Quantum chemical exploration of formaldehyde clusters (H2CO)n (n = 2–4)

Global exploration of equilibrium structures and interconversion pathways on the quantum chemical potential energy surface (PES) is performed for (H₂CO)n (n = 2–4) by using the Scaled Hypersphere Search‐Anharmonic Downward Distortion Following (SHS‐ADDF) method. Density functional theoretical (DFT) calculations with empirical dispersion corrections (D3) yielded comparable results for formaldehyde dimer in comparison with recent detailed studies at CCSD(T) levels. Based on DFT‐D3 calculations, trimer and tetramer structures and their stabilities were studied. For tetramer, a highly symmetrical S₄ structure was found as the most stable form in good accordance with experimentally determined tetramer unit in the formaldehyde crystal. © 2018 Wiley Periodicals, Inc.     

Electronic,bonding, and optical properties of 1d [CuCN]n (n = 1–10) chains, 2d [CuCN]n (n = 2–10) nanorings,and 3d [Cun(CN)n]m (n = 4, m = 2, 3; n = 10, m = 2) tubes studied by DFT/TD‐DFT methods

The electronic, bonding, and photophysical properties of one‐dimensional [CuCN]_n (n = 1–10) chains, 2‐D [CuCN]_n (n = 2–10) nanorings, and 3‐D [Cu_n(CN)_n]_m (n = 4, m = 2, 3; n = 10, m = 2) tubes are investigated by means of a multitude of computational methodologies using density functional theory (DFT) and time‐dependent‐density‐functional theory (TD‐DFT) methods. The calculations revealed that the 2‐D [CuCN]_n (n = 2–10) nanorings are more stable than the respective 1‐D [CuCN]_n (n = 2–10) linear chains. The 2‐D [CuCN]_n (n = 2–10) nanorings are predicted to form 3‐D [Cu_n(CN)_n]_m (n = 4, m = 2, 3; n = 10, m = 2) tubes supported by weak stacking interactions, which are clearly visualized as broad regions in real space by the 3D plots of the reduced density gradient. The bonding mechanism in the 1‐D [CuCN]_n (n = 1–10) chains, 2‐D [CuCN]_n (n = 2–10) nanorings, and 3‐D [Cu_n(CN)_n]_m (n = 4, m = 2, 3; n = 10, m = 2) tubes are easily recognized by a multitude of electronic structure calculation approaches. Particular emphasis was given on the photophysical properties (absorption and emission spectra) of the [CuCN]_n chains, nanorings, and tubes which were simulated by TD‐DFT calculations. The absorption and emission bands in the simulated TD‐DFT absorption and emission spectra have thoroughly been analyzed and assignments of the contributing principal electronic transitions associated to individual excitations have been made. © 2015 Wiley Periodicals, Inc.     

Search for the global minimum structures of AlB3H2n (n = 0 − 6) clusters

The global minimum structures of AlB₃H_2n (n = 0–6) clusters are determined using the stochastic search method at the B3LYP/6–31G level of theory. These initially specified geometries are recalculated using B3LYP and CCSD(T) methods using the 6–311++G^** basis set. The structural and electronic properties of the two lowest‐lying isomers are presented. The structural parameters obtained for aluminum borohydride are compared with the experimental and theoretical results. The H₂ fragmentation energies of the most stable isomers are investigated. Chemical bonding analyses for the global minimum of AlB₃H_2n (n = 0–6) clusters are performed using the adaptive natural density partitioning method. © 2014 Wiley Periodicals, Inc.     

