441 research outputs found
Zero temperature quantum properties of small protonated water clusters (H2O)(n)H+ (n=1-5)
No full textThe structure and vibrational motion of the protonated water clusters (H2O)nH+ (n=1-5) were studied using the OSS3 interaction potential and the diffusion Monte Carlo (DMC) method. The aim was to simulate their ground state eigenfunctions. An attempt was also made to compute the ground state energy for neutral water clusters using the same potential to obtain reference values for their proton affinity. The resultant data wer analyzed in detail
Chemical reaction dynamics
No full textGuest editors Xueming Yang, David Clary and Daniel Neumark introduce the chemical reaction dynamics themed issue of Chemical Society Reviews.</p
Torsional anharmonicity in the conformational analysis of beta-D-galactose
No full textSchemes to include a treatment of torsional anharmonicity in the conformational analysis of biological molecules are introduced. The approaches combine ab initio electronic energies and harmonic frequencies with anharmonic torsional partition functions calculated using the torsional path integral Monte Carlo method on affordable potential energy surfaces. The schemes are applied to the conformational study of the monosaccharide -D-galactose in the gas phase. The global minimum structure is almost exclusively populated at 100 K, but a large number of conformers are present at ambient and higher temperatures. Both quantum mechanical and anharmonic effects in the torsional modes have little effect on the populations at all temperatures considered, and it is, therefore, expected that standard harmonic treatments are satisfactory for the conformational study of monosaccharides
Quantum scattering and quasi-classical trajectory calculations for the H2 + OH ⇆ H2O + H reaction on a new potential surface
No full textSix-dimensional (6D) quantum scattering calculations of reaction probabilities are reported for the OH + H2 ⇆ H2O + H reaction. An arrangement channel hyperspherical coordinate method is used. A new potential energy surface due to Ochoa and Clary is employed. The results agree well with those calculated using the rotating bond approximation (RBA) and the quasi-classical trajectory (QCT) method. 6D quantum, RBA and QCT calculations of rate constants for the OH + H2 reaction agree well with experiment. In addition, RBA calculations of differential cross sections for the OH + D2 → HOD + D reaction and the photodetachment spectrum for H3O- also agree well with experiment. These results suggest that the new potential surface is reliable for this reason.Fil: Pogrebnya, Sergei K.. Colegio Universitario de Londres; Reino UnidoFil: Palma, Juliana Isabel. Colegio Universitario de Londres; Reino UnidoFil: Clary, David C.. Colegio Universitario de Londres; Reino UnidoFil: Echave, Julián. Universidad Nacional de Quilmes. Centro de Estudios e Investigación; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin
Reduced dimensionality spin-orbit dynamics of CH3 + HCl reversible arrow CH4 Cl on ab initio surfaces
No full textA reduced dimensionality quantum scattering method is extended to the study of spin-orbit nonadiabatic transitions in the CH3 + HCl reversible arrow CH4 + Cl(P-2(J)) reaction. Three two-dimensional potential energy surfaces are developed by fitting a 29 parameter double-Morse function to CCSD(T)/IB//MP2/cc-pV(T+d)Z-dk ab initio data; interaction between surfaces is described by geometry-dependent spin-orbit coupling functions fit to MCSCF/cc-pV(T+d)Z-dk ab initio data. Spectator modes are treated adiabatically via inclusion of curvilinear projected frequencies. The total scattering wave function is expanded in a vibronic basis set and close-coupled equations are solved via R-matrix propagation. Ground state thermal rate constants for forward and reverse reactions agree well with experiment. Multi-surface reaction probabilities, integral cross sections, and initial-state selected branching ratios all highlight the importance of vibrational energy in mediating nonadiabatic transition. Electronically excited state dynamics are seen to play a small but significant role as consistent with experimental conclusions. (C) 2011 American Institute of Physics. [doi:10.1063/1.3592732]</p
Nuclear quantum effects on the structure and energetics of (H2O)6H+.
