Biochemie und Chemie
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- PE Spectra (8)
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- ESR/ENDOR Spectra (5)
- MNDO Calculations (5)
- Cyclic Voltammetry (4)
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- 1,2,4,5-Tetracyanobenzene (1)
- 1,2-Dimesitoylbenzene (1)
- 1.10-Phenanthrolin-5,6-dione (1)
- 1H NMR Spectra (1)
- 2,5-Bis(trimethylsilyl)-p-quinone and -hydroquinone-monosodium Salt (1)
- 2H-Azirine (1)
- 7,7,8,8-Tetracyano-p-quinodimethane (1)
- AM1 Calculations (1)
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- Azidoacetonitrile Trimethylenetetrazole and Tetrazolo[1,5-a]pyridine (1)
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- Benzoquinone Radical Anion (1)
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- Cyanine Distortion of C6-Rings (1)
- Cyanogen Azide (1)
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- Dialkylamino-substituted π-Systems (1)
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- ENDOR Spectra (1)
- ENDOR and Triple Spectra (1)
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- Electron Transfer (1)
- Gas Phase Conformation (1)
- Gas Phase Dehydrochlorination (1)
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- Gasphase Pyrolyses (1)
- Heterogeneous Catalysis (1)
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- MO Interpretation (1)
- Methyleneaminoacetonitrile Monomer and Trimer (1)
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- One-Electron Reduction of 9,9′-Bianthryl (1)
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- Reduction of 1-Sila-2,5-diazacyclopentane-3,4-dithione and of Tetrakis(isopropylthio)-p-benzoquinone (1)
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- Temperature Dependence (1)
- Tetra(2'-pyridyl)pyrazine (1)
- Tetra(methylthio)thiophene (1)
- Tetraketone Radical Anion (1)
- Tetraphenyl-p-benzoquinone Reduction (1)
- Tetraphenyl-p-quinodimethane Dianion (1)
- Tetraphenylbutatriene Radical Anion and Dianion Salts (1)
- Thermal Retrotrimerization (1)
- Thiocarbonates (1)
- Tin Organic Compounds (1)
- Tricyanomethane Derivatives (1)
- Trifluoromethylazide (1)
- Triphenylphosphane (1)
- U V /V IS Spectra (1)
- Vinyl Azide (1)
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- hyperconjugation (1)
- methylene derivatives (1)
- photoelectron spectra (1)
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Institute
- Biochemie und Chemie (44)
Photoelectron (PE) spectra of ethylene and vinylene carbonates and thiocarbonates as well as of methylene trithiocarbonate and some open-chain derivatives are reported.
The low energy bands, well separated in the unsaturated compounds, are assigned to lone pair and π type ionizations. The assignment is based on comparison of PE spectra, modified CNDO calculations, and sulfur Κβ emission spectra. The pronounced substituent effects due to which the first ionization potential varies from 8.4 eV to 11.1 eV are discussed.
The well-resolved helium(I) photoelectron spectrum of H3C-Se—Se-CH3 exhibits distinct bands corresponding to 11 of the total 13 valence electron ionizations. The unequivocal assignment is supported by EHMO calculations including spin/orbit coupling. The two selenium lone pair ionizations differ by 0.23 eV; a split observed also for dimethyl disulfide and discussed within a general model for interactions between adjacent lone pairs.
The photoelectron spectrum of H5C6-Te-CH3 displays in its low energy region overlapping bands of gas-phase conformers. Depending on the dihedral angle between the plane of the phenyl ring and the tellurium lone pair, the π conjugation amounts to only 0.1 eV and 0.3 eV, respectively. These values are compared to the considerably larger ones found for the analogous phenyl derivatives H5C6-X-CH3 with X = O, S and Se.
