Wolfgang Schmickler
- Published in print:
- 1996
- Published Online:
- November 2020
- ISBN:
- 9780195089325
- eISBN:
- 9780197560563
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780195089325.003.0021
- Subject:
- Chemistry, Physical Chemistry
The traditional electrochemical techniques are based on the measurement of current and potential, and, in the case of liquid electrodes, of the surface ...
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The traditional electrochemical techniques are based on the measurement of current and potential, and, in the case of liquid electrodes, of the surface tension. While such measurements can be very precise, they give no direct information on the microscopic structure of the electrochemical interface. In this chapter we treat several methods which can provide such information. None of them is endemic to electrochemistry; they are mostly skillful adaptations of techniques developed in other branches of physics and chemistry. The scanning tunneling microscope (STM) is an excellent device to obtain topographic images of an electrode surface . The principal part of this apparatus is a metal tip with a very fine point, which can be moved in all three directions of space with the aid of piezoelectric crystals. All but the very end of the tip is insulated from the solution in order to avoid tip currents due to unwanted electrochemical reactions. The tip is brought very close, up to a few Ångstroms, to the electrode surface. When a potential bias ΔV, usually of the order of a few hundred millivolts, is applied between the electrode and the tip, the electrons can tunnel through the thin intervening layer of solution, and a tunneling current is observed. The situation is illustrated in Fig. 15.2: A potential energy barrier exists between the tip and the substrate. Application of a bias potential shifts the two Fermi levels of the tip and of the substrate. Electrons can tunnel from the metal with the higher Fermi level through the barrier to empty states on the other metal. Roughly speaking, electrons with energies between the two Fermi levels can be transferred. A detailed calculation shows that the current is proportional to the electronic density of states at the Fermi level of the substrate. The tip is moved slowly in the yz direction parallel to the metal surface, and simultaneously the distance x from the electrode is adjusted in such a way that the tunneling current is constant (constant-current mode).
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The traditional electrochemical techniques are based on the measurement of current and potential, and, in the case of liquid electrodes, of the surface tension. While such measurements can be very precise, they give no direct information on the microscopic structure of the electrochemical interface. In this chapter we treat several methods which can provide such information. None of them is endemic to electrochemistry; they are mostly skillful adaptations of techniques developed in other branches of physics and chemistry. The scanning tunneling microscope (STM) is an excellent device to obtain topographic images of an electrode surface . The principal part of this apparatus is a metal tip with a very fine point, which can be moved in all three directions of space with the aid of piezoelectric crystals. All but the very end of the tip is insulated from the solution in order to avoid tip currents due to unwanted electrochemical reactions. The tip is brought very close, up to a few Ångstroms, to the electrode surface. When a potential bias ΔV, usually of the order of a few hundred millivolts, is applied between the electrode and the tip, the electrons can tunnel through the thin intervening layer of solution, and a tunneling current is observed. The situation is illustrated in Fig. 15.2: A potential energy barrier exists between the tip and the substrate. Application of a bias potential shifts the two Fermi levels of the tip and of the substrate. Electrons can tunnel from the metal with the higher Fermi level through the barrier to empty states on the other metal. Roughly speaking, electrons with energies between the two Fermi levels can be transferred. A detailed calculation shows that the current is proportional to the electronic density of states at the Fermi level of the substrate. The tip is moved slowly in the yz direction parallel to the metal surface, and simultaneously the distance x from the electrode is adjusted in such a way that the tunneling current is constant (constant-current mode).
Kathleen Araújo
- Published in print:
- 2018
- Published Online:
- November 2020
- ISBN:
- 9780199362554
- eISBN:
- 9780197562901
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780199362554.003.0007
- Subject:
- Environmental Science, Environmental Sustainability
Today’s energy sectors hold different potentials for saving on energy, carbon, and other greenhouse gases (GHGs). Buildings, for instance, represent more than 40% of ...
