Recent Progress on the Design, Characterization, and Application of Superalkalis

Electron superhalogens as positronium superhalogens

Positronium (Ps) exhibits the ability to form energetically stable complexes with atoms and molecules before annihilation occurs. In particular, F, a halogen, shows the highest reported positronium binding energy (2.95 eV) in the periodic table. Superhalogens are defined as molecules with electron affinities exceeding that of Cl (3.61 eV), the atom with the highest electron affinity. Building upon the concept of superhalogens, we can define Ps-superhalogens as molecules with Ps binding energies surpassing that of F. This study explores structural and energetic aspects of positronium and positron binding to neutral and anionic superhalogen molecules of the MXk+1 family (M = Li, Na, Be, Mg, B, Al, Si, P; X = F, Cl, Br), respectively and where k represents the highest formal valence of M. We perform multicomponent MP2 calculations for positron systems, which reveal how positron affinities vary with the type and number of halogen atoms present. The analysis of the results emphasizes the predominant role of electrostatic interactions in determining the positron affinity, with negligible effects of electronic and geometric relaxation upon positron attachment. We predict the energetic stability of 22 of the 24 PsMXk+1 complexes with respect to the chemically relevant dissociation channels: e+ emission, Ps emission and M-X bond breaking. Our findings reveal six MFk+1 systems that qualify as Ps-superhalogens, showing a positronium binding energy exceeding 2.95 eV. Of these, AlF4 stands out by setting a new record for the highest positronium binding energy among neutral molecules, reaching 4.36 eV.

Suzuki-Miyaura Cross-Coupling Reaction Catalyzed by Al12M (M = Be, Al, C, and P) Superatoms with Different Numbers of Valence Electrons

The exploration of low-cost, efficient, environmentally safe, and selective catalysts for the activation of carbon-halogen bonds has become an important and challenging topic in modern chemistry. With the help of density functional theory (DFT), it is found that phenyl bromide (PhBr) can be efficiently chemisorbed by the Al12M (M = Be, Al, C, and P) superatoms via forming highly polarized Al-Br covalent bonds, where the C-Br bonds of PhBr can be effectively activated through the electron transfer from Al12M. The different electronic structures of these four Al12M superatoms pose a substantial effect on their performances on the activation of PhBr and the catalytic mechanisms of the Suzuki-Miyaura (SM) reaction. Among them, the alkali-metal-like superatom Al12P exhibits the best performance for the activation of PhBr. In particular, Al13 and Al12P with open-shell electronic structures exhibit catalytic performances comparable to those of previously reported catalysts for this coupling reaction. Hence, it is highly expected that Al13 and Al12P could be used as novel superatom catalysts for C-C coupling reactions and, therefore, open up new possibilities to use nonprecious superatoms in catalyzing the activation and transformation of carbon-halogen bonds.

Superalkali nature of the Si9M5 (M = Li, Na, and K) Zintl clusters: a theoretical study on electronic structure and dynamic nonlinear optical properties

Zintl clusters have attracted widespread attention because of their intriguing bonding and unusual physical properties. We explore the Si9 and Si9M5 (where M = Li, Na, and K) Zintl clusters using the density functional theory combined with other methods. The exothermic nature of the Si9M5 cluster formation is disclosed, and the interactions of alkali metals with pristine Si9 are shown to be noncovalent. The reduced density gradient analysis is performed, in which increased van der Waals interactions are observed with the enlargement of the size of alkali metals. The influence of the implicit solvent model is considered, where the hyperpolarizability (βo) in the solvent is found to be about 83 times larger than that in the gas phase for Si9K5. The frequency-dependent nonlinear optical (NLO) response for the dc-Kerr effect is observed up to 1.3 × 1011 au, indicating an excellent change in refractive index by an externally applied electric field. In addition, natural bonding orbitals obtained from the second-order perturbation analysis show the charge transfer with the donor-acceptor orbitals. Electron localization function and localized orbital locator analyses are also performed to better understand the bonding electrons in designed clusters. The studied Zintl clusters demonstrate the superalkali character in addition to their remarkable optical and nonlinear optical properties.

