Minor Channel Research Articles

C–Cl bond fission dynamics and angular momentum recoupling in the 235 nm photodissociation of allyl chloride

The photodissociation dynamics of allyl chloride at 235 nm producing atomic Cl((2)P(J);J=1/2,3/2) fragments is investigated using a two-dimensional photofragment velocity ion imaging technique. Detection of the Cl((2)P(1/2)) and Cl((2)P(3/2)) products by [2+1] resonance enhanced multiphoton ionization shows that primary C-Cl bond fission of allyl chloride generates 66.8% Cl((2)P(3/2)) and 33.2% Cl((2)P(1/2)). The Cl((2)P(3/2)) fragments evidenced a bimodal translational energy distribution with a relative weight of low kinetic energy Cl((2)P(3/2))/high kinetic energy Cl((2)P(3/2)) of 0.097/0.903. The minor dissociation channel for C-Cl bond fission, producing low kinetic energy chlorine atoms, formed only chlorine atoms in the Cl((2)P(3/2)) spin-orbit state. The dominant C-Cl bond fission channel, attributed to an electronic predissociation that results in high kinetic energy Cl atoms, produced both Cl((2)P(1/2)) and Cl((2)P(3/2)) atomic fragments. The relative branching for this dissociation channel is Cl((2)P(1/2))/[Cl((2)P(1/2))+Cl((2)P(3/2))]=35.5%. The average fraction of available energy imparted into product recoil for the high kinetic energy products was found to be 59%, in qualitative agreement with that predicted by a rigid radical impulsive model. Both the spin-orbit ground and excited chlorine atom angular distributions were close to isotropic. We compare the observed Cl((2)P(1/2))/[Cl((2)P(1/2))+Cl((2)P(3/2))] ratio produced in the electronic predissociation channel of allyl chloride with a prior study of the chlorine atom spin-orbit states produced from HCl photodissociation, concluding that angular momentum recoupling in the exit channel at long interatomic distance determines the chlorine atom spin-orbit branching.

Participatory management: Would it be a turning point in the history of the Gezira scheme?

AbstractSince its establishment in 1925 decisions in the Sudan's Gezira irrigated scheme have been top‐down from the central headquarters of the Sudan Gezira Board (SGB). The irrigation system is divided into the upper system (US) that includes the dam, main, branch and major canals and the lower system that comprises the minor, tertiary and field channels. The Ministry of Irrigation and Water Resources (MOIWR) is responsible for the O&M of the US in addition to the maintenance of the Minor canals. The operation of the minor canals is handled by the SGB. The maintenance of the minor canals was given to the SGB in 1999, thus unifying O&M for the first time in 75 years. This opened the door for the O&M to be handed to tenants in a pilot project (PP) supported by the FAO during 2000–2002. Crop yields in the PP outyielded the neighboring blocks as well as the scheme average during the two years of the project. The PP is to be extended to at least one block in each of the 18 Gezira administrative groups. The PP established the base of legalization and institutional arrangements that govern the water users' activities and resulted in a great change in tenants' attitudes towards water management. Copyright © 2004 John Wiley & Sons, Ltd.

