Infrared Spectra of Magnesium Hydride Molecules, Complexes, and Solid Magnesium Dihydride

Spectroscopic Study of [Mg, H, N, C, O] Species: Implications for the Astrochemical Magnesium Chemistry

Magnesium is an abundant metal element in space, and magnesium chemistry has vital importance in the evolution of interstellar medium (ISM) and circumstellar regions, such as the asymptotic giant branch star IRC+10216 where a variety of Mg compounds bearing H, C, N, and O have been detected and proposed as the important components in the gas-phase molecular clouds and solid-state dust grains. Herein, we report the formation and infrared spectroscopic characterization of the Mg-bearing molecules HMg, [Mg, N, C], [Mg, H, N, C], [Mg, N, C, O], and [Mg, H, N, C, O] from the reactions of Mg/Mg+ and the prebiotic isocyanic acid (HNCO) in the solid neon matrix. Based on their thermal diffusion and photochemical behavior, a complex reactivity landscape involving association, decomposition, and isomerization reactions of these Mg-bearing molecules is developed, which can not only help understand the chemical processes of the magnesium (iso)cyanides in astrochemistry but also provide implications on the presence of magnesium (iso)cyanates in the ISM and the chemical model for the dust grain surface reactions. It also provides a new paradigm of the key intermediate nature of the cationic complexes in the formation of neutral interstellar species.

A Holistic View of the Interactions between Electron-Deficient Systems: Clustering of Beryllium and Magnesium Hydrides and Halides

In the search for common bonding patterns in pure and mixed clusters of beryllium and magnesium derivatives, the most stable dimers and trimers involving BeX2 and MgX2 (X = H, F, Cl) have been studied in the gas phase using B3LYP and M06-2X DFT methods and the G4 ab initio composite procedure. To obtain some insight into their structure, stability, and bonding characteristics, we have used two different energy decomposition formalisms, namely MBIE and LMO-EDA, in parallel with the analysis of the electron density with the help of QTAIM, ELF, NCIPLOT, and AdNDP approaches. Some interesting differences are already observed in the dimers, where the stability sequence observed for the hydrides differs entirely from that of the fluorides and chlorides. Trimers also show some peculiarities associated with the presence of compact trigonal cyclic structures that compete in stability with the more conventional hexagonal and linear forms. As observed for dimers, the stability of the trimers changes significantly from hydrides to fluorides or chlorides. Although some of these clusters were previously explored in the literature, the novelty of this work is to provide a holistic approach to the entire series of compounds by using chemical bonding tools, allowing us to understand the stability trends in detail and providing insights for a significant number of new, unexplored structures.

Eco-Friendly Superhydrophobic Coupling Conversion Coating with Corrosion Resistance on Magnesium Alloy

An eco-friendly superhydrophobic conversion coating is fabricated to enhance the corrosion resistance of the AZ31B Mg alloy by combining the deep eutectic solvent pretreatment and electrodeposition. The coral-like micro-nano structure formed by reacting deep eutectic solvent and Mg alloy provides a structural basis for constructing a superhydrophobic coating. Cerium stearate with low surface energy is deposited on the structure, providing the coating's superhydrophobicity and the corrosion inhibition effect. Electrochemical test results demonstrate that the as-prepared superhydrophobic conversion coating (water contact angle at 154.7°) with a 99.68% protection effect significantly improves anticorrosion properties for the AZ31B Mg alloy. The corrosion current density decreases from 1.79 × 10^-4 A·cm^-2 of Mg substrate to 5.57 × 10^-7 A·cm^-2 of the coated sample. Besides, the electrochemical impedance modulus reaches the value of 1.69 × 10³ Ω·cm² and increases approximately 23 times in magnitude compared with the Mg substrate. Furthermore, the corrosion protection mechanism is attributed to the coupling effect of water-repellency barrier protection and corrosion inhibition, resulting in excellent corrosion resistance. Results demonstrate a promising strategy for the corrosion protection of Mg alloys by replacing the chromate conversion coating with the superhydrophobic coupling conversion coating.

