The pterin-containing molybdoenzymes catalyze the net exchange of an oxygen atom between water and substrate and there is evidence to support the involvement of oxygen atom transfer (OAT or oxo transfer) in the reactions of dimethyl sulfoxide reductases (DMSOR), sulfite oxidase, and nitrate reductase. [2][3][4][5][6][7] Accordingly, many model studies have focused on OAT reactions, most notably the reduction of dimethyl sulfoxide by oxo-Mo-(IV) complexes and the oxidation of tertiary phosphines by dioxo-Mo(VI) complexes. 3 Indeed, Schultz et al. 4 have shown that the DMSOR from Rhodobacter sphaeroides couples these reactions during catalysis of OAT from Me 2 SO to the watersoluble tertiary phosphine 1,3,5-triaza-7-phosphatricyclo[3.3.1.1 3,7 ]-decane. As well, the crystal structure of Me 2 S-soaked crystals of oxidized DMSOR from R. capsulatus has revealed the presence of an Mo-bound Me 2 SO molecule formed upon incomplete OAT from Mo to Me 2 S. 5 Putative oxo(phosphine oxide) intermediates formed during the oxidation of phosphines by enzyme or model systems have never been detected or isolated.The reactions of LMo VI O 2 X (L ) hydrotris(3,5-dimethylpyrazol-1-yl)borate) with PPh 3 are second-order and produce OPPh 3 and "LMo IV OX", which may be trapped, e.g., as LMo IV OX-(solvent) or LMo IV OX (as monodentate X becomes bidentate). 8-10 An associative mechanism has been proposed for the overall OAT reaction. 9 However, it is not clear whether the intermediate, LMoOX(OPPh 3 ), gives the product by an associative or dissociative mechanism. The reaction of LMoO 2 (SPh) with phosphines led Hall and co-workers 6 to examine the reaction of MoO 2 (NH 3 ) 2 -(SH) 2 with PMe 3 by computational methods. In the first step of the reaction, nucleophilic attack of PMe 3 on a π* ModO orbital perpendicular to the MoO 2 unit and at an Mo-O‚‚‚P angle of ca. 130°takes place. This results in a transition state with a weakened Mo-O bond (1.83 Å), an O-P interaction (2.43 Å), and a P-OModO torsion angle of 89.7°; the remaining ModO bond becomes stronger consistent with a "spectator oxo" function. 11 The OPMe 3 ligand then rotates about the Mo-O bond, breaking the Mo-O π interaction to generate an intermediate with Mod O ) 1.67 Å, Mo-O ) 2.18 Å, O-P ) 1.53 Å, and P-O-Mod O torsion ) 0.5°. The intermediate was 68.9 kcal‚mol -1 lower in energy than the reactants. At this stage replacement of OPMe 3 by water is predicted to take place by an associatiVe mechanism.Here, we report the detection of oxo(phosphine oxide) intermediates in the OAT reactions of LMoO 2 X and PPh 3 by fast atom bombardment mass spectrometry (FABMS) and the use of this technique to assess the stability of the intermediates and examine the kinetics of decay for unstable species. 12 Also, we report the isolation and characterization of L Pr Mo IV O(OPh)(OPEt 3 ) (L Pr ) hydrotris(3-isopropylpyrazol-1-yl)borate), the first stable oxo-(phosphine oxide) complex to be synthesized by incomplete OAT in a molybdoenzyme model system.Intermediates in the reactions of LMoO ...

