Metastable Radicals Research Articles

Expeditious and single step synthesis of ceria quantum dots by microplasma discharge method for supercapacitor applications

In the present study, CeO2 quantum dots (QDs) were synthesized by microplasma discharge method out of cerium nitrate precursor suspension using argon gas as plasma medium. The highly energetic electrons in the microplasma plume generate highly reactive metastable OH radicals, which further combine with electrons to form OH− and react with the cerium ions (III) in the precursor solution to form CeO2 QDs. The radical produced during the process was recorded using optical emission spectroscopy. The structural features, oxidation states, morphology, elemental composition and surface area of the synthesized QDs were identified by XRD, Raman, XPS, FE-SEM, HR-TEM, EDS, PL and BET analysis, respectively. The results obtained revealed the formation of cubic phase CeO2 QDs, which was confirmed by XRD and Raman analysis. Agglomerated ceria QDs of spherical shape with an average size of around 5 nm were analyzed from FE-SEM and HR-TEM observations. The electrochemical performance of CeO2 QDs as electrode material was investigated through cyclic voltammetry analysis by an electrochemical workstation. The material achieved a highest specific capacitance of 388 Fg-1 at 1 Ag-1 current density. The commendable electrochemical performance of microplasma synthesized CeO2 QDs was attributed to the minuscule size, high surface area and the presence of Ce3+ ions. This study highlights the synthesis of CeO2 QDs through a single, rapid and amicable process without the use of secondary treatments and suggests the use of CeO2 as a good electrode material for electrochemical applications.

Diverse metastable diarylacetonitrile radicals generated by polymer mechanochemistry

Diverse diarylacetonitrile radicals were generated by polymeric mechanochemical reactions. Fluorescence wavelengths could be tuned by changing the substituents on the aromatic rings of the generated radicals.

Predicting isotopologue abundances in the products of organic catagenesis with a kinetic Monte-Carlo model

Thermal cracking of complex organic matter can occur under many geological settings. However, the role of thermal cracking vs. other chemistries (e.g., metallic catalysis or Fischer-Tropsch-type reactions) in petroleum formation remains controversial. Realistic modeling of isotope effects in chemical reaction networks involving thermal cracking might shed light on this problem, especially given the recent progress on measurements of intramolecular stable isotope distributions in organic compounds. Previously published models of thermal cracking in petroleum formation are incapable of predicting the intramolecular isotope patterns of products because they do not incorporate realistic precursors, elementary reactions and patterns of inheritance. In this study, we develop a kinetic Monte-Carlo (kMC) model to address this problem. We simulate thermal breakdown of different types of organic matter that is represented by molecular models. At the onset of each simulation, we initialize the model parent organic molecules with isotopic substitutions, and then subject them to free radical chain reactions in a many-step process. Every simulation captures a possible route of thermal degradation and tallies the numbers of each unique isotopologue of all product molecules at the end. Although this model produces data that contain information for all molecules and isotopic forms, in this study we focus on the proportions of many of the isotopologues of all of the C1-C7n-alkanes. We use two chemistry schemes that differ in complexity. The basic scheme (scheme A) includes only homolytic cleavage and capping of metastable radicals by hydrogen atoms. The more sophisticated scheme (scheme B) includes most reactions of importance in the free radical chain mechanism of thermal cracking. We find significant differences between the schemes A and B in predicted molecular and isotopic compositions of products. Scheme B is more consistent with natural data than scheme A, suggesting that full radical mechanisms should be considered in models of thermal cracking in natural hydrocarbon formation. Our model is further validated by reproducing results from hydrous pyrolysis experiments. For simulating natural petroleum formation, we found that the kMC model is acceptable at low maturity, but cannot match compositions and time estimates of mature gas formation, suggesting the influence of alternative mechanisms in late-stage, high-thermal-maturity catagenesis. Using our model, we provide mechanistic explanations for some of the existing observations, such as the evolution of intramolecular and intermolecular carbon isotope compositions with thermal maturity. Our study also makes predictions of intramolecular isotope compositions for higher-order alkanes (C4+) that have been little studied to date but present attractive targets for future measurements.

Steric Hindrance Favors σ Dimerization over π Dimerization for Julolidine Dicyanomethyl Radicals.