The photoelectron (PE) spectra of dicyano methane and of its dimethyl derivative are tentatively assigned on the basis of a simple MO model. The interactions defined therein between the two cyano groups as well as with the R2C-framework can be parametrized using the PE data. Thus the hyperconjugation πCN/πCR₂ is estimated to amount to 1.7 eV in both compounds. Hyperconjugative effects in methane derivatives H3CX and H2CX2 with X = Br, Cl and CN are compared.
The He(I) photoelectron spectra of the following molecules with S·̱·̱̱·̱·̱·̱̱·̱N multiple bonds ... are assigned by radical cation state comparison between the chemically related compounds as well as by MO models based on CNDO calculations. From the ionisation energies of the O=S=O/HN=S=O pair a parameter απSN can be deduced, which proves to be useful in the discussion of other SN compounds like R3C-N=S=O and RN=S=NR.
Triphenylphosphane 1, its oxide 2 and sulfide 3 undergo one-electron reduction at a mercury cathode in DMF to yield the corresponding radical anions. ESE analysis of the paramagnetic species is facilitated by deuteration and suggests a pyramidal geometry of the radicals. Reduction with potassium metal in DME at low temperature yields also radical anions for 2 and 3. The phosphane 1, however, reacts under phenyl cleavage and potassiumphenyl-assisted ring closure to the dianion of 5H-dibenzophosphole 4. This radical 4· ⊖⊖ is also obtainod by alkali metal reduction of P-phenyldibenzophosphole o, and its spin distribution is compared to iso-.-π-electronic radicals containing CH, N, O, S, or Se links instead of the phosphorus atom.
Semiquinone radical anions are prototype compounds for contact ion pair formation with metal counter cations. In order to investigate the still open question whether bulky alkyl groups can sterically interfere, diphenoquinone derivatives O=C(RC=CH)2C=C(HC=CR)2C=O with R = C(CH3)3, CH(CH3)2 and CH3 have been selected and the following ESR/ENDOR results are obtained for the alkaline metal cations: The tetrakis(tert-butyl)-substituted radical anion only adds Li⊕ and Na⊕, while K⊕ forms no ion pair. The 3,3ʹ,5,5ʹ-tetra(isopropyl)diphenoquinone radical anion is accessible to all cations Me⊕, although Rb⊕ and Cs⊕ seem to be present solvent-separated in solution. The tetramethyl-substituted radical anion unfortunately polymerizes rapidly. Additional information concerns the ESR/ENDOR proof for ion triple radical cation formation [Li⊕ M•⊖Li⊕]•⊕, or the difference in the coupling constants upon Me⊕ docking at one δ⊖O=C group, suggesting that about 87% of the spin density is located in the cation-free molecular half of the diphenoquinone radical anion. Based on the wealth of ESR/ENDOR information, crystallization of the contact ion pairs and their structural characterization should be attempted.
The reactive intermediate methyleneaminoacetonitrile H2C = N - C H2 - CN has been generated via thermal retrotrimerization of N ,N',N"-tris(cyanomethyl)hexahydro-s-triazine and characterized by its photoelectron, mass and low-temperature NMR spectra. A fully geometryoptimized MNDO calculation allows to assign the observed ionization energies and yields estimates for other molecular properties, e.g. a rather high dipole moment.
The following mixed-stack donor/acceptor complexes {D···A}∞ have been crystallized and their structures determined: {hexamethylbenzene···3,5-dicyano-1-nitrobenzene hexamethylbenzene···3,5-dinitro-1-cyanobenzene}∞, {pyrene···3,5-dinitro-1-cyanobenzene}∞, {anthracene···(3,5-dinitro-1-cyanobenzene)2}∞, {N,N-dimethylanilin···3,5-dinitro- 1-cyanobenzene}∞ and { 1-3-phenylenediamine···3,5-dinitro-1-cyanobenzene}∞. Their lattice packing consists of parallel layers, which contain either donors and acceptors as for hexamethylbenzene and pyrene or composite ones as in the 1:2 complex of anthracene with each one of the acceptors above and below its peripheral rings. The isostructural hexamethylbenzene complexes exhibit almost identical packing coefficients as well as a hexagonal coplanar arrangement of the C6(CH3)6 donors. Weak intermolecular van der Waals interactions are also observed between antiparallel cyano substituents. The interplanar n distances range between 334 and 353 pm, i. e. around 340 pm of two van der Waals n radii. In none of the complexes, however, significant structural changes in either the donor or the acceptor components due to the complex formation are observed. In both the crystals as well as in solution, the donor/acceptor complexes exhibit colours between yellow and red; their long-wavelength charge transfer absorption maxima, therefore, correspond to a lowering in excitation energy of only up to 1 eV relative to that of the components. The different charge transfer in the ground and the CT excited states is also discussed referring to other data such as vertical first ionization energies or interplanar distances {D···A}, as well as to results from semiempirical calculations based on the crystal structure data determined and including approximate configuration interaction.
