Heme charge-transfer band III is vibronically coupled to the Soret band

A complete resonance Raman excitation profile of the heme charge-transfer band known as band III is presented. The data obtained throughout the near-infrared region show preresonance with the Q-band, but the data also clearly show the enhancement of a number of modes in the spectral region of band III. Only nontotally symmetric modes are observed to have resonance enhancement in the band III region. The observed resonance enhancements in modes of B(1g) symmetry are compared with the enhancements of those same modes in the excitation profiles of the Q-band of deoxy myoglobin, also presented here for this first time. The Q-band data agree well with the theory of vibronic coupling in metalloporphyrins (Shelnutt, J. A. J. Chem. Phys. 1981, 74, 6644-6657). The strong vibronic coupling of the Q-band of the deoxy form of hemes is discussed in terms of the enhancement of modes with both B(1g) and A(2g) symmetry. The comparison between the Q-band and band III reveals that, consistent with the theory, only modes of B(1g) symmetry are enhanced in the vicinity of band III. These results show that band III is vibronically coupled to the Soret band. The coupling of band III to modes with strong rhombic distortion of the heme macrocycle calls into question the hypothesis that the axial iron out-of-plane displacement is primarily responsible for the structure-dynamics correlations observed in myoglobin.     

Heterocumulene metathesis by iridium guanidinate and ureylene complexes: catalysis involving reversible insertion to form six-membered metallacycles

The four-membered metallacycles Cp*Ir((NAr)2C=Z) have been prepared for Z = NAr and Z = O. These complexes undergo facile exchange with free heterocumulenes. This exchange process can be employed to effect the rapid, catalytic metathesis of aryl carbodiimides at room temperature and the metathesis of aryl carbodiimides with aryl isocyanates at slightly elevated temperature. The exchange process appears to proceed via a novel associative mechanism involving ring expansion to form a six-membered metallacycle rather than cycloreversion to give an imido complex. Observation of key intermediates and the results of crossover experiments support this hypothesis.     

Argon rectification and the cause of light emission in single-bubble sonoluminescence

In single-bubble sonoluminescence, repeated brief flashes of light are produced in a gas bubble strongly driven by a periodic acoustic field. A startling hypothesis has been made by Lohse and co-workers [Phys. Rev. Lett. 78, 1359 (1997)] that the non-noble gases in an air bubble undergo chemical reaction into soluble products, leaving only argon. In the present work, this dissociation hypothesis is supported by simulations, although the associated temperatures of about 7000 K seem too low for bremsstrahlung, which has been proposed as the dominant light emission mechanism. This suggests that emission from water vapor and its reaction products, heretofore not included, may play an important role.     

Crystal structure and luminescence of 2,7-dimethylnaphthalene-1-carbonitrile

The title compound crystallizes in the monoclinic space group P2 ₁/c: M _r = 181.2; a = 7.119(1), b = 18.389(4), c = 7.5385(6) Å = 91.661(7)° V = 986.4(5) ų; and Z = 4. The purified material shows fluorescence similar to other naphthalene derivatives: monomer fluorescence (_max 350-370 nm) in the solid state and in dilute solutions, and excimer fluorescence (_max 421 nm) in concentrated solutions. Intense blue-green luminescence (_max 490 nm) is observed in some partially purified crystalline samples. This is attributed to phosphorescence from two isomeric bromodimethylnaphthalenecarbonitrile impurities detected by GC-MS analysis.     

Enantioselective synthesis of the 4-hydroxy buteneolide terminus of mucocin and related annonaceous acetogenins

The 4-hydroxy buteneolide terminus 3, applicable to mucocin 1 and related annonaceous acetogenins, was prepared in an expeditious manner from the selenocarbonate 2 via an intramolecular acyl radical cyclization followed by an enantioselective Lewis-acid catalyzed Keck-allylation reaction.     

Formation of paramagnetic mononuclear iron nitrosyl complexes from diamagnetic di- and tetranuclear iron-sulphur nitrosyls: characterisation by EPR spectroscopy and study of thiolate and nitrosyl ligand exchange reactions

The diamagnetic Roussin esters Fe₂(SR)₂(NO)₄ readily underwent exchange with thiols R′SH to yield Fe₂(SR′)₂(NO)₄: the exchange was faster in polar, coordinating solvents where paramagnetic, mononuclear complexes of types [Fe(NO)₂(solvent)₂]⁺ and Fe(NO) ₂(SR)(solvent) were formed. With the corresponding thiolate anions RS^-, the esters Fe₂(SR)₂(NO)₄ formed the mononuclear complexes [Fe(SR)₂(NO)₂]^-, which were fully characterised by EPR spectroscopy for R = H, Me, Et, i-Pr, t-Bu and PhCH₂: assignments of hyperfine couplings were confirmed by use of ¹⁵N. With Fe₂(SR)₂(NO)₄ and a different set of thiolate anion, R′S ^-, in excess, thiol exchange occurred to give [Fe(SR′)₂(NO)₂]^-. A mechanism for formation of Fe₂(SR′)₂(NO)₄ from Fe₂(SR)₂(NO)₄ has been proposed. The paramagnetic mononuclear complexes [Fe(SR)₂(NO)₂] were also readily formed from the diamagnetic clusters [Fe₄S₃(NO)₇]^- and Fe₄S₄(NO)₄, together with [Fe(SR)₃(NO)]^-, and additionally from [Fe(CO)₃NO]^-. [Fe(SMe)₂(NO)₂]^-. was found to be a precursor of isolable Fe₂(SMe)₂(NO)₄, and [Fe(SH)₂ (NO)₂]^- to be the common precursor of both Roussin′s red anion [Fe₂S₂(NO)₄]^- and Roussin's black anion [Fe₄S₃ (NO)₇]^- interconvertible by appropriate adjustment of pH. The nitrosyl groups in these complexes were freely labile, and mononitrosyliron and dinitrosyliron fragments were readily interconvertible: FE(NO) fragments were favoured by the dimethyldithiocarbamate ligand (Me₂NCS ₂) and Fe(NO)₂ fragments by thiolate ligands, RS^-, regardless of the origin of the Fe(NO)_x(x = 1,2) fragment: both mono- and dinitrosyliron complexes persisted with [(i-PrO)₂S₂]^- as ligand. Isotopic labelling showed the occurrence of rapid exchange of nitrogen between nitrosyl ligands and added nitrite in Fe(NO)(S₂CNMe₂)₂ and [Fe(SR)₂(NO)₂]^-     

Hydrocarbon metal sulphide complexes : VI. A new type of rearrangement of a cyclopentadienylruthenium complex

The complexes C₅H₅Ru(CO)₂SR (R = C₆F₅ or CH₃) undergo photochemical conversion to binuclear and trinuclear derivatives which exhibit isomerism. A novel rearrangement has been detected for [C₅H₅Ru(CO)(SC₆F₅)]₂.