  Atom tunneling in the hydrogen atom transfer reaction of the
2,4,6-tri-tert-butylphenyl radical to 3,5-di-tert-butylneophyl, which has a
short but strongly curved reaction path, was investigated using instanton
theory. We found the tunneling path to deviate qualitatively from the classical
intrinsic reaction coordinate, the steepest-descent path in mass-weighted
Cartesian coordinates. To perform that comparison, we implemented a new variant
of the predictor-corrector algorithm for the calculation of the intrinsic
reaction coordinate. We used the reaction force analysis method as a means to
decompose the reaction barrier into structural and electronic components. Due
to the narrow energy barrier, atom tunneling is important in the abovementioned
reaction, even above room temperature. Our calculated rate constants between
350 K and 100 K agree well with experimental values. We found a H/D kinetic
isotope effect of almost 106 at 100 K. Tunneling dominates the protium transfer
below 400 K and the deuterium transfer below 300 K. We compared the lengths of
the tunneling path and the classical path for the hydrogen atom transfer in the
reaction HCl + Cl and quantified the corner cutting in this reaction. At low
temperature, the tunneling path is about 40% shorter than the classical path.