마젤란 분지의 백악기 심해저 하도 퇴적계의 퇴적상 및 진화

The Lago Sofia Conglomerate encased in the 2km thick hemipelagic mudstones and thinbedded turbidites of the Cretaceous Cerro Toro Formation, southern Chile, is a deposit of a gigantic submarine channel developed along a foredeep trough. It is hundreds of meters thick kilometers wide, and extends for more than 120km from north to south, representing one of the largest ancient submarine channels in the world. The channel deposits consist of four major facies, including stratified conglomerates (Facies A), massive or graded conglomerates (Facies B), normally graded conglomerates with intraformational megaclasts (Facies C), and thick-bedded massive sandstones (Facies D). Conglomerates of Facies A and B show laterally inclined stratification, foreset stratification, and hollow-fill structures, reminiscent of terrestrial fluvial deposits and are suggestive of highly competent gravelly turbidity currents. Facies C conglomerates are interpreted as deposits of composite or multiphase debris flows associated with preceding hyperconcentrated flows. Facies D sandstones indicate rapidly dissipating, sand-rich turbidity currents. The Lago Sofia Conglomerate occurs as isolated channel-fill bodies in the northern part of the study area, generally less than 100m thick, composed mainly of Facies C conglomerates and intercalated between much thicker fine-grained deposits. Paleocurrent data indicate sediment transport to the east and southeast. They are interpreted to represent tributaries of a larger submarine channel system, which joined to form a trunk channel to the south. The conglomerate in the southern part is more than 300 m thick, composed of subequal proportions of Facies A, B, and C conglomerates, and overlain by hundreds of m-thick turbidite sandstones (Facies D) with scarce intervening fine-grained deposits. It is interpreted as vertically stacked and interconnected channel bodies formed by a trunk channel confined along the axis of the foredeep trough. The channel bodies in the southern part are classified into 5 architectural elements on the basis of large-scale bed geometry and sedimentary facies: (1) stacked sheets, indicative of bedload deposition by turbidity currents and typical of broad gravel bars in terrestrial gravelly braided rivers, (2) laterally-inclined strata, suggestive of lateral accretion with respect to paleocurrent direction and related to spiral flows in curved channel segments around bars, (3) foreset strata, interpreted as the deposits of targe gravel dunes that have migrated downstream under quasi-steady turbidity currents, (4) hollow fills, which are filling thalwegs, minor channels, and local scours, and (5) mass-flow deposits of Facies C. The stacked sheets, laterally inclined strata, and hollow fills are laterally transitional to one another, reflecting juxtaposed geomorphic units of deep-sea channel systems. It is noticeable that the channel bodies in the southern part are of feet stacked toward the east, indicating eastward migration of the channel thalwegs. The laterally inclined strata also dip dominantly to the east. These features suggest that the trunk channel of the Lago Sofia submarine channel system gradually migrated eastward. The eastward channel migration is Interpreted to be due to tectonic forcing imposed by the subduction of an oceanic plate beneath the Andean Cordillera just to the west of the Lago Sofia submarine channel.

Hyperthermal Reactions of O+(4S3/2) with CD4 and CH4: Theory and Experiment

Theoretical and experimental methods are applied to the study of the reaction dynamics in hyperthermal collisions of O+ (4S3/2) with methane. Measurements of the absolute reaction cross sections for the interaction of O+ with CD4 and CH4 were obtained at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) technique. Product recoil velocity distributions, using the GIB time-of-flight (TOF) methods, were determined for several product ions at selected collision energies. The main reaction channel, charge transfer, proceeds via large impact parameter collisions. A number of minor channels, involving more intimate collisions, were also detected. Ab initio electronic structure calculations have been performed with different levels of theory and basis sets, including high-level coupled-cluster calculations to determine the energies of reaction intermediates and transition states for reaction. Several reaction paths on both quartet and doublet electronic states of (O·CH4)+ are found, and these provide a reasonable qualitative interpretation of the experiments. Although most of the products can be produced via spin-allowed pathways, the appearance of CH3+ at low energies suggests that intersystem crossing plays some role.

Competing Pathways in the 248 nm Photodissociation of Propionyl Chloride and the Barrier to Dissociation of the Propionyl Radical

Photofragment translational spectroscopy was used to study the photodissociation of propionyl chloride at 248 nm. The crossed laser-molecular beam experiment with VUV photoionization showed two primary dissociation channels, C−Cl bond fission and HCl elimination. Following cleavage of the C−Cl bond, unimolecular dissociation of the propionyl radical produced CH3CH2 and CO. The energy barrier to the CH3CH2CO → CH3CH2 + CO reaction was estimated to be in the range of 16.3 ± 1.5 kcal/mol by determining the internal energy distribution of surviving propionyl radicals. No other secondary dissociation channels were observed for the propionyl radical. The HCl elimination channel, previously reported only for the condensed phase of propionyl chloride, was observed as the minor primary dissociation channel in the gas phase. The cofragment to the HCl elimination, CH3CHCO or CH2CH2CO, underwent secondary dissociation to produce CO and CH2CH2 with a significant amount of energy partitioned into translational motion.

