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Radicals of the lightest group 13 element, boron, are well established and observed in numerous forms. In contrast to boron, radical chemistry involving the heavier group 13 elements (aluminum, gallium, indium, and thallium) remains largely underexplored, primarily attributed to the formidable synthetic challenges associated with these elements. Herein, we report the synthesis and isolation of planar and twisted conformers of a doubly CAAC (cyclic alkyl(amino)carbene)‐radical‐substituted dialane. Extensive characterization through spectroscopic analyses and X‐ray crystallography confirms their identity, while quantum chemical calculations support their open‐shell nature and provide further insights into their electronic structures. The dialane‐connected diradicals exhibit high susceptibility to oxidation, as evidenced by electrochemical measurements and reactions with o‐chloranil and a variety of organic azides. This study opens a previously uncharted class of dialuminum systems to study, broadening the scope of diradical chemistry and its potential applications. [ABSTRACT FROM AUTHOR]

We report the preparation of a new organic σ‐donor with a C6H4‐linker between an N‐heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N‐heterocyclic quinodimethane (NHQ). The aromatization of the C6H4‐linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N‐heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head‐to‐head dimerization by C−C coupling of the methylidene groups. DFT computations indicate that this reaction proceeds via an open‐shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N‐heterocyclic bis‐quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E1/2=−1.71 V vs. Fc/Fc+) and exhibits a remarkable small singlet–triplet gap of ΔES→T=4.4 kcal mol−1. [ABSTRACT FROM AUTHOR]

Herein, reactivity studies of a cyclic bis‐hydridostannylene [(ADC)SnH]2 (1‐H2) (ADC=PhC{(NDipp)C}2; Dipp=2,6‐iPr2C6H3) with various unsaturated organic substrates are reported. Reactions of terminal alkynes (RC≡CH) with 1‐H2 afford mixed acetylide‐vinyl‐functionalized bis‐stannylenes via dehydrogenation and hydrostannylation. Treatment of 1‐H2 with PhC≡CCH3 gives a unique distannabarrelene via dehydrogenative C(sp3)−H stannylation and hydrostannylation of the C≡CCH3 moiety. 1‐H2 undergoes dehydrogenative [2+2]‐cycloaddition reactions with diphenylacetylene, azobenzene, acetone, benzophenone, and benzaldehyde to form the 1,4‐distannabarrelene derivatives. The elimination of H2 in these reactions suggests the masked‐diradical property of 1‐H2. In fact, these [2+2]‐cycloaddition products are also accessible on treatments of the Sn(I) diradicaloid [(ADC)Sn]2 (1) with appropriate reagents. All compounds have been characterized by multinuclear NMR spectroscopy and single crystal X‐ray diffraction. Moreover, the catalytic activity of 1‐H2 has been shown for the hydroboration of unsaturated substrates. [ABSTRACT FROM AUTHOR]

Phosphorus‐centered disbiradicals, in which the radical sites exist as individual spin doublets with weak spin‐spin interaction have not been known so far. Starting from monoradicals of the type [⋅P(μ‐NTer)2P−R], we have now succeeded in linking two such monoradical phosphorus centers by appropriate choice of a linker. To this end, biradical [⋅P(μ‐NTer)2P⋅] (1) was treated with 1,6‐dibromohexane, affording the brominated species {Br[P(μ‐NTer)]2}2C6H12 (3). Subsequent reduction with KC8 led to the formation of the disbiradical {⋅[P(μ‐NTer)]2}2C6H12 (4) featuring a large distance between the radical phosphorus sites in the solid state and formally the highest biradical character observed in a P‐centered biradical so far, approaching 100 %. EPR spectroscopy revealed a three‐line signal in solution with a considerably larger exchange interaction than would be expected from the molecular structure of the single crystal. Quantum chemical calculations revealed a highly dynamic conformational space; thus, the two radical sites can approach each other with a much smaller distance in solution. Further reduction of 4 resulted in the formation of a potassium salt featuring the first structurally characterized P‐centered distonic radical anion (5−). Moreover, 4 could be used in small molecule activation. [ABSTRACT FROM AUTHOR]

Radicals based on arylamine cyclophanes can be used as functional materials and show application potential in fields such as synthetic chemistry, molecular electronic components, organic light-emitting diodes, and catalytic chemistry. Using a Buchwald–Hartwig palladium-catalyzed aryl halide amination method, we synthesized a series of neutral hexaazacyclophane compounds 1–3 with different substituents in the meta–meta–meta positions of the phenyl rings. Three characteristic high-spin hexaazacyclophane diradical dications were obtained by two-electron oxidation using AgSbF6: 12·+•2[SbF6]−, 22·+•2[SbF6]−, and 32·+•2[SbF6]−. The electronic structures and physical properties of these compounds were then investigated by 1H and 13C nuclear magnetic resonance spectroscopy, cyclic voltammetry, electron paramagnetic resonance spectroscopy, superconducting quantum interferometry, ultraviolet–visible spectroscopy, and density functional theory calculations. The findings provide new ideas for designing radical species with novel physical properties and electronic structures. Importantly, the obtained radical species are not sensitive to air, making them valuable functional materials for practical applications. [ABSTRACT FROM AUTHOR]

The dinitrene 2,3,5,6‐tetrafluoro‐1,4‐phenylenedinitrene was synthesized by photolysis of matrix‐isolated 1,4‐diazido‐2,3,5,6‐tetrafluorobenzene at 365 or 405 nm in high yields at cryogenic temperatures and characterized by FT‐IR, UV‐Vis, and EPR spectroscopy. The electronic structure of the dinitrene is described best as a quinoidal diradical with an open‐shell singlet ground state and a triplet state laying only 650 ± 5 cal/mol higher in energy. Irradiation with λ = 254 nm results in fragmentation of the dinitrene into FCCF and several olefins which on prolonged photolysis fragment into FCCCN, FCCNC, and FCN. [ABSTRACT FROM AUTHOR]

