  Investigation of intermolecular electron spin interaction is of fundamental
importance in both science and technology.Here, radical pairs of all-trans
retinoic acid molecules on Au(111) are created using an ultra-low temperature
scanning tunneling microscope. Antiferromagnetic coupling between two radicals
is identified by magnetic-field dependent spectroscopy.The measured exchange
energies are from 0.1 to 1.0 meV. The biradical spin coupling is mediated
through O-H$\cdots$O hydrogen bonds, as elucidated from analysis combining
density functional theory calculation and a modern version of valence bond
theory.
