Time to start the New Year, and wish the RSC a great 175th anniversary. If we think of the last 175 years, chemistry has made serious changes, both to itself and to our lives. Nevertheless, at heart, chemistry is about molecules, and molecules are groups of atoms held together by what we call chemical bonds. The recent Chemistry World has an article that shows how our understanding of bonding has evolved, but I am not convinced it shows how hard this was. Don't forget, as it stood then, the concept of an electron moving around a nucleus violated one of the greatest triumphs of 19th century physics, namely Maxwell's electromagnetic theory. Would you have been prepared to propose something as radical? Or has the quest to discover died out? It is a lot easier to grasp something when everyone tells you it is correct, but what about when nobody knows? I may annoy a number of people, but I believe we still do not properly understand even the simplest of chemical bonds, and I thought one contribution I could make to the year would be to illustrate a problem over a small number of posts. Amongst other things, I hope to show you how hard it is to form new theories.
What I am going to do is to focus on the dimeric molecules of the Group 1 metals. These are of interest to me because they are the simplest molecules, with the fewest complicating issues. Or so I thought. Some of what I am going to put in the following posts was submitted to two journals and rejected by both on what I consider spurious grounds. The first said these molecules were not very interesting; the second said nobody would be interested in what I was proposing (which involved a hitherto unrecognized quantum effect). There were no adverse comments about the physics! I wonder were these editors right? Perhaps nobody is interested? Perhaps the urge to overturn the wrong has gone? If it looks OK, then do not disturb! Welcome back, Claudius Ptolemy! Of course, just because nobody is arguing, that may be because it is correct. Maxwell's electromagnetic theory is correct, right? Apart from that pesky issue about electrons around atoms, that is. But you know why that is, don't you? Do you really? Maybe in detail things are not quite what you think. Care to try out some problems?
So here goes thought number one. Atoms have a characteristic covalent radius, so the bond distance of an A – B molecule is the arithmetic mean of the A – A and B – B bond distances. Do you agree with that statement?
Let's test it. The literature contains the necessary A – A bond distances, although of varying degrees of accuracy. However, recently a review of the bond properties of the A – B molecules of the group 1 metals has been published (Fedorov, D. A., Derevianko, A., Varganov, S. A. J. Chem Phys. 140: 184315 (2014).). So, let's check. First, the A – A bond distances. The following is from various literature sources with distances in pm:
Li2 133.6; Na2 153.9; K2 193.5; Rb2 210.5; Cs2 230
Now, let us look at the A – B molecules. In the following, the column labelled "mean" is the arithmetic mean of the relevant A – A molecules, the column labelled observed is the measured values from Federov et al., and δ is what must be added to the calculated value to get the observed value.
Mean Obs δ
Li – Na 287.5 (288.9) 1.4
Li – K 328.1 (332.3) 4.2
Li – Rb 344.1 (346.6) 2.5
Li – Cs 363.6 (366.8) 3.2
Na – K 347.4 (349.9) 2.5
Na – Rb 364.4 (364.3) -0.1
Na – Cs 383.9 (385) 1.1
K – Rb 404 (406.9) 2.9
K – Cs 423.5 (428.4) 5.9
Rb – Cs 440.5 (437.1) -3.4
What do you make of that? There are three options:
(1) The bond distance is the sum of the covalent radii, and the discrepancies are observational error, including in the A – A molecules.
(2) The bond distance is the sum of the covalent radii, and the discrepancies are partly observational error, including in the A – A molecules, and partly some very small additional effect.
(3) The bond distance is not the sum of the covalent radii, as shown by the lack of agreement.
What do you opt for? Can you discern any trends? This probably seems fairly obvious to you, but soon it will be less so. The question is, is anyone interested? Were the journal editors right in that nobody cares about the nature of the chemical bond? Will anyone respond?
