Weighing Down Pyridine

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Science  12 Dec 2008:
Vol. 322, Issue 5908, pp. 1611
DOI: 10.1126/science.322.5908.1611b

A common technique for unraveling the mechanism of complex organic reactions is to substitute the hydrogen atoms on different carbon centers with heavier deuterium isotopes. The mass shift can influence reaction rates through changes in the molecule's vibrational structure, thereby revealing key sites of reactivity. In general, though, isotopic substitution tends not to perturb the overall outcome of chemical or physical transformations. Crawford et al. uncover an intriguing exception, in which deuteration of the five carbons in pyridine leads to a distinct crystalline arrangement at low temperature. Through a careful series of x-ray and neutron diffraction studies on single-crystal and powder samples, the authors established that below 215 K, the deuterated isotopologue crystallizes in a layered phase along the (110) planes; above this temperature, the solid rearranges to align along the (100) planes, as observed across the whole temperature range for protio pyridine. Although such isotopic polymorphism has previously been observed in a number of polyatomic salts, the behavior is rare in neutral organics. — JSY

Angew. Chem. Int. Ed. 47, 10.1002/anie.200803589 (2008).

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