Temperature Dependent Electron Binding in (H 2 O) 8 Marcelo A. Carignano,* Anis Mohammad, and Sabre Kais Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed: February 04, 2009; ReVised Manuscript ReceiVed: June 22, 2009 We combine classical molecular dynamics simulations and quantum density functional theory calculations to study the temperature effects on the electron affinity of the water octamer. The atomistic simulations provide a sample of the cluster’s conformations as a function of the temperature, on which the density functional calculations are carried on. As the temperature increases, the cluster undergoes its characteristic phase change from a cubic, solidlike structure to a liquidlike state. This phase change is also reflected by an increase on the total dipole moment of the cluster. The quantum calculations indicate that the large dipole moment conformations have a positive electron affinity. Relaxing the high temperature conformations of the cluster anion to its local minimum, the average vertical detachment energy is calculated and shows a clear tendency to increase as the temperature increases. The analysis of the high temperatures conformations reveals that origin of higher values of the vertical detachment energy is not the stability of the negative octamer but the high energy of the corresponding neutral cluster. I. Introduction Since the first observation of negatively charged water clusters in 1981, 1 the interest on water clusters and their ability to bind an excess electron has never declined. 2-13 Clusters have the advantage that they are more accessible to exhaustive compu- tational studies than bulk systems, and therefore many theoretical works are dedicated to them. Theoretical results combined with spectroscopic techniques allowed the characterization of water cluster anions according to their vertical detachment energy (VDE). 9,8,14-16 These isomeric types are labeled I, II, and III in order of decreasing VDE. Water cluster anions are also interesting because they are expected to contribute to the understanding of the hydrated electron, which plays a significant role in radiation induced processes, biological reactivity, atmospheric chemistry in water droplets, and charge induced reactivity. In their pioneer work, Armbruster et al. 1 created water cluster anions by injecting electrons from a radioactive foil into a condensation chamber with warm water vapor, which is then supersonically expanded through a nozzle. The authors claim that eight water molecules are sufficient to capture an excess electron. Subsequent experiments performed Haberland et al. 3,4 with a similar approach, but using low energy electrons of less than 1 eV, failed to observe the octamer anion. With Xe as carrier gas, the octamer anion is observed, but it is missing if the carrier gas is Ar. Knapp et al. 5 were able to attach an electron to existing cold neutral water clusters with at least 11 molecules. Their technique requires the use of cold electrons, with energy close to 0 eV. In conclusion, the early experiments are somewhat contradictory with respect to the existence of a stable (H 2 O) 8 - . In their discussion, Kanpp et al. 5 speculate on the role of temperature on this particular cluster size and state that hot octamers may be able to trap low energy electrons. The water octamer represents a very interesting case since its lowest energy structures, the D2d or S4 cubic conformations, 17,18 have zero total dipole moment and no electron binding is possible. As the temperature increases, the cluster undergoes a phase change 17,19 and is free to adopt conformations of finite total dipole moment that favor the binding of an electron. An ideal dipole will bind an electron if the dipole moment is larger than the critical value μ c ) 1.6648 D. 20 However, subsequent experimental and computational studies taking into account corrections to the Born-Oppenheimer approximation give the more realistic estimate of μ c ) 2.5 D. 2,21 Upon the binding of an electron, the system’s potential energy surface changes, and this change may induce a conformational evolution to a different state that may or may not be energetically favorable for the cluster anion. In consequence, the electron may remain bound to the cluster or be released. A recent ab initio study by Lee and Kim 22 has addressed the structure and electronic properties of (H 2 O) 8 - . They have found that the most stable structures of the anion are cubic, as is the case for the neutral cluster, but with a different arrangement of hydrogen bonds. All the cubic structures for the water octamer have four “daa”-type (donor-acceptor-acceptor) molecules, and four “dda”-type (donor-donor-acceptor) molecules. It is the relative position of these molecules that most affects the binding of an extra electron. Namely, the ability of the cluster to bind an extra electron is affected by the position of the dangling hydrogen atoms. According to ref 22, the lowest energy system (Cdh) has the electron bound to a central dangling H atom, on a structure characterized by having the dangling H atoms sharing three edges of the cubic cluster and converging to a central dangling H atom. Two other low energy conformers were found (Cd and Cd′′) with the four dangling H atoms on the face of the cube. The VDE energies calculated using MP2/ aug-cc-pVDZ+(2sp/s) are larger for Cd and Cd′′ conformers (0.40 and 0.38 eV) than for the Cdh lowest energy structure (0.25 eV). Another structure (Cb) having yet higher energy, and involving 11 hydrogen bonds to form a deformed cube leaving one “aa”-type molecule to bind the electron with an even higher VDE (0.73 eV). Isomer-specific spectroscopy experiments by Roscioli and Johnson 23 reveal that the type I isomers of (H 2 O) 8 - are consistent with the red-shift signature of an “aa”-type molecule, while the type II isomers show a red shift consistent with the electron bound to a single dangling hydrogen. * Corresponding author. E-mail: cari@purdue.edu. J. Phys. Chem. A 2009, 113, 10886–10890 10886 10.1021/jp901047y CCC: $40.75  2009 American Chemical Society Published on Web 09/01/2009