If you want to understand unstable molecules, one way is to systematically create slightly different versions of a similar stable molecule and then investigate each new molecule with identical analysis and experiments. That is exactly what researchers from JILA and CU are doing with a series of ringed molecules. JILA researchers are Graduate Student Adam Gianola, Postdoctoral Research Associate Takatoshi Ichino, and Fellow Carl Lineberger. The CU collaborators are Lecturer Rebecca Hoenigman, Senior Research Associate Shuji Kato, and Professor Veronica Bierbaum.

The investigators began their work with a ring of 5 carbons (C), each with hydrogens (H) attached to it (C₅H₆). Then, they evaluated a ring with a nitrogen (N) atom substituted for one of the C-H groups in the ring, creating pyrrole (C₄H₄N), as shown below. Next, they substituted a second N atom into the ring, creating imidazole (C₃H₃N₂), also shown below. By substituting additional N atoms into the ring in a stepwise fashion, they eventually hope to study a 5-member ring made of all N atoms, pentazole (N₅H). This latter structure's properties are so difficult to measure that no one has ever succeeded in measuring them (so far).

Gianola and his colleagues are using a two-step process to investigate their series of ringed molecules. First they use hydroxide (OH^-) to remove the proton (H⁺) from the N atom, which leaves the ringed molecules as negative ions. Second, they bombard the negative ions with a laser photon beam, which removes the extra electrons from the ions, returning the molecules to a neutral-charge state. The experimenters collect the photodetached electrons and measure their energy distribution. These measurements provide information about the resulting neutral molecules, which are unstable.

The researchers also measure the energy changes that occur when the proton is removed in step 1. It is not only possible to remove a proton from the N atom, but also from a C atom. The energy distribution of electrons removed in step 2 allows the researchers to precisely determine which protons were removed.

Thus far, it appears that as you add more nitrogen atoms to the ring, it gets easier to remove a proton from one of them in step 1 but harder to remove the extra electron in step 2. The researchers are currently investigating a ringed molecule with three N atoms. They are working with Postdoctoral Research Associate Jeff Rathbone and Graduate Students Django Andrews and Ryan Calvi. —Julie Phillips

Reference:
Gianola, A. J., Ichino, T., Hoenigman, R. L., Kato, S., Bierbaum, V. M., and Lineberger, W. C., Photoelectron spectra and ion chemistry of imidazolide, Journal of Physical Chemistry A, 109 (50) 11504-11514 (2005).

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