Fluxional bonds in quasi-planar and half-sandwich (M = K,Rb, and Cs)

Detailed molecular orbital and bonding analyses reveal the existence of both fluxional σ- and π-bonds in the global minima C_s ( 1 ) and C_s MB₁₈ ( 3 ) and transition states C_s ( 2 ) and C_s ( 4 ) of dianion and monoanions (M = K, Rb, and Cs). It is the fluxional bonds that facilitate the fluxional behaviors of the quasi-planar and half-sandwich which possess energy barriers smaller than the difference of the corresponding zero-point corrections. © 2019 Wiley Periodicals, Inc.     

On the accurate estimation of free energies using the jarzynski equality

The Jarzynski equality is one of the most widely celebrated and scrutinized nonequilibrium work theorems, relating free energy to the external work performed in nonequilibrium transitions. In practice, the required ensemble average of the Boltzmann weights of infinite nonequilibrium transitions is estimated as a finite sample average, resulting in the so-called Jarzynski estimator, . Alternatively, the second-order approximation of the Jarzynski equality, though seldom invoked, is exact for Gaussian distributions and gives rise to the Fluctuation-Dissipation estimator . Here we derive the parametric maximum-likelihood estimator (MLE) of the free energy considering unidirectional work distributions belonging to Gaussian or Gamma families, and compare this estimator to . We further consider bidirectional work distributions belonging to the same families, and compare the corresponding bidirectional to the Bennett acceptance ratio () estimator. We show that, for Gaussian unidirectional work distributions, is in fact the parametric MLE of the free energy, and as such, the most efficient estimator for this statistical family. We observe that and perform better than and , for unidirectional and bidirectional distributions, respectively. These results illustrate that the characterization of the underlying work distribution permits an optimal use of the Jarzynski equality. © 2018 Wiley Periodicals, Inc.     

Substitutional carbon defects in silicon: A quantum mechanical characterization through the infrared and Raman spectra

The infrared (IR) and Raman spectra of eight substitutional carbon defects in silicon are computed at the quantum mechanical level by using a periodic supercell approach based on hybrid functionals, an all electron Gaussian type basis set and the CRYSTAL code. The single substitutional C _s case and its combination with a vacancy (C _sV and C _sSiV) are considered first. The progressive saturation of the four bonds of a Si atom with C is then examined. The last set of defects consists of a chain of adjacent carbon atoms C, with i = 1–3. The simple substitutional case, C _s, is the common first member of the three sets. All these defects show important, very characteristic features in their IR spectrum. One or two C related peaks dominate the spectra: at 596 cm⁻¹ for C _s (and C _sSiV, the second neighbor vacancy is not shifting the C _s peak), at 705 and 716 cm⁻¹ for C _sV, at 537 cm⁻¹ for C and C (with additional peaks at 522, 655 and 689 for the latter only), at 607 and 624 cm⁻¹, 601 and 643 cm⁻¹, and 629 cm⁻¹ for SiC, SiC, and SiC, respectively. Comparison with experiment allows to attribute many observed peaks to one of the C substitutional defects. Observed peaks above 720 cm⁻¹ must be attributed to interstitial C or more complicated defects.     

Mono-silicon isoelectronic replacement in CAl4: van't hoff/le bel carbon or not?

In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology. The well-known penta-atomic 18 valence electron (ve) species and its Al⁻/Si or Al/Si⁺ isoelectronically replaced clusters CAl₃Si⁻, CAl₂Si₂, , and , all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure. In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl₄ with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl₃X (X = Ga/In/Tl). At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl₂SiX⁺ (X = Al/Ga/In/Tl) maintains thC as the parent CAl₄ does. Instead, X = Al/Ga globally favors an unusual ptC structure that has one long C─X distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon. The frustrated formation of thC in these clusters is ascribed to the CSi bonding that prefers a planar fashion. Inclusion of chloride ion would further stabilize the ptC of CAl₂SiAl⁺ and CAl₂SiGa⁺. The unexpectedly disclosed CAl₂SiAl⁺ and CAl₂SiGa⁺ represent the first type of 16ve-cationic ptCs with multiple bonds. © 2019 Wiley Periodicals, Inc.     

