Observation of the rate of transition of a molecule from reactant to product has provided the first experimental evidence for a critical prediction of one chemistry's most fundamental theories. Results of the experiment will help chemists in their on-going effort to understand and predict chemical reaction rates and products.
It is no exaggeration to say that a society's standard of living is largely dependent upon its knowledge of chemical reactions. For more than fifty years, much of our knowledge of chemical reactions has been derived from transition state theory.
Says Bradley Moore, a chemist with LBL's Chemical Sciences Division and professor of chemistry with UC Berkeley, "The theory tells us how the movement of atoms in a molecule during a reaction determines the outcome of the reaction."
Despite the heavy reliance on transition state theory, few of the theory's predictions have been tested quantitatively because, until recently, chemists lacked the tools to do so.
A team of LBL-UCB chemists led by Moore and including Edward Lovejoy and Sang Kyu Kim has now confirmed one key prediction -- that the rate of a reaction is proportional to the number of different ways a molecule can vibrate at the transition state.
"The reaction rate increases in steps as the energy increases through the sequence of transition state vibrational quantum levels," says Moore. "Each of these quantum energy levels is a threshold which separates reactant from product."
Using a technique called photofragment spectroscopy, the team was able, for the first time, to directly observe transition state quantum energy levels in a unimolecular reaction -- one involving only one molecule as the reactant -- and deduce the molecular motions associated with them.
The chemists conducted their tests on ketene (CH2CO), a reactive gas that absorbs energy from ultraviolet photons and separates into CH2 and CO.
Says Kim, "To see the transition state energy levels, you have to be able to add energy to a molecule in small, precisely defined increments. This can't be done with many molecules, but it can be done with ketene."
The chemists began their study by cooling a sample of ketene molecules in a supersonic jet expansion to near absolute zero temperatures in order to ensure that all of the molecules started out at the same energy. Ultraviolet laser light was then flashed on the molecules to stimulate a reaction.
"A cold ketene molecule absorbing energy from ultraviolet photons becomes highly vibrationally excited," says Lovejoy. "The excited molecules pass into the transition state by localizing energy in the C-C bond. The CH2 and the CO components of ketene then begin to separate, forming the products of the unimolecular reaction."
The excitation energies pumped into the molecules corresponded to the energy levels predicted in transition state theory. Reaction rates were measured by monitoring the appearance of CO fragments at each energy level with vacuum ultraviolet laser-induced fluorescence. A photofragmentation spectrum of the CO was also taken.
Says Lovejoy, "Both experiments illustrated clearly that the rate of the reaction increased in a stepwise manner with increasing energy, which is consistent with the predictions of transition state theory."
Data from this experiment also verified some, but not all, of the theoretical predictions made for the intermediate structures assumed by ketene in the transition state.
"The ab initio quantum chemistry calculations predicting a strongly bent C-C-O framework (about 116 degrees versus a 180 degree linear structure for ground state energy) and the C-C-O bending frequency were dead-on," says Moore. "The threshold energy predicted for carbon bond breaking was also very good."
These quantum chemistry calculations were performed by former LBL researchers Henry Schaefer and Wesley Allen while they were at the Lab.
In addition to the bending and bond-breaking motions, ab initio calculations also predict a low frequency torsional motion. This too was observed but the predictions were not so accurate.
Says Moore, "Future experiments will focus on improving the accuracy of these predictions by obtaining a better quantitative understanding of the phenomenon, and on probing some of the more subtle features of atomic motions in the region of the transition state."