1,720,973 research outputs found
Calculations on the kinetics, thermodynamics, and selectivity of methyl radical addition to olefins coordinated to d8 and d0 transition-metal fragments: Two distinct and opposite anti-evans-polanyi effects with potential practical implications
No full textDFT calculations predict that methyl radical addition to the propene and isobutene adducts of the [Rh(PCP)] and [(Cp)2Zr(OtBu)] + fragments affords different kinetic and thermodynamic products. The regioselectivity in the Zr adducts is predicted to be reversed compared to that in the uncoordinated or the Rh-coordinated systems. © 2008 American Chemical Society.[Anonymous], 1988, PHYS REV A, DOI DOI 10.1103-PHYS.REV.B.37-785; ARNAUD R, 1986, J CHEM SOC PERK T 2, P1517, DOI 10.1039-p29860001517; Bennett JL, 1997, J AM CHEM SOC, V119, P10696, DOI 10.1021-ja9707419; BYERS JH, 2006, ORG LETT, V8, P3455; CLARK T, 1986, J CHEM SOC CHEM COMM, P1774, DOI 10.1039-c39860001774; Clark T, 2006, J AM CHEM SOC, V128, P11278, DOI 10.1021-ja063204+; DELBECQ F, 1985, J AM CHEM SOC, V107, P1623, DOI 10.1021-ja00292a027; Evans MG, 1938, T FARADAY SOC, V34, P0011, DOI 10.1039-tf9383400011; Fischer H, 2001, ANGEW CHEM INT EDIT, V40, P1340, DOI 10.1002-1521-3773(20010417)40:81340::AID-ANIE13403.0.CO;2-#; Fossey J., 1995, FREE RADICALS ORGANI; GIESE B, 1983, ANGEW CHEM INT EDIT, V22, P573; Giese B., 1986, RADICALS ORGANIC SYN; Gomez-Balderas R, 2004, J PHYS CHEM A, V108, P2874, DOI 10.1021-jp036375z; Hasanayn F, 2008, J AM CHEM SOC, V130, P511, DOI 10.1021-ja072704f; Henry DJ, 2004, J AM CHEM SOC, V126, P1732, DOI 10.1021-ja039139a; JONES WD, 1993, J AM CHEM SOC, V115, P554, DOI 10.1021-ja00055a027; Lalevee J, 2003, J AM CHEM SOC, V125, P9377, DOI 10.1021-ja034750v; Matyjaszewski K, 2001, CHEM REV, V101, P2921, DOI 10.1021-cr940534g; Merlic CA, 2001, J AM CHEM SOC, V123, P4904, DOI 10.1021-ja000600y; Nsouli NH, 2008, ORGANOMETALLICS, V27, P2004, DOI 10.1021-om701079m; Ogoshi S, 1998, J AM CHEM SOC, V120, P3514, DOI 10.1021-ja972895u; Reid SJ, 2003, DALTON T, P3975, DOI 10.1039-b307308d; SCHMALZ HG, 1995, ANGEW CHEM INT EDIT, V34, P2383, DOI 10.1002-anie.199523831; Shaik S, 1999, ANGEW CHEM INT EDIT, V38, P586; SHAIK SS, 1990, J AM CHEM SOC, V112, P1446, DOI 10.1021-ja00160a023; Stoebenau EJ, 2006, J AM CHEM SOC, V128, P8162, DOI 10.1021-ja0575225; Stoebenau EJ, 2006, J AM CHEM SOC, V128, P8638, DOI 10.1021-ja057524p; Torraca KE, 2000, COORDIN CHEM REV, V206, P469, DOI 10.1016-S0010-8545(99)00253-2; Vatamanu M, 2008, J AM CHEM SOC, V130, P454, DOI 10.1021-ja0742683; Vyakaranam K, 2006, J AM CHEM SOC, V128, P5610, DOI 10.1021-ja060067+; Vyakaranam K, 2006, J AM CHEM SOC, V128, P5680, DOI 10.1021-ja054967k; Warren JJ, 2008, J AM CHEM SOC, V130, P2774, DOI 10.1021-ja711057t; WIBERG KB, 1993, J COMPUT CHEM, V14, P1504, DOI 10.1002-jcc.540141213; Zhao HT, 2006, INORG CHIM ACTA, V359, P3527, DOI 10.1016-j.ica.2005.12.013; Zytowski T, 1997, J AM CHEM SOC, V119, P12869, DOI 10.1021-ja973128y87
Combined Self-Consistent-Field and Spin-Flip Tamm–Dancoff Density Functional Approach to Potential Energy Surfaces for Photochemistry
Get PDFWe present a new approach to calculating potential energy surfaces for photochemical reactions by combining self-consistent-field calculations for single-reference ground and excited states with symmetry-corrected spin-flip Tamm-Dancoff approximation calculations for multireference electronic states. The method is illustrated by an application with the M05-2X exchange-correlation functional to cis-trans isomerization of the penta-2,4-dieniminium cation, which is a model (with three conjugated double bonds) of the protonated Schiff base of retinal. We find good agreement with multireference configuration interaction-plus-quadruples (MRCISD+Q) wave function calculations along three key paths in the strong-interaction region of the ground and first excited singlet states. © 2012 American Chemical Society
Conical Intersection and Potential Energy Surface Features of a Model Retinal Chromophore: Comparison of EOM-CC and Multireference Methods
