Nature or number of species in a transition state: the key role of catalytically active molecules in hydrogen transfer stages in atmospheric aldehyde reactions.

For the first time, the two factors (the number of sites in the transition state and the nature of the catalytically active species) that affect the energy barriers (Ea and ΔG‡) in atmospheric aldehyde reactions are proposed. The contribution of each factor to the energy barriers of the ammonization and amination stages, dehydration, and intramolecular hydrogen transfer is studied using the example of the acetaldehyde and glyoxal interactions with ammonia in aqueous solution. A regular decrease in energy barriers is observed in a series of 4-, 6-, and 8-membered transition states (TSs) regardless of the nature of the catalytically active species and their numbers. The 8-membered TSs of ammonization, amination, and dehydration reactions are the most efficient catalytic systems. The role of the nature of catalytically active species is secondary and is expressed in different cases through the influence of entropy and different acidity/basicity of catalytically active species and their structures. The regularities for the stage of intramolecular hydrogen transfer stand out from those for the ammonization, amination, and dehydration stages. The intramolecular hydrogen transfer is organized by three atoms in TSs without the participation of catalytically active species, while the 5- and 7-membered TSs are formed with the participation of such species. A proportional decrease in energy barrier with a sequential increase in the number of TS sites (3-, 5-, and 7-) is not observed. A sharp decrease in the barriers occurs only during the formation of the 7-membered TSs, while the 5-membered structures lie above the 3-membered catalytically inactive structures on the potential energy surface (PES) regardless of the nature of the species forming these structures.

The reaction of acetaldehyde, glyoxal, and ammonia to yield 2-methylimidazole: thermodynamic and kinetic analyses of the mechanism.

The most thermodynamically and kinetically favorable pathways for the formation of 2-methylimidazole (2MI) in the reaction of glyoxal and acetaldehyde with ammonia in aqueous solution have been determined. The formation of 2MI proceeds through a number of successive intermediates of acyclic and cyclic structures, and the most favorable route (thermodynamically and kinetically) for the formation of the imidazole ring is the condensation of amine intermediates, in contrast to the existing concepts of imine structures. The limiting stage is the stage of cyclization involving the intramolecular attack by the amino group of the precyclic intermediate on the carbon atom bound to the hydroxyl group with the simultaneous release of a water molecule according to the SN2 mechanism. Further stages of stepwise dehydration lead to the formation of a cyclic diazine, the intramolecular migration of the proton of the tertiary carbon atom to the nitrogen atom of which completes the formation of 2MI. Experimental studies on the effect of the order of mixing of initial reagents on the 2MI yield confirmed the quantum-chemically substantiated favorable pathway for the formation of 2MI during the interaction of amine intermediates, and also revealed that the selectivity of the 2MI formation is achieved by successive mixing of acetaldehyde with ammonia until the formation of hydroxyamine products and their further interaction with glyoxal.

A cocrystal of L-ascorbic acid with picolinic acid: the role of O-H...O, N-H...O and C-H...O hydrogen bonds and L-ascorbic acid conformation in structure stabilization.

A new 1:1 cocrystal (L-Asc-Pic) of L-ascorbic acid (vitamin C) with picolinic acid was prepared as a powder and as single crystals. The crystal structure was solved and refined from single-crystal X-ray diffraction (SCXRD) data collected at 293 (2) and 100 (2) K. The samples of the L-Asc-Pic cocrystal were characterized by elemental (HCNS) analysis and titrimetric methods, TG/DTG/DSC, and IR and Raman spectroscopy. The asymmetric unit comprises a picolinic acid zwitterion and an L-ascorbic acid molecule. The stabilization energy of intermolecular interactions involving hydrogen bonds, the vibrational spectrum and the energies of the frontier molecular orbitals were calculated using the GAUSSIAN09 and the CrystalExplorer17 programs. The charge distribution on the atoms of the L-Asc-Pic cocrystal, L-ascorbic acid itself and its 12 known cocrystals (structures from Version 5.40 of the Cambridge Structural Database) were calculated by the methods of Mulliken, Voronoi and Hirshfeld charge analyses (ADF) at the bp86/TZ2P+ level of theory. The total effective charges and conformations of the L-ascorbic acid molecules in the new and previously reported cocrystals were compared with those of the two symmetry-independent molecules in the crystals of L-ascorbic acid. A correlation between molecular conformation and its effective charge is discussed.

Theoretical analysis of glyoxal condensation with ammonia in aqueous solution.

The reactions of glyoxal with ammonia, ammonium salts, and amines cause the formation of the secondary organic aerosol (SOA) components (imidazole and its derivatives) in the atmosphere. The interaction of glyoxal and ammonia in aqueous solution is a primary reaction for these processes, and the explanation of its mechanism will allow developing the methods to control the formation of the SOA components. A detailed mechanism for the formation of key intermediates, namely, ethanediimine, diaminoethanediol, and aminoethanetriol, required for the imidazole ring cyclization, is proposed, and its potential energy surface (PES) has been constructed. This mechanism includes the experimentally identified intermediate compounds and takes into account the conformational and hydration equilibria of glyoxal. The schemes are proposed for further conversion of the key intermediates to the products of condensation between glyoxal and ammonia in the aqueous solution, C-N cyclic oligomers, that were identified. The products are shown to correspond to low positions on the PES in terms of Gibbs free energy, from -30.8 to -68.3 kcal mol-1, which confirms the high probability of their formation. The preferable thermodynamic pathway for formation of the imidazole products does not comprise the conversion of the diimine intermediate with the participation of the proton, but rather the interaction of either the diaminoalcohol with glyoxal monohydrate or two monoamine derivatives between themeselves (aminoethantriol and aminohydroxyacetaldehyde).

Acetaldehyde-Ammonia Interaction: A DFT Study of Reaction Mechanism and Product Identification.

The product of acetaldehyde and ammonia reaction, namely, 2,4,6-trimethyl-1,3,5-hexahydrotriazine trihydrate, was synthesized and identified using a combination of experimental (NMR spectroscopy, IR spectroscopy, melting point determination) and DFT-based theoretical approaches. A reaction mechanism was proposed. The reaction was shown to proceed via the formation of aminoalcohol, imine, and geminal diamine intermediates accompanied by cyclization of these species. The calculation results allowed us to build a potential energy surface of the acetaldehyde and ammonia interaction and determine the most energetically favorable pathway to yield acetaldehyde ammonia trimer. The reaction product was found in an energy minimum (-53.5 kcal/mol).