Magnesium: properties and rich chemistry for new material synthesis and energy applications

Magnesium (Mg) has many unique properties suitable for applications in the fields of energy conversion and storage. These fields presently rely on noble metals for efficient performance. However, among other challenges, noble metals have low natural abundance, which undermines their sustainability. Mg has a high negative standard reduction potential and a unique crystal structure, and its low melting point at 650 °C makes it a good candidate to replace or supplement numerous other metals in various energy applications. These attractive features are particularly helpful for improving the properties and limits of materials in energy systems. However, knowledge of Mg and its practical uses is still limited, despite recent studies which have reported Mg's key roles in synthesizing new structures and modifying the chemical properties of materials. At present, information about Mg chemistry has been rather scattered without any organized report. The present review highlights the chemistry of Mg and its uses in energy applications such as electrocatalysis, photocatalysis, and secondary batteries, among others. Future perspectives on the development of Mg-based materials are further discussed to identify the challenges that need to be addressed.

Access to a Labile Monomeric Magnesium Radical by Ball-Milling

In order to isolate a monometallic Mg radical, the precursor (Am)MgI⋅(CAAC) (1) was prepared (Am=tBuC(N-DIPP)2 , DIPP=2,6-diisopropylphenyl, CAAC=cyclic (alkyl)(amino)carbene). Reduction of a solution of 1 in toluene with the reducing agent K/KI led to formation of a deep purple complex that rapidly decomposed. Ball-milling of 1 with K/KI gave the low-valent MgI complex (Am)Mg⋅(CAAC) (2) which after rapid extraction with pentane and crystallization was isolated in 15 % yield. Although a benzene solution of 2 decomposes rapidly to give Mg(Am)2 (3) and unidentified products, the radical is stable in the solid state. Its crystal structure shows planar trigonal coordination at Mg. The extremely short Mg-C distance of 2.056(2) Å indicates strong Mg-CAAC bonding. Calculations and EPR measurements show that most of the spin density is in a π* orbital located at the C-N bond in CAAC, leading to significant C-N bond elongation. This is supported by calculated NPA charges in 2: Mg +1.73, CAAC -0.82. Similar metal-to-CAAC charge transfer was calculated for M0 (CAAC)2 and [MI (CAAC)2 + ] (M=Be, Mg, Ca) complexes in which the metal charges range from +1.50 to +1.70. Although the spin density of the radical is mainly located at the CAAC ligand, complex 2 reacts as a low-valent MgI complex: reaction with a I2 solution in toluene gave (Am)MgI⋅(CAAC) (1) as the major product.

Reductive Dimerization of CO by a Na/Mg(I) Diamide

Sodium reduction of [{SiNDipp}Mg] [{SiNDipp} = {CH2SiMe2N(Dipp)}2; Dipp = 2,6-i-Pr2C6H3] provides the Mg(I) species, [{SiNDipp}MgNa]2, in which the long Mg-Mg bond (>3.2 Å) is augmented by persistent Na-aryl interactions. Computational assessment indicates that this molecule is best considered to comprise a contiguous tetrametallic core, a viewpoint borne out by its reaction with CO, which results in ethynediolate formation mediated by the dissimilar metal centers.

Density Functional Theory Modeling of Reactions of Addition of H2 Molecules to Magnesium Clusters Mg17M Doped with Atoms M of Transition 3d Elements

Density functional theory calculations of potential energy surface (PES) have been performed for elementary hydrogenation reactions Mg₁₇M + H₂ → Mg₁₇MH₂ of magnesium clusters Mg₁₇M doped by transition 3d metals (M = Ti-Ni), and for consecutive reactions Mg₁₇Ni + nH₂ → Mg₁₇NiH_2n of addition of n hydrogen molecules to Ni-doped clusters Mg₁₇Ni and Mg₁₇NiH₂. Energetic, geometric, and spectroscopic features of intermediates and transition states along the minimum energy pathway have been found, and their trends were analyzed with dopants changing along the 3d series and with increasing number of atoms H attached to the surface positions of the magnesium backbone.

