 
Summary: J. Am. Chem. SOC. .1987, 109, 13171322 1317
subtracted from all carbon atom equivalents. To circumvent this
problem, the MP2/63 lG*//HF/321G energy of ethaneg0and
its heat of formations9are added to the set of equations used to
determine the atom equivalents.
The resulting HF/321G, HF/631G*, and MP2/631G*//
HF/321G heats of formation at 298 K are compared with ex
periment in Table XVI. For example, the theoretical heat of
formation of CHzFC1is obtained by taking the ab initio energy
and subtracting C(H)2(R)z, H(C) twice, F(C), and CI(C).
Multiplying the result by 627.509 converts the heat of formation
(298 K) from hartrees to kcal/mol. The average absolute errors
between theory and experiment for these compounds are 5.6
kcal/mol at HF/321G, 3.5 kcal/mol at HF/631G*, and 1.2
kcal/ mol at MP2/ 631G*//HF/ 321G. The excellent agreement
between experiment and the MP2/63 IG*//HF/321G values
is due to both the improved theoretical energy caused by adding
electron correlation and the fact that only these data (and C2H6
to remove the additive constant) are used to determine the atom
equivalents. It is interesting to note that the results in Tables XI11
