Computational Chemistry for Industry and Defense
Published in Interface. Vol. 2, No. 1, Feb 1994
Dr. Koop Lammertsma, University of Alabama at Birmingham
Introduction. Accurate modeling of molecular structures and chemical reactions requires detailed ab initio quantum mechanical theories. Accurate geometries, vibrational frequencies, NMR chemical shifts, binding energies, acid and base strengths, heats of formation, and other observables can be calculated reliably with ab initio theory. Its strength lies, however, in exploring the chemistry of molecules that cannot be observed directly as well as in the characterization of reaction paths. For small molecular systems, the accuracy of sophisticated calculations lies well within experimental error. It is then no surprise that ab initio quantum mechanical calculations are increasingly used to predict chemical properties and processes. With the assistance of the Alabama Supercomputer Authority and codes such as Gaussian, we have concentrated our efforts on areas where the technology gained from computational modeling can be applied to industrial and defense applications. Three topics are highlighted.
Carbodications. Carbodications are doubly-charged hydrocarbons and can be viewed both as doubly protonated and as dioxidized compounds. Initially, such systems were considered merely of academic interest because of their anticipated instability. However, in spite of expected electrostatic repulsion in these doubly charged systems, dications were found to display an "unexpected" kinetic stability and to have unconventional structures. Theoretical calculations proved essential in shedding light on their chemistry because only larger dications are observable in superacids, and structural details cannot be obtained from gas phase studies.
The persistent theoretical calculations in this area have stimulated closer scrutiny of dications under experimental conditions. Recent work, spearheaded by Professor Olah at the University of Southern California, has shown that even the t-butyl cation can be protonated to a dication in superacids. There is now rapidly mounting evidence that many common acid-catalyzed electrophilic processes like alkylations, hydrocracking, acetylations, formylations, nitrations, etc. are in fact governed by dication reaction pathways. The theoretical calculations in this area are unraveling these industrially important liquid and solid acid-catalyzed processes.
Materials Science. The manufacturing of ever smaller devices and materials for electronics, catalysis, ceramics, optics, and coatings requires an understanding of quantum mechanical effects at the atomic level. In recent years, we have concentrated on providing a basic understanding of the chemical properties of binary clusters, which are small atomic aggregates containing two elements as in the semiconductor SiC and GaAs bulk materials. Our exploratory ab initio calculations in this materials science area revealed the existence of nearly indistinguishable rhombic structures with vastly different electronic properties for a large variety of elemental combinations. Calculations on small clusters of the light elements Li, Be, and B uncovered evidence of metallic bonding. It was found that cluster build-ups can be predicted from the topologies (graphs) of their electron densities.
High energy density compounds. The increased demand for high energy compounds for aerospace, defense, and commercial applications directs the search for new solid rocket propellants and insensitive high energy explosives. Ab initio calculations provide valuable insights in this heretofore largely trial-and-error endeavor. Theory predicts that high energy density materials can be composed of light elements with high specific impulse, which may be useful as propellant additives.
We have concentrated on elucidating the still unresolved detonation mechanism of the conventional explosive TNT (trinitrotoluene). Such detailed insight may enable us to design new high energy explosives with low impact sensitivities in addition to the currently popular NTO (3-nitro-1,2,4-triazole-5-one), RDX, and MDX explosives.
Conclusion. Even for small molecular systems, the calculations described above can be extremely demanding. However, the rapid advancement of compute power in the past decade has made computational chemistry a common tool in the laboratory. Molecular modeling now complements and even directs experimental approaches.
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