Raney nickel, a highly reactive and air-sensitive solid, if prepared and investigated under oxygen-free conditions, exhibits interesting catalytic properties. Using photoelectron spectroscopy for real-time gas analysis in a flow reactor, the following results are obtained with alkyl and acylhalides: Dehydrohalogenation temperatures are lowered relative to thermal HHal elimination up to 350 K. Monochloro and bromo propanes and butenes yield propene and butadiene, respectively. 1,1-Dichloro ethane or 1,1-dibromo propane only split off one HHal and form chloroethene or 1-bromopropene-2. HCl elimination from 2-methyl propionic acid chloride, expectedly, produces dimethyl ketene. Most interesting, however, is the ring opening of monobromo cyclobutane to 1-bromo-butene-3, observed already at room temperature, which strongly suggests the intermediate formation of a chemisorbed surface carbene at Raney nickel. The formation of hexadiene-1,5 as a by-product in the HCl elimination of 1-chloropropane, i. e. a surface carbene dimer, indicates their presence also in other dehydrohalogenations heterogeneously catalyzed by Raney nickel.
Raney nickel, a highly reactive and air-sensitive solid, if prepared and investigated under oxygen-free conditions, exhibits interesting catalytic properties. Using photoelectron spectroscopy for real-time gas analysis in a flow reactor, the following results are obtained with alkyl and acylhalides: Dehydrohalogenation temperatures are lowered relative to thermal HHal elimination up to 350 K. Monochloro and bromo propanes and butenes yield propene and butadiene, respectively. 1,1-Dichloro ethane or 1,1-dibromo propane only split off one HHal and form chloroethene or l-brom opropene-2. HCl elim ination from 2-methyl propionic acid chloride, expectedly, produces dimethyl ketene. Most interesting, how ever, is the ring opening of monobromo cyclobutane to 1-brom o-butene-3, observed already at room temperature, which strongly suggests the intermediate formation of a chem isorbed surface carbene at Raney nickel. The formation of hexadiene-1,5 as a by-product in the HCl elim ination of 1-chloropropane, i.e. a surface carbene dimer, indicates their presence also in other dehydrohalogenations heterogeneously catalyzed by Raney nickel.
The following mixed-stack donor/acceptor complexes {D · · · A }∞ have been crystallized and their structures determined: { 1 ,2,4,5-tetramethylbenzene · · · tetrabromo-p -benzoquinone}∞ , {hexamethylbenzene · · · tetrabromo-p-benzoquinone}∞ , { ( 1 ,2 ,4,5-tetramethyl-benzene)2 · · · tetrachloro -p -benzoquinone}∞ , {pyrene · · · tetrafluoro-p-benzoquinone}∞ , {pyrene · · · tetrabromo-p-benzoquinone}∞ and {perylene · · · tetrabromo-p-benzoquinone}∞ . They exhibit an interesting lattice packing, especially the 2:1 tripeldecker sandwich of tetrachloro-p-benzoquinone, which crystallizes in a herringbone pattern. Their interplanar distances are around 340 pm, i. e. two van der Waals π radii. None of them , however, exhibits in neither the donor nor the acceptor components significant structural changes due to complex formation. Their colours range from orange-red to black in the crystal and to green in H2CCl2 solution. Their long-wavelengths charge transfer absorption maxim a correspond to a lowering in excitation energy of up to 2 eV relative to that of the components. The different charge transfer in the ground and excited states of the donor/acceptor complexes investigated is further discussed referring to data such as cyclovoltammetric reduction potentials as w ell as to results from semiempirical calculations based on the crystal structure data determined and including configuration interaction.
