Density-functional study of the sign preference of the binding of 1-propanol to tungsten oxide seed particles
Introduction
The motivation for this study was twofold. The first motivation was that nucleation on ions is an important process in the atmosphere. Even though the majority of atmospheric nucleation is believed to happen via neutral pathways [1], [2], ion-induced nucleation may play some part, especially in regions where air ion or ion cluster concentrations are relatively high. Also, the majority of detection methods for nucleating clusters rely on charging, which makes understanding processes that involve ions vital.
Ions of opposite sign were observed to exhibit different nucleation rates as early as 1897 [3], but the reason for this sign preference remained a mystery for more than a century. In a recent paper by Nadykto et al. [4] it was demonstrated that the sign effect can be predicted by carrying out relatively simple quantum chemical calculations. This means we can use quantum chemical methods to help us understand the role of ion-induced nucleation in atmospheric conditions. Recently done calculations for sulfuric acid by Kurtén et al. [5] are another example of how quantum chemical methods can be applied to atmospheric systems. However, the computational methods in use today are generally iterative methods that employ a variety of approximations in order to keep the cost in computational resources reasonable. Because of this, theoretical predictions need to be compared with high-quality experimental results whenever possible, in order to gain reliable insight on the initial steps of ion-induced nucleation.
In this study, we have examined the sign preference of the binding of a single 1-propanol molecule to small tungsten oxide molecules with different charge. In experimental studies such as the one performed by Winkler et al. [6], tungsten oxide particle generators can be used to produce nearly monodispersed particles smaller than 2 nm in diameter. Nanoparticles such as these are valuable when studying the initial steps of nucleation. Thus, while tungsten oxide particles probably have no direct relevance to the real atmosphere, they still have an important role to play in atmospheric sciences. This brings us to the second motivation for this study: the choice of method for doing simulations is non-trivial.
While accurate simulation of transition metals, such as tungsten, generally requires the use of a very high level of theory due to the strong multireference nature of their wave functions, the computational cost for multireference methods such as multireference configuration interaction quickly becomes unfeasible as we move from single atoms and dimers to systems with a larger number of atoms. One solution to this problem is to use density functional theory (DFT), which scales more favorably with system size, and has had some success [7] in treating transition metal systems. Care has to be taken, however. In DFT calculations, the choice of density functional and basis set plays a major role, since a single density functional does not generally work equally well in every situation. Thus, the quality of results may vary greatly. This leads to a need to perform the simulations using different density functionals and basis sets, unless one can be relatively sure that the system in question is not very sensitive to the choice of theoretical method.
Section snippets
Methods
A quantum chemical study of the structure and electronic energies of WO, WO+, WO2, , , WO3, , , W3O9, , , (WO2)(C3H8O), ()(C3H8O), ()(C3H8O), (WO3)(C3H8O), ()(C3H8O), ()(C3H8O), (W3O9)(C3H8O), ()(C3H8O) and ()(C3H8O) was performed employing the quantum chemistry programs Spartan [8], Gaussian 03 [9], Gaussian 09 [10], SIESTA [11] and ADF [12], [13], [14] with ADF-GUI [15]. Molecular visualization programs Molden 4.6 [16] and Molekel 4.3 [17]
Results and discussion
As can be seen from Table 1a, with the exception of a considerably larger bond angle given by the RPBE/DZP method for the negatively charged WO2 molecule, our results for free tungsten oxide molecules are quite similar to the results obtained by other groups using different methods. The same can be seen from Table 1b, where deviations from the results by Huang et al. remained relatively small except for the vertical detachment energy (VDE), which is the energy of the neutral molecule minus the
Conclusions
We have performed a quantum chemical study of the optimized geometries and energies of several tungsten oxide species using the methods BLYP/DZP, RPBE/DZP and TPSSTPSS/SDD for both geometry optimizations and energies. For the energies, a combined method of RPBE/DZP optimization and TPSSTPSS/def2-QZVPP single point energy calculations was also used. The results were compared with each other and results obtained previously by other groups and overall the agreement was quite good. Furthermore, we
Acknowledgment
We thank the CSC – IT Center for Science Ltd. for computer time and technical assistance. The financial support by the Academy of Finland Centre of Excellence program (Project No. 1118615), the Research Foundation of the University of Helsinki and ERC StG 257360-MOCAPAF is gratefully acknowledged.
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