An ab initio investigation on the zinc-blende MnAs nanocrystallite

Among a new class of spintronics materials, nanoscale dots of the zinc-blende (ZB) MnAs have recently been synthesized on the GaAs(001) surface. We apply an ab initio pseudopotential method with the generalized gradient approximation to study magnetic and structural stability for the Mn-centered ZB MnAs4Mn12As24 cluster, which is relaxed both atomically and electronically. The ferromagnetic cluster is shown to have characteristics of a half metal with a large magnetic moment 4μB per Mn atom. The Mn-As bond length is slightly longer than the one in the bulk, in accord with experimental observation. The “semiconducting” gap 1.83 eV is opened up in the minority channel, due to the large bonding-antibonding splitting from the p-d hybridization, similar to the ZB bulk MnAs and Ga1−xMnxAs. We also find an antiferromagnetic coupling in this system such that the moment of the central Mn atom is flipped with respect to those of the second-shell Mn atoms.

H and D release in approximately 243.1 nm photolysis of vibrationally excited 3nu1, 4nu1, and 4nuCD overtones of propyne-d3.

The photofragmentation of propyne-d(3), D(3)C-C[Triple Bond]C-H, following approximately 243.1 nm photodissociation of rovibrationally excited molecules promoted to the second (3nu(1)) and third (4nu(1)) acetylenic C-H overtone and to the third (4nu(CD)) methyl overtone has been investigated. The resulting H and D photoproducts were detected via (2+1) resonantly enhanced multiphoton ionization. The measured room-temperature photoacoustic and jet-cooled action spectra allowed derivation of the molecular parameters of the C-H overtones and the Doppler profiles revealed the translational energies associated with the H(D) photofragments and the H to D branching ratios. Propensities toward the latter were encountered, while the translational energy disposal in both photofragments was essentially identical for a given preexcitation. This behavior agrees with that found for the almost isoenergetic 193.3 nm photolysis of propyne [Qadiri et al., J. Chem. Phys. 119, 12842 (2003)], but contradicts previous findings. The bond fission of C-H and C-D is preceded by internal conversion to, and isomerization on, the ground-state potential energy surface (PES), followed by extensive intramolecular vibrational redistribution. For molecules preexcited to 3nu(1) and 4nu(1) an additional minor channel opens, where elimination of H occurs directly on the accessed excited PES, while that of D on the ground state.

Defect Enhanced Carbon Monoxide Oxidation at Elevated Oxygen Pressures on a Pt/Al2O3 Thin Film

Carbon monoxide oxidation has been studied, using in situ soft X-ray techniques and ultrahigh vacuum (UHV) temperature-programmed reaction spectroscopy (TPRS), on a well-characterized supported platinum thin film over the100-1000 K temperature range and in pressures ranging from UHV to 0.01 Torr. In high oxygen pressures the reaction is enhanced by oxidation at defect sites. Temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) studies of preadsorbed CO coverages heated in pressures of flowing oxygen, ranging from I x 10 - 5 to I x 10 - 2 Torr, have been performed. Unlike with the Pt(111) surface, the onset temperature for oxidation of a saturated coverage of CO on the platinum thin film decreases dramatically with increasing oxygen pressure. The CO oxidation onset temperature decreased from 340 K in 1 x 10 - 5 Torr of flowing oxygen to 230 K in 1 x 10 - 2 Torr of flowing oxygen. Therefore, oxidation is not limited by CO desorption for high CO coverages, and reactions at defect sites control the rate of CO oxidation at high oxygen pressures. Isothermal oxidation experiments in 0.002 Torr of flowing oxygen have yielded an activation energy of 8.5 kcal/mol. TPRS of coadsorbed carbon monoxide and oxygen has indicated a primary oxidation channel at 350 K corresponding to the oxidation of CO on Pt( 111) terrace sites and minor oxidation channels above 400 K. The thin Pt/Al 2 O 3 film has been characterized using both chemical and structural methods. CO and O 2 temperature-programmed desorption (TPD) experiments indicate the Pt film surface is similar to Pt(111), but with a higher defect site concentration. Plane-view and cross-section transmission electron microscopy (TEM) experiments have confirmed a 100 A Pt film consisting of ∼450 A, well-ordered grains, as indicated by local area electron diffraction patterns, with the (111) orientation parallel to the substrate.