Recent studies have shown that the UV/vis photolysis of nitryl chloride (ClNO2) can be a major source of reactive chlorine in the troposphere. The present work reports measurements of the ClNO2 absorption spectrum and its temperature dependence between 210 and 296 K over the wavelength range 200–475 nm using diode array spectroscopy. The room temperature spectrum obtained in this work was found to be in good agreement with the results from Ganske et al. (J. Geophys. Res. 1992, 97, 7651) over the wavelength range common to both studies (200–370 nm) but differs systematically from the currently recommended spectrum for use in atmospheric models. The present results lead to a decrease in the calculated atmospheric ClNO2 photolysis rate by 30%. Including the temperature dependence of the ClNO2 spectrum decreases the calculated atmospheric photolysis rate at lower temperatures (higher altitudes) even further. A parametrization of the wavelength and temperature dependence of the ClNO2 spectrum is presented. O(3P) quantum yields, Φ(ClNO2)(O), in the photolysis of ClNO2 at 193 and 248 nm were measured at 296 K using pulsed laser photolysis combined with atomic resonance fluorescence detection of O(3P) atoms. Φ(ClNO2)(O)(λ) was found to be 0.67 ± 0.12 and 0.15 ± 0.03 (2σ error limits, including estimated systematic errors) at 193 and 248 nm, respectively, indicating that multiple dissociation channels are active in the photolysis of ClNO2 at these wavelengths. The Φ(ClNO2)(O)(λ) values obtained in this work are discussed in light of previous ClNO2 photodissociation studies and the differences are discussed.

Oxalyl chloride, (ClCO)(2), has been used as a Cl atom photolytic precursor in numerous laboratory kinetic and photochemical studies. In this study, the UV/vis absorption spectrum of (ClCO)(2) and the Cl atom quantum yields in its photolysis at 193, 248, and 351 nm are reported. The UV∕vis spectrum was measured between 200 and 450 nm at 296 K using diode array spectroscopy in conjunction with an absolute cross section obtained at 213.9 nm. Our results are in agreement with the spectrum reported by Baklanov and Krasnoperov [J. Phys. Chem. A 105, 97-103 (2001)], which was obtained at 11 discrete wavelengths between 193.3 and 390 nm. Cl atom quantum yields, Φ(λ), were measured using pulsed laser photolysis coupled with time resolved atomic resonance fluorescence detection of Cl. The UV photolysis of (ClCO)(2) has been shown in previous studies to occur via an impulsive three-body dissociation mechanism, (COCl)(2) + hv → ClCO* + Cl + CO (2), where the excited ClCO radical, ClCO*, either dissociates or stabilizes ClCO* → Cl + CO (3a), → ClCO (3b). ClCO is thermally unstable at the temperatures (253-298 K) and total pressures (13-128 Torr) used in our experiments ClCO + M → Cl + CO + M (4) leading to the formation of a secondary Cl atom that was resolvable in the Cl atom temporal profiles obtained in the 248 and 351 nm photolysis of (ClCO)(2). Φ(193 nm) was found to be 2.07 ± 0.37 independent of bath gas pressure (25.8-105.7 Torr, N(2)), i.e., the branching ratio for channel 2a or the direct formation of 2Cl + 2CO in the photolysis of (ClCO)(2) is >0.95. At 248 nm, the branching ratio for channel 2a was determined to be 0.79 ± 0.15, while the total Cl atom yield, i.e., following the completion of reaction (4), was found to be 1.98 ± 0.26 independent of bath gas pressure (15-70 Torr, N(2)). Φ(351 nm) was found to be pressure dependent between 7.8 and 122.4 Torr (He, N(2)). The low-pressure limit of the total Cl atom quantum yield, Φ(0)(351 nm), was 2.05 ± 0.24. As part of this work, rate coefficients for the thermal decomposition of ClCO were measured between 253 and 298 K at total pressures between 13 and 128 Torr (He and N(2) bath gases). The N(2) bath gas results were combined with the data reported in Nicovich et al. [J. Chem. Phys. 92, 3539-3544 (1990)] to yield k(4)(T, N(2)) = (4.7 ± 0.7) × 10(-10) exp [-(2987 ± 16)/T] cm(3) molecule(-1) s(-1), while the He bath gas data fit yielded k(4)(T, He) = (2.3 ± 2.1) × 10(-10) exp [-(2886 ± 218)/T] cm(3) molecule(-1) s(-1). The quoted uncertainties are at the 2σ level from the precision of the fit. In addition, the room temperature rate coefficient for the Cl + ClNO reaction was measured in this work to be (1.03 ± 0.10) × 10(-10) cm(3) molecule(-1) s(-1).