No full textThe energetics and structure of the protonated water hexamer (H2O)6H+ have been examined employing both model potentials and high-level ab initio methods. To select candidate structures for this cluster, Parallel-Tempering and the OSS2 potential were used as devices to complement the set of stationary points previously optimized by Hodges and Wales, Chem. Phys. Lett., 2000, 324, 279. Structures of these local minima were successively re-optimized using OSS3, B3LYP/aug-cc-pVDZ, and MP2/aug-cc-pVDZ, the latter providing a reference to benchmark the performance of the empirical models and B3LYP method. We found that both OSS2 and OSS3 require a re-parameterization to adequately describe the energetics of some isomers. Zero point energy was found to be important in defining the relative stability of the optimized isomers. The effect of the anharmonicity on the vibrational ground state of (H2O)6H+ was also examined by means of diffusion Monte Carlo (DMC) and the OSS3 potential, and we found that it accounts for a decrease in total energy of roughly 0-4.4 mE(h). This is a significant effect on the energetics considering that many isomers are nearly degenerate. Including the anharmonic corrections computed with DMC, the branched species were found to be the most stable isomers. The height of the barriers separating a cage or cyclic isomer from a branched one was found to vary from 1.5 to 5.8 mE(h)
Substance use, self-stigma, and help seeking among military members: Exploring the utility of the emerging adulthood theory
No full textBackground: Emerging adults (18-29 year olds) have the highest rates of substance use across the lifespan and many military members often have even higher rates of substance use and related consequences. Unfortunately, those most in need of mental health treatment perceive greater stigma and are less likely to pursue treatment for their past trauma or current mental health or substance use needs. Moreover, few treatments address the unique military culture or one’s developmental status when intervening. This study explored the utility of the five Emerging Adulthood Theory dimensions, a human developmental lens, to military members. It sought to understand developmental factors related to high rates of substance use among military populations, while also providing culturally responsive implications for social workers assisting this vulnerable population. In addition, it evaluated if one interview had a positive impact on substance use frequency, general help seeking behaviors, and stigma on help seeking behaviors.
Methods: To answer these research questions, I completed in-depth qualitative interviews utilizing motivational interviewing spirit with 26 emerging adult military members who have high risk substance use behaviors. I asked about their experiences developing into an adult, substance use behaviors, barriers to receiving mental health treatment, and ways to reduce mental health stigma. Most interviews were conducted via Zoom (n=22) with some in-person (n=4). I collected pre-test and post-test data on substance use frequency, self-stigma, and help-seeking behaviors, with a 100% follow-up rate. Qualitative interviews were analyzed following reflexive-thematic driven analysis and were coded using Nvivo. Rigorous coding procedures integrated inter-rater and intra-rater reliability procedures. Quantitative data were cleaned and analyzed in both Excel and SPSS using dependent samples t-tests.
Results: We found that this sample related to the five Emerging Adulthood Theory dimensions, but in different ways than is proposed by the theory. We believe these various themes could be attributed to their immersion in military culture. Moreover, data shows that a one-time interview has positive effects on substance use behaviors, help-seeking and stigma about mental health treatment. Follow-up data shows that the research study was well received by participants as they could share their stories, think critically about military culture, and have a genuine conversation with a social worker regarding their military experiences.
Implications: Findings provide the framework to effectively support emerging adult military members with high risk substance behaviors. Future research should integrate these findings to produce culturally adaptive interventions to reduce substance use behaviors and stigma around mental health treatment while also encouraging help seeking behaviors. This study fills the literature gap of more thoroughly understanding the utility of the theory among military members. Further, these data shed light on the acceptability and feasibility of brief interventions with military members.Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2022-05-01The student, Kelly Clary, accepted the attached license on 2020-03-26 at 08:05.The student, Kelly Clary, submitted this Dissertation for approval on 2020-03-26 at 08:32.This Dissertation was approved for publication on 2020-03-27 at 16:04.DSpace SAF Submission Ingestion Package generated from Vireo submission #14919 on 2020-08-25 at 17:27:12Made available in DSpace on 2020-08-26T23:51:26Z (GMT). No. of bitstreams: 3
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Semiclassical transition state theory for chemical reactions
No full textFirst-principles calculations of chemical reaction rate constants have useful applications in a variety of areas. Quantum reactive scattering approaches are the most accurate, but their computational costs scale significantly with system size. Semiclassical Transition State Theory (SCTST) constitutes a more efficient approach. In SCTST, rate constants are calculated from frequencies and anharmonic constants, which depend on the second, third, and fourth derivatives of the reaction's potential energy surface (PES) at the transition state. Further gains in efficiency can be achieved by calculating anharmonic constants for only a subset of degrees of freedom. SCTST can be improved by imposing more realistic behaviour of the reactant and product asymptotes of the potential barrier used in calculations.