Photodissociation of benzene under collision-free conditions: an ab initio/Rice-Ramsperger-Kassel-Marcus study.

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of benzene at various wavelengths upon absorption of one or two UV photons followed by internal conversion into the ground electronic state. Reaction pathways leading to various decomposition products have been mapped out at the G2M level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for C(6)H(5)+H, C(6)H(4)+H(2), C(4)H(4)+C(2)H(2), C(4)H(2)+C(2)H(4), C(3)H(3)+C(3)H(3), C(5)H(3)+CH(3), and C(4)H(3)+C(2)H(3) have been calculated subsequently using both numerical integration of kinetic master equations and the steady-state approach. The results show that upon absorption of a 248 nm photon dissociation is too slow to be observable in molecular beam experiments. In photodissociation at 193 nm, the dominant dissociation channel is H atom elimination (99.6%) and the minor reaction channel is H(2) elimination, with the branching ratio of only 0.4%. The calculated lifetime of benzene at 193 nm is about 11 micros, in excellent agreement with the experimental value of 10 micros. At 157 nm, the H loss remains the dominant channel but its branching ratio decreases to 97.5%, while that for H(2) elimination increases to 2.1%. The other channels leading to C(3)H(3)+C(3)H(3), C(5)H(3)+CH(3), C(4)H(4)+C(2)H(2), and C(4)H(3)+C(2)H(3) play insignificant role but might be observed. For photodissociation upon absorption of two UV photons occurring through the neutral "hot" benzene mechanism excluding dissociative ionization, we predict that the C(6)H(5)+H channel should be less dominant, while the contribution of C(6)H(4)+H(2) and the C(3)H(3)+C(3)H(3), CH(3)+C(5)H(3), and C(4)H(3)+C(2)H(3) radical channels should significantly increase.

Three-Dimensional Numerical Modeling of Deltaic Depositional Sequences 1: Influence of the Rate and Magnitude of Sea-Level Change

ABSTRACT A three-dimensional numerical model of deltaic deposition is used to investigate the influence of sea-level changes on delta development and sequence variability. Results illustrate the three-dimensional morphology of key stratal surfaces and architecture of stratal units (systems tracts) and highlight the importance of the rate and magnitude of sea-level change in controlling the evolution of deltaic depositional sequences. High rates of sea-level fall lead to the development of a limited number of major incised channels that focus sediment supply to a few elongate, finger-like forced regressive lobes separated by large areas of nondeposition. In contrast, low rates of sea-level fall cause only minor channel incision, which occurs late during sea-level fall. As a result, sediment is supplied more uniformly to the delta front, leading to an attached, laterally continuous forced regressive apron. During lowstand and subsequent sea-level rise, the delta morphology and internal geometry are strongly controlled by the rate of rise. High rates lead to: i) poorly developed lowstand wedges that are drowned early, ii) high-magnitude transgressions, and iii) the late development of maximum flooding surfaces. The stratigraphy developed during sea-level rise is also strongly influenced by the incised-valley system created during the preceding sea-level fall. If deep, major valleys developed that captured most of the sediment supply, the resultant stratigraphy has well developed lowstand wedges that are flooded relatively late during sea-level rise. Even within a single delta, systems tracts and key stratal surfaces show three-dimensional variability and two-dimensional sections often lack significant elements of the stratigraphy. As a result, analysis of two-dimensional sections can often lead to miscorrelation and erroneous interpretations of the controlling processes.

