Interaction of the NH₂ Radical with the Surface of a Water Droplet

We present a comprehensive computational study of NH₂ (radical) solvation in a water nanodroplet. The ab initio Born-Oppenheimer molecular dynamics simulation shows that NH₂ tends to accumulate at the air-water interface. The hydrogen-bonding analysis shows that compared to the hydrogen bond of HNH··OH₂, the hydrogen bond of HOH··NH₂ is the dominant interaction between NH₂ and water. Due to the loose hydrogen-bonding network formed between NH₂ and the droplet interface, the NH₂ can easily move around on the droplet surface, which speeds up the dynamics of NH₂ at the air-water interface. Moreover, the structural analysis indicates that the NH₂ prefers an orientation such that both N atom and one of its H atoms interact with the water droplet, while the other H atom is mostly exposed to the air. As a result, the NH₂ radical becomes more accessible for reaction at the water interface. More importantly, the solvation of NH₂ modifies the amplitude of vibration of the N-H bond, thereby affecting the Mulliken charges and electrophilicity of NH₂. As such, reactive properties of the NH₂ are altered by the droplet interface, and this can either speed up reactions or allow other reactions processes to occur in the atmosphere. Hence, the solvation of NH₂ on water droplets, in chemistry of the atmosphere, may not be negligible when considering the effects of clouds.