A first principles molecular dynamics study of excess electron and lithium atom solvation in water–ammonia mixed clusters: Structural, spectral, and dynamical behaviors of [(H $_2$2O) $_5$5NH $_3]^-$3]− and Li(H $_2$2O) $_5$5NH $_3$3 at finite temperature
Authors: Pratihar Subha , Chandra Amalendu 
Published: January 2011

First principles molecular dynamics simulations are carried out to investigate the solvation of an excess electron and a lithium atom in mixed water–ammonia cluster (H \documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2O) \documentclass[12pt]{minimal}\begin{document}$_5$\end{document}5NH \documentclass[12pt]{minimal}\begin{document}$_3$\end{document}3 at a finite temperature of 150 K. Both [(H \documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2O) \documentclass[12pt]{minimal}\begin{document}$_5$\end{document}5NH \documentclass[12pt]{minimal}\begin{document}$_3$\end{document}3] \documentclass[12pt]{minimal}\begin{document}$^-$\end{document}− and Li(H \documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2O) \documentclass[12pt]{minimal}\begin{document}$_5$\end{document}5NH \documentclass[12pt]{minimal}\begin{document}$_3$\end{document}3 clusters are seen to display substantial hydrogen bond dynamics due to thermal motion leading to many different isomeric structures. Also, the structures of these two clusters are found to be very different from each other and also very different from the corresponding neutral cluster without any excess electron or the metal atom. Spontaneous ionization of Li atom occurs in the case of Li(H \documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2O) \documentclass[12pt]{minimal}\begin{document}$_5$\end{document}5NH \documentclass[12pt]{minimal}\begin{document}$_3$\end{document}3. The spatial distribution of the singly occupied molecular orbital shows where and how the excess (or free) electron is primarily localized in these clusters. The populations of single acceptor (A), double acceptor (AA), and free (NIL) type water and ammonia molecules are found to be significantly high. The dangling hydrogens of these type of water or ammonia molecules are found to primarily capture the free electron. It is also found that the free electron binding motifs evolve with time due to thermal fluctuations and the vertical detachment energy of [(H \documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2O) \documentclass[12pt]{minimal}\begin{document}$_5$\end{document}5NH \documentclass[12pt]{minimal}\begin{document}$_3$\end{document}3] \documentclass[12pt]{minimal}\begin{document}$^-$\end{document}− and vertical ionization energy of Li(H \documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2O) \documentclass[12pt]{minimal}\begin{document}$_5$\end{document}5NH \documentclass[12pt]{minimal}\begin{document}$_3$\end{document}3 also change with time along the simulation trajectories. Assignments of the observed peaks in the vibrational power spectra are done and we found a one to one correlation between the time-averaged populations of water and ammonia molecules at different H-bonding sites with the various peaks of power spectra. The frequency-time correlation functions of OH stretch vibrational frequencies of these clusters are also calculated and their decay profiles are analyzed in terms of the dynamics of hydrogen bonded and dangling OH modes. It is found that the hydrogen bond lifetimes in these clusters are almost five to six times longer than that of pure liquid water at room temperature.