## 3 Results
### Substituted Benzenes
#### 3.3.10 Indoles  
Experimental and model data for indole (Figure 2, left-hand structure) and iodine (Figure 2, right-hand structure) and several of its derivatives are shown in Table 17. Following our earlier approach, both structures are considered a Group by themselves. Except for 1-methylindoline, the data in Table 17 reveal agreement between experimental and G4 results within chemical accuracy, indicating that there is little doubt about their correctness (within the respective error bars). The GC model values for 2-methylindole and 2-methylindoline are also in good agreement with the experimental values. For the 1-methyl substituted species, we need to consider which Groups are to be involved. In previous sections on amines, we established different Group parameters for RNH\({}_{2}\), R\({}_{2}\)NH, and R\({}_{3}\)N. Thus, substitution at the 1-position in indole/ine requires a modification in the Group contribution. In order to avoid the introduction of new parameters, for 1-methylindole  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Furans** & **Exp.** & **Model** & \begin{tabular}{c} **ABS** \\ **(Model – Exp.)** \\ \end{tabular} & \begin{tabular}{c} **ABS** \\ **(G4 Avgd. – Exp.)** \\ \end{tabular} & \begin{tabular}{c} **G4** \\ **Avgd. – Exp.)** \\ \end{tabular} & \begin{tabular}{c} **G4** \\ **Avgd.** \\ \end{tabular} & \
\begin{tabular}{c} **G4 Boltzm.** \\ **Pedley** \\ \end{tabular} &
\begin{tabular}{c} **Error from** \\ **Pedley** \\ \end{tabular} \\ \hline furan & \(-34.9\) & & & 0.4 & \(-34.5\) & \(-34.5\) & 0.7 \\ \hline
2-furanmethanol & \(-211.8\) & \(-220.5\) & \(8.7\) & \(7.0\) & \(-220.7\) & \(-218.8\) & 2.1 \\ \hline
2-furancarbaldehyde (furfurral) & \(-151.0\) & \(-158.9\) & \(7.9\) & \(5.1\) & \(-156.8\) & \(-156.1\) & 4.6 \\ \hline furancarboxylic acid & \(-389.9\) & \(-425.9\) & \(36.0\) & \(19.0\) & \(-409.2\) & \(-408.9\) & 2.3 \\ \hline
2-furancarboxylic acid methyl ester & \(-404.8\) & \(-403.3\) & \(1.5\) & \(13.5\) & \(-391.5\) & \(-391.3\) & 1.0 \\ \hline vinylfuran & \(27.8\) & \(27.6\) & \(0.2\) & \(9.8\) & \(17.1\) & \(18\) & 3.6 \\ \hline \hline \end{tabular}
\end{table}
Table 16: Experimental, present GC model, and conformationally averaged G4 model values for the heat of formation for furans. All values in kJ/mol. All experimental data and errors in experimental results (last column) are from Pedley c.s. [34] and all G4 results from the present study. Substituted furan sue furan as a Group in the GC model, i.e., value \(-34.9\) kJ/mol.  
(and similar for 1-methylindoline) we made the attempt to take the indole Group, subtract the contribution for R\({}_{2}\)NH, and subsequently add the parameter value for the R\({}_{3}\)N Group. In addition, we added the additional interaction parameter of a magnitude of \(-7\) kJ/mol for the NH\({}_{2}\)-benzene pair which we adopted from the aniline series discussed earlier. Although there was no guarantee that this would work, we obtained a very good result for 1-methylindole, very much within chemical accuracy (1.2 kJ/mol). For 1-methylindoline, the deviation is as high as 11.1 kJ/mol. It remains open whether this is a real problem, as the G4 result lies more or less precisely in between the experimental and the GC model value. Thus, taking into account the good performance of the G4 method, if the G4 value were the correct one, the GC model value would almost be within chemical accuracy. More data are needed to put this on more solid ground.