Quantification of hydroxycarbonyls from OH-isoprene reactions.

Hydroxycarbonyls arising from OH-initiated reactions of isoprene have been quantified by the technique of a flow reactor coupled to proton-transfer reaction mass spectrometry (PTR-MS) detection. The yields of C5- and C4-hydroxycarbonyls are (19.3 +/- 6.1)% and (3.3 +/- 1.6)%, respectively, measured at a flow tube pressure of about 100 Torr and at a temperature of 298 +/- 2 K. A yield of (8.4 +/- 2.4)% is obtained for the unsaturated carbonyl C5H8O, confirming that internal OH addition represents the minor channel in the initial OH-isoprene reaction. The results show that those carbonyl compounds account for the most previously unquantified carbon, enabling the isoprene carbon closure. The study also reveals novel aspects of the delta-hydroxyalkoxy radical degradation mechanism, which is essential for modeling tropospheric O3 formation. In addition, this work demonstrates the application of PTR-MS for quantification of products of hydrocarbon reactions, which should have profound impacts on elucidation of the chemistry of atmospheric anthropogenic and biogenic hydrocarbons.

Reaction of Cu+ with dimethoxyethane: competition between association and multiple dissociation channels.

The reaction of Cu+ with dimethoxyethane (DXE) is studied using kinetic-energy dependent guided ion beam mass spectrometry. The bimolecular reaction forms an associative Cu(+)(DXE) complex that is long-lived and dissociates into several competitive channels: C4H9O2(+)+CuH, Cu(+)(C3H6O)+CH3OH, back to reactants, and other minor channels. The kinetic-energy dependences of the cross sections for the three largest product channels are interpreted with several different models (including rigorous phase space theory) to yield 0 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, Doppler broadening, and dissociation lifetimes. These values are compared with bond energies obtained from collision-induced dissociation (CID) studies of the Cu(+)(DXE) complex and found to be self-consistent. Although all models provide reasonable thermochemistry, phase space theory reproduces the details of the cross sections most accurately. We also examine the dynamics of this reaction using time-of-flight methods and a retarding potential analysis. This provides additional insight into the unimolecular decay of the long-lived Cu(+)(DXE) association complex. Comparison of results from this study with those from the complementary CID study, thus forming the same energized Cu(+)(DXE) complex in two distinct ways, allows an assessment of the models used to interpret CID thresholds.

Competition between C−C and C−H Activation in Reactions of Neutral Yttrium Atoms with Cyclopropane and Propene

Branching ratios between C−C and C−H bond activation were measured for reactions of ground-state Y (a2D, s2d1) atoms with two C3H6 isomers (cyclopropane and propene) in crossed molecular beams. For both isomers, C−C bond activation led to formation of YCH2 + C2H4, whereas C−H activation led to YC3H4 + H2 and YH2 + C3H4. The angular and velocity distributions for all three product channels and for nonreactive collisions were measured at several collision energies (Ecoll). For Y + cyclopropane, the branching ratio for YCH2 + C2H4 increased relative to YC3H4 + H2 with increasing Ecoll, this C−C activation channel becoming dominant at Ecoll ≥ 19 kcal/mol. For the propene reaction, φYCH2/φYC3H4 also increased with Ecoll, reaching 0.75:1.00 at Ecoll = 28.8 kcal/mol. For both C3H6 reactants, formation of YH2 + C3H4 was observed as a minor channel at the highest collision energies. Experimental results and Rice−Ramsperger−Kassel−Marcus (RRKM) modeling indicate that for propene reactions all three channels involve initial formation of π-association complexes that undergo insertion into one of the sp3-hybridized β-C−H bonds in the methyl substituent. Decay of the yttrium allyl hydride intermediate by β-H migration leads to the two C−H activation products, YC3H4 + H2 and YH2 + C3H4. We propose that the YCH2 + C2H4 channel involves reverse β-H migration forming the same strongly bound metallacyclobutane intermediate formed in the Y + cyclopropane reaction.

