Subject: bond strengths

From: "E. Lewars" <[email-masked]>
Date: Sun, 22 May 1994 15:52:05 -0400

P. Subramanian asks how to calcualte bond strengths, i.e. (I suppose) bond
energies.  These are usually defined as the energy needed to produce
homolytic cleavage.  The standard way to calculate these seems to be to
use isodesmic reactions, reactions with the same number of each kind of
bond on the left-hand and right-hand sides of the eqn.  For example, to 
calc E(R-Cl), find a reaction for which you *know* the C-Cl cleavage energy,
e.g. CH3-Cl --> CH3. + .Cl   Then calculate the energy of R-Cl + CH3. -->
R. + CH3-Cl   Now add the experimentally known and the calc reactions (and
their E's) to get the E for R-Cl --> R. + .Cl  
Why not just calc R-Cl --> R. + .Cl ?  Because here we have no free radicals
on the left, and free radicals on the right, and the errors in deltaE (delta
H of form for semiempirical calcs) is bigger than for an isodesmic reaction.
Try it for several reactions of known bond energy and see.
For small molecules you *could* use the actual reaction R-X --> R. + .X and
the high-quality calc E's from G1 or G2 ab initio theory; see "Exploring
Chemistry with Electronic Structure Methods" by Frisch et al.  The calc of
bond strengths is also discussed somewhere in "Ab Initio Molecular Orbital
Theory" by Hehre et al.
Do several calcs with known systems to get a feel for the reliability of the
results.
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