Thermal behavior of some N,N-dimethylbiguanide derivatives displaying antimicrobial activity

N,N-Dimethylbiguanide derivatives (HDMBG)X, where X=CH₃COO (1), Cl (2) and NO₃ (3) respectively, exhibit in vitro antimicrobial activity on representative bacterial and fungal strains. The presence of N,N-dimethylbiguanidium ion for all derivatives was evidenced by IR and ¹H NMR spectra. Thermal analysis gave information on their decomposition steps and also on the accompanying thermodynamic effects. According to TG and DTG curves processes as melting, oxidative degradation as well as oxidative condensation of –C=N– units occur. The different nature of the anions results different melting points. Paracyanide formation at various condensation degrees was observed.     

Mechanisms for fluorescence depolarization in dendrimers

We have investigated the fluorescence properties of dendrimers (Gn is the dendrimer generation number) containing four different luminophores, namely terphenyl (T), dansyl (D), stilbenyl (S), and eosin (E). In the case of T, the dendrimers contain a single p-terphenyl fluorescent unit as a core with appended sulfonimide branches of different size and n-octyl chains. In the cases of D and S, multiple fluorescent units are appended in the periphery of poly(propylene amine) dendritic structures. In the case of E, the investigated luminophore is noncovalently linked to the dendritic scaffold, but is encapsulated in cavities of a low luminescent dendrimer. Depending on the photophysical properties of the fluorescent units and the structures of the dendrimers, different mechanisms of fluorescence depolarization have been observed: (i) global rotation for GnT dendrimers; (ii) global rotation and local motions of the dansyl units at the periphery of GnD dendrimers; (iii) energy migration among stylbenyl units in G2S; and (iv) restricted motion when E is encapsulated inside a dendrimer, coupled to energy migration if the dendrimer hosts more than one eosin molecule.     

Azomethine Ylides from Nitrones: Using Catalytic nBuLi for the Totally Stereoselective Synthesis of trans‐2‐Alkyl‐3‐oxazolines

The cycloaddition of azomethine ylide N‐oxides (nitrone ylides) with aldehydes provides 3‐oxazolines in a completely stereoselective manner in the presence of a catalytic amount of n‐butyllithium. The process involves an initial nucleophilic attack on the aldehyde, followed by intramolecular oxygen addition to the nitrone moiety and lithium‐assisted elimination of water, regenerating the catalytic species. Various Li‐based catalytic systems are possible and the in situ generated water is required for continuing the catalytic cycle. The best results are observed with 20 mol % of n‐butyllithium, whereas the use of stoichiometric amounts inhibit the rate of catalysis. Experimental, spectroscopic, and computational mechanistic studies have provided evidence of lithium‐ion catalysis and rationalized several competing catalytic pathways     

Structural Modifications of cis‐Glycofused Benzopyran Compounds and Their Influence on the Binding to Amyloid‐β Peptide

A small library of glycofused tricyclic compounds with a central pyran ring chemically modified in the position para to the ring oxygen has been synthesised. The influence of the chemical modification on the structural conformation of the compounds and on their ability to bind Aβ peptide has been evaluated respectively using molecular mechanics (MM) and molecular dynamics (MD) simulations, and STD NMR spectroscopy. The introduction of particularly polar/charged groups leads to the loss of binding ability, without a significant change in the conformation, whilst other substitutions does not significantly affect either the structural conformation or the binding.     

