Gronert1 has published a scathing criticism of the concept of “protobranching” (see my previous blog post) put forth by Schleyer, Houk and Ma2 – SHM for short. As a review, protobranching is the term coined by SHM for attractive 1,3-interactions in alkanes. They argue that these attractive 1,3-interactions are the reason for the energetic stability of the branched alkanes over the straight-chain alkanes. Their argument largely rests on the fact that Reaction 1 is exothermic by 2.8 kcal mol-1.
2 CH2CH3 → CH4 + CH3CH2CH3 Reaction 1
Gronert’s arguments are many and I will discuss only some of them. First, he notes that choosing ethane and methane as the reference molecules leads to all alkanes being stabilized. The stabilization energy of n-heptane is 5.7 kcal mol-1 and that of n-heptane is 14.1 kcal mol-1; is this a difference that is meaningful? Under the protobranching method, the stabilization energies of norbornane and n-heptane are quite similar (13.8 and 14.1 kcal mol-1, respectively) – does that mean they are equally strained? Similarly, protobranching leads to an extraordinary prediction for the resonance energy of benzene: 69 kcal mol-1. (I find these arguments quite compelling – the use of protobranching extenuates to magnitude of many chemical effects like ring strain, π-conjugation and resonance energy to the point that they become unusable.)
Gronert notes that the C-C-C angle in propane is larger than 109.5°, suggestive of a repulsive force, and one that is in fact much larger than suggested by SHM. The “attractive interaction” is not reproduced in intermolecular models. He points out the SHM attribute the attractive 1,3-interaction in alkenes to hyperconjugation and not to protobranching, and further notes that SHM correct for the strength of the C-H bond in ethyne but not for the Csp-C bond in propyne, nor do they make any such corrections for the alkenes.
But Gronert’s main complaint rests on the fact that there is simply no evidence for an attractive 1,3-interaction. All previous suggestions for this have been refuted by many others over the past 30 years. SHM’s main support rests on the ability to fit the thermodynamic trends, but Gronert points out that many other possibilities exist for doing so, including a repulsive model. There is ample evidence to support a repulsive interaction. It seems to me that Schleyer, Houk and Ma have their work cut out for them to carefully rebut Gronert’s arguments.
References
(1) Gronert, S., "The Folly of Protobranching: Turning Repulsive Interactions into Attractive Ones and Rewriting the Strain/Stabilization Energies of Organic Chemistry," Chem. Eur. J. 2009, DOI: 10.1002/chem.200800282
(2) Wodrich, M. D.; Wannere, C. S.; Mo, Y.; Jarowski, P. D.; Houk, K. N.; Schleyer, P. v. R., "The Concept of Protobranching and Its Many Paradigm Shifting Implications for Energy Evaluations," Chem. Eur. J. 2007, 13, 7731-7744, DOI: 10.1002/chem.200700602
Henry Rzepa responded on 02 May 2009 at 2:35 am #
Perhaps this is a tad off-topic,but the debate about protobranching seems similar to the 20 year old debates about the meaning of critical points in electron densities (the Bader AIM method). Applied to densely packed molecules, one finds bond critical points appearing almost everywhere. Do they have any meaning? In particular, BCPs tend to often appear between two hydrogen atoms which have approached to within 2.2A of each other. This tends to be quite common in highly branched globular hydrocarbons. One line of thought tends to arguing that the presence (or indeed absence) of a BCP tells one nothing about either the bonding in the system, or indeed any stabilization effects that might be operating. An (almost opposite) line is that one can usefully calibrate the value of ρ(r) at any BCP against the interaction energy involved. Thus in π-π stacking, the stacking/binding energy can be related to the ρ(r) values of the BCPs in the stacking region. And again, BCPs defining what is conventionally called a hydrogen bond can often be correlated with the strength of that hydrogen bond. Thus AIM does appear to have utility, but whether it illuminates bonding continues to be controversial. By the way, one effect which I have noted previously is a phenomenon called self-annihilation (which occurs between a bond and a ring critical point), and which makes both vanish, even though one might assume that there is interesting bonding going on which these critical points should describe (if only they were actually present). Its complications like this which make AIM analysis so fascinating!