Dibridged, Monobridged, Vinylidene-Like, and Linear Structures for the Alkaline Earth Dihydrides Be2H2, Mg2H2, Ca2H2, Sr2H2, and Ba2H2. Proposals for Observations

This research reports a search for peculiar monobridged structures of the E₂H₂ molecules (E = Be, Mg, Ca, Sr, Ba). For Be₂H₂ and Mg₂H₂, the monobridged geometry is not an equilibrium but rather a transition state between the vinylidene-like structure and the global minimum HE-EH linear geometry. However, for Ca₂H₂, Sr₂H₂, and Ba₂H₂, this situation changes significantly; the linear structure is no longer the global minimum but lies higher in energy than two other equilibria, the dibridged and monobridged structures. The planar dibridged structures of both Sr₂H₂ and Ba₂H₂ should be observable via IR spectroscopy. Although the remarkable monobridged structures lie 8.3 (Sr) and 7.6 kcal/mol (Ba) higher, the large IR intensities of the terminal E-H stretching frequencies may make the monobridged structures observable. The monobridged structures have sizable permanent dipole moments (3.07 and 3.06 D for Sr and Ba, respectively) and also should be observable via microwave spectroscopy.

Watching in situ the hydrogen diffusion dynamics in magnesium on the nanoscale

Active plasmonic and nanophotonic systems require switchable materials with extreme material contrast, short switching times, and negligible degradation. On the quest for these supreme properties, an in-depth understanding of the nanoscopic processes is essential. Here, we unravel the nanoscopic details of the phase transition dynamics of metallic magnesium (Mg) to dielectric magnesium hydride (MgH₂) using free-standing films for in situ nanoimaging. A characteristic MgH₂ phonon resonance is used to achieve unprecedented chemical specificity between the material states. Our results reveal that the hydride phase nucleates at grain boundaries, from where the hydrogenation progresses into the adjoining nanocrystallites. We measure a much faster nanoscopic hydride phase propagation in comparison to the macroscopic propagation dynamics. Our innovative method offers an engineering strategy to overcome the hitherto limited diffusion coefficients and has substantial impact on the further design, development, and analysis of switchable phase transition as well as hydrogen storage and generation materials.

Covalent Surface Functionalization of Bovine Serum Albumin to Magnesium Surface to Provide Robust Corrosion Inhibition and Enhance In Vitro Osteo-Inductivity

Herein, we describe precisely a covalent modification of pure magnesium (Mg) surface and its application to induce in vitro osteogenic differentiation. The new concept of a chemical bonding method is proposed for developing stable chemical bonds on the Mg surface through the serial assembly of bioactive additives that include ascorbic acid (AA) and bovine serum albumin (BSA). We studied both the physicochemical and electrochemical properties using scanning electron microscopy and other techniques to confirm how the covalent bonding of BSA on Mg can, after coating, significantly enhance the chemical stability of the substrate. The modified Mg-OH-AA-BSA exhibits better anti-corrosion behavior with high corrosion potential (E_corr = -0.96 V) and low corrosion current density (I_corr = 0.2 µA cm^-2) as compared to the pure Mg (E_corr = -1.46 V, I_corr = 10.42 µA cm^-2). The outer layer of BSA on Mg protects the fast degradation rate of Mg, which is the consequence of the strong chemicals bonds between amine groups on BSA with carboxylic groups on AA as the possible mechanism of peptide bonds. Collectively, the results suggest that the surface-modified Mg provides a strong bio-interface, and enhances the proliferation and differentiation of pre-osteoblast (MC3T3-E1) cells through a protein-lipid interaction. We therefore conclude that the technique we describe provides a cost-effective and scalable way to generate chemically stable Mg surface that inherits a biological advantage in orthopedic and dental implants in clinical applications.

Convergent energies and anharmonic vibrational spectra of Ca2H2 and Ca2H4 constitutional isomers

