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1.
Chemistry ; 2020 Jun 24.
Artigo em Inglês | MEDLINE | ID: mdl-32578914

RESUMO

Among all C-, N-, and O-centered polyradicals, high-spin nitrenes possess the largest magnetic anisotropy and are of considerable interest as multi-level molecular spin systems for exploration of organic molecular magnetism and quantum information processing. Although the first representatives of quintet and septet nitrenes were obtained almost 50 years ago, the experimental and theoretical studies of these highly reactive species became possible only recently, owing to new achievements in molecular spectroscopy and computational chemistry. Meanwhile, dozens of various quintet dinitrenes and septet trinitrenes were successfully characterized by IR, UV-vis, and EPR spectroscopy, thus providing important information about the electronic structure, magnetic properties and reactivity of these compounds.

2.
Chemistry ; 2020 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-32293763

RESUMO

The Cope rearrangement of selectively deuterated isotopomers of 1,5-dimethylsemibullvalene 2 a and 3,7-dicyano-1,5-dimethylsemibullvalene 2 b were studied in cryogenic matrices. In both semibullvalenes the Cope rearrangement is governed by heavy-atom tunneling. The driving force for the rearrangements is the small difference in the zero-point vibrational energies of the isotopomers. To evaluate the effect of the driving force on the tunneling probability in 2 a and 2 b, two different pairs of isotopomers were studied for each of the semibullvalenes. The reaction rates for the rearrangement of 2 b in cryogenic matrices were found to be smaller than the ones of 2 a under similar conditions, whereas differences in the driving force do not influence the rates. Small curvature tunneling (SCT) calculations suggest that the reduced tunneling rate of 2 b compared to that of 2 a results from a change in the shape of the potential energy barrier. The tunneling probability of the semibullvalenes strongly depends on the matrix environment; however, for 2 a in a qualitatively different way than for 2 b.

3.
J Phys Chem A ; 2020 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-32208698

RESUMO

The photoionization of phenylnitrene was investigated by photoion mass-selected threshold photoelectron spectroscopy in the gas phase. Flash vacuum pyrolysis of phenyl azide at 480 °C produces the nitrene, which subsequently rearranges at higher temperatures affording three isomeric cyanocyclopentadienes, in contrast to low-temperature trapping experiments. Temperature control of the reactor and threshold photoelectron spectra allows for optimizing the generation of phenylnitrene or its thermal rearrangement products, as well as obtaining vibrational information for the corresponding ions. The adiabatic ionization energies (AIE) of the triplet nitrene (3A2) to the radical cation in its lowest-energy doublet (2B2) and quartet (4A1) spin states were determined to 8.29 ± 0.01 and 9.73 ± 0.01 eV, respectively. Vibrational frequencies of ring breathing modes were measured at 500 ± 80 and 484 ± 80 cm-1 for both the [Formula: see text](2B2) and [Formula: see text](4A1) cationic states, respectively. The AIE differ from the values previously reported; hence, we revise the doublet-quartet energy splitting of the phenylnitrene radical cation to 1.44 eV, in excellent agreement with composite methods and coupled cluster calculations, but considerably higher than the literature reference (1.1 eV).

4.
J Org Chem ; 84(24): 16013-16018, 2019 Dec 20.
Artigo em Inglês | MEDLINE | ID: mdl-31730349

RESUMO

5-Methoxy-2H-benzazirine was prepared via irradiation of the corresponding phenyl azide, isolated in an argon matrix at cryogenic temperatures. It undergoes ring expansion to the corresponding ketenimine in the dark at T < 30 K despite a calculated activation barrier of 4.9 kcal mol-1 [B3LYP/6-311++G(d,p)]. Since this rearrangement proceeds with a rate constant in the order of 10-4 s-1, exhibiting only a shallow temperature dependence, the results are interpreted in terms of heavy-atom tunneling. Of the four isomeric benzazirines resulting from the initial photolysis, only one can be observed to rearrange; this conformer specificity is explained by the other potentially observable rearrangements being either too fast or too slow to be detected due to the differences in heights and widths of their respective activation barriers.

