Abstract:

The qualitative description of the Molecular Orbital Theory is a topic usually found by General Chemistry textbooks. In fact, the presence of a chapter dealing with a qualitative approach to the Molecular Orbital paradigm in the Chemistry curriculum, seems to be far from questionable nowadays. Moreover, this topic is directly connected to the description of the chemical bond. Unfortunately, probably because of a certain lack of mathematical abilities in the students, this qualitative approach is not based on the fact that molecular orbitals (MOs) are just mathematical entities and not physical observables. In this statement we exclude atomic hydrogen-like systems, where the wave function may be rigorously split into parts where the square of the 1-electron part represents the electron density, a real physical observable, obtainable experimentally, e.g. through X-ray diffraction experiments. The fact that MOs are not presented from the beginning as what they are, elements in a mathematical approximation to a physical problem which cannot be solved exactly, explains why so many chemists speak about them as real objects. In fact, we often listen/read that one electron moves from MO to another in an electronic excitation, or that electron density is transferred from HOMO to LUMO, etc.

Worse than these, when MOs are employed to provide basic interpretation of the chemical bond in diatomic molecules, textbooks provide a long series of systems with “detailed” qualitative MO descriptions. Some of these examples are outside the scope where the Hartree-Fock method (a quantitative reference for the MO model) is applicable. C2 is one of these cases. Many of those who had to study the MO explanation of the chemical bond in this molecule were probably puzzled by a funny feature: according to the MO framework, the configuration of the ground electronic state of C2 is KKσ2(σ*)2π2π2. If we now make use of the “bond order” (BO) “definition” (another undesirable definition usually spread by textbooks) we obtain that C-C in C2 is a double bond. And now, we have that the bond is (according to the MO model) “exclusively” due to π electron density, as Shaik et al. have said “C2 is supposedly bond by two suspended π-bonds, like levitating halves of a sandwich with nothing in it”.

The chemical bond in heteroatomic molecules is another point that should be considered. Here, the qualitative MO explanation is biased so much from the quantitative MOs data that it denies the calculations. HF is a common example in textbooks. The MO electronic configuration obtained is now 1s(F)2 2s(F)2σ2π2π2. In this case the π MO are denoted as nonbonding as, because of symmetry (and this is mathematically correct), 2px and 2py “occupied orbitals of F” do not combine with any “occupied orbital of H”. Moreover, the second MO with “lower energy” is also denoted as “nonbonding” because it basically corresponds with the 2s fluorine orbital, and this is not true according to MO calculations. Probably, this is said, just to obtain a value of 1 for the F-H BO employing the simple definition initially presented, without realising that this is just one among the huge number of BO definitions in chemistry.

Keywords:

Molecular Orbitals, Chemical Bond, Chemistry, Physical Chemistry.