Anil K. Kandalam

Hydrogen storage and the 18-electron rule

We show that the 18-electron rule can be used to design new organometallic systems that can store hydrogen with large gravimetric density. In particular, Ti containing organic molecules such as C(4)H(4), C(5)H(5), and C(8)H(8) can store up to 9 wt % hydrogen, which meets the Department of Energy target for the year 2015. More importantly, hydrogen in these materials is stored in molecular form with an average binding energy of about 0.55 eV /H(2) molecule, which is ideal for fast kinetics. Using molecular orbitals we have analyzed the maximum number of H(2) molecules that can be adsorbed as well as the nature of their bonding and orientation. The charge transfer from the H(2) bonding orbital to the empty d(xy) and d(x(2)-y(2) ) orbitals of Ti has been found to be singularly responsible for the observed binding of the hydrogen molecule. It is argued that early transition metals are better suited for optimal adsorption/desorption of hydrogen.

Hyperhalogens : Discovery of a new class of highly electronegative species

Electron affinity (EA), defined as the amount of energy necessary to remove an electron from an anion, plays a dominant role in reactivity. This is evidenced by halogen atoms, whose electron affinities are larger than those of any other element in the periodic table. However, there exists a class of molecules whose electron affinities are even larger. Bartlett and Lohmann were among the first to study such molecules nearly half a century ago. They showed that PtF6 can even oxidize O2 molecules [1] and Xe atoms. Several years later, Gutsev and Boldyrev coined the term superhalogen to describe these highly electronegative species. According to these authors, a superhalogen consists of a central metal atom surrounded by halogen atoms. When the number of these halogen atoms exceeds the maximal valence of the metal atom, the molecule possesses electron affinities that are much larger than that of the halogen atoms. In a series of subsequent theoretical studies, Boldyrev and coworkers showed that a large number of superhalogens, in which the central metal atom is typically an sp element, are possible. The first photoelectron spectrum of MX2 anions (M = Li, Na; X = Cl, Br, I) was reported by Wang and coworkers. Subsequent photoelectron spectroscopic and theoretical studies have further confirmed the existence of superhalogens in the gas phase. In a very recent theoretical study, it was proposed that the hydrogen atom can act as the central atom to form a superhalogen, and it was shown that the vertical detachment energies of [HnFn+1] anions can be extremely high. Numerous other superhalogen anions, such as permanganate (MnO4 ), perchlorate (ClO4 ), hexafluorides (AuF6 and PtF6 ), BO2 , and MgxCly [20] have also been reported. In a joint experimental and theoretical study, we have presented unusually stable Aun(BO2) clusters (n = 1–5) that exhibit superhalogen characteristics. Because of their high EAs, superhalogens almost always exist as negative ions, usually as the anionic portions of salts. Because salts composed of superhalogens have highly oxidative properties, there is considerable interest in the synthesis of species with high EAs. Herein we show that a new class of highly electronegative species can be synthesized if the peripheral halogen atoms are replaced by superhalogen moieties. We name this new class of electronegative species “hyperhalogens”, because their electron affinities can even be larger than those of their superhalogen building blocks and thus can serve as ingredients in the synthesis of new superoxidizing agents. Using density functional theory (DFT) and photoelectron spectroscopy (PES) experiments with a cluster beam, we demonstrate this possibility by concentrating on a gold atom and also a gold cluster decorated with BO2 superhalogens. The BO2 molecule, like MnO4, has a large electron affinity of 4.32 eV, 21] whilst its anionic counterpart, BO2 , being isoelectronic with CO2, is a very stable anion. It has been shown recently that the EA of an XFn cluster (X = Cu, Ag, Au; n = 1–6) increases as the central coinage metal atom is decorated successively with fluorine atoms. This happens as the extra electron is delocalized over several halogen atoms. We questioned whether the electron affinity would increase even further if the metal atom is decorated with superhalogen molecules instead. In this case, the extra electron will be delocalized over superhalogen moieties. What would happen if some but not all of the halogens atoms were replaced with superhalogen molecules; would the electron affinity lie in between the two? For example, would the electron affinity of Au(BO2)2 be much larger than that of AuO2? Similarly, would the electron affinity of AuO(BO2) be in between that of AuO2 and Au(BO2)2? From our DFT-based calculations (Table 1), we found that the electron affinity of Au(BO2)2 is 5.54 eV, which is 1.6 times larger than that of AuO2. [24] On the other hand, the electron affinity of AuO(BO2) is 4.21 eV, which lies between that of

