Subject: Calculating effects of isotopic substitution

From: "E. Lewars" <[email-masked]>
Date: Mon, 6 Jun 1994 01:39:51 -0400

S. Shapiro asks why H, D and T (hydrogen-1, -2, and -3) substituents all
give the same results in his calculations.  The answer is that the normal
procedures of computational chemistry do not take isotopic masses into account.
Thus the calculated bond orders of H-X and D-X will be the same (altho' in
real life they are presumably slightly different).  If you look under the hood
of your program (e.g.MOPAC) you will see that the masses of the atomic nuclei
do not figure in the calculations.  This gives pretty good results in most
cases, since chemistry is ruled by the electromagnetic force, which is far,
far stronger than the gravitational force.  The one common type of calculation 
that does use nuclear masses is a frequency calc, since freqs depend on force
constants *and* nuclear masses.  I suppose the mass effect on freqs shows up
in zero point energies and so affects relative E's when one bothers to correct
for ZPE.
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Errol Lewars  Chem Dept  Trent U  Peterborough  Ontario  Canada  K9J 7B8
D-day  1:40
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