Three constitutional isomers of both Ca₂H₂ and Ca₂H₄ have been characterized with molecular electronic structure theory. Correlation methods as complete as CCSDT(Q) and basis sets as large as cc-pwCV5Z have been used to converge the relative energies within chemical accuracy (≤1 kcal mol^-1). Anharmonic vibrational frequencies were computed using second-order vibrational perturbation theory employing CCSD(T)/cc-pwCVTZ cubic and quartic force-fields and a CCSD(T)/cc-pwCVQZ quadratic force field. The monobridged [Ca(μ₂-H)CaH] and dibridged [Ca(μ₂-H)₂Ca] isomers of Ca₂H₂ were predicted to lie 6.5 and 12.9 kcal mol^-1 below the energy of the classical HCaCaH linear isomer, respectively. Despite the energetic favorability of the bridged Ca₂H₂ isomers, we conclude (surprisingly) that only the higher energy linear structure has been observed in the laboratory. At 0 K, the tribridged [Ca(μ₂-H)₃CaH] isomer of Ca₂H₄ is predicted to be enthalpically favored by 0.9 kcal mol^-1 in comparison to the enthalpy of the dibridged [HCa(μ₂-H)₂CaH] structure. Comparison of experiment with our computed frequencies suggests that the observed vibrational features arise from both the dibridged and the tribridged Ca₂H₄ structures.

Activation of Small Molecules (H2, CO2, N2O, CH4, and C6H6) by a Porphyrinoid-Based Dimagnesium(I) Complex, an Electride

A density functional theory-based computation has been carried out to reveal the geometrical and electronic structures of Mg₂EP (1), where EP is an extended (3.1.3.1) porphyrinoid system. EP is a 22 π electronic system and is aromatic in nature. Here, we have studied the thermodynamic and kinetic stabilities of EP^2--supported Mg₂ ²⁺ ion. The nature of bonding has been studied using natural bond orbital and atoms in molecule schemes. The presence of a covalent Mg(I)-Mg(I) σ-bond in Mg₂EP is confirmed. The occurrence of a non-nuclear attractor (NNA) with large electron population, negative Laplacian of electron density at NNA, and presence of an electron localization function basin along with large nonlinear optical properties prompt us to classify Mg₂EP as the first porphyrinoid-based organic electride. Further five small molecules, viz., dihydrogen (H₂), carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄), and benzene (C₆H₆), are found to be activated by the electron density between the two Mg atoms in Mg₂EP.

Facile preparation of β-/γ-MgH₂ nanocomposites under mild conditions and pathways to rapid dehydrogenation

A magnesium hydride composite with enhanced hydrogen desorption kinetics can be synthesized via a simple wet chemical route by ball milling MgH2 with LiCl as an additive at room temperature followed by tetrahydrofuran (THF) treatment under an Ar atmosphere. The as-synthesized composite comprises ca. 18 mass% orthorhombic γ-MgH2 and 80 mass% tetragonal β-MgH2 as submicron-sized particles. The β-/γ-MgH2 nanocomposite exhibits a dehydrogenation capacity of 6.6 wt% and starts to release hydrogen at ∼260 °C; ca. 140 °C lower than that of commercial MgH2. The apparent activation energy for dehydrogenation is 115 ± 3 kJ mol(-1), which is ca. 46% lower than that of commercial MgH2. Analysis suggests that the meta-stable γ-MgH2 component either directly dehydrogenates exothermically or first transforms into stable β-MgH2 very close to the dehydrogenation onset. The improved hydrogen release performance can be attributed both to the existence of the MgH2 nanostructure and to the presence of γ-MgH2.

Electronic structures and second hyperpolarizabilities of alkaline earth metal complexes end-capped with NA2 (A = H, Li, Na)

The ground state structures and NLO properties of a number of alkaline earth metal complexes end-capped with NA₂ groups (A = H, Li, Na) are calculated by employing the CAM-B3LYP, wB97XD and B2PLYP functionals along with MP2 and CCSD(T) for 6-311++G(d,p), 6-311++G(3df,3pd), aug-cc-pVTZ, aug-pc-2 and Hypol basis sets.

Infrared spectra of NgBeS (Ng = Ne, Ar, Kr, Xe) and BeS2 in noble-gas matrices

Laser-ablated beryllium atom has been codeposited at 4 K with hydrogen sulfide in excess noble gas matrices. Four noble-gas compounds NgBeS (Ng = Ne, Ar, Kr, Xe) and the BeS(2) molecule are identified on the basis of the S-34 isotopic substitution, DFT and CCSD(T) theoretical predictions, and a comparison of noble-gas substitution. The agreement between the experimental and calculated vibrational frequencies supports the identification of these molecules. The dissociation energies are calculated at 1.6, 12.6, 10.7, and 13.4 kcal/mol for NeBeS, ArBeS, KrBeS, and XeBeS, respectively, at the CCSD(T) level. The BeS Lewis acid molecule favors strong chemical binding between the Be and Ng atoms.

