Spinning in Place

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Science  03 Oct 2008:
Vol. 322, Issue 5898, pp. 16
DOI: 10.1126/science.322.5898.16a

Unlike macroscopic objects, molecules vibrate and rotate in discrete increments. To uncover the underlying quantum-mechanical restrictions governing such behavior, spectroscopists induce specific patterns of motion through light absorption. Thus, the molecules under study must be free to move about, but unless they are to some degree restricted, the flurry of different movements can be hard to disentangle. A promising compromise is the use of para hydrogen (p-H2) matrices. When p-H2 (H2 with oppositely oriented nuclear spins) is cooled to low temperature, it forms an unusual medium, termed a quantum solid, in which the nuclei delocalize in space. Consequently, guest molecules embedded in a matrix of this solid retain a certain amount of flexibility. Lee et al. show through infrared absorption spectroscopy that CH3F molecules can rotate about the C-F axis in such a matrix, but are restricted from tumbling in orthogonal directions. The study bolsters the utility of p-H2 matrices for precise spectral characterization of small molecules. — JSY

J. Chem. Phys. 129, 104502 (2008).

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