Pulsed versus continuous laser multiphoton dissociation of iodobenzene cations

In the two-photon visible dissociation of the iodobenzene cation, laser pulse shape is unimportant when collisional relaxation is slow compared to the time between pulses but becomes important when the relaxation rate is comparable to time between pulses. The dissociation can be simulated using an internal energy-governed computer model.     

Direct α-hydroxylation of ketones using iodosobenzene

Ketones are converted smoothly into the corresponding acyloin with a high degree of selectivity upon treatment with iodosobenzene or phenyliodosodiacetate in methanolic sodium hydroxide.     

Stereoelectronic control of the rate of ring opening in azidofurans

5-Azido-2-furyl alkyl ketones are converted into nitrogen and 4,5-dioxo-2-alkenenitriles in a reaction whose facility depends on the nature of the alkyl group.     

A theoretical approach to substituent effects. A comparison of the isoelectronic BH,CH3, and NH groups and their interaction with substituents in disubstituted benzenes

Ab initio molecular orbital theory with the STO -3G basis set is used to examine both charge and energy interactions in a series of meta- and para-substituted phenylborate anions and toluenes. Comparison of the results is made with data for substituted anilinium cations. It is concluded that whereas NH is a powerful σ acceptor, with essentially no π interaction, BH is primarily a π donor, and, to a slight extent only, a π donor. CH₃ is indicated to be both a weak σ and π donor. Energies of interaction of BH and NH with a series of substituents are an order of magnitude larger than corresponding values for CH₃. Interaction energies for BH are of opposite sign to those for NH. The results may be understood qualitatively using perturbation molecular orbital (PMO ) theory.     

BLEACHING OF PURPLE MEMBRANE WITH O-SUBSTITUTED HYDROXYLAMINES

Abstract The retinal Schiff base of bacteriorhodopsin, in the purple membrane from Halobacterium halobium , can be cleaved by hydroxylamine in the presence of light. We have further investigated this reaction with a series of O -substituted hydroxylamines, RONH₂, where R = -H (HA), -CH₃ (MHA), -SO₃− (HAS), benzyl- (BHA), p -nitrobenzyl- (NBHA), and pentafluorobenzyl- (FBHA). All except MHA caused light-induced bleaching of the purple membrane and the chromophore could be regenerated from apomembrane and all- trans retinal. Relative bleaching rate constants were obtained from V = QI _a k ₀ X /( k _r+ k ₀ X ), where V = bleaching rate, Q = quantum yield, I a = absorbed light intensity, X = hydroxylamine concentration, k ₀= rate constant for bleaching and k _r= rate constant for return of photoexcited bacteriorhodopsin to the initial state. This equation fits the time-, concentration- and intensity-dependences of the bleaching reactions in 0.02 M phosphate, pH 7.0. The rate constants k ₀ relative to HA were: MHA: 0; HAS: 0.3; HA: 1.0; BHA: 1.8; FBHA: 10.1; NBHA: 10.8. The relative rate constants do not correlate with the basicity of the derivatives. Instead, the results suggest that the retinal Schiff base is near a non-polar cavity into which an aromatic group can be inserted.     

Untersuchungen zurFriedel-Crafts-Reaktion. XVI. Darstellung isomerer 2-Acyl- und 3-Acyl-4-methoxyphenole

Acylation of 4-methoxy phenol according toFriedel andCrafts, as well as the rearrangement of its esters according toFries lead always to 2-acyl-4-methoxy phenols or to their demethylated compounds. The unknown 3-acyl-4-methoxy phenols were prepared in two steps: First, the ester is acylated with the corresponding acyl chloride and SnCl₄ in nitromethane. In the second step the resulting ketoesters are hydrolysed. This is a general method. The yields ofmeta-acylphenols are between 40 and 90%. The isomeric 2-acyl-4-methoxy-phenols which were partly unknown or accessible only in low yields have also been prepared for comparative spectral studies (UV, IR, NMR, MS) ofortho- andmeta-acylphenols.

     

Tuning the singlet oxygen quantum yield of near-IR-absorbing porphyrazines

We report the quantum yields for singlet oxygen production by a series of porphyrazines (pz) of the form M[pz(An;B4-n)] (Scheme 1), where the peripheral substituent A is [S-R]2 with R = (CH2CH2O)3H, B is a fused alpha,alpha'-dialkoxybenzo group and M = 2H, Mg or Zn. These compounds show intense near-IR absorbance/emission (longest wavelength emission, approximately 830 nm). Their solubilities vary with R, whereas their optical properties do not. We show that singlet oxygen sensitization by these luminescent compounds can be "tuned" from essentially off to on by varying n and selection among M = 2H, Mg or Zn. The quantum yields vary ca 60-fold within the set of compounds studied, from phidelta = 0.007 for compound 3 to phidelta = approximately 0.4 for compound 11.     

The dapE-encoded N-succinyl-l,l-diaminopimelic acid desuccinylase from Haemophilus influenzae is a dinuclear metallohydrolase

The Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra, of the dapE-encoded N-succinyl-l,l-diaminopimelic acid desuccinylase (DapE) from Haemophilus influenzae have been recorded in the presence of one or two equivalents of Zn(II) (i.e. [Zn_(DapE)] and [ZnZn(DapE)]). The Fourier transforms of the Zn EXAFS are dominated by a peak at ca. 2.0 A, which can be fit for both [Zn_(DapE)] and [ZnZn(DapE)], assuming ca. 5 (N,O) scatterers at 1.96 and 1.98 A, respectively. A second-shell feature at ca. 3.34 A appears in the [ZnZn(DapE)] EXAFS spectrum but is significantly diminished in [Zn_(DapE)]. These data show that DapE contains a dinuclear Zn(II) active site. Since no X-ray crystallographic data are available for any DapE enzyme, these data provide the first glimpse at the active site of DapE enzymes. In addition, the EXAFS data for DapE incubated with two competitive inhibitors, 2-carboxyethylphosphonic acid and 5-mercaptopentanoic acid, are also presented.     

Ambident reactivity of the thiocyanate anion revisited: can the product ratio be explained by the hard soft acid base principle?

Laser flash photolysis and stopped-flow methods have been employed to determine the kinetics of the reactions of benzhydrylium ions with both termini of the thiocyanate ion. In contrast to previous investigations which reported sulfur/nitrogen ratios of k(S)/k(N) = 2-10 for the reactions of carbocations with SCN(-), values of k(S)/k(N) = 10(3)-10(4) are now derived from absolute rate constants. This discrepancy is explained by the fact that the data determined in this investigation are the first which refer to activation-controlled attack of carbocations at both termini of the thiocyanate ion, while previous reactivity ratios included diffusion-controlled reactions. It is concluded that the selectivities of the reactions of carbocations with the thiocyanate ion cannot be explained by the hard soft acid base principle.     

The dual-site alternating cyclocopolymerization of 1,3-butadiene with ethylene

The copolymerization of ethylene with 1,3-butadiene was studied with the series of ansa-metallocenes Me2Si(Cp)(9-Flu)ZrCl2 (1), Me2Si(1-Ind)(9-Flu)ZrCl2 (2), and Me2Si(9-Flu)2ZrCl2 (3) with methylaluminoxane (MAO) as cocatalyst. The catalyst 2/MAO yields a cyclocopolymer composed of two ethylene monomer units for every one butadiene in a novel periodic architecture of 1,2-enchained cyclopentane units separated by three methylenes. The high butadiene content in the copolymer and the high selectivity for alternating cyclocopolymerization to form methylene-1,2-cyclopentane units implicate a dual-site mechanism where butadiene and ethylene are enchained at different coordination sites.