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51.
Olga Dmitrenko Robert D. Bach Rafal R. Sicinski Wolfgang Reischl 《Theoretical chemistry accounts》2003,109(4):170-175
B3LYP calculations in conjunction with natural bond orbital population analysis have been performed for a previtamin D model
and corresponding transition structures for the [1,7]-hydrogen migration. In addition the 19,19-difluoro, 19-methoxy and 19-fluoro
substituted analogs were investigated. The calculated activation barriers decrease in the following order: CHF2>CH3>CH2OCH3 (24.8, 23.5 and 20.1 kcal/mol). This is in qualitative agreement with experiments. It has been suggested that a decrease
of the barrier by a 19-methoxy substituent and its increase by a 19,19-difluoro substituent are phenomena of different origin.
In the case of 19-methoxy substitution, the effect is due to the charge redistribution in the triene system and the decrease
of the C(19)–H bond energy. The effect of two fluorine substituents at C-19 on the activation barrier is suggested to originate
from the combination and balance of several factors: electrostatic repulsion between the negative fluorine atom and the π-electron
cloud over the conjugated system, an increase of the HOMO–LUMO gap, and geminal difluoro substitution affecting C–F and C–C
bond energies.
Received: 17 May 2002 / Accepted: 11 September 2002 /
Published online: 14 February 2003 相似文献
52.
Magnucki Krzysztof Lewinski Jerzy Cichy Rafal 《Archive of Applied Mechanics (Ingenieur Archiv)》2019,89(10):2103-2114
Archive of Applied Mechanics - The paper is devoted to simply supported beams subjected to non-uniformly distributed loads. Shapes of bisymmetrical cross sections of the beams are expressed by... 相似文献
53.
54.
Kamila Pruszkowska Olga A. Stasyuk Anna Zep Adam Krówczyński Rafal R. Sicinski Miquel Solà Michał K. Cyrański 《Chemphyschem》2022,23(2):e202100741
Two new square planar ONNO nickel(II) complexes C2_core and C3_core have been synthesized and characterized by single crystal X-ray diffraction, NMR spectroscopy, thermogravimetry, and DFT calculations. The experimental results revealed the effect of the length of diamine bridge in the ligand on the behavior of the studied complexes in the reaction with N-heterocyclic aromatic amines, while DFT calculations provided a basis for the rationalization of this observation. The complex with propylenediamine bridge ( C3_core ) readily reacts with pyridine and its derivatives to form high-spin (paramagnetic) complexes with octahedral geometry as characterized by X-ray diffraction; electron-donating substituents on the pyridine ring facilitate the coordination of axial ligands. In contrast, the complex with ethylenediamine bridge ( C2_core) does not undergo such a reaction because of the high deformation energy of the core required for the formation of C2_Py complex. 相似文献
55.
Rafal Kruszynski Wojciech Czestkowski 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(5):402-406
The title compound, C21H26FN3O7, is assembled by N—H...O and O—H...O hydrogen bonds into well‐separated two‐dimensional layers of about 15 Å thickness. The crescent conformation of the molecules is stabilized by weak intramolecular C—H...O and C—H...F hydrogen bonds. The uridine moiety adopts an anti conformation. The ribofuranose ring exists in an envelope conformation. All the endocyclic uracil bonds are shorter than normal single C—N and C—C bonds, and five of them have comparable lengths, which implies a considerable degree of delocalization of the electron density within this ring. 相似文献
56.
57.
Adsorption and structural properties of periodic mesoporous organosilica with Im3m symmetry (SBA-16) and large heterocyclic bridging groups in cage-like mesopores are studied. The core of this bridging group is an isocyanurate ring integrated with three trimethoxysilyls through flexible propyl chains. 相似文献
58.
59.
Two series of ordered mesoporous organosilica (OMO) SBA-15 materials with surface and bridging groups were fabricated by varying
the organic precursor addition at different synthesis stages. The consequence of the delayed introduction of organic precursor
on the structural and adsorption properties of the resulting OMOs was investigated. The OMOs studied were synthesized via
co-condensation of tetraethyl orthosilicate (TEOS) and ureidopropyltrimethoxysilane (UPS) as well as TEOS and bis(triethoxysilylpropyl)
disulfide (BTDS) in the presence of poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide) triblock copolymer Pluronic
P123 (EO20PO70EO20). The aforementioned OMOs were characterized by nitrogen adsorption-desorption isotherms at −196 °C and powder X-ray diffraction
(XRD). Nitrogen adsorption isotherms were used to estimate the pore volume, mesopore diameter and the BET specific surface
area, whereas the XRD data provided information about structural ordering and unit cell of the samples studied.
Ryan Felix, undergraduate student from Oberlin College (Oberlin, OH, USA) participating in the NSF-REU program during Summer
2006. 相似文献
60.
Polanski J Gieleciak R Magdziarz T Bak A 《Journal of chemical information and computer sciences》2004,44(4):1423-1435
Shape analysis is a powerful tool in chemistry and drug design, and molecular surface defines shape in the molecular scale. In the current publication we presented a novel formalism for the comparative molecular surface analysis (s-CoMSA). The method enables both quantitative modeling of 3D-QSAR and finding possible pharmacophoric sites. The method provides very predictive models for the CBG activity of the benchmark steroid series, tinctorial properties of the heterocyclic azo dyes and anti-HIV activity of the HEPT series. 相似文献