Computer-modeling studies hydrocarbons

Carter, W. P. L and R. Atkinson, Computer Modeling Study of Incremental Hydrocarbon Reactivity, Enciron. Sci. Technoi, 23, 864-880 (1989). 

Hough, A. M., and R. G. Derwent, Computer Modeling Studies of the Distribution of Photochemical Ozone Production between Different Hydrocarbons, Atmos. Environ., 21, 2015-2033 (1987). 

Carter, W. P. L. and R. Atkinson A computer modeling study of incremental hydrocarbon reactivity, Environ. Soi. Technol.,23,864(1989)... 

Due to their demanding synthesis, diamondoids are helpful models to study structure-activity relationships in carbocations and radicals, to develop empirical computational methods for hydrocarbons, and to investigate orientational disorders in molecular crystals as well [5,32]. 

The overwhelming majority of the theoretical studies were performed on cluster models of the catalytic site, hi spite of the fact that the role of space confinement and the secondary interactions with the framework atoms is well-known, there are only a few electronic structure calculations on lattice models involving hydrocarbons, using either periodic DFT calculations, or embedding methods. In this brief account of the subject we attempt to overview some of the recent computational results of the literature and present some new data obtained from ab initio DFT pseudopotential plane wave calculations on Cl - C4 alkanes in the chabazite framework. 

Lastly, the single-events theory, which was historically designed for the activation of carbon-carbon bonds, does not currently cover the reactivity of C-S and C-N bonds. Some computer models have been produced to represent the possible presence of hetero atoms in hydrocarbon structures. Avenues are therefore open for a very wide field of application, that of HDT reactions and sulphide catalysis. They must nevertheless be based on an in-depth, improved description of the heterolytic mechanisms, also studied (Blanchin et al., 2001) under IFP supervision. 

Lii J-H and N L Allinger 1989. Molecular Mechanics. The MM3 Force Field for Hydrocarbons 2 Vibrational Frequencies and Thermod5mamics Journal of the American Chemical Society 111-8566-8582 London F 1930 Zur Theori und Systematik der Molekularkrafte Zeiischrift fur Physik 63 245-279 Luckhurst G R, R A Stephens and R W Phippen 1990 Computer Simulation Studies of Anisotropic Systems XIX Mesophases Formed by the Gay-Beme Model Mesogen Liquid Crystals 8-451-464 Luque F J, F lias and M Orozco 1990 Comparabve Study of the Molecular Electrostatic Potential Obtained from Different Wavefunctions - Reliability of the Semi-Empirical MNDO Wavefunction Journal of Computational Chemistry 11-416-430. 

For the modeling of the FT reaction, by which CO with H2 is converted to longer hydrocarbons, several mechanistic issues have to be resolved. While there are many outstanding experimental and computational studies available that directly probe details of the reaction mechanism, detailed understanding of many of its aspects is still lacking. This is very relevant to any kinetics modeling study since it requires a mechanistic scheme to set up a kinetics model. In a predictive approach where the aim is not to make predictive statements on conversion and selectivity based on kinetics parameters obtained by fitting to experimental data, such parameters will have to be based on molecular reactivity data of surface intermediates. 

Tarek et al. [388] studied a system with some similarities to the work of Bocker et al. described earlier—a monolayer of n-tetradecyltrimethylammonium bromide. They also used explicit representations of the water molecules in a slab orientation, with the mono-layer on either side, in a molecular dynamics simulation. Their goal was to model more disordered, liquid states, so they chose two larger molecular areas, 0.45 and 0.67 nm molecule Density profiles normal to the interface were calculated and compared to neutron reflectivity data, with good agreement reported. The hydrocarbon chains were seen as highly disordered, and the diffusion was seen at both areas, with a factor of about 2.5 increase from the smaller molecular area to the larger area. They report no evidence of a tendency for the chains to aggregate into ordered islands, so perhaps this work can be seen as a realistic computer simulation depiction of a monolayer in an LE state. 

The modification of theoretical gas-phase reaction techniques to study gas-surface reactions continues to hold promise. In particular, the LEPS formalism appears to capture a sufficient amount of realistic bonding characteristics that it will continue to be used to model gas-surface reactions. One computational drawback of the LEPS-style potentials is the need to diagonalize a matrix at each timestep in the numerical integration of the classical equations of motion. The size of the matrix increases dramatically as the number of atoms increases. Many reactions of more direct practical interest, such as the decomposition of hydrocarbons on metal surfaces, are still too complicated to be realistically modeled at the present time. This situation will certainly change in the near future as advances in both dynamics techniques and potential energy surfaces continue. 

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