Previous Density Functional Theory Calculations Research Articles

Solubility of NaCl in water vapor at 400–700 °C

Water significantly impacts the chemical evolution of the Earth’s crust, affecting environments from volcanic settings to hydrothermal systems. These fluids transport elements essential for geological processes, such as metal ore deposit formation. At high temperatures, as water transitions from liquid to vapor, its molecular structure changes, drastically reducing its capacity to dissolve solids and solvate ions. Here, we report experimental results of halite (NaCl(s)) solubility in water vapor at 400–700 °C and 30–300 bar using a novel U-tube flow-through reactor system. The results show that halite solubility is low (xNaCl,tot = 3.2 × 10−9 to 2.9 × 10−4 mol/mol) and increases with temperature and pressure, attributed to the dissolution of NaCl followed by its hydration according to the reaction:NaCl(s)+nH2O(g)⇋NaCl·H2On(g)Thermodynamic modeling of the experimental results indicates a preference for specific hydrated species, including NaClg, NaCl·H2O4g, NaCl·H2O6g and NaCl·H2O8g. The less hydrated gaseous NaCl species become more prevalent with increasing temperature and decreasing pressure. At temperatures below 600 °C and pressures above 50 bar, halite solubility is close to stoichiometric, whereas at higher temperatures and lower pressures, the hydrolysis of NaCl(s) to form NaOH(lq,s) or a NaClxOH(1-x)(lq,s) (x < 1) solid solution is evident, resulting in the formation of HCl(g). The logarithm of the halite equilibrium solubility constant (logKn) ranges from −10.18 to −4.19 for NaCl(g), and from −13.12 to −11.94, −15.90 to −17.28, and −19.67 to –22.73 for NaCl·H2O4(g), NaCl·H2O6(g) and NaCl·H2O8(g), respectively. Standard thermodynamic properties derived from this data reveal a temperature-independent enthalpy (ΔHn,ro), entropy (ΔSn,ro) and heat capacity (ΔCp,n,ro) of reaction for each NaCl species. While the enthalpy of reaction is nearly constant, the entropy and heat capacity decrease with an increasing degree of hydration, indicating the decreasing stability of these species with increasing temperature. Our thermodynamic model results and gas species stoichiometries are in excellent agreement with previous density functional theory (DFT) calculations (Suleimenov et al., 2006). The halite solubility data obtained here compare well with prior studies and thermodynamic models for the NaCl-H2O system.

London Dispersion Effects on the Structure and Properties of Nonlinear Optical BiB3O6 Crystal.

α-BiB3O6 (BiBO) is an important nonlinear optical (NLO) material with high efficiency for applications in harmonic generations and quantum technology. Owing to its low symmetry and cooperative Bi3+ lone pair arrangement, it has also exceptional large piezoelectric and electro-optic coefficients and strong anisotropies on other material characteristics. Previous theoretical calculations on its physical (mainly optical) properties often gave confusing results. It is found here that London dispersion (LD) tends to stabilize structures with closer pack entities like lone pair heavy ion Bi3+ with large polarizabilities, which is ignored in most previous density functional theory (DFT) calculations. Present study shows that without considering the LD effect, the structure of α-BiB3O6 (BiBO) was predicted with an over-estimated (by over 10 %) unique b-axis while underestimates a and overestimates c in a less amount. Consequently it is not possible to use the calculated structure to obtain meaningful properties of this important material. By applying a modified post-DFT LD correction based on linear combination of atomic orbitals (LCAO) and B3LYP functional, the experimental structure is well reproduced with the theoretical optimized one. Many important material property tensors of BiBO crystal are calculated in unprecedented precisions, including: dielectric constants (static and in THz range), elastic and elasto-optic constants, piezoelectric constants, refractive indices, NLO and electro-optic (EO) coefficients. Among them, theoretical calculation of the refractive indices in the THz range by diagonalizing the clamped-ion dielectric constants was firstly achieved at least for BiBO crystal. The calculation also confirms that BiBO has an exceptional large piezoelectric constant d22=40 pC/N and largest free EO coefficients , , on the order of 10 pm/V among borate crystals. The calculation also reveals that the large free EO coefficients are largely originated from the piezoelectric induced photo-elastic effect and for practical high speed applications only the clamped-ion EO coefficients take effect. The clamped ion EO coefficient of =-4.17 pm/V, =-2.61 pm/V are obtained for the first time and may be consulted if one seeks to design BiBO crystal as a high-speed EO modulator. Furthermore, full tensor matrix of the elasto-optic constants was obtained on the first time. Together with the calculated elastic constants, it can help to design acoustic optic modulating devices with preferable figure of merits 10 times that of traditional quartz crystal.

