## 3 Results
### Substituted Benzenes
#### 3.3.5 Methoxyphenols  
Experimental and computed data for methoxyphenols are collected in Table 12. First of all, we observe that the G4 results compare very favourably with the experimental values, with an averaged absolute difference of 1.9 kJ/mol and not a single deviation beyond chemical accuracy, which is yet another demonstration that the G4 method can be used to fill gaps where experimental data are not available or are insufficiently reliable. The GC model for these species obviously involves the phenol-related correction (OH-phenyl) parameter (+7 kJ/mol) as well as the methoxy Group parameter (\(-\)152 kJ/mol). With the OH-benzene and methoxy-benzene interaction parameters established previously for the methoxybenzenes and phenols, for the methoxyphenols, we find four out of eight GC model values within chemical accuracy from the experimental ones, viz. columns 3 and 4 in Table 12. The most unexpected difference is that for 4-methoxyphenol as no steric effects are to be expected, but the difference with 2-methoxyphenol is well represented by B3 LYP relative energy calculations (12 kJ/mol difference compared to 13 kJ/mol based on the experimental values), and thus it is to be considered genuine. For the dimethoxyphenols, we observe a larger difference between experimental and GC model values for 2,6- and 3,4-dimethoxyophenol. The value for 3,5-dimethoxyophenol is somewhat beyond chemical accuracy. We observed similar significant deviations for the Marrero-Gani GC method [10] for the same species (in fact, larger deviations than for our GC model), viz. Table 12, and we found that this also holds for the Constantinou and Gani [9] and the Joback and Reid [8] GC methods (all these GC results are from the ICAS23 software suite [41]). This indicates that current GC methods cannot account for these species at present. However, when we now introduce the methoxy-methoxy interaction parameters as established in the previous section for the methoxybenzes, i.e., +11 kJ/mol for adjacent \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Methoday-Phenols**} & \multirow{2}{*}{**Exp.**} & \multirow{2}{*}{**Model**} & \multirow{2}{*}{**Model – Exp.**} & \multicolumn{2}{c}{**ABS (Model +**} & \multirow{2}{*}{**ABS (Exp. – G4)**} & \multirow{2}{*}{**ABS**} & \multirow{2}{*}{**G4**} & \multirow{2}{*}{**MG in ICAS23**} \\  & & & & & & & & & \\ \hline
2-methoxyphenol & \(-\)247.3 & \(-\)244.0 & 3.3 & 3.3 & 3.3 & 0.0 & \(-\)247.3 & \(-\)249 \\ \hline
3-methoxyphenol & \(-\)241.2 & \(-\)244.0 & \(-\)2.8 & 2.8 & 2.8 & 3.6 & \(-\)244.8 & \(-\)250.8 \\ \hline
4-methoxyphenol & \(-\)234.3 & \(-\)244.0 & \(-\)9.7 & 9.7 & 9.7 & 1.9 & \(-\)236.2 & \(-\)251.0 \\ \hline
2,3-dimethoxyphenol & \(-\)393.0 & \(-\)396.0 & \(-\)3.0 & 3.0 & 8.0 & 3.5 & \(-\)389.5 & \(-\)398.4 \\ \hline
2,4-dimethoxyphenol & \(-\)398.3 & \(-\)396.0 & 2.3 & 2.3 & 1.8 & 1.1 & \(-\)397.2 & \(-\)401.7 \\ \hline
2,5-dimethoxyphenol & \(-\)396.0 & & & & 1.8 & & \(-\)397.8 & \(-\)401.7 \\ \hline
2,6-dimethoxyphenol & \(-\)384.8 & \(-\)396.0 & \(-\)11.2 & 11.2 & 11.2 & 4.1 & \(-\)388.9 & \(-\)398.4 \\ \hline
3,4-dimethoxyphenol & \(-\)380.9 & \(-\)396.0 & \(-\)15.1 & 15.1 & 1.6 & 0.4 & \(-\)380.5 & \(-\)401.7 \\ \hline
3,5-dimethoxyphenol & \(-\)401.2 & \(-\)396.0 & 5.2 & 5.2 & 1.1 & 0.8 & \(-\)402.0 & \(-\)403.9 \\ \hline averaged absolute difference & & & & 6.6 & 4.6 & 1.9 & \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Experimental, present GC model, and G4 model values for the heat of formation for methoxyphenols. All values in kJ/mol. Experimental and G4 quantum data are from [PERSON] c.s. [22]. Due to the absence of an experimental value, for 2,5-dimethoxyphenol, the G4 value was used to calculated the difference with the GC model value. The last column shows the Marrero–Gani GC model values generated using the ICAS23 software suite [41].