  Chemical selectivity is a phenomenon displayed by potential energy surfaces
(PES) that is relevant for many organic chemical reactions whose PES feature a
valley-ridge inflection point (VRI) in the region between two sequential
index-1 saddles. In this letter we describe the underlying dynamical phase
space mechanism that qualitatively determines the product distributions
resulting from bifurcating reaction pathways. We show that selectivity is a
consequence of the heteroclinic and homoclinic connections established between
the invariant manifolds of the families of unstable periodic orbits (UPOs)
present in the system. The geometry of the homoclinic and heteroclininc
connections is determined using the technique of Lagrangian descriptors, a
trajectory-based scalar technique with the capability of unveiling the
geometrical template of phase space structures that characterizes transport.