Henry Rzepa responded on 02 May 2009 at 8:51 am #
Bond paths often show extreme curvatures, and one can even get bond paths terminating at another bond critical point (the symmetric norbornyl cation is the famous example). Whilst some feel that this is in fact a description of a π-complex, it is also possible to show that some of these path solutions are unstable to tiny perturbations. That seems to be the crux of the problem, that AIM seems often to be on the edge of chaos. I personally prefer use of the ELF function, but that has its own issues.
By the way, I am not sure Bader actually claimed insight into the chemical bond, which he recognized will always probably be a matter of interpretation.
Henry Rzepa responded on 02 May 2009 at 12:25 pm #
Steve notes that AIM can lead to more than 4 bond paths at a single carbon. Is there anything particularly sacred for tetravalent carbon? At the recent ACS meeting in SLC, one talk dwelt on the possibility of creating a compound with a genuinely pentavalent carbon. This is slightly different from species such as CH5(+), where two-electron-3-center bonds are involved. Silicon after all can do this easily; the argument was based on the observation that C is actually too small to easily form a pentavalent centre. There is also a recent article in JACS (10.1021/ja710423d) on the synthesis of hexacoordinate carbon. This is particularly significant since they measured the electron density very accurately using high angle X-ray diffraction, and performed the AIM analysis on the experimental rather than computed electron density. They obtained six bond paths to the central carbon. Arguably of course, of these six paths, four could be called bonds, and the other two interactions. But you can see how the argument can descend to counting angels on the head of a pin.
sbachrach responded on 02 May 2009 at 1:58 pm #
My next post will actually take on this topic – the presence of bond critical points (bcp) between close hydrogens that are formally non-bonded. I have always felt that Bader’s contention that a bcp in a local minimum energy structure is the necessary and sufficient condition for a chemical bond. Some of my work as a post-doc described organolithium systems with greater than 4 bond paths terminating at a single carbon.
Computational Organic Chemistry » Protobranching once again! responded on 13 Apr 2010 at 11:27 am #
[…] I tend to side more with Schleyer2 in his rebuttal of these charges, and so will present from this perspective. First off, Schleyer argues that he can define protobranch anyway he wants! (He in fact cites a quote of Humpty Dumpty from Lewis Carroll to support this stance!) Schleyer is of course correct. Fishtik should really have argued “Does Schleyer’s definition of protobranch add to our understanding of strain?” So Fishtik claims that there is an internal inconsistency in Schleyer’s definition – taking the view point that the C-(C)2(H)2 group is identical to the protobranch. Schleyer counters that no, the protobranch is this group along with the caveat that the two terminal carbons are not connected, like they are in cyclopropane. I really prefer Gronert’s approach here – where he argues for just what are the implications of Schleyer’s definition (see this post). […]
Larry Bartell responded on 09 Jul 2012 at 12:55 pm #
After a lifetime as a structural chemist, I argue from a different standpoint, one that is confirmed by spectroscopy and structural studies. Ligands around a smallish central atom repel each other inasmuch as they are closer than the sum of their van der Waals radii. This is confirmed by by the fact that vibrational spectra are in much better accord with Urey-Bradley force fields (based on ligand-ligand repulsions) than with force fields based on valence angle displacements – – and by the well established ligand-close-packing model of molecular structure (in which bond angles are extablished by the close-packing of repelling ligands rather than by any consideration of hybridization. The Schleyer explanation of protobranching requires what had been called a “mysterius attraction” between neighboring methly ligands, a concept at odds with the compelling evidence that ligands around carbon repel each other. The alternative interpretation of Gronert correctly identifies the physical cause of “protobranching” stabilization without violating any principles of structural chemistry