Real-Space Interpretation of Interatomic Charge Transfer and Electron Exchange Effects by Combining Static and Kinetic Potentials and Associated Vector Fields**

Intricate behaviour of one-electron potentials from the Euler equation for electron density and corresponding gradient force fields in crystals was studied. Channels of locally enhanced kinetic potential and corresponding saddle Lagrange points were found between chemically bonded atoms. Superposition of electrostatic and kinetic potentials and electron density allowed partitioning any molecules and crystals into atomic - and potential-based -basins; -basins explicitly account for the electron exchange effect, which is missed for -ones. Phenomena of interatomic charge transfer and related electron exchange were explained in terms of space gaps between zero-flux surfaces of - and -basins. The gap between - and -basins represents the charge transfer, while the gap between - and -basins is a real-space manifestation of sharing the transferred electrons caused by the static exchange and kinetic effects as a response against the electron transfer. The regularity describing relative positions of -, -, and - basin boundaries between interacting atoms was proposed. The position of -boundary between - and -ones within an electron occupier atom determines the extent of transferred electron sharing. The stronger an H⋅⋅⋅O hydrogen bond is, the deeper hydrogen atom's -basin penetrates oxygen atom's -basin, while for covalent bonds a -boundary closely approaches a -one indicating almost complete sharing of the transferred electrons. In the case of ionic bonds, the same region corresponds to electron pairing within the -basin of an electron occupier atom.     

Temperature Dependence of 1H Paramagnetic Chemical Shifts in Actinide Complexes,Beyond Bleaney's Theory: The AnVIO22+–Dipicolinic Acid Complexes (An=Np,Pu) as an Example

Actinide +VI complexes ( = , and ) with dipicolinic acid derivatives were synthesized and characterized by powder XRD, SQUID magnetometry and NMR spectroscopy. In addition, and complexes were described by first principles CAS based and two-component spin-restricted DFT methods. The analysis of the ¹H paramagnetic NMR chemical shifts for all protons of the ligands according to the X-rays structures shows that the Fermi contact contribution is negligible in agreement with spin density determined by unrestricted DFT. The magnetic susceptibility tensor is determined by combining SQUID, pNMR shifts and Evans’ method. The SO-RASPT2 results fit well the experimental magnetic susceptibility and pNMR chemical shifts. The role of the counterions in the solid phase is pointed out; their presence impacts the magnetic properties of the complex. The temperature dependence of the pNMR chemical shifts has a strong contribution, contrarily to Bleaney's theory for lanthanide complexes. The fitting of the temperature dependence of the pNMR chemical shifts and SQUID magnetic susceptibility by a two-Kramers-doublet model for the complex and a non-Kramers-doublet model for the complex allows for the experimental evaluation of energy gaps and magnetic moments of the paramagnetic center.     

The Apparently Unreactive Substrate Facilitates the Electron Transfer for Dioxygen Activation in Rieske Dioxygenases

Rieske dioxygenases belong to the non-heme iron family of oxygenases and catalyze important cis-dihydroxylation as well as O-/N-dealkylation and oxidative cyclization reactions for a wide range of substrates. The lack of substrate coordination at the non-heme ferrous iron center, however, makes it particularly challenging to delineate the role of the substrate for productive activation. Here, we studied the role of the substrate in the key elementary reaction leading to activation from a theoretical perspective by systematically considering (i) the 6-coordinate to 5-coordinate conversion of the non-heme Fe^II upon abstraction of a water ligand, (ii) binding of , and (iii) transfer of an electron from the Rieske cluster. We systematically evaluated the spin-state-dependent reaction energies and structural effects at the active site for all combinations of the three elementary processes in the presence and absence of substrate using naphthalene dioxygenase as a prototypical Rieske dioxygenase. We find that reaction energies for the generation of a coordination vacancy at the non-heme Fe center through thermoneutral H₂O reorientation and exothermic binding prior to Rieske cluster oxidation are largely insensitive to the presence of naphthalene and do not lead to formation of any of the known reactive Fe-oxygen species. By contrast, the role of the substrate becomes evident after Rieske cluster oxidation and exclusively for the 6-coordinate non-heme Fe sites in that the additional electron is found at the substrate instead of at the iron and oxygen atoms. Our results imply an allosteric control of the substrate on Rieske dioxygenase reactivity to happen prior to changes at the non-heme Fe in agreement with a strategy that avoids unproductive activation.     

Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain**

Azobenzene and its derivatives are one of the most widespread molecular scaffolds used in a range of modern applications, as well as in fundamental research. After photoexcitation, azo-based photoswitches revert back to the most stable isomer on a timescale ( ) that determines the range of potential applications. Attempts to bring to extreme values prompted the development of azobenzene and azoheteroarene derivatives that either rebalance the E- and Z-isomer stabilities, or exploit unconventional thermal isomerization mechanisms. In the former case, one successful strategy has been the creation of macrocycle strain, which tends to impact the E/Z stability asymmetrically, and thus significantly modify . On the bright side, bridged derivatives have shown an improved optical switching owing to the higher quantum yields and absence of degradation. However, in most (if not all) cases, bridged derivatives display a reversed thermal stability (more stable Z-isomer), and smaller than the acyclic counterparts, which restricts their potential interest to applications requiring a fast forward and backwards switch. In this paper, the impact of alkyl bridges on the thermal stability of phenyl-azoheteroarenes is investigated by using computational methods, and it is revealed that it is indeed possible to combine such improved photoswitching characteristics while preserving the regular thermal stability (more stable E-isomer), and increased values under the appropriate connectivity and bridge length.     

Supramolecular Assembly of Metal Complexes by (Aryl)I⋅⋅⋅d [PtII] Halogen Bonds

The theoretical data for the half-lantern complexes [{Pt( )(μ- )}₂] [ 1 – 3 ; is cyclometalated 2-Ph-benzothiazole; is 2-SH-pyridine ( 1 ), 2-SH-benzoxazole ( 2 ), 2-SH-tetrafluorobenzothiazole ( 3 )] indicate that the Pt ⋅⋅⋅ Pt orbital interaction increases the nucleophilicity of the outer d orbitals to provide assembly with electrophilic species. Complexes 1 – 3 were co-crystallized with bifunctional halogen bonding (XB) donors to give adducts ( 1 – 3 )₂ ⋅ (1,4-diiodotetrafluorobenzene) and infinite polymeric [ 1⋅ 1,1′-diiodoperfluorodiphenyl]_n. X-ray crystallography revealed that the supramolecular assembly is achieved through (Aryl)I ⋅⋅⋅ d [Pt^II] XBs between iodine σ-holes and lone pairs of the positively charged (Pt^II)₂ centers acting as nucleophilic sites. The polymer includes a curved linear chain ⋅⋅⋅ Pt₂ ⋅⋅⋅ I(arene^F)I ⋅⋅⋅ Pt₂ ⋅⋅⋅ involving XB between iodine atoms of the perfluoroarene linkers and (Pt^II)₂ moieties. The ¹⁹⁵Pt NMR, UV/Vis, and CV studies indicate that XB is preserved in CH(D)₂Cl₂ solutions.     

Measurements and Utilization of Consistent Gibbs Energies of Transfer of Single Ions: Towards a Unified Redox Potential Scale for All Solvents

Utilizing the “ideal” ionic liquid salt bridge to measure Gibbs energies of transfer of silver ions between the solvents water, acetonitrile, propylene carbonate and dimethylformamide results in a consistent data set with a precision of 0.6 kJ mol⁻¹ over 87 measurements in 10 half-cells. This forms the basis for a coherent experimental thermodynamic framework of ion solvation chemistry. In addition, we define the solvent independent - and the values that account for the electronating potential of any redox system similar to the value of a medium that accounts for its protonating potential. This scale is thermodynamically well-defined enabling a straightforward comparison of the redox potentials (reducities) of all media with respect to the aqueous redox potential scale, hence unifying all conventional solvents′ redox potential scales. Thus, using the Gibbs energy of transfer of the silver ion published herein, one can convert and unify all hitherto published redox potentials measured, for example, against ferrocene, to the scale.     