Metastable radicals exist in a steady-state equilibrium in solution with dimers, which can be either σ dimers or π dimers. Here, we show that steric hindrance at the para position causes julolidine-derived dicyanomethyl radicals to form σ dimers rather than π dimers, the opposite behavior as seen in other carbon-centered radicals, where steric hindrance typically favors pimerization. The change in dimerization mode can be attributed to weaker London dispersion forces and a decreased orbital overlap in the sterically hindered dicyanomethyl radical π dimers, while the bulky groups exert relatively little effect on the energy of the σ dimer.

Applications of Cold Atmospheric Pressure Plasma in Dentistry

Plasma is an electrically conducting medium that responds to electric and magnetic fields. It consists of large quantities of highly reactive species, such as ions, energetic electrons, exited atoms and molecules, ultraviolet photons, and metastable and active radicals. Non-thermal or cold plasmas are partially ionized gases whose electron temperatures usually exceed several tens of thousand degrees K, while the ions and neutrals have much lower temperatures. Due to the presence of reactive species at low temperature, the biological effects of non-thermal plasmas have been studied for application in the medical area with promising results. This review outlines the application of cold atmospheric pressure plasma (CAPP) in dentistry for the control of several pathogenic microorganisms, induction of anti-inflammatory, tissue repair effects and apoptosis of cancer cells, with low toxicity to healthy cells. Therefore, CAPP has potential to be applied in many areas of dentistry such as cariology, periodontology, endodontics and oral oncology.

Chemically stimulated luminescence from food proteins

AbstractBackground and objectivesProtein powders contain metastable radicals that are released upon hydration. The influence of protein source, free radical content, and hydration condition on luminescence was investigated.FindingsHydration of rice, soy, pea, and egg albumin generated a burst of luminescence in the absence of added enhancers/stimulates. Rice proteins were generated from 3 to 10 times more luminescence than other protein samples. Hydrating proteins with 25 mM H2O2 increased the luminescence intensities from all food proteins. Luminescence maximums occurred between 550–600 nm for rice protein, 500–600 nm for soy protein, and two peaks from egg albumin (500–550 nm and <450 nm). Carbon‐centered metastable radicals were most abundant in rice protein which contained between 3.87 × 1016–53.44 × 1016 spins per gram. Plotting intrinsic luminescence versus spins per gram (including all types of proteins examined) revealed an r‐square of 0.771.ConclusionsBased on peak shape, g‐value (2.0050 ± 0.0003) and power saturation (4–8 mW), metastable radicals in proteins were identified as carbon‐centered. Both intrinsic and added sources of oxidative stimuli strongly correlate with the generation of chemically stimulated luminescence from hydrated proteins.Significance and noveltyThese findings are the first to correlate the quantity of metastable radicals in rice proteins with intrinsic luminescence generated upon hydration and provide spectral characterization of luminescence from these proteins.

Where Does the Electron Go? Stable and Metastable Peptide Cation Radicals Formed by Electron Transfer.

Electron transfer to doubly and triply charged heptapeptide ions containing polar residues Arg, Lys, and Asp in combination with nonpolar Gly, Ala, and Pro or Leu generates stable and metastable charge-reduced ions, (M + 2H)+●, in addition to standard electron-transfer dissociation (ETD) fragment ions. The metastable (M + 2H)+● ions spontaneously dissociate upon resonant ejection from the linear ion trap, giving irregularly shaped peaks with offset m/z values. The fractions of stable and metastable (M + 2H)+● ions and their mass shifts depend on the presence of Pro-4 and Leu-4 residues in the peptides, with the Pro-4 sequences giving larger fractions of the stable ions while showing smaller mass shifts for the metastables. Conversion of the Asp and C-terminal carboxyl groups to methyl esters further lowers the charge-reduced ion stability. Collisional activation and photodissociation at 355 nm of mass-selected (M + 2H)+● results in different dissociations that give sequence specific MS3 spectra. With a single exception of charge-reduced (LKGLADR + 2H)+●, the MS3 spectra do not produce ETD sequence fragments of the c and z type. Hence, these (M + 2H)+● ions are covalent radicals, not ion-molecule complexes, undergoing dramatically different dissociations in the ground and excited electronic states. The increased stability of the Pro-4 containing (M + 2H)+● ions is attributed to radicals formed by opening of the Pro ring and undergoing further stabilization by hydrogen atom migrations. UV-VIS photodissociation action spectroscopy and time-dependent density functional theory calculations are used in a case in point study of the stable (LKGPADR + 2H)+● ion produced by ETD. In contrast to singly-reduced peptide ions, doubly reduced (M + 3H)+ ions are stable only when formed from the Pro-4 precursors and show all characteristics of even electron ions regarding no photon absorption at 355 nm or ion-molecule reactions, and exhibiting proton driven collision induced dissociations. Graphical Abstract ᅟ.

