Spectroscopic Evidence for a Covalent Sigma Au–C Bond on Au Surfaces Using (13)C Isotope Labeling

[Image: see text] The Au–C linkage has been demonstrated as a robust interface for coupling thin organic films on Au surfaces. However, the nature of the Au–C interaction remains elusive up to now. Surface-enhanced Raman spectroscopy was previously used to assign a band at 412 cm(–1) as a covalent sigma Au–C bond for films generated by spontaneous reduction of the 4-nitrobenzenediazonium salt on Au nanoparticles. However, this assignment is disputed based on our isotopic shift study. We now provide direct evidence for covalent Au–C bonds on the surface of Au nanoparticles using (13)C cross-polarization/magic angle spinning solid-state NMR spectroscopy combined with isotope substitution. A (13)C NMR shift at 165 ppm was identified as an aromatic carbon linked to the gold surface, while the shift at 148 ppm was attributed to C–C junctions in the arylated organic film. This demonstration of the covalent sigma Au–C bond fills the gap in metal–C bonds for organic films on surfaces, and it has great practical and theoretical significance in understanding and designing a molecular junction based on the Au–C bond.