A Density Functional Theory Study on Nonlinear Optical Properties of Double Cage Excess Electron Compounds: Theoretically Design M[Cu(Ag)@(NH3)n](M = Be,Mg and Ca; n = 1–3)

In this work, we investigated the nonlinear optical (NLO) properties of excess electron electride molecules of M[Cu(Ag)@(NH₃)_n](M = Be, Mg and Ca; n = 1–3) using density functional theory (DFT). This electride molecules consist of an alkaline-earth (Be, Mg and Ca) together with transition metal (Cu and Ag) doped in NH₃ cluster. The natural population analysis of charge and their highest occupied molecular orbital suggests that the M[Cu(Ag)@(NH₃)_n] compound has excess electron like alkaline-earth metal form double cage electrides molecules, which exhibit a large static first hyperpolarizability () (electron contribution part) and one of which owns a peak value of 216,938 (a.u.) for Be[Ag@(NH₃)₂] and vibrational harmonic first hyperpolarizability () (nuclear contribution part) values and the ratio of /, namely, η values from 0.02 for Be[Ag@(NH₃)] to 0.757 for Mg[Ag@(NH₃)₃]. The electron density contribution in different regions on values mainly come from alkaline-earth and transition metal atoms by first hyperpolarizability density analysis, and also explains the reason why values are positive and negative. Moreover, the frequency-dependent values β(−2ω,ω,ω) are also estimated to make a comparison with experimental measures. © 2018 Wiley Periodicals, Inc.     

Mechanism of the O2(1Δg) generation from the Cl2/H2O2 basic aqueous solution explored by the combined ab initio calculation and nonadiabatic dynamics simulation

In the present work, mechanism of the O₂(¹Δ_g) generation from the reaction of the dissolved Cl₂ with H₂O₂ in basic aqueous solution has been explored by the combined ab initio calculation and nonadiabatic dynamics simulation, together with different solvent models. Three possible pathways have been determined for the O₂(¹Δ_g) generation, but two of them are sequentially downhill processes until formation of the OOCl⁻ complex with water, which are of high exothermic character. Once the complex is formed, singlet molecular oxygen is easily generated by its decomposition along the singlet-state pathway. However, triplet molecular oxygen of O₂() can be produced with considerable probability through nonadiabatic intersystem crossing in the ¹Δ_g/ intersection region. It has been found that the coupled solvent, heavy-atom, and nonadiabatic effects have an important influence on the quantum yield of the O₂(¹Δ_g) generation. © 2018 Wiley Periodicals, Inc.     

A Computational Analysis of the Intrinsic Plasticity of Five-Coordinate Cu(II) Complexes and the Factors Leading to the Breakdown of the Orbital Directing Effect in Paddlewheel Secondary Building Units

Quantum chemical calculations on model copper paddlewheel (CPW) complexes of general formula [Cu₂(μ₂-O₂CR)₄L₂] establish two local coordination geometries at the metal centers depending on the balance between equatorial and axial ligand fields. When the equatorial field is stronger than the axial field (large ligand field asymmetry), dominates the stereochemical activity of the d⁹ shell resulting in a relatively rigid, “orbitally directed” planar or square pyramidal structure. However, if the axial field is significantly increased, or the equatorial field moderately weakened, a small ligand field asymmetry results and both and are involved in the stereochemical activity. This results in a “plastic,” distorted trigonal bipyramidal geometry where the former axial ligand moves into one of the original four equatorial positions. Linkers already used to synthesize zinc-dabco MOFs (dabco = 1,4-diazabicyclo[2.2.2]octane) are shown to generate plastic CPW secondary building unit analogs with potential implications for conferring breathing behavior for MOFs which would currently be assumed to be rigid. © 2019 Wiley Periodicals, Inc.     

Efficient singular-value decomposition of the coupled-cluster triple excitation amplitudes

We demonstrate a novel technique to obtain singular-value decomposition (SVD) of the coupled-cluster triple excitations amplitudes, . The presented method is based on the Golub-Kahan bidiagonalization strategy and does not require to be stored. The computational cost of the method is comparable to several coupled cluster singles and doubles (CCSD) iterations. Moreover, the number of singular vectors to be found can be predetermined by the user and only those singular vectors which correspond to the largest singular values are obtained at convergence. We show how the subspace of the most important singular vectors obtained from an approximate triple amplitudes tensor can be used to solve equations of the CC3 method. The new method is tested for a set of small and medium-sized molecular systems in basis sets ranging in quality from double- to quintuple-zeta. It is found that to reach the chemical accuracy (≈1 kJ/mol) in the total CC3 energies as little as 5 − 15% of SVD vectors are required. This corresponds to the compression of the amplitudes by a factor of about 0.0001 − 0.005 . Significant savings are obtained also in calculation of interaction energies or rotational barriers, as well as in bond-breaking processes. © 2019 Wiley Periodicals, Inc.     