Two series of regioisomeric dicyanomethylene substituted dithienodiazatetracenes with formal para‐ or ortho‐quinodimethane subunits were synthesized and characterized. Whereas the para‐isomers (p‐n, diradical index y0=0.01) are stable and isolable, the ortho‐isomer (y0=0.98) dimerizes into a covalent azaacene cage. Four elongated σ‐CC bonds are formed and the former triisopropylsilyl(TIPS) ‐ethynylene groups transformed into cumulene units. The azaacene cage dimer (o‐1)2 was characterized by X‐ray single crystal structure analysis and temperature‐dependent infrared (IR), electron paramagnetic resonance (EPR, solid‐state), nuclear magnetic resonance (NMR) and ultraviolet‐visible (UV/Vis) spectroscopies (solution) indicating reformation of o‐1. [ABSTRACT FROM AUTHOR]

The multifaceted little Na−B bond in NaBH3‐ cluster has been the subject of many recent studies. Many proposals have been put forward for the complete understanding of the chemical bonding in NaBH3‐ cluster. However, none of the studies provided a complete picture. Herein, the missing recipe of the chemical bonding in NaBH3‐ cluster has been identified which completes the description. Our calculations reveal that the chemical bonding in NaBH3‐ cluster is more multifaceted than ever thought. Our finding suggests that the NaBH3‐ cluster features a Na−B one electron bond which is assisted by three 3‐centre‐2‐electron Na‐B−H bond. Detailed topological analyses of electron density and electron localization function at the correlated level reveal such a bonding situation. This missing 3‐centre‐2‐electron Na‐B−H bond completes the description of chemical bonding in NaBH3‐ cluster which is at‐per with the well established bonding scenario for electron deficient compounds. Similar bonding scenario is observed in KBH3−, CuBH3‐ and AuBH3− clusters, further lending support to the complete picture of bonding. [ABSTRACT FROM AUTHOR]

A heteroatomic analog of 1,1,2,3,3-pentamethylenepropane, the diradical N-[bis(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)methylene]-2-methylpropane-2-amine oxide (1) with two C(sp2)-bonded nitronyl nitroxide fragments in magnetically concentrated (single crystal) and magnetically diluted (frozen solution) forms was studied by EPR spectroscopy. The longitudinal and transverse relaxation times of diradical 1 in the frozen solution were measured for the first time; a significant difference in the intramolecular exchange interaction was demonstrated for the cases of the single crystal and the frozen solution. The influence of solvation on the value of the intramolecular exchange interaction in this class of compounds was discussed. [ABSTRACT FROM AUTHOR]

The first SnI diradical [(ADCPh)Sn]2 (4) based on an anionic dicarbene (ADCPh={CN(Dipp)}2CPh; Dipp=2,6‐iPr2C6H3) scaffold has been isolated as a green crystalline solid by KC8 reduction of the corresponding bis‐chlorostannylene [(ADCPh)SnCl]2 (3). The six‐membered C4Sn2‐ring of 4 containing six π‐electrons shows a diatropic ring current, thus 4 may also be regarded as the first 1,4‐distannabenzene derivative. DFT calculations suggest an open‐shell singlet (OS) ground state of 4 with a remarkably small singlet–triplet energy gap (ΔEOS–T=4.4 kcal mol−1), which is consistent with CASSCF (ΔES–T=6.6 kcal mol−1 and diradical character y=37 %) calculations. The diradical 4 splits H2 at room temperature to yield the bis‐hydridostannylene [(ADCPh)SnH]2 (5). Further reactivity of 4 has been studied with PhSeSePh and MeOTf. [ABSTRACT FROM AUTHOR]

Pyrrolidine is a suitable model substance featuring a five-membered N-heterocycle representing the typical structure of the N-containing compounds in biomass. Previous studies have provided ambiguous arguments on the reaction mechanism of pyrrolidine thermal decomposition. Knowledge on the fate of the most dominant decomposition product, the unstable diradical · CH 2 NHCH 2 · , is lacking. In this work, a high-level potential energy surface of the unimolecular reactions of · CH 2 NHCH 2 · , including isomerization and decomposition channels, was explored. Then, the rate coefficients of various channels were obtained by the RRKM/master equation method over 500–2000 K and 0.001–100 atm. The results show that the thermal stabilization of cyc-C 2 H 5 N is highly favored over other isomerization and decomposition channels. The channels isomerizing to CH 3 NCH 2 , cis-HNCHCH 3 and trans-HNCHCH 3 compete with each other, and the rate constants are at least two orders of magnitude lower than the formation of cyc-C 2 H 5 N. Being thermodynamically unstable, cyc-C 2 H 5 N will mainly isomerize back into the diradical at temperatures ≤ 1200 K at 1 atm or isomerize to cis-HNCHCH 3 when the temperature is higher. To validate the postulated reaction pathways, a pyrolysis experiment of pyrrolidine was conducted in a SiC reactor with a short residence time (40–60 μ s) at 1050 K and 0.263 atm. The experimental result confirms the collisional stabilization of H 2 NCHCH 2 and cyc-C 2 H 5 N + CH 3 NCH 2. The diradical · CH 2 NHCH 2 · was not readily detectable due to its low concentration, which falls below the detection limit of current analytical techniques, while the stabilization of cis-HNCHCH 3 and trans-HNCHCH 3 was not sure because of their extremely low photoionization cross section under the studied energy range. The rate constants of the isomerization and decomposition reactions of diradical · CH 2 NHCH 2 · and cyc-C 2 H 5 N are provided, which are valuable for developing the mechanism for pyrrolidine and deepening our understanding of the mechanism of N-heterocyclic compounds pyrolysis/combustion. [ABSTRACT FROM AUTHOR]

