Thermal stability of the ionic liquids is of great importance for their industrial applications. In this work density functional theory calculations have been used to evaluate thermal decomposition mechanisms and activation energies of N7,N9-dimethyladeninium cation and amino acid anions ([dMA]+[AA]- (AA=Gly, Ala,Val, Leu, Ileu). The relative thermal stability of these ionic liquids and different aspects of the corresponding transition states of thermal decomposition were considered to determine the role of alkyl side chain length of the amino acid anions on the energy barrier. The results showed that [dMA][Gly] pyrolysis has the highest activation Gibbs free energy while [dMA][Ala] has the lowest one. Increase in the alkyl side chain in Val, Leu and Ileu elevates the calculated activation Gibbs free energies. Finally, several correlations between the Gibbs free energy of activation and interaction energy, E(2),at the bond critical points of C-O and N-Cbond as well as electron density at the transition states have been obtained by the quantum theory of atoms in molecules and natural bond orbital methods.

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