Formation and stability of proton-amine-inorganic anion complexes in aqueous solution

Interaction of some amines with inorganic ligands was studied potentiometrically in aqueous solutions at 25°C at different ionic strengths. The systems taken into account were: meta-PP, meta-TPP, en-PP, en-TPP, en-HPO ₄ ^2– , dap-SO ₄ ^2– , tetren-HPO ₄ ^2– , 6da-PP and 8da-TPP (meta=methylamine, en=ethylenediamine, tetren=tetraethylenepentamine, dap=1,2-diaminopropane, 6da=1,6-hexanediamine, 8da=1,8-octanediamine, PP=pyrophosphate, TPP=tripolyphosphate). Several ALH_r species are formed for all the studied mixed systems (A=amine, L =inorganic ligand), and in some cases A_pL_qH_r species were also found. The stability of mixed proton-ligand-ligand species is proportional to the charges involved in the complexation reaction and, to some extent, it depends on steric characteristic of the ligands. A simple relationship has been found between formation constants of mixed ligand species and charges involved in the complexation reaction. The ionic strength dependence of formation constants has been taken into account, and it was found that, also for these types of mixed complexes, a general equation independent of the nature of the reactants can be used.     

Synthesis of new 2,2′‐disubstituted 5,5′‐dimethyl‐4,4′‐bitriazoles and 2‐(4‐Triazolyl)quinoxalines

Thermal rearrangement of 3‐acylisoxazole arylhydrazones allowed facile preparation of 2H‐1,2,3‐triazoles which were firstly reacted with isoamyl nitrite and then with an opportune arylhydrazine to produce the corresponding α‐hydroxyiminohydrazones 8a‐h . The reaction of compounds 8a‐h with phosphorus pentachloride afforded the desired 4,4′‐bitriazoles 1a‐h . The α‐hydroxyiminoketone derivative 7 or the α‐diketone 14 reacted easily with 1,2‐phenylenediamine to afford 1,2,3‐triazoles 2a‐c bearing the quinoxaline moiety at position 4. Improved yields of the quinoxalines 2a‐c were obtained when 1,2‐phenylenediamine was reacted with the dioxime 15.     

Aryl sulfoxide radical cations. Generation, spectral properties, and theoretical calculations

Aromatic sulfoxide radical cations have been generated by pulse radiolysis and laser flash photolysis techniques. In water (pulse radiolysis) the radical cations showed an intense absorption band in the UV region (ca. 300 nm) and a broad less intense band in the visible region (from 500 to 1000 nm) whose position depends on the nature of the ring substituent. At very low pulse energy, the radical cations decayed by first-order kinetics, the decay rate increasing as the pH increases. It is suggested that the decay involves a nucleophilic attack of H(2)O or OH(-) (in basic solutions) to the positively charged sulfur atom to give the radical ArSO(OH)CH(3)(*). By sensitized [N-methylquinolinium tetrafluoborate (NMQ(+))] laser flash photolysis (LFP) the aromatic sulfoxide radical cations were generated in acetonitrile. In these experiments, however, only the band of the radical cation in the visible region could be observed, the UV band being covered by the UV absorption of NMQ(+). The lambda(max) values of the bands in the visible region resulted almost identical to those observed in water for the same radical cations. In the LFP experiments the sulfoxide radical cations decayed by second-order kinetics at a diffusion-controlled rate, and the decay is attributed to the back electron transfer between the radical cation and NMQ(*). DFT calculations were also carried out for a number of 4-X ring substituted (X = H, Me, Br, OMe, CN) aromatic sulfoxide radical cations (and their neutral parents). In all radical cations, the conformation with the S-O bond almost coplanar with the aromatic ring is the only one corresponding to the energy minimum. The maximum of energy corresponds to the conformation where the S-O bond is perpendicular to the aromatic ring. The rotational energy barriers are not very high, ranging from 3.9 to 6.9 kcal/mol. In all radical cations, the major fraction of charge and spin density is localized on the SOMe group. However, a substantial delocalization of charge and spin on the ring (almost 50% for the 4-methoxy derivative and around 30% for the other radical cations) is also observed. This suggests some conjugative interaction between the MeSO group and the aromatic system that may become very significant when a strong electron donating substituent like the MeO group is present. The ionization energies (IE) of the 4-X ring substituted neutral aromatic sulfoxides were also calculated, which were found to satisfactorily correlate with the experimental E(p) potentials measured by cyclic voltammetry.     