Ab initio structure and vibration-rotation dynamics of germylene,GeH2

Accurate structure and potential energy surface of germylene, GeH₂, in its ground electronic state ¹A₁ were determined from ab initio calculations using the coupled-cluster approach in conjunction with the correlation-consistent basis sets up to sextuple-zeta quality. The Born-Oppenheimer equilibrium structural parameters for the ¹A₁ state are estimated to be r_e(GeH) = 1.5793 Å and ∠_e(HGeH) = 91.19^∘. The term value T_e for the lowest excited electronic state ã³B₁ of GeH₂ is predicted to be 9140 cm^–1. The vibration-rotation energy levels for the ¹A₁ state of the ⁷⁴GeH₂, ⁷⁴GeD₂, ⁷²GeH₂, and ⁷⁰GeH₂ isotopologues were determined using a variational approach and compared with the experimental data. The role of the core-electron correlation, higher-order valence-electron correlation, scalar relativistic, spin-orbit, and adiabatic effects for prediction of the structure and vibration-rotation dynamics of the GeH₂ molecule is discussed. © 2019 Wiley Periodicals, Inc.     

Topological analysis of chemical bonding in the layered FePSe3 upon pressure-induced phase transitions

Two pressure-induced phase transitions have been theoretically studied in the layered iron phosphorus triselenide (FePSe₃ ). Topological analysis of chemical bonding in FePSe₃ has been performed based on the results of first-principles calculations within the periodic linear combination of atomic orbitals (LCAO) method with hybrid Hartree-Fock-DFT B3LYP functional. The first transition at about 6 GPa is accompanied by the symmetry change from to C2/m , whereas the semiconductor-to-metal transition (SMT) occurs at about 13 GPa leading to the symmetry change from C2/m to . We found that the collapse of the band gap at about 13 GPa occurs due to changes in the electronic structure of FePSe₃ induced by relative displacements of phosphorus or selenium atoms along the c-axis direction under pressure. The results of the topological analysis of the electron density and its Laplacian demonstrate that the pressure changes not only the interatomic distances but also the bond nature between the intralayer and interlayer phosphorus atoms. The interlayer P–P interactions are absent in two non-metallic FePSe₃ phases while after SMT the intralayer P–P interactions weaken and the interlayer P–P interactions appear.     

Thermal rate constant for the C(3P) + OH(X2Π) → CO(X1Σ) + H(2S) reaction using stochastic energy grained master equation method

In the present work, the kinetic mechanism of the reaction is studied. The rate constants were determined using the Master Equation Solver for Multi-Energy Well Reactions (MESMER). The master equation modeling was also employed to examine the pressure dependence for each pathway involved. The theoretical analysis shows that the overall rate coefficient is practically independent of pressure up to 100 Torr for the temperature range 125-500 K. The unusual dependence of the overall rate constant with temperature was fit with the d-Arrhenius expression , where cm³molecule⁻¹s⁻¹, , and  kJ·mol⁻¹, for 125⩽ T ⩽ 500 K. The thermal rate constant results are in relatively good agreement with other theoretical studies.     

Reversible Metal-Hydride Transformation in Mg-Ti-H Nanoparticles at Remarkably Low Temperatures

We study the kinetics of hydrogen sorption in Mg-Ti-H nanoparticles prepared by gas phase condensation of mixed Mg-Ti vapors under a H₂-containing atmosphere. Four samples with different Ti contents from 14 to 63 at.% Ti are examined in the 100–150 °C range. The hydrogen absorption kinetics coupled with the formation of MgH₂ can be described by a nucleation and growth model. The activation energy is in the range kJ/mol and the rate constant (at 150 °C) increases from s⁻¹ to s⁻¹ with increasing Ti content. Hydrogen desorption is well modeled by a sequence of surface-limited and contracting-volume kinetics, except at the highest Ti content where nucleation and growth is observed. The activation energy of surface-limited kinetics is /mol. The rate constant (at 150 °C) increases from s⁻¹ to s⁻¹ with the Ti content. These results open an unexplored kinetic window for Mg-based reversible hydrogen storage close to ambient temperature.     

