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Energetics of bond dissociation, especially the R-H bond heterolysis free energy (pKa), has played a central role in promoting chemistry to become a rational science. Despite the oceans of acidity studies in the literature, the current knowledge is limited to that in the classical molecular solvents and is unable to be extended to anticipate the acidity changes in ionic media. As the latter is now very popular for replacing volatile organic solvents, it becomes highly desirable to know how the driving force of bond cleavage is varied as the medium composition is switched from neutral molecules to the charged ions. Here we describe a general approach to measure absolute pKa's in pure ionic liquid (IL). The standard conditions warranting accurate measurement were outlined. The pKa's of the selected 18 C-H type indicator acids in four ILs were determined and a convenient indicator platform was constructed for easy expansion of acidity scales. These absolute pKa's make possible, for the first time, direct comparisons of bond energies in IL with those in molecular solvent and in the gas phase and should be able to serve as the standard parameters for calibrating computational methods suitable for the studies in ionic media. The effect of cation and anion in IL in relation to structure was analyzed. [ABSTRACT FROM AUTHOR]

We present here an energetic and atomistic description of how d-ornithine 4,5-aminomutase (OAM), an adenosylcobalamin (AdoCbl; coenzyme B12)-dependent isomerase, employs a large-scale protein domain conformational change to orchestrate the homolytic rupture of the Co-C bond. Our results suggest that in going from the open form (catalytically inactive) to the closed form (catalytically active), the Rossmann domain of OAM effectively approaches the active site as a rigid body. It undergoes a combination of a ∼52° rotation and a ∼14 Å translation to bring AdoCbl-initially positioned ∼25 Å away—into the active-site cavity. This process is coupled to repositioning of the Ado moiety of AdoCbl from the eastern conformation to the northern conformation. Combined quantum mechanics and molecular mechanics calculations further indicate that in the open form, the protein environment does not impact significantly on the Co-C bond homolytic rupture, rendering it unusually stable, and thus catalytically inactive. Upon formation of the closed form, the Co-C bond is activated through the synergy of steric and electrostatic effects arising from tighter interactions with the surrounding enzyme. The more pronounced effect of the protein in the closed form gives rise to an elongated Co-C bond (by 0.03 Å), puckering of the ribose and increased "strain" energy on the Ado group and to a lesser extent the corrin ring. Our computational studies reveal novel strategies employed by AdoCbl-dependent enzymes in the control of radical catalysis. [ABSTRACT FROM AUTHOR]

The first heterolytic and homolytic N(O)-NO2 bond dissociation energy scales of three types Y-nitro (Y = N, O) compounds and corresponding radical anions in acetonitrile were established by using titration calorimetry combined with relevant electrochemical data through proper thermodynamic cycles. [ABSTRACT FROM AUTHOR]

Two new organically templated layered copper(I) sulfites, namely, ~H2pip}~Cu3(CN)3(SO3)} (1) and {H2pip}-{NaCu2(SO3)2Br(H2O)}·2H2O (2) (pip = piperazine), have been synthesized by hydrothermal reactions of copper-(I) cyanide or copper(I) bromide with NaHSO3 and piperazine. Both compounds exhibit a layered structure. The 2D layer of {Cu3(CN)3(SO3)}2- in 1 is composed of 1 D chains of copper(I) cyanide interconnected by sulfite anions via both Cu-S and Cu-O bonds, whereas the 2D layer of {NaCu2(SO3)2Br}2- in 2 is formed by 1D chains of copper(I) bromide and 1D sodium(I) aqua chains that are interconnected by sulfite anions via Na-O, Cu-S, and Cu-O bonds. Chemical bonding in 1 and 2 has been also investigated by theoretical calculations based on DFT methods. [ABSTRACT FROM AUTHOR]

An n-Bu4NI catalyzed domino reaction that involves selective dual amination of sp³ C-H bonds has been developed. The protocol affords a facile and efficient approach to the synthesis of imidazo[1,5-c]-quinazolines under mild conditions. [ABSTRACT FROM AUTHOR]

The title compound, C17H12ClNO, is a Schiff base compound derived from the condensation of equimolar quantities of 5-chloro­salicyl­aldehyde and 1-naphthyl­amine. The structure displays a trans configuration with respect to the imine C=N double bond. The N atom is also involved in an intramolecular O—H·N hydrogen bond. In the crystal structure, the mol­ecules stack along the b axis. [ABSTRACT FROM AUTHOR]

A form ofdl-aspartic acid, C4H7NO4 is presented, which crystallizes in the same space group as the previously reported form but with a slightly different a axis. Its structure exhibits a three-dimensional supramolecular network of N—H...O and O—H...O bonds, which stabilizes the crystal packing. [ABSTRACT FROM AUTHOR]