Controlled Conversion

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Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1865e
DOI: 10.1126/science.310.5756.1865e

In the absence of a magnetic field, the two nuclear spin states of an isolated hydrogen atom are completely equivalent. However, in molecules with more than one hydrogen atom, the spins interact with one another, and the total energy changes slightly with their relative orientations. In low-pressure conditions, such as interstellar space, interconversion of such isomers is poorly understood. Sun et al. (p. 1938; see the Perspective by Hougen and Oka) have used the differential absorption of infrared light by the four nuclear spin isomers of ethylene (C2H4) to produce a nonequilibrium population, depleted in one isomer. By monitoring the evolution of this gaseous sample, they find that the isomers of similar inversion symmetry can interconvert efficiently, but do not transform to isomers of opposite symmetry.

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