Estudio de propiedades termoquímicas de compuestos nitratos de peroxiacilo (PANs, RC(O)OONO2 ) de relevancia atmosférica.

Abstract

Peroxyacyl nitrates (PANs) are considered unstable secondary pollutants, a product of the reaction of peroxy alkyl radicals (from the oxidative degradation of aldehydes and ketones) and nitrogen dioxide via photolysis. The first compound was detected and named substance "X" in 1953 using a long-range IR spectrophotometer. Until 1960, by means of the gas chromatography technique it was named as peroxyacetyl nitrate (PAN), being this the most abundant of the PAN family found worldwide both in the troposphere and in polluted urban regions. PAN has been extensively studied at the environmental and laboratory level. Since the 1970s, various investigations have been carried out using techniques such as long-range infrared (IR) spectroscopy, gas chromatography with electron capture detection, chemiluminescent detection, mass spectrometry, nuclear magnetic resonance and other methods to detect derivative compounds. PAN such as propionyl peroxynitrate (PPN), isobutyryl peroxynitrate (PiBN), methacryloyl peroxynitrate (MPAN), benzoyl peroxynitrate (PBzN), and peroxycrotonyl nitrate (CPAN). This family of compounds is characterized by being stable at low temperatures and unstable at high temperatures, providing large amounts of NOX that contribute to the photochemical production of O3. They are considered transporters of nitrogen oxides (NOX) over long distances, markers for the identification of volatile organic compounds (VOCs), powerful respiratory and eye irritants, and harmful agents in vegetation. Considering the importance of this family in the atmosphere and the scarcity of physicochemical data, it is of great interest to carry out studies through computational chemistry and high precision chemical theories to estimate structural, vibrational and thermodynamic data that will help to know the chemical properties of this family of PANs. In this work, 8 compounds belonging to the peroxyacyl nitrate family were studied: methoxyiformyl peroxynitrate (MoPAN), acryloyl peroxynitrate (APAN), (PPN), n-butyryl peroxynitrate (PnBN), (CPAN), (PiBN ), (MPAN) and peroxy-n-valeryl nitrate (PnVN). Our calculations used density functional theory (DFT) through the hybrid functionals, B3LYP and M06-2X in combination with the base set 6-311 ++ G (3df, 3pd).Our results allowed us to estimate geometric parameters that show us very flexible structures that have a stable conformation in their cis isomerism in relation to the carbonyl group and the nitro group (NO2) of these compounds, the estimated vibrational frequencies allowed us to characterize the bands of the group peroxyacyl nitrate -C (O) OONO2 which appear in the average frequencies for the C = O band (1875 cm-1), two bands for the nitro group NO2 symmetric (1353 cm-1) NO2 asymmetric (1803 cm-1) , a band for OO (982 cm-1) and a band for CO (1076 cm-1) that are in agreement with that reported in the literature. Molecular orbitals were explored and allowed us to calculate the binding orders of the molecules studied, focusing mainly on the -C (O) OONO2 group and the  systems of the PANs derivatives. Regarding the estimated thermochemical parameters, the enthalpy of formation values ​​increase with molecular complexity in a range from -32.8 kcal / mol to -100.7 kcal / mol and two of our molecules are in accordance with the data reported by other researchers while the rest of the values ​​reported here can be taken into account as reference values.