Mechanistic study of styrene aziridination by iron( <small>IV</small> ) nitrides

Crandell, Douglas ,W.,Muñ,oz III, Salvador B.,Smith, Jeremy M.,Baik, Mu-Hyun Royal Society of Chemistry 2018 Chemical Science Vol.9 No.45

<▼1><P>A combined experimental and computational investigation reveals that styrene aziridination by an iron(<SMALL>IV</SMALL>) nitride occurs by a stepwise mechanism involving multistate character.</P></▼1><▼2><P>A combined experimental and computational investigation was undertaken to investigate the mechanism of aziridination of styrene by the tris(carbene)borate iron(<SMALL>IV</SMALL>) nitride complex, PhB(<SUP><I>t</I></SUP>BuIm)<SUB>3</SUB>Fe 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 N. While mechanistic investigations suggest that aziridination occurs <I>via</I> a reversible, stepwise pathway, it was not possible to confirm the mechanism using only experimental techniques. Density functional theory calculations support a stepwise radical addition mechanism, but suggest that a low-lying triplet (<I>S</I> = 1) state provides the lowest energy path for C–N bond formation (24.6 kcal mol<SUP>–1</SUP>) and not the singlet ground (<I>S</I> = 0) state. A second spin flip may take place in order to facilitate ring closure and the formation of the quintet (<I>S</I> = 2) aziridino product. A Hammett analysis shows that electron-withdrawing groups increase the rate of reaction <I>σ</I><SUB>p</SUB> (<I>ρ</I> = 1.2 ± 0.2). This finding is supported by the computational results, which show that the rate-determining step drops from 24.6 kcal mol<SUP>–1</SUP> to 18.3 kcal mol<SUP>–1</SUP> when (<I>p</I>-NO<SUB>2</SUB>C<SUB>6</SUB>H<SUB>4</SUB>)CH 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CH<SUB>2</SUB> is used and slightly increases to 25.5 kcal mol<SUP>–1</SUP> using (<I>p</I>-NMe<SUB>2</SUB>C<SUB>6</SUB>H<SUB>4</SUB>)CH 00000000000

The mechanism of selective catalytic reduction of NO_x on Cu-SSZ-13 – a computational study

Crandell, Douglas W.,Zhu, Haiyang,Yang, Xiaofan,Hochmuth, John,Baik, Mu-Hyun The Royal Society of Chemistry 2017 Dalton Transactions Vol.46 No.2

<▼1><P>The copper-exchanged aluminosilicate zeolite SSZ-13 is a leading catalyst for the selective catalytic reduction of NO.</P></▼1><▼2><P>The copper-exchanged aluminosilicate zeolite SSZ-13 is a leading catalyst for the selective catalytic reduction of NO. Density functional theory calculations are used to construct a complete catalytic cycle of this process paying special attention to the coordination geometries and redox states of copper. N2 can be produced in the reduction half-cycle <I>via</I> a nitrosamine intermediate generated from the reaction of the additive reductant NH3 with a NO<SUP>+</SUP> intermediate stabilized by the zeolite lattice. The decomposition of this nitrosamine species can be assisted by incipient Brønsted acid sites generated during catalysis. Our calculations also suggest that the reoxidation of Cu(i) to Cu(ii) requires the addition of both NO and O2. The production of a second equivalent of N2 during the oxidation half-cycle proceeds through a peroxynitrite intermediate to form a Cu–nitrite intermediate, which may react with an acid, either HNO2 or NH4<SUP>+</SUP> to close the catalytic cycle. Models of copper neutralized by an external hydroxide ligand are also examined. These calculations form a key basis for understanding the mechanism of NO reduction in Cu-SSZ-13 in order to develop strategies for rationally optimizing the performance in future experiments.</P></▼2>

Intramolecular Oxyl Radical Coupling Promotes O–O Bond Formation in a Homogeneous Mononuclear Mn-based Water Oxidation Catalyst: A Computational Mechanistic Investigation

Crandell, Douglas W.,Xu, Song,Smith, Jeremy M.,Baik, Mu-Hyun American Chemical Society 2017 Inorganic chemistry Vol.56 No.8

<P>The mechanism of water oxidation performed by a recently discovered manganese pyridinophane catalyst [Mn((Py2NBu2)-Bu-t)(H2O)(2)](2+) is studied using density functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a Mnv- bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O-O bond formation is found on the quintet potential energy surface and involves an intramolecular coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin Mnill ion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a critical role for catalysis. It enables facile electron transfer between the oxo moieties and the Mn-center and promotes O-O bond formation via a radical coupling reaction with a calculated reaction barrier of only 14.7 kcal mol(-1). This mechanism of O-O coupling is unprecedented and provides a novel possible pathway to coupling two oxygen atoms bound to a single metal site.</P>

Near-ground wind and its characterization for engineering applications

Crandell, Jay H.,Farkas, William,Lyons, James M.,Freeborne, William Techno-Press 2000 Wind and Structures, An International Journal (WAS Vol.3 No.3

This report presents the findings of a one-year monitoring effort to empirically characterize and evaluate the nature of near-ground winds for structural engineering purposes. The current wind engineering practice in the United States does not explicitly consider certain important near-ground wind characteristics in typical rough terrain conditions and the possible effect on efficient design of low-rise structures, such as homes and other light-frame buildings that comprise most of the building population. Therefore, near ground wind data was collected for the purpose of comparing actual near-ground wind characteristics to the current U.S. wind engineering practice. The study provides data depicting variability of wind speeds, wind velocity profiles for a major thunderstorm event and a northeaster, and the influence of thunderstorms on annual extreme wind speeds at various heights above ground in a typical rough environment. Data showing the decrease in the power law exponent with increasing wind speed is also presented. It is demonstrated that near-ground wind speeds (i.e., less than 10 m above ground) are likely to be over-estimated in the current design practice by as much as 20 percent which may result in wind load over-estimate of about 50% for low-rise buildings in typical rough terrain. The importance of thunderstorm wind profiles on determination of design wind speeds and building loads (particularly for buildings substantially taller than 10 m) is also discussed. Recommendations are given for possible improvements to the current design practice in the United States with respect to low-rise buildings in rough terrain and for the need to study the impact of thunderstorm gust profile shapes on extreme value wind speed estimates and building loads.

The origin of the ligand-controlled regioselectivity in Rh-catalyzed [(2 + 2) + 2] carbocyclizations: steric vs. stereoelectronic effects

Crandell, D.,Mazumder, S.,Evans, P. A.,Baik, M. H. THE ROYAL SOCIETY OF CHEMISTRY 2015 Chemical Science Vol.6 No.12

Important role of ancillary ligand in the emission behaviours of blue-emitting heteroleptic Ir(iii) complexes

Cho, Yang-Jin,Kim, So-Yoen,Kim, Jin-Hyoung,Crandell, Douglas W.,Baik, Mu-Hyun,Lee, Jiwon,Kim, Chul Hoon,Son, Ho-Jin,Han, Won-Sik,Kang, Sang Ook Royal Society of Chemistry 2017 Journal of Materials Chemistry C Vol.5 No.18