Possible Roles of Transition Metal Cations in the Formation of Interstellar Benzene via Catalytic Acetylene Cyclotrimerization

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous interstellar molecules. However, the formation mechanisms of PAHs and even the simplest cyclic aromatic hydrocarbon, benzene, are not yet fully understood. Recently, we reported the statistical and dynamical properties in the reaction mechanism of Fe+-catalyzed acetylene cyclotrimerization, whereby three acetylene molecules are directly converted to benzene. In this study, we extended our previous work and explored the possible role of the complex of other 3d transition metal cations, TM+ (TM = Sc, Ti, Mn, Co, and Ni), as a catalyst in acetylene cyclotrimerization. Potential energy profiles for bare TM+-catalyst (TM = Sc and Ti), for TM+NC--catalyst (TM = Sc, Ti, Mn, Co, and Ni), and for TM+-(H2O)8-catalyst (TM = Sc and Ti) systems were obtained using quantum chemistry calculations, including the density functional theory levels. The calculation results show that the scandium and titanium cations act as efficient catalysts in acetylene cyclotrimerization and that reactants, which contain an isolated acetylene and (C2H2)2 bound to a bare (ligated) TM cation (TM = Sc and Ti), can be converted into a benzene-metal-cation product complex without an entrance barrier. We found that the number of electrons in the 3d orbitals of the transition metal cation significantly contributes to the catalytic efficiency in the acetylene cyclotrimerization process. On-the-fly Born-Oppenheimer molecular dynamics (BOMD) simulations of the Ti+-NC- and Ti+-(H2O)8 complexes were also performed to comprehensively understand the nuclear dynamics of the reactions. The computational results suggest that interstellar benzene can be produced via acetylene cyclotrimerization reactions catalyzed by transition metal cation complexes.

Probing Cluster-π Interactions between Cun- and C2H2/C2H4 for Gas Separation

Copper-related materials are used for separation of ethylene and acetylene gases in chemistry; however, the precise mechanism regarding selectivity is elusive to be fully understood. Here, we have conducted a joint experimental and theoretical study of the Cu_n^- (n = 7-30) clusters in reacting with C₂H₄ and C₂H₂. It is found that all of the Cu_n^- clusters readily react with C₂H₂, giving rise to C₂H₂-addition products; however, Cu₁₈^- and Cu₁₉^- do not react with C₂H₄. We illustrate the superatomic stability of Cu₁₈^- and advocate its availability to separate C₂H₄ from C₂H₂. Further, we demonstrate the atomically precise mechanism regarding selectivity by fully unveiling the size-dependent cluster-π interactions.

Impact sensitivity of aryl diazonium chlorides: Limitations of molecular and solid-state approach

The mechanism of the compression-induced decomposition of aryl diazonium chlorides is proposed on the basis of quantum-chemical calculations of both the isolated cations and crystalline materials. The electron transfer from the anion to the cation, followed by the crystal decomposition, is observed with the rise of pressure. Taking the known nature of the structural changes in cations undergone upon reduction, five structural, vibrational and electronic determinants of impact sensitivity are found. Thus, a correlation (R² = 0.79) between the experimentally known impact sensitivity of 40 different aryl diazonium cations and the developed empirical function Ω, which includes the above-mentioned parameters, is obtained. Meanwhile, an abnormal impact sensitivity of 4-nitrobenzenediazonium chloride (4 J) compared to the parent benzenediazonium chloride (3 J) is rationalized on the basis of first-principles calculations of the latter and its three nitro derivatives. Using our recently proposed solid-state criteria of impact sensitivity, another empirical function Ω was developed being able to predict impact sensitivity of these four salts with very good confidence (R² = 0.97).

A unified model of impact sensitivity of metal azides

A comprehensive theoretical study of 18 metal azides is reported.

Significance of crystal habit sphericity in the determination of the impact sensitivity of bistetrazole-based energetic salts

The effect of crystal habit sphericity (Ψ) on impact sensitivity is described quantitatively. The developed empirical function (Ω), which includes five quantities obtained from first-principles calculations, correlates well with the experimentally measured impact sensitivities of 20 crystalline energetic salts based on 5,5′-bistetrazole derivatives.

Spontaneous disproportionation of lithium biphenyl in solution: a combined experimental and theoretical study

Spontaneous disproportionation of lithium biphenyl radical anion into a lithium biphenyl dianion plus neutral biphenyl, according to 2LiBiph ⇄ Li₂Biph + Biph, has been evidenced experimentally by UV-vis spectroscopy and backed up theoretically using TDDFT methods.

Super high-energy density single-bonded trigonal nitrogen allotrope—a chemical twin of the cubic gauche form of nitrogen

A new ambient-pressure metastable single-bonded nitrogen allotrope was predicted using reliable theoretical methods. The predicted allotrope has a number of similarities with the experimentally detected cubic gauche nitrogen allotrope.