Carbon-centered radicals and diradicaloids based on classical N-heterocyclic carbene (NHC) scaffolds are readily accessible as crystalline solids. The presence of an aryl (Ar) substituent at the C2-position is the key to the remarkable stability of these open-shell species as it provides appropriate room for the spin-density delocalization. Two catalytic as well as high-yielding protocols have been developed to install a suitable aryl group at the C2-position of NHCs. The spin-density in mono-radicals (NHCAr)• is mostly located on the parent carbene carbon (C2) atom. The bridging of two NHCs through a phenylene spacer (C6 H4) n enables the isolation of various p -quinodimethane (p -QDM) derivatives, which may be considered as open-shell Kekulé diradicaloids. The diradical character of these NHC-analogues of Thiele (n = 1), Chichibabin (n = 2), and Müller (n = 3) hydrocarbons [(NHC)(C6 H4) n (NHC)] can be tuned by a rational choice of the size and/or the topology of spacers. In this account, the synthesis, structure, and properties of this new class of radical hydrocarbons is presented. 1 Introduction 2 NHC-Monoradicals 3 NHC-Diradicaloids 4 Conclusion [ABSTRACT FROM AUTHOR]

• Highly selective synthesis of disulfide bonds in the gas phase. • Covalent bond formation to identify sites of noncovalent crown ether binding. • Both biradical and diradical species can be used for bond synthesis. Photoinitiated radical chemistry has proven to be useful for breaking covalent bonds within many biomolecules in the gas phase. Herein, we demonstrate that radical chemistry is useful for bond synthesis in the gas phase. Single peptides containing two cysteine residues capped with propylmercaptan (PM) often form disulfide bonds following ultraviolet excitation at 266 nm and loss of both PM groups. Similarly, noncovalently bound peptide pairs where each peptide contains a single cysteine residue can be induced to form disulfide bonds. Comparison with disulfide bound species sampled directly from solution yields identical collisional activation spectra, suggesting that native disulfide bonds have been recapitulated in the gas phase syntheses. Another approach utilizing radical chemistry for covalent bond synthesis involves creation of a reactive diradical that can first abstract hydrogen from a target peptide, creating a new radical site, and then recombine the second radical with the new radical to form a covalent bond. This chemistry is illustrated with 2-(hydroxymethyl-3,5-diiodobenzoate)-18-crown-6 ether, which attaches noncovalently to protonated primary amines in peptides and proteins. Following photoactivation and crosslinking, the site of noncovalent adduct attachment can frequently be determined. The ramifications of these observations on peptide structure and noncovalent attachment of 18-crown-6-based molecules is discussed. [ABSTRACT FROM AUTHOR]

CarH is a cobalamin-based photoreceptor which has attracted significant interest due to its complex mechanism involving its organometallic coenzyme-B 12 chromophore. While several experimental and computational studies have sought to understand CarH's mechanism of action, there are still many aspects of the mechanism which remain unclear. While light is needed to activate the Co-C 5′ bond, it is not entirely clear whether reaction pathway involves singlet or triplet diradical states. A recent experimental study implicated triplet pathway and importance of intersystem crossing (ISC) as a viable mechanistic route for photoproduct formation in CarH. Herein, a combined quantum mechanics/molecular mechanics approach (QM/MM) was used to explore the involvement of triplet states in CarH. Two possibilities were explored. The first possibility involved photo-induced homolytic cleavage of the Co-C 5′ where the radical pair (RP) would deactivate to a triplet state (T 0) on the ground state potential energy surface (PES). However, a pathway for the formation of the photoproduct, 4′,5′-anhydroadenosine (anhAdo), on the triplet ground state PES was not energetically feasible. The second possibility involved exploring a manifold of low-lying triplet excited states computed using TD-DFT within the QM/MM framework. Viable crossings of triplet excited states with singlet excited states were identified using semiclassical Landau-Zener theory and the effectiveness of spin-orbit coupling by El-Sayed rules. Several candidates along both the Co-N Im potential energy curve (PEC) and Co-C 5′ /Co-N Im PES were identified, which appear to corroborate experimental findings and implicate the possible role of triplet states in CarH. [Display omitted] • QM/MM calculations employed to explore the role of triplet states in photoreceptor CarH. • Excited state deactivation mechanism involving a triplet diradical was explored. • Manifold of low-lying triplet excited states in CarH computed using TD-DFT. • Landau-Zener theory and El-Sayed rules were used to estimate effective spin-orbit coupling. • Multiple triplet states identified as potential candidates for intersystem crossing in CarH. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic behavior of the fluorene bridged verdazyl diradicals has been studied theoretically in their neutral and deprotonated states. The deprotonation of C9‐H site of the fluorene ring opens a new coupling pathway, which changes the nature of magnetic coupling. The transmission spectra analysis reveals that the transmission through fluorine has also increased due to the opening of a new coupling path after deprotonation. [ABSTRACT FROM AUTHOR]

The behavior of organo-aluminum species with 2,3-bis(2-pyridyl) quinoxaline (DPQ), a well-known polyazine capable of performing interesting radical transformations, was examined in the presence and absence of chromium. In spite of proving the presence of chromium as essential for reactivity, only organic radicals, coupled to aluminum-containing residues, have been isolated and characterized. The electronic structure of the organic radicals has been elucidated by a combination of crystallographic, DFT calculations and EPR studies. Experimental and computational work has highlighted the co-existence of both singlet and triplet forms in one di-radical complex. [ABSTRACT FROM AUTHOR]