Supercrystallization of KCl from solution irradiated by soft X-rays

The X-rays influence on KCl crystallization in a saturated water solution has been investigated for the aim of comparing it with previously considered NaCl crystallization. The rate of crystallization has been measured in the drying drop in the solution activated by the irradiation. We have measured the influence of the irradiation time of the solution on the rates of KCl crystallization as well as the beginning of the crystallization processes on drying drops. For a longer irradiation time of the solution early crystallization in the drops occurs. A saturated water solution of KCl was irradiated with the diffractometer DRON-3M (Russian device) and this had a great influence on the two-step processes of crystallization. The ionization of the solution by soft X-rays can produce ions, metastable radicals in water, excited crystals' seeds and vacancies in growing crystals by Auger's effect. The X-rays generate a very fast crystallization in the drying drop.

Probing the Time Scale of FPOP (Fast Photochemical Oxidation of Proteins): Radical Reactions Extend Over Tens of Milliseconds.

Hydroxyl radical (⋅OH) labeling with mass spectrometry detection reports on protein conformations and interactions. Fast photochemical oxidation of proteins (FPOP) involves ⋅OH production via H2O2 photolysis by UV laser pulses inside a flow tube. The experiments are conducted in the presence of a scavenger (usually glutamine) that shortens the ⋅OH lifetime. The literature claims that FPOP takes place within 1 μs. This ultrafast time scale implies that FPOP should be immune to labeling-induced artifacts that may be encountered with other techniques. Surprisingly, the FPOP time scale has never been validated in direct kinetic measurements. Here we employ flash photolysis for probing oxidation processes under typical FPOP conditions. Bleaching of the reporter dye cyanine-5 (Cy5) served as readout of the time-dependent radical milieu. Surprisingly, Cy5 oxidation extends over tens of milliseconds. This time range is four orders of magnitude longer than expected from the FPOP literature. We demonstrate that the glutamine scavenger generates metastable secondary radicals in the FPOP solution, and that these radicals lengthen the time frame of Cy5 oxidation. Cy5 and similar dyes are widely used for monitoring the radical dose experienced by proteins in solution. The measured Cy5 kinetics thus strongly suggest that protein oxidation in FPOP extends over a much longer time window than previously thought (i.e., many milliseconds instead of one microsecond). The optical approach developed here should be suitable for assessing the performance of future FPOP-like techniques with improved temporal labeling characteristics. Graphical Abstract ᅟ.

Thermally Stimulated Luminescence in Powdered Soy Proteins

Heating powder isolated soy proteins (ISPs) in a N₂ environment produced thermally stimulated luminescence (TSL), in 2 major temperature regions, 50 to 250°C (region R1) and 250 to 350°C (region R2). In soy protein 7S fraction, strong TSL was detected in both regions with glow peak maximum (T(m)) at 150 ± 15°C and at 300 ± 10°C. Two additional satellite or shoulder peaks were detected from the ISP and 7S protein fraction within region R1 at T(m) = 90°C and T(m) = 210°C. The soy protein 11S fraction produced a broad, poorly defined TSL peak in the low-temperature region. Electron paramagnetic resonance spectroscopy data from the control ISP sample, deuterium sulfide-treated ISP, ISP stored in either N₂ or O₂, and defatted soy flour, indicated that the trapped radicals present in ISP is associated with the production of the primary TSL peak at 150 ± 15°C. Activation energies required to release the trapped charges (for luminescence to occur) are approximately 0.70, 0.78, 1.50, and 1.8 eV for TSL at Tm = 100, 150, 200, and 300°C, respectively. The reaction mechanism that leads to the release of the trapped charges for TSL to occur followed a mixed order kinetic, between 1.5 and 1.8. The frequency factor varied between 10⁷/s and 10¹⁷/s.

