We can now calculate the rates for reactions of interest to us within days to a week, compared to six months to a year previously.

—Stephen Klippenstein, report author

These chemists are performing basic research on radical-radical reactions relevant to the combustion of hydrocarbons. Accurate experimental measurements of these reaction rates are challenging because the radicals are difficult to produce in the laboratory. Consequently, only a small number of radical-radical reaction rates have been measured accurately. Previous theoretical methods required long computer simulations and could only be applied to small radicals.

The Argonne technique couples efficient quantum chemistry and reaction rate theory with large-scale parallel computing. The three chemists adapted a faster but less accurate method for calculating the needed radical-radical interaction potentials with a simple correction to obtain accurate results.

The new method has been successfully applied to both self- and cross-combinations of methyl, ethyl, iso-propyl and tert-butyl radicals, answering a long-standing debate about temperature dependence of these reactions. The reaction rates decrease with increasing temperature.

—senior chemist Larry Harding

This finding is the opposite of expected behavior because most reactions speed up as the temperature increases. This new information is critical because in the past, combustion models have often used extrapolations of room temperature measurements.

With the new approach, the team also:

• Validated the geometric mean rule first postulated in the 1960s as a way to relate the rates of cross reactions to the rates of corresponding self reactions. .

• Demonstrated that the effect of methyl substituents adjacent to the radical site follows a simple rule: each additional substituent slows the reaction by a factor of two. For example, the reaction of methyl (CH 3 ) with ethyl (C 2 H 5 ) is twice as fast as the reaction of methyl with iso -propyl (i-C 3 H 7 ), which has one more methyl group.

The researchers are moving on to new territory. The chemists have so far only looked at hydrocarbon radicals and now want to investigate oxygenated radicals. Another topic to be addressed in the near future is resonance-stabilized radicals.

The work is supported by the Division of Chemical Sciences, Geosciences and Biosciences in DOE’s Office of Basic Energy Sciences. Research was also performed at Sandia National Laboratory.

Resources:

• Physical Chemistry Chemical Physics