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Páxinas persoais  »  Emilio Martínez Nuñez

Automated methods to discover reaction mechanisms

Over the last few years we have been involved in the development of automated methods to discover complex reaction mechanisms. The methods are based on the use of accelerated dynamics simulations and Graph Theory. The procedure, termed tsscds, has been applied to different systems and a computer program is available: tsscds-2018.


The cobalt-catalyzed hydroformylation of ethylene has been studied with our procedure, explaining not only the major mechanism, but also side  reactions like hydrogenation. (Chem. Sci. 2017, 8, 3843)


The method has been also employed to elucidate the different HCN and HNC elimination channels from methyl cyanoformate (MCF). The simulated vibrational energy distributions agree very well with those measured experimentally, as can be observed below. MCF was postulated as a possible source of HCN/HNC in the inter-stellar medium (ApJ, 2017, 849,15).

 


The potential energy surface of protonated uracil has been explored using our automated method, resulting in the finding of 1398 stationary points (see some of them on the left) and 751 reactive channels. The KMC/RRKM product abundances show that the major fragmentation channels are isocyanic acid and ammonia losses, in good agreement with experiments. The third predominant channel is water loss according to both theory and experiments (PCCP, 2016, 18, 14980). 

 

Theoretical determination of rate coefficients at low temperatures

The kinetics of the reaction of methanol with hydroxyl radicals has been studied at very low temperatures. The rate constants exhibit an approximately 102-fold increase when the temperature decreases from 200 to 50. Our calculations show that methanol dimers are much more reactive to hydroxyl than monomers and imply that a dimer concentration of about 30% of the equilibrium concentration can account quantitatively for the observed rates (PCCP, 2016, 18, 22712).

 

 Energy transfer models in gas-surface reactions

An energy transfer model is applied to gas-surface reactions (see equation below). The model relies on simple gas-phase scattering models. When energy transfer is analyzed in the limit of high incident energies, the following results are found in this study. The percent of energy transfer to vibration (and rotation) of light diatomic projectiles decreases as the projectile’s mass increases, while this transfer is almost independent of the mass for heavier projectiles. For small projectiles (less than 10 atoms), transfer to vibration increases as a function of the projectile’s size. However, for larger projectiles, the percent transfer to vibration is nearly constant, a result that can be attributed to a mass effect and also to the fact that only a reduced subset of “effective” vibrational dof is being activated in the collisions (JPC C2014118, 2609).