This thesis investigates the accuracy of one- and two-dimensional SCTST in relation to accurate quantum scatteirng calculations performed on the same PESs. Calculations are performed for four H atom abstraction and exchange reactions: H + CH4, H + C2H6, H + cyc-C3H6, and CH3 + CH4. Two methods are presented for calculating frequencies and anharmonic constants from PESs developed previously for these reactions, which are defined in Jacobi coordinates. 2-D SCTST rate constants exhibit acceptable agreement with results from previous quantum scattering calculations. 1-D SCTST rate constants agree very well with 2-D results, although barrier corrections must be applied in the case of the CH3 + CH4 reaction. These corrections did not significantly improve the accuracy of other calculations. Agreement between 2-D and 1-D results suggests the validity of reducing the number of degrees of freedom considered in an SCTST calculation.
A practical ab initio approach is presented to facilitate evaluation of the accuracy of 1-D SCTST in relation to FD SCTST. Richardson extrapolation is discussed as an approach to improving the accuracy of purely numerical calculations of third and fourth derivatives. FD calculations for the H + CH4 and H + C2H6 reactions exhibit good agreement with previous results and with results from two different 1-D methods. The second of these methods requires only 4 single-point energy calculations in addition to those required for TST. The agreement of 1-D and FD results suggests the potential applicability of 1-D SCTST to larger systems.</p
Reduced dimensionality quantum dynamics of chemical reactions
No full textIn this thesis a reduced dimensionality quantum scattering model is applied to the study of polyatomic reactions of type X + CH4 XH + CH3.Two dimensional quantum scattering of the symmetric hydrogen exchange reaction CH3+CH4 CH4+CH3 is performed on an 18-parameter double-Morse analytical function derived from ab initio calculations at the CCSD(T)/cc-pVTZ//MP2/cc-pVTZ level of theory. Spectator mode motion is approximately treated via inclusion of curvilinear or rectilinear projected zero-point energies in the potential surface. The close-coupled equations are solved using R-matrix propagation. The state-to-state probabilities and integral and differential cross sections show the reaction to be primarily vibrationally adiabatic and backwards scattered. Quantum properties such as heavy-light-heavy oscillating reactivity and resonance features significantly influence the reaction dynamics. Deuterium substitution at the primary site is the dominant kinetic isotope effect. Thermal rate constants are in excellent agreement with experiment.The method is also applied to the study of electronically nonadiabatic transitions in the CH3 + HCl CH4 + Cl(2PJ) reaction. Electrovibrational basis sets are used to construct the close-coupled equations, which are solved via Rmatrix propagation using a system of three potential energy surfaces coupled by spin-orbit interaction. Ground and excited electronic surfaces are developed using a 29-parameter double-Morse function with ab initio data at the CCSD(T)/ccpV( Q+d)Z-dk//MP2/cc-pV(T+d)Z-dk level of theory, and with basis set extrapolated data, both corrected via curvilinear projected spectator zero-point energies. Coupling surfaces are developed by fitting MCSCF/cc-pV(T+d)Z-dk ab initio spin orbit constants to 8-parameter functions. Scattering calculations are performed for the ground adiabatic and coupled surface models, and reaction probabilities, thermal rate constants and integral and differential cross sections are presented. Thermal rate constants on the basis set extrapolated surface are in excellent agreement with experiment. Characterisation of electronically nonadiabatic nonreactive and reactive transitions indicate the close correlation between vibrational excitation and nonadiabatic transition. A model for comparing the nonadiabatic cross section branching ratio to experiment is discussed
Fossil fuel decarbonization and plastics-waste conversion to hydrogen and high-value carbons: pure science behind two emerging disruptive technologies
No full textFossil fuels are renewable only over geological time scales. Their oxidation, via aerial combustion, in considerable amounts since the dawn of the industrial revolution has led to an accumulation of CO2 in the atmosphere with an anthropogenic influence on the Earth’s climate. We highlight here our innovations in the catalytic conversion or recycling of CO2 captured from the air together with green or biogenic hydrogen to produce Sustainable Aviation Fuel (SAF). Releasing CO2 back to the atmosphere following the combustion of SAFs recreates the natural carbon cycle and promises a net-zero future for aviation. We also highlight fossil fuels as potent, natural sources of sustainable hydrogen; so also, plastics-waste, now seen as an equally attractive anthropogenic source of clean hydrogen. This chapter reviews our advances in the microwaveinitiated catalytic pyrolysis or “hydrogen-stripping” from fossil fuels and plastics-waste. This innovation is a rapid and highly-effective route to clean hydrogen and high-value solid carbon nanomaterials, in many cases accompanied by low- to zero-CO2 emissions. Underpinning these advances are the remarkable physicochemical properties of mesoscale catalyst particles close to the Size-Induced Metal-Insulator Transition
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