Mg2H2: new insight on the Mg-Mg bonding and spectroscopic study

The six dimensional potential energy surface of the electronic ground state X̃(1)Σ(g)(+) of Mg(2)H(2) has been generated by the coupled-cluster approach with single, double and perturbative triple excitations [CCSD(T)] combined with the aug-cc-pCVTZ basis set for Mg atoms and the aug-cc-pVTZ basis set for the H atoms. The analytical representation of this surface was used in variational calculations of the rovibrational energies of Mg(2)H(2), Mg(2)D(2), and HMg(2)D for J = 0 and 1. For Mg(2)H(2), the rotational constant B(0) is computed to be 0.1438 cm(-1), and the fundamental anharmonic wavenumbers are calculated to be ν(1) = 1527.3 cm(-1) (Σ(g)(+)), ν(2) = 275.3 cm(-1) (Σ(g)(+)), ν(3) = 1503.6 cm(-1) (Σ(u)(+)), ν(4) = 312.9 cm(-1) (Π(g)), and ν(5) = 256.5 cm(-1) (Π(u)). In addition, the electronic ground states of Mg(2)H, MgH(2), Mg(2), and MgH have been investigated in order to compute the bonding energies of Mg(2)H(2) and to explain the strength of the Mg-Mg bond in this tetra-atomic molecule. The nature of the low-lying excited states of Mg(2)H(2) is also studied.

Effect of Magnesium Hydride on the Corrosion Behavior of Pure Magnesium in 0.1 M NaCl Solution

The effect of magnesium hydride on the corrosion behavior of pure magnesium in 0.1 M NaCl solution was investigated using the gas collection method, potentiostatic current decay test, and in situ Raman spectrum. The formation of magnesium hydride (MgH2,Mg2H4) was observed at the cathodic region. Applying anodic potential leads to decomposition of magnesium hydride. Magnesium hydride plays an important role on the negative difference effect (NDE) in both the cathodic and anodic regions.

A computational study of 2J(HH)(gem) indirect spin-spin coupling constants in simple hydrides of the second and third periods

Several theoretical methods have been used to compute (2)J(HH) in neutral, anionic and cationic HXH hydrides, X being the 14 nuclei from Li to Cl (28 molecules). Since the calculations also provide (1)J(XH) spin-spin coupling constants (SSCC), these have also been analyzed. The best results were obtained using Second-order polarization propagator approximation (SOPPA)/sadJ. The geminal coupling constants appear to be dependent on the electronegativity of the X-atom.

Infrared Spectra and Theoretical Calculations of Lithium Hydride Clusters in Solid Hydrogen, Neon, and Argon

A matrix isolation IR study of laser-ablated lithium atom reactions with H2 has been performed in solid para-hydrogen, normal hydrogen, neon, and argon. The LiH molecule and (LiH)(2,3,4) clusters were identified by IR spectra with isotopic substitution (HD, D(2), and H(2) + D(2)) and comparison to frequencies calculated by density functional theory and the MP2 method. The LiH diatomic molecule is highly polarized and associates additional H(2) to form primary (H(2))(2)LiH chemical complexes surrounded by a physical cage of solid hydrogen where the ortho and para spin states form three different primary complexes and play a role in the identification of the bis-dihydrogen complex and in characterization of the matrix cage. The highly ionic rhombic (LiH)(2) dimer, which is trapped in solid matrices, is calculated to be 22 kcal/mol more stable than the inverse hydrogen bonded linear LiH-LiH dimer, which is not observed here. The cyclic lithium hydride trimer and tetramer clusters were also observed. Although the spontaneous reaction of 2 Li and H(2) to form (LiH)(2) occurs on annealing in solid H(2), the formation of higher clusters requires visible irradiation. We observed the simplest possible chemical reduction of dihydrogen using two lithium valence electrons to form the rhombic (LiH)(2) dimer.