2,960 research outputs found
Semiclassical reaction rate constant calculations: investigation of anharmonicity and quantum effects
No full textSemiclassical transition state theory (SCTST) is a relatively simple method for the computation from first principles of reactive rate constants, including quantum effects while accounting for anharmonicity and the coupling between reactive and bound modes.[1-3] In this talk, I will illustrate how we have developed this technique for practical applications[4-7] involving the study of phenomena like kinetic isotope effects, heavy atom tunneling, and elusive conformer lifetimes.[5,6,8]
While many approximate reaction rate theories reduce to the parabolic barrier estimate for the tunneling correction at high temperatures, SCTST, which is based on vibrational perturbation theory (VPT2), gives the exact limit when one considers the leading order term in an expansion of powers of ħ2 of the tunneling transmission coefficient.[9-11] Our investigation of molecular reactive systems assesses the importance of the non-linear corrections to the parabolic barrier estimate of the transmission coefficient. When the reaction barrier is significantly anharmonic, it is mandatory to account for non-linear corrections; otherwise, the transmission coefficient overlooks a high-temperature regime which may be dominated by quantum reflection.[12] These results highlight the importance of having a theory such as SCTST that includes the correct high-temperature limit.
[1] W.H. Miller Faraday Discuss. Chem. Soc. 62, 40 (1977).
[2] W.H. Miller J. Chem. Phys. 62, 1899 (1975)
[3] R. Hernandez et al., Chem. Phys. Lett. 214, 129 (1993).
[4] C. Aieta, F. Gabas, M. Ceotto, J. Phys. Chem. A 120, 4853 (2016).
[5] C. Aieta F. Gabas, M. Ceotto, J. Chem. Theory Comput. 15, 2142 (2019).
[6] G. Mandelli, C. Aieta, M. Ceotto J. Chem. Theory Comput. 18, 623 (2022).
[7] J.R. Barker, MultiWell-2023 software suite; University of Michigan: Ann Arbor, Michigan, USA, 2023; http://clasp-research.engin.umich.edu/multiwell/
[8] G. Mandelli, L. Corneo, C. Aieta J. Phys. Chem. Lett. 14, 9996 (2023).
[9] E. Pollak, J. Cao, Phys. Rev. A, 107, 022203 (2023).
[10] E. Pollak, S Upadhyayula J. Chem. Phys. 160, (2024).
[11] E. Pollak J. Chem. Phys. 160, 150902 (2024).
[12] C. Aieta, M. Ceotto, E. Pollak, in preparation
A ritual of building. An experimentation into non-conventional geometry made of recycled material
No full textIs it possible to build geometrically complex architecture using waste? This is the question our research team posed to a small group of students within an experimental workshop. Their answer has been a self-built pavilion with a tensile roof made of recycled cardboard.
The design purpose was to provide a temporary structure that could act as a multi-religious building and host for one week several rituals and cultural activities. This building has been inaugurated, used and dismantled during the summer 2008.
Besides providing a material answer to a multifaceted and not yet clearly expressed need, the design process has been seen as an experiment on how to conciliate simple techniques with complex geometries. Apparently a contrast
Assembly of Cytochrome c
No full textTransport of apocytochrome c across the outer mitochondrial membrane and conversion to holocytochrome c were studied in vitro. Apocytochrome c was synthesized in a cell-free homogenate from Neurospora crassa. Transfer in vitro was accomplished in a reconstituted system consisting of the postribosomal supernatant of the cell-free homogenate and of isolated and purified mitochondria from Neurospora. The reconstituted system has the following characteristics:
* 1.
Apocytochrome c is rapidly cleared from the supernatant and holocytochrome c appears in the mitochondria with the same kinetics. More than 80% of the apocytochrome c employed is converted to holocytochrome c. No transient accumulation of apocytochrome c is found in mitochondria.
* 2.
The heme group becomes covalently linked to apocytochrome c in the reconstituted system as demonstrated by analysis of tryptic peptide maps of the apoprotein and holoprotein.
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Deuterohemin added to the reconstituted system but not deuteroporphyrin inhibits the formation of holocytochrome c. This inhibition is reversed by protohemin.
* 4.
