Yeon Jae Ko

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 ,

Photoelectron spectroscopy of europium-silicon cluster anions, EuSin− (3⩽n⩽17)

We report the photoelectron spectra of EuSi(n) (-) cluster anions (3<or=n<or=17). They reveal dramatic electronic rearrangements over the size range n=10-12. In particular, a marked increase in the adiabatic electron affinity of EuSi(12) (2.8 eV) compared to its stoichiometric neighbor, EuSi(11) (1.9 eV), is observed. We propose that a significant geometric reorganization due to the encapsulation of a europium atom occurs in this size range and is responsible for the detected changes in the electronic structure. In light of this interpretation, EuSi(12) is the smallest fully endohedral europium-silicon cluster.

Electronic structure and properties of isoelectronic magic clusters: Al13X (X=H,Au,Li,Na,K,Rb,Cs)

The equilibrium structure, stability, and electronic properties of the Al(13)X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable us to identify the ground state geometries of these clusters both in neutral and anionic configurations.

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.

Low‐Energy‐Barrier Proton Transfer Induced by Electron Attachment to the Guanine⋅⋅⋅Cytosine Base Pair

The photoelectron spectrum of the anion of the guanine...cytosine base pair (GC)(*-) is recorded for the first time. The observed variation in the spectral peak-height ratios with the source conditions suggests the presence of two or more anionic isomers. Two maxima of the broad bands in the photoelectron spectrum were measured at about 1.9 and about 2.6 eV. These values are very well reproduced by the vertical detachment energies of the B3LYP/6-31++G(d,p) calculated low-energy anionic structures, which are 1) the Watson-Crick base-pair anion with proton transferred from N1 of guanine to N3 of cytosine, 2) its analogue in which the proton is transferred from N9 of guanine to N7 of guanine, and 3) the global minimum geometry, which is formed from the latter anion by rotation of guanine about the axis approximately defined by C2 of guanine and C4 of cytosine. Furthermore, a minor difference in the stabilities of the two lowest energy anions explains the experimentally observed source (temperature) dependence of the PES spectrum. A rational procedure, based on the chemistry involved in the formation of anionic dimers, which enables the low-energy anions populated in the photoelectron spectrum to be identified is proposed. In contrast to the alternative combinatorial approach, which in the studied case would lead to carrying out quantum chemical calculations for 2000-2500 structures, the procedure described here reduces the computational problem to only 15 geometries.

Valence Anions of 9-Methylguanine-1-Methylcytosine Complexes. Computational and Photoelectron Spectroscopy Studies

The photoelectron spectrum for the radical anion of 9-methylguanine-1-methylcytosine, MGMC(-), was recorded for the first time. To interpret the experimental results, B3LYP/6-31++G(d,p) level computational studies were carried out for the neutral and anionic complexes of MGMC/MGMC(-) stabilized by three hydrogen bonds and comprising canonical or low-energy tautomeric forms of the methylated nucleobases. The visualization of singly occupied molecular orbitals for the MGMC(-) anions indicates that they are valence-bound species since the excess electron is localized on a pi* orbital of cytosine. All but one of the studied anionic complexes are adiabatically stable at the applied B3LYP level of theory. The intensity maximum of the broad band in the photoelectron spectrum was measured at 2.1 eV. This value is very well reproduced by the calculated vertical detachment energy of the calculated global minimum geometry of the MGMC(-) anion. This structure is the Watson-Crick base pair anion with proton transferred from the N1 atom of guanine to the N3 site of cytosine. The calculated adiabatic electron affinities span a range of 0.39-0.60 eV. The consequences of electron attachment to the AT or GC base pairs incorporated within DNA are briefly discussed in the context of DNA damage by low-energy electrons.

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.

Photoelectron spectroscopy of boron aluminum hydride cluster anions

Boron aluminum hydride clusters are studied through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations. Boron aluminum hydride cluster anions, BxAlyHz(-), were generated in a pulsed arc cluster ionization source and identified by time-of-flight mass spectrometry. After mass selection, their photoelectron spectra were measured by a magnetic bottle-type electron energy analyzer. The resultant photoelectron spectra as well as calculations on a selected series of stoichiometries reveal significant geometrical changes upon substitution of aluminum atoms by boron atoms.

Evolution of superhalogen properties in PtCln clusters

We have systematically calculated the ground state geometries, relative stability, electronic structure, and spectroscopic properties of PtCl(n) (n = 1-7) clusters. The bonding in these clusters is dominated by covalent interaction. In neutral clusters, chlorine atoms are chemically bound to Pt up to n = 5. However, in neutral PtCl(6) and PtCl(7) clusters, two of the chlorine atoms bind molecularly while the remaining bind as individual atoms. In the negative ions, this happens only in the case of PtCl(7) cluster. The geometries of both neutral and anionic clusters can be considered as fragments of an octahedron and are attributed to the stabilization associated with splitting of partially filled d orbitals under the chloride ligand field. The electron affinity of PtCl(n) clusters rises steadily with n, reaching a maximum value of 5.81 eV in PtCl(5). PtCl(n) clusters with n ≥ 3 are all superhalogens with electron affinities larger than that of chlorine. The accuracy of our results has been verified by carrying out photoelectron spectroscopy experiments on PtCl(n)(-) anion clusters.

Combined experimental and theoretical investigation of three-dimensional, nitrogen-doped, gallium cluster anions.

Anion photoelectron spectra of Ga(x)N(y)(-) cluster anions, in which x = 4-12, y = 1 and x = 7-12, y = 2, were measured. Ab initio studies were conducted on Ga(x)N(y)(-) cluster anions in which x = 4-7, y = 1 and Ga(7)N(2)(-), providing their structures and electronic properties. The photoelectron spectra were interpreted in terms of the computational results. This allowed for identification of the isomers present in the beam experiments for specific Ga(x)N(-) cluster anions (x = 4-7). The unexpected presence of Ga(x)N(2)(-) species is also reported.