Characterizing the origin band spectrum of isoquinoline with resonance enhanced multiphoton ionization and electronic structure calculations.

We report the experimental resonance enhanced multiphoton ionization spectrum of isoquinoline between 315 and 310nm, along with correlated electronic structure calculations on the ground and excited states of this species. This spectral region spans the origin transitions to a π-π* excited state, which previous work has suggested to be vibronically coupled with a lower lying singlet n-π* state. Our computational results corroborate previous density functional theory calculations that predict the vertical excitation energy for the n-π* state to be higher than the π-π* state; however, we find an increase in the C-N-C angle brings the n-π* state below the energy of the π-π* state. The calculations find two out-of-plane vibrational modes of the n-π* state, which may be brought into near resonance with the π-π* state as the C-N-C bond angle increases. Therefore, the C-N-C bond angle may be important in activating vibronic coupling between the states. We fit the experimental rotational contour with a genetic algorithm to determine the excited state rotational constants and orientation of the transition dipole moment. The fits show a mostly in-plane polarized transition, and the projection of the transition dipole moment in the a-b plane is about 84° away from the a axis. These results are consistent with the prediction of our electronic structure calculations for the transition dipole moment of the π-π* excited state.

Ambient-Pressure X-ray Photoelectron Spectroscopy Study of Acetic Acid Thermal Decomposition on Pd(111)

Acetic acid on Pd(111) is an excellent model system to study the effect of water on thermal decomposition of fatty acids and other oxygenates. Acetic acid is a simple molecule that has both methyl and acid groups, where its decomposition involves many of the key bond-breaking steps involved in more complex chemical decompositions. Here, we use ambient-pressure X-ray photoelectron spectroscopy and mass spectrometry supported by density functional theory (DFT) calculations of the core-level binding energies to study the effect of co-adsorbed water on the decomposition of acetic acid. The addition of water to acetic acid results in an increased acetic acid/acetate coverage and a decreased CO coverage. Moreover, when water is co-dosed with acetic acid, the gas phase composition from the reaction shows an approximately 30% increase in the CO2/CO ratio, suggesting a shift toward CO2 production in the presence of water. These observations are supported by previous DFT calculations, which show that the presence of co-adsorbed water lowers the barriers for O–H bond breaking but increases the barriers for C–O bond breakage, leading to increased CO2 production.

Dependence of predicted bulk properties of hexagonal hydroxyapatite on exchange–correlation functional

Hydroxyapatite (HA) is the main component of human bones and teeth. HA has also been widely applied in various technological fields due to its unique properties. Reliable computational simulations for this critical material often require input parameters obtained from the first-principles density functional theory (DFT) calculations with appropriate exchange–correlation functionals. Previous DFT calculations are insufficient to verify the reliabilities of various functionals, particularly as they fail to assess predictions for multiple properties of the material. In this paper, we first select 18 different functionals to calculate geometric, elastic, electronic, and thermodynamic properties of hexagonal HA bulk crystal. We find that the results from optB86b-vdW and optB88-vdW functionals with dispersion corrections have the overall best agreement with available experimental data. Then, we choose optB88-vdW functional, as well as PBE functional without dispersion corrections as a comparison, to perform extensive first-principles DFT phonon calculations under the quasiharmonic approximation. Various thermodynamic properties (including phonon contributions to internal energy, entropy, and Helmholtz free energy) and thermal parameters (including thermal expansions of volume, thermal expansion coefficients, heat capacities, isothermal bulk moduli, etc.) versus temperature are consequently obtained. By comparing these quantities, we find that the results from optB88-vdW functional have significantly better agreement with available experimental data than those from PBE functional although the latter has been widely used in previous DFT calculations for HA-based material systems.

Unravelling the surface structure of β-Ga2O3 (100).

