Reaction Kinetics of the Benzylation of Adenine in DMSO: Regio-Selectivity Guided by Entropy.

The factors governing the regio-selectivity of the alkylation of adenine have been of interest for many years due to the biological importance of adenine derivatives, however, no reaction kinetic studies have been conducted. Herein, we report the rate constants and activation parameters of the benzylation of adenine under basic conditions in DMSO in the absence and presence of 15-crown-5 ether using real-time 1H NMR spectroscopy. The reaction is second-order for the formation of the N9- and N3-benzyladenine products, with a regio-selectivity factor 2.3 in favour of the N9-adduct. The Gibbs free energy of activation amounts to 87±2 kJ mol-1 for both reactions. The formation of the N9-adduct is more activated by 7 kJ mol-1, but its effect is offset by a less negative activation entropy, demonstrating that the long-contested reason for the regioselectivity in the benzylation of adenine is dominated by compensation of entropy and enthalpy in the transition state. The kinetic parameters obtained in the presence of the 15-crown-5 ether indicate that the crown ether forms a complex with an adenine-sodium ion-pair, increasing the activation barrier. However, the Gibbs free energy in the absence and presence of the crown ether remains constant.

Uncovering thermally activated purple-to-blue luminescence in Co-modified MgAl-layered double hydroxide.

Thermally activated blue-to-purple luminescence of Co-modified nano-sandrose MgAl-layered double hydroxides (LDHs) is concentration dependent, occurring only for MgCoAl-LDH with a molar metal cation concentration of 15% Co. Temperature sweep luminescence spectroscopy between 83 K and 298 K shows that the luminescence is strongest at room temperature, increasing with an activation energy of 1 kJ mol-1 between these temperatures. The luminescence occurs in a broad, but fine-structured band below the conduction band (CB) edge at 3.0 eV after excitation at 5.0 eV.

Preserving Cultural Diversity in Rural Africa Using Renewable Energy.

Ninety percent of the large interior, rural part of Africa is not an abundant user of fossil fuels and is not connected to an electricity grid. This limits habitability and leads to significant migration to larger cities in attempts to improve economic and social welfare, which happens at the cost of its rich cultural diversity by inevitable adaption and mixing of societies. A direct transition from a firewood to an off-grid renewable electricity age can mitigate this detrimental development. This perspective discusses the interdisciplinary requirements linking cultural, sociological, economic, and technical aspects for a transition to modern life without loss of valuable traditions. Photovoltaic power and wind energy can provide local affordable electricity in off-grid locations. Intermediate storage for day-night cycles is catered for by novel types of batteries. Purifying and recycling water, refrigerating food and medicine, and benefitting from contact with the world via electronic media permit a tremendous increase in living conditions and significantly lower the pressure of migration into cities. Access to energy is a fundamental requirement for the preservation of the rich cultural diversity with family and tribal bindings, local languages, traditions, and religions, and allows for a more moderate transition to a modern society.

What Is Heat? Can Heat Capacities Be Negative?

In the absence of work, the exchange of heat of a sample of matter corresponds to the change of its internal energy, given by the kinetic energy of random translational motion of all its constituent atoms or molecules relative to the center of mass of the sample, plus the excitation of quantum states, such as vibration and rotation, and the energy of electrons in excess to their ground state. If the sample of matter is equilibrated it is described by Boltzmann's statistical thermodynamics and characterized by a temperature T. Monotonic motion such as that of the stars of an expanding universe is work against gravity and represents the exchange of kinetic and potential energy, as described by the virial theorem, but not an exchange of heat. Heat and work are two distinct properties of thermodynamic systems. Temperature is defined for the radiative cosmic background and for individual stars, but for the ensemble of moving stars neither temperature, nor pressure, nor heat capacities are properly defined, and the application of thermodynamics is, therefore, not advised. For equilibrated atomic nanoclusters, in contrast, one may talk about negative heat capacities when kinetic energy is transformed into potential energy of expanding bonds.

Towards a Molecular Understanding of Cation-Anion Interactions and Self-aggregation of Adeninate Salts in DMSO by NMR and UV Spectroscopy and Crystallography.