We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated oxidation of isoprene in the presence of O 2 and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photolysis of monodeuterated/nondeuterated 2-iodo-2-methylbut-3-en-1-ol results exclusively in the dominant OH-isoprene addition product, providing important insight into the oxidation mechanism. On the basis of kinetic analysis of OH cycling experiments, we have determined the rate constant for O 2 addition to the hydroxyalkyl radical to be 1.0 -0.5 +1.7 × 10 -12 cm 3 s -1 , and we find a value of 8.1 -2.3 +3.4 × 10 -12 cm 3 s -1 for the overall reaction rate constant of the resulting hydroxyperoxy radical with NO. We also report the first clear experimental evidence of the (E) form of the δ-hydroxyalkoxy channel through isotopic labeling experiments and quantify its branching ratio to be (10 ( 3)%. This puts a rigorous upper limit on the branching of the (E)-δ-hydroxyalkoxy radical channel. Since our measured isomeric-selective rate constants for the dominant outer channel in OH-initiated isoprene chemistry are similar to the overall rate constants derived from nonisomeric kinetics, we predict that the remaining outer addition channel will have similar reactivity.

We have examined in detail the nonrestoration of symmetry at high temperature in a finite-temperature Littlest Higgs model, without and with T-parity, by evaluating the one-loop-order finite-temperature integrals of the effective potential numerically, without the high-temperature approximation, . i m T >> We observe that in the model without T-parity it is not possible to find a transition temperature within the allowed temperature range of the model (0

We report isomer-selective kinetics and mechanistic details for the hydroxyl radical-initiated oxidation of isoprene, in the presence of O(2) and NO, employing complementary experimental and theoretical techniques. Using a recently demonstrated photolytic route to initiate isomer-selective kinetics in OH-initiated oxidation of unsaturated hydrocarbons via the UV photolysis of iodohydrins, the photolysis of 1-iodo-2-methyl-3-buten-2-ol results in a single isomer of the possible four OH-isoprene adducts, specifically the minor channel associated with OH addition to one of the inner carbon atoms. Employing both the laser-photolysis/laser-induced fluorescence (LP/LIF) technique and time-dependent multiplexed photoionization mass spectrometry, we find clear experimental evidence supporting the prompt rearrangement of the initially formed beta-hydroxyalkyl radicals to alpha-hydroxyalkyl radicals, in agreement with Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation predictions. We have determined a rate constant of (3.3 +/- 0.5) x 10(-11) cm(3) molecule(-1) s(-1) for molecular oxygen to abstract a hydrogen atom from the alpha-hydroxyalkyl radical to form 4-penten-2-one and HO(2). This reaction provides a mechanistic route to C(5) carbonyl species as first-generation end products for the addition of hydroxyl radical to isoprene in the presence of O(2) and NO.

We derive a single field slow-roll inflaton potential in three intersecting D7 branes configuration under type IIB/F-theory compactification. Among three resulting Kähler moduli corresponding to three orthogonal directions, two are stabilized via perturbative corrections in Kähler potential arising from large volume scenario (α'^3) and four graviton scattering amplitude upto one loop level and the remaining Kähler modulus is stabilized by KKLT-type non-perturbative correction in superpotential. The symmetric combination of two canonically normalized and perturbatively stabilized Kähler moduli gives the inflaton field and the anti-symmetric combination manifests as an auxiliary field.

We study electroweak baryogenesis within the framework of the littlest Higgs model with T parity. This model has shown characteristics of a strong first-order electroweak phase transition, which is conducive to baryogenesis in the early Universe. In the T parity symmetric theory, there are two gauge sectors, viz., the T -even and the T -odd ones. We observe that the effect of the T -parity symmetric interactions between the T -odd and the T -even gauge bosons on gauge-higgs energy functional is quite small, so that these two sectors can be taken to be independent. The T -even gauge bosons behave like the Standard Model gauge bosons, whereas the T -odd ones are instrumental in stabilizing the Higgs mass. For the T -odd gauge bosons in the symmetric and asymmetric phases and for the T -even gauge bosons in the asymmetric phase, we obtain, using the formalism of Arnold and McLerran, very small values of the ratio, (Baryon number violation rate/Universe expansion rate). We observe that this result, in conjunction with the scenario of inverse phase transition in the present work and the value of the ratio obtained from the lattice result of sphaleron transition rate in the symmetric phase, can provide us with a plausible baryogenesis scenario. PACS numbers: 98.80Cq, 12.15Ji, 12.60Cn, 12.60Fr.