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RISS 인기검색어
- 검색키워드
- 저자 : E. Leroy (검색결과 6 건)
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Positive Cooperativity in the Template-Directed Synthesis of Monodisperse Macromolecules
Belowich, Matthew E.,Valente, Cory,Smaldone, Ronald A.,Friedman, Douglas C.,Thiel, Johannes,Cronin, Leroy,Stoddart, J. Fraser American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.11
<P>Two series of oligorotaxanes R and R' that contain -CH(2)NH(2)(+)CH(2)- recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the -NH(2)(+)- recognition sites are separated by trismethylene bridges, whereas in the R' series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 ? apart from one another so as to facilitate efficient [ππ] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in the R' series, the [3]-, [4]-, [8]-, and [12]rotaxanes were isolated after reaction times of <5-30 min in yields of 72-85%, in the R series, the [3]-, [4]-, [5]-, [8]-, [12]-, [16]-, and [20]rotaxanes were isolated in <5 min to 14 h in 88-98% yields. It follows that while in the R' series the higher order oligorotaxanes are formed in lower yields more rapidly, in the R series, the higher order oligorotaxanes are formed in higher yields more slowly. In the R series, the high percentage yields are sustained throughout, despite the fact that up to 39 components are participating in the template-directed self-assembly process. Simple arithmetic reveals that the conversion efficiency for each imine bond formation peaks at 99.9% in the R series and 99.3% in the R' series. This maintenance of reaction efficiency in the R series can be ascribed to positive cooperativity, that is, when one ring is formed it aids and abets the formation of subsequent rings presumably because of stabilizing extended [ππ] stacking interactions between the arene units. Experiments have been performed wherein the dumbbell is starved of the macrocyclic components, and up to five times more of the fully saturated rotaxane is formed than is predicted based on a purely statistical outcome, providing a clear indication that positive cooperativity is operative. Moreover, it would appear that as the R series is traversed from the [3]- to the [4]- to the [5]rotaxane, the cooperativity becomes increasingly positive. This kind of cooperative behavior is not observed for the analogous oligorotaxanes in the R' series. The conventional bevy of analytical techniques (e.g., HR-MS (ESI) and both (1)H and (13)C NMR spectroscopy) help establish the fact that all the oligorotaxanes are pure and monodisperse. Evidence of efficient [ππ] stacking between contiguous arene units in the rings in the R series is revealed by (1)H NMR spectroscopy. Ion-mobility mass spectrometry performed on the R and R' series yielded the collisional cross sections (CCSs), confirming the rigidity of the R oligorotaxanes and the flexibility of the R' ones. The extended [ππ] stacking interactions are found to be present in the solid-state structures of the [3]- and [4]rotaxanes in the R series and also on the basis of molecular mechanics calculations performed on the entire series of oligomers. The collective data presented herein supports our original design in that the extended [ππ] stacking between contiguous arene units in the rings of the R series of oligorotaxanes facilitate an essentially rigid rod-like conformation with evidence that positive cooperativity improves the efficiency of their formation. This situation stands in sharp contrast to the conformationally flexible R' series where the oligorotaxanes form with no cooperativity.</P>
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RANK INEQUALITIES OVER SEMIRINGS
LeRoy B. Beasley,Alexander E. Guterman 대한수학회 2005 대한수학회지 Vol.42 No.2
Inequalities on the rank of the sum and the product of two matrices over semirings are surveyed. Preferences are given to the factor rank, row and column ranks, term rank, and zero-term rank of matrices over antinegative semirings.
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RANK INEQUALITIES OVER SEMIRINGS
BEASLEY LeRoy B.,GUTERMAN ALEXANDER E. Korean Mathematical Society 2005 대한수학회지 Vol.42 No.2
Inequalities on the rank of the sum and the product of two matrices over semirings are surveyed. Preferences are given to the factor rank, row and column ranks, term rank, and zero-term rank of matrices over antinegative semirings.
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Linear preservers of zeros of matrix polynomials
Beasley, LeRoy B.,Guterman, Alexander E.,Lee, Sang-Gu,Song, Seok-Zun Elsevier 2005 Linear algebra and its applications Vol.401 No.-
<P><B>Abstract</B></P><P>We classify linear operators on matrices with semiring entries that preserve the zeros of multivariable matrix polynomials. These matrix polynomials are defined via the adjoint operator ([<I>X</I>,<I>Y</I>]=<I>XY</I>−<I>YX</I>) as if the matrices were being considered as if over a field. Also we compare the results over fields with the results over semirings.</P>
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A. Lusson,N. Haneche,V. Sallet,P. Galtier,V. Munoz-Sanjose,J. Zuniga-Perez,S. Agouram,J. A. Bastos Segura,E. Leroy 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1
Zn(1-x)CdxO solid solutions with a composition ranging from pure ZnO up to x = 0.07 have been grown on ZnO substrates by using metalorganic chemical vapor deposition (MO-CVD). The alloy concentration was carefully measured using a Castaing electron microprobe. The measured lateral homogeneity of the alloy was around 0.1 %. Two dierent photoluminescence lines were observed on Zn(1-x)CdxO alloys. At low temperature, a localized exciton line was identied and a second line appeared at higher energy for intermediate temperatures. This eect is interpreted as a delocalization eect of excitons trapped in potential uctuations related to a local alloy disorder. Zn(1-x)CdxO solid solutions with a composition ranging from pure ZnO up to x = 0.07 have been grown on ZnO substrates by using metalorganic chemical vapor deposition (MO-CVD). The alloy concentration was carefully measured using a Castaing electron microprobe. The measured lateral homogeneity of the alloy was around 0.1 %. Two dierent photoluminescence lines were observed on Zn(1-x)CdxO alloys. At low temperature, a localized exciton line was identied and a second line appeared at higher energy for intermediate temperatures. This eect is interpreted as a delocalization eect of excitons trapped in potential uctuations related to a local alloy disorder.
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