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.