Superalkalis with Hydrogen as Central Electronegative Atom and their Possible Applications: Ab Initio and DFT Study

Superalkalis are unusual species having ionization energies lower than that of the alkali metals. These species with various applications are of great importance in chemistry due to their low ionization energies and strong reducing property. A typical superalkali contains a central electronegative core decorated with excess metal ligands. In the quest for novel superalkalis, we have designed the superalkalis HLi2, HLiNa and HNa2 using hydrogen as central electronegative atom for the first time employing high level ab initio (CCSD(T), MP2) and density functional theory (ωB97X-D) methods. The superalkalis exhibit very low ionization energies, even lower than that of cesium. Stability of these species is verified from binding energy and dissociation energy values. The superalkalis are capable of reducing SO2, NO, CO2, CO and N2 molecules by forming stable ionic complexes and therefore can be used as catalysts for the reduction or activation of systems possessing very low electron affinities. The superalkalis form stable supersalts with tailored properties when interact with a superhalogen. They also show remarkably high non-linear optical responses, hence could have industrial applications. It is hoped that this work will enrich the superalkali family and spur further theoretical and experimental research in this direction.

Design of High-Performance Inorganic-Organic Hybrid Nonlinear Optical Materials Using Superhalogen Al13 and Dianhydrides

Novel inorganic-organic hybrid complexes Al13-X (X represents the dianhydrides PMDA, NTCDA, and PTCDA) are theoretically designed and studied using density functional theory (DFT) and time-dependent DFT. These conjugated dianhydrides containing four acceptor carbonyl groups are commonly used as electron acceptor materials. These compounds possess large binding energies, reflecting the sufficient binding of Al13 to the dianhydride molecule. The binding nature of the complexes is of charge transfer type, i.e., electrons are transferred from the aluminum cluster to the dianhydride. All of the aimed complexes have large mean polarizability (α0) and first hyperpolarizability (β0). The β0 values are explained on the basis of electronic transitions in crucial excited states using the TD-DFT method. Additionally, the hole-electron distribution was analyzed, revealing the nature of electronic excitation. Absorption spectra analysis shows that these complexes have an excellent infrared (IR) transparent region (1000-5000 nm). Therefore, these inorganic-organic hybrid complexes with high stability can be considered as potential candidates for new IR nonlinear optical molecules.

Dual-Valence Characteristics of Be11: Tin/Lead-like Superatom

To enhance the superatom family, the new superatom analogue Be11 of group IVA elements has been developed. Be11 can exhibit multiple valence states (+2 and +4), similar to carbon-group elements, and is capable of forming stable ionic compounds with other atoms such as carbon, chalcogen, (super)halogen, and hydroxyl. This resembles how tin and lead atoms combine with these elements to form stable molecules. Their special stability can be rationalized from the perspective of a cluster shell model. Sn or Pb could be the nearest atomic analogue to Be11 in group IVA, as the +2 oxidation state is more stable than the +4 oxidation state. This comparative investigation highlights the resemblance between Be11 and carbon-group elements, which encourages additional exploration within the superatom family.

Superalkali-alkaline earthide ion pairs of δ+(AM-HMHC)-AM'δ- (AM = Li, Na and K; AM' = Be, Mg and Ca) possessing large NLO responses and excellent electronic stabilities and alkalide characteristics: a DFT study

To identify superalkali-alkaline earthide ion pairs, it's theoretically shown that, as a novel class of excess electron superalkali compounds, both chair and boat forms of (AM-HMHC)-AM' (AM = Li, Na, and K; AM' = Be, Mg, and Ca; HMHC = 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane) are good candidates. An attractive superalkali-alkaline earthide ion pair in δ+(AM-HMHC)-AM'δ- is firstly exhibited, which possesses alkaline-earthide characteristics and nonlinear optical response superior to similar M+(calix[4]pyrrole)M'- (M = Li, Na, and K; M' = Be, Mg, and Ca) with high stability. The electronic and vibrational second order hyperpolarizabilities and the frequency-dependent first hyperpolarizabilities of δ+(AM-HMHC)-AM'δ- are presented. For each pair of (AM-HMHC)-AM', the boat conformation is preferred to its chair one in the case of Hyper-Rayleigh scattering response (βHRS). These alkaline earthides suggest prominently high βHRS up to 2.59 × 104 a.u. (boat forms of δ+(Na-HMHC)-Caδ-). We expect that this work will inspire the preparation and characterization of these new alkaline earthides as high-performance NLO materials.