The neutral title compound, 8,8-bis(dimethylamino)dibenzo-[a,d]-heptafulvene, exhibits a first vertical ionization potential of only 6.98 eV and, therefore, can also be oxidized by AlCl3 in H2CCl2 solution. The radical cation generated shows a complex multiplet signal pattern, which is assigned based on additional ENDOR measurements. The photoelectron (PE) and ESR spectra of the 112 valence electron molecule are interpreted by “pararneter-optimized” HMO and by geometry-optimized MNDO calculations, which both suggest a non-planar π-type ground state with most of the charge and the spin distributed over the dibenzoheptatriene part of the radical cation.
Di(methylthio)acetylene H3CS-C≡C-SCH3 reacts with S2C12 in AlCl3/H2CCl2 solution to the tetra(thiomethyl)thiophene radical cation (H3CS)4C4S·⊕ and with H3CSCl to the tetra(thiomethyl)ethene radical cation (H3CS)2C·=⊕C(SCH3)3. Their ESR spectra are assigned by comparison with literature data or those of analogous products obtained from other acetylene derivatives R-C≡C-R with R = SCH2CH3, CH3, C6H5 as well as based on HMO arguments. The possible course of the oxidative sulfuration is discussed.
The structures of seven di- or tetrasubstituted p-benzoquinone derivatives O=C(XC=CH )2C=O and O=C(XC=CX)2C=O with substituents X = -OCH3, -N(CH2)5, - N(CH2CH2)2O, -Cl, -CN and -⊕N(HC=CH)2C-N(CH3)2 are presented and discussed in comparison with published ones substituted by X = -Si(CH3)3, -C6H5, -N(CH3)2, -⊕N(HC=CH)2CN(CH3)2, -O⊖ , and - NO2. Based on the introduction, in which halfwave-reduction potentials, geometry-optimized quantum-chemical calculations on substituent perturbation and known structural data of p-benzoquinone derivatives are used to characterize their molecular ground states. The structural changes indicate how substituent perturbations might be rationalized. Of the categories defined - imperturbed, donor, donor/acceptor and acceptor perturbed - the donorsubstituted p-benzoquinones do exhibit the largest differences, often called cyanine distorsion. In very satisfactory agreement with extensive semiempirical calculations, all effects determined experimentally are discussed in terms of varying charge distribution. With respect to the biochemical importance of p-benzoquinone derivatives, this first structural summary points out important facets.
From the electron and proton transfer equilibria network of quinones in solution a novel intermediate can be prepared by deprotonation of 2,5-bis(trimethylsilyl)hydroquinone to its monoanion using sodium metal. The sodium salt crystallizes in polymer strings connected via O⊖···(H)O hydrogen bridges, which are capped additionally by twofold dimethoxy-ethanesolvated Na⊕ countercations. The single crystal structure determination reveals one of the shortest O⊕ ··· HO distances observed so far of only 246 pm. MNDO calculations further confirm the assignment of hydroquinone monoanion building blocks in the polymer chain. For structural comparison as well as for attempts of its sodium reduction, 2,5-bis(trimethylsilyl)-p- benzoquinone has been synthesized. Its single crystal structure is reported, which does not show any cyanine distortion.