Unimolecular thermal fragmentation of ortho-benzyne

The ortho-benzyne diradical, o-C(6)H(4) has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o-C(6)H(4)+Delta--> products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o-benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o-C(6)H(4)+Delta-->HC triple bond CH+HC triple bond C-C triple bond CH. The experimental Delta(rxn)H(298)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH) is found to be 57+/-3 kcal mol(-1). Further experiments with the substituted benzyne, 3,6-(CH(3))(2)-o-C(6)H(2), are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation of o-benzyne has been investigated by rigorous ab initio electronic structure computations. These calculations used basis sets as large as [C(7s6p5d4f3g2h1i)H(6s5p4d3f2g1h)] (cc-pV6Z) and electron correlation treatments as extensive as full coupled cluster through triple excitations (CCSDT), in cases with a perturbative term for connected quadruples [CCSDT(Q)]. Focal point extrapolations of the computational data yield a 0 K barrier for the concerted, C(2v)-symmetric decomposition of o-benzyne, E(b)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH)=88.0+/-0.5 kcal mol(-1). A barrier of this magnitude is consistent with the experimental results. A careful assessment of the thermochemistry for the high temperature fragmentation of benzene is presented: C(6)H(6)-->H+[C(6)H(5)]-->H+[o-C(6)H(4)]-->HC triple bond CH+HC triple bond C-C triple bond CH. Benzyne may be an important intermediate in the thermal decomposition of many alkylbenzenes (arenes). High engine temperatures above 1500 K may crack these alkylbenzenes to a mixture of alkyl radicals and phenyl radicals. The phenyl radicals will then dissociate first to benzyne and then to acetylene and diacetylene.     

The peculiar trend of cyclic perfluoroalkane electron affinities with increasing ring size

The adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) of cyclic perfluoroalkanes, c-C(n)F(2n) (n = 3-7), and their monotrifluoromethyl derivatives were computed using various pure and hybrid density functionals with DZP++ (polarization and diffuse function augmented double-zeta) basis sets. The theoretical AEA of c-C(4)F(8) at KMLYP/DZP++ is 0.70 eV, which exhibits satisfactory agreement with the 0.63 +/- 0.05 eV experimental value. The nonzero-point-corrected AEA of c-C(4)F(8) is predicted to be 0.41 eV at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory, which shows a slight deviation of 0.11 eV from the KMLYP estimated value of 0.52 eV for the same. With the zero-point correction from the MP2/6-311G(d) [Gallup, G. A. Chem. Phys. Lett. 2004, 399, 206] level of theory combined with the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ result, the most reliable estimate of AEA of c-C(4)F(8) is 0.60 eV. c-C(3)F(6)(-), c-C(4)F(8)(-), and c-C(5)F(10)(-) are unusual in preferring planar to near planar ring structures. The ZPE-corrected AEAs of c-C(n)F(2n) increase from n = 3 (0.24 eV) to n = 5 (0.77 eV), but then dramatically fall off to 0.40 eV for both n = 6 and n = 7. All of the other functionals predict the same trend. This is due to a change in the structural preference: C(s)() c-C(6)F(12)(-) and C(1) c-C(7)F(14)(-) are predicted to favor nonplanar rings, each with an exceptionally long C-F bond. (There also is a second, higher energy D3d minimum for C(6)F(12)(-).) The SOMOs as well as the spin density plots of the c-PFA radical anions reveal that the "extra" electron is largely localized on the unique F atoms in the larger n = 6 and n = 7 rings but is delocalized in the multiatom SOMOs of the three- to five-membered ring radical anions. The computed AEAs are much larger than the corresponding VEAs; the latter are not consistent with different functionals. The AEAs are substantially larger when a c-C(n)()F(2)(n)() fluorine is replaced by a -CF(3) group. This behavior is general; PFAs with tertiary C-F bonds have large AEAs. The VDEs for all the anions are substantial, ranging from 1.89 to 3.64 eV at the KMLYP/DZP++ level.     

