On the tunneling instability of a hypercoordinated carbocation

Kozuch, S. Phys. Chem. Chem. Phys. 2015, 17, 16688-16691
Contributed by Steven Bachrach.
Reposted from Computational Organic Chemistry with permission

Last year I wrote a post on the possibility of a stable hypercoordinated carbon in the C(CH₃)₅⁺ molecule as proposed by Schleyer and Schaefer.¹ Kozuch has re-examined this molecule with an eye towards examining the lifetime of this proposed “fleeting” molecule.²

The computed barriers for either (1) loss of a methane molecule leaving behind the (CH₃)₂C⁺CH₂CH₃cation or (2) loss of an ethane molecule leaving behind the t-butyl cation are small: 1.65 and 1.37 kcal mol^-1, respectively. Kozuch employed canonical variational theory with and without small curvature tunneling (SCT). Without the tunneling correction, the pentamethylmethyl cation is predicted to have a long (millennia) lifetime at very low temperatures (<20 K). However, when tunneling is included, the half-life is reduced to 6 and 40 μs for degradation along the two pathways. Clearly, this is not a fleeting molecule – its lifetime is really too short to consider it as anything.

Interestingly, perdeuterating the molecule ((CD₃)₅C⁺) substantially increases the half-life to 4 ms, a thousand-fold increase. Tritium substitution would further increase the half-life to 0.2 s – a long enough time to really identify it and perhaps justify the name “molecule”. What is perhaps the most interesting aspect here is that H/D substitution has such a large effect on the tunneling rate even though no C-H bond is broken in the TS! This results from a mass effect (a CH₃ vs. a CD₃ group is migrating) along with a zero-point vibrational energy effect.

References

(1) McKee, W. C.; Agarwal, J.; Schaefer, H. F.; Schleyer, P. v. R. "Covalent Hypercoordination: Can Carbon Bind Five Methyl Ligands?," Angew. Chem. Int. Ed. 2014, 53, 7875-7878, DOI: 10.1002/anie.201403314.

(2) Kozuch, S. "On the tunneling instability of a hypercoordinated carbocation," Phys. Chem. Chem. Phys.2015, 17, 16688-16691, DOI: 10.1039/C5CP02080H.

InChIs

C(CH₃)₅⁺: InChI=1S/C6H15/c1-6(2,3,4)5/h1-5H3/q+1
InChIKey=GGCBGJZCTGZYFV-UHFFFAOYSA-N

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