Design and Investigation of Superatoms for Redox Applications: First-Principles Studies

A superatom is a cluster of atoms that acts like a single atom. Two main groups of superatoms are superalkalis and superhalogens, which mimic the chemistry of alkali and halogen atoms, respectively. The ionization energies of superalkalis are smaller than those of alkalis (<3.89 eV for cesium atom), and the electron affinities of superhalogens are larger than that of halogens (>3.61 eV for chlorine atom). Exploring new superalkali/superhalogen aims to provide reliable data and predictions of the use of such compounds as redox agents in the reduction/oxidation of counterpart systems, as well as the role they can play more generally in materials science. The low ionization energies of superalkalis make them candidates for catalysts for CO2 conversion into renewable fuels and value-added chemicals. The large electron affinity of superhalogens makes them strong oxidizing agents for bonding and removing toxic molecules from the environment. By using the superatoms as building blocks of cluster-assembled materials, we can achieve the functional features of atom-based materials (like conductivity or catalytic potential) while having more flexibility to achieve higher performance. This feature paper covers the issues of designing such compounds and demonstrates how modifications of the superatoms (superhalogens and superalkalis) allow for the tuning of the electronic structure and might be used to create unique functional materials. The designed superatoms can form stable perovskites for solar cells, electrolytes for Li-ion batteries of electric vehicles, superatomic solids, and semiconducting materials. The designed superatoms and their redox potential evaluation could help experimentalists create new materials for use in fields such as energy storage and climate change.

Polymeric tungsten carbide nanoclusters as potential non-noble metal catalysts for CO oxidation

The discovery of tungsten carbide (WC) as an analog of the noble metal Pt atom is of great significance toward designing novel highly-active catalysts from the viewpoint of the superatom concept. The potential of such a superatom to serve as building blocks of replacement catalysts for Pt has been evaluated in this work. The electronic properties, adsorption behaviors, and catalytic mechanisms towards the CO oxidation of (WC)_n and Pt_n (n = 1, 2, 4, and 6) were compared. Counterintuitively, these studied (WC)_n clusters exhibit quite different electronic properties and adsorption behaviours from the corresponding Pt_n species. For instance, (WC)_n preferentially adsorbs O₂, whereas Pt_n tends to first combine with CO. Even so, it is interesting to find that the catalytic performances of (WC)_n are always superior to the corresponding Pt_n, and especially, the largest (WC)₆ cluster exhibits the best catalytic ability towards CO oxidation. Therefore, assembling superatomic WC clusters into larger polymeric clusters can be regarded as a novel strategy to develop efficient superatom-assembled catalysts for CO oxidation. It is highly expected to see the realization of non-noble metal catalysts for various reactions in the near future experiments by using superatoms as building blocks.

Recent advances in in silico design and characterization of superalkali-based materials and their potential applications: A review

In the advancement of novel materials, chemistry plays a vital role in developing the realm where we survive. Superalkalis are a group of clusters/molecules having lower ionization potentials (IPs) than that of the cesium atom (3.89 eV) and thus, show excellent reducing properties. However, the chemical industry and material science both heavily rely on such reducing substances; an in silico approach-based design and characterization of superalkalis have been the focus of ongoing studies in this area along with their potential applications. However, although superalkalis have been substantially sophisticated materials over the past couple of decades, there is still room for enumeration of the recent progress going on in various interesting species using computational experiments. In this review, the recent developments in designing/modeling and characterization (theoretically) of a variety of superalkali-based materials have been summarized along with their potential applications. Theoretically acquired properties of some novel superalkali cations (Li3 +) and C6Li6 species, etc. for capturing and storing CO2/N2 molecules have been unveiled in this report. Additionally, this report unravels the first-order polarizability-based nonlinear optical (NLO) response features of numerous computationally designed novel superalkali-based materials, for instance, fullerene-like mixed-superalkali-doped B12N12 and B12P12 nanoclusters with good UV transparency and mixed-valent superalkali-based CaN3Ca (a high-sensitivity alkali-earth-based aromatic multi-state NLO molecular switch, and lead-founded halide perovskites designed by incorporating superalkalis, supersalts, and so on) which can indeed be used as a new kind of electronic nanodevice used in designing hi-tech NLO materials. Understanding the mere interactions of alkalides in the gas and liquid phases and the potential to influence how such systems can be extended and applied in the future are also highlighted in this survey. In addition to offering an overview of this research area, it is expected that this review will also provide new insights into the possibility of expanding both the experimental synthesis and the practical use of superalkalis and their related species. Superalkalis present the intriguing possibility of acting as cutting-edge construction blocks of nanomaterials with highly modifiable features that may be utilized for a wide-ranging prospective application.

