Example: CH 2 ¼CHOH/CH 3 CHO,DE(MP2 opt, including ZPE)¼71.6 kJ mol#^1 ,
total time 1,064 s;DE(MP2 single point on MMFF geometries)¼70.7 kJ mol#^1 ,
total time¼48 s (G98 on a now-obsolescent Pentium 3). What role does this leave
for semiempirical calculations?
If the above approach really has wide applicability then it could be a very useful
way to get relative energies at only modest cost in time. However, it could be used
only for species for which the MMFF gives reliable geometries. This excludes
exotic molecules and transition states. Whatever the deficiencies of SE methods in
these two categories, at least they do permit such calculations.
Chapter 6, Harder Questions, Answers
Q10
Semiempirical methods are untrustworthy for “exotic” molecules of theoretical
interest. Give an example of such a molecule and explain why it can be considered
exotic. Why cannot SE methods be trusted for molecules like yours? For what other
kinds of molecules might these methods fail to give good results?
A simple exotic molecule is pyramidane:
C
C CC CH HH H1This is exotic because one of the carbon atoms is forced to have very unusual
pyramidalbonding: tetracoordinate carbon normally has its four bonds directed
toward the corners of a tetrahedron, but the apical carbon of 1 has all four bonds
pointing forward. Without any further investigation of 1 we can thus characterize it
as exotic. Of course without further investigation we cannot assert with confidence
if it can exist, much less what its properties might be. Semiempirical and low-level
ab initio [1, 2] and higher-level ab initio [3] studies on pyramidane have been
published, and work on this and related molecules is reviewed [4]. SE methods
cannot be trusted for molecules like pyramidane because they are parameterized
using information, whether experimental or calculated, for normal molecules.
Other kinds of molecules besides 1 (which has unusual bond stereochemistry)
for which these methods might fail to give good results are hypercoordinate
molecules like NF 5 , molecules with noble gas atoms, particularly those of helium
and neon, molecules with highly twisted C¼C bonds, extraordinarily crowded
molecules like hexaphenylethane, unknown dimers, trimers etc. of small familiar
molecules, like C 2 O 2 and N 6 , and very highly strained molecules. All these cases
are discussed in a book on exotic molecules [4].
Answers 635