Interaction of gaseous nitric acid and water on thiol-based molecular overlayers on C₇₀

We present a theoretical investigation of the stability and interactions of nitric acid and water on molecular overlayers formed by alkanethiols adsorbed on C₇₀. Density functional theory calculations reveal that nitric acid is efficiently trapped by thiol-functionalized C₇₀, with adsorption energies ranging from -62 to -90 kJ mol^-1. The binding strength is governed by the polarity of the thiol terminations, with mixed -OH/-NH₂ overlayers exhibiting the strongest adsorption (-90 kJ mol^-1) due to cooperative hydrogen bonding. In all cases, the random mixture of thiols on the C₇₀ surface is more stable than segregated domains by 100-150 kJ mol^-1, a phenomenon driven by the anisotropic curvature of the fullerene which enhances dipole-dipole coupling and suppresses phase separation. Nitrate formation via full dissociation into NO₃^- and H₃O⁺ is thermodynamically unfavourable; however, a feasible pathway involving partial proton transfer to the thiol sulfur atom is identified, with activation barriers of 70-100 kJ mol^-1 that are lowered by 10-20 kJ mol^-1 in the presence of water. Born-Oppenheimer molecular dynamics simulations at 300 K reveal that confinement within the C₇₀ cage enables transient hydronium formation, whereas the acid remains molecular in the inter-chain regions of the thiol layer. Infrared spectral analysis demonstrates the evolution of HNO₃ vibrational modes upon hydration, showing broadening and red-shifts of 20-40 cm^-1 consistent with strong hydrogen bonding. The results establish functionalized C₇₀ as a model for probing confined proton dynamics and provide quantitative insight into acid adsorption on curved carbon interfaces.