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

Abstract

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.