Two isomeric solid carbon nitrides with 1 : 1 stoichiometry which exhibit strong mechanical anisotropy

Two isomeric layered carbon nitride polymorphs are characterized using reliable theoretical methods. The NCNC phase, which is predicted for the first time, has a number of differences with the isomeric NCCN polymorph in its electronic, spectral and mechanical properties.

Hetero-dinuclear complexes of 3d metals with a bridging dinitrogen ligand: theoretical prediction of the characteristic features of geometry and spin multiplicity

Spin multiplicities and coordination structures of dinitrogen-bridged hetero-dinuclear complexes of 3d metals, (μ-N2)[M(1)(AIP)][M(2)(AIP)] (AIPH = (Z)-1-amino-3-imino-prop-1-ene; M(1), M(2) = V(i) to Co(i)), were investigated using the CASPT2 method. (μ-N2)[V(AIP)][Cr(AIP)] has a low spin doublet ((2)B2) ground state with an η(2)-side-on dinitrogen coordination structure but (μ-N2)[Mn(AIP)][Fe(AIP)] has a high spin octet ((8)A2) ground state with an η(1)-end-on coordination structure. These results are similar to those of the homo-dinuclear Cr and Fe analogues, respectively. In (μ-N2)[Cr(AIP)][M(AIP)] (M = Mn(i), Fe(i), or Co(i)) consisting of an early 3d metal (Cr) and a late one (Mn to Co), on the other hand, we found characteristic features in the geometry and the ground state electronic structure which are different from those of homo-dinuclear analogues. The Cr-Mn complex has a high spin decet ((10)B1) ground state with an η(2)-side-on structure. This decet state has the highest spin multiplicity in the dinuclear transition metal complexes, to our knowledge. The A2 state with a doublet spin multiplicity is moderately less stable than the (10)B1 state. The optimized structures and the molecular orbitals indicate that the Cr atom strongly interacts with the N2 moiety in the (10)B1 state but the Mn atom strongly interacts with the N2 moiety in the (2)A2 state. The Cr-Fe complex has a high spin nonet ((9)B1) ground state with an η(2)-side-on structure like the Cr-Mn complex, but only the Cr-Co complex has a medium spin quartet (4)A2 ground state with an η(2)-side-on structure. The different ground electronic state of the Cr-Co complex arises from the presence of 3d orbitals at low energy. Based on these results, it is concluded that the geometry is determined by the Cr center but the electronic structure and the spin multiplicity are determined by the combination of early and late 3d metals in the dinitrogen-bridged hetero-dinuclear chelates of 3d metals.

CASPT2 study of inverse sandwich-type dinuclear 3d transition metal complexes of ethylene and dinitrogen molecules: similarities and differences in geometry, electronic structure, and spin multiplicity

The spin multiplicities and coordination structures of inverse sandwich-type complexes (ISTCs) of ethylene and dinitrogen molecules with 3d transition metal elements (Sc to Ni), (μ-C2H4)[M(AIP)]2 and (μ-N2)[M(AIP)]2 (AIPH = (Z)-1-amino-3-iminoprop-1-ene; M = Sc to Ni) were investigated by the CASPT2 method. In both ethylene and dinitrogen ISTCs of the early 3d transition metals (Sc to Cr), sandwiched ethylene and dinitrogen ligands coordinate with two metal atoms in an η(2)-side-on form and their ground states have an open-shell singlet spin multiplicity. The η(1)-end-on coordination structure of dinitrogen ISTCs is considerably less stable than the η(2)-side-on form in these metals. For the late 3d transition metals (Mn to Ni), ethylene and dinitrogen ISTCs exhibit interesting similarities and differences in spin multiplicity and structure as follows: in ethylene ISTCs of Mn to Ni, the ground state has an open-shell singlet spin multiplicity like those of the ISTCs of early transition metals. However, the ethylene ligand is considerably distorted, in which the ethylene carbon atoms have a tetrahedral-like structure similar to sp(3) carbon and each of them coordinates with one metal in a μ-η(1):η(1) structure. These geometrical features are completely different from those of ISTCs of the early transition metals. In dinitrogen ISTCs of Mn to Ni, on the other hand, the ground state has a high spin multiplicity from nonet (Mn) to triplet (Ni). The η(2)-side-on coordination structure of the dinitrogen ligand is as stable as the η(1)-end-on form in the Mn complex but the η(1)-end-on structure is more stable than the η(2)-side-on form in the Fe to Ni complexes. All these interesting similarities and differences between ethylene and dinitrogen ISTCs and between the early and late transition metal elements arise from the occupation of several important molecular orbitals.