Page 283 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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264 for organic molecules, known as MM4, the computations involve iterations to locate
an energy minimum. Precautions must be taken to establish that a true (“global”)
CHAPTER 3
minimum, as opposed to a local minimum energy, is found. This can be accomplished
Structural Effects on by using a number of different initial geometries and comparing the structures of the
Stability and Reactivity
minima that are located. As with the group equivalent approach, MM calculations of
o
H are grounded in experimental measurements of a limited number of molecules
f
that were used to optimize the parameters. The original parameters pertained to hydro-
carbons, but as the method has developed, the parameters have been extended to
many functional groups. MM calculations specifically take molecular geometry into
account, including nonbonded and dipolar interactions. Van der Waals interactions are
described in terms of energy functions and parameters that describe the interaction of
the approaching atoms. Polar interactions are modeled as electrostatic interactions.
Heats of formation are calculated as a sum of the bond energies and other stabi-
lizing and destabilizing (e.g., strain) increments for the structure. MM4 calculations
include terms for contributions of higher-energy conformations. 19 For a set of hydro-
carbons ranging from methane and ethane to adamantane and bicyclo[2.2.2]octane,
the heats of formation are calculated with a standard deviation of 0.353 kcal/mol. The
MM4 system has also been applied to alkenes, 20 aldehydes, 21 and ketones. 22

3.1.2.5. Thermodynamic Data from MO and DFT Computations. MO and DFT calcu-
lations provide another approach to obtaining thermodynamic data. The accuracy with
which the various computational methods reproduce molecular energies varies. Of the
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26
23
24
semiempirical methods only MINDO, MNDO , AM1, and PM3 provide reliable
estimates of energies and the range of applicability is open to some discussion. 27
Among the ab initio methods the level of accuracy generally increases with larger basis
sets and treatment of correlation effects. G1, G2, and G3 computations can achieve
a level of accuracy that permits comparison of energy data among related molecules.
DFT calculations have also been applied to various compounds. 28 Users of computa-
tional thermochemical data must critically assess the reliability of the method being
applied in the particular case under study.
A large series of compounds, including hydrocarbon derivatives, was studied at
the G2 and G2(MP2,SVP) levels and compared with results from the B3LYP method. 29
Another group carried out a similar comparison on a smaller set of molecules. 30
18
N. L. Allinger, K. Chen, and J. -H. Lii, J. Comput. Chem., 17, 642 (1996).
19 N. L. Allinger, L. R. Schmitz, I. Motoc, C. Bender, and J. Labanowski, J. Phys. Org. Chem., 3, 732
(1990); N. L. Allinger, L. R. Schmitz, I. Motoc, C. Bender, and J. Labanowski, J. Am. Chem. Soc., 114,
2880 (1992).
20
N. Nevins, K. Chen, and N. L. Allinger, J. Comput. Chem., 17, 695 (1996).
21 C. H. Langley, J. H. Lii, and N. L. Allinger, J. Comput. Chem., 22, 1396 (2001).
22
C. H. Langley, J. H. Lii, and N. L. Allinger, J. Comput. Chem., 22, 1426, 1451, 1476 (2001).
23 R. C. Bingham, M. J. S. Dewar, and D. H. Lo, J. Am. Chem. Soc., 97, 1294 (1975).
24 M. J. S. Dewar and G. P. Ford, J. Am. Chem. Soc., 101, 5558 (1979).
25
M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, and J. J. P. Stewart, J. Am. Chem. Soc., 107, 3902 (1985).
26
J. J. P. Stewart, J. Comput. Chem., 10, 221 (1989).
27 J. A. Pople, J. Am. Chem. Soc., 97, 5307 (1975); T. A. Halgren, D. A. Kleier, J. H. Hall, Jr., L. D. Brown,
and W. L. Lipscomb, J. Am Chem. Soc., 100, 6595 (1978); M. J. S. Dewar and D. M. Storch, J. Am.
Chem. Soc., 107, 3898 (1985).
28
K. Raghavachari, B. B. Stefanov, and L. A. Curtiss, Molec. Phys., 91, 555 (1997);B. S. Jursic, Theochem,
391, 75 (1997); B. S. Jursic, Theochem, 417, 99 (1997); J. Andzelm, J. Baker, A. Scheiner, and
M. Wrinn, Int. J. Quantum Chem., 56, 733 (1995).
29 L. A. Curtiss, K. Raghavachari, P. C. Redfern, and J. A. Pople, J. Chem. Phys., 106, 1063 (1997).
30
J.-W. Pan, D. W. Rogers, and F. J. McLafferty, Theochem., 468, 59 (1999).