On the gas-phase reactivity of complexed OH+ with halogenated alkanes

OH(+) is an extraordinarily strong oxidant. Complexed forms (L--OH(+)), such as H(2)OOH(+), H(3)NOH(+), or iron-porphyrin-OH(+) are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L--OH(+) towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L--OH(+). With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas-phase pseudo-first-order reaction rates of H(2)OOH(+), H(3)NOH(+), and protonated 4-picoline-N-oxide towards isobutane and different halogenated alkanes C(n)H(2n+1)Cl (n=1-4), HCF(3), CF(4), and CF(2)Cl(2) have been determined by means of Fourier transform ion cyclotron resonance measurements. Reaction rates for H(2)OOH(+) are generally fast (7.2x10(-10)-3.0x10(-9) cm(3) mol(-1) s(-1)) and only in the cases HCF(3) and CF(4) no reactivity is observed. In contrast to this H(3)NOH(+) only reacts with tC(4)H(9)Cl (k(obs)=9.2x10(-10)), while 4-CH(3)-C(5)H(4)N-OH(+) is completely unreactive. While H(2)OOH(+) oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH(2)OH, a series of alkanes C(n)H(2n+2) (n=1-4), and halogenated alkanes C(n)H(2n+1)Cl (n=1-4), HCF(3), CF(4), and CF(2)Cl(2), were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C(n)H(2n+1) (+) (n=1-4) and C(n)H(2n)Cl(+) (n=1-4). The hypersurface for the reaction of H(2)OOH(+) with CH(3)Cl and C(2)H(5)Cl was calculated at the B3 LYP, MP2, and G3(m*) level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.     

Trifluoromethyl‐Radical‐Mediated Carbonylation of Alkanes Leading to Ethynyl Ketones

The carbonylation of alkanes 1 under radical‐reaction conditions was examined by using ethynyl triflone A as the unimolecular chain‐transfer (UMCT) reagent. Good to moderate yields of ethynyl ketones 2 were prepared by means of this three‐component coupling reaction. Higher CO pressures as well as lower concentrations of triflone A improved the efficiency of the reaction over the direct addition, the latter leading to alkylated ethynes 3 . In contrast to the reaction with A , the reaction of cyclohexane ( 1a ) with allyl triflone B (= ethyl 2‐methylene‐3‐[(trifluoromethyl)sulfonyl]propanoate) in the presence of CO gave a mixture of carbonylation products, including 8a formed from two molecules each of cyclohexane, CO, and allyl triflone B .     

Formation of secondary ozonides in the gas-phase ozonolysis of simple alkenes

Secondary propene ozonide and isobutene ozonide were formed in the gas-phase ozonolysis of ethene with added acetaldehyde and acetone, respectively. Combined with the formation of hydroperoxymethyl formate and methoxymethyl hydroperoxide in the ethene-ozone reaction system in the presence of HCOOH and CH₃OH, respectively, formation of the secondary ozonides reveals a close similarity between the gas-phase and the liquid-phase ozonolysis of alkenes.     

Fine structures of zeolite-Linde-L (LTL): surface structures, growth unit and defects

High-resolution electron microscopy (HREM) has been used to image the surface structure of nano- and micrometer-sized synthetic crystals of zeolite-Linde-L (LTL). Columnar holes and rotational, nano-sized, wheel-like defects were observed within the crystals, where the hole has a minimum size equal to that of the rotational defect. Predictions of surface structure from atomistic computer simulation concur with the observations from HREM and provide insight into the crystal growth mechanism of perfect and defective LTL. Analysis of the energetics of the formation of rotational defect structures reveals that the driving force for defect creation is thermodynamic and furthermore, the rotational defects could be created in high concentrations. Formation of a columnar hole is found to be slightly energetically unfavourable and therefore we speculate that the incidence of both rotational and nano-sized vacancy defects is strongly dependent on kinetic factors and reaction conditions. The morphology of nano- and microcrystalline LTL is contradistinct and we use insights from simulation to propose an explanation of the disparity in crystal shape.     

Design, synthesis, and binding affinities of pyrrolinone-based somatostatin mimetics

[structure: see text] Tetrapyrrolinone somatostatin (SRIF) mimetics (cf. 1), based on a heterochiral (D,L-mixed) pyrrolinone scaffold, were designed, synthesized, and evaluated for biological activity. The iterative synthetic sequence, incorporating the requisite functionalized coded and noncoded amino acid side chains, comprised a longest linear synthetic sequence of 23 steps. Binding affinities at two somatostatin receptor subtypes (hsst 4 and 5) reveal micromolar activity, demonstrating that the d,l-mixed pyrrolinone scaffold can be employed to generate functional mimetics of peptide beta-turns.     

Structural studies of the carbon monoxide complex of [NiFe]hydrogenase from Desulfovibrio vulgaris Miyazaki F: suggestion for the initial activation site for dihydrogen

