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

Abstract

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.

Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface

“…Interestingly, this proposed mechanism for the chemistry of MA at the air/liquid water interface also suggests a new source for NH 2 radicals at aerosol surfaces, other than by reaction of absorbed NH 3 . This provides important support for the recent work of Martin-Costa et al 77 and Zhong et al 68 on the interaction of NH 2 radical and volatile organic compounds at the surface of water droplet. Future work is planned to examine the influence of both temperature and humidity on the position/orientation of such by-products at aerosol droplets.…”

“…Interestingly, the same feature has been observed in accurate (yet extremely expensive and short simulation time) Born Oppenheimer molecular dynamics of the NH 2 radical on a small liquid droplet, 68 also showing that NH 2 is preferentially located at the interface. Moreover, we observed the primary amine group bounded to a small number of water molecules, especially at the interface.…”

“…These angular variables are used to describe the water arrangement around the solute and to obtain insights about the hydrogen bond network between solute and water. [65][66][67][68] As an initial reference, Figure 10 shows the bivariate distribution of an H 2 O around a water molecule over the course of 50 ns NPT simulation, recording the (θ µ , φ) angular position for the vector connecting the water oxygen and the oxygen of H 2 O within a distance of 0.35 nm from the water's oxygen. The pattern shows two peaks, one about (θ µ , φ) (50 • , 90 • ) and another at (θ µ , φ) (130 • , 0 • ).…”

“…Nevertheless, this work reveals a need for new methods for better describing the solvent around solute molecules than that presented here using the (θ µ , φ) bivariate distribution. This set of angular variables was designed to study simple solutes (water, 65,66 NH 2 radical, 68 Cl − , 67 etc.) in which the water distribution around the solute can be simply described using only two angular coordinates.…”

“…Nevertheless, Figure 13 points also to the possibility of another channel with the formation of NH 2 radical and formaldehyde, which are also known to be preferentially located at the surface of liquid water. 68,77 It is known from studies 83 of substituted alkoxy radicals that C-X bonds are weaker than C-H ones because of the lower activation energy barrier for bond cleavage, suggesting that the release of NH 2 radical is also possible. Interestingly, this proposed mechanism for the chemistry of MA at the air/liquid water interface also suggests a new source for NH 2 radicals at aerosol surfaces, other than by reaction of absorbed NH 3 .…”