Abstract: One‐electron reductions of Mes*As=Fl* (1; Fl*=2,7‐di‐tert‐butylfluorenylidene, Mes*=2,4,6‐tBu3C6H2) and diarsaalkenes [1,2‐b]‐IF(=AsMes*)2 (2; IF=indenofluorene) with potassium led to the isolation of the arsenic‐centered radical anion salts 1K and 2K, respectively. The diradical dianion salts 2K2 and 3K2 were afforded by the reduction of 2 and 2,8‐tBu2‐[2,1‐b]‐IF(=AsMes*)2 (3) with an excess amount of KC8. The radicals have been investigated by single‐crystal X‐ray crystallography, EPR, and UV/Vis absorption spectroscopy, along with theoretical calculations. The calculations revealed that 2K2 and 3K2 feature open‐shell singlet ground states with singlet‐triplet energy gaps of 2.1 and 1.0 kcal mol−1, respectively. They are readily thermally excited to triplet states as demonstrated by EPR spectroscopy. The obtained diradicals represent the first examples of heavier Group 15 element centered diradicals with experimentally observable triplet states. [ABSTRACT FROM AUTHOR]

A domino fashioned allylic rearrangement, intramolecular decarboxylative coupling, propargyl-allene rearrangement and diradical 1,3-dien-5-yne cycloaromatization of aryl propiolates of the Baylis-Hillman alcohols in the presence of catalytic Pd(PPh 3 ) 4 and DBU leading to the formation 4-benzylnaphthoates has been described. Interestingly, the meta -substituted propiolates ( 1i – t ) provided sterically hindered cyclization products ( 12 – 23 ) via the unusual ortho -selective cycloaromatization; while the meta -alkoxy ( 1i – m , 1r – t ) group provided predominantly the ortho -selective 1,3-dien-5-yne cycloaromatization products, meta -Me group ( 1 n – q ), products formed both via ortho -selective cyclization and para -selective cyclization products in comparable ratio. A novel mechanism involving diradical species has been proposed to explain the unusual ortho -selectivity in 1,3-dien-5-yne cycloaromatization. Preference for sterically hindered ortho -selective cyclization may be attributed to the formation of a transition state radical possessing more stable ortho -methoxy cyclohexadiene radical moiety (driving force). [ABSTRACT FROM AUTHOR]

To explore stable organic diradicaloids, meso-thienylquinonoid-substituted porphyrins Pn and hexaphyrins Hn, where 'n' denotes the number of thienyl units in the meso-substituents, were synthesized. P0 was identified as a closed-shell quinonoid, whereas P1 was shown to possess significant diradical character with diradical character index ( y) of 0.99 and quite small singlet-triplet energy gap (Δ ES-T) of −0.13 kcal mol−1. P1 was certainly stable, allowing its isolation, but decomposed gradually in solution. In the hexaphyrin series, it was shown that H0 and H1 were closed-shell quinonoids, but H2 was a highly stable diradicaloid with y=0.85 and Δ ES-T of −3.72 kcal mol−1. The high stability of H2 was ascribed to effective spin delocalization over the entire conjugated network. Characteristically, H2 displays an intense absorption band in NIR region at λmax=1175 nm with molar absorption coefficient ( ϵ) of 8.81×104 mol−1 L cm−1, a narrow HOMO-LUMO gap of 0.69 eV, and nine reversible redox potential waves. [ABSTRACT FROM AUTHOR]

Two new diboranes, 2,6-bis(BMes2)mesitylene ( 1) and 3,3′-bis(BMes2)bimesitylene ( 3), were synthesized. Two-electron reduction of 1 with elemental potassium afforded the C−H activation product [(18-c-6)K(THF)2]2+⋅ 22− bearing a BC3 four-membered ring as colorless crystals, whereas the reduction of 3 with potassium led to the isolation of [(18-c-6)K(THF)2]2+⋅ 32−.. as dark blue crystals. Both reduction products were characterized by structural and spectroscopic methods. Electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations revealed that the electron spin density of 32−.. mainly resides on the two boron nuclei and features a triplet ground state, which was confirmed by superconducting quantum interference device (SQUID) measurements as well as theoretical calculations. 32−.. represents the first structurally characterized boron-centered diradical with a triplet ground state. In addition, the reactivity of [(18-c-6)K(THF)2]2+⋅ 32−.. toward PhSeSePh and nBu3SnH was investigated, which is consistent with its radical character. [ABSTRACT FROM AUTHOR]

The purpose of this study was to explore regioselectivity and stereoselectivity in the Paternò-Büchi reaction of pyrrole derivatives. The formation of a bicyclic oxetane in the Paternò-Büchi reaction of 2-siloxypyrrole with benzophenone was reported for the first time, and a mechanism involving the distribution of the intermediary triplet diradicals was proposed to account for the regioselectivity of the reaction. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In order to predict the energies of the low-lying electronic states of the dioxa derivative of trimethylenemethane (DOTMM), we have carried out (U)B3LYP, CASPT2, and (U)CCSD(T) calculations, using the aug-cc-pVTZ basis set. Our calculations predict that DOTMM has a triplet ground state, with one unpaired electron occupying a b2 σ MO and the other a b1 π MO. An open-shell singlet state, with the same orbital occupancy as the triplet, is calculated to lie very close to the 3A2 ground state. However, this open-shell singlet (1A2) is predicted to be the transition structure for methylene rotation and to lead to the barrierless formation of an equivalent pair of α-lactones. We also report the results of some calculations on the of DOTMM to CH2 + CO2. Our predictions about DOTMM could be tested experimentally by generating the DOTMM•− radical anion in the gas phase and obtaining its negative ion photoelectron spectrum. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

