Observing the intricate chemical transformation of an individual molecule as it undergoes a
complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe
microscopy now provide the tools to visualize not only the frontier orbitals of chemical
reaction partners and products, but their internal covalent bond configurations as well. We
used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed
internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a (100) oriented
silver surface as they underwent a series of cyclization processes. Our images reveal the complex
surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes.
Calculations using ab initio density functional theory provide additional support for the proposed
reaction pathways.