The present work is on a comprehensive surface atomic structure investigation of β-Ga2O3 (100). The β-Ga2O3 single crystal was studied by a structural model system in the simulations and in situ characterization via X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) allowed for probing the outermost layers' properties. In situ XPD characterization allows for the collection of valuable element-specific short-range information from the β-Ga2O3 surface, and the results were compared to a systematic and precise multiple scattering simulation approach. The experiments, characterizations, and simulations indicated strong evidence of considerable structural variations in the interatomic layer's distances. Such atomic displacement could clarify the electronic phenomena observed in theoretical studies. The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicated that the β-Ga2O3 surface has two possible terminations. The best fits to the photoelectron diffraction curves are used to calculate the interplanar relaxation in the first five atomic layers. The results show good agreement with previous density functional theory calculations, establishing XPD as a useful tool for probing the atomic structure of oxide surfaces.

Insights into the structure and growth of Lu-doped germanium clusters: comparing density functional theory calculations with photoelectron spectroscopy experiments

ABSTRACT The structure and growth of a series of Lu-doped germanium clusters, LuGe n q (n = 2–14, q = 0, −1) have been investigated by previous photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The ground states of the anionic LuGe n – clusters obtained from DFT calculations are verified by comparing simulated PES with experimental results. The simulated PES for smaller clusters LuGe n – (n ≤ 6) display relatively simple spectral patterns, suggesting high symmetry structures. It is observed that the pentagonal bipyramid shape is the basic framework for the nascent growth process of LuGe n – (n = 2–8). The structures of LuGe n – (n = 2–13) clusters are all exohedral structures with the Lu atom adsorbed at the surface of the bare Ge n – clusters, while LuGe14 – is the smallest endohedral Lu-doped germanium cluster with the Lu atom completely fallen into the germanium frame. It is found that the LuGe n – clusters with even n are more stable than those with odd n and in the LuGe n clusters there is an opposite trend. Especially, the LuGe n q (n = 9, 12, q = 0, −1) clusters are extremely stable compared to other size clusters. HOMO–LUMO gap shows that the chemical stability of bare Ge n – (n = 2–14) clusters are stronger than that of LuGe n q (n = 2–14, q = 0, −1) clusters owing to the doping of a Lu atom.

Facile Benzylic Alkylation of Arenes with Alcohols by Catalysis with Spirocyclic NHC IrIII Pincer Complex.

A facile benzylic alkylation of indenes and other arenes was developed from readily available primary and secondary alcohols using our newly investigated CCC pincer IrIII catalyst (SNIr-H). Excellent regioselectivity and yield (89 %) of the C3-alkylated indenes were obtained. Additionally, the challenging sp2 C-alkylation was readily accomplished. This method could be utilized for the synthesis of the analogs of a histamine H1 receptor antagonist and the functional material template molecule, indeno[2,1-a]indene. A hemilabile IrIII -dihydride intermediate was proposed based on control experiments and previous density functional theory (DFT) calculations for the borrowing hydrogen mechanism and is key to the success of this IrIII catalyst in the reduction of unactivated multi-substituted olefin intermediates.

Facile Benzylic Alkylation of Arenes with Alcohols by Catalysis with a Spirocyclic NHC IrIII Pincer Complex

AbstractA facile benzylic alkylation of indenes and other arenes was developed from readily available primary and secondary alcohols using our newly investigated CCC pincer IrIII catalyst (SNIr−H). Excellent regioselectivity and yield (89 %) of the C3‐alkylated indenes were obtained. Additionally, the challenging sp2C‐alkylation was readily accomplished. This method could be utilized for the synthesis of the analogs of a histamine H1 receptor antagonist and the functional material template molecule, indeno[2,1‐a]indene. A hemilabile IrIII‐dihydride intermediate was proposed based on control experiments and previous density functional theory (DFT) calculations for the borrowing hydrogen mechanism and is key to the success of this IrIII catalyst in the reduction of unactivated multi‐substituted olefin intermediates.

First Principle Calculation of Accurate Electronic and Related Properties of Zinc Blende Indium Arsenide (zb-InAs).

We carried out a density functional theory (DFT) study of the electronic and related properties of zinc blende indium arsenide (zb-InAs). These related properties include the total and partial densities of states and electron and hole effective masses. We utilized the local density approximation (LDA) potential of Ceperley and Alder. Instead of the conventional practice of performing self-consistent calculations with a single basis set, albeit judiciously selected, we do several self-consistent calculations with successively augmented basis sets to search for and reach the ground state of the material. As such, our calculations strictly adhere to the conditions of validity of DFT and the results are fully supported by the theory, which explains the agreement between our findings and corresponding, experimental results. Indeed, unlike some 21 previous ab initio DFT calculations that reported zb-InAs band gaps that are negative or zero, we found the room temperature measured value of 0.360 eV. It is a clear achievement to reproduce not only the locations of the peaks in the valence band density of states, but also the measured values of the electron and hole effective masses. This agreement with experimental results underscores not only the correct description of the band gap, but also of the overall structure of the bands, including their curvatures in the vicinities of the conduction band minimum (CBM) and of the valence band maximum (VBM).