Insights into the nonlinear optical (NLO) response of pure Aum (2 ≥ m ≤ 7) and copper-doped Au m -xCu x clusters

A series of small pure Au m (2 ≥ m ≤ 7) and copper-doped Au m-x Cu x clusters was evaluated by density functional theory (DFT) at the CAM-B3LYP/LANL2DZ level for their geometric, electronic, and nonlinear optical (NLO) properties. The charge transfer for the Au cluster significantly improved by reducing the HOMO-LUMO energy gap from 3.67 eV to 0.91 eV after doping with Cu atoms. The doping of Cu also showed noteworthy impacts on other optical and NLO properties, including a decrease in the excitation energy and increase in the dipole moment and oscillator strength. Furthermore, changes in the linear isotropic and anisotropic polarizabilities (α iso and α aniso) and first and second NLO hyperpolarizabilities (β static, γ static) were also observed in the pure and Cu-doped clusters, which enhanced the NLO response. The nonlinear optical properties of the clusters were evaluated by calculating the static and frequency dependent second- and third-order NLO polarizabilities at 1064 nm wavelength. Among all the doped structures, the Au3Cu1 cluster showed the largest static first hyperpolarizability of β (total) = 4.73 × 103 au, while the Au1Cu6 cluster showed frequency dependent first hyperpolarizability of β (-2w;w,w) = 1.26 × 106 au. Besides this, large static and frequency-dependent second hyperpolarizability values of 6.30 × 105 au and 1.05 × 10 au were exhibited by Cu7 and Au1Cu6, respectively. This study offers an effective approach to design high-performance NLO materials utilizing mixed metal clusters which might have broad applications in the fields of optoelectronics and electronics.

Designing Special Nonmetallic Superalkalis Based on a Cage-like Adamanzane Complexant

In this study, to examine the possibility of using cage-like complexants to design nonmetallic superalkalis, a series of X@3⁶adz (X = H, B, C, N, O, F, and Si) complexes have been constructed and investigated by embedding nonmetallic atoms into the 3⁶adamanzane (3⁶adz) complexant. Although X atoms possess very high ionization energies, these resulting X@3⁶adz complexes possess low adiabatic ionization energies (AIEs) of 0.78–5.28 eV. In particular, the adiabatic ionization energies (AIEs) of X@3⁶adz (X = H, B, C, N, and Si) are even lower than the ionization energy (3.89 eV) of Cs atoms, and thus, can be classified as novel nonmetallic superalkalis. Moreover, due to the existence of diffuse excess electrons in B@3⁶adz, this complex not only possesses pretty low AIE of 2.16 eV but also exhibits a remarkably large first hyperpolarizability (β₀) of 1.35 × 10⁶ au, indicating that it can also be considered as a new kind of nonlinear optical molecule. As a result, this study provides an effective approach to achieve new metal-free species with an excellent reducing capability by utilizing the cage-like organic complexants as building blocks.

Novel and Polynuclear K- and Na-Based Superalkali Hydroxides as Superbases Better Than Li-Related Species and Their Enhanced Properties: An Ab Initio Exploration

Hydroxides of superalkalis (particularly, K- and Na-related species) are shown for the first time to function as superbases. A new small series of hydroxides (XM n+1OH) is designed based on superalkali species (XM n+1) where M (K and Na) is alkali metal atoms, n is the maximal formal valence of the central atom X (F, O, and N), and n ≥ 1. To probe whether such fascinating polynuclear superalkali hydroxides (SAHs), especially the K- and Na-associated moieties are as basic as the representative alkali metal hydroxides (KOH, NaOH, and LiOH) as well as similar Li-based SAHs, a comprehensive computational exploration (in the gas phase) has been reported using the framework of an ab initio method. The ab initio calculations reveal that both the K- and Na-related SAHs consisting of larger gas-phase proton affinity (PA) and gas-phase basicity (GB) values demonstrate stronger basic character compared to the LiOH and Li-based SAHs. However, the available SAHs act as strong bases as well as superbases; among the proposed K- and Na-based SAHs, remarkably, the OK3OH moiety having the highest PA (1168.4 kJ/mol) and GB (1146.9 kJ/mol) values shows the evidence of the strongest basicity (i.e., superbase/hyperbase), which exceed enough (ΔPA: 142.1 kJ/mol and ΔGB: 146.9 kJ/mol) the IUPAC-defined superbasicity threshold values (PA: 1026.3 kJ/mol and GB: 1000 kJ/mol) of 1,8-bis(dimethylamino)naphthalene (DMAN). Furthermore, theoretical signatures have been predicted via the electronic structure calculation approach in probing the dissociation energy, ionization potential, electron affinity, HOMO-LUMO gap, and chemical hardness as well as the NCI plot and QTAIM tools are used for the bonding feature analysis and such parameters are well linked with the basicity analyzing parameters. In this study, the ab initio-based computational experiments provide some new insights into the basicity features and understanding of the structural and electronic features of a small series of designed K- and Na-related SAHs. Design and synthesis of such theoretically examined SAHs may pave alternative routes for the experimentally rewarding applications.

