J. Phys. Chem. B 2008, 112, 3767-3772
3767
Solvation of Excess Electrons in LiF Ionic Pair Matrix: Evidence for a Solvated Dielectron from Ab Initio Molecular Dynamics Simulations and Calculations Liang Zhang,† Shihai Yan,† R. I. Cukier,‡ and Yuxiang Bu*,†,‡ Key Laboratory for Colloid and Interface Chemistry of Ministry of Education, The Modeling & Simulation Chemistry DiVision, School of Chemistry & Chemical Engineering, Shandong UniVersity, Jinan, 250100, P. R. China, and Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824 ReceiVed: NoVember 9, 2007
Ab initio molecular dynamics simulations and first-principles calculations reveal the existence of a solvated dielectron species, (2e)s, in an LiF ionic matrix. The nature of the solvation mechanism and the stability of the species was explored. In addition to electrostatic interactions, a hole-orbital coupling among solvent molecules may significantly enhance the stability of the solvated electrons and govern the extent of electron solvation. This hole-orbital coupling is different from either an electrostatic coupling or conventional chemical bonding, and it may be described as a transition between them.
Introduction The solvation of excess electrons in various media has attracted widespread interest due largely to its importance for understanding phenomena associated with chemical and biological electron transfers, radical reactions, and polarons.1-12 The significance of solvated electrons has sparked much experimental activity aimed at characterizing its energetics and dynamics through various spectroscopic methods.13-21 In addition, this species stands as a most fundamental quantum mechanical solute, such that modeling its properties through various computational approaches such as coupled quantumclassical molecular dynamics and Car-Parrinello molecular dynamics22 provides new insights into condensed-phase dynamics.23-24 Electron solvation in various media such as methanol,14 alcohol,15,16 supercritical water,24 acetonitrile,16,25 ammonia,26 and HF27 has been studied, and a consistent physical picture of its structural, spectral, and dynamic properties has emerged, bolstered in part by details extracted from electrons in clusters. A consensus has been reached that an excess electron in solution is trapped inside a well-formed solvent cavity. The density of the trapped electron could be described as a s-like function for the ground state and a p-like function for the possible first excited electronic state.11,18,19 Clearly, the ability to stabilize and localize an excess electron depends strongly on the solvent properties. Generally, qualified solvents should have a large positive electrostatic region or they should be highly polar. Another important condition may be the availability of low-lying unoccupied orbitals, which are welllocalized. Accordingly, as proved both experimentally and theoretically, molecules with X-H (X ) halogen, O, N, etc.) may suffice for binding an excess electron within a deep potential well. On the basis of the larger polarity of LiF relative to HF, and by the electron trapping phenomenon in ionic crystals, molten salts, and clusters of alkali-halides,28,29 one can predict that metal halides should have an enhanced ability to trap excess electrons. Excess electron solvation in solid or liquid * Corresponding author. E-mail:
[email protected]. † Shandong University. ‡ Michigan State University.
matrixes offers the possibility of designing new materials with unprecedented and intriguing electromagnetic properties.29-30 Thus, it is of great interest to extend excess electron solvation studies to other systems with very large positive charge regions, such as metal halides. Our initial qualitative examination of excess electron solvation revealed the intriguing fact that two electrons may be trapped in a LiF molecular cavity, which, together with the preliminiary predictions regarding a solvated dielectron,31 motivated us to carry out a more detailed exploration. Thus, in this work, we present a computational approach to address the following issues: (1) the structural characteristics of electron solvation, and (2) the nature of the forces that drive this solvation in a LiF ionic pair matrix. Computational Details Ab initio molecular dynamics (AIMD) simulations were performed for the (LiF)30 system with 1 or 2 excess electrons in a cubic cell (15.00 Å × 15.00 Å × 15.00 Å), corresponding to a density of 0.383 g cm-1. Periodic boundary conditions (PBC) were applied in all three directions to reproduce the bulk LiF ionic matrix. The primitive cell size had been chosen with the following two considerations: (1) to be as large as possible to minimize interactions between periodic images, and (2) to be small enough to allow for a reasonable calculation time. All electrons were included in the spin-unrestricted calculation. The electronic structure was described by nonlocal density functional theory (DFT), using the BLYP functional, where the exchange functional was given by Becke32 and the correlation energy expression by Lee, Yang, and Parr,33 and thus can be categorized as a generalized gradient approximated (GGA) functional. The accuracy of integration points used to integrate the wavefunction in reciprocal space (K-point) was set to medium. A double numerical plus p-functions (DNP) atomic orbital basis set was employed to provide accurate results.34 Simulations were carried out within the canonical (NVT) ensemble, and the system temperature was kept around 2000 K by using a Nose´-Hoover chain of thermostats.35,36 The integration time step was 2 fs, to ensure constant temperature throughout the whole process. The system was first equilibrated for 2 ps without an excess electron.