5.
Angew Chem Int Ed Engl ; 58(42): 14855-14859, 2019 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-31412153

RESUMO

The control of the spin states of molecules opens the path to tuning selectivity in chemical reactions and to developing novel magnetically switchable materials. 3-Methoxy-9-fluorenylidene is a carbene that is generated in cryogenic matrices both in its lowest energy singlet and triplet states, and the ratio of these states can be shifted by selective irradiation. The interconversion of the nearly degenerate spin states is induced by a conformational change of the methoxy group: switching the methoxy group into the "up" position results in the singlet state and switching into the "down" position in the triplet state. The spin control via a remote functional group makes this carbene unique for the study of spin-specific reactions, which is demonstrated for the hydrogenation reaction. Spin switching by switching the conformation of a remote functional group is a novel phenomenon with potential applications in the design of functional materials.

6.
Angew Chem Int Ed Engl ; 58(37): 12994-12998, 2019 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-31265166

RESUMO

The septet ground state trinitrenes 1,3,5-trichloro-2,4,6-trinitrenobenzene and 1,3,5-tribromo-2,4,6-trinitrenobenzene were isolated in inert (Ar, Ne, and Xe) as well as reactive matrices (H2 , O2 , and H2 O) at cryogenic temperatures. These trinitrenes were obtained in high yields by UV photolysis of the corresponding triazides and characterized by IR and UV/Vis spectroscopy. The trinitrenes, despite bearing six unpaired electrons, are remarkably unreactive towards molecular oxygen and hydrogen and are persistent in water ice up to 160 K where the water matrix starts to sublime off.

7.
J Org Chem ; 84(18): 11450-11457, 2019 09 20.
Artigo em Inglês | MEDLINE | ID: mdl-31343881

RESUMO

We investigate the effects of small admixtures of protic solvent molecules, such as water and alcohols, on the ultrafast dynamics of diphenylcarbene in acetonitrile at room temperature. Broadband transient absorption measurements and quantum mechanics/molecular mechanics molecular dynamics simulations allow elucidating the dominant reaction mechanism of an intermediate hydrogen-bonded complex between singlet diphenylcarbene and a protic solvent molecule, thus competing with intersystem crossing. Analysis of the data indicates that complex formation is a diffusion-controlled process with orientational requirements. The reaction path involving a benzhydryl cation is less likely in neat bulkier alcohols, as it requires the interaction of the carbene with a protic solvent molecule being part of a hydrogen-bonded network. The simulations indicate a further reaction path toward O-H insertion and two side reactions depending on the involved protic solvent species. Thus, we established that not only the number but also the chemical nature of the protic solvent molecule determine which reaction path is pursued.

8.
Chemphyschem ; 20(13): 1664-1670, 2019 07 02.
Artigo em Inglês | MEDLINE | ID: mdl-31045298

RESUMO

Solvent and temperature can affect the structural properties of cyclic peptides by controlling their flexibility. Here, we investigate two cyclic peptides, featuring beta turns. Using temperature-dependent NMR and FT-IR, we observed a pronounced temperature effect on the conformation of the cyclic peptide D-1 in CHCl3 but a much smaller effect in CH3 CN. Almost no effect was observed for its diastereomer L-1 within a similar temperature range and using the same solvents. With the aid of Replica Exchange Molecular Dynamics simulations and Quantum Mechanics/Molecular Mechanics calculations, we were able to explain this behavior based on the increased flexibility of D-1 (in CHCl3 ) in terms of intramolecular hydrogen bonding. The largest temperature dependence is observed for D-1 in CHCl3 , while the temperature effect is less pronounced for L-1 in CHCl3 and for both peptides in CH3 CN. This work provides new insights into the role of the environment and temperature on the conformations of cyclic peptides.