Effects of uniaxial strain on structural, electronic, and optical properties of LiNbO3: Ab-initio calculations

The semiconductor LiNbO3 has recently attracted a great deal of interest in the material science community. In this study, the effect of uniaxial strain Ɛc on the structural and optoelectronic properties of the material is studied by the first-principles full-potential linearized augmented plane wave method. The results are obtained in a density functional theory (DFT) framework, using a generalized gradient approximation (GGA-PBE) based on total energy minimization, as implemented in the WIEN2k code. The research results show that the unstrained LiNbO3 is an indirect band gap semiconductor with energy of 3.544 eV, which improves the results of some previous DFT calculations, but is still lower than the experimental data. It is found that the strain between −8% and +8% will affect the value of the LiNbO3 band gap. In fact, the results show that the electronic properties are closely related to the external compressive and tensile stress (CTS), causing the electronic band gap of LiNbO3 to increase and decrease, indeed the gap energy goes from 3.119 eV for 8% to 3.746 eV for −8%. The optical coefficients in the in both x-z directions are studied. For unstrained LiNbO3 they are similar but significantly distinct. The applied uniaxial strain increases the anisotropy significantly, and the difference between the xx and zz directions becomes much larger. It has also been found that strain can cause considerable changes in the optical band gap of the material. In addition, the absorption capacity and optical conductivity is improved under tensile stress, which is more efficient and more suitable for optical applications in the visible and ultraviolet spectrum.

Reflection absorption infrared spectroscopy of the surface chemistry of furfural on Pd(111)

The hydrolysis and subsequent acidic dehydration of biomass leads to the production of smaller oxygenates, including furfural, which can undergo subsequent reactions such as hydrogenation to produce value-added products. Palladium has been found to be an active catalyst for this process. As a result, the surface chemistry of furfural is investigated on a Pd(111) single-crystal surface using reflection-absorption infrared spectroscopy as a basis for understanding the catalytic conversion of furfural to value-added products. Following adsorption at 90 K, furfural adopts a flat-lying geometry at low coverages, but converts to a tilted species as the coverage approaches saturation. Heating to ∼175 K forms a tilted η1(O) species that appears to deprotonate on heating to above 200 K to form an intermediate with a tilted furyl ring and a carbonyl group close to parallel to the surface. Further heating to ∼250 K and above caused this species to decarbonylate to form adsorbed carbon monoxide and an infrared invisible furyl intermediate. This can then undergo a ring-opening reaction to produce further CO and form a C3H3 intermediate that can hydrogenate to produce propylene. This reaction sequence is in good agreement with previous density functional theory calculations and with the products observed in temperature-programmed desorption.

Ground State Properties of the Wide Band Gap Semiconductor Beryllium Sulfide (BeS).

We report the results from self-consistent calculations of electronic, transport, and bulk properties of beryllium sulfide (BeS) in the zinc-blende phase, and employed an ab-initio local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). We obtained the ground state properties of zb-BeS with the Bagayoko, Zhao, and Williams (BZW) computational method, as enhanced by Ekuma and Franklin (BZW-EF). Our findings include the electronic energy bands, the total (DOS) and partial (pDOS) densities of states, electron and hole effective masses, the equilibrium lattice constant, and the bulk modulus. The calculated band structure clearly shows that zb-BeS has an indirect energy band gap of 5.436 eV, from Γ to a point between Γ and X, for an experimental lattice constant of 4.863 Å. This is in excellent agreement with the experiment, unlike the findings of more than 15 previous density functional theory (DFT) calculations that did not perform the generalized minimization of the energy functional, required by the second DFT theorem, which is inherent to the implementation of our BZW-EF method.

