Last year James Garvey and his colleagues at the University of Buffalo presented some alarming findings about air pollution: the sum of certain emissions is more dangerous than its parts. Indeed, the group simulated how certain tiny gas-phase clusters react in the upper atmosphere, and found that even fairly benign reagents could lead to dangerous products. In particular, when they mixed nitric oxide¿one of the most common industrial emissions and a component of acid rain¿with methanol, the unexpected combination was nitrous acid and methyl nitrate, a known carcinogen. Now they have more bad news about nitric oxide, which appears in this week's issue of the Journal of the American Chemical Society.

This time, Garvey and his co-workers examined the pairing of nitric oxide with another alcohol, ethanol, and found that the molecules were highly reactive. To simulate the effects of sunlight in the upper atmosphere, they used laser light to generate mixed gas-phase clusters of the two ingredients. "It turns out nitric acid is insidious because it engages in more than simple bimolecular reactions," Garvey said. "It has its own unique chemistry inside of gas-phase clusters, and that may be something environmental regulators will need to take into account."

In addition, they discovered that the reactive site between a positively charged nitric oxide ion and an alcohol will vary depending on how many solvent molecules surround them. This fundamental finding may help scientists better tailor certain reactions. "Chemists want to be able to control where the chemistry occurs," Garvey added. "These results provide us with a new way to do just that. It turns out that subtle changes in the solvent 'cage' around the reactants bring about huge changes in how and where the reaction occurs. As the number of solvent molecules increases, we saw a direct change in where the reaction was occurring on the ethanol."

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