FIRST-PRINCIPLES APPROACHES TO THE STRUCTURE AND REACTIVITY OF ATMOSPHERICALLY RELEVANT AQUESOUS INTERFACES
The field of atmospheric science is very rich in problems ranging from the molecular to the regional and global scale. These problems are often extremely complex, and although the statement of a particular atmospheric science question may be clear, finding a single, concise computational approach to address this question can be daunting. As a result, the broad scope of scientific problems that lie within the umbrella of atmospheric science require a multi-discipline approach. Of particular interest to atmospheric chemists is the role that heterogeneous chemistry plays in the important processes that take place throughout the atmosphere. The definition of heterogeneous is: consisting of dissimilar elements or parts. The chemical environment induced by the presence of the interface can be dramatically different than the corresponding gas- or condensed phase homogeneous environment and can give rise to novel chemistry. Although the importance of heterogeneous chemistry in the atmosphere has been known for decades, a challenge to both experimentalists and theorists in provide simplified models and experiments that can yield insight into the field measurements of the atmospheric process. The use of molecular modeling has been widely used to provide a particle-based picture of atmospherically relevant interfaces to deduce the novel chemistry that is taking place. Unfortunately, even with the most computationally efficient particle-based approach, it is still impossible to model the full ice-crystal in the stratosphere or the sea-salt aerosol in the troposphere. Figure 1 depicts a caricature of the actual system of interest, and highlights the region where efficient molecular modeling can be employed. Although there is seemingly a large disconnect between reality and the model, we hope to convince the reader that there is still much insight to be gained from a particle-based picture. There is a myriad of different approaches to molecular modeling that have been successfully applied to studying the complex problems put forth by atmospheric chemists. To date, the majority of the molecular models of atmospherically relevant interfaces have been comprised of two genres of molecular models. The first is based on empirical interaction potentials. The use of an empirical interaction potential suffers from at least two shortcomings. First, empirical potentials are usually fit to reproduce bulk thermodynamic states, or gas phase spectroscopic data. Thus, without the explicit inclusion of charge transfer, it is not at all obvious that empirical potentials can faithfully reproduce the structure at a solid-vapor, or liquid-vapor interface where charge rearrangement is known to occur (see section 5). One solution is the empirical inclusion of polarization effects. These models are certainly an improvement, but still cannot offer insight into charge transfer processes and are usually difficult to parameterize. The other shortcoming of empirical models is that, in general, they cannot describe bond-making/breaking events, i.e. chemistry. In order to address chemistry one has to consider an ab initio (to be referred to as first-principles throughout the remaining text) approach to molecular modeling that explicitly treats the electronic degrees of freedom. First-principles modeling also give a direct link to spectroscopic data and chemistry, but at a large computational cost. The bottle-neck associated with first-principles modeling is usually determined by the level of electronic structure theory that one chooses to study a particular problem. High-level first-principles approaches, such as MP2, provide accurate representation of the electronic degrees of freedom but are only computationally tractable when applied to small system sizes (i.e. 10s of atoms). Nevertheless, this type of modeling has been extremely useful in deducing reaction mechanisms of atmospherically relevant chemistry that will be discussed in this review (see section 4). However, to solve problems relating to heterogeneous chemistry at interfaces where the interfacial system is large enough to include the effects of the semi-infinite system (see Figure 1), dramatically reduces the number of electronic structure methods available.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 945893
- Report Number(s):
- UCRL-JRNL-212909; CHREAY; TRN: US200903%%862
- Journal Information:
- Chemical Reviews, Vol. 106, Issue 4; ISSN 0009-2665
- Country of Publication:
- United States
- Language:
- English
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