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Physical Chemistry

Care In Using Quantum Chemical Calculations

September 26, 2011 | APPEARED IN VOLUME 89, ISSUE 39

The article beseeching chemists to exercise great caution when using quantum chemical (QC) calculations was long overdue and is so very important (C&EN, Aug. 15, page 36).

One of the greatest challenges that the inexperienced theoretician faces when using QC calculations has to do with bond lengths. Since, theoretically, the electron density around any atom can extend to infinity, and “bonding” is seen as the sharing of electron density between any two atoms, then we often see references to “long” covalent bonds when QC theoreticians examine diffraction-derived structures in their unit cells, and even intramolecularly.

Then, if any two atoms of separate structures in the unit cell are seen to be “close enough together,” they are also seen as being somehow bonded to each other. Similarly, two atoms in the same molecule are in danger of being seen as bonded together if the interatomic distance is greater than the normal distance but still “close enough together for some kind of bonding.”

The simplest of experimental tests to prove, or disprove, the existence of these “long” covalent bonds is their existence when the “molecule” is in solution. NMR can tell us that for intermolecular situations, but close packing in a unit cell cannot. Similarly, intramolecularly, the actual chemistry of a molecule is a better arbiter of which atoms are mutually bonded, especially when bond lengths that are longer than the normal ranges are involved.

It is truly an important requirement that QC calculations recognize that all covalent bonds have discrete ranges of lengths. Bonds cannot be arbitrarily long, or infinitely long. Until then, many QC calculations will yield suspect, and even ridiculous, data. Until then, I might be forced to spend time wondering which, if any, of the atoms in my body are currently bonded to some atom in the sun.

By Vernon G. S. Box
New York City



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