Steric energy and the semi‐empirical method of calculating steric energy are discussed in the language of potential surfaces. Equations are given relating ΔH00 for several types of reactions commonly used to exhibit steric effects experimentally to the steric energy of the molecules involved in the reactions. The equations of Westheimer and Mayer for the calculation of the steric energy of a molecule are generalized in several respects. The difference in ΔH00 (formation) between cis and trans−2‐butene is calculated as a steric effect. Because of various complications the application of the method in its present form to molecules such as H2O, NH3, PF3, etc., is rather unsatisfactory.
REFERENCES
1.
2.
3.
4.
5.
6.
(a) If the molecule under discussion happens to be an activated complex it is to be understood that the words “equilibrium” and “minimum” are to be replaced by “saddle‐point,” and that the reaction coordinate is to be omitted in calculating zero‐point vibrational energy and thermodynamic functions.
(b) An approximate calculation of essentially this type has been carried out for See
J. H.
Van Vleck
and P. C.
Cross
, J. Chem. Phys.
1
, 357
(1934
).7.
(a) Force constants obtained on the assumption of valence forces (with or without cross products) will be most convenient.
(b) The introduction of the saddle‐point property into may sometimes be awkward. Because of the special nature of the reaction coordinate in Westheimer’s case,3 this complication did not arise.
8.
In most cases the zero‐point vibrational energies should largely cancel. One exception is a reaction involving an activated complex, since one vibrational frequency is missing here. The cancellation is especially effective where one takes first and second differences in values in comparing related reactions (see below).
9.
The two separate assumptions are not necessary. It is obviously sufficient to make the single assumption which should be even more exact.
10.
In rate studies there is ordinarily available the experimental activation energy defined by See Glasstone, Laidler, and Eyring, Theory of Rate Processes (McGraw‐Hill Book Company, Inc., New York, 1941), pp. 197–199. See also footnote 12 of paper I.
11.
In these equations the symbol represents the value of for the reference reaction, in conformity with our notation. It is not to be confused with
12.
A. P. I. Research Project 44, National Bureau of Standards. Selected Values of Properties of Hydrocarbons. Table 8w (Part 1), 1944–1946.
13.
14.
For an excellent discussion of valence force constants and generalizations, see G. Herzberg, Infrared and Raman Spectra (D. Van Nostrand Company, Inc., New York, 1945), pp. 168–191.
15.
16.
(b) Strictly speaking, the H–H van der Waals interaction chosen includes already some dipole interaction. However, this van der Waals curve does not give any angular dependence to the dipole interaction as it properly should. Also, since the van der Waals parameters are in any case very uncertain for this interaction, we ignore this complication in the present case.
17.
(a) L. Pauling, Nature of the Chemical Bond (Cornell University Press, Ithaca, 1945).
(b) Personal communication to F. H. Westheimer.
18.
The same possibility exists and is, of course, more practical in connection with ordinary catalysts and smaller molecules.
19.
20.
Method 2 leads to essentially the same results.
21.
One is tempted to make the following classification: the reasonable result for is probably fortuitous; for some reason P, As, and Sb compounds are anomolous; methyl compounds are not expected to give good results in view of difficulties (b) and (e) listed above.
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© 1948 American Institute of Physics.
1948
American Institute of Physics
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