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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]

• 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]

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]

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]

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]

Highlights: [•] Photoactive magnetic property in GFP chromophore coupled diradicals are designed. [•] The magnetic property can be tuned by the exposure of light of appropriate wavelength. [•] Such systems are suitable for different biological applications. [ABSTRACT FROM AUTHOR]

Highlights: [•] Concerted mechanism of isoprene cycloaddition is more favorable than stepwise one. [•] Asynchronous characters of 30 concerted transition structures are analyzed. [•] Conformation transition is important to the closure of stepwise intermediate. [•] Influence of configuration and conformation of the transition states are discussed. [•] Calculation support the experimental result that sylvestrene is favorable. [Copyright &y& Elsevier]

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]

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]

Abstract: There is evidence that the crystalline-state thermochromic behavior of dispiropyracene acr-1 with an ultralong C–C bond [1.791(3)Å at 413K, 1.771(3)Å at 93K] might be due to thermal generation of a bond-dissociated triplet diradical. The C1–C2 bond lengths (d) in the newly prepared 1,1,2,2-tetraarylpyracenes 1a–e [1.717(4)–1.761(4)Å at 113/123/153K] are also much larger than the standard value for C(sp3)–C(sp3) (1.54Å). The fact that there is no correlation between d and the radical-stabilizing parameter (σ•) provides evidence that there is no contribution from a crystallographic artifact caused by contamination of the diradical in the crystal of 1 at room temperature or below. Further indication for the covalent nature of the ultralong C–C bond in tetraphenylpyracene 1c was attained by Raman spectroscopy, showing the far red-shifted C1–C2 stretching vibration (638cm−1) and by the theoretical prediction of very large vertical singlet–triplet energy gap (58.3kcalmol−1 at the UB3LYP/6-31G∗ level). However, at elevated temperatures bond dissociation might occur forming a triplet diradical. [Copyright &y& Elsevier]

Abstract: Calix[4]arenes constrained to 1,3-alternate conformation and functionalized at the upper rim with four and two nitronyl nitroxides have been synthesized, and characterized by X-ray crystallography, magnetic resonance (EPR and 1H NMR) spectroscopy, and magnetic studies. Such calix[4]arene tetraradicals and diradicals provide scaffolds for through-bond and through-space intramolecular exchange couplings. [Copyright &y& Elsevier]

Abstract: Two novel, thioether containing benzannulated enediyne ligands, 1,2-bis(3-(quinolin-8-ylsulfanyl)prop-1-ynyl)benzene (1), and 1,2-bis(3-(pyridin-2-ylmethylsulfanyl)prop-1-ynyl)benzene (2) have been synthesized and characterized along with their corresponding Cu(I), Ag(I), Cu(II), and Pd(II) metalloenediyne complexes (3–10). Single crystal X-ray structures of the d10 and d8 N2S2 complexes show thioether coordination leading to distorted tetrahedral and trans square planar geometries about the metal centers, respectively. Based on their X-ray structures, all complexes are expected to exhibit high Bergman cyclization temperatures (>200 °C), and indeed this is the case for the Cu(I) and Ag(I) complexes (211–218 °C). In contrast, the corresponding divalent, tetragonal and trans square planar complexes of Cu(II) and Pd(II) exhibit more facile reactivity and lower solid state Bergman cyclization temperatures (186–190 °C) which more closely resemble values expected for cis-complexes. The unusual order in the thermal reactivities of these constructs can be attributed to a previously evaluated mechanism by which weak thioether coordination permits an elevated temperature trans-to-cis geometric rearrangement prior to Bergman cyclization. [Copyright &y& Elsevier]

Isomerization of soluble precursor compounds to produce fused-ring systems is an attractive approach for preparing conjugated polymers and oligomers. Cycloaromatization chemistry has previously been explored in this capacity employing reactions based on the Bergman cyclization. Using ethynyl sulfides with a terminal o-diethynylbenzene unit, an alternative strategy is demonstrated that offers selectivity advantages in the kinetically controlled radical cyclizations. The products are acene-fused thiophenes in which the diethynylsulfide acts as a relay for the diradical produced in a Bergman cyclization. [Copyright &y& Elsevier]

Diradical and low-lying first triplet excited-state energy E (T 1) are served as important elements toward tuning singlet fission (SF) efficiency, necessitating a rational and precise design strategy toward their fabrication. Herein, the tunable current density vectors (CDV) strategy is utilized to identify a series of tunable diradicaloid SF switches between dihydro-tetraaza-acenes isomers. Theoretical calculations indicate that the independent local diatropic CDV (ILD-CDV) on the S 0 -state and T 1 -state is the origin of the radical site and transition defect, respectively. Importantly, the diradical parameter y 0 related to ground state is adjusted by tuning the discontinuous diatropic CDV on zigzag edge of the sandwiched rings, while low-lying first triplet excited-state energy E (T 1) depends on the primary charge-transfer related to delocalized CDV region. Our ultimate goal is to emphasize the CDV analysis method that rationalizes the radical and S 0 → T 1 transition site, which provides new application perspectives for precise designing new material in organic electronics. Image 1 • Tunable current density vectors strategy is utilized to identify a series of diradical SF switches between dihydro-tetraaza-acenes isomers. • The CDV is instrumental in locating radical and excluding the S 0.→ T 1 transition site. • The tunable CDV could tune the diradical parameter and the E (T 1) level. [ABSTRACT FROM AUTHOR]