Late Wisconsinan Sedimentation in the Québec City Region: Evidence for Energetic Subaqueous Fan Deposition During Initial Deglaciation

In the St. Lawrence Valley just west of Québec City, initial deglaciation was accompanied by an energetic northward discharge of meltwater into a body of water, possibly subglacial, that was present in the axis of the valley. At Pointe Saint-Nicolas, a thick (> 35 m) ice-proximal sandy subaqueous fan was deposited during this event. The subaqueous fan is composed primarily of northwest-dipping turbidite sheets, with minor hyperconcentrated underflow channel fills and gravel-outwash deposits. By 11 200 14C BP, subaqueous fan sedimentation had stopped, and massive to rhythmically interbedded glaciomarine muds were being deposited. Introduction of muds into the basin promoted subaqueous slope failure and debris flowing locally. Between 10 950 and 10 800 14C BP, the Laurentide Ice Sheet readvanced southward onto the south shore, and deposited the subglacial St. Nicolas Till. With continued isostatic rebound and associated forced regression, tidal currents began to rework emergent coastal zones, and fluvial systems started to incise their valleys.

The ground and excited state potential energy surfaces of nitromethane related to its dissociation dynamics after excitation at 193 nm

The relevant low-lying singlet and triplet potential energy surfaces in the photolysis of nitromethane have been studied by using the multistate extension of the multiconfigurational second-order perturbation theory in conjunction with large atomic natural orbital-type basis sets. The proposed mechanism for the photolytic decomposition of CH3NO2 provides a consistent and reinterpreted picture of the available experimental results. Two reaction paths are found in the photolysis of nitromethane after excitation at 193 nm: (1) Major Channel, CH3NO2(1A′)+hν(193 nm)→CH3NO2(2A″)→ lim ICCH3NO2(2A′)→CH3(1A1′)+NO2(1 2B1)→ lim −hν′ICCH3(1A1′)+NO2(1 2A1)→ lim 193 nmhνCH3(1A1′)+NO(A 2Σ+)+αO(3P)+βO(1D). (2) Minor Channel, CH3NO2(1A′)+hν(193 nm)→CH3NO2(2A″)→CH3(1A1′)+NO2(1 2A2)→CH3(1A1′)+NO(X 2Π)+αO(3P)+βO(1D), being α and β fractional numbers. No ionic species are found in any dissociation path. Additionally, the respective low-lying Rydberg states of nitromethane and nitrogen dioxide have been studied too.

Dual-level direct dynamics studies for the reactions of CH3OCH3 and CF3OCH3 with the OH radical

The reactions of CH3OCH3+OH (R1) and CF3OCH3+OH (R2) via two hydrogen abstraction channels are investigated theoretically using the dual-level direct dynamics approach. The minimum energy path calculation is carried out at the MP2/6-311G(d,p) level, and energetic information is further refined by the G3 theory. For each reaction hydrogen abstraction is favored for the out-of-plane hydrogen, while the abstraction from the in-plane hydrogen is a minor channel. Hydrogen-bonded complexes are present on the reactants and products sides of the primary channel, indicating that the reactions may proceed via an indirect mechanism. By means of variational transition state theory with interpolated single-point energies method the dynamic results of all channels are obtained, and the small-curvature tunneling is included. The total rate constants calculated from the sum of the individual rate constants are in good agreement with the experimental data and are fitted to be k1=3.33×10−20 T2.91 exp(−409.7/T) and k2=1.23×10−24 T3.93exp(−188.2/T) cm3 molecule−1 s−1 over the temperature range 230–2000 K. The calculation indicates the CH3OCH3+OH reaction may proceed much easier than the CF3OCH3+OH reaction and fluorine substitution decreases the reactivity of the C–H bond.

