Search

Ground state singlet carbenes commonly feature σ2π0 orbital occupations and are known for their concerted σ-bond insertion and cycloaddition reactions. Despite the facility of these transformations, orbital symmetry conservation forces them into non-least-motion π-approach reaction pathways. This situation completely changes when the singlet σ0π2 electron configuration becomes the ground state, which we show here by means of high-level CCSD(T) geometry optimizations. Carbenes like the experimentally known 2H-imidazol-2-ylidene react with H2 and ethylene with negligible or no barrier in a σ-fashion, which effectively corresponds to a least-motion reaction trajectory.

In 1983, F. Duncan M. Haldane predicted a singlet ground state for isolated integer-spin one-dimensional antiferromagnets with low single-ion anisotropy D. Since then, a lot of species containing c...

Ionization potentials (IPs) for MO3 and MO2 for M = U, Mo, W, and Nd have been predicted using the Feller-Peterson-Dixon (FPD) approach at the coupled cluster CCSD(T)/complete basis set level including additional corrections. The additional corrections are mostly small, with spin-orbit effects contributing less than 0.05 eV, except for NdO2 where the correction lowers the IP by 0.26 eV. The IPs for UO3 and UO2 are calculated to be 9.59 and 6.09 eV, respectively. The calculated IPs for MoO3 and WO3 are very similar, 11.13 and 11.11 eV, respectively, and MoO2 and WO2 are 8.51 and 8.79 eV, respectively. MoO2 has a triplet ground state, whereas WO2 has a singlet ground state. The calculated IP for NdO2 is 7.90 eV. NdO3 does not achieve a high +VI formal oxidation state on the lanthanide and has an IP of 7.80 eV. These calculated IPs are expected to have error bars of ±0.04 eV.

The photophysics of the isolated trianion Ag29(BDT)123– (BDT = benzenedithiolate), a ligand-protected cluster comprising BDT-based ligands, terminating a shell of silver thiolates and a core of sil...

Zethrenes are model diradicaloids with potential applications in spintronics and optoelectronics. Despite a rich chemistry in solution, on-surface synthesis of zethrenes has never been demonstrated. We report the on-surface synthesis of super-heptazethrene on Au(111). Scanning tunneling spectroscopy investigations reveal that super-heptazethrene exhibits an exceedingly low HOMO-LUMO gap of 230 meV and, in contrast to its open-shell singlet ground state in the solution phase and in the solid-state, likely adopts a closed-shell ground state on Au(111).

The isolation and characterization of the first silicon analogue of the aromatic cyclobutadiene dication is reported. The aromatic 2π-electron tetrasilacyclobutadiene dication in [(LPhSi)4](BPh4)2 ...

Diindeno-fused dibenzo[a,h]anthracene 6 and diindeno-fused dibenzo[c,l]chrysene 9 contain the key moieties 1,4-quinodipropene (1,4-QDP) and 2,6-naphthoquinodipropene (2,6-NQDP), respectively, and they both have an open-shell singlet ground state. The latter compound exhibits a strong biradical character and interesting properties, including a low ΔET-S (2.44 kcal mol-1 ), a small HOMO-LUMO gap (1.06 eV), a wide photoabsorption range (250-1172 nm), and a large two-photon absorption cross-section (σ=1342±56 GM). This work verifies that 6 has a slightly larger HOMO-LUMO gap and ΔET-S than its helical isomer diindeno[2,1-f:1',2'-j]picene (DIP), but is a much stronger two-photon absorber, verifying the important effect of geometry on the photophysical properties.

The active materials of organic solar cells are widely recognized to show closed-shell singlet ground state and their electron spin resonance signals are attributed to the defects and impurities. Herein, we disclose the inherent open-shell singlet ground state of donors and the closed-shell structure of acceptors via the combination of variable temperature NMR, electron spin resonance, superconducting quantum interference device and theoretical calculation, providing a new perspective to understand the intrinsic molecular structure in organic solar cells.

The cyclic Ge(I) compound [(ADCPh)Ge]2 (4) (ADCPh = {CN(Dipp)}2CPh, Dipp = 2,6-iPr2C6H3) containing a 6π-electron C4Ge2 framework has been isolated as a red crystalline solid. CASSCF calculations reveal a closed-shell singlet ground state for 4 with a considerable diradical character (y = 34%). Thus, the diradicaloid 4 readily splits dihydrogen at room temperature to yield the elusive bis-hydridogermylene [(ADCPh)GeH]2 (5).