Assuntos
Acetonitrilos/química , Clorofórmio/química , Peptídeos Cíclicos/química , Solventes/química , Ligação de Hidrogênio , Espectroscopia de Ressonância Magnética , Simulação de Dinâmica Molecular , Maleabilidade , Conformação Proteica , Teoria Quântica , Espectroscopia de Infravermelho com Transformada de Fourier , Temperatura
9.
Magn Reson Chem ; 57(8): 472-478, 2019 08.
Artigo em Inglês | MEDLINE | ID: mdl-30997697

RESUMO

Complex multicomponent, multispin molecular system, consisting of a septet trinitrene, two quintet dinitrenes, and three triplet mononitrenes, was obtained by the photolysis of 2,4,6-triazido-3-cyano-5-fluoropyridine in solid argon. To identify these paramagnetic products, electron paramagnetic resonance spectroscopy in combination with line-shape spectral simulations and density functional theory calculations was used. The products of the photolysis was found to be triplet 2,4-diazido-3-cyano-5-fluoropyridyl-6-nitrene (DT  = 1.000 cm-1 , ET  = 0), triplet 2,4-diazido-3-cyano-5-fluoropyridyl-2-nitrene (DT  = 1.043 cm-1 , ET  = 0), triplet 2,6-diazido-3-cyano-5-fluoropyridyl-4-nitrene (DT  = 1.128 cm-1 , ET  = 0 cm-1 ), quintet 4-azido-3-cyano-5-fluoropyridyl-2,6-dinitrene (DQ  = 0.211 cm-1 , EQ  = 0.0532 cm-1 ), quintet 2-azido-3-cyano-5-fluoropyridyl-4,6-dinitrene (DQ  = 0.208 cm-1 , EQ  = 0.0386 cm-1 ), and septet 3-cyano-5-fluoropyridyl-2,4,6-trinitrene (DS  = -0.1017 cm-1 , ES  = -0.0042 cm-1 ) in a 38:4:7:22:14:4 ratio, respectively.

10.
J Am Chem Soc ; 140(49): 17271-17277, 2018 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-30430835

RESUMO

The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in radical-type reactions and unwanted rearrangements, which diminish the use of arylnitrenes as intermediates in organic synthesis. While the closed-shell singlet states of arylnitrenes are expected to undergo useful chemical transformations (comparable to the closed-shell singlet states of carbenes), these states are too high in energy to be chemically accessible. When triplet pentafluorophenylnitrene is interacting with the Lewis acid BF3 under the conditions of matrix isolation, a Lewis acid-base complex consisting of the closed-shell singlet state of the nitrene and two molecules of BF3 is formed. Although the closed-shell singlet state of pentafluorophenylnitrene is calculated (CCSD(T)) to lie more than 25 kcal/mol above its triplet ground state, the reaction with BF3 results in switching the spin state from triplet to singlet. The formation of the singlet complex was monitored by IR, UV-vis, and EPR spectroscopy. DFT, CCSD(T), and CASPT2 calculations confirm the experimental findings.

11.
Nat Commun ; 9(1): 4750, 2018 11 12.
Artigo em Inglês | MEDLINE | ID: mdl-30420598

RESUMO

Controlled switching of the spin state of transition metal ions, particularly of FeII and FeIII, is a prerequisite to achieve selectivity, efficiency, and catalysis in a number of metalloenzymes. Here we report on an iron(III) porphyrin with a photochromic axial ligand which, upon irradiation with two different wavelengths reversibly switches its spin state between low-spin (S = 1/2) and high-spin (S = 5/2) in solution (DMSO-acetone, 2:598). The switching efficiency is 76% at room temperature. The system is neither oxygen nor water sensitive, and no fatigue was observed after more than 1000 switching cycles. Concomitant with the spin-flip is a change in redox potential by ~60 mV. Besides serving as a simple model for the first step of the cytochrome P450 catalytic cycle, the spin switch can be used to switch the spin-lattice relaxation time T1 of the water protons by a factor of 15.