A proton-shuttle mechanism mediated by the porphyrin in benzene hydroxylation by cytochrome p450 enzymes.

Benzene hydroxylation is a fundamental process in chemical catalysis. In nature, this reaction is catalyzed by the enzyme cytochrome P450 via oxygen transfer in a still debated mechanism of considerable complexity. The paper uses hybrid density functional calculations to elucidate the mechanisms by which benzene is converted to phenol, benzene oxide, and ketone, by the active species of the enzyme, the high-valent iron-oxo porphyrin species. The effects of the protein polarity and hydrogen-bonding donation to the active species are mimicked, as before (Ogliaro, F.; Cohen, S.; de Visser, S. P.; Shaik, S. J. Am. Chem. Soc. 2000, 122, 12892-12893). It is verified that the reaction does not proceed either by hydrogen abstraction or by initial electron transfer (Ortiz de Montellano, P. R. In Cytochrome P450: Structure, Mechanism and Biochemistry, 2nd ed.; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York, 1995; Chapter 8, pp 245-303). In accord with the latest experimental conclusions, the theoretical calculations show that the reactivity is an interplay of electrophilic and radicalar pathways, which involve an initial attack on the pi-system of the benzene to produce sigma-complexes (Korzekwa, K. R.; Swinney, D. C.; Trager, W. T. Biochemistry 1989, 28, 9019-9027). The dominant reaction channel is electrophilic and proceeds via the cationic sigma-complex,( 2)3, that involves an internal ion pair made from a cationic benzene moiety and an anionic iron porphyrin. The minor channel proceeds by intermediacy of the radical sigma-complex, (2)2, in which the benzene moiety is radicalar and the iron-porphyrin moiety is neutral. Ring closure in these intermediates produces the benzene oxide product ((2)4), which does not rearrange to phenol ((2)7) or cyclohexenone ((2)6). While such a rearrangement can occur post-enzymatically under physiological conditions by acid catalysis, the computations reveal a novel mechanism whereby the active species of the enzyme catalyzes directly the production of phenol and cyclohexenone. This enzymatic mechanism involves proton shuttles mediated by the porphyrin ring through the N-protonated intermediate, (2)5, which relays the proton either to the oxygen atom to form phenol ((2)7) or to the ortho-carbon atom to produce cyclohexenone product ((2)6). The formation of the phenol via this proton-shuttle mechanism will be competitive with the nonenzymatic conversion of benzene oxide to phenol by external acid catalysis. With the assumption that (2)5 is not fully thermalized, this novel mechanism would account also for the observation that there is a partial skeletal retention of the original hydrogen of the activated C-H bond, due to migration of the hydrogen from the site of hydroxylation to the adjacent carbon (so-called "NIH shift" (Jerina, D. M.; Daly, J. W. Science 1974, 185, 573-582)). Thus, in general, the computationally discovered mechanism of a porphyrin proton shuttle suggests thatthere is an enzymatic pathway that converts benzene directly to a phenol and ketone, in addition to nonenzymatic production of these species by conversion of arene oxide to phenol and ketone. The potential generality of protonated porphyrin intermediates in P450 chemistry is discussed in the light of the H/D exchange observed during some olefin epoxidation reactions (Groves, J. T.; Avaria-Neisser, G. E.; Fish, K. M.; Imachi, M.; Kuczkowski, R. J. Am. Chem. Soc. 1986, 108, 3837-3838) and the general observation of heme alkylation products (Kunze, K. L.; Mangold, B. L. K.; Wheeler, C.; Beilan, H. S.; Ortiz de Montellano, P. R. J. Biol. Chem. 1983, 258, 4202-4207). The competition, similarities, and differences between benzene oxidation viz. olefin epoxidation and alkanyl C-H hydroxylation are discussed, and comparison is made with relevant experimental and computational data. The dominance of low-spin reactivity in benzene hydroxylation viz. two-state reactivity (Shaik, S.; de Visser, S. P.; Ogliaro, F.; Schwarz, H.; Schröder, D. Curr. Opin. Chem. Biol. 2002, 6, 556-567) in olefin epoxidation and alkane hydroxylation is traced to the loss of benzene resonance energy during the bond activation step.