Density functional theory calculations are employed to explore the reactivity of metalla-N-heterocyclic carbenes (MNHCs) towards activation of a variety of small molecules (H2, NH3, PH3, SiH3Ph and CH4). All the MNHCs considered are found to have a stable singlet ground state and possess suitable electronic properties for their application in small molecule activation. The calculated energy barriers of E–H (E = H, C, N, Si, P) activation for the MNHCs are found to be in agreement with those of the experimentally evaluated cyclic alkyl(amino)carbene (CAAC) and diamidocarbenes (DACs), thereby indicating the activating effect of the incorporation of an ancillary metal center within a cyclic NHC, and highlighting a new, underexplored strategy in achieving difficult bond activations with carbenes.

The energy profile and electronic structure in the [Cu2O2]2+ core have been studied through the peroxo to bis-μ-oxo isomerization process, using multiconfigurational methods. Different strategies were performed to obtain the absolute energies of the singlet ground state using the accuracy potential of the Difference Dedicated Configuration Interaction (DDCI) method. For each structure, the DDCI energies for the ground states were obtained. The results indicate that the minimum bis-μ-oxo is approximately 8 kcal/mol more stable than the peroxo. The analysis of the electronic structure of the peroxo and bis-μ-oxo forms reveals the existence of different extents of charge transfer between the O2 molecule and the Cu, leading to different oxidation states in the Cu and O2 molecule.

(12/12)CASPT2, (16/14)CASPT2, B3LYP, and CCSD(T) calculations have been carried out on 1,8-Naphthoquinone (1,8-NQ), to predict the low-lying electronic states and their relative energies in this non-Kekule quinone diradical. CASPT2 predicts a 1 A1 ground state, with three other electronic states-3 B2 , 3 B1 , and 1 B1 -within about 10 kcal/mol of the ground state in energy. On the basis of the results of these calculations, it is predicted that NIPES experiments on 1,8-NQ •- will find that 1,8-NQ is a diradical with a singlet ground state. © 2018 Wiley Periodicals, Inc.

Although N-heterocyclic carbenes have been well-studied, the simplest aminocarbene, aminomethylene H−C−NH2, has not been spectroscopically identified to date. Herein we report the gas-phase preparation of aminomethylene by high-vacuum flash pyrolysis of cyclopropylamine and subsequent trapping of the pyrolysate in an inert argon matrix at 12 K. Aminomethylene was characterized by matching matrix IR and UV/Vis spectroscopic data with ab initio coupled cluster computations. After UV irradiation of the matrix aminomethylene rearranges to its isomer methanimine (formaldimine) H2C=NH. Based on our experimental results and computations aminomethylene has a singlet ground state with a reaction barrier of almost 46 kcal mol−1 to methanimine so that H-tunneling is excluded.

The two new diboranes 1 and 2 connected by a pyrene moiety at the 1,6- and 1,3-positions, respectively, were synthesized, and their two-electron-reduction reactions were investigated. The doubly reduced species 1••2– is silent in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic measurements, suggesting a quasi-quinoidal structure with a diradical character of 1••2–, which has a singlet–triplet gap of 6.6 kcal mol–1 as determined by theoretical calculations. In contrast, the reduction product 2••2– is EPR active and theoretical calculations indicate that 2••2– has an open-shell singlet ground state with a singlet–triplet energy gap of 4.9 kcal mol–1.