More
Today’s energy sectors hold different potentials for saving on energy, carbon, and other greenhouse gases (GHGs). Buildings, for instance, represent more than 40% of energy use worldwide and one-third of GHGs (United Nations Environment Programme [UNEP], n.d.a). Improvements in heating, cooling, and powering of buildings, as well as industrial processes, can deliver substantial and cost-effective savings. In line with this, geothermal energy represents a more unusual form of renewable energy in that it can directly contribute to heating, cooling, and electricity services. Unlike a number of its counterparts, geothermal energy can provide a more stable and renewable form of energy that is largely unaffected by weather. The chapter focuses on geothermal energy adoption in Iceland, “a little country that roars,” according to UNFCCC Executive Secretary Christina Figueres (Iceland Monitor, 2014), when discussing leadership in renewable energy use and related action. In developing its renewable energy leadership, Iceland has wrestled, like many countries, with tradeoffs in energy, the environment, and economic development. The chapter highlights the interplay of these interests and explores the innovative engineering and industrial spillovers in Iceland’s geothermal adoption. Iceland is a country of roughly 333,000 people, and is a global leader in renewable energy use (Islandsbanki, 2010; Ministry of the Environment, 2010; Statistics Iceland, 2017). Two-thirds of the country’s primary energy consists of geothermal energy, with roughly nine out of ten Icelandic homes heated by the fuel source and a quarter of the country’s electricity powered by it (Orkustofnun, 2015; Ragnarsson, 2015). The nation leads globally in terms of geothermal heat capacity per capita and serves as a principal source of international training and consulting on geothermal energy, with a diverse industrial cluster that has developed around the technology (Gekon, n.d.; United Nations University Geothermal Training Programme [UN- GTP], n.d). The country’s low carbon development pathway reflects choices and debate about how to manage its natural resources and allow for foreign investment. Iceland began the 20th century as one of the poorest nations in Europe and is now a top-ranked country in the United Nations Development Program’s Human Development Index (Hannibalsson, 2008; United Nations Development Program [UNDP], 2015).
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Today’s energy sectors hold different potentials for saving on energy, carbon, and other greenhouse gases (GHGs). Buildings, for instance, represent more than 40% of energy use worldwide and one-third of GHGs (United Nations Environment Programme [UNEP], n.d.a). Improvements in heating, cooling, and powering of buildings, as well as industrial processes, can deliver substantial and cost-effective savings. In line with this, geothermal energy represents a more unusual form of renewable energy in that it can directly contribute to heating, cooling, and electricity services. Unlike a number of its counterparts, geothermal energy can provide a more stable and renewable form of energy that is largely unaffected by weather. The chapter focuses on geothermal energy adoption in Iceland, “a little country that roars,” according to UNFCCC Executive Secretary Christina Figueres (Iceland Monitor, 2014), when discussing leadership in renewable energy use and related action. In developing its renewable energy leadership, Iceland has wrestled, like many countries, with tradeoffs in energy, the environment, and economic development. The chapter highlights the interplay of these interests and explores the innovative engineering and industrial spillovers in Iceland’s geothermal adoption. Iceland is a country of roughly 333,000 people, and is a global leader in renewable energy use (Islandsbanki, 2010; Ministry of the Environment, 2010; Statistics Iceland, 2017). Two-thirds of the country’s primary energy consists of geothermal energy, with roughly nine out of ten Icelandic homes heated by the fuel source and a quarter of the country’s electricity powered by it (Orkustofnun, 2015; Ragnarsson, 2015). The nation leads globally in terms of geothermal heat capacity per capita and serves as a principal source of international training and consulting on geothermal energy, with a diverse industrial cluster that has developed around the technology (Gekon, n.d.; United Nations University Geothermal Training Programme [UN- GTP], n.d). The country’s low carbon development pathway reflects choices and debate about how to manage its natural resources and allow for foreign investment. Iceland began the 20th century as one of the poorest nations in Europe and is now a top-ranked country in the United Nations Development Program’s Human Development Index (Hannibalsson, 2008; United Nations Development Program [UNDP], 2015).
Michael B. McElroy
- Published in print:
- 2016
- Published Online:
- November 2020
- ISBN:
- 9780190490331
- eISBN:
- 9780197559642
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780190490331.003.0018
- Subject:
- Environmental Science, Environmental Sustainability
As discussed in Chapter 3, the transportation sector accounts for approximately a third of total emissions of CO2 in the United States, with a smaller fraction but a ...