Effect of Sequestering Intrinsic Iron on the Electron Paramagnetic Resonance Signals in Powdered Soy Proteins

This investigation examined iron in powdered soy protein products using electron paramagnetic resonance (EPR) spectroscopy, and the effect that selectively binding free iron in isolated soy protein (ISP) had on the occurrence of metastable radicals in powdered soy proteins. EPR analyses of soybean defatted flour, commercial ISP and laboratory ISP samples revealed a peak at g = 4.3 characteristic of high-spin ferric iron in a rhombic-coordinated environment. Commercial ISP samples examined contained higher levels of the rhombic ferric iron than laboratory-prepared ISP samples. During the first 6 wk of storage the primary singlet EPR signal at g = 2.0049 in the commercial ISP samples approximately doubled, and the laboratory prepared samples increased by about 9-fold. The EPR signal was initially about 4-times higher in the freshly prepared commercial samples compared to the corresponding laboratory ISP. Laboratory ISP samples prepared with added deferoxamine to sequester endogenous iron exhibited a large increase in the high-spin ferric iron EPR signal at g = 4.3. ISP treated with deferoxamine also exhibited a multiple-line EPR signal at about g = 2.007, instead of the typical singlet signal at g = 2.0049. The power at which the signal amplitude was half-saturated also changed from about 1 mW in the control ISP to about 20 mW in the deferoxamine treated ISP. The multiple-line EPR spectrum from the ISP treated with deferoxamine increased during storage over a 6-wk period by about 6-fold. The observed changes in EPR line-shape, g-value, and power saturation with the deferoxamine treatment indicate that the primary free-radical signal in powdered ISP samples may be from stabilized tyrosine radicals with spin densities distributed over the aromatic ring.

Plasma interactions with aminoacid (l-alanine) as a basis of fundamental processes in plasma medicine

Plasma interactions with l-alaine have been studied as a basis of fundamental processes in plasma medicine. The plasma interactions with l-alaine have been examined for investigations of molecular degradations induced by direct exposures with Ar plasma and exposures with UV–VUV photons emitted from the Ar plasma via chemical bonding states analyses using X-ray photoelectron spectroscopy (XPS). The direct Ar-plasma exposure resulted in significant degradations of COOH group and CNH2 group. Separate experiments via irradiation with photons in UV and VUV regions from the Ar plasma showed that the molecular degradation via irradiation with photons in VUV region was much more significant than via irradiation with photons in UV region. These experiments have indicated that the causality of the molecular degradation of the l-alanine during the Ar plasma exposure is considered to be significant in the following order; ions > VUV photons > UV photons ∼ meta-stable radicals. Furthermore, the exposure with Ar–O2 mixture plasma resulted in insignificant change in the XPS C1s spectra for variation of the exposure time ranging from 30 s to 300 s, indicating that the surface etching process is much more considerable than the chemical degradation process.

Effect of Moisture, Lipids, and Select Amino Acid Blocking Agents on the Formation and Stability of Metastable Radicals in Powdered Soy Proteins

Incremental increases in the moisture content of powdered soy protein products from 4.4% to 13.4% produced an inverse effect on the ability of soy proteins to maintain metastable free radicals. The corresponding reduction in electron paramagnetic resonance signal was not due to dielectric loss in the range of moisture contents examined. Subsequent evaluations of various treatments were conducted after drying soy proteins with molecular sieve to a water activity below 0.085 in order to minimize the influence from variations in moisture. Isolated soy protein (ISP) samples, prepared with "defatted flour" that had been further extracted with chloroform/methanol (2: 1), had a 96% reduction in total lipids compared to the control ISP samples. The initial rate of radical accumulation in the "reduced-lipid" ISP for the first 3 wk was not significantly different from the initial rate of radical increases in the control ISP. After 3 wk, radical accumulation in the "reduced-lipid" ISP continued to increase, but at a rate that was less than the control. These findings indicate that the initial reactions contributing to the formation of metastable radicals in the powdered ISP are not strongly dependent on associated lipids. Blocking sulfhydryl groups during ISP preparation with N-ethylmaleimide did not significantly slow the rate of radical accumulation compared to the control ISP. Blocking arginine residues in ISP samples with phenylglyoxal caused an increased rate of radical accumulation for the first 4 wk. Levels of metastable radicals in powdered soy protein products typically range from 10 to 100 times greater than the free radicals in other food protein sources. This current research examines various compositional and treatment parameters that might be used to minimize the content of free radicals in foods containing soy proteins.