An Aza-Wittig reaction of 1,1′-bis(triphenylphosphoranylidenamino)-ferrocene with 3-isocyanato-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-oxyl and subsequent intramolecular [2+2]-cycloaddition of the bis(carbodiimide) intermediate was used for the synthesis of a 1,3-diazetidine-2,4-diimine moiety bearing two radical groups and bridging the cyclopentadienyl (Cp) rings. According to single-crystal X-ray data, the planar 2,4-diimino-1,3-diazetidine moiety adopts a perpendicular orientation with respect to the eclipsed Cp rings. One of the radical groups lies almost within the plane of the four-membered 1,3-diazetidine ring, while the other has a conformationally preferred orientation with dihedral angles ±62.3°. The two Cp rings are constricted by the 1,3-diazetidine-2,4-diimine moiety so that the angle between their planes is 11.0°. The synthesized diradical contains ferrocene and nitroxide redox signaling units that can be oxidized step-by-step at E 1/2 = 0.26 and 0.48 V ( vs Fc/Fc + ), respectively. Electron spin resonance spectroscopy revealed a moderate exchange interaction (| J | ∼ a N ) between the two nitroxide radical moieties and the following values of zero-field splitting parameters: ∣D∣ = 3.5 mT and E/D = 0 were obtained. These data were in agreement with density functional theory calculations. The newly developed approach to multispin systems may be interesting for the construction of weakly coupled rigid polyradicals for quantum technologies, the molecular design of magnets, and the creation of ferrocene-based electron-paramagnetic-resonance–active chemical sensors. [ABSTRACT FROM AUTHOR]

Upon irradiation at λ = 254 nm in acetonitrile solution, the title compounds formed 8-(2-formyloxyethyl)-substituted 2,4-diazabicyclo[4.2.0]oct-1(8)-en-3,5-diones, which underwent thermal cyclobutene ring opening to 6-substituted pyrimidine-2,4-diones. The reaction cascade results in an unprecedented formal 1,5-shift of the substituent at the pyrimidine-2,4-dione core. It was shown that the reaction is likely to proceed via the [2 + 2] photocycloaddition products of the orotates, which could be intercepted in good yields (85% and 98%) upon irradiation at λ = 300 nm. At shorter wavelength a cleavage of the bond between the lactone carbonyl carbon atom and the α-carbon atom is induced, which leads – as shown by deuterium labelling experiments – by intramolecular hydrogen abstraction to the above-mentioned cyclobutenes. [ABSTRACT FROM AUTHOR]

To explore the prospect of molecules involving silicon-silicon multiple bonds as nonlinear optical molecular systems, the relationship between the structure and the second hyperpolarizabilities γ of the oligomeric systems composed of carbon and silicon π-structures is investigated using the density functional theory method. It is found that these compounds indicate intramolecular charge transfer (ICT) from the silicon units to the carbon units together with nonzero diradical characters. The γ values of these compounds are shown to be 2–13 times as large as those of the carbon analogs. Although asymmetric carbon and silicon π-systems exhibit comparable enhancement to the corresponding symmetric systems, donor-π-donor structures exhibit remarkable enhancement of γ despite of their both-end short silicon π-chain moieties (donor units). Further analysis using the odd electron and γ densities clarifies that the intermediate diradical character also contributes to the enhancement of γ. These results predict that even short π-conjugated silicone moieties can cause remarkable enhancement of γ by introducing them into π-conjugated hydrocarbon structures. [ABSTRACT FROM AUTHOR]

Higher acenes have drawn much attention as promising organic semiconductors with versatile electronic properties. However, the nature of their ground state and electronic excited states is still not fully clear. Their unusual chemical reactivity and instability are the main obstacles for experimental studies, and the potentially prominent diradical character, which might require a multireference description in such large systems, hinders theoretical investigations. Here, we provide a detailed answer with the particle-particle random-phase approximation calculation. The ¹Ag ground states of acenes up to decacene are on the closed-shell side of the diradical continuum, whereas the ground state of undecacene and dodecacene tilts more to the open-shell side with a growing polyradical character. The ground state of all acenes has covalent nature with respect to both short and long axes. The lowest triplet state ³B2u is always above the singlet ground state even though the energy gap could be vanishingly small in the polyacene limit. The bright singlet excited state ¹B2u is a zwitterionic state to the short axis. The excited ¹Ag state gradually switches from a double-excitation state to another zwitterionic state to the short axis, but always keeps its covalent nature to the long axis. An energy crossing between the ¹B2u and excited ¹Ag states happens between hexacene and heptacene. Further energetic consideration suggests that higher acenes are likely to undergo singlet fission with a low photovoltaic efficiency; however, the efficiency might be improved if a singlet fission into multiple triplets could be achieved. [ABSTRACT FROM AUTHOR]

DFT broken-symmetry calculations framework at the B3LYP/6-31G(d) level were carried out to evaluate the magnetic exchange coupling constants of four chalcogen-centered radical-C60 diadducts. The interactions of radicals, including O ̇ H, S ̇ H, S ̇ CH3, S ̇ C6H5 occurred in different relative positions. The calculated exchange coupling constants indicate that the coupling interaction regime changes when the radical centers and the relative distance of binding sites on C60 coupler change. In fact, the most intense ferromagnetic interaction occurs when radicals form a right-angle with the fullerene center. The shapes of singly occupied molecular orbital (SOMO) and spin density distribution of structures have also been studied. The energy splitting of the singly occupied molecular orbitals (Δ E SS ) confirm the broken symmetry results. The calculated isotropic hyperfine coupling constants (hfccs) show some differences in diradical-coupler complexes that can be employed in the experimental electron paramagnetic resonance (EPR) spectroscopy. [ABSTRACT FROM AUTHOR]

Bis-2,5-dimethylene-2,5-dihydrothiophenes have been generated in the gas-phase by flash vacuum pyrolysis (FVP) of diester precursors. These thiophene-based bis-( p -quinodimethanes) are shown to undergo reactions leading to macrocycles. The conversions are consistent with a mechanism involving cyclic diradical intermediates followed by disproportionation of the radical centers. [ABSTRACT FROM AUTHOR]

