## 3 Results
### Alkyl Esters
Whereas for the linear (non-branched) alkanoic acid methyl esters we should always have the same set of Groups constituting the molecule, the Marrero-Gani method turned out to have a third-order contribution CH\({}_{3}\)-(CH\({}_{2}\))m-CH\({}_{2}\)COO in some cases, CH\({}_{3}\)-CHm-COO in some others, and yet still others in which both contributions are taken into account. As these are the only differences within the series, this must be related to the irregularly varying dHf, even though one should simply have an approximately 20 kJ/mol difference upon one more (or less) CH\({}_{2}\) group. Table 7 shows that for the first 50% of the entries, we find excellent agreement between the experimental value and our GC model. The four available G4 results are also in very good agreement with both the experimental and the GC model values. But going down the table, we find somewhat unexpected results. For the first 50% of the entries in Table 7, we note that the increment per CH\({}_{2}\) group is close to 20 kJ/mol, which is in accordance with all other alkyl like systems, and also the same for the different GC models. However, when we look at the sequence pentanoic acid ethyl ester-pentanoic acid butyl ester-pentanoic acid butyl ester, the increment is about 27 kJ/mol which cannot be understood from any known and common sense physico-chemical argument. The same increment, 27 kJ/mol, is found for the pair butanoic acid 1-methylpropyl ester and pentanoic acid 1-methylpropyl ester (last two entries in Table 7). There is, as we think, only one possible explanation, which is that long-range interactions cause this effect at the side of the ester group (and not at the opposite side; see the first nine entries in Table 7). We can have an initial verification by using quantum calculations on relative stabilities. We compare three molecules with identical chemical formulae but different alkyl length at the ester side. For pentanoic acid ethyl ester, butanoic acid propyl ester, and propanoic acid butyl ester, both the B3 LYP and \(\omega\)B97X-D yield marginal difference in the total energies, namely, within 3 kJ/mol, which cannot account for the much larger differences we see in Table 7.  
Next, when we consider pentanoic acid methyl ester (one of these nine), 2-methylbutanoic acid methyl ester, and 3-methylbutanoic acid methyl ester (the three entries in green in Table 7), where all species have the same chemical formulae, the results from our GC model suggest that these three have close dHf values with a range of 7 kJ/mol. The experimental values show a range of 26 kJ/mol, with the difference between our GC model and experimental values for the methylbutanoic methyl esters being 14.8 and 18.1 kJ/mol. As it was not clear why this difference is so large, we performed quantum calculations of the B3 LYP//6-311+G** and \(\omega\)B97X-D//6-311+G** methods. Both methods reveal differences between the three structures of several kJ/mol only, which does not support the large differences between the experimental values. The quantum calculations also reveal that, in accordance with the experimental and GC values, 3-methylbutanoic acid methyl ester is the most stable one of the three structures. Whereas steric effects due to methyl substitution next to the carboxylic group as in 2-methylbutanoic acid ethyl ester and 2-methylbutanoic acid methyl ester may be an explanation for the differences between experimental and GC model values, the fact that for 2,2-dimethylpropanoic acid methyl ester we have pretty good agreement between the experimental and our GC model values (4.7 kJ/mol, just beyond chemical accuracy) means that steric effects do not seem to be involved. A similar reasoning applies to the three ethyl esters (the three entries in blue in Table 7).