Chemists depend on sophisticated molecules to develop life-saving drugs, produce advanced high-tech materials, and recreate processes found in living systems. For decades, however, chemists have faced a basic limitation in how electron transfer works. When two molecules are competing to receive an electron, the electron typically goes to the molecule that is easier to reduce. "Our calculations reveal how the decisive selectivity emerges after electron transfer has already occurred," says Paton. By changing how chemists think about where and when reaction selectivity is determined, the method could expand the range of molecules that can be connected through electron-transfer chemistry.