Rare anionic forms of nucleic acids play a significant biological role and lead to spontaneous mutations and replication and translational errors. There is a lack of information surrounding the stability and reactivity of these forms. Ion pairs of mono-sodium and -potassium salts of adenine exist in DMSO solution with possible cation coordination sites at the N1, N7 and N9 atoms of the purine ring. At increasing concentrations π-π stacked dimers are the predominant species of aggregates followed by higher order aggregation governed by coordination to metal cations in which the type of counter ion present has a central role in the aggregate formation.

Superatom chemistry: promising properties of near-spherical noble metal clusters.

Properties of near-spherical metal clusters are best understood on the basis of the concept of conventional atoms. Their conduction electrons occupy cluster orbitals that remind of hydrogen-like orbitals since they have the same angular dependence. When populated with electrons, maxima in their ionization potentials and minima in electron affinities reveal the closing of shells in the same sense as for noble gases. This suggests that the periodic table of elements should be amended by a third dimension reflecting the number of atoms in a cluster of the element. In a bonded situation the symmetry of cluster atoms is broken, and the atomic orbital momentum is quenched to a large extent. However, if superatoms are axially symmetric, there are superatomic orbital angular moments that are locked along this symmetry axis. If their z-component is non-zero, this leads to large magnetic moments and to significant spin-orbit interactions, which greatly complicate spectroscopic observation. This magnetic interaction is anisotropic and may lead to hysteresis loops with corresponding blocking temperatures up to room temperature. The number of unpaired electrons in such a system is crucial, and it may be influenced by doping with different atoms or by chemical bonds to capping ligands. Stable superatom clusters with size-tuned, tailored band gaps and band edge energies may be attractive replacements for toxic or rare elements in photovoltaic cells or batteries; they form chemically inert and well-defined stoichiometric complexes with various ligands. This reminds of the established transition metal complexes and may lead to a novel branch of chemistry in which the central ion of organometallic complexes is replaced by a metal superatom.

In command of non-equilibrium.

The second law of thermodynamics is well known for determining the direction of spontaneous processes in the laboratory, life and the universe. It is therefore often called the arrow of time. Less often discussed but just as important is the effect of kinetic barriers which intercept equilibration and preserve highly ordered, high energy non-equilibrium states. Examples of such states are many modern materials produced intentionally for technological applications. Furthermore, all living organisms fuelled directly by photosynthesis and those fuelled indirectly by living on high energy nutrition represent preserved non-equilibrium states. The formation of these states represents the local reversal of the arrow of time which only seemingly violates the second law. It has been known since the seminal work of Prigogine that the stabilisation of these states inevitably requires the dissipation of energy in the form of waste heat. It is this feature of waste heat dissipation following the input of energy that drives all processes occurring at a non-zero rate. Photosynthesis, replication of living organisms, self-assembly, crystal shape engineering and distillation have this principle in common with the well-known Carnot cycle in the heat engine. Drawing on this analogy, we subsume these essential and often sophisticated driven processes under the term machinery of life.

Understanding catalysis.

The large majority of chemical compounds underwent at least one catalytic step during synthesis. While it is common knowledge that catalysts enhance reaction rates by lowering the activation energy it is often obscure how catalysts achieve this. This tutorial review explains some fundamental principles of catalysis and how the mechanisms are studied. The dissociation of formic acid into H2 and CO2 serves to demonstrate how a water molecule can open a new reaction path at lower energy, how immersion in liquid water can influence the charge distribution and energetics, and how catalysis at metal surfaces differs from that in the gas phase. The reversibility of catalytic reactions, the influence of an adsorption pre-equilibrium and the compensating effects of adsorption entropy and enthalpy on the Arrhenius parameters are discussed. It is shown that flexibility around the catalytic centre and residual substrate dynamics on the surface affect these parameters. Sabatier's principle of optimum substrate adsorption, shape selectivity in the pores of molecular sieves and the polarisation effect at the metal-support interface are explained. Finally, it is shown that the application of a bias voltage in electrochemistry offers an additional parameter to promote or inhibit a reaction.

Platinum-hydrogen vibrations and low energy electronic excitations of 13-atom Pt nanoclusters.

Two Pt-H vibrational bands at 1679 cm(-1) and 1392 cm(-1) observed with 13-atom Pt clusters supported in LTL zeolite by Fourier Transform Infrared (FTIR) spectroscopy confirms that H atoms bridge two Pt atoms across the edges of the metal cluster. An additional broad absorption band centred near 2200 cm(-1) which exhibits some substructure is assigned to low energy electronic excitations across the HOMO-LUMO gap of the developing band structure of the nanocluster.