Metastable Radicals and Intrinsic Chemiluminescence from Soy Proteins

Chemiluminescence from various powdered food proteins were examined without the addition of any external source of free radicals or luminescent agents. In the solid-state, soy and whey proteins produced more intrinsic chemiluminescence than casein, sodium caseinate, or egg albumin. However, when these same food proteins were hydrated, intrinsic chemiluminescence from soy proteins was about 4- to 8-times greater than other source proteins. Quenching the alkyl-radicals in the powdered soy proteins with hydrogen sulfide reduced the typical electron paramagnetic resonance spectra from soy proteins below detectable levels, and reduced the chemiluminescence from the hydrated soy proteins by about 65%. Antioxidants also reduced chemiluminescence in hydrated soy proteins by about 50% to 92%, with ellagic acid being the most effective. The reduction in chemiluminescence from both quenching radicals in the solid state, and by the addition of antioxidants to aqueous mixtures, indicate that the chemiluminescence produced when soy proteins are hydrated is a free radical catalyzed event. Based on the production of chemiluminescence, the radicals from soy protein were largely released within 30 min of hydration at 23 °C. Elevating the hydration temperatures increased chemiluminescence by as much as 280% at 70 °C, and decreased the half-life of the light-emitting reaction by about 9-fold. Levels of metastable radicals in powdered soy proteins typically range from to 10 to 100 times greater than free radicals from other food protein sources. This research focuses on the types of reactions these radicals catalyze when soy proteins are hydrated, and the radicals suddenly become reactive. The findings suggest that a portion of the energy released from metastable radicals when powdered soy proteins are hydrated is involved in the generation of chemically-induced light.

Accumulation of Stark-decelerated NH molecules in a magnetic trap

Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH (a1Δ) radicals is produced and brought to a near standstill at the center of a quadrupole magnetic trap using a Stark decelerator. There, optical pumping of the metastable NH radicals to the X3Σ− ground state is performed by driving the spin-forbidden A3Π ← a1Δ transition, followed by spontaneous A → X emission. The resulting population in the various rotational levels of the ground state is monitored via laser induced fluorescence detection. A substantial fraction of the ground-state NH molecules stays confined in the several milliKelvin deep magnetic trap. The loading scheme allows one to increase the phase-space density of trapped molecules by accumulating packets from consecutive deceleration cycles in the trap. In the present experiment, accumulation of six packets is demonstrated to result in an overall increase of only slightly over a factor of two, limited by the trap-loss and reloading rates.

Kinetics of H 2–O 2–H 2O redox equilibria and formation of metastable H 2O 2 under low temperature hydrothermal conditions

Hydrothermal experiments were conducted to evaluate the kinetics of H 2(aq) oxidation in the homogeneous H 2–O 2–H 2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55–250 °C and 242–497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H 2(aq) and O 2(aq), are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H 2(aq)–O 2(aq) equilibrium become slower with decreasing temperature, allowing excess H 2(aq) to remain in solution. Sluggish reaction rates of H 2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H 2(aq) and O 2(aq), even at temperatures lower than 40 °C due to H 2(aq) persistence in the seawater/vent fluid mixtures. For these H 2–O 2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H 2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites. More importantly, when H 2(aq)–O 2(aq) disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H 2O 2(aq) synthesis appears to be enhanced under conditions of elevated H 2(aq)/O 2(aq) molar ratios that correspond to abundant H 2(aq) concentrations. Formation of metastable H 2O 2 is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe ++ by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.