Bimetallic superalkali substitution in the CsPbBr3 perovskite: Pseudocubic phases and tunable bandgap

Perovskites attract attention as efficient light absorbers for solar cells due to their high-power conversion efficiency (up to 24%). The high photoelectric conversion efficiency is greatly affected by a suitable band structure. Cation substitution can be an effective approach to tune the electronic band structure of lead halide perovskites. In this work, superalkali cations were introduced to replace the Cs⁺ cation in the CsPbBr₃ material. The bimetallic superalkalis (LiMg, NaMg, LiCa, and NaCa) were inserted since they are structurally simple systems and have a strong tendency to lose one electron to achieve a closed-shell cation. The cation substitution in the lead halide perovskite leads to changes in the shape of both valence and conduction bands compared to CsPbBr₃. Introducing superalkali cations produces extra electronic states close to the Fermi level, which arise from the formation of alkali earth metal states at the top of the valence band. Our first-principles computations reveal that bimetallic superalkali substitution decreases the bandgap of the perovskite. The bandgaps of MgLi-PbBr₃ (1.35 eV) and MgNa-PbBr₃ (1.06 eV) are lower than the bandgap of CsPbBr₃ (2.48 eV) and within the optimal bandgap (i.e., 1.1-1.4 eV) for single-junction solar cells. Thus, the MgLi-PbBr₃ and MgNa-PbBr₃ inorganic perovskites are promising candidates for high-efficiency solar cells.

Designing an alkali-metal-like superatom Ca3B for ambient nitrogen reduction to ammonia

Converting earth-abundant nitrogen (N₂) gas into ammonia (NH₃) under mild conditions is one of the most important issues and a long-standing challenge in chemistry. Herein, a new superatom Ca₃B was theoretically designed and characterized to reveal its catalytic performance in converting N₂ into NH₃ by means of density functional theory (DFT) computations. The alkali-metal-like identity of this cluster is verified by its lower vertical ionization energy (VIE, 4.29 eV) than that of potassium (4.34 eV), while its high stability was guaranteed by the large HOMO-LUMO gap and binding energy per atom (E_b). More importantly, this well-designed superatom possesses unique geometric and electronic features, which can fully activate N₂via a "double-electron transfer" mechanism, and then convert the activated N₂ into NH₃ through a distal reaction pathway with a small energy barrier of 0.71 eV. It is optimistically hoped that this work could intrigue more endeavors to design specific superatoms as excellent catalysts for the chemical adsorption and reduction of N₂ to NH₃.

Small lithium-chloride clusters: Superalkalis, superhalogens, supersalts and nanocrystals

In the present paper, the results of the theoretical investigation of the small lithium-chloride clusters are reported. The geometrical structures, electronic and thermodynamic stability of superalkalis, superhalogens, and their single and double charged ions are obtained using efficient and accurate quantum chemistry methods. Further, low-lying isomers of the Li_n Cl_n ( n = 2 - 5 ) clusters and their stability parameters are calculated. Two ways of formation of the Li_n Cl_n clusters, polymerization of LiCl fragments and combination of superalkalis and superhalogen clusters, are compared. By examination the lattice energy and the average Li-Cl bond length in rectangular Li_n Cl_n ( n ≤ 60 ) clusters, it was concluded that already 50 LiCl are enough to mostly resembles the structure and stability of the bulk LiCl.