Get PDFThis work investigates the performance of equation-of-motion coupled-cluster (EOM-CC) methods for describing the changes in the potential energy surfaces of the penta-2,4-dieniminium cation, a reduced model of the retinal chromophore of visual pigments, due to dynamical electron correlation effects. The ground-state wave function of this model includes charge-transfer and diradical configurations whose weights vary along different displacements and are rapidly changing at the conical intersection between the ground and the first excited states, making the shape of the potential energy surface sensitive to a balanced description of nondynamical and dynamical correlation. Recently, variational (MRCISD) and perturbative (MRPT2) approaches for including dynamical correlation in CASSCF-based calculations were tested along three representative ground state paths. Here, we use the same three paths to compare the performance of single-reference EOM-CC methods against MRCISD and MRCISD+Q We find that the spin-flip variant of EOM-CCSD with perturbative inclusion of triple excitations (dT or IT) produces potential energy profiles of the two lowest electronic states in quantitative agreement with MRCISD+Q (our highest-quality reference method). The nonparallelity errors and differences in vertical energy differences of the two surfaces along these scans are less than 1.4 kcal/mol (EOM-SF-CCSD(dT) versus MRCISD+Q). For comparison, the largest error of MRCISD versus MRCISD+Q is 1.7 kcal/mol. Our results show that the EOM-CC methods provide an alternative to multireference approaches and may be used to study photochemical systems like the one used in this work
A Conical Intersection Controls the Deactivation of the Bacterial Luciferase Fluorophore
No full textThe photophysics of flavins is highly dependent on their environment. For example, 4a-hydroxy flavins display weak fluorescence in solution, but exhibit strong fluorescence when bound to a protein. To understand this behavior, we performed temperature-dependent fluorescent studies on an N(5)-alkylated 4a-hydroxy flavin: the putative bacterial luciferase fluorophore. We find an increase in fluorescence quantum yield upon reaching the glass transition temperature of the solvent. We then employ multiconfigurational quantum chemical methods to map the excited-state deactivation path of the system. The result reveals a shallow but barrierless excited state deactivation path that leads to a conical intersection displaying an orthogonal out-of-plane distortion of the terminal pyrimidine ring. The intersection structure readily explains the observed spectroscopic behavior in terms of an excited-state barrier imposed by the rigid glass cavity. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Learning from photobiology how to design molecular devices using a computer
No full textBiological photoreceptors and fluorescent proteins provide striking examples of how non-covalent interactions could be exploited for tuning the photochemistry and photophysics of organic chromophores. In this tutorial review we show how the construction of computer models of such natural supramolecular systems not only provides atomic-level information on the mechanisms of their function, but also principles useful for designing light-responsive components of artificial supramolecular systems. Using a few complementary case studies, the intellectual process leading to the implementation of such an engineering target is followed up to the actual construction of a working prototype of a biomimetic molecular switch. This journal is © the Partner Organisations 2014
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Quantum–classical simulations of rhodopsin reveal excited-state population splitting and its effects on quantum efficiency
No full textThe activation of rhodopsin, the light-sensitive G-protein-coupled receptor responsible for dim-light vision in vertebrates, is driven by an ultrafast excited-state double-bond isomerization with a quantum efficiency of almost 70%. The origin of such light sensitivity is not understood and a key question is whether in-phase nuclear motion controls the quantum efficiency value. In this study we used hundreds of quantum–classical trajectories to show that, 15 fs after light absorption, a degeneracy between the reactive excited state and a neighbouring state causes the splitting of the rhodopsin population into subpopulations. These subpopulations propagate with different velocities and lead to distinct contributions to the quantum efficiency. We also show here that such splitting is modulated by protein electrostatics, thus linking amino acid sequence variations to quantum efficiency modulation. Finally, we discuss how such a linkage that in principle could be exploited to achieve higher quantum efficiencies would simultaneously increase the receptor thermal noise leading to a trade-off that may have played a role in rhodopsin evolution. [Figure not available: see fulltext.]
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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