In the presence of deuterohemin about half of the apocytochrome c remains in the supernatant; the other half becomes associated with the mitochondria. The latter portion is tightly bound and is specifically released upon incubation of the mitochondria with excess apocytochrome c. It is converted to holocytochrome c after addition of protohemin.
We conclude from these observations that apocytochrome c is transported across the outer mitochondrial membrane via receptor sites. In the presence of the heme analogue deuterohemin, binding to the receptor sites on the cytoplasmic surface of the outer mitochondrial membrane still takes place but translocation does not. The latter step is apparently coupled to the covalent linkage of the heme group. We suggest that the formation of the thioether bonds between apoprotein and heme is catalysed by an enzyme in the intermembrane space and that deuterohemin can compete with protohemin for binding to the enzyme. Finally, the data indicate that it is the heme group and not the porphyrin group which is coupled to the apoprotein
Geometrical indeterminacy as a way to structural and architectural performance
No full textStructural systems with complex geometries are generally applied to very prestigious works of architecture and realized with sophisticated technologies.
The design process presented here aimed at investigating the possibility of building geometrically complex architecture using simple techniques and very cheap materials, yet achieving a meaningful structural and architectural performance.
Within an experimental workshop a team of 5 students supervised by the 2 authors built a real scale artefact, a temporary multi-religious pavilion with a tensile roof made of recycled cardboard
A counting proof of the Graham-Pollak Theorem
No full textWe give a counting based proof of the Graham-Pollak Theorem. (C) 2013 Published by Elsevier B.V
From Eggs to the Stars
No full textJane Pollak is a Westport, Connecticut, artist who started her career as a high school art teacher. She has now branched out into public speaking, is the author of two books, and embraces the life of entrepreneur as a sole proprietor of her rapidly expanding business of decorating eggs. For Jane, her life path has been one of hope and unexpected personal and business achievements.</jats:p
Cr-CO photodissociation in Cr(CO)6: Reassessment of the role of ligand-field excited states in the photochemical dissociation of metal-ligand bonds.
No full textDensity functional calculations have been performed on the excited states of Cr(CO)6. In contrast to the generally accepted assignment of the spectrum by Gray and Beach1,2 but in agreement with recent CASSCF/CASPT2 calculations by Pierloot et al.3 we find the low-intensity absorption at the low-energy side of the first charge-transfer (CT) band not to be due to ligand-field (LF) excited states, but to symmetry forbidden CT excitations. In Cr(CO)6 as in other d6 metal−carbonyl complexes,4-6 the LF states are at high energy. The calculations show that two states arising from the low-energy CT configuration have dissociative potential energy surfaces, in agreement with the experimentally observed photodissociation of the Cr−CO bond upon low-energy absorption. The photodissociation is therefore occurring from CT and not from LF states. This leads to a reassessment of the role of LF states in metal−ligand photodissociation: it is not necessary to excite to LF states in order to induce photodissociation of ligands, and such dissociation, when observed, does not prove that the excitation was to a LF state
From Eggs to the Stars
No full textInterview of artist Jane Pollak by Shawn Blau and Laurence Weinstein.
Jane Pollak is a Westport, Connecticut, artist who started her career as a high school art teacher. She has now branched out into public speaking, is the author of two books, and embraces the life of entrepreneur as a sole proprietor of her rapidly expanding business of decorating eggs. For Jane, her life path has been one of hope and unexpected personal and business achievements
Gary Becker's Contributions to Family and Household Economics
No full textGary Becker's influence on the economics of the family has been pervasive. His ideas have dominated research in the economics of the family, shaping the tools we use, the questions we ask, and the answers we give. The foundational assumptions of Becker's economic approach to the family -- maximizing behavior and equilibrium -- as well as such primary auxiliary assumptions as household production and interdependent preferences, are now widely accepted not only by economists but also by family sociologists, demographers, and others who study the family. Yet the interesting and provocative implications of Becker's economic approach to the family do not follow from the foundational assumptions or from the primary auxiliary assumptions. Instead they depend on contested auxiliary assumptions to which neoclassical economics has no commitment and which lack empirical support. This paper discusses the crucial role of auxiliary assumptions in Becker's analysis of the family, first in the context of preferences, then in the context of household production, and finally in the context of family or household collective choice.
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