Assuntos
Ferro/química , Luz , Ligantes , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Porfirinas/química , Prótons , Piridinas/química , Fatores de Tempo
12.
Angew Chem Int Ed Engl ; 57(50): 16334-16338, 2018 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-30307715

RESUMO

We have observed the inversion of the solvation environment of a one-dimensional solid by low-temperature scanning tunneling microscopy. Adsorption of 3-methoxy-9-diazofluorene on Ag(111) yields highly oriented supramolecular chains, which are then exposed to water molecules. The annealing of dry and water-decorated chains results in diametrically opposed outcomes. While the former simply leads to an increase in chain length and number, the latter results in a complete loss of order and produces water clusters decorated with the organic molecule.

13.
Chemistry ; 24(70): 18801-18808, 2018 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-30312518

RESUMO

The hydrogenation reactions of diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 were investigated in solid H2 and D2 matrices and in H2 - and D2 -doped argon matrices at cryogenic temperatures. The reactivity of the carbenes towards H2 increases in the order 1<3<2. Whereas 1 is stable in solid H2 , 2 and 3 react fast under the same conditions via quantum chemical tunneling. In D2 both 1 and 3 are stable, whereas 2 slowly reacts. The different reactivity of the three carbenes is rationalized in terms of differing carbene stabilization energies.

14.
Chemistry ; 24(68): 18043-18051, 2018 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-30230615

RESUMO

The reactions of the three triplet ground state arylcarbenes diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 with the Lewis acids H2 O, ICF3 , and BF3 were studied under the conditions of matrix isolation. H2 O was selected as typical hydrogen bond donor, ICF3 as halogen bond donor, and BF3 as strong Lewis acid. H2 O forms hydrogen-bonded complexes of the singlet carbenes with 1 and 2, but not with 3. This is rationalized by the larger singlet-triplet gap of 3, which does not allow to stabilize the singlet state below the triplet state by hydrogen bonding. With ICF3 , both 1 and 3 form halogen-bonded complexes of the singlet states of the carbenes. This indicates that halogen bonding stabilizes singlet carbenes more than hydrogen bonding. Carbene 2 reacts differently from 1 and 3 by forming an iodonium ylide, thus avoiding antiaromatic destabilization of the fluorenyl unit. With BF3 , all three carbenes form zwitterionic Lewis acid/base complexes.

15.
J Org Chem ; 83(15): 7586-7592, 2018 Aug 03.
Artigo em Inglês | MEDLINE | ID: mdl-30019897

RESUMO

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CF3I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF3I at 3 K. Bimolecular reactions between the carbenes and CF3I were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF3 group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25-30 K.

16.
Phys Chem Chem Phys ; 19(40): 27384-27388, 2017 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-28972604

RESUMO

The current study focuses on the generation, identification, and characterization of the phenylselenyl radical using the matrix isolation technique in combination with density functional theory (B3LYP/cc-pVTZ) computations. The hitherto unknown phenylselenyl peroxy radical was synthesized by co-condensation of the phenylselenyl radical with molecular ground state triplet oxygen from the gas phase and subsequent trapping in argon matrices at 10 K. The experimental IR spectra including 18O isotopically labelled materials compare well with the data obtained from B3LYP/cc-pVTZ computations. Upon 312 nm irradiation, the phenylselenyl peroxy radical isomerizes to the thermodynamically more stable equally novel phenylselenoyl radical.

