The COMP Division is excited to announce the Chemical Computing Group Excellence Award for Graduate Students winners for the New Orleans ACS meeting (spring 2013). Please visit the COMP award winners and the other excellent COMP posters at the COMP Poster Session on Tuesday, April 9, 2013 from 6pm to 8pm at a location to be determined.
Toward effective CO2/CH4 separations by sulfur-containing PIMs: insight from molecular simulations
Kyle E Hart and Coray M Colina (Advisor), Department of Materials Science and Engineering, Penn State University
The separation of CO2 from CH4 is an industrially important process, and polymeric membranes have the potential to greatly increase the efficiency of current separation techniques. In this work, we evaluate four novel sulfur-containing polymers of intrinsic microporosity (PIMs) for use in CO2/CH4 gas separation membranes through the use of molecular simulations. Using a three-step multi-scale approach, we found that the CO2/CH4 separation ability of PIMs is directly correlated with the number of polar groups per repeat unit, and sulfur PIMs may be profoundly improved upon by a post-polymerization oxidation to contain sulfonyl functionalities. In addition, a quantitative structure-property relationship model was constructed to correlate the polymer's essential characteristic to the selectivity under those conditions, which, moving forward, will allow for the predictive screening of many PIM functionalities. The results presented in this work should facilitate the design of new intrinsically microporous polymer membranes with increased CO2/CH4 gas separation performance.
Examining the 4d transition metals and the lower p-block with a pseudopotential-based composite method: Atomic and molecular applications of rp-ccCA
Marie L Laury and Angela K Wilson (Advisor), Department of Chemistry, University of North Texas
By combining relativistic pseudopotentials and the correlation consistent composite approach (ccCA), a first-principles-based composite approach for main group and first-row transition metal species, accurate energetic and thermodynamic data for heavier elements, including transition metals and lower p-block (5p and 6p) elements, are obtainable. Relativistic pseudopotential ccCA (rp-ccCA) is formulated and tested on a subset of the G3/05 set that contains 4p elements (Ga-Kr). The time savings and accuracy of the methodology, as compared to ccCA and ccCA-TM, is gauged before extending the method to the 4d transition metals and lower p-block elements. The TM-4d set, composed of 30 experimental enthalpies of formation, is employed for the second row transition metal study, while the LP80 set, composed of atomic ionization potentials and electron affinities and molecular dissociation energies and enthalpies of formation, is utilized for the lower p-block study. The accuracy and utility of rp-ccCA for energetic and thermodynamic studies of elements further down the periodic table will be demonstrated for 4d metals and lower p-block molecules.
Effect of trans and cis conformational defects on the localization of electronic excitations in ∏-conjugated organic polymers
Iffat H Nayyar,1,2 Enrique R Batista,1 Sergei Tretiak (Advisor),1 Avadh Saxena,1 Darryl L Smith,1 and Richard L Martin1
1Los Alamos National Laboratory; 2University of Central Florida
Electro-optical devices with ∏-conjugated polymers are in demand for use in light-emitting diodes (LED), solar cells and lasers. A recent study predicted differences in the response of the hyperfine field by opposite charges in organic LEDs. The improved fluorescence exhibited by different isomeric forms of PPV derivatives in optoelectronic devices motivated us to investigate the influence of various conformational distortions of trans and cis nature on the energetics and localization of positive (P+) and negative (P-) polarons. We observe the P+ and P- states are highly sensitive on the structural conformation and atomic charge distributions. The P- state is observed to be more localized than P+ in consistent with experiment with the inclusion of the polarizable dielectric environment. These defects not only break the particle-hole symmetry but demonstrate higher charge-carrier mobilities for holes than electrons. Thus, we can tune the charge-transport and photo-physical properties in organic materials by understanding their structure-property correlations for technological applications.
Fast protein refolding observed in pressure-jump molecular dynamics simulation
Yanxin Liu, Martin Gruebele and Klaus Schulten (Advisor), Department of Physics, University of Illinois at Urbana-Champaign
Pressure jump is known to induce fast protein folding. For a five-helix bundle λ-repressor fragment, a short refolding time of ~2 μs was reported in an earlier pressure-jump experiment. To investigate this pressure-jump induced fast folding behavior, all-atom molecular dynamics simulations of more than 33 μs in explicit solvent were carried out on the same λ-repressor construct. High-pressure denatured states, generated through a high-temperature unfolding and high-pressure equilibration simulation procedure, were found to contain a significant amount of helical structure. Upon pressure drop, the protein refolded into the native state in 20 μs. The folding in the simulation is slower than the one measured in pressure-jump experiment, but faster than the folding time of 80 μs measured in temperature-jump experiment. A complete unfolding and refolding process was observed in the trajectory, which permitted the characterization of high-pressure denatured states and refolding pathway. The pressure jump simulations carried out for this study can be employed in the future to investigate slow-folding proteins through 10∼100 μs molecular dynamics simulations by inducing a fast folding phase.
Development and Application of the generalized Connectivity-Based Hierarchy: Accurate Thermochemistry for Organic Molecules
Raghunath O Ramabhadran and Krishnan Raghavachari (Advisor), Department of Chemistry, Indiana University
The long-standing problem of constructing a reliable and automated computational procedure to obtain the various thermochemical properties of organic molecules have been solved by our Connectivity-Based Hierarchy (CBH) – an automated method developed by us recently.
The excellent performance of our hierarchy offers scope to accurately compute thermochemical properties for important systems such as biologically relevant amino acids and carbohydrates. In an initial application on amino acids, we have calculated the enthalpies of formation of the two naturally occurring sulfur containing amino acids. Our results confirm that MP2 or B3LYP methods with modest sized basis-sets are adequate in yielding reliable geometries and thermochemistry.
The poster presents a detailed account of (a) the construction of the hierarchy, (b) results for reaction energies and heats of formations, (c) applications to amino acids and (d) future directions in CBH.