Chelate complexes of 1,2-dimesitoylbenzene radical anion with alkali metal cations exhibit in aprotic solution extremely large ESR /ENDOR metal coupling constants. For rationalization, structures of both the neutral molecule (H3C)3H2C6 - CO - C6H4 - CO - C6H2(CH3)3, in which the two carbonyl groups are twisted out of the benzene ring plane by dihedral angles of ± 3̄7̄°, and a sodium contact ion quadruple have been determined. One of the dimers [dimesitoylbenzeneH⊖ (Na⊕H2N H2C - CH2NH2)]2, although generated by Na metal mirror reduction of 1,2-dimesitoylbenzene in aprotic DME solution with added ethylendiamine for better electron transfer, surprisingly contains two 245 pm short (!) hydrogen bridges ⊖O ··· (H)O and in addition two solvation bridges e ⊖O ··· Na⊕(H2NH2C - CH2NH2) ··· O⊖. Results of MNDO calculations based on the experimental coordinates support the proposed concept.
Crystals of lemon yellow dipotassium nitranilate and of yellow disodium nitranilate dihydrate have been grown and their structures determined at 290 and 200 K. The six-member- ed, O2N-disubstituted rings show a pronounced cyanine distortion with all four CO bonds identical and the two (OCC(NO2)CO)⊖ chains connected by single CC bonds of each 156 pm length. In the anhydrous K⊕ salt, the ring is planar, but in the Na⊕ hydrate salt it exhibits a twist conformation. Quantum chemical calculations allow to reproduce the structure in every detail, demonstrate strong charge alternation along the cyanine chains with considerable delocalization into the O2N acceptor substituents, and suggest that the rather long connecting CC bonds contain positively charged carbon centers on both ends. In addition, metal ion coordination effects as well as the rather high pKa value of nitranilic acid are rationalized.
1,4-Bis(trimethylsiloxy)benzene has been crystallized both by vacuum sublimation and from «-heptane solution, which each yielded colourless plates with identical monoclinic unit cell dimensions (P2/n, Z = 4). The conformation of C[ symmetry shows the two (H3C)3SiO-substituents to be conrotationally twisted around the O-( C6H4)-O axis by dihedral angles o f ± 60°. According to the photoelectron spectroscopic ionisation pattern and its Koopmans’ assignment, IEVn = -εJAM 1, by AM 1 eigenvalues, the gas phase structure should also be of C, symmetry. The results of geometry-optimized MNDO , AM 1 or PM 3 calculations for the monosubstituted derivative H5C6-OS i(CH3)3 are compared with respect to the quality of their fit to the measured data.
The reduction potentials of 40 aromatic nitro compounds Rπ(NO2)n with Rπ = benzene, naphthalene, anthracene, fluorene and carbazole and n = 1 to 4 nitro groups are determined by cyclic voltammetry in DMF under aprotic conditions. The perturbation by the strongly electron accepting substituents can be rationalized via correlation with HMO eigenvalues. Based on reversibility criteria, the electrochemical behaviour is discussed and the compounds are classified with respect to reversible or irreversible one-electron transfer as well as up to 4 (quasi)-reversible reduction steps. The CV data measured can be used to predict redox reactions of aromatic nitro compounds in inert solvents.
UV/VIS and ESR spectra of electron transfer reaction products in aprotic (cH⊕ < 0,1 ppm) solution can be measured in an especially designed and sealed glass apparatus and provide information on unknown facets of the microscopic pathway through the network of interdependent equilibria. For tetraphenyl-p-benzoquinone in tetrahydrofuran, single-electron reduction by a sodium metal mirror produces a red solution and, unexpectedly, after addition of 2.2.2. cryptand, contact with a potassium metal mirror generates a green (!) one. For both, ESR/ENDOR spectra prove the presence of tetraphenyl-p-benzoquinone radical anion. UV/VIS measurements provide the clue: In the equilibrium revealed by repetetive spectra recording, M·⊖solv + Me⊕solv ⇄ [M·⊖···Me⊖]solv, the radical anion is green (vm = 16900 cm-1) and the contact ion pair red (vm=18900 cm-1 ). On ion pair formation, therefore, the excitation energy of the radical anion increases by 0.25 eV.