First-principles feasibility assessment of a topological insulator at the InAs/GaSb interface

First-principles simulations are conducted to shed light on the question of whether a two-dimensional topological insulator (2DTI) phase may be obtained at the interface between InAs and GaSb. To this end, the InAs/GaSb interface is compared and contrasted with the HgTe/CdTe interface. Density functional theory (DFT) simulations of these interfaces are performed using a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. For the HgTe/CdTe interface, our simulations show that band crossing is achieved and an inverted gap is obtained at a critical thickness of 5.1 nm of HgTe, in agreement with experiment and previous DFT calculations. In contrast, for InAs/GaSb, the gap narrows with increasing thickness of InAs; however, the gap does not close for interfaces with up to 50 layers (about 15 nm) of each material. When an external electric field is applied across the InAs/GaSb interface, the GaSb-derived valence band maximum is shifted up in energy with respect to the InAs-derived conduction band minimum until eventually the bands cross and an inverted gap opens. Our results show that it may be possible to reach the topological regime at the InAs/GaSb interface under the right conditions. However, it may be challenging to realize these conditions experimentally, which explains the difficulty of experimentally demonstrating an inverted gap in InAs/GaSb.

Unveiling the Active Structure of Single Nickel Atom Catalyst for Electrochemical CO2 Reduction

A single nickel atom embedded in graphene is one of the most representative single-atom catalysts, and it has a high activity and selectivity for electrochemical CO2 reduction (CO2R) to CO. However, the catalytic origin, especially the coordination structure of Ni, remains highly puzzling, as previous density functional theory (DFT) calculations showed that all the possible structures should be inactive and/or nonselective. Here, using ab initio molecular dynamics (AIMD) and a “slow-growth” sampling approach to evaluate the reaction kinetic barriers, we show that the charge capacity (of the site) and hydrogen bonding (with the intermediates), which were neglected/oversimplified in previous DFT calculations, play crucial roles, and including their effects can resolve the catalytic origin. Particularly, a high charge capacity allows the catalytic site to carry more charges than required for the electrochemical step, lowering the electrochemical barrier, and hydrogen bonding promotes the reaction that produces polar intermediates by stabilizing the intermediates and facilitating the H transfer from water, explaining the high selectivity for CO2R over the hydrogen evolution reaction. Consequently, we find that a hybrid coordination environment (with one nitrogen and three carbon atoms) for the Ni-atom is most active and selective for CO2R. Our work not only explains a long-standing puzzle for an important catalyst but also highlights the crucial roles of charge capacity and hydrogen bonding, which can help elucidate the mechanisms of other heterogeneous electrocatalysts in aqueous solution and enable more effective catalyst design [1,2].[1] X. Zhao, Y. Liu, J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b13872[2] D. Kim, J. Shi, Y. Liu, J. Am. Chem. Soc., 2018, 140, 9127-9131

Aluminum Hugoniot from model calculations including semicore electrons based on density functional theory

We present a method to obtain Hugoniot from model calculations based on density functional theory, and apply the method to aluminum Hugoniot. Technological advances have extended the experimental research of high energy density physics, and call for quantitative theoretical analysis. However, direct computation of Hugoniot from density functional theory is very difficult. We propose two step calculations of Hugoniot from density functional theory. The first step is molecular dynamics simulations with an ambient temperature for electrons. The second step is total energy calculations of a crystal with desired high temperatures for electrons and with the ambient temperature for electrons. We treated the semicore 2s and 2p electrons of aluminum as valence electrons only for the total energy calculations of the aluminum crystal. The aluminum Hugoniot from our model calculations is in excellent agreement with available experimental data and the previous density functional theory calculations in the literature.

Dipolar coupling of nanoparticle-molecule assemblies: An efficient approach for studying strong coupling.

Strong light-matter interactions facilitate not only emerging applications in quantum and non-linear optics but also modifications of properties of materials. In particular, the latter possibility has spurred the development of advanced theoretical techniques that can accurately capture both quantum optical and quantum chemical degrees of freedom. These methods are, however, computationally very demanding, which limits their application range. Here, we demonstrate that the optical spectra of nanoparticle-molecule assemblies, including strong coupling effects, can be predicted with good accuracy using a subsystem approach, in which the response functions of different units are coupled only at the dipolar level. We demonstrate this approach by comparison with previous time-dependent density functional theory calculations for fully coupled systems of Al nanoparticles and benzene molecules. While the present study only considers few-particle systems, the approach can be readily extended to much larger systems and to include explicit optical-cavity modes.

