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Donor Ability (donor + ability)

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Chemistry100%

Selected Abstracts

Prediction of solvent effect on the reaction rate and endo/exo selectivity of a Diels,Alder reaction using molecular surface electrostatic potential

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2003
M. R. Gholami
Abstract Molecular surface electrostatic potential was used to predict the solvent effect on the reaction rate, endo/exo selectivity and diastereomeric excess of a Diels,Alder reaction. It is shown that these quantities can be expressed in terms of molecular surface electrostatic potentials of solvents which are obtained computationally by the HF/6,31++G* procedure. Regression analyses and an experimental database are used to obtain analytical representation of rate constant, endo/exo selectivity and diastereomeric excess. The models obtained show that the hydrogen bond donor ability of solvents on the above mentioned properties is substantial, whereas solvophobicity only affects the reaction rate and endo/exo selectivity of the reaction. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Isoamyl­cobalamin,acetone,water (1/0.385/12.650)

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2004
Christopher B. Perry
The title compound, [Co(C5H11)(C62H88N13O14P)]·0.385C3H6O·12.650H2O, contains the isoamyl (3-methyl­butyl) anion bonded to the CoIII ion through a C atom. The compound is thus a structural analog of the two biologically important vitamin B12 coenzymes adenosyl­cobalamin and methyl­cobalamin. The lower axial Co,N bond length [2.277,(2),Å] is one of the longest ever reported for a cobalamin and reflects the strong ,-donor ability of the isoamyl group. [source]

Interaction of cyanine dyes with nucleic acids.

BIOPOLYMERS, Issue 4 2001

Abstract The spectral luminescent properties of two groups of monomethine cyanine dyes were studied in the presence of DNA. The first group included five dyes with 5,6-methylenedioxy-[d]-benzo-1,3-thiazole heterocycle and their unsubstituted analogs. Five monomethine pyrylium cyanines and their N -methyl-pyridine analogs were included in the second group. In each pair the pyrylium and pyridine dyes had similar geometry but differed in charge density distribution. The results presented some evidence in favor of the half-intercalation interaction mode between the studied dyes and DNA. When the benzothiazole residue had the lowest electron donor ability between the two heterocycles in the dye molecule, its substitution with the bulky methylenedioxy group led to a significant decrease in fluorescence enhancement of the dye,DNA complex. On the contrary, when the substituents that create steric hindrance (e.g., methylenedioxy and methyl groups) were introduced into the heterocycle with the higher electron donor ability, the fluorescence enhancement value of the dye,DNA complex was virtually unchanged. The changes in the Stock's shift values upon the formation of the dye,DNA complexes were in agreement with the proposed half-intercalation model. Interestingly, in the dye,DNA complexes the pyrylium dyes probably resided in a place similar to the pyridine ones. It is possible that the benzothiazole (or benzooxazole) ring intercalated between the DNA bases and the pyrylium (or pyridine) residue was located in the DNA groove closer to the phosphate backbone. © 2001 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 62: 219,227, 2001 [source]

The Ever-Surprising Chemistry of Boron: Enhanced Acidity of Phosphine,Boranes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2009
Marcela Hurtado Dr.
Abstract The acidity-enhancing effect of BH3 in gas-phase phosphine,boranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas-phase acidities (GA) of MePH2 and MePH2,BH3 differ by about 118,kJ,mol,1 (see picture). The gas-phase acidity of a series of phosphines and their corresponding phosphine,borane derivatives was measured by FT-ICR techniques. BH3 attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110,kJ,mol,1). This acidity-enhancing effect of BH3 is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High-level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH3 association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH3 group is present. Our study also shows that deprotonation of ClCH2PH2 and ClCH2PH2,BH3 is followed by chloride departure. For the latter compound deprotonation at the BH3 group is found to be more favorable than PH2 deprotonation, and the subsequent loss of Cl, is kinetically favored with respect to loss of Cl, in a typical SN2 process. Hence, ClCH2PH2,BH3 is the only phosphine,borane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid. [source]