Anomalous diamagnetic susceptibility in 13-atom platinum nanocluster superatoms.

We are used to being able to predict diamagnetic susceptibilities χD to a good approximation in atomic increments since there is normally little dependence on the chemical environment. Surprisingly, we find from SQUID magnetization measurements that the χD per Pt atom of zeolite-supported Pt13 nanoclusters exceeds that of Pt(2+) ions by a factor of 37-50. The observation verifies an earlier theoretical prediction. The phenomenon can be understood nearly quantitatively on the basis of a simple expression for diamagnetic susceptibility and the superatom nature of the 13-atom near-spherical cluster. The two main contributions come from ring currents in the delocalized hydride shell and from cluster molecular orbitals hosting the Pt 5d and Pt 6s electrons.

Hydrogen storage by physisorption on dodecahydro-closo-dodecaboranes.

Hydrogen physisorption on dodecahydro-closo-dodecaborane units is studied using ab initio quantum chemical calculations based on Møller-Plesset perturbation theory. After adding zero-point energy corrections, the adsorption energy due to the charge-quadrupole and the charge-induced dipole interaction is somewhat larger than the more common dispersion interaction with spacer molecules in molecular framework compounds. Furthermore, the energy landscape on the surface of the near-spherical B12H12(2-) permits considerable residual dynamics with corresponding configurational entropy that releases partly the requirements on the magnitude of the adsorption energy. If it can be made fully accessible in an open architecture the system promises an enormous storage capacity. An experimental test for Cs2B12H12 dispersed in the cages of a dealuminated faujasite zeolite amends the theoretical study.

Application of a contact mode AFM for spatially resolved electrochemical impedance spectroscopy measurements of a Nafion membrane electrode assembly.

A Nafion fuel cell membrane is investigated by means of electrochemical atomic force microscopy in different gas atmospheres. From chronoamperometric experiments with a point contact electrode spatially resolved electrochemical impedance spectra are obtained from which information about electrode processes and proton transport in the membrane is derived. In the first part the oxygen reduction reaction is investigated. Due to the absence of diffusion limitation, which is partly a result of the small electrode size, a low frequency inductive loop is observed, which is normally masked in macroscopic electrochemical impedance spectra. The influence of water formation from the oxygen reduction reaction at the cathode is discussed. The second part focuses on a hydrogen/oxygen fuel cell setup. A qualitative explanation is given for the necessity of an applied voltage in addition to the electrochemical potential. Electrochemical impedance spectra obtained at two different positions are compared and fitted based on a Randles-like equivalent circuit. A strongly inhomogeneous performance is observed which is attributed to the properties of the Nafion membrane. The electrolyte resistance and the Nernst impedance are restrictive parameters which describe the diffusion through the membrane.

Maximum hydrogen chemisorption on KL zeolite supported Pt clusters.

Platinum clusters supported on KL zeolites were characterized by EPR, HRTEM, and EXAFS. Two kinds of hydrogen chemisorption experiments both result in a saturation value of 2.9 hydrogen atoms per platinum atom, significantly more than that reported so far. A hydrogen coverage-dependent cluster restructuring is suggested.

Reversible transient hydrogen storage in a fuel cell-supercapacitor hybrid device.

A new concept is investigated for hydrogen storage in a supercapacitor based on large-surface-area carbon material (Black Pearls 2000). Protons and electrons of hydrogen are separated on a fuel cell-type electrode and then stored separately in the electrical double layer, the electrons on the carbon and the protons in the aqueous electrolyte of the supercapacitor electrode. The merit of this concept is that it works spontaneously and reversibly near ambient pressure and temperature. This is in pronounced contrast to what has been known as electrochemical hydrogen storage, which does not involve hydrogen gas and where electrical work has to be spent in the loading process. With the present hybrid device, a H(2) storage capacity of 0.13 wt% was obtained, one order of magnitude more than what can be stored by conventional physisorption on large-surface-area carbons at the same pressure and temperature. Raising the pressure from 1.5 to 3.5 bar increased the capacity by less than 20%, indicating saturation. A capacitance of 11 µF cm(-2), comparable with that of a commercial double layer supercapacitor, was found using H(2)SO(4) as electrolyte. The chemical energy of the stored H(2) is almost a factor of 3 larger than the electrical energy stored in the supercapacitor. Further developments of this concept relate to a hydrogen buffer integrated inside a proton exchange membrane fuel cell to be used in case of peak power demand. This serial setup takes advantage of the suggested novel concept of hydrogen storage. It is fundamentally different from previous ways of operating a conventional supercapacitor hooked up in parallel to a fuel cell.

