The electron spin state significantly influences the adsorption energy and orbital interactions of intermediates on metal single-atom catalysts. The splitting degree of d–σ interaction determines bond strength, and d–π* interaction influences the bond energy of structure at different spin states; the bond length and frequency shift exhibit a quasi-linear relationship.

Electron spin at metal sites plays a critical role in surface/interface reaction activity. Herein, a series of metal (Fe, Co, Ni, Pd, Pt, Cu, Ag, and Au) single-atom catalysts as activity center and CO as a probe molecule, to systematically investigate the role of spin electrons by calculations of structure stability, orbital energy level, and electron transfer. Fe and Pt single-atom structures are most stable in low-spin states, while others are stable in high-spin states. The bond energy of CO influenced by spin state demonstrates the same trend, and the splitting degree of d–σ interaction determines the strength of bond energy. Furthermore, it is found that there is a quasi-linear relationship between frequency shift and bond length of adsorbed CO. This work offers an example of how spin electrons influence orbital interaction of molecular adsorption and helps to understand the role of electron spin at metal sites in reaction.