The increasing accuracy of the electronic structure methods is allowing to tackle a great variety of chemical problems. In our research group we are contributing to the development of an integrated computational code able to simulate multiple aspects of chemical reactions dynamics. The methods developed can be applied to: organometallic catalysis, gas-phase mass spectrometry, the simulation of microwave spectra, the calculation of thermodynamic properties and to the study of many chemical reactions occurring in the gas phase. At the moment we are concentrating our efforts, although not exclusively, in applying the methods to:
– Combustion chemistry
– Chemistry at ultra-low temperatures
Emission reduction of noxious chemicals that are produced during the combustion of fossil fuels has become a world priority. One of the most promising substitutes are biofuels (fuels obtained from biomass). The study of the decomposition, oxidation, hydrogen abstraction and isomerization reactions of different biofuel candidates (for instance, alcohols, ethers and esters) is an important step to understand the combustion process as a whole.
However, the computer simulation of the combustion, which involves several hundreds of elementary reactions is a challenging task from several points of view. An example of how we handle this type of reactions is the decomposition reaction of the 1-propanol radicals (J. Phys. Chem. A, 2018, 122, 4790-4800). The flowchart below shows the process we adopted to design the theoretical kinetics mechanism for combustion:
Chemistry at ultra-low temperatures
While in combustion reactions, factors as the conformational flexibility or torsional anharmonicity, play an important role in the reaction mechanism, at cryogenic temperatures the reaction processes are determined by quantum effects. In these conditions, reactions with an activation barrier can only occur by quantum mechanical tunneling (QMT), effect that allows particles to penetrate through classically forbidden regions of the potential barrier. The simulation of reactions occurring at very-low temperature require a fine tune of the potential energy surface, since the reaction occurs from the zero-point energy level of reactants. The combination of semiclassical methods to treat QMT and variational transition state theory (VTST) is able to mimic the reaction times of hydrogen transfer reactions. An example is the isomerization reaction that takes place in urea derivatives (selenourea and thiourea) in which the enol form is obtained by UV excitation and tautomerizes to the keto form (see figure below, where blue, brown, gray and white are the N, S, C and H atoms respectively). The reaction also involves subtle differences in stability between the enol conformers (Phys. Chem. Chem. Phys, 2020, 22, 24951-24963).
We are also studying bimolecular reactions in which participates a small radical (for instance, H or OH) and a small neutral molecule (“Kinetics of the Methanol Reaction with OH at Interstellar, Atmospheric, and Combustion Temperatures”, J. Am. Chem. Soc., 2018, 140, 8, 2906-2918).
For some of these reactions (mainly the ones occurring with the radical OH) the thermal rate constants at ultra-low temperatures may be much larger than the ones at room temperature, which may be the reason for the formation of some of molecules in the interstellar space.