PREDICTION OF THE ATMOSPHERIC LIFETIMES OF HALON REPLACEMENTS.

Scientists at NIST have developed a new technology to allow a reliable estimation of the atmospheric lifetimes of halon replacements based on ab initio quantum mechanical calculations. The potential suitability of new industrial compounds depends in large part on an assessment of their environmental suitability. For gaseous compounds, such as potential halon replacements, a key element in this assessment is the atmospheric lifetime, which is based on the reactivity of the compound toward the hydroxyl radical. This parameter is the starting point in calculating the ozone depletion and global warming potentials of these substances. Unfortunately, simple correlation schemes have proven inadequate to predict this parameter, particularly when new functional groups are involved. Thus, it appeared that laborious laboratory measurements would be required for any new class of halon replacement considered in the Next Generation Fire Suppression Technology Program of the Department of Defense. The efforts at NIST will s ignificantly decrease the need for these laboratory measurements.

This effort involved the calculation of rate constants at various levels of theory for a large set of reactions for which the kinetics were well known. From these calculations, an optimum approach was determined; optimum in this case meaning of sufficient accuracy but also with a sufficiently frugal use of computational resources to make routine use practical. These studies, which appeared in the Journal of Physical Chemistry A, Vol. 104 (2000), covered 13 partially halogenated methanes, with rate constants calculated over the temperature range 250 K to 400 K. The results agreed with experimental values typically better than a factor of two and always within a factor of four.

The technique has now been applied to five potential fire suppressants, all bromine-containing methanes, for which there was no experimental data. From the ab initio calculations, atmospheric lifetimes were estimated to range from 0.12 years to 1.88 years. These lifetimes, in turn, can be incorporated into atmospheric models to estimate the ozone depletion potentials of these compounds if it is decided to consider them further as halon replacements. A manuscript on this work appears in the February 2001 issue of the Journal of Physical Chemistry A.

Efforts are under way to extend the approach to additional classes of potential fire suppressants and to further simplify the computations.