Estudo computacional dos mecanismos de formação do púrpura de ruhemann

Abstract

Ninhydrin is often used to identify latent fingerprints (IDL) in crime scenes using Ruhemann's Purple pigment (RR) formation. This reaction occurs due to the several amino acids released by the eccrine gland whose excretory channel is directly into the pores of the skin. Ninhydrin was applied as IDL developer agent for the first time in 1954, since then it has become a popular reagent for the detection of fingerprints on porous surfaces. The literature reports three different proposals for mechanisms and there is no consensus about the preferential reaction route. The only computational work that addresses attention the mechanisms does not consider the importance of the thermal corrections that are indispensable to support its conclusions. In this sense, this work makes a computational effort in order to describe the thermodynamics of the mechanisms of McCaldin, Lamothe and Friedman to provide a better understanding of the reaction route. The values of total energy variation (ΔE) and the thermodynamic parameters ΔrG, ΔrH and ΔrS of the steps present in the McCaldin, Lamothe and Friedman mechanisms were calculated with B3LYP hybrid functional and base function 6-31++G(d,p) and compared to the ΔE values obtained in the literature at ab initio HF/6-31G* calculation level. It is important to mention that both HF and B3LYP results of the present work agree in some steps and disagree in others, which can be attributed to the use of a correlated B3LYP method and larger basis function 6-31++G(d,p). In this sense, the results obtained in this work provide greater reliability because of the more complete descriptions of the organic molecules present in the mechanisms. The calculation indicates that RP formation step, the dehydration of an amine intermediate known as Strecker amine will react with another ninhydrin molecule to form the desired product, the step being more energetic. We also note that the amine intermediate can be consumed to form another by-product, present in the mechanisms of McCaldin, Lamothe, and Friedman. In all three cases, the increase in temperature favors the path of RR formation. The mechanisms were endothermic (+34.41 kJ/mol) and exergonic (-94.69 kJ/mol) at the calculation levels used in this work, in agreement with the experimental observations