Structure and further fragmentation of significant [a3 + Na − H]+ ions from sodium‐cationized peptides

A good understanding of gas‐phase fragmentation chemistry of peptides is important for accurate protein identification. Additional product ions obtained by sodiated peptides can provide useful sequence information supplementary to protonated peptides and improve protein identification. In this work, we first demonstrate that the sodiated a₃ ions are abundant in the tandem mass spectra of sodium‐cationized peptides although observations of a₃ ions have rarely been reported in protonated peptides. Quantum chemical calculations combined with tandem mass spectrometry are used to investigate this phenomenon by using a model tetrapeptide GGAG. Our results reveal that the most stable [a₃ + Na ? H]⁺ ion is present as a bidentate linear structure in which the sodium cation coordinates to the two backbone carbonyl oxygen atoms. Due to structural inflexibility, further fragmentation of the [a₃ + Na ? H]⁺ ion needs to overcome several relatively high energetic barriers to form [b₂ + Na ? H]⁺ ion with a diketopiperazine structure. As a result, low abundance of [b₂ + Na ? H]⁺ ion is detected at relatively high collision energy. In addition, our computational data also indicate that the common oxazolone pathway to generate [b₂ + Na ? H]⁺ from the [a₃ + Na ? H]⁺ ion is unlikely. The present work provides a mechanistic insight into how a sodium ion affects the fragmentation behaviors of peptides. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical study of the stability and properties of magic numbers (m = 5, n = 2) and (m = 6, n = 3) of bimetallic bismuth‐copper nanoclusters; Bim Cun

Inspired by the experimental discovery of magic numbers we present a first study using density functional theory for the structure and properties of neutral and cationic Bi₆Cu₃ and Bi₅Cu₂ clusters. Our results confirm predictions based on Wade's rules. The closed electron shells, characteristic of cationic clusters help impose enhanced stability, while also complying with Wade's rules. Charge distribution analysis, as well as electrostatic potential maps show that in almost all cases, Bi atoms donate charges to Cu atoms. According to the analysis of condensed Fukui indices, Cu atoms inside both clusters are not reactive. Contrastingly, Bi atoms are reactive and may be targeted by different types of attack. This study of the electronic properties may thus help to determine experimental strategies with the capacity to enhance the synthesis of catalysts.     

Theoretical investigation of the structures and spectroscopic properties of (H2O4)n (n = 1–4) clusters

Density functional theory and ab initio calculations were performed to elucidate the hydrogen interactions in (H₂O₄)_n (n = 1–4) clusters. The optimized geometries, binding energies, and harmonic vibrational frequencies were predicted at various levels of theory. The trans conformer of the H₂O₄ monomer was predicted to be the most stable structure at the CCSD(T)/aug‐cc‐pVTZ level of theory. The binding energies per H₂O₄ monomer increased in absolute value by 9.0, 10.1, and 11.8 kcal/mol from n = 2 to n = 4 at the MP2/cc‐pVTZ level of theory (after the zero‐point vibrational energy and basis set superposition error corrections). This result implies that the intermolecular hydrogen bonds were stronger in the long‐chain clusters, that is, the formation of the longer chain in the (H₂O₄)_n clusters was more energetically favorable.     

(Rg = He ∼ Rn,n = 1–4): In quest of the potential trapping ability of the aromatic ring

A new series of divalent boron‐rare gas cations (Rg = He ∼ Rn, n = 1–4) have been predicted theoretically at the B3LYP, MP2, and CCSD(T) levels to present the structures, stability, charge distributions, bond natures, and aromaticity. The Rg B bond energies are quite large for heavy rare gases and increase with the size of the Rg atom. Because of steric hindrance new Rg atoms introduced to the B₄ ring will weaken the Rg B bond. Thus in the Rg B bond has the largest binding energy 90–100 kcal/mol. p‐ has a slightly shorter Rg B bond length and a larger bond energy than o‐ . NBO and AIM analyses indicate that for the heavy Rg atoms Ar ∼ Rn the B Rg bonds have character of typical covalent bonds. The energy decomposition analysis shows that the σ‐donation from rare gases to the boron ring is the major contribution to the Rg B bonding. Adaptive natural density partitioning and nuclear‐independent chemical shift analyses suggest that both and have obvious aromaticity.     

