Chapter 3, Harder Questions, Answers
Q8
What factor(s) might cause an electronic structure calculation (e.g. ab initio or
DFT) to give geometries or relative energies very different from those obtained
from MM?
The most likely factor is electronic: since MM makes no reference to electrons, it
should not be expected to reflect structural and energetic effects arising from, say,
aromaticity and antiaromaticity, encapsulated in the 4nþ2 and the corollary 4n
rules [1–3].
References
- Minkin V, Glukhovtsev MN, Simkin B. Ya (1994) Aromaticity and antiaromaticity: electronic
and structural aspects. Wiley, New York - Randic ́M (2003) Chem Rev 103:344
- (2005) Chem Rev 105(10); whole issue devoted to aromaticity, antiaromaticity and related
topics
Chapter 3, Harder Questions, Answers
Q9
Compile a list of molecular characteristics/properties that cannot be calculated
purely by MM.
Among these properties are:
UV spectra
Dipole moment (by pure MM)
Delocalization energy (this is related to aromaticity and antiaromaticity)
Transition state structures and energies (see the hedge below)
The properties are listed in approximate order of simplicity of connection with
electronic behavior:
UV spectraarise from electronic transitions, automatically placing them outside the
accessible to MM.
Dipole momentsarise from uneven distribution of electric charge in a molecule,
which in turn is due to nuclear charges not being “matched” spatially by electron
distribution. This would seem to automatically rule out probing by MM.
Answers 607