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Science  03 Feb 2006:
Vol. 311, Issue 5761, pp. 581
DOI: 10.1126/science.311.5761.581a

Terahertz (Thz) radiation, which bridges the infrared and microwave regions of the electromagnetic spectrum, can penetrate most clothing and packaging materials. Researchers have therefore sought to develop THz spectroscopy for security screening, which would require a precise understanding of the absorption spectra that would signal the presence of drugs or explosives. However, the spectra are hard to analyze because they comprise many overlapping modes, arising both from intramolecular vibrations and delocalized lattice motion.

One approach has been to model the individual molecules computationally, as though they were in the gas phase, in order to discern which spectral features correspond to intramolecular modes, but Allis et al. uncover a problem with this method. Using several variants of density functional theory, they simulate the THz absorption spectrum of crystalline HMX explosive, a solid composed of eight-membered rings with alternating CH2 and N(NO2) groups. Modeling of the isolated molecule fails to reproduce any of the experimental absorption features, whereas more computationally demanding methods, which treat the extended solid lattice, yield reasonable agreement with the measured spectrum. The results suggest that packing forces in the lattice shift the orientation of NO2 substituents and thereby affect intramolecular mode frequencies in addition to lattice modes. — JSY

J. Phys. Chem. A 10.1021/jp0554285 (2006).

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