Structural connectivity and formation mechanism of monometallic cluster fullerenes YCN@Cn (n = 68–84)

Excited by the recently experimental reports of monometallic cluster fullerenes, we examined the electronic and geometrical properties of monometallic cluster fullerenes YCN@C_n with size from C₆₈ to C₈₄ by density functional theory and statistical thermodynamic calculations. The calculations demonstrate that the thermodynamically favored isomers of YCN@C_n are in good agreement with available experimental results. Morphology analysis shows that the lowest‐energy YCN@C_n species are structurally connected by C₂ insertion/extrusion and Stone–Wales rotation, which can be promoted under high temperature; enthalpy–entropy interplay can change the relative abundances of low‐energy isomers significantly at high temperature. All the results suggest that there is a structural evolution among these metallic cluster fullerenes in discharge condition, and thus, can rationalize their structural diversity in the soot and partly disclose their formation mechanism. The geometrical structures, electronic properties of these endohedral fullerene were discussed in detail.     

Formation of positive cluster ions LinBr (n = 2–7) and ionization energies studied by thermal ionization mass spectrometry

Clusters of the type Li_nX (X = halides) can be considered as potential building blocks of cluster‐assembly materials. In this work, Li_nBr (n = 2–7) clusters were obtained by a thermal ionization source of modified design and selected by a magnetic sector mass spectrometer. Positive ions of the Li_nBr (n = 4–7) cluster were detected for the first time. The order of ion intensities was Li₂Br⁺ > Li₄Br⁺ > Li₅Br⁺ > Li₆Br⁺ > Li₃Br⁺. The ionization energies (IEs) were measured and found to be 3.95 ± 0.20 eV for Li₂Br, 3.92 ± 0.20 eV for Li₃Br, 3.93 ± 0.20 eV for Li₄Br, 4.08 ± 0.20 eV for Li₅Br, 4.14 ± 0.20 eV for Li₆Br and 4.19 ± 0.20 eV for Li₇Br. All of these clusters have a much lower ionization potential than that of the lithium atom, so they belong to the superalkali class. The IEs of Li_nBr (n = 2–4) are slightly lower than those in the corresponding small Li_n or Li_nH clusters, whereas the IEs of Li_nBr are very similar to those of Li_n or Li_nH for n = 5 and 6. The thermal ionization source of modified design is an important means for simultaneously obtaining and measuring the IEs of Li_nBr (n = 2–7) clusters (because their ions are thermodynamically stable with respect to the loss of lithium atoms in the gas phase) and increasingly contributes toward the development of clusters for practical applications. Copyright © 2012 John Wiley & Sons, Ltd.     

Electronic and structural properties of Lin@Be2B8 (n = 1–14) and Lin@Be2B36 (n = 1–21) nanoflakes shed light on possible anode materials for Li‐based batteries

Systematic addition of Li atoms to the Be₂B₈ and Be₂B₃₆ backbones has been studied by density functional theory‐based calculations with the aim to investigate properties of interest on possible anode materials for Li‐based batteries. For the Be₂B₈Li_n (n = 1–8) and the Be₂B₃₆Li_n (n = 1–20) systems, lithium salts are dominant whereas a clear electride feature shows up for Be₂B₈Li_n (n = 9–14) and Be₂B₃₆Li₂₁. Addition of hydrogen radicals to these systems shows that the Be₂B₈Li₁₄ electride becomes a Be₂B₈Li₁₄H₂ hydride electride whereas Be₂B₃₆Li₂₁ leads to a Be₂B₃₆Li₂₁H salt. Moreover, for the addition of Li atoms to Be₂B₈ and the Be₂B₃₆ backbones, large values of the interaction and of the adsorption energy per Li atom, high specific capacity of Be₂B₈Li₁₄ and of Be₂B₃₆Li₂₁ (1860 and 1017 mAh g⁻¹, respectively) and low and flat voltage associated with lithiation have been found. Likewise, the considerable thermodynamic driving force (ΔG° = −29.66 kcal/mol) and the small energy barrier ( = 0.26 kcal/mol) associated with electron transfer in Be₂B₃₆Li₂₁ and Be₂B₃₆ species confirm that boron rich species have potential abilities to be used in the Li‐based battery. © 2018 Wiley Periodicals, Inc.     