17.
J Phys Chem A ; 121(40): 7477-7493, 2017 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-28892389

RESUMO

We irradiated binary ice mixtures of ammonia (NH3) and oxygen (O2) ices at astrophysically relevant temperatures of 5.5 K with energetic electrons to mimic the energy transfer process that occurs in the track of galactic cosmic rays. By monitoring the newly formed molecules online and in situ utilizing Fourier transform infrared spectroscopy complemented by temperature-programmed desorption studies with single-photon photoionization reflectron time-of-flight mass spectrometry, the synthesis of hydroxylamine (NH2OH), water (H2O), hydrogen peroxide (H2O2), nitrosyl hydride (HNO), and a series of nitrogen oxides (NO, N2O, NO2, N2O2, N2O3) was evident. The synthetic pathway of the newly formed species, along with their rate constants, is discussed exploiting the kinetic fitting of the coupled differential equations representing the decomposition steps in the irradiated ice mixtures. Our studies suggest the hydroxylamine is likely formed through an insertion mechanism of suprathermal oxygen into the nitrogen-hydrogen bond of ammonia at such low temperatures. An isotope-labeled experiment examining the electron-irradiated D3-ammonia-oxygen (ND3-O2) ices was also conducted, which confirmed our findings. This study provides clear, concise evidence of the formation of hydroxylamine by irradiation of interstellar analogue ices and can help explain the question how potential precursors to complex biorelevant molecules may form in the interstellar medium.

18.
J Am Chem Soc ; 139(37): 13024-13030, 2017 09 20.
Artigo em Inglês | MEDLINE | ID: mdl-28826206

RESUMO

The reaction of triplet tetrachlorocyclopentadienylidene with BF3 in rare gas matrices yields a zwitterion consisting of a cyclopentadienyl cation bearing a positive charge and a negatively charged BF3 unit. IR and UV-vis spectra as well as the absence of EPR signals demonstrate a singlet ground state of the zwitterion, and its calculated geometry and magnetic properties clearly reveal a strong antiaromatic character. The zwitterion is highly labile and by visible or IR irradiation rearranges via a 1,2-fluorine migration from boron to carbon. Interaction with a second molecule of BF3 stabilizes the zwitterion and suppresses the fluorine migration, thus providing a convenient and efficient synthesis of an antiaromatic molecule under very mild conditions.

19.
J Am Chem Soc ; 139(35): 12310-12316, 2017 09 06.
Artigo em Inglês | MEDLINE | ID: mdl-28789527

RESUMO

p-Tolyl(trifluoromethyl)carbene and the related fluorenyl(trifluoromethyl)carbene were synthesized in solid argon and characterized by IR, UV-vis, and electron paramagnetic resonance spectroscopy as well as by quantum mechanical calculations. The carbenes can be generated in both their triplet and singlet states, and both states coexist under the conditions of matrix isolation. According to our calculations, the singlet and triplet states of these carbenes are energetically nearly degenerate in the gas phase. Warming of matrices containing pure triplet p-tolyl(trifluoromethyl)carbene from 3 to 25 K leads to an interconversion of up to 20% of the triplet into the singlet state. This interconversion is thermally irreversible, and cooling back to 3 K does not change the singlet to triplet ratio. Irradiation at 365 nm results in a complete singlet to triplet interconversion, whereas 450 nm irradiation produces again up to 20% of the singlet state. An alternative way to generate the singlet carbene is the reaction of the triplet with water molecules by annealing water-doped matrices at 25 K. This results in the irreversible formation of a hydrogen-bonded complex between the singlet carbene and water. For fluorenyl(trifluoromethyl)carbene, very similar results are obtained, but the yield of the singlet state is even higher. Magnetic bistability of carbenes seems to be a general phenomenon that only depends on the singlet-triplet gap rather than on the nature of the carbene.

20.
J Phys Chem A ; 121(34): 6405-6412, 2017 Aug 31.
Artigo em Inglês | MEDLINE | ID: mdl-28771351

RESUMO

The benzhydryl radical is generated in high yields by flash-vacuum thermolysis of 1,1,2,2-tetraphenylethane with subsequent trapping of the product in argon or amorphous water at 3-4 K. Photoionization of the radical with various UV lights and electron sources produces the benzhydryl cation, which was identified by IR and UV-vis spectroscopy. In solid argon, the formation of the benzhydryl cation is irreversible, whereas in amorphous water-ice the electron transfer is reversible, and irradiation into the major absorption band at 443 nm of the cation leads back to the radical by electron attachment. Applications of ionization of organic matter trapped in water-ice to icy environments in astrophysics and planetary sciences, including Earth, are discussed.

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