Reaction of Pertechnetate in Highly Alkaline Solution: Synthesis and Characterization of the Nitridotrioxotechnetate Ba[TcO3N]

The preparation of novel technetium oxides, their characterization and the general investigation of technetium chemistry are of significant importance, since fundamental research has so far mainly focused on the group homologues. Whereas the structure chemistry of technetium in strongly oxidizing media is dominated by the anion, our recent investigation yielded the new anion. Brown single crystals of Ba[TcO₃N] were obtained under hydrothermal conditions starting from Ba(OH)₂ ⋅ 8H₂O and NH₄[TcO₄] at 200 °C. crystallizes in the monoclinic crystal system with the space group P2₁/n (a=7.2159(4) Å, b=7.8536(5) Å, c=7.4931(4) Å and β=104.279(2)°). The crystal structure of consists of isolated tetrahedra, which are surrounded by Ba²⁺ cations. XANES measurements complement the oxidation state +VII for technetium and Raman spectroscopic experiments on Ba[TcO₃N] single crystals exhibit characteristic Tc−O and Tc−N vibrational modes.     

Hydrosilane σ-Adduct Intermediates in an Adaptive Zinc-Catalyzed Cross-dehydrocoupling of Si−H and O−H Bonds

Three-coordinate ^PhBOX ZnR (^PhBOX =phenyl-(4,4-dimethyl-oxazolinato; R=Me: 2 a , Et: 2 b ) catalyzes the dehydrocoupling of primary or secondary silanes and alcohols to give silyl ethers and hydrogen, with high turnover numbers (TON; up to 10⁷) under solvent-free conditions. Primary and secondary silanes react with small, medium, and large alcohols to give various degrees of substitution, from mono- to tri-alkoxylation, whereas tri-substituted silanes do not react with MeOH under these conditions. The effect of coordinative unsaturation on the behavior of the Zn catalyst is revealed through a dramatic variation of both rate law and experimental rate constants, which depend on the concentrations of both the alcohol and hydrosilane reactants. That is, the catalyst adapts its mechanism to access the most facile and efficient conversion. In particular, either alcohol or hydrosilane binds to the open coordination site on the ^PhBOX ZnOR catalyst to form a ^PhBOX ZnOR(HOR) complex under one set of conditions or an unprecedented σ-adduct ^PhBOX ZnOR(H−SiR′₃) under other conditions. Saturation kinetics provide evidence for the latter species, in support of the hypothesis that σ-bond metathesis reactions involving four-centered electrocyclic 2σ–2σ transition states are preceded by σ-adducts.     

Azobenzene Isomerization-Induced Photomodulation of Electronic Properties of N-Heterocyclic Carbenes

Azobenzene-based protonated N-heterocyclic carbenes (NHCs), N,N’-bis(azobenzene)imidazolium chlorides (IAz-X⋅HCl; X=OMe, Br, H) were successfully synthesized and switching abilities of the attached azobenzene units along with the concomitant photoinduced generation of geometric isomers were studied. Upon irradiation with 365 nm UV light, a p-methoxy-azobenzene derivative get transformed from all-trans isomer to nearly all-cis isomer at the photostationary state. The extent of photomodulation of electronic properties in the NHC ring of the p-methoxy-azobenzene derivative is determined through the Tolman Electronic Parameter (TEP). Iridium complex, [(IAz-OMe)IrCl(CO)₂] was synthesized and infrared spectra were recorded in dichloromethane solution as film in NaCl crystals and by drop-casting in an ATR crystal. Comparison in averaged carbonyl stretching frequency between all-trans isomer ( ) and all-cis isomer ( ) indicates a significant decrement of Δ^tt–cc _av=2.7 cm⁻¹ (film) and 3.8 cm⁻¹ (ATR). Therefore, moderate to excellent enhancement of electron density (Δ^tt–cc TEP=2.3 cm⁻¹ [film] and 3.2 cm⁻¹ [ATR]) at the C-2 position of the NHC is achieved through trans→cis isomerization of the remotely placed azobenzene units. This fine phototuning of electron-donating ability at the catalytic center would pave the way to control NHC/NHC-metal catalyzed organic transformations through external stimuli.     

