P. Jena

Spin Conservation Accounts for Aluminum Cluster Anion Reactivity Pattern with O2

The reactivity pattern of small (∼10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters AlnH–, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd-n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.

Origin of the unusual stability of MnO4

Abstract The attachment of an electron to manganese tetroxide cluster is found to lower its total energy by as much as 5 eV, thus putting MnO 4 into the class of superhalogens. This result, predicted by first-principles calculations based on density functional theory and generalized gradient approximation, is verified experimentally by photodetachment spectroscopy. The combined theoretical and experimental studies not only allow a fundamental understanding of the origin of the unusual stability of MnO 4 − but also provide information on ground-state geometries, vibrational frequencies, and electronic structure of MnO 4 , MnO 4 − , and their isomers.

Giant magnetic moments of nitrogen-doped Mn clusters and their relevance to ferromagnetism in Mn-doped GaN.

Calculations based on density-functional theory show that the stability and magnetic properties of small Mn clusters can be fundamentally altered by the presence of nitrogen. Not only are their binding energies substantially enhanced, but also the coupling between the magnetic moments at Mn sites remains ferromagnetic irrespective of their size or shape. In addition, these nitrogen-doped Mn clusters carry giant magnetic moments ranging from 4mu(B) in MnN to 22mu(B) in Mn5N. It is suggested that the giant magnetic moments of MnxN clusters may play a key role in the ferromagnetism of Mn-doped GaN which exhibit a wide range (10-940 K) of Curie temperatures.

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.

Interactions of Au cluster anions with oxygen

Experimental and theoretical evidence is presented for the nondissociative chemisorption of O2 on free Au cluster anions (Aun-, n=number of atoms) with n=2, 4, 6 at room temperature, indicating that the stabilization of the activated di-oxygen species is the key for the unusual catalytic activities of Au-based catalysts. In contrast to Aun- with n=2, 4, 6, O2 adsorbs atomically on Au monomer anions. For the Au monomer neutral, calculations based on density functional theory reveal that oxygen should be molecularly bound. On Au dimer and tetramer neutrals, oxygen is molecularly bound with the O-O bond being less activated with respect to their anionic counterparts, suggesting that the excess electron in the anionic state plays a crucial role for the O-O activation. We demonstrate that interplay between experiments on gas phase clusters and theoretical approach can be a promising strategy to unveil mechanisms of elementary steps in nanocatalysis.

Appearance of bulk properties in small tungsten oxide clusters

Contrary to the conventional understanding that atomic clusters usually differ in properties and structure from the bulk constituents of which they are comprised, we show that even a dimer of tungsten oxide (WO(3))(2) possesses bulklike features and the geometry of a small cluster containing only 4 tungsten and 12 oxygen atoms bears the hallmarks of crystalline tungsten oxide, WO(3). This observation, based on a synergistic approach involving mass distributions under quasisteady state conditions, photoelectron spectroscopy, and first principles molecular orbital theory, not only illustrates the existence of a class of strongly covalent or ionic materials whose embryonic forms are tiny clusters but also lends the possibility that a fundamental understanding of complex processes such as catalytic reactions on surfaces may be achieved on an atomic scale with clusters as model systems.

Anomalous magnetism in small Mn clusters

Abstract Unlike any other transition metal clusters, and in sharp contrast to its bulk behavior, Mn clusters containing up to 5 atoms are found to retain their atomic magnetic moments. Mn 2 , Mn 3 , Mn 4 and Mn 5 clusters in their ground states are coupled ferromagnetically and carry a moment of 10, 15, 20 and 25 μ B , respectively. Several low-lying excited states have also been identified. These results are based on all electron first-principles self-consistent calculations using density functional theory and generalized gradient approximation. Predictive capability of this approach is established by comparing the calculated energetics, electronic and magnetic properties of Mn 2 + with experiment.

Atomic and electronic structures of neutral and charged boron and boron-rich clusters

Ab initio molecular orbital theory based on both density functional formalism and quantum chemical methods has been used to calculate the equilibrium geometries, binding energies, ionization potentials, fragmentation patterns, and electronic structures of neutral and charged boron clusters containing up to six atoms. Calculations have also been performed on restricted geometries for BnX (n=1,5,12; X=Be, B, C) and B20 clusters to see if clusters can be designed so as to increase their stability. Energetics of doubly charged Bn++ clusters have also been studied to find the critical size for Coulomb explosion. The results are compared with existing experimental and theoretical data.

Electronic structure of chromium oxides, CrOn− and CrOn(n=1–5) from photoelectron spectroscopy and density functional theory calculations

The electronic structure of CrOn− and CrOn (n=1–5) was investigated using anion photoelectron spectroscopy and density functional theory. Photoelectron spectra of CrOn− were obtained at several photon energies and yielded electron affinities, vibrational and electronic structure information about the neutral CrOn species. Density functional theory calculations were carried out for both the neutrals and anions and were used to interpret the experimental spectra. Several low-lying electronic states of CrO were observed and assigned from photodetachment of the CrO− ground state (6∑+) and an excited state (4∏), which is only 0.1 eV higher. The main spectral features of CrO2− were interpreted based on a C2v CrO2− (4B1). A very weak Cr(O2)− isomer was also observed with lower electron binding energies. Relatively simple and vibrationally resolved spectra were observed for CrO3−, which was determined to be D3h. The CrO3 neutral was calculated to be C3v with the Cr atom slightly out of the plane of the three O at...

Atomic and electronic structure of neutral and charged SinOm clusters

Using molecular orbital approach and the generalized gradient approximation in the density functional theory, we have calculated the equilibrium geometries, binding energies, ionization potentials, and vertical and adiabatic electron affinities of SinOm clusters (n⩽6,m⩽12). The calculations were carried out using both Gaussian and numerical form for the atomic basis functions. Both procedures yield very similar results. The bonding in SinOm clusters is characterized by a significant charge transfer between the Si and O atoms and is stronger than in conventional semiconductor clusters. The bond distances are much less sensitive to cluster size than seen for metallic clusters. Similarly, calculated energy gaps between the highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) of (SiO2)n clusters increase with size while the reverse is the norm in most clusters. The HOMO-LUMO gap decreases as the oxygen content of a SinOm cluster is lowered eventually approaching the visible range. The photolum...