Credit: © 2008 AAAS

The conformation of a molecule has a strong bearing on its reactivity, so a fundamental understanding of the mechanisms and rates at which molecules can change shape is useful. Rotational (microwave) spectroscopy can be used to elucidate molecular shape but so far it has not been advanced enough to study changes in molecular shape caused by isomerization. Now, Brooks Pate and co-workers at the University of Virginia have developed1 a microwave spectroscopy method that can measure a much larger frequency range (11 GHz) in the rotational spectrum of a molecule than previous methods.

The technique, chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy, reduces sample acquisition time and enables the investigation of picosecond-scale conformational isomerization reactions. The improvements have been exploited to look at the isomerization dynamics of cyclopropane carboxaldehyde (CPCA) — a compound that can adopt two stable stereoisomeric conformations (syn and anti) that can interconvert through the rotation of a single carbon–carbon bond. A single conformer of CPCA first had to be vibrationally excited by a laser, causing the energized molecules to rapidly alternate between the syn and anti conformers. As they slowly relaxed to a distribution of the two, an 'averaged' rotational spectrum was recorded using CP-FTMW. Even though only one conformer had been excited, information on both conformers was obtained because of the large frequency range measured by the new technique. From this, Pate and co-workers were able to elucidate isomerization rates, finding that the rate is 16 times slower than predicted by theory.

This study certainly shows the need for further measurements on similar systems to aid the currently insufficient theories on such isomerization reactions. Owing to its speed, this technique also has potential for other analytical applications.