Synthesis,Structure and Nickel Carbonyl Complexes of Dialkylterphenyl Phosphines

The experimental and computational characterization of a series of dialkylterphenyl phosphines, PR₂Ar′ is described. The new P-donors comprise five compounds of general formula PR₂Ar (R=Me, Et, iPr, c-C₅H₉ and c-C₆H₁₁); Ar = 2,6-C₆H₃-(3,5-C₆H₃-(CMe₃)₂)₂), and another five PR₂Ar′ phosphines containing the bulky alkyl groups iPr, c-C₅H₉ or c-C₆H₁₁, in combination with Ar′=Ar , Ar , or Ar ( L1 – L10 ). Steric and electronic parameters have been determined computationally and from IR and X-ray data obtained for the phosphines and for some derivatives, including tricarbonyl and dicarbonyl nickel complexes, Ni(CO)₃(PR₂Ar′) and Ni(CO)₂(PR₂Ar′). In the solid state, the free phosphines PR₂Ar′ adopt one of the three possible structures formally related by rotation around the C_ipso−P bond. Details on their relative energies and on the influence of the free phosphine structure on its coordination chemistry towards Ni(CO)_n (n = 2, 3) fragments has been obtained by experimental and computational methods.     

Surprising Homolytic Gas Phase Co−C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Aryl-cobalamins are a new class of organometallic structural mimics of vitamin B₁₂ designed as potential ‘antivitamins B₁₂’. Here, the first cationic aryl-cobinamides are described, which were synthesized using the newly developed diaryl-iodonium method. The aryl-cobinamides were obtained as pairs of organometallic coordination isomers, the stereo-structure of which was unambiguously assigned based on homo- and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the ‘beta’ or at the ‘alpha’ face of the cobalt-center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas-phase bond dissociation energies (BDEs) of the coordination-isomeric phenyl- and 4-ethylphenyl-cobinamides were determined by ESI-MS threshold CID experiments, furnishing (Co−C )-BDEs of 38.4 and 40.6 kcal mol⁻¹, respectively, for the two β-isomers, and the larger BDEs of 46.6 and 43.8 kcal mol⁻¹ for the corresponding α-isomers. Surprisingly, the observed (Co−C )-BDEs of the Co_β-aryl-cobinamides were smaller than the (Co−C )-BDE of Co_β-methyl-cobinamide. DFT studies and the magnitudes of the experimental (Co−C )-BDEs revealed relevant contributions of non-bonded interactions in aryl-cobinamides, notably steric strain between the aryl and the cobalt-corrin moieties and non-bonded interactions with and among the peripheral sidechains.     

Oxidation of methylamine

A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH + O CHNH / CHNH + HO, CHNH + H CH + NH, CHNH CHNH, and CHNH + O CHNH + HO were calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNH and its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NH group in CHNH compared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NH in methylamine oxidation.     

Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides

Tetrahydrides crystallizing in the ThCr₂Si₂ structure type have been predicted to become stable for a plethora of metals under pressure, and some have recently been synthesized. Through detailed first-principles investigations we show that the metal atoms within these symmetry MH₄ compounds may be divalent, trivalent or tetravalent. The valence of the metal atom and its radius govern the bonding and electronic structure of these phases, and their evolution under pressure. The factors important for enhancing superconductivity include a large number of hydrogenic states at the Fermi level, and the presence of quasi-molecular H units whose bonds have been stretched and weakened (but not broken) via electron transfer from the electropositive metal, and via a Kubas-like interaction with the metal. Analysis of the microscopic mechanism of superconductivity in MgH₄, ScH₄ and ZrH₄ reveals that phonon modes involving a coupled libration and stretch of the H units leading to the formation of more complex hydrogenic motifs are important contributors towards the electron phonon coupling mechanism. In the divalent hydride MgH₄, modes associated with motions of the hydridic hydrogen atoms are also key contributors, and soften substantially at lower pressures.     

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.     