Geometry and electronic structure of Vn(Bz)m complexes

First-principles calculations based on the generalized gradient approximation to the density functional theory are performed to explore the global geometries, ground-state spin multiplicities, relative stabilities, and energetics of neutral and anionic V(n)(Bz)(m) (n=1-3, m=1-4, with n<m) complexes. The calculated results show that the V(n)(benzene)(m) complexes clearly prefer sandwich structures to rice-ball structures. The ground-state spin multiplicities of the V(n)(benzene)(n+1) complexes increased linearly with the size of the system (i.e., n). In the anionic complexes, the V(benzene)(2) complex is found to be unstable against the autodetachment of the extra electron. The energy difference between adiabatic and vertical electron affinities is found to be very less, indicating negligible ionization-induced structural changes in the ground-state geometries of V(n)(benzene)(n+1) complexes.

Manganese-Based Magnetic Superhalogens†

The unusual properties of nanoscale materials brought about by their reduced sizes have ushered in a new era in materials science where materials with tailored properties can be synthesized. A fundamental understanding of how their properties evolve one atom and/or one electron at a time can be best studied with atomic clusters. Numerous studies of clusters over the past 30 years have demonstrated their unique properties, which can be tailored by fixing their size and composition. One of the most important properties of atomic clusters is that they exhibit unusual stability at a specific size and composition. There are two main classes of these stable clusters, which are often referred to as magic clusters. Clusters of simple metals such as sodium exhibit unusual stability at 2, 8, 18, 20, 34, 40, ... atoms, while clusters of noble gas atoms exhibit stability at 13, 55, 147, ... atoms. The former series is due to electronic shell closure, while the latter is due to atomic shell closure. It has been suggested that magic clusters, owing to their enhanced stabilities, can form building blocks of new cluster assembled materials. Herein we present the discovery of a new class of magnetic magic clusters with molecular composition (MnxCl2x+1) (x = 1, 2, 3, 4, ...). Supported by photoelectron spectroscopy experiments and density functional theory (DFT)-based calculations, we show that these magic clusters owe their unusual stability neither to the conventional electronic shell closing nor to the atomic shell closing, but to the superhalogen character of their corresponding neutral species and to the d configuration of each of the manganese atoms. These molecular anions have the potential to serve as building blocks of a new class of salts with magnetic and super-oxidizing properties. (MnxCly) cluster anions were produced in a pulsed-arc cluster ion source and mass analyzed by a time-of-flight (TOF) mass spectrometer. The mass ion intensity distribution is shown in Figure 1. The prominent peaks occur at MnCl3 ,

Structures and photoelectron spectroscopy of Cun(BO2)m− (n, m = 1, 2) clusters: Observation of hyperhalogen behavior

The electronic structures of CuBO(2)(-), Cu(BO(2))(2)(-), Cu(2)(BO(2))(-), and Cu(2)(BO(2))(2)(-) clusters were investigated using photoelectron spectroscopy. The measured vertical and adiabatic detachment energies of these clusters revealed unusual properties of Cu(BO(2))(2) cluster. With an electron affinity of 5.07 eV which is larger than that of its BO(2) superhalogen (4.46 eV) building-block, Cu(BO(2))(2) can be classified as a hyperhalogen. Density functional theory based calculations were carried out to identify the ground state geometries and study the electronic structures of these clusters. Cu(BO(2)) and Cu(BO(2))(2) clusters were found to form chainlike structures in both neutral and anionic forms. Cu(2)(BO(2)) and Cu(2)(BO(2))(2) clusters, on the other hand, preferred a chainlike structure in the anionic form but a closed ringlike structure in the neutral form. Equally important, substantial differences between adiabatic detachment energies and electron affinities were found, demonstrating that correct interpretation of the experimental photoelectron spectroscopy data requires theoretical support not only in determining the ground state geometry of neutral and anionic clusters, but also in identifying their low lying isomers.