Two rival solutions have emerged from applications of spin-coupled (SC) theory, also known as the full-generalized valence bond (full-GVB) approach, to the π system of O3. The lowest solution (SC d) corresponds to significant diradical character but there is a second solution (SC c) which lies fairly close in energy. Combining these two descriptions, it turns out that both of them make significant contributions, with neither being dominant. The situation is entirely different for SO2, with the SC c description being overwhelmingly dominant over SC d. It can be concluded from an examination of various combinations of rival π-space descriptions for both molecules that neither of the SC c nor SC d solutions provides a very satisfactory description on its own of the π system in O3, but that SO2 is already fairly well described by SC c before the explicit incorporation of any SC d character. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Using a two-site valence configuration interaction model, we reveal the diradical character (y) dependences of the third-harmonic generation (THG) spectra of singlet diradicaloids, which are described by the photon energy-dependent THG second hyperpolarizabilities γ. We particularly examine three- and two-photon resonance peaks associated with the dipole-allowed transition to the first excited state and with the dipole-forbidden transition to the second excited state, respectively. When increasing y, both peaks move to lower-energy region and become close to each other. Although the THG γ spectral intensities show resonance enhancements for any y value, we observe for THG γ the same bell-shape behavior as a function of y, as for the static γ, irrespective of the photon energy. [ABSTRACT FROM AUTHOR]

Azaborolyl anions, the five-membered BN heterocycles, have attracted a considerable attention due to their aromaticity and isoelectronic relationship with ubiquitous cyclopentadienyl ligands. Besides their syntheses and applications in the preparation of metal complexes, the other aspects of their chemistry have been virtually unexplored. Reduction of the azabutadienyl chelate boron dichloride [ArNC(R)CHC(R)]BCl2 ( 2, Ar=2,6-Me2C6H3, R= tBu) with two equivalents of potassium yielded the novel 2-chloro-azaborolyl anion [ArNC(R)CHC(R)BCl]K(thf) ( 3) as a stable product in good yield. Reaction of 3 with 1,3,4,5-tetramethylimidazol-2-ylidene (NHC) yielded the first NHC-azaborole adduct with the elimination of KCl. The salt elimination reaction was also observed in the reactions with H2O and the organic azide ArN3, leading to the formation of an oxo-bridged 3 H-1,2-diazaborole and an intramolecular donor-stabilized iminoborane, demonstrating that 3 is a source of the unexplored 1,2-azaborole isosteric to cyclopentadienylidene. [ABSTRACT FROM AUTHOR]

We predict the magnetic exchange coupling constant ( J) for 27 m-phenylene-based nitronyl nitroxide (NN) diradicals with nine different substituents in three unique (common ortho, ortho- meta and common meta) positions on the coupler unit by using the broken-symmetry density functional methodology. For all investigated diradicals, J values are computed using B3LYP, B3LYP-D3 and M06-2X functionals with 6-311+G(d,p) basis set. The J value is larger than J and closer to the observed value for the unsubstituted species. Substitutions at common ortho position always produce a greater angle of twist between the spin source and the coupler units. When the twist angle is very large, the nature of intramolecular magnetic interaction changes from ferromagnetic to antiferromagnetic. In these cases, the coupler-NN bond order becomes small. Substitution at the common meta position of m-phenylene in the diradical has little steric and hydrogen-bonding effects. Electron-withdrawing groups reveal a specific trend for single-atom substitution. An ortho substitution generally decreases J and a meta substitution always increases J with a decreasing −I effect. Variation of J with planarity as well as Hammett constant is investigated. The nucleus-independent chemical shift value is found to decrease from the corresponding mono-substituted phenyl derivatives. The dependence of J on these factors is explored. [ABSTRACT FROM AUTHOR]

The polycyclic p-quinodimethanes are proposed to be the novel candidates of the high-performance nonlinear optical (NLO) materials because of their large third order polarizabilities ( γ). We investigate the switchable NLO responses of a series of polycyclic p-quinodimethanes with redox properties by employing the density functional theory (DFT). The polycyclic p-quinodimethanes are forecasted to exhibit obvious pure diradical characters because of their large y0 index (the y0 index is a value between 0 [closed-shell state] and 1 [pure biradical state]). The γ values of these polycyclic p-quinodimethanes and their corresponding one-electron and two-electron reduced/oxidized species are calculated by the (U)BHandHLYP method. The γ values of polycyclic p-quinodimethanes and their corresponding one-electron reduced species are all positive and significantly different. The large differences of the γ values are due to a change in the transition energy and are related to the different delocalization of the spin density, which demonstrates that the NLO switching is more effective on one-electron reduction reactions. Therefore, the study on these polycyclic p-quinodimethanes provides a guideline for a molecular design of highly efficient NLO switching. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

What could be simpler than C2, a diatomic molecule that has the second strongest homonuclear bond? This molecule turns out to be a microcosm of the bonding issues that bother chemists, as is shown in this trialogue. Join the three authors in their lively debate, light a candle, as Faraday did, and see the excited states of C2! [ABSTRACT FROM AUTHOR]

Enediynes undergo cycloaromatization to p-benzyne diradicals. A new reaction of p-benzynes is here presented. Instead of their usual radical reactivity, they add nucleophiles, followed by protonation. Kinetic evidence showed that the rate-limiting step is the cycloaromatization, followed by rapid addition of a nucleophile. The unusual feature of nucleophilic addition to a radical center in a singlet diradical is discussed. Calculations that ignore solvation suggest that the addition has no activation barrier, but one is created by the necessity for desolvation of the anionic nucleophile. Relative reactivities of several nucleophiles have been measured under competition conditions. The observation of deuterium incorporation from DMSO- d6 or CD3CN shows that the species that is protonated is the aryl anion, not the p-benzyne. The extent of deuterium incorporation increases as the nucleophile is changed from iodide to bromide to chloride. This remarkable instance of selectivity is discussed. These studies of p-benzyne reactivity are exploring the nonradical chemistry of this diradical. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Total synthesis of ceratopicanol ( 1) was achieved with a tandem cycloaddition reaction of allenyl diazo compound 6 via a trimethylenemethane (TMM) diyl intermediate. The TMM diyl mediated [2+3] cycloaddition reaction furnished the consecutive quaternary carbon centers and showed an unusual diastereoselectivity. [ABSTRACT FROM AUTHOR]

