Investigation of Molecule-Surface Interactions With Overtone Absorption Spectroscopy and Computational Methods
Final rept. Jan 2009-Jan 2010
EDGEWOOD CHEMICAL BIOLOGICAL CENTER ABERDEEN PROVING GROUND MD
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The objective of this study is to determine the optimal methodology for the computational modeling of a molecule surface interaction. As a starting case, we use neat l ,3.5-trinitrotoluenc TNT where the fundamental and first overtone transition of the C-H stretches are measured with standard FTIR measurements, as well as laser photoacoustic spcctroscopy and compared with spectra that are predicted from computational models. Using the Harmonically Coupled Anharmonic Oscillator HCAO model for the C-H stretches, spectra have been predicted for isolated TNT molecules, X member TNT clusters, and 64 member clusters, where a single TNT molecule is treated quantum mechanically and the rest of the cluster is treated with a classical force field. The standard frequency analysis did not yield predicted spectra that could be compared with experiment. The spectrum predicted by the single TNT molecule model agreed with the experimental data the best whereas, the large cluster model agreed with the data the least. The disagreement from a cluster arises either from an overestimation of nonbonding interactions, poor selection of the boundary between quantum and classical model layers, not averaging predicted spectra over several molecules in the cluster models, or the use of a harmonic oscillator basis set in the HCAO intensity calculations.
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