Estudio teórico de la estabilidad relativa de alcanos ramificados

Abstract

In this paper, we study a relatively new concept on the intramolecular interactions between the fragments methyl, methylene, methine and quaternary carbon and its possible quantification in organic molecules linear and branched; the protobranching. This concept suggests that the relative stability of the alkanes, is determined by the number of interactions 1,3 -alkyl-alkyl mentioned between the fragments of each molecule. On the basis that the branched alkanes are more stable than the linear alkanes, the validity of the concept of protobranching has been analyzed in order to explain the relative stability of linear and branched alkanes using isodesmic reactions, verifying that there is a very good correlation between the enthalpies of reaction and the number of interactions 1,3 present in the alkane, so that the alkane is more stable as more of such interactions are present. Calculations have been made of the enthalpies of reaction with ab initio methods, including correlation and electronic energies of dispersion. Calculations have been made with the methods HF, B3LYP, B3LYP-D3, PBE and MP2. To quantify the interactions that give rise to protobranching model, two new magnitudes have been used regarding the previous articles: the DI and the DECs. Using the DIs and DECs between intramolecular fragments, it has been observed that when the contributions associated with interactions at a greater distance (1,4 and higher) are included, it significantly improves the correlation. These results indicate that the concept of protobranching contributions can be extended to 1,4 and higher, only when these can be quantified. However, a bilinear analysis has shown that the best correlations are obtained by distinguishing between different interactions of type 1,3 , in this case those that involve fragments methyl and those that do not. Through the interpretation of the Dis, it has been observed that they are virtually independent of the calculation method used. In contrast, the results obtained with the DECs depend on the method, observing a systematic improvement of the results to HF level if including the electronic correlation (PBE and B3LYP) and the corrections of dispersion (B3LYP-D3)