Kinetics of the C3H7O2 + NO Reaction: Temperature Dependence of the Overall Rate Constant and the i-C3H7ONO2 Branching Channel

The temperature dependence of the overall rate constant for the C3H7O2 + NO reaction and the rate constant for the minor branching channel resulting in the production of i-C3H7ONO2 have been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. The temperature dependence of the overall rate constant for the C3H7O2 + NO reaction was investigated between 298 and 213 K at 100 Torr pressure, and the data were fit by the following Arrhenius expression (with 2 standard deviation error limits indicated): × 10-12 exp[(268 ± 56)/T] cm3 molecule-1 s-1. This expression agrees well with previous isomer-specific measurements of the n-C3H7ONO2 and i-C3H7O2 + NO rate constants made at lower pressures. The temperature dependence of the rate constant for the minor reaction channel i-C3H7O2 + NO → i-C3H7ONO2 was investigated between 298 and 213 K at 100 Torr pressure. The following Arrhenius expression was determined for the minor channel: × 10-16 exp[(1380 ± 230)/T] cm3 molecule-1 s-1. The Arrhenius expressions for the overall rate and the i-C3H7ONO2 producing channel indicate a branching ratio of about 0.006 at 298 K and 0.020 at 213 K at 100 Torr pressure, which is in good agreement with the predictions of a recently revised empirical model for alkyl nitrate branching ratios.

Simultaneous spectroscopic detection of adsorbed and gas-phase species during atmospherically relevant heterogeneous reactions

The application of a recently developed ATR-IR flow cell coupled with an FTIR gas phase detection system to simultaneously monitor the gas-phase and surface composition during the well-known reaction of nitrogen oxide gases NO2 and N2O5 with NaCl powder is reported. Infrared absorption band assignments for the major products are presented for species in the gas and solid phase, and correlations between the appearance of gas phase and surface species are used to follow the reactions qualtitatively in real time. The reaction of NO2 with NaCl produces the expected surface NO3− and gaseous ClNO products, but is also observed to produce in a minor reaction channel a second surface nitrate species correlated with surface water loss as a result of heterogeneous hydrolysis. The reaction of N2O5 similarly produces ClNO2 in the gas phase and a surface nitrate in one reaction channel, with simultaneous production of a significant amount of gas-phase HNO3 in a second heterogeneous hydrolysis reaction channel. These observations confirm the usefullness of the combined surface and gas phase analytical methodology in following the mechanistic and kinetic detail of atmospherically relevant reactions.

Ab initio mechanism and thermal rate constants for the reaction of atomic H with Ge2H6

The reaction of digermane Ge2H6 with atomic H has been studied theoretically. The detailed mechanism has been revealed for the first time. This reaction involves not only abstraction but also substitution. The calculation shows that there are two transition states for the substitution reaction: (1) frontside attack of the Ge–Ge bond by the hydrogen atom and (2) backside attack of the GeH3 group by the hydrogen atom along the Ge–Ge axis, forming a transition state structure with C3v symmetry. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction path for each channel are discussed and compared. On the basis of the ab initio data, the rate constants of each channel have been deduced by canonical variational transition state theory (CVT) with the small-curvature tunneling (SCT) correction method over the temperature range of 200–3000 K. The theoretical results have been compared with available experimental data. The kinetics calculations show that the variational effect is small for all the channels and in the lower temperature range, the small-curvature tunneling effect is important for the abstraction channel and the substitution channel with backside attack. At lower temperatures, the major product channel is direct hydrogen abstraction leading to Ge2H5 and H2, whereas the substitution reaction with frontside attack becomes the dominant channel at higher temperatures. The substitution reaction with backside attack is a minor channel over the whole studied temperature range.