The choice and justification thereof is very much an individual matter: what kind
of chemistry fascinates you? You can read about some of the molecules that
fascinate other chemists in the books by Hopf [1] and by me [2].
References
- Hopf H (2000) Classics in hydrocarbon chemistry. Wiley-VCH, Weinheim, New York
- Lewars E (2008) Modelling marvels. Computational anticipation of novel molecules. Springer,
The Netherlands
Chapter 2, Harder Questions, Answers
Q1
The Born–Oppenheimer principle [1] is often said to be a prerequisite for the
concept of a PES. Yet the idea of a PES [2] predates the Born–Oppenheimer
principle (1927). Discuss.
The Born–Oppenheimer principle (Born–Oppenheimer approximation) [1] says
that the electrons in a molecule move so much faster than the nuclei that the two
kinds of motion are independent: the electrons see the nuclei as being stationary,
and so each electron doesn’t have to adjust its motion to maintain a minimized
electron–nucleus interaction energy. Thus we can calculate the purely electronic
energy of a molecule, then the internuclear repulsion energy, and add the separate
energies to get the total molecular energy.
The concept of a PES can be based simply on the concept of molecular struc-
ture, without specific reference to nuclei and electrons: if one thinks of a molecule as
being defined by the relative positions (in a coordinate system) of its atoms (no
reference to nuclei and electrons), then it is intuitively apparent that as these positions
are altered the energy of the collection of atoms will change. This is probably how
Marcelin thought of molecules [2]. On the mathematical surface defined by Energy¼
f(atomic coordinates), minima, transition states etc., defined by first and second
derivatives, emerge naturally. On the other hand, if one insists on going beyond
mere atoms, and thinks of a molecule as a collection of nuclei and electrons, then
molecular shape (geometry) has meaning only if the nuclei (the hallmark of “atoms”)
are more or less fixed. This stricture is violated in CH 5 +, which has no clear shape [3].
References
- Born M, Oppenheimer JR (1927) Ann Phys 84:457
- Marcelin R (1915) Ann Phys 3:152. Potential energy surface: p 158
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