Effects of Th doping on mechanical properties of U&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Th&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;O&lt;sub&gt;2&lt;/sub&gt;: An atomistic simulation

Since thorium (Th) owns high conversion ratio in thermal neutron spectrum, high melting temperature, high thermal conductivity and good corrosion resistance in high-temperature water, it can be doped into UO&lt;sub&gt;2&lt;/sub&gt; based fuel to initiate the fission reaction, and improve the physical properties of UO&lt;sub&gt;2&lt;/sub&gt;. Owing to the challenging experimental conditions and technologies, molecular dynamics (MD) simulations are conducted to investigate the influences of Th doping on the mechanical properties of U&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Th&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;O&lt;sub&gt;2&lt;/sub&gt;. The phase transition from initial fluorite structure to the metastable scrutinyite phase when loading along the [001] direction is observed, which accords well with the previous density functional theory calculations. However, if U&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Th&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;O&lt;sub&gt;2&lt;/sub&gt; is loaded along the [111] direction, only brittle fracture is observed. It is found that both the elastic modulus and fracture stress decrease linearly with elevating temperature but the fracture strain increases. As the Th concentration increases from 0 to 0.55, the elastic modulus first decreases and then increases; if the Th concentration is larger than 0.1, the fracture stress increases and the fracture strain decreases monotonically. The cracks are nucleated with an angle of 45º to the loading direction, propagate rapidly, and are characteristic of brittle fracture, which accords well with the classical failure criteria and experimental results for brittle materials. By comparison, the uniaxial tensile loading is also performed for polycrystalline U&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Th&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;O&lt;sub&gt;2&lt;/sub&gt;. It is found that the elastic modulus and fracture stress decrease as the temperature increases. However, the elastic modulus is not sensitive to the Th concentration and the fracture increases as the Th concentration increases. The brittle intergranular fracture is observed in each of all polycrystalline samples. The obtained physical parameters are useful for designing the fuels in nuclear reactors.

Raman studies of hydrogen trapped in As4O6·2H2 at high pressure and low temperature.

Raman spectroscopic measurements of the arsenolite-hydrogen inclusion compound As4O6·2H2 were performed in diamond anvil cells at high pressure and variable temperature down to 80 K. The experimental results were complemented by ab initio molecular dynamics simulations and phonon calculations. Observation of three hydrogen vibrons in As4O6·2H2 is reported in the entire temperature and pressure range studied (up to 24 GPa). While the experiments performed with protium and deuterium at variable temperatures allowed for the assignment of two vibrons as Q1(1) and Q1(0) transitions of ortho and para spin isomers of hydrogen trapped in the inclusion compound, the origin of the third vibron could not be unequivocally established. Low-temperature spectra revealed that the lowest-frequency vibron is actually composed of two overlapping bands of Ag and T2g symmetries dominated by H2 stretching modes as predicted by our previous density functional theory calculations. We observed low-frequency modes of As4O6·2H2 vibrations dominated by H2 "librations," which were missed in a previous study. A low-temperature fine structure was observed for the J = 0 → 2 and J = 1 → 3 manifolds of hydrogen trapped in As4O6·2H2, indicating the lifting of degeneracy due to an anisotropic environment. A non-spherical distribution was captured by molecular dynamics simulations, which revealed that the trajectory of H2 molecules is skewed along the crystallographic ⟨111⟩ direction. Last but not least, low-temperature synchrotron powder x-ray diffraction measurements on As4O6·2H2 revealed that the bulk structure of the compound is preserved down to 5 K at 1.6 GPa.

CASPT2 Potential Energy Curves for NO Dissociation in a Ruthenium Nitrosyl Complex.

Ruthenium nitrosyl complexes are fascinating photoactive compounds showing complex photoreactivity, such as N→O linkage photoisomerism and NO photorelease. This dual photochemical behavior has been the subject of many experimental studies in order to optimize these systems for applications as photoswitches or therapeutic agents for NO delivery. However, despite recent experimental and computational studies along this line, the underlying photochemical mechanisms still need to be elucidated for a more efficient design of these systems. Here, we present a theoretical contribution based on the calculations of excited-state potential energy profiles for NO dissociation in the prototype trans-[RuCl(NO)(py)4]2+ complex at the complete active space second-order perturbation theory (CASPT2). The results point to a sequential two-step photon absorption photorelease mechanism coupled to partial photoisomerization to a side-on intermediate, in agreement with previous density functional theory calculations.