Dependence of the Physical Properties of GaZnO/Polyethersulfoneon the Ar Gas Flow Rate During Sputtering Deposition Process

The effects of Ar gas flow rate during the sputtering deposition of Ga-doped ZnO thin films, which were grown on polyethersulfone substrates, on their physical properties were investigated. For measurements of X-ray diffraction, samples deposited under higher Ar gas flux showed the relevant ZnO (002) peak while samples deposited under lower Ar gas flux revealed ZnGa2O4 (311) secondary phase. The samples showed a quite strong dependency of the electrical resistivity on the Ar gas flux; i .e., samples deposited under higher Ar gas flux showed a lower resistivity, but samples deposited under lower Ar gas flux showed a quite high resistivity. This result was confirmed to originate from the insulating behavior of the unexpectedly formed ZnGa2O4 spinels. The formation of ZnGa2O4 spinels might result from the generation of metastable radicals during the sputtering process because supplying an unduly insufficient amount of Ar gas gives rise to a bombardment of metastable species from the target material.

Early-Stage Evolution of the EPR Spectrum of Crystalline Sucrose at Room Temperature after High-Dose X Irradiation

Abstract X irradiation of sucrose single crystals at room temperature leads to the production of stable radicals, which give rise to the dosimetric electron paramagnetic resonance (EPR) signal. In the first few hours after irradiation, however, the shape of the EPR spectrum changes drastically. Based on two-dimensional field-frequency electron nuclear double resonance (FF-ENDOR) measurements, we demonstrate that, after high-dose ( approximately 5 kGy) and high-dose-rate irradiation, several species with limited stability at room temperature are produced next to the stable radicals. For two of these species, the main characteristics could be determined. Analysis of the time evolution of the FF-ENDOR and room-temperature EPR spectra in the first few hours after irradiation leads to the conclusion that these meta-stable radicals mainly recombine into diamagnetic species, while transformation into stable radicals is at most a marginal process.

Electron Super-Rich Radicals. III. On the Peculiar Behavior of the Aminodihydroxymethyl Radical in the Gas Phase

In contrast to previously reported electron-super-rich trihydroxy-, triamino- and diaminohydroxymethyl radicals, the title aminodihydroxymethyl radical (1) generates a fraction of metastable species in the form of their deuterium isotopologues. The lifetimes of metastable radicals produced by femtosecond collisional electron transfer to aminodihydroxymethyl cations exceed 4 mus. The main fraction of 1 dissociates by fast loss of a hydroxyl hydrogen atom to form carbamic acid. Loss of an amino hydrogen atom is less facile and becomes <10% competitive at high internal energies or if the main dissociation is slowed down by deuterium isotope effects. RRKM calculations of unimolecular rate constants on a CCSD(T)/aug-cc-pVTZ potential energy surface gave a reasonably good fit for the competitive dissociations of 1 but not for the fraction of nondissociating radicals. The metastable species are attributed to excited electronic states which are predicted to have favorable Franck-Condon factors for being formed by collisional electron transfer.

Ignition of premixed hydrogen/air by heated counterflow under reduced and elevated pressures

The temperature of an inert jet required to ignite a counterflowing lean premixed hydrogen/air jet was experimentally determined over the pressure range of 0.6 to 7 atm and computationally simulated using detailed chemistry and transport. Results show that, compared to the homogeneous explosion limits, ignition takes place at higher temperatures and exhibits five limits over the pressure range investigated. The first and second ignition limits resemble the corresponding first and second homogeneous explosion limits, except they have steeper slopes in the pressure–temperature response, with the first limit being affected by the significant transport loss of the H radical and the second limit modified by the activation of the otherwise metastable HO2 radicals by the diffusively enriched H2. The third and fifth ignition limits are respectively manifestations of the low- and high-pressure responses of the third homogeneous explosion limit behavior, which is nevertheless punctuated by the fourth ignition limit characterized by the HO2–H reactions. Furthermore, the fourth ignition limit runs fairly parallel to the crossover temperature, but is shifted to lower temperatures. An explicit expression, 2k1={2k10/(k10+k11)}k9[M], was derived and found to describe well this limit as well as the extended second limit observed in previous flow reactor studies. It is further shown that, since transport effects are inherently important for the present premixed system because of the diffusive loss of H to the hot, inert side of the counterflow, the ignition temperature increases substantially with increasing strain rate at all pressures and that such a sensitivity can be moderated by doping the inert flow with a small amount of oxygen.