Reduction of the imine functions of meso-[(3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradec-4,11-diene)nickel(II)] by NaBH4 yields the complex nickel(II) cations of three stereo-isomers of 2,5,5,7,9,12,14,14-octamethyl-1,4,8,11-tetrazacyclotetradecane (L): 2RS,7SR,9SR,14RS (mm1-L), 2RS,7RS,9SR,14SR (mm2-L) and 2RS,7RS,9SR,14RS (mr-L), in ca. 16, 37 and 23% yields respectively. The preparation and structure of [Ni(mm1-L)](ClO4)2 has been reported, but no metal-ion compounds of mm2-L have been reported. The preparations and structures are reported for square-planar singlet ground state salts: [Ni(mm1-L)]ZnCl4, [Ni(mm2-L)]ZnCl4, [Ni(mm2-L)](ClO4)2, [Ni(mm1-L)]Cl2·2H2O and [Ni(mm1-L)](CNS)2. Preparations and structures are reported for the triplet ground state octahedral compounds trans-[Ni(mm2-L)X2], X− = Cl−, N3−, ONO−, and CH3CO2−, trans-[Ni(mm2-L)(OH2)2](NO3)2, trans-[Ni(mm1-L)(N3)2] and trans-[Ni(mm1-L)(NCS)2]·0.5H2O, all with the nitrogen configuration 1R,4S,8S,11R. The mm1-L compounds are all centrosymmetrical with the C2 and C9 methyl substituents axially oriented. The mm2-L compounds, except for [Ni(mm2-L)]–ZnCl4 and –(ClO4)2, have centrosymmetrical octahedral molecules/cations with the C2 and C9 methyl substituents equatorially oriented. The structure of 2RS,7RS,9SR,14SR-2,5,5,7,9,12,12,14-octamethyl-1,4,8,11-tetraazacyclotetradecane dihydrate, mm2-L·2H2O, released from the nickel(II) compound by reaction with cyanide, is reported. Structures are also reported for trans-[Ni(teta)X2] (teta = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetrazacyclotetradecane, X = NO2− and CH3CO2−.

Iron porphyrins can act as potent electrocatalysts for CO2 functionalization. The catalytically active species has been proposed to be a formal Fe(0) porphyrin complex, [Fe(TPP)]2– (TPP = tetraphenylporphyrin), generated by two-electron reduction of [FeII(TPP)]. Our combined spectroscopic and computational investigations reveal that the reduction is ligand-centered and that [Fe(TPP)]2– is best formulated as an intermediate-spin Fe(II) center that is antiferromagnetically coupled to a porphyrin diradical anion, yielding an overall singlet ground state. As such, [Fe(TPP)]2– contains two readily accessible electrons, setting the stage for CO2 reduction.

Incorporating magnetism into two dimensional (2D) van der Waals (VdW) heterostrutures is crucial for the development of functional electronic and magnetic devices. Here we show that Nb3X8 (X = Cl, Br) is a family of 2D layered trimerized kagom\'e magnets that are paramagnetic at high temperatures and undergo a first order phase transition on cooling to a singlet magnetic state. X-ray diffraction shows that a rearrangement of the VdW stacking accompanies the magnetic transition, with high and low temperature phases consistent with STEM images of the end members {\alpha}-Nb3Cl8 and \b{eta}-Nb3Br8. The temperature of this transition is systematically varied across the solid solution Nb3Cl8-xBrx (x = 0-8), with x = 6 having transitions near room temperature. The solid solution also varies the optical properties, which are further modulated by the phase transition. As such, they provide a platform on which to understand and exploit the interplay between dimensionality, magnetism, and optoelectronic behavior in VdW materials.

Theoretical studies predict that the stability of the singlet state of metalla-N-heterocyclic carbenes (MNHCs) is strongly influenced not only by the substituents at the α-nitrogen atoms and the nature of ligands at the transition metal center but also by the substituents at the carbenic backbone. All the MNHCs were found to have a stable singlet ground state and the computed ΔES–T values for some of the MNHCs were found to be significantly large and lie within the range of experimentally known carbenes (31.0–84.0 kcal mol−1). Furthermore, they were found to have superior σ-donation ability to conventional NHCs. The calculated proton affinities, gallium pyramidalization, pKa and nucleophilicity index values for the MNHCs were found to be in good agreement with their calculated electron donation ability.

Experiments on the lowest lying singlet and triplet states of didehydropyrazine isomers (that focus on energy gaps, geometries, and vibrational frequencies) have been carried out computationally by implementing the improved virtual orbital-based multireference (MR) ab initio methods. Pyrazines possess a reasonable MR nature making their description challenging with the conventional quantum chemical approaches. Although wave functions of the diradicals usually have two dominant configurations, a larger reference space is warranted to consistently and accurately describe pyrazine diradicals indicating the complex nature of the systems. Present calculations predict a singlet ground state for ortho- and para-pyrazines, while a triplet ground state is suggested for the meta isomer. The adiabatic singlet-triplet energy gap for the para form is found to be notably higher (by 28.4 kcal/mol) compared to ortho (2.0 kcal/mol) and meta isomers (-5.1 kcal/mol). Accurate and reliable computations are imperative for forecasting the state-ordering in such diradicals. The structural properties obtained from the present calculations lend strong support toward future experimental explorations on these systems.

