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Quantum mechanical tunneling in the automerization of cyclobutadiene.

Schoonmaker, R. and Lancaster, T. and Clark, S. J. (2018) 'Quantum mechanical tunneling in the automerization of cyclobutadiene.', Journal of chemical physics., 148 (10). p. 104109.


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.

Item Type:Article
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Publisher 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
Date accepted:21 February 2018
Date deposited:13 March 2018
Date of first online publication:12 March 2018
Date first made open access:12 March 2019

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