Quantum Nuclear Delocalization and its Rovibrational Fingerprints

Quantum mechanics dictates that nuclei must undergo some delocalization. In this work, emergence of quantum nuclear delocalization and its rovibrational fingerprints are discussed for the case of the van der Waals complex . The equilibrium structure of is planar and T-shaped, one He atom solvating the quasi-linear He−H⁺−He core. The dynamical structure of , in all of its bound states, is fundamentally different. As revealed by spatial distribution functions and nuclear densities, during the vibrations of the molecule the solvating He is not restricted to be in the plane defined by the instantaneously bent chomophore, but freely orbits the central proton, forming a three-dimensional torus around the chromophore. This quantum delocalization is observed for all vibrational states, the type of vibrational excitation being reflected in the topology of the nodal surfaces in the nuclear densities, showing, for example, that intramolecular bending involves excitation along the circumference of the torus.     

Activation Strain Analyses of Counterion and Solvent Effects on the Ion-Pair SN2 Reaction of and CH3Cl

We have computationally studied the bimolecular nucleophilic substitution (S_N2) reactions of M_nNH₂⁽ⁿ⁻¹⁾ + CH₃Cl (M⁺ = Li⁺, Na⁺, K⁺, and MgCl⁺; n = 0, 1) in the gas phase and in tetrahydrofuran solution at OLYP/6-31++G(d,p) using polarizable continuum model implicit solvation. We wish to explore and understand the effect of the metal counterion M⁺ and of solvation on the reaction profile and the stereochemical preference, that is, backside (S_N2-b) versus frontside attack (S_N2-f). The results were compared to the corresponding ion-pair S_N2 reactions involving F⁻ and OH⁻ nucleophiles. Our analyses with an extended activation strain model of chemical reactivity uncover and explain various trends in S_N2 reactivity along the nucleophiles F⁻, OH⁻, and , including solvent and counterion effects. © 2019 Wiley Periodicals, Inc.     

A Multiconformational Transition State Theory Approach to OH Tropospheric Degradation of Fluorotelomer Aldehydes

Experimental work on the OH-initiated oxidation reactions of fluorotelomer aldehydes (FTALs) strongly suggests that the respective rate coefficients do not depend on the size of the C_xF_2x+1 fluoroalkyl chain. FTALs hence represent a challenging test to our multiconformer transition state theory (MC-TST) protocol based on constrained transition state randomization (CTSR), since the calculated rate coefficients should not show significant variations with increasing values of . In this work we apply the MC-TST/CTSR protocol to the cases and calculate both rate coefficients at 298.15 K with a value of cm³ molecule⁻¹ s⁻¹, practically coincident with the recommended experimental value of k_exp= cm³ molecule⁻¹ s⁻¹. We also show that the use of tunneling corrections based on improved semiclassical TST is critical in obtaining Arrhenius-Kooij curves with a correct behavior at lower temperatures.     

Analyzing Anisotropic Exchange in a Pentanuclear Os2Ni3 Complex

Spin Hamiltonian parameters of a pentanuclear Os Ni cyanometallate complex are derived from ab initio wave function based calculations, namely valence-type configuration interaction calculations with a complete active space including spin-orbit interaction (CASOCI) in a single-step procedure. While fits of experimental data performed so far could reproduce the data but the resulting parameters were not satisfactory, the parameters derived in the present work reproduce experimental data and at the same time have a reasonable size. The one-centre parameters (local matrices and single-ion zero field splitting tensors) are within an expected range, the anisotropic exchange parameters obtained in this work for an Os−Ni pair are not exceedingly large but determine the low-T part of the experimental χT curve. Exchange interactions (both isotropic and anisotropic) obtained from CASOCI have to be scaled by a factor of 2.5 to obtain agreement with experiment, a known deficiency of such types of calculation. After scaling the parameters, the isotropic Os−Ni exchange coupling constant is cm⁻¹ and the D parameter of the (nearly axial) anisotropic Os−Ni exchange is ⁻¹, so anisotropic exchange is larger in absolute size than isotropic exchange. The negative value of the isotropic J (indicating antiferromagnetic coupling) seemingly contradicts the large-temperature behaviour of the temperature dependent susceptibility curve, but this is caused by the negative g value of the Os centres. This negative g value is a universal feature of a pseudo-octahedral coordination with configuration and strong spin-orbit interaction. Knowing the size of these exchange interactions is important because Os(CN) is a versatile building block for the synthesis of / magnetic materials.