International audience; A new triethylbenzene-derived tetraazamacrocycle containing pyridyl spacers, L, was prepared and its dinuclear copper(II) complex was used as receptor for the recognition of phosphorylated peptides in aqueous solution. A detailed study of the acid-base behaviour of L and its copper(II) complexation properties as well as of the cascade species with phosphorylated anions including two peptidic substrates was carried out in H2O/MeOH (50:50 v/v) solution using different techniques, such as potentiometry, X-band EPR and DFT calculations. The association constants of the dinuclear receptor with the phosphorylated peptides and other anionic species revealed a clear preference towards phenylic phosphorylated substrates, with values ranging 3.96–5.35 log units. Single-crystal X-ray diffraction determination of the dicopper(II) complex of L showed the copper centres at a distance of 5.812(1) Å from one another, with the phosphate group of the (PhPO4)2– substrate well accommodated between them. X-band EPR studies indicated a similar structure for this cascade complex and for the other cascade complexes with the phosphorylated anions studied. DFT studies of the [Cu2L(μ-OH)]3+ complex revealed a different conformation of the ligand that brings the two copper centres at a very short distance of 3.94 Å aided by the presence of a bridging hydroxide anion that provides a Cu···O···Cu angle of 167.3º. This complex is EPR silent, in line with the singlet ground state obtained using CASSCF(2,2) calculations and DFT calculations with the broken-symmetry approach. This species coexists in solution with a complex in a different conformation, and having a Cu···Cu distance of 6.63Å, in lower percentage.

Variable-temperature NMR and ESR spectroscopic studies reveal that bis(dibenzo[a,i]fluorenylidene) 1 possesses a singlet ground state, 1(S0), while the 90° twisted triplet 1(T1) is populated to a small extent already at room temperature. Analysis of the increasing amount of paramagnetic 1(T1) at temperatures between 300 and 500 K yields the exchange interaction Jex/h c=3351 cm−1 and a singlet–triplet energy splitting of 9.6 kcal mol−1, which is in excellent agreement with calculations (9.3 kcal mol−1 at the UKS BP86/B3LYP/revPBE level of theory). In contrast, the zero-field splitting parameter D is very small (calculated value −0.018 cm−1) and unmeasurable.

Four isomeric azulenylcarbenes were synthesized in argon matrices by photolysis of the corresponding diazo precursors, and the photochemistry of these carbenes was studied. The carbenes and their rearranged products were characterized by IR, UV/Vis, and EPR spectroscopy, and the experimental data were compared to results from DFT calculations. While 2-, 5- and 6-azulenylcarbene show triplet ground states, 1-azulenylcarbene exhibits a singlet ground state, in accord with theoretical predictions. The rearrangements of the azulenylcarbenes give access to a number of unusual C11 H8 isomers, such as other carbenes and strained allenes.

The fundamental relationship between structure and diradical character is important for the development of open-shell diradicaloid-based materials. In this work, we synthesized two structural isomers bearing a 2,6-naphthoquinodimethane or a 1,5-naphthoquinodimethane bridge and demonstrated that their diradical characters and chemical reactivity are quite different. The mesityl-or pentafluorophenyl-substituted octazethrene derivatives OZ-M/OZ-F and their isomer OZI-M (with mesityl substituents) were synthesized via an intramolecular Friedel-Crafts alkylation followed by oxidative dehydrogenation strategy from the key building blocks 4 and 11. Our detailed experimental and theoretical studies showed that both isomers have an open-shell singlet ground state with a remarkable diradical character (y0 = 0.35 and 0.34 for OZ-M and OZ-F, and y0 = 0.58 for OZI-M). Compounds OZ-M and OZ-F have good stability in an ambient environment, while OZI-M has high reactivity and can be easily oxidized to a dioxo product 15, which can be correlated to their different diradical characters. Additionally, we investigated the physical properties of OZ-M, OZ-F, and 15.