Superhalogen properties of CumCln clusters: Theory and experiment

Using a combination of density functional theory and anion photoelectron spectroscopy experiment, we have studied the structure and electronic properties of CuCl(n)(-) (n = 1-5) and Cu(2)Cl(n)(-) (n = 2-5) clusters. Prominent peaks in the mass spectrum of these clusters occurring at n = 2, 3, and 4 in CuCl(n)(-) and at n = 3, 4, and 5 in Cu(2)Cl(n)(-) are shown to be associated with the large electron affinities of their neutral clusters that far exceed the value of Cl. While CuCl(n) (n ≥ 2) clusters are conventional superhalogens with a metal atom at the core surrounded by halogen atoms, Cu(2)Cl(n) (n ≥ 3) clusters are also superhalogens but with (CuCl)(2) forming the core. The good agreement between our calculated and measured electron affinities and vertical detachment energies confirm not only the calculated geometries of these superhalogens but also our interpretation of their electronic structure and relative stability.

Origin of the Unusual Stability of B12 and B13+ Clusters

A novel way to analyze the boron clusters is proposed in which the cluster is partitioned as inner and outer rings. Fragment molecular orbital analysis, based on this fragmentation, reveals that the delocalized valence electrons in B(12) and B(13)(+) clusters can be trifurcated as 6pi-6sigma(delo)-6sigma(3-ring), leading to triple aromaticity, which is unique to these clusters.

Origin of the Unusual Properties of Aun(BO2) Clusters

We report the discovery of a new class of clusters consisting of Au(n)(BO(2)) that forms during the oxygenation of gold clusters when boron nitride is used as insulation in a pulsed-arc cluster ion source (PACIS). Photoelectron and mass spectroscopy of these clusters further revealed some remarkable properties: instead of the expected Au(n)O(m) peaks, the mass spectra contain intense peaks corresponding to Au(n)(BO(2)) composition. Some of the most predominant features of the electronic structure of the bare Au clusters, namely even-odd alternation in the electron affinity, are preserved in the Au(n)(BO(2)) species. Most importantly, Au(n)(BO(2)) [odd n] clusters possess unusually large electron affinity values for a closed-shell cluster, ranging from 2.8-3.5 eV. The open-shell Au(n)(BO(2)) [even n] clusters on the other hand, possess electron affinities exceeding that of F, the most electronegative element in the periodic table. Using calculations based on density functional theory, we trace the origin of these species to the unusual stability and high electron affinity of the BO(2) moiety. The resulting bond formed between BO(2) and Au(n) clusters preserves the geometric and electronic structure of the bare Au(n) clusters. The large electron affinity of these clusters is due to the delocalization of the extra electron over the Au(n) cluster.

Aluminum Zintl anion moieties within sodium aluminum clusters.

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have established that aluminum moieties within selected sodium-aluminum clusters are Zintl anions. Sodium-aluminum cluster anions, Na(m)Al(n)(-), were generated in a pulsed arc discharge source. After mass selection, their photoelectron spectra were measured by a magnetic bottle, electron energy analyzer. Calculations on a select sub-set of stoichiometries provided geometric structures and full charge analyses for both cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra), and fragment molecular orbital based correlation diagrams.

The viability of aluminum Zintl anion moieties within magnesium-aluminum clusters

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have investigated the extent to which the aluminum moieties within selected magnesium-aluminum clusters are Zintl anions. Magnesium-aluminum cluster anions were generated in a pulsed arc discharge source. After mass selection, photoelectron spectra of MgmAln (-) (m, n = 1,6; 2,5; 2,12; and 3,11) were measured by a magnetic bottle, electron energy analyzer. Calculations on these four stoichiometries provided geometric structures and full charge analyses for the cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra). Calculations revealed that, unlike the cases of recently reported sodium-aluminum clusters, the formation of aluminum Zintl anion moieties within magnesium-aluminum clusters was limited in most cases by weak charge transfer between the magnesium atoms and their aluminum cluster moieties. Only in cases of high magnesium content, e.g., in Mg3Al11 and Mg2Al12 (-), did the aluminum moieties exhibit Zintl anion-like characteristics.