A hybrid hydrazine–nitroxide monoradical and the corresponding heterohydrazyl– nitroxide diradical were used as probes in 4-cyano-4′-pentylbiphenyl liquid crystal. EPR data (hyperfine constants, rotational time, and anisotropy parameter values) are reported in the range of 25°C–55°C. It was shown by EPR spectrometry that the nitroxide moiety gives an anisotropic triplet line, while the hydrazyl moiety cannot be detected under these conditions. [ABSTRACT FROM AUTHOR]

A hybrid hydrazine-nilroxide monoradical and the corresponding heterohydrazyl-nitroxide diradical were used as probes in 4-cyano-4'-pentylbiphenyl liquid crystal. EPR data (hyperfine constants, rotational time, and anisotropy parameter values) are reported in the range of 25°C-55°C. It was shown by EPR spectrometry that the nitrox-ide moiety gives an anisotropic triplet line, while the hydrazyl moiety cannot be detected under these conditions. [ABSTRACT FROM AUTHOR]

The antiaromatic compounds have received a great deal of attention for several decades because of their unusual electronic structures. The electronic structures and properties of antiaromatic pentalene and its six nitrogen heterocyclic derivatives were systematically studied by the density functional theory at the Becke, three-parameter, Lee-Yang-Parr level with 6-31G* basis set. The results indicated that all the monomers have stable singlet states and remarkable bond-length alternations. From the dimer to polymer in those molecules, pentalene(P), cyclopenta[ b]pyrrole(CPP), cyclopenta[ d]imidazole(CPI), pyrrolo[2,3- b]pyrrole(PP1) and pyrrolo[3,2- d]imidazole(PI) are stable diradical structures; pyrrolo[3,2- b]pyrrole (PP2) and imidazo[4,5- d]imidazole(II) are stable singlet ground states. The electronic properties including bond length, bond-length alternation, electron density at bond critical points, Wiberg bond index and nucleus-independent chemical shift were analyzed. It was found that in diradical molecules the bond-length alternations are diminished, the charge tends to equilibrate, the π-electron delocalization and conjugation are strengthened. The electronic properties of singlet ground state molecules have nearly no variations from monomers to polymers. The band structure analysis shows that diradical structure molecules have small band gaps (<1.0 eV), wide bandwidth and small effective masses of holes and electrons which suggest that diradical structure molecules are very good candidates for conductive materials. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The thermal C2-C6/ene cyclization of enyne-allenes has become an exciting venue for both mechanistic and synthetic studies. While at first most efforts were aimed at establishing the diradical nature of the thermal process and to derive therefrom efficient protocols for synthetic schemes, recent evidence has disclosed the C2-C6 cyclization as a unique reaction heavily influenced by nonstatistical dynamic effects. Depending on the substituents at the enyne-allene, one can readily identify transitions between classical and nonstatistical behaviors on the basis of experimental data. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We have developed a method for analyzing the (hyper)polarizabilities of open-shell molecular systems. This method employs the (hyper)polarizability densities based on the natural orbitals and occupation numbers, which enables us to analyze the contributions of odd electrons having various open-shell (diradical) characters. Within broken-symmetry, i.e., spin-unrestricted, single-determinant molecular orbital and density functional theory approaches, we can also remove the spin contamination effects on these quantities through spin projection. To do that, an approximate spin projected method has been elaborated and applied to the analysis of the (hyper)polarizability of multi-radical systems. As examples, typical open-shell singlet systems, 1,3-dipoles and rectangular graphene nanoflakes, are examined. [ABSTRACT FROM AUTHOR]

Abstract: To develop the redox-switching system of intramolecular magnetic interaction, 1,2-bis[3-(N-tert-butyl-N-oxylamino)phenyl]-1,2-bis[4-{N,N-bis(4-methoxyphenyl)amino}phenyl]ethylene, tetraarylethylene with two nitroxide radical groups at the meta-position, was synthesized, and characterized by the electrochemical method and ESR spectroscopy. Cyclic voltammetry showed the tetraarylethylene core has the lower oxidation potential than the substituted nitroxide radical moiety. ESR spectroscopy in frozen solution revealed that the neutral form shows the fine-structured spectrum characteristic of the spin triplet species, while the dicationic form shows the anisotropic hyperfine-structured spectrum characteristic of the randomly-oriented nitroxide radical, indicating the drastic change of intramolecular magnetic interaction. [Copyright &y& Elsevier]

