Are bond properties additive?
In the last post, I presented data for the covalent bonds of the A – B compounds of the Group 1 elements that showed to a reasonable degree that the atoms each had a characteristic covalent energy, in the same way there is a covalent radius, and that the bond energy of the A – B bond is the sum of the A and B contributions. This goes against all the standard textbook writings. In an earlier post I stated that previously I had submitted a paper that would lead to a method for readily calculating these bond energies, but the paper was rejected by the editors of some journals on the grounds that either these are not very important molecules, or alternatively (or both) nobody would be interested. This annoyed me at the time, but is seems to me they had a point. These blog posts have received absolutely no comment. Either nobody cares, or nobody is reading the posts. Either way, it is hardly encouraging.
Now, the next point that could have been made is that when we get to more common problems, the bond energies are not additive in that way. Or are they? One problem I see is the actual data are not really suitable for reaching a conclusion.
Let's consider the P –P bond energy, which is needed for considering the bond additivity of any phosphorous compounds. I made a quick calculation of the P – P bond energy in diphosphine, on the assumption that the P – H bond energy was the same as in phosphine, and I got the energy 242 kJ/mol. If you look up some bond energy tables, you find the energy is quoted as 201 kJ/mol. How did they get that? If you consider the heats of atomization of phosphorus, the bond energy is 221 kJ/mol, but if we assume that is in the P4 form, it would be in the tetrahedrane structure, which will be strained (although the strain will also stabilize lone pairs) and of course the standard state will be a solid, so in principle energy should be added to get it into the gas phase before atomizing to make the comparison, so it is reasonable to assume that the real bond energy will be stronger than that indicated by that calculation.
The problem is obvious: to make any sense of this, we need more accurate data. We also need the data to involve energies of atomization, and not rely on the more easily obtained bond dissociation energies. But as far as I can see, the chemical community has given up trying to establish this data. Does it matter? I think it does. For me, a problem with modern chemical theory, which is essentially extremely complicated computations, is that it offers little assistance to the issues that matter for the chemist because there are no principles enunciated, but merely results and comments on various computational programs. The principles are needed, even if the calculations are not completely accurate, so that chemists can draw conclusions, and use these to formulate new plans of action. How many really think they understand why many synthetic reactions work that way? Do we care about the very fundamental component of our discipline? And, for that matter, does anyone care whether I write this blog?
Posted by
Ian Miller on Feb 29, 2016 2:14 AM Europe/London
