Geometry changes that occur along the tunneling reaction pathway from 1-methylcyclobutylfluorocarbene to 1-fluoro-2-methylcyclopentene. For the portion of the reaction pathway in which the reactant is tunneling through the barrier that separates it from the product, the carbenic carbon turns purple (Thanks go to David Hrovat, University of Washington). Further details of this reaction may be found in P. S. Zuev, R. S. Sheridan, T. V. Albu, D. G. Truhlar, D. A. Hrovat, and W. T. Borden, Science 299, 867-870 (2003).