Computatioal Study of Autoionization in Liquid Water

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Computatioal Study of Autoionization in Liquid Water

Category: Article Review

Subcategory: Biochemistry

Level: PhD

Pages: 3

Words: 825

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Computational Study of Autoionization in Liquid Water
Different studies in Chemistry deal with rare and unique events of chemical dissociation. Among those is separation or breaking up of the water molecule into liquid water (Chandler et al. 245). This reaction is also termed as the core reaction for the determination the pH of a solution. However, the dynamics and dissociation mechanism have been explained through different routes. This paper looks at the computation dynamics and dissociative mechanism of the water molecule through TIS and CP2K methods.
Literature Review
The dissociative chemistry of the water molecule with acid-based chemical equilibria and is a substantial factor in determining the pH of water. Geissler et al. (2121) have noted that any random water molecules dissociate into liquid water thereby generating a hydroxide (OH-) ion and a hydronium ion (H3O+) within a 10-hour time span. The event occurs for femtosecond scale based on molecular motions. However, Eigen and De Maeyer (986) have argued that the formation of intermediate state together with ionic diffusions is a relatively “facile” phenomenon.
A similar study was also conducted by Aziz et al. (89-91) that addressed the recombination aspects of hydroxide (OH-) ions and hydronium ions (H3O+). However, the paper used Grotthuss dynamic simulation model for assessing the recombination mechanism of the water molecule (Agmon 456-457). The process uses diffusive transport and proton transfer methods for the elaboration of acid-base neutralization reactions on computation scale. As a universal solvent, water is quite commonly used in many activities. It possesses a unique dissociation phenomenon due to the hydrogen bond present in each molecule (Hassanali et al. 21410).The rapid event of bond breaking and recombination is mainly because of the small KW value of 1×10-14 that shows that water can readily dissociate and recombine within a few picoseconds. A significant part of the “DFT calculation is its contribution to the energy, occasioned by interactions between pairs of electrons” (Hutter 17). Moreover, the interaction of molecules was also found to occur within one picosecond.
The phenomenon of dissociation and recombination of water molecules is vividly described by Chandler et al. (246). Among the commonly used dynamic simulation tools is Car and Parrinello’s molecular dynamics (CPMD) modeling. Dellago and Chandler (1-13) have explained the potential significance of CPMD modeling by delineating transition path sampling method. This modeling technique allows for better opportunities for studying “rare, but important events occurring in complex systems for which prior mechanistic knowledge are unavailable” (Dellago and Chandler 6). The method is particularly valuable because of the additional factor of reactivity that is kept under control throughout the reaction path, thereby reducing the computational time for generation of trajectories. Morrone and Roberto (13) have also seconded the notion of using CPMD simulations. Dynamics on the molecular scale have become quite essential for exploration of time-based transition state crossing of atomic motions. Dellago and Chandler (8) have shown that there are two steps associated with the process of dissociation of water molecules. The first part entails the creation of a rare solvent with a destabilized electric field that affects the OH bond. Afterward, a Proton is transferred to the neighboring water molecule with a hydrogen bond. As the solvent fluctuates with the changing electric field, it influences an ionic pair that separates the proton and hydroxyl ion further apart. Geissler et al. (2121-2124) also established that the dynamics of hydrogen bond and electric fields are quite important factors for the Autoionization mechanism of water. Rare fluctuations in electric fields lead to the dissociation of oxygen-hydrogen bonds. Therefore, the ionization that occurs through this route is commonly recombined spontaneously because the salvation fluctuations last for tens of femtoseconds. In a case where the variation coincides with a break in the hydrogen bond wire, the process occurs after every picosecond. Therefore, rapid recombination is no longer a possibility. Hassanali et al. (20414) have proposed a computational simulation involving CP2K package. As part of the studies, Hassanali and his coworkers have a molecular simulation of Born-Oppenheimer dynamics. They have concluded that the neutralization leads to a concerted triple jump of protons followed by an enhanced compressive state of several water molecules.
Bakker and Neinhuys (588) also studied the O-H bond stretch vibrations for examining the interactions of O-H…O bond. The study concluded that around 20% of the energy of dissociation is utilized in breaking the hydroxyl bond in the water molecule in the gaseous phase. The high delocalization displays enhanced O-H vibration, and it can second the proton transfer mechanism following Car-Parrinello Molecular Dynamics (CPMD) simulation that starts with the auto-dissociation of water as follows:
23856958001000H2O + H2O H3O+ + OH-
All in all, water dissolution and reformation is surely a rare but exceptional phenomenon in Chemistry. This phenomenon can be described through models like Car and Parrinello’s molecular dynamics, Grotthuss dynamic simulation model, and CP2K. Each of these has a slightly different understanding of bond breakage and bond formation mechanisms. Nevertheless, the models have provided a critical but important insight on molecular interaction, together with electric field and hydrogen bonding aspects.

Work Cited
Agmon, Noam. “The grotthuss mechanism.” Chemical Physics Letters 244.5 (1995): 456-462.
Aziz, Emad F., et al. “Interaction between liquid water and hydroxide revealed by core-hole de-
excitation.” Nature 455.7209 (2008): 89-91.
Bakker, H. J., and H-K. Nienhuys. “Delocalization of protons in liquid water.”Science 297.5581 (2002): 587-590.
Chandler, David, Christoph Dellago, and Phillip Geissler. “Ion dynamics: Wired-up water.” Nature Chemistry 4.4 (2012): 245-247.
Dellago, Christoph, and David Chandler. “Bridging the time scale gap with transition path sampling.” Bridging Time Scales: Molecular Simulations for the Next Decade. Springer Berlin Heidelberg, 2002. 321-333.
Eigen, M., and L. De Maeyer. “Untersuchungen über die kinetik der neutralisation. I.” Zeitschrift für Elektrochemie, Berichte der Bunsengesellschaft für physikalische Chemie 59.10 (1955): 986-993.
Geissler, Phillip L., et al. “Autoionization in liquid water.” Science 291.5511 (2001): 2121-2124.
Hassanali, Ali, et al. “On the recombination of hydronium and hydroxide ions in water.” Proceedings of the National Academy of Sciences 108.51 (2011): 20410-20415.
Hutter, Jürg, et al. “CP2K: atomistic simulations of condensed matter systems.” Wiley
Interdisciplinary Reviews: Computational Molecular Science4.1 (2014): 15-25.
Morrone, Joseph A., and Roberto Car. “Nuclear quantum effects in water.”Physical review
letters 101.1 (2008): 017801.

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