Spectral Signatures of Oxidation States in a Manganese-Oxo Cubane Water Oxidation Catalyst

We report IR and UV/Vis spectroscopic signatures that allow discriminating between the oxidation states of the manganese-based water oxidation catalyst [(Mn₄O₄)(V₄O₁₃)(OAc)₃]³⁻. Simulated IR spectra show that V=O stretching vibrations in the 900–1000 cm⁻¹ region shift consistently by about 20 cm⁻¹ per oxidation equivalent. Multiple bands in the 1450–1550 cm⁻¹ region also change systematically upon oxidation/reduction. The computed UV/Vis spectra predict that the spectral range above 350 nm is characteristic of the managanese-oxo cubane oxidation state, whereas transitions at higher energy are due to the vanadate ligand. The presence of absorption signals above 680 nm is indicative of the presence of Mn^III atoms. Spectroelectrochemical measurements of the oxidation from [Mn Mn ] to [Mn ] showed that the change in oxidation state can indeed be tracked by both IR and UV/Vis spectroscopy.     

Environmentally Friendly,Photochemical Access to [ ]AuIII Pincer Complexes By Oxidative Addition

Starting from commercially available DMSAuCl and diazonium salts, cationic [ ]Au^III complexes were synthesized in a selective, photosensitizer-free, photochemical reaction by irradiation with blue LED light. This new protocol represents the first easy synthesis of these types of pincer complexes in moderate to excellent yield starting from a readily available gold(I) precursor with nitrogen as the only by-product. Owing to the disadvantages of known protocols, especially the toxicity in the case of a transmetalation with mercury or the necessity for a mostly twofold excess of a gold precursor, this method offers an attractive alternative towards this kind of gold(III) complexes. In addition, the first arylated [ ]Au(III) pincer complex was synthesized by using this technology.     

On normalized Laplacians,multiplicative degree-Kirchhoff indices,and spanning trees of the linear [n]phenylenes and their dicyclobutadieno derivatives

Let be the molecular graph of the linear [n] phenylene with n hexagons and n − 1 squares, and let be the graph obtained by attaching four-membered rings to the terminal hexagons of . In this article, the normalized Laplacian spectrum of consisting of the eigenvalues of two symmetric tridiagonal matrices of order 3n is determined. An explicit closed-form formula of the multiplicative degree-Kirchhoff index (respectively the number of spanning trees) of is derived. Similarly, explicit closed-form formulas of the multiplicative degree-Kirchhoff index and the number of spanning trees of are obtained. It is interesting to see that the multiplicative degree-Kirchhoff index of (respectively ) is approximately to one half of its Gutman index.     

Manipulating Slow Magnetic Relaxation by Light in a Charge Transfer {Fe2Co} Complex

Some cyanide-bridged complexes are known for exhibiting slow magnetic relaxation behavior in a light-induced metastable state. Herein, an unexpected reverse effect is observed for the first time in the S= {Fe^II_LS-Co^III_LS-Fe^III_LS} (HS=high spin, LS=low spin) ground state of a novel V-shaped trinuclear cyanide-bridged {Fe₂Co} complex. In this complex, light-switchable iron-cobalt charge transfer with repeatable off/on switching of slow magnetic relaxation is discovered upon alternating laser irradiation at 785 and 560 nm. An important characteristic of the present compound is that the S= ground state exhibits slow magnetic relaxation before irradiation, whereas this is accelerated after irradiation. This is different from the typical behavior, where the light-induced metastable state exhibits slow magnetic relaxation.     

Photoionization Bands of Cyanogen: Multi-Mode Vibronic Coupling and Renner-Teller Effects

Multi-mode vibronic coupling in the , , and electronic states of Cyanogen radical cation (C N ) is investigated with the aid of ab initio quantum chemistry and first principles quantum dynamics methods. The electronic degenerate states of Π symmetry of C N undergo Renner-Teller (RT) splitting along degenerate vibrational modes of π symmetry. The RT split components form symmetry allowed conical intersections with those from nearby RT split states or with non-degenerate electronic states of Σ symmetry. A parameterized vibronic Hamiltonian is constructed using standard vibronic coupling theory in a diabatic electronic basis and symmetry rules. The parameters of the Hamiltonian are derived from ab initio calculated adiabatic electronic energies. The vibronic spectrum is calculated, assigned and compared with the available experimental data. The impact of various electronic coupling on the vibronic structure of the spectrum is discussed.