Tetraphenylhexaazaanthracene (TPHA), a fluorescent zwitterionic biscyanine with a closed-shell singlet ground state, on treatment with manganese dioxide or phenyliodine bis(trifluoroacetate) (PIFA), undergoes oxidative dimerization to give a near-zero dipole scissor 5,5′-dimer DI-TPHA. Both acene components of the new dimer DI-TPHA maintain their biscyanine closed-shell singlet ground state motifs, as judged by analysis of both single crystal X-ray crystallographic and density functional theory computational studies however, unlike TPHA, DI-TPHA is only very weakly fluorescent. © 2016 American Chemical Society. 18 5 1116 1119

We report the crystal field levels of several newly discovered rare-earth kagome compounds: ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$, ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$, and ${\mathrm{Pr}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$. We determine the crystal electric field (CEF) Hamiltonian by fitting to neutron scattering data using a point charge Hamiltonian as an intermediate fitting step. The fitted Hamiltonians accurately reproduce bulk susceptibility measurements, and the results indicate easy-axis ground-state doublets for ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$ and ${\mathrm{Nd}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Zn}}_{2}{\mathrm{O}}_{14}$ and a singlet ground state for ${\mathrm{Pr}}_{3}{\mathrm{Sb}}_{3}{\mathrm{Mg}}_{2}{\mathrm{O}}_{14}$. These results provide the groundwork for future investigations of these compounds and a template for CEF analysis of other low-symmetry materials.

Tetraphenylhexaazaanthracene, TPHA-1, is a fluorescent zwitterionic biscyanine with a closed-shell singlet ground state. TPHA-1 overcomes its weak 16π antiaromaticity by partitioning its π system into 6π positive and 10π negative cyanines. The synthesis of TPHA-1 is low yielding and accompanied by two analogous TPHA isomers: the deep red, non-charge-separated, quinoidal TPHA-2, and the deep green TPHA-3 that partitions into two equal but oppositely charged 8π cyanines. The three TPHA isomers are compared. © 2015 American Chemical Society. 17 16 4026 4029 Cited By :5

The geometries and electronic structures of selenolate-protected Au nanoclusters, Au24(SeR)20 and Au20(SeR)16, and their thiolate analogues are theoretically investigated with DFT and SCS-MP2 methods, to elucidate the electronic structure of their unusual Au8 core and the reason why they have the unusual entangled "staple-like" chain ligands. The Au8 core is understood to be an [Au4](2+) dimer in which the [Au4](2+) species has a tetrahedral geometry with a closed-shell singlet ground state. The SCS-MP2 method successfully reproduced the distance between two [Au4](2+) moieties, but the DFT with various functionals failed it, suggesting that the dispersion interaction is crucial between these two [Au4](2+) moieties. The SCS-MP2-calculated formation energies of these nanocluster compounds indicate that the thiolate staple-like chain ligands are more stable than the selenolate ones, but the Au8 core more strongly coordinates with the selenolate staple-like chain ligands than with the thiolate ones. Though Au20(SeR)16 has not been reported yet, its formation energy is calculated to be large, suggesting that this compound can be synthesized as a stable species if the concentration of Au(SeR) is well adjusted. The aurophilic interactions between the staple-like chain ligands and between the Au8 core and the staple-like chain ligand play an important role for the stability of these compounds. Because of the presence of this autophilic interaction, Au24(SeR)20 is more stable than Au20(SeR)16 and the unusual entangled ligands are involved in these compounds.

N,N′-Diphenylbenzothiazinophenothiazine exhibited two reversible oxidation waves at −0.06 and +0.42 V vs. Fc/Fc+ in CH2Cl2. Chemical oxidation using two equivalents of tris(4-bromophenyl)aminium he...

WOS: 000440607800023, Nano silicon is emerging as an active element for UV applications due to quantum confinement-induced widening of the Si bandgap, amenability to integration on Si, and less sensitivity to temperature. NanoSi-based UV applications include deep space exploration, high temperature propulsion, solar photovoltaics, and particle detection in high energy accelerators. However, the viability of the technology is limited by a complex nanoSi optical quenching instability. Here, we examined the time dynamics of UV-induced luminescence of sub 3-nm nanoSi. The results show that luminescence initially quenches, but it develops a stability at similar to 50% level with a time characteristic of minutes. Upon isolation, partial luminescence recovery/reversibility occurs with a time characteristics of hours. To discern the origin of the instability, we perform first principles atomistic calculations of the molecular/electronic structure in 1-nm Si particles as a function of Si structural bond expansion, using time dependent density functional theory, with structural relaxation applied in both ground and excited states. For certain bond expansion/relaxation, the results show that the low-lying triplet state dips below the singlet ground state, providing a plausible long-lasting optical trap that may account for luminescence quenching as well as bond cleavage and irreversibility. Time dynamics of device-operation that accommodates the quenching/recovery time dynamics is suggested as a means to alleviate the instability and allow control of recovery, which promises to make it an effective alternative to UV-enhanced Si or metal-based wide-bandgap sensing technology. Published by AIP Publishing.