with the aldehyde group. The now larger deviation for 3,4-dimethoxybenzaldehyde, compared to the model value without the additional methoxy-methoxy interaction parameters, remains unaccounted for now.  
It would be possible to account for the remaining deviations by introducing additional GC parameters but that would be based on a few data only and is, therefore, not pursued.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Methody-Benzaldehyde** & **Exp.** & **Model** & **Model – Exp.** & **ABS (Model – Exp.)** & **ABS (Exp. – G4)** & **G4** & **MG in ICA523** \\ \hline benzaldehyde & \(-38.4\) & \(-39.5\) & \(-1.1\) & \(1.1\) & \(1.1\) & \(0.3\) & \(-38.7\) & \(-37.8\) \\ \hline
2-methoxybenzaldehyde & \(-190.1\) & \(-191.5\) & \(-1.4\) & \(1.4\) & \(1.4\) & \(4.4\) & \(-185.7\) & \(-188.5\) \\ \hline
3-methoxybenzaldehyde & \(-193.7\) & \(-191.5\) & \(2.2\) & \(2.2\) & \(2.2\) & \(0.5\) & \(-193.2\) & \(-190.4\) \\ \hline
4-methoxybenzaldehyde & \(-196.5\) & \(-191.5\) & \(5.0\) & \(5.0\) & \(5.0\) & \(1.7\) & \(-194.8\) & \(-190.6\) \\ \hline
2,3-dimethoxybenzaldehyde & & \(-343.5\) & \(-20.1\) & \(20.1\) & \(9.1\) & & \(-323.4\) & \(-338.0\) \\ \hline
2,4-dimethoxybenzaldehyde & \(-346.9\) & \(-343.5\) & \(3.4\) & \(3.4\) & \(0.7\) & \(0.4\) & \(-347.3\) & \(-341.2\) \\ \hline
2,5-dimethoxybenzaldehyde & & \(-338.1\) & \(-343.5\) & \(-5.4\) & \(5.4\) & \(5.4\) & \(0.6\) & \(-337.5\) & \(-341.2\) \\ \hline
2,6-dimethoxybenzaldehyde & & \(-343.5\) & \(-20.4\) & \(20.4\) & \(20.4\) & & \(-323.1\) & \(-338.0\) \\ \hline
3,4-dimethoxybenzaldehyde & \(-343.1\) & \(-343.5\) & \(-0.4\) & \(0.4\) & \(10.6\) & \(0.1\) & \(-341.2\) & \(-341.2\) \\ \hline
3,5-dimethoxybenzaldehyde & & \(-343.5\) & \(6.1\) & \(6.1\) & \(2.0\) & & \(-349.6\) & \(-343.4\) \\ \hline \multicolumn{7}{c}{averaged absolute difference} & \multicolumn{1}{c}{\(6.5\)} & \multicolumn{1}{c}{\(5.8\)} & \multicolumn{1}{c}{\(1.1\)} \\ \hline \multicolumn{7}{c}{averaged absolute difference without two exceptions 2,3- and} \\ \multicolumn{7}{c}{2,6-dimethoxybenzaldehyde} & \multicolumn{1}{c}{\(2.1\)} \\ \hline \hline \end{tabular}
\end{table}
Table 13: Experimental, present GC model, and G4 model values for the heat of formation for methoxybenzaldehyde. All values in kJ/mol. Experimental and G4 quantum data are from [PERSON] c.s. [44] for benzaldehyde, and all other values from [PERSON] [22]. The last column shows the Marrero–Gani GC model values generated using the ICAS23 software suite [41].