Multidentate,V-Shaped Pyridine Building Blocks as Tectons for Crystal Engineering

The formation of supramolecular structural units through self-assembly is a powerful method to design new architectures and materials endowed with specific properties. With the aim of adding a group of versatile tectons to the toolkit of crystal engineers, we have devised and synthesised four new V-shaped building blocks characterised by an aryl acetylene scaffold comprising three substituted pyridine rings connected by two triple bonds. The judicious choice of different substituents on the pyridine rings provides these tectons with distinctive steric, electrostatic and self-assembly properties, which influence their crystal structures and their ability to form co-crystals. Co-crystals of the tectons with tetraiododifluorobenzene were obtained both via traditional and mechanochemical crystallisation strategies, proving their potential use in crystal engineering. The energetic contributions of the supramolecular interactions at play in the crystal lattice have also been evaluated to better understand their nature and strength and to rationalise their role in designing molecular crystals.     

Chiral receptors for phosphate ions

The binding tendencies of the enantiomeric forms, R,R and S,S, of the neutral receptor 1 towards anions were investigated through UV-vis and 1H NMR titration experiments in DMSO. Both enantiomers form stable H-bond complexes with carboxylates and phosphates. In particular, receptor 1 strongly binds two H2PO4- ions according two stepwise equilibria, in which logK2 is higher than logK1. Such an unusual cooperativity effect is to be ascribed to the formation of strong H-bond interactions between the two H2PO4- anions, when bound to the two urea subunits of the receptor, as demonstrated by the crystal and molecular structures of the 1 : 2 complex salt: [Bu4N]2[R,R-1...(H2PO4)2]. The S,S enantiomer forms an H-bond complex with the biologically relevant D-2,3-diphosphoglycerate anion, whose association constant is twice that of the R,R complex. Such an effect is ascribed to the different structural features of the two diastereomeric complexes in solution, as shown by 31P NMR studies.     

Charge remote fragmentation of fatty acids cationized with alkaline earth metal ions

Fatty acids can be collisionally activated as [M ? H + Cat]⁺, where Cat is an alkaline earth metal, by using tandem mass spectrometry. High-energy collisional activation induces charge remote fragmentation to give structural information. In the full scan mass spectra molecular ions are easily identified, particularly when barium is used as a cationizing agent; ions are shifted to a higher mass, lower chemical noise region of the mass spectrum. Moreover, the isotopic pattern of barium is characteristic, and the high mass defect of barium allows an easy separation of the cationized analyte from any remaining interfering ions (chemical noise), provided medium mass-resolving power is available. An additional advantage is that most of the ion current is localized in [M ? H + Cat]⁺ species. Structural analysis of fatty acids can be performed when the sample size is as low as 1 ng.     

Reactivity of aminopyrroles: Protonation

The behaviour of 2- and 3-aminopyrroles towards protonation is similar. In dimethyl sulphoxide/trifluoroacetic acid they are protonated at the exocyclic nitrogen, whereas in pure trifluoroacetic acid, protonation at the 5- and/or 3-position of the ring takes place.     

Time scales and other problems in linking simulations of simple chemical systems to more complex ones

We shall start with very small systems like H₂ and H₃, computed with very accurate methods (Hylleraas–CI ) or atomic systems up to Zn with accurate methods (CI ), then move to more complex ones, like C₆₀, but now with somewhat less accurate methods, specifically Hartree–Fock with density functionals, the latter for the correlation energy but not for the exchange energy. For even more complex tasks like geometry optimization of C₆₀, we have resorted to even simpler and parametrized methods, like local density functionals. Then, we could use quantum mechanics either to provide interaction potentials for classical molecular dynamics or to directly solve dynamical systems, in a quantum molecular dynamics approximation. Having demonstrated that we can use the computational output from small systems as input to larger ones, we discuss in detail a new model for liquid water, which is borne out entirely from ab initio methods and nicely links spectroscopic, thermodynamics, and other physicochemical data. Concerning time scales, we use classical molecular dynamics to determine friction coefficients, and with these we perform stochastic dynamic simulations. The use of simulation results from smaller systems to provide inputs for larger system simulations is the “global simulation” approach, which, today, with the easily available computers, is becoming more and more feasible. Projections on simulations in the 1996–1998 period are discussed, new computational areas are outlined, and a N⁴ complexity algorithm is compared to density functional approaches. © 1993 John Wiley & Sons, Inc.     

Improved activity in acidic media of immobilized lysozyme

Polyethylene glycols (PEGs) of various chain length were used to crosslink lysozyme onto an insoluble support such as oxirane. A very high degree of modification and no inactivation of lysozyme were obtained with PEG 20000, but enzymatic activity increased up to 20 times at pH 3.0, at which point the activity of the native enzyme was lower when using Leuconostok oenus as a macromolecular substrate.