Analysis of T-2 and HT-2 toxins in cereal grains by immunoaffinity clean-up and liquid chromatography with fluorescence detection

A sensitive, precise and accurate method has been developed for the simultaneous determination of T-2 and HT-2 toxins in cereal grains at ppb levels using high-performance liquid chromatography (HPLC) with fluorescence detection and 1-antroylnitrile (1-AN) as labeling reagent after immunoaffinity clean-up. Cereal samples were extracted with methanol/water (90:10, v/v), and the extracts were cleaned-up through commercially available immunoaffinity columns containing monoclonal anti-T-2 antibodies (T-2 test HPLC, Vicam). T-2 and HT-2 toxins were quantified by reversed-phase HPLC with fluorometric detection (excitation wavelength 381 nm, emission wavelength 470 nm) after derivatization with 1-AN. The monoclonal antibody showed 100% cross-reactivity with both T-2 and HT-2 toxin, and the immunoaffinity column clean-up was effective up to 1.4 microg of both toxins. The method was successfully applied to the analysis of T-2 and HT-2 toxins in wheat, maize and barley. Recoveries from spiked samples with toxin levels from 25 to 500 microg/kg ranged from 70% to 100%, with relative standard deviation generally lower than 8%. The limit of detection of the method was 5 microg/kg for T-2 toxin and 3 microg/kg for HT-2 toxin, based on a signal-to-noise ratio 3:1. HT-2 toxin was detected in ten naturally contaminated wheat samples out of 14 samples analyzed, with toxin levels ranging from 10 to 71 microg/kg; three of them contained also T-2 toxin up to 12 microg/kg.     

Catalysis and corrosion: the theoretical surface-science context

Numerous experiments in ultra-high vacuum as well as (T=0 K, p=0) theoretical studies on surfaces have been performed over the last decades in order to gain a better understanding of the mechanisms, which, for example, underlie the phenomena of catalysis and corrosion. Often the results achieved this way cannot be extrapolated directly to the technologically relevant situation of finite temperature and high pressure. Accordingly, modern surface science has realized that bridging the so-called pressure gap (getting out of the vacuum) is the inevitable way to go. Of similar importance are studies in which the temperature is changed systematically (warming up and cooling down). Both aspects are being taken into account in recent experiments and ab initio calculations.

In this paper we stress that there is still much to learn and important questions to be answered concerning the complex atomic and molecular processes which occur at surfaces and actuate catalysis and corrosion, although significant advances in this exciting field have been made over the years. We demonstrate how synergetic effects between theory and experiment are leading to the next step, which is the development of simple concepts and understanding of the different modes of the interaction of chemisorbed species with surfaces. To a large extent this is being made possible by recent developments in theoretical methodology, which allow to extend the ab initio (i.e., starting from the self-consistent electronic structure) approach to poly-atomic complexes with 10,000 and more atoms, time scales of seconds, and involved statistics (e.g., ab initio molecular dynamics with 10,000 and more trajectories). In this paper we will

1. sketch recent density–functional theory based hybrid methods, which bridge the length and time scales from those of electron orbitals to meso- and macroscopic proportions, and

2. present some key results on properties of surfaces, demonstrating their role in corrosion and heterogeneous catalysis. In particular we discuss

◦ the influence of the ambient gas phase on the surface structure and stoichiometry,

◦ adsorbate phase transitions and thermal desorption, and

◦ the role of atoms' dynamics and statistics for the surface chemical reactivity.

Logical Analysis of Legislation Using Flow Diagrams

The Department of the Environment/Department of Transport (DOE/DTp) OR group uses flow diagrams to analyse legislation. Taking the bus deregulation sections of the 1985 Transport Act as the main example, this paper shows how a complex course of action prescribed in legal language may be simply represented by a series of yes/no questions. Traffic Area staff who implement the legislation have the diagrams as working guidelines to resolve their own, potentially conflicting, interpretations.     

Some new aspects of the Boyer reaction

[reaction: see text] The reaction outcome of 2-azidoethanol and aliphatic aldehyde is found to be dependent on the catalyst and the structure of the azido alcohol. Under the catalysis of Cu(II) triflate, the corresponding acetal is obtained. A similar reaction between 2-aryl-2-azidoethanol and aldehyde catalyzed by BF3 yields a mixture of 3-oxazoline and 2-oxazoline. The latter reaction has been used for the preparation of 3-oxazolines in good enantioselectivity.