10.1021/jp800381a CCC: $40.75 © 2008 American Chemical Society Published on Web 03/04/2008
3768 J. Phys. Chem. B, Vol. 112, No. 12, 2008 After further pre-equilibrium of 3 ps with one injected electron, simulation trajectory data were collected for another 3 ps to analyze the structural characteristics of the electron solvation process. All simulations were done with the Cerius2 4.6 suite of program periodic DMol3 package37 from Accelrys, Inc. Complementary to AIMD simulations, traditional quantum chemical calculations can provide additional insights into the energetics and the structure of smaller negatively charged ionic matrix clusters. Although the properties of these clusters can be strikingly different from the simulated system, the quantum chemical cluster results can be very useful for predicting and understanding the properties of larger systems. In this regard, we performed high-level ab initio calculations using the Gaussian 03 suite of programs.38 Møller-Plesset second-order perturbation theory (MP2)39 was used in the geometry optimizations and harmonic vibrational frequency determinations. Furthermore, the values of the vertical detachment energy (VDE) and other energy-related parameters were determined at the coupled cluster level of theory with the use of iterative singlet, double excitations (CCSD)40 on the basis of the MP2 optimized geometries. A Gaussian-type basis set including diffuse basis functions (6-311+G* basis set) was utilized in both the MP2 and CCSD calculations. In addition, DFT results (unpublished) were also obtained at the B3LYP level of theory41 with a 6-311+G* basis set and found to give qualitatively similar results with minor differences in geometric parameters. Results and Discussions Ab initio molecular dynamics (AIMD) simulations were carried out by starting with a pre-equilibrated (LiF)30 system in a cubic box and then adding one or two excess electrons. The initial LiF matrix was randomly generated to correspond with matrixes obtained experimentally by matrix isolation techniques. In the preliminary simulation run at a temperature of 2000 K without excess electron injection, the equilibrated (LiF)30 vibrated in the vicinity of its initial position and behaved as a matrix of ionic pairs. The hole-orbitals (HOs) are distributed in an approximate band structure (Figure 1a). We started the electron injection simulations with the normal LiF ionic matrix equilibrated for 2 ps. First, single-electron solvation was examined. After excess electron injection, little rearrangement of the equilibrated (LiF)30 structure occurs for about 280 fs. Then, the electropositive Li atoms are attracted to the negative charge, while the electronegative F atoms are repelled within the 3 ps pre-equilibrium. With no hydrogen bonds to break or reform, the phenomenon observed in this {(LiF)30+e} system is slightly different from that in the hydrated electron case24 in which there is a rapid (1 ps time scale) major reorganization of the hydrogen-bonding network to minimize the system energy. After pre-equilibrium of 3 ps with the injected electron, the simulation trajectory was collected for the following 3 ps. As the simulation proceeds, a subset of the LiF ionic pairs gradually reorganizes and gradually forms a cavity-like structure around the electron. When a single lobe appears for the electron wavefunction, several LiF ionic pairs reorganize in such a way that all of them point their Li ends toward the electron cloud, forming a solvated singleelectron cavity. After this reorganization was completed, the delocalized state became localized and a cavity-like structure formed in the ionic matrix. Unlike hydrated electrons, the cavities that are formed mainly exhibit as oligomers, here defined as dimer-to-trimer (2-3mer) cavities with lifetimes of ∼200 fs (Figure 1b). Along the course of the simulation, the inner- and outer-shell ligands mutually exchange incessantly,
Zhang et al.
Figure 1. Some snapshots for the solvated electron(s) cavities extracted from the simulation trajectory. (a) Hole-orbital distribution after 2 ps pre-equilibration; (b) single-electron solvation; (c) S-state dielectron solvation; (d) T-state dielectron solvation. The LiF molecules are represented with a ball-and-stick mode. Purple and cyan balls indicate Li and F atoms, respectively. Orbitals (b-d) are plotted using a contour value of 0.015 au, while (a) is significantly more diffuse and is plotted using a contour value of 0.005 au. Sketches of the approximate shape of the different solvated electron cavity structures are presented near the plotted orbitals.
and thus the cavity dynamically reorganizes. During the reorganization process, multimer [tetramer-to-hexamer (46mer)] and even more metastable heptamer (7mer) or octamer (8mer) short-lifetime (
