Regioselective and Stepwise [8 + 2] Cycloaddition Reaction between Alkynyl-Fischer Carbene Complexes and Tropothione

The formal [8 + 2] cycloaddition reaction between alkynyl Fischer carbene complexes and tropothione leads to the regioselective formation of novel 3aH-cyclohepta[b]thiophene carbene complexes. Computational DFT calculations indicate that the process proceeds stepwise via antiaromatic zwitterionic intermediates.     

On the lifetime of the transients (NP)-(CH3)n (NP = Ag0, Au0, TiO2 nanoparticles) formed in the reactions between methyl radicals and nanoparticles suspended in aqueous solutions

Methyl radicals react in fast reactions, with rate constants k>1×10(8) M(-1) s(-1), with Au(0), Ag(0) and TiO(2) nanoparticles (NPs) dispersed in aqueous solutions to form intermediates, (NP)-(CH(3))(n), in which the methyl groups are covalently bound to the NPs. These intermediates decompose to form ethane. As n≥2 is required for the formation of C(2)H(6), the minimal lifetime (τ) of the methyls bound to the NPs, (NP)-CH(3), can be estimated from the rate of production of the CH(3)(·) radicals and the NPs concentration. The results obtained in this study, using a very low dose rate γ-source for NP = Ag(0), Au(0), and TiO(2) point out that τ of these intermediates is surprisingly long, for example, ≥8 and ≥188?sec for silver and gold, respectively. These data point out that the NP-C bond dissociation energies are ≥70?kJ mol(-1). Under low rates of production of CH(3)(·), that is, when the rate of formation of ethane is very low, other reactions may occur, consequently the mechanism proposed is "broken". This is observed in the present study only for TiO(2) NPs. These results have to be considered whenever alkyl radicals are formed near surfaces. Furthermore, the results point out that the rate of reaction of methyl radicals with (NP)-(CH(3))(n) depends on n, that is, the number of methyl radicals bound to the NPs affect the properties of the NPs.     

Electronic energy transfer from the S2 state of rhodamine 6G

In this paper we present the results of an experimental study of intermolecular electronic energy transfer (EET) from the short-lived Second excited singlet state of rhodamine 6G (R6G) to the ground state of 2,5-bis [5′-tert-butyl-2-benzoxazolyl] thiophene (BBOT). The S₂ state of the donor was excited by sequential, time-delayed, two-photon excitation (STDTPE) utilizing the second harmonic and the first harmonic of a mode-locked Nd³⁺: glass laser, while the EET process was interrogated by monitoring the enhancement of the S₁ → S₀ fluorescence of BBOT. The enhancement of the fluorescence intensity of BBOT was found to be linear in the energies of the two exciting pulses, and linear in the concentration of the energy acceptor (over the BBOT concentration range of (0.3–7) × 10^?5 M), which is in accord with the predictions of the Forster—Dexter mechanism for resonant EET from an ultrashort-lived donor state at low acceptor concentrations. Quantitative measurements of the S₂ → S₀ fluorescence yield in R6G solution directly excited by STDTPE and of the S₁ → S₀ fluorescence of BBOT from R6G + BBOT solutions resulting from EET led to the values of Y_D(S₂ → S₀) = (2.1 ± 0.5) × 10^?6 for the emission quantum yield of the S₂ state of R6G and τr_D(S₂) ≈ 3 × 10^?14 s for the lifetime of the metastable S₂ state of this molecule.     

Synthesis of 2-amino-6-chloro- and 2,6-diammopunne 3-oxides

N-Oxidation of 2-amino-6-chloropurine to the 3-oxide provided a convenient intermediate for the synthesis of 2-amino-6-substituted purine 3-oxides, including the previously unavailable 2,6-diaminopurine 3-oxide. Thiation of the 6-halogen was accompanied by reduction of the N-oxide. The properties of the 1- and 3-oxides of 2,6-diaminopurine are compared.     

Laser desorption/ionization mass spectrometry fingerprinting of complex hydrocarbon mixtures: application to crude oils using data mining techniques

Crude oil fingerprints were obtained from four crude oils by laser desorption/ionization mass spectrometry (LDI-MS) using a silver nitrate cationization reagent. Replicate analyses produced spectral data with a large number of features for each sample (>11,000 m/z values) which were statistically analyzed to extract useful information for their differentiation. Individual characteristic features from the data set were identified by a false discovery rate based feature selection procedure based on the analysis of variance models. The selected features were, in turn, evaluated using classification models. A substantially reduced set of 23 features was obtained through this procedure. One oil sample containing a high ratio of saturated/aromatic hydrocarbon content was easily distinguished from the others using this reduced set. The other three samples were more difficult to distinguish by LDI-MS using a silver cationization reagent; however, a minimal number of significant features were still identified for this purpose. Focus is placed on presenting this multivariate statistical method as a rapid and simple analytical procedure for classifying and distinguishing complex mixtures.     

Correlation between Hammett substituent constants and directly calculated pi-conjugation strength

The results of an energy decomposition analysis of ortho-, meta-, and para-substituted benzylic cations and para-substituted benzylic anions H2C-C6H4Rq (R = H, F, CN, Me, OH, NH2, NO2, CHO, CO2H; q = +, -) are presented and discussed. The calculated values for the pi bonding between CH2(q) and C6H4R show for substituents which have pi orbitals a linear correlation with the Hammett sigma(p), sigma(+)(p), and sigma(m) constants.     

The Interplay between Steric and Electronic Effects in SN2 Reactions

Myths of steric hindrance : In contrast with current opinion, energy decomposition analysis shows that the presence of bulky substituents at carbon leads to the release of steric repulsion in the transition state shown in the graphic. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier.