Julie K. Pearce

Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

The scattering dynamics leading to the formation of Cl (2P(3/2)) and Cl* (2P(1/2)) products of the CH(3)+HCl reaction (at a mean collision energy =22.3 kcal mol(-1)) and the Cl (2P(3/2)) products of the CD(3)+HCl reaction (at =19.4 kcal mol(-1)) have been investigated by using photodissociation of CH(3)I and CD(3)I as sources of translationally hot methyl radicals and velocity map imaging of the Cl atom products. Image analysis with a Legendre moment fitting procedure demonstrates that, in all three reactions, the Cl/Cl* products are mostly forward scattered with respect to the HCl in the center-of-mass (c.m.) frame but with a backward scattered component. The distributions of the fraction of the available energy released as translation peak at f(t)=0.31-0.33 for all the reactions, with average values that lie in the range =0.42-0.47. The detailed analysis indicates the importance of collision energy in facilitating the nonadiabatic transitions that lead to Cl* production. The similarities between the c.m.-frame scattering and kinetic energy release distributions for Cl and Cl* channels suggest that the nonadiabatic transitions to a low-lying excited potential energy surface (PES) correlating to Cl* products occur after passage through the transition state region on the ground-state PES. Branching fractions for Cl* are determined to be 0.14+/-0.02 for the CH(3)+HCl reaction and 0.20+/-0.03 for the CD(3)+HCl reaction. The difference cannot be accounted for by changes in collision energy, mass effects, or vibrational excitation of the photolytically generated methyl radical reagents and instead suggests that the low-frequency bending modes of the CD(3)H or CH(4) coproduct are important mediators of the nonadiabatic couplings occurring in this reaction system.

Reactive transport in porous media for CO2 sequestration

Abstract Injection of CO 2 subsurface may lead to chemical reactivity of rock where CO 2 is dissolved in groundwater. This process can modify pore networks to increase or decrease porosity through mineral dissolution and precipitation. A lattice Boltzmann (LB) based computational model study on the pore scale reactive transport in three dimensional heterogeneous porous media (sandstone consisting of both reactive and non-reactive minerals) is described. This study examines how fluid transport in porous materials subject to reactive conditions is affected by unsteady state local reactions and unstable dissolution fronts. The reaction of a calcite cemented core sub-plug from the Hutton Sandstone of the Surat Basin, Australia, is used as a study case. In particular, the work studies the interaction of acidic fluid (an aqueous solution with an elevated concentration of carbonic acid) with reactive (e.g. calcite) and assumed non-reactive (e.g. quartz) mineral surfaces, mineral dissolution and mass transfer, and resultant porosity change. The proposed model is implemented in our custom LBM code and suitable for studies of multiple mineral reactions with disparate reaction rates. A model for carbonic acid reaction with calcite cemented sandstone in the CO 2 -water-rock system is verified through laboratory experimental data including micro-CT characterization before and after core reaction at reservoir conditions. The experimentally validated model shows: (1) the dissolution of calcite cement forms conductive channels at the pore scale, and enables the generation of pore throats and connectivity; (2) the model is able to simulate the reaction process until the reaction equilibrium status is achieved (around 1440 days); (3) calcite constituting a volume of around 9.6% of the whole core volume is dissolved and porosity is consequently increased from 1.1% to 10.7% on reaching equilibrium; (4) more than a third of the calcite (constituting 7.4% of the total core volume) is unaffected, which suggests that this calcite is not connected with open pores (at the resolution of the model) that the acidic fluid can access. The model enables exploration of the porosity change in systems as they react, which has applications for analysis of the induced permeability change at the macroscale.

SO2 and O2 co-injection with potential carbon storage target sandstone from a fresh-water aquifer

Oceanic anoxic events (OAEs) were a frequent occurrence in the Cretaceous greenhouse ocean. Based on a variety of paleoredox indicators, euxinic water column conditions are commonly invoked for these OAEs. However, in a high resolution study of OAE3 deep sea sediments [1], revised paleoredox indicators suggest that euxinic conditions fluctuated with anoxic ferruginous conditions on orbital timescales. Building upon this, we here present new data for a continental shelf setting at Tarfaya, Morocco, that spans a period prior to, and during, the onset of OAE2. We again find strong evidence for orbital transitions from euxinic to ferruginous conditions. The presence of this distinct cyclicity during OAE2 and OAE3 in shallow and deep water settings, coupled with its occurrence on the anoxic shelf prior to the global onset of anoxia, suggests that these fluctuations were a fundamental feature of anoxia in the Cretaceous ocean. The observed redox cyclicity has major implications for the cycling of phosphorus, and hence the maintenance and longevity of OAEs. However, despite this significance, controls on the observed redox cyclicity are essentially unknown. Here, we utilize S isotope measurements (pyrite S and carbonate-associated S) from the deep sea and shelf settings to model oceanic sulphate concentrations across the redox transitions. Perhaps surprisingly, we find no evidence to suggest that ferruginous conditions arose due to extensive drawdown of seawater sulphate (as pyrite-S and organic-S) under euxinic conditions. Instead, S isotope systematics in the deep sea imply increased sulphate concentrations during ferruginous intervals. Based on these observations and other major element data, we infer that the redox cyclicity instead relates to orbitally-paced fluctuations in continental hydrology and weathering, linking the redox state of the global ocean to climate-driven processes on land. [1] Marz et al (2008) GCA, 72, 3703-3717.