Theoretical investigations on the superhalogen properties and interaction of PdOn (n = 1–5) species

Density functional calculations on the ground state geometries and stabilities of PdO_n species (n = 1–5) are performed in neutral as well as anionic forms. Calculations reveal that Pd can bind stably with four O atoms indicating the maximum oxidation state of Pd as high as +8. The electron affinities of PdO_n suggest that these species behave as superhalogens for n ≥ 2. The large electron affinities of PdO_n species along with stability of their anions point toward the synthesis of new class of compounds having unusual oxidizing capabilities. This possibility is explored by considering the interaction of PdO₂ superhalogen with Ca atom which forms a stable CaPdO₂ complex. In this complex, PdO₂ unit closely mimics the behavior of O atom when compared with CaO molecule. © 2013 Wiley Periodicals, Inc.     

A high‐accuracy theoretical study of the CHnP Systems n = 1–3

We have performed high‐level electronic structure computations on the most important species of the CH_nP systems n = 1–3 to characterize them and provide reliable information about the equilibrium and vibrationally averaged molecular structures, rotational constants, vibrational frequencies (harmonic and anharmonic), formation enthalpies, and vertical excitation energies. Those chemical systems are intermediates for several important reactions and also prototypical phosphorus‐carbon compounds; however, they are often elusive to experimental detection. The present results significantly complement their knowledge and can be used as an assessment of the experimental information when available. The explicitly correlated coupled‐cluster RCCSD(T)‐F12 method has been used for geometry optimizations and vibrational frequency calculations. Vibrational configuration interaction theory has been used to account for anharmonicity effects. Basis‐set limit extrapolations have been carried out to determine accurate thermochemical quantities. Electronic excited states have been calculated with coupled‐cluster approaches and also by means of the multireference configuration interaction method. © 2013 Wiley Periodicals, Inc.     

Electronic and magnetic properties of small RhnCa (n = 1–9) clusters: A DFT study

The equilibrium geometries, relative stabilities, electronic and magnetic properties of small Rh_nCa (n = 1–9) clusters have been investigated by DFT calculations. The obtained results show that the three‐dimensional geometries are adopted for the lowest‐energy Rh_nCa clusters, and the doped Ca atom prefers locating on the surface of the cluster. Based on the analysis of the second‐order difference of energies, fragmentation energies and the HOMO‐LUMO energy gaps, we identify that the Rh₄Ca, Rh₆Ca, and Rh₈Ca clusters are relatively more stable than their neighboring clusters, and the doping of Ca enhances the chemical reactivity of the pure Rh_n clusters, suggesting that the Rh_nCa clusters can be used as nanocatalysts in many catalytic reactions. The magnetic moment for these clusters is mostly localized on the Rh atoms, and the doping Ca atom has no effect on the total magnetic moment of Rh_nCa clusters. The partial density of states, VIP, VEA, and η of these clusters in their ground‐state structures were also calculated and discussed. © 2015 Wiley Periodicals, Inc.     

Theoretical insight of nitric oxide adsorption on neutral and charged Pdn (n = 1–5) clusters

Density functional theory (DFT) calculations within the framework of generalized gradient approximation have been used to systematically investigate the adsorption of nitric oxide (NO) molecule on neutral, cationic, and anionic Pd_n (n = 1–5) clusters. NO coordinate to one Pd atom of the cluster by the end‐on mode, where the tilted end‐on structure is more favorable due to the additional electron in the π* orbital. On the contrary, in the neutral and cationic Pd₂ system, NO coordinates to the bridge site of cluster preferably by the side‐on mode. Charge transfer between Pd clusters and NO molecule and the corresponding weakening of N? O bond is an essential factor for the adsorption. The N? O stretching frequency follow the order of cationic > neutral > anionic. Binding energy of NO on anionic clusters is found to be greater than those of neutral and cationic clusters. © 2015 Wiley Periodicals, Inc.     