On the Role of Alkali-Metal-Like Superatom Al12 P in Reduction and Conversion of Carbon Dioxide

Developing efficient catalysts for the conversion of CO₂ into fuels and value-added chemicals is of great significance to relieve the growing energy crisis and global warming. With the assistance of DFT calculations, it was found that, different from Al₁₂ X (X=Be, Al, and C), the alkali-metal-like superatom Al₁₂ P prefers to combine with CO₂ via a bidentate double oxygen coordination, yielding a stable Al₁₂ P(η² -O₂ C) complex containing an activated radical anion of CO₂ (i.e., CO₂ ^.- ). Thereby, this compound could not only participate in the subsequent cycloaddition reaction with propylene oxide but also initiate the radical reaction with hydrogen gas to form high-value chemicals, revealing that Al₁₂ P can play an important role in catalyzing these conversion reactions. Considering that Al₁₂ P has been produced in laboratory and is capable of absorbing visible light to drive the activation and transformation of CO₂ , it is anticipated that this work could guide the discovery of additional superatom catalysts for CO₂ transformation and open up a new research field of superatom catalysis.

DFT and QTAIM studies on the reduction of carbon monoxide by superalkalis

Carbon monoxide (CO) is a toxic gas molecule with no positive electron affinity, which makes it difficult to reduce it into CO¯. In this work, we perform density functional theory (DFT) and quantum theory of atoms in molecule (QTAIM) based studies on the interaction of CO molecule with superalkali (SA) clusters. Our findings suggest that this interaction results in SA(CO) complexes, which are stabilized by purely ionic as well as partially covalent bonds although their binding energy decreases with the increase in the size of SA clusters. In these ionic complexes, the electron is transferred from the SA cluster to the CO molecule. This suggests the single-electron reduction of the CO molecule by interacting with superalkalis. This work may offer some novel insights into the detection and reduction of stable CO molecule and related systems.

On the Interaction between Superatom Al12Be and DNA Nucleobases/Base Pairs: Bonding Nature and Potential Applications in O2 Activation and CO Oxidation

The interaction between quasi-chalcogen superatom Al₁₂Be and DNA nucleobases/base pairs has been explored by searching for the most stable Al₁₂Be-X (X = DNA bases and base pairs) complexes. Our results reveal that Al₁₂Be prefers to combine with guanine by two Al-O and Al-N bonds rather than the other DNA bases, no matter in free state or base pair. The formed Al-N and Al-O bonds between Al₁₂Be and DNA bases proved to be strong polar covalent bonds by the Wiberg bond index, nature bond orbitals, atoms in molecules theory, localized molecular orbitals, and electron localization functions analyses. More importantly, it is found that the formed global minimum of Al₁₂Be-G has the ability to activate an oxygen molecule into a peroxide dianion ¹O₂ ^2-, which can further catalyze the CO oxidation via the Eley-Rideal mechanism with a small energy barrier of 7.78 kcal/mol. We hope that this study could not only provide an in-depth understanding on the intermolecular interaction between metallic superatoms and DNA at the molecular level but also attract more interest in designing and synthesizing superatom-based heterogeneous catalysts with DNA/nucleobases as basic building blocks.

Intramolecular Reactions in Ionized Ammonia Clusters: A Direct Ab Initio Molecular Dynamics Study

Ammonia cluster cations are a chemical species that has recently attracted considerable research attention as an ion-molecule reaction species in the planetary atmosphere, surface reaction species in materials chemistry, and super-alkali species. Reactions of the radical cation of an ammonia cluster, [(NH₃)_n]⁺ (n = 2-6), following the ionization of the parent neutral cluster, were investigated using direct ab initio molecular dynamics to elucidate the reactions of the ammonia cluster cation under astrochemical conditions. The calculations showed that two competing reaction channels-proton transfer (PT) channel and complex formation channel-operate after the ionization of neutral clusters. In the PT channel, a proton of NH₃⁺ was transferred to a neighboring ammonia molecule. The PT channel was found in all clusters (n = 2-6). Reaction via the PT channel became faster with increasing cluster size and saturated around n = 5-6. In the complex formation channel, a face-to-face complex having a H₃N-NH₃⁺ structure (with a N-N bond) was formed. This channel was found only in larger clusters (n = 5-6). Time scales of PT and complex formation channels were calculated to be 20-30 and 40-50 fs, respectively. The reaction mechanism was discussed based on the results of theoretical calculations.