The isomeric 2-dehydro- and 4-dehydro-1,3-benzoquinodimethanes, 18 and 19, were generated by irradiation of the corresponding triiodo compounds in cryogenic argon matrices and characterized by electron paramagnetic resonance spectroscopy. In agreement with multireference computations, both systems possess quartet ground states, whereas in the isomeric 5-dehydro- m-xylylene 20 the 2B2 doublet state lies energetically below the 4B2 state, so that no characteristic quartet signals could be observed for this triradical under similar experimental conditions. The best estimates for the adiabatic doublet-quartet energy splittings (CAS(7,7)-AQCC/cc-pVTZ//CAS(9,9)-RS2c/cc-pVTZ) of 18-20 are 10.4, 7.7, and −1.3 kcal/mol, respectively. The measured zero-field splitting parameters of 18 and 19 are discussed in terms of the contributions of carbenoid resonance structures (spin polarization of the π-system) to the resonance hybrid of the title triradicals. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Three fluoranthenes and one substituted fluoranthene, 2,2-dimethyl-2 H-dibenzo[ cd,k]fluoranthene, were investigated using the unrestricted symmetry-broken and complete active space methods. It was shown that four Kekuléan hydrocarbons are diradicals, implying that their ground state is a triplet. In the energetically less favorable singlet state these hydrocarbons exhibit pronounced diradical character. This occurance is explained with the tendency of the investigated molecules to delocalize their π-electrons. This leads to aromatic stabilization which is stronger than destabilization due to unpaired electrons. Our results for 2,2-dimethyl-2 H-dibenzo[ cd,k]fluoranthene are in excellent accord with experimental findings of McMaster et al. concerning this compound. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Mukherjee-type (Mk) state specific (SS) multi-reference (MR) coupled-cluster (CC) calculations of 1,n-didehydropolyene diradicals were carried out to elucidate singlet-triplet energy gaps via through-bond coupling between terminal radicals. Spin-unrestricted Hartree-Fock (UHF) based coupled-cluster (CC) computations of these diradicals were also performed. Comparison between symmetry-adapted MkMRCC and broken-symmetry (BS) UHF-CC computational results indicated that spin-contamination error of UHF-CC solutions was left at the SD level, although it had been thought that this error was negligible for the CC scheme in general. In order to eliminate the spin contamination error, approximate spin-projection (AP) scheme was applied for UCC, and the AP procedure indeed eliminated the error to yield good agreement with MRCC in energy. The CCD with spin-unrestricted Brueckner's orbital (UB) was also employed for these polyene diradicals, showing that large spin-contamination errors at UHF solutions are dramatically improved, and therefore AP scheme for UBD removed easily the rest of spin-contaminations. Pure- and hybrid-density functional theory (DFT) calculations of the species were also performed. Three different computational schemes for total spin angular momentums were examined for the AP correction of the hybrid DFT. The AP DFT calculations yielded the singlet-triplet energy gaps that were in good agreement with those of MRCC, AP UHF-CC and AP UB-CC. Chemical indices such as the diradical character were calculated with all these methods. Implications of the present computational results are discussed in relation to previous RMRCC calculations of diradical species and BS calculations of large exchange coupled systems. [ABSTRACT FROM AUTHOR]

It is shown that some Kekuléan fluoranthenes are diradicals and that their ground state is a triplet. In the energetically less favorable singlet state, these hydrocarbons also exhibit pronounced diradical character. The diradical character y of the compounds under investigation was estimated using the unrestricted symmetry-broken (yPUHF) and complete active space (yNOON) methods. It was found that the yPUHF values better reproduce the diradical character of the investigated hydrocarbons. It was shown that the SOMO and SOMO-1 of a diradical structure occupy different parts of space with a small shared region, resulting in a spin density distribution over the entire molecule. The spatial diradical distribution in the singlet diradical structures was examined by inspecting the HOMOs and LUMOs for  and β spin electrons. It was shown that the -HOMO and the β-LUMO (as well as the β-HOMO and the -LUMO) occupy practically the same part of space. In this way, there are no unpaired electrons in a singlet diradical structure, yet two of them occupy different parts of space, thus allowing the -electrons to delocalize. [ABSTRACT FROM AUTHOR]

Symmetry and broken symmetry (BS) in molecular orbital description of transition structures and intermediates in oxygenation reactions have been revisited to elucidate states correlation diagrams and mechanisms for addition reactions of molecular oxygen and metal-oxo M&dbond;O (M = Mn(II) and Fe(II)) species to C&dbond;C double bonds. Relative stabilities between diradical (DR) and perepoxide (PE) intermediates were thoroughly investigated by several BS hybrid DFT (HDFT) methods and BS CCSD(T) method with and without spin projection. It has been found that recovery of spin symmetry, namely eliminating spin contamination error from the BS solutions, is crucial for the elucidation of reasonable state correlation diagrams and energy differences of the key structures in the oxygenation reactions because the singlet-triplet energy gap for molecular oxygen is large (22 kcal/mol). The BS HDFT followed by spin correction reproduced activation barriers for transition structures along both PE and DR reaction pathways by the use of the CASPT2 method. Basis set dependence on the relative stability between PE and DR intermediates were also examined thoroughly. Solvation effect for DR and PE intermediates was further examined with self-consistent reaction field (SCRF) and SCIPCM methods. Both BS HDFT and CASPT2 have concluded that the DR mechanism is favorable for the addition reaction of singlet oxygen to ethylene, supporting our previous conclusions. The BS HDFT with spin correction was concluded to be useful enough for theoretical investigations of mechanisms of oxygenation reactions. Implications of the computational results were discussed in relation to the theoretical framework (four configuration model) for elucidation of possible mechanisms of epoxidation reactions with Fe(IV)&dbond;O cores in metalloenzymes on the basis of isolobal analogies among O, O&dbond;O, and Fe(IV)&dbond;O. Correspondence between magnetic coupling mode and radical pathway in oxygenations with these species was clarified based on the BS MO interaction diagrams, leading to local singlet and triplet diradical mechanisms for epoxidations. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Abstract: Based on the mild, thermal rearrangement of 1,2-dialkynylimidazoles to reactive carbene or diradical intermediates, a series of 1,2-dialkynylimidazoles were designed as potential irreversible p38 MAP kinase α-isoform (p38α) inhibitors. The synthesis of these dialkynylimidazoles and their kinase inhibition activity is reported. The 1-ethynyl-substituted dialkynylimidazole 14 is a potent (IC50 =200nM) and selective inhibitor of p38α. Moreover, compound 14 covalently modifies p38α as determined by ESI-MS after 12h incubation at 37°C. The unique kinase inhibition, covalent kinase adduct formation, and minimal CYP450 2D6 inhibition by compound 14 demonstrate that dialkynylimidazoles are a new, promising class of p38α inhibitors. [Copyright &y& Elsevier]