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Quantum mechanical tunneling in the automerization of cyclobutadiene

Schoonmaker, R.; Lancaster, T.; Clark, S.J.


R. Schoonmaker


Cyclobutadiene has a four-membered carbon ring with two double bonds, but this highly strained molecular configuration is almost square and, via a coordinated motion, the nuclei quantum mechanically tunnels through the high-energy square state to a configuration equivalent to the initial configuration under a 90° rotation. This results in a square ground state, comprising a superposition of two molecular configurations, that is driven by quantum tunneling. Using a quantum mechanical model, and an effective nuclear potential from density functional theory, we calculate the vibrational energy spectrum and the accompanying wavefunctions. We use the wavefunctions to identify the motions of the molecule and detail how different motions can enhance or suppress the tunneling rate. This is relevant for kinematics of tunneling-driven reactions, and we discuss these implications. We are also able to provide a qualitative account of how the molecule will respond to an external perturbation and how this may enhance or suppress infra-red-active vibrational transitions.


Schoonmaker, R., Lancaster, T., & Clark, S. (2018). Quantum mechanical tunneling in the automerization of cyclobutadiene. The Journal of Chemical Physics, 148(10), Article 104109.

Journal Article Type Article
Acceptance Date Feb 21, 2018
Online Publication Date Mar 12, 2018
Publication Date Mar 12, 2018
Deposit Date Mar 13, 2018
Publicly Available Date Mar 13, 2018
Journal Journal of Chemical Physics
Print ISSN 0021-9606
Electronic ISSN 1089-7690
Publisher American Institute of Physics
Peer Reviewed Peer Reviewed
Volume 148
Issue 10
Article Number 104109


Accepted Journal Article (2.1 Mb)

Copyright Statement
© 2018 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Schoonmaker, R., Lancaster, T. & Clark, S. J. (2018). Quantum mechanical tunneling in the automerization of cyclobutadiene. The Journal of Chemical Physics 148(10): 104109 and may be found at

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