How do the structures of polyatomic molecules affect their reaction dynamics

The reactions of Cl atoms with three organic ethers, dimethyl ether (CH3OCH3), oxirane (c-C2H2O) and oxetane (c-C3H6O) provide an opportunity to study the effects of different molecular structural motifs on the chemical dynamics. The rotational excitation of the nascent HCl reaction product has been measured for all three reactions, and the results are compared with direct dynamics trajectory calculations that allow mechanisms to be visualized with the aid of trajectory animations. Reaction of oxirane, a strained, three-membered ring compound, gives rise to HCl that is markedly rotationally cooler (Trot = 168 ? 7?K) than the products of the two other reactions (Trot = 418 ? 25?K for Cl + dimethyl ether and 399 ? 23?K for Cl + oxetane). Possible reasons are discussed in terms of the reorientational dynamics of the polar HCl and organic radical products in the post-transition state regions of the reaction potential energy surfaces.

Imaging the nonadiabatic dynamics of the CH3 + HCl reaction

LAB-frame velocity distributions of Cl-atoms produced in the photoinitiated reaction of CH(3) radicals with HCl have been measured for both the ground Cl ((2)P(3/2)) and excited Cl* ((2)P(1/2)) spin-orbit states using a DC slice velocity-map ion imaging technique. The similarity of these distributions, as well as the average internal excitation of methane co-products for both Cl and Cl* pathways, suggest that all the reactive flux proceeds through the same transition state on the ground potential energy surface (PES) and that the couplings which promote nonadiabatic transitions to the excited PES correlating to Cl* occur later in the exit channel, beyond the TS region. The nature of these couplings is discussed in light of initial vibrational excitation of CH(3) radicals as well as previously reported nonadiabatic reactivity in other polyatomic molecule reactions. Furthermore, the scattering of the reaction products, derived using the photoloc method, suggests that at the high collision energy of our experiment (E(coll) = 22.3 kcal mol(-1)), large impact parameter collisions are favoured with a reduced kinematic constraint on the internal excitation of the methane co-product.

Nonadiabatic dynamics in the CH3+HCl→CH4+Cl(PJ2) reaction

Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl({sup 2}P{sub 3/2}) and Cl{sup *}({sup 2}P{sub 1/2}) products. Reaction was initiated by photodissociation of CH{sub 3}I at 266 nm within a single expansion of a dilute mixture of CH{sub 3}I and HCl in argon, giving a mean collision energy of 7800 cm{sup -1} in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl({sup 2}P{sub J}) atoms formed in the J=(1/2) level at this collision energy was 0.150{+-}0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH{sub 4}+Cl({sup 2}P{sub 3/2}) products.Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl(2P(3/2)) and Cl*(2P(1/2)) products. Reaction was initiated by photodissociation of CH(3)I at 266 nm within a single expansion of a dilute mixture of CH(3)I and HCl in argon, giving a mean collision energy of 7800 cm(-1) in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl(2P(J)) atoms formed in the J=1/2 level at this collision energy was 0.150+/-0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH(4)+Cl(2P(3/2)) products.

Nonadiabatic dynamics in the CH{sub 3}+HCl{yields}CH{sub 4}+Cl({sup 2}P{sub J}) reaction

Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl({sup 2}P{sub 3/2}) and Cl{sup *}({sup 2}P{sub 1/2}) products. Reaction was initiated by photodissociation of CH{sub 3}I at 266 nm within a single expansion of a dilute mixture of CH{sub 3}I and HCl in argon, giving a mean collision energy of 7800 cm{sup -1} in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl({sup 2}P{sub J}) atoms formed in the J=(1/2) level at this collision energy was 0.150{+-}0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH{sub 4}+Cl({sup 2}P{sub 3/2}) products.Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl(2P(3/2)) and Cl*(2P(1/2)) products. Reaction was initiated by photodissociation of CH(3)I at 266 nm within a single expansion of a dilute mixture of CH(3)I and HCl in argon, giving a mean collision energy of 7800 cm(-1) in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl(2P(J)) atoms formed in the J=1/2 level at this collision energy was 0.150+/-0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH(4)+Cl(2P(3/2)) products.

Non-adiabatic dynamics in the CH 3 + HCl → CH 4 + Cl( 2 P J ) reaction

Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl({sup 2}P{sub 3/2}) and Cl{sup *}({sup 2}P{sub 1/2}) products. Reaction was initiated by photodissociation of CH{sub 3}I at 266 nm within a single expansion of a dilute mixture of CH{sub 3}I and HCl in argon, giving a mean collision energy of 7800 cm{sup -1} in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl({sup 2}P{sub J}) atoms formed in the J=(1/2) level at this collision energy was 0.150{+-}0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH{sub 4}+Cl({sup 2}P{sub 3/2}) products.Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl(2P(3/2)) and Cl*(2P(1/2)) products. Reaction was initiated by photodissociation of CH(3)I at 266 nm within a single expansion of a dilute mixture of CH(3)I and HCl in argon, giving a mean collision energy of 7800 cm(-1) in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl(2P(J)) atoms formed in the J=1/2 level at this collision energy was 0.150+/-0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to CH(4)+Cl(2P(3/2)) products.