Nuclear Spin Conversion in Molecules

Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1913-1914
DOI: 10.1126/science.1122110

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In molecules with identical atoms, such as H, the nuclear spins can take on different arrangements called nuclear spin modifications. For hydrogen, the spin modifications are ortho (with spins aligned parallel) and para (spins aligned antiparallel). In their Perspective, Hougen and Oka discuss results reported in the same issue by Sun et al. in which conversion among different spin modifications of ethylene is studied. Laser light is used to separate different spin modification to create a gas depleted in one modification, and then the nuclear interconversion is observed over time. Because ethylene has four nuclear spin modifications, the subtle influences of molecular symmetry can be examined in greater detail than previously possible.