Chemists rewrite boron chemistry reference
07/01/1998
Chemists rewrite boron chemistry reference
A recent computation by chemists with the National Partnership for Advanced Computational Infrastructure (NPACI) suggests that a definitive reference on a fundamental property of boron is wrong. Specifically, this recent work shows that the accepted heat of formation for boron (i.e., 132.7 ? 3.0 kcal/mol) in the JANAF (Joint Army Navy Air Force) Thermochemical Tables has an error of > 2%. It is interesting that a similar heat of formation value was suggested by a 1977 experiment (Storms & Mueller, Journal of Physical Chemistry, 81:318), but rejected by the compilers of the JANAF tables. The new work, with an error bar of < 0.5%, almost exactly matches the rejected 20-year-old value (see table).
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The work, published in the April 30th Journal of Physical Chemistry, was conducted on the CRAY T90 at the San Diego Supercomputer Center, the leading-edge site for the Partnership. (The computations in this work required 60 Gbits of disk space.) Funded by the National Science Foundation through the Partnership and a chemistry grant, the research has produced the first NPACI publication.
Peter Taylor, computational chemist at SDSC, leader of the Partnership`s molecular science thrust and the individual who conducted the boron computation, said, "It came as a surprise to me that for an element as important and simple as boron that there was any uncertainty, but there is." Taylor worked with Jan Martin of Israel`s Weizmann Institute of Science. "If you do industrial chemistry for a living, this is a key piece of information," said Taylor.
In developing an industrial process, chemists must calculate whether a given chemical reaction releases heat or requires heat to be added. To make this
calculation, a chemist must know the "heats of formation" for the compounds and elements in the reaction, and this information can be found in standard reference tables. "Even the generous error bar in the reference is not enough to hold the higher value, meaning the currently accepted value is just wrong," Taylor said.
The new result comes from an ab initio computation - one that works from fundamental physics equations describing the atoms of a reaction for which the initial reactants` heats of formation are precisely known, in this case boron trifluoride and fluorine. Taylor and Martin first calculated a binding energy for boron trifluoride accurate to within 0.1%.
With the computed binding energy, a heat of formation for boron can be easily calculated with an error of < 0.5% - considerably more reliable than the JANAF reference.
"This is excellent work that resolves an important and long-standing experimental controversy," said Karl K. Irikura of the Physical and Chemical Properties Division at the National Institute of Standards and Technology, the compilers of the JANAF tables. "The authors have demonstrated that theory has superseded experiment for this type of chemical problem. As these methods become adopted elsewhere, we can expect the quality of thermochemical data to increase rapidly." - P.B.