Ab initio investigations on the stabilities of AuOnq− (q = 0 to 3; n = 1 to 4) species: Superhalogen behavior of AuOn (n ≥ 2) and their interactions with an alkali metal

Theoretical density functional calculations are performed on AuO_n^q? species for q = 0–3 and n = 1–4 in various spin states. AuO_n species are found to be relatively more stable in their mono‐anionic forms and behave as superhalogens for n ≥ 2. The maximum oxidation state of Au is found to be +7 in these species, but limited to +5. This fact is explained by considering interactions of AuO_n superhalogens with K atom and which leads to the formation of more stable KAuO_n complex up to n = 3, only. Thus, the present study is expected not only to motivate the synthesis of a new class of salts but also to assign the maximum oxidation state of gold. © 2013 Wiley Periodicals, Inc.     

Computational studies of stable hexanuclear CulAgmAun (l + m + n = 6; l,m, n > 0) clusters

A DFT study was carried out on the ground state structures of ternary Cu_lAg_mAu_n (l + m + n = 6) clusters, with the aim of investigating changes of thermal and kinetic stabilities as an effect of composition, as well as the composition dependence of the electrostatic potential, of stable planar structures. DFT optimizations were performed using the PBE functional and the SDD basis set. All the optimized structures adopt planar geometries with bent triangular structures. Calculated binding energy values are in the range 1.5–1.9 eV/atom, which shows their thermal stability. The predicted HOMO‐LUMO energy gap values are in the semiconductor region, providing a qualitative indication of a moderate kinetic stability. NBO analyses indicate the existence of two mechanisms promoting planar structural stability, one due to bonding‐antibonding orbital interaction, and the other one due to the well‐known spd hybridization. Wiberg indices were obtained showing interatomic bonding. Electrostatic potential calculations show the existence of nucleophilic attack regions preferentially around silver and copper atoms located at the vertices while electrophilic attack regions are found in the vicinity of gold atoms over the cluster plane. Apparently, charge transfer occurs toward gold from silver and copper atoms when the concentration is favorable in the proximity of gold atoms. In particular, if the small ternary clusters discussed here contain only one gold atom, then a high electron density is observed at the site of this gold atom. © 2016 Wiley Periodicals, Inc.     

Structures and electronic properties of the small rubidium‐doped silicon RbSin (n = 1–12) clusters

The geometries, relative stabilities, and electronic properties of small rubidium‐doped silicon clusters RbSi_n (n = 1–12) have been systematically investigated using the density functional theory at the B3LYP/GENECP level. The optimized structures show that lowest‐energy isomers of RbSi_n are similar with the ground state isomers of pure Si_n clusters and prefer the three‐dimensional for n = 3–12. The relative stabilities of RbSi_n clusters have been analyzed on the averaged binding energy, fragmentation energy, second‐order energy difference, and highest occupied molecular orbital‐lowest unoccupied molecular orbital energy gap. The calculated results indicate that the doping of Rb atom enhances the chemical activity of Si_n frame and the magic number is RbSi₂. The Mulliken population analysis reveals that the charges in the corresponding RbSi_n clusters transfer from the Rb atom to Si atoms. The partial density of states and chemical hardness are also discussed. © 2014 Wiley Periodicals, Inc.     

Lanthanide metals in the boron cages: Computational prediction of M@Bn (M = Eu,Gd; n = 38, 40)

Endohedral metalloborofullerenes (EMBFs) are novel boron analogues of the famous endohedral metallofullerenes (EMFs). Many EMBFs have been proposed by theoretical calculations thus far. However, in sharp contrast to EMFs, which trap most of the lanthanides with f electrons inside the cages, the corresponding lanthanide‐based EMBFs have never been reported. In this work, the encapsulation of Eu and Gd in the B₃₈ and B₄₀ fullerenes was studied by means of density functional theory calculations. Our results revealed that Gd@B₃₈(⁹A), Eu@B₄₀(⁸B₂), and Gd@B₄₀(⁷A″) all favor the endohedral configuration, and the electronic structures can be described as Gd³⁺@ , Eu²⁺@ , and Gd³⁺@ with jailed f electron spins. The large binding energies and sizable HOMO–LUMO gaps suggest that they may be achieved experimentally. They feature σ and π double aromaticity, and their excellent stabilities were confirmed by the Born–Oppenheimer molecular dynamics simulations. Finally, the infrared and UV/vis spectra were simulated to assist experimental characterization.