The carbon monoxide complex of [NiFe]hydrogenase from Desulfovibrio vulgaris Miyazaki F has been characterized by X-ray crystallography and absorption and resonance Raman spectroscopy. Nine crystal structures of the [NiFe]hydrogenase in the CO-bound and CO-liberated forms were determined at 1.2-1.4 A resolution. The exogenously added CO was assigned to be bound to the Ni atom at the Ni-Fe active site. The CO was not replaced with H(2) in the dark at 100 K, but was liberated by illumination with a strong white light. The Ni-C distances and Ni-C-O angles were about 1.77 A and 160 degrees, respectively, except for one case (1.72 A and 135 degrees ), in which an additional electron density peak between the CO and Sgamma(Cys546) was recognized. Distinct changes were observed in the electron density distribution of the Ni and Sgamma(Cys546) atoms between the CO-bound and CO-liberated structures for all the crystals tested. The novel structural features found near the Ni and Sgamma(Cys546) atoms suggest that these two atoms at the Ni-Fe active site play a role during the initial H(2)-binding process. Anaerobic addition of CO to dithionite-reduced [NiFe]hydrogenase led to a new absorption band at about 470 nm ( approximately 3000 M(-1)cm(-1)). Resonance Raman spectra (excitation at 476.5 nm) of the CO complex revealed CO-isotope-sensitive bands at 375/393 and 430 cm(-1) (368 and 413 cm(-1) for (13)C(18)O). The frequencies and relative intensities of the CO-related Raman bands indicated that the exogenous CO is bound to the Ni atom with a bent Ni-C-O structure in solution, in agreement with the refined structure determined by X-ray crystallography.     

Relationship between symmetry of porphyrinic pi-conjugated systems and singlet oxygen (1Delta g) yields: low-symmetry tetraazaporphyrin derivatives

We have investigated the excited-state properties and singlet oxygen ((1)Delta(g)) generation mechanism in phthalocyanines (4M; M = H(2), Mg, or Zn) and in low-symmetry metal-free, magnesium, and zinc tetraazaporphyrins (TAPs), that is, monobenzo-substituted (1M), adjacently dibenzo-substituted (2AdM), oppositely dibenzo-substituted (2OpM), and tribenzo-substituted (3M) TAP derivatives, whose pi conjugated systems were altered by fusing benzo rings. The S(1)(x) and S(1)(y) states (these lowest excited singlet states are degenerate in D(4)(h) symmetry) split in the low-symmetry TAP derivatives. The excited-state energies were quantitatively determined from the electronic absorption spectra. The lowest excited triplet (T(1)(x)) energies were also determined from phosphorescence spectra, while the second lowest excited triplet (T(1)(y)) states were evaluated by using the energy splitting between the T(1)(x) and T(1)(y) states previously reported (Miwa, H.; Ishii, K.; Kobayashi, N. Chem. Eur. J. 2004, 10, 4422-4435). The singlet oxygen quantum yields (Phi(Delta)) are strongly dependent on the pi conjugated system. In particular, while the Phi(Delta) value of 2AdH(2) is smallest in our system, that of 2OpH(2), an isomer of 2AdH(2), is larger than that of 4Zn, in contrast to the heavy atom effect. The relationship between the molecular structure and Phi(Delta) values can be transformed into a relationship between the S(1)(x) --> T(1)(y) intersystem crossing rate constant (k(ISC)) and the energy difference between the S(1)(x) and T(1)(y) states (DeltaE(S)(x)(T)(y)). In each of the Zn, Mg, and metal-free compounds, the Phi(Delta)/tau(F) values (tau(F): fluorescence lifetime), which are related to the k(ISC) values, are proportional to exp(-DeltaE(S)(x)(T)(y)), indicating that singlet oxygen ((1)Delta(g)) is produced via the T(1)(y) state and that the S(1)(x) --> T(1)(y) ISC process follows the energy-gap law. From the viewpoint of photodynamic therapy, our methodology, where the Phi(Delta) value can be controlled by changing the symmetry of pi conjugated systems without heavy elements, appears useful for preparing novel photosensitizers.     

Evaluation of dissociation constants of ammonium ion in aqueous lithium perchlorate and lithium chloride-sodium chloride mixed solutions

The Pitzer approach has been applied to the evaluation of dissociation constants of ammonium ion in lithium perchlorate and lithium chloride-sodium chloride mixed solutions at 25°C. The calculated values showed good agreement with the observed values, provided all the higher-order interaction terms ('s and 's) concerned were introduced. The unknown (NH₄LiClO₄) value was determined from the isopiestic measurements of NH₄ClO₄–LiClO₄ mixed solutions. Parameters in the Pitzer formalism for ammonia-ion interactions involved in LiCl and NaCl media were determined by use of the activity coefficients of ammonia measured in LiCl–NaCl mixed solutions by a transpiration method.     

Determination of rare earth elements in seawater by on-line column preconcentration inductively coupled plasma mass spectrometry

A home made column of commercially available iminodiacetate resin, Muromac A-1 (50–100 mesh) was used to concentrate rare earth elements (REEs) (15 elements: Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) in seawater. An automated low pressure flow analysis method with on-line column preconcentration/inductively coupled plasma mass spectrometry (ICP-MS) is described for the determination of REEs in seawater. Sample solutions (adjusted to pH of 3.0) passed through the column. After washing the column with water, the adsorbed elements were subsequently eluted into the plasma with 0.7 M nitric acid. Calibration curves were accomplished by means of purified artificial seawater with a sample loading time of 120 s. Detection limits (DLs) of the on-line column preconcentration/ICP-MS by eight replicate operations were between 0.040 and 0.251 pg ml⁻¹ for REEs in the artificial seawater. The precision was less than 8.9% for REEs and one sample can be processed in 7 min using a 7 ml of sample. The proposed method was applied to determine REEs in coastal seawater of Hiroshima Bay, Japan.     

InGaAsP/InP optical waveguide switch operated by a carrier-induced change in the refractive index

Waveguided semiconductor optical switches operated by a carrier-induced change in the refractive-index associated with the plasma dispersion are proposed. InGaAsP/InP four-port switches having two intersecting single-mode channel waveguides are fabricated by selective liquid-phase epitaxy and investigated at 1.5m wavelength. Optical switching is observed as a result of mode interference in the waveguide intersection region.