Steric congestion, rather than orbital interactions, is the driving force behind CC and CH bonds contracting when the number of substituents around the carbon center decreases, new research shows.1
It is a known phenomenon that CC and CH bonds decrease in length as the carbon center of sp3rd sp2 hybridized to sp. Historically, this difference has been explained by the increasing amount of s-character in the hybrid orbital that makes up the bond. Since s orbitals are more contracted than p orbitals, a shorter bond length is required for optimal orbital overlap.
Now, using the Kohn-Sham molecular orbital theory, a team of researchers from the Netherlands has suggested that the decreasing bond length is actually the result of lower steric repulsion when there are fewer substituents around the carbon center.
To evaluate their theory, the team performed DFT calculations on a range of saturated and unsaturated hydrocarbons. Their calculations confirmed that interactions between singly occupied molecular orbitals suggest that shorter bond lengths are required to achieve maximum orbital overlap. However, the team identified two notable contradictions with hybridization theory: the bond lengths of the CC and CH interactions differed significantly from the distance at which maximum orbital overlap was achieved; and that when the hybridization changed, the variation in CC and CH bond length was much smaller than the variation in the distance of maximum orbital overlap. They therefore had to conclude that something other than orbital interactions affects the bond length.
“Interestingly, we actually find that, for example, the CH bond overlap reaches its optimum with ever shorter bond distances along the row ethane, ethene, ethine,” says Matthias Bickelhaupt from the VU University of Amsterdam, who headed the work. “However, this happens at very short distances, around 0.7 to 0.8 Å, well below the actual equilibrium distance of around 1.1 Å. Our quantitative binding analyzes show that the steric [Pauli] the repulsion with closed-shell orbitals on the carbon fragment is a dominant factor that shifts the equilibrium CH distance to the usual and significantly longer values. ‘
The idea that Pauli repulsion could be responsible for the changes in CC and CH bond lengths is not new. Lawrence Bartell proposed a similar concept in the 1960s, which he called the sequence of unbound interactions.2 but the theory has never been quantitatively proven before.
“I think the most important advance was the development of a physically sound, quantitative molecular orbital model that not only reproduces the observable trends, but also covers all relevant underlying causal mechanisms and quantifies their importance,” says Bickelhaupt. “I can only speculate why Bartell’s suggestion has gone unnoticed in our textbooks over time, although steric factors are mentioned on many other occasions. One reason could be that Bartell’s proposal was based on the assumption of a soft-sphere model using model potentials, the origin of which was not clearly clarified, although they are very plausible. The hybridization model, on the other hand, provided an attractive causal mechanism that is directly and even directly rooted in the orbital electron structure. “
While the study confirms the assumption that hybridization promotes shorter bond lengths, the notion that this is not the dominant factor would turn the currently taught bond length on its head.
“As faculty, we seek to provide useful mental models for students to apply to wide-ranging problems of structure and attachment, and the hybridization model is useful,” said James Ashenhurst, founder of MasterOrganicChemistry.com, an online educational resource for chemists. “This paper provides an example of other models that can be applied to gluing that may ultimately prove to be more accurate that advanced students should look at.”
However, Ashenhurst is not confident that these results will have a major impact on the wider discourse within the community. “Given the intuitive nature of the hybridization model and its prevalence in introductory organic chemistry education, I think this article will change the course of introductory organic chemistry education no more than a pebble thrown against the hull of a moving supertanker.”
Thanks to István Hargittai, Professor Emeritus at the Budapest University of Technology and Economics in Hungary, for his guidance on the work of Lawrence Bartell.