Impedance Spectroscopic Investigation of Proton Conductivity in Nafion Using Transient Electrochemical Atomic Force Microscopy (AFM).

Spatially resolved impedance spectroscopy of a Nafion polyelectrolyte membrane is performed employing a conductive and Pt-coated tip of an atomic force microscope as a point-like contact and electrode. The experiment is conducted by perturbing the system by a rectangular voltage step and measuring the incurred current, followed by Fourier transformation and plotting the impedance against the frequency in a conventional Bode diagram. To test the potential and limitations of this novel method, we present a feasibility study using an identical hydrogen atmosphere at a well-defined relative humidity on both sides of the membrane. It is demonstrated that good quality impedance spectra are obtained in a frequency range of 0.2-1,000 Hz. The extracted polarization curves exhibit a maximum current which cannot be explained by typical diffusion effects. Simulation based on equivalent circuits requires a Nernst element for restricted diffusion in the membrane which suggests that this effect is based on the potential dependence of the electrolyte resistance in the high overpotential region.

Spin trapping of hydroxyl radicals on Cu/HY zeolites suspended in aqueous solution.

Hydroxyl radical intermediates are trapped in calcined Cu/HY zeolites in the presence of oxygen and water. This suggests that hydrogen peroxide is formed in situ from oxygen. Brønsted acids enhance the formation of the radicals.

Adsorption of oxygen on copper in Cu/HZSM5 zeolites.

The present study focuses on the characterization of the active sites for oxygen adsorption in both copper-free and copper-containing HZSM5 zeolites. FTIR, EPR, EXAFS and UV-Vis measurements offer insight into the initial state of the catalyst before oxygen adsorption. Both liquid and solid state ion exchanged samples contain a certain amount of Cu(ii) and Cu(i) ions in the alpha3, alpha4 and gamma6 position, their population ratio depending on the ion exchange temperature. They are accessible for interaction with the adsorbate, as the copper-oxygen spin exchange demonstrates. Both the sample magnetization and the EXAFS analysis indicate that 10-30% of the Cu(ii) exists in the form of oxygen bridged Cu-Cu pairs. UV-Vis measurements prove that two different antiferromagnetically coupled copper peroxide complexes are formed during the sample preparation process, the bis(mu-oxo)- and (mu-eta(2):eta(2)-peroxo) dimers. One of the complexes is susceptible to oxygen adsorption, which cleaves it irreversibly into two individual Cu(ii)-O(2)(-) units, while Cu(i) ions are oxidised to the same species. The Brønsted acid sites are also able to adsorb oxygen both at room and low temperatures. The presence of the different active sites may be an explanation for the high catalytic activity of the Cu/HZSM5 zeolite. The Brønsted sites near copper centers could protonate the peroxide complexes, leading to the in situ formation of hydrogen peroxide, a common oxidant. This peroxide would be a highly active species for catalytic reactions.

EPR spectroscopic investigation of radical-induced degradation of partially fluorinated aromatic model compounds for fuel cell membranes.

EPR spectroscopic investigations of reactions between monomeric model compounds representing typical structural moieties of poly(aryl) ionomers and photochemically generated hydroxyl radicals are reported. Deoxygenated solutions of the model compounds (in a water/methanol mixture) containing hydrogen peroxide at defined pH values were exposed to UV light in the flow cell within the cavity of an EPR spectrometer. Spectra were analyzed by computer simulation and the formed radicals were assigned by comparing their g-factors and hyperfine coupling constants (hfccs) with those from the literature and from density functional theory (DFT) calculations. The relevance for polymer electrolyte membrane fuel cells (PEMFCs) and alkaline-anion exchange membrane fuel cells (AAEMFCs) is discussed.