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As discussed in Chapter 3, the transportation sector accounts for approximately a third of total emissions of CO2 in the United States, with a smaller fraction but a rapidly growing total in China. Combustion of oil, either as gasoline or diesel, is primarily responsible for the transportation- related emissions of both countries. Strategies to curtail overall emissions of CO2 must include plans for a major reduction in the use of oil in the transportation sector. This could be accomplished (1) by reducing demand for trans¬portation services; (2) by increasing the energy efficiency of the sector; or (3) by transitioning to an energy system less reliant on carbon- emitting sources of energy. Assuming continuing growth in the economies of both countries, option 1 is unlikely, certainly for China. Significant success has been achieved already in the United States under option 2, prompted by the application of increasingly more stringent corporate average fuel economy (CAFE) standards. And the technological advances achieved under this program are likely to find application in China and elsewhere, given the global nature of the automobile/ truck industry. The topic for discussion in this chapter is whether switching from oil to a plant- or animal- based fuel could contribute to a significant reduction in CO2 emissions from the transportation sector of either or both countries, indeed from the globe as a whole. The question is whether plant- based ethanol can substitute for gasoline and whether additional plant- and animal- derived products can cut back on demand for diesel. The related issue is whether this substitution can contribute at acceptable social and economic cost to a net reduction in overall CO2 emissions when account is taken of the entire lifecycle for production of the nonfossil alternatives. There is an extensive history to the use of ethanol as a motor fuel. Nicolas Otto, cred¬ited with the development of the internal combustion engine, used ethanol as the energy source for one of his early vehicle inventions in 1860. Henry Ford designed his first auto¬mobile, the quadricycle, to run on pure ethanol in 1896.
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As discussed in Chapter 3, the transportation sector accounts for approximately a third of total emissions of CO2 in the United States, with a smaller fraction but a rapidly growing total in China. Combustion of oil, either as gasoline or diesel, is primarily responsible for the transportation- related emissions of both countries. Strategies to curtail overall emissions of CO2 must include plans for a major reduction in the use of oil in the transportation sector. This could be accomplished (1) by reducing demand for trans¬portation services; (2) by increasing the energy efficiency of the sector; or (3) by transitioning to an energy system less reliant on carbon- emitting sources of energy. Assuming continuing growth in the economies of both countries, option 1 is unlikely, certainly for China. Significant success has been achieved already in the United States under option 2, prompted by the application of increasingly more stringent corporate average fuel economy (CAFE) standards. And the technological advances achieved under this program are likely to find application in China and elsewhere, given the global nature of the automobile/ truck industry. The topic for discussion in this chapter is whether switching from oil to a plant- or animal- based fuel could contribute to a significant reduction in CO2 emissions from the transportation sector of either or both countries, indeed from the globe as a whole. The question is whether plant- based ethanol can substitute for gasoline and whether additional plant- and animal- derived products can cut back on demand for diesel. The related issue is whether this substitution can contribute at acceptable social and economic cost to a net reduction in overall CO2 emissions when account is taken of the entire lifecycle for production of the nonfossil alternatives. There is an extensive history to the use of ethanol as a motor fuel. Nicolas Otto, cred¬ited with the development of the internal combustion engine, used ethanol as the energy source for one of his early vehicle inventions in 1860. Henry Ford designed his first auto¬mobile, the quadricycle, to run on pure ethanol in 1896.
Philip Coppens
- Published in print:
- 1997
- Published Online:
- November 2020
- ISBN:
- 9780195098235
- eISBN:
- 9780197560877
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780195098235.003.0014
- Subject:
- Chemistry, Physical Chemistry
Small molecules consisting of light-, few-electron atoms were the first species beyond atoms to yield to quantum-mechanical methods. Similarly, crystals of small ...
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Small molecules consisting of light-, few-electron atoms were the first species beyond atoms to yield to quantum-mechanical methods. Similarly, crystals of small light-atom molecules have served as most useful test cases of charge density mapping. The small number of core electrons in first-row atoms enhances the relative contribution of valence electron scattering to the diffraction pattern. Early studies, done just after automated diffractometers became widely available, were concerned with molecular crystals such as uracil (Stewart and Jensen 1967), s-triazine (Coppens 1967), oxalic acid dihydrate (Coppens et al. 1969), decaborane (Dietrich and Scheringer 1978), fumaramic acid (Hirshfeld 1971), glycine (Almlof et al. 1973), and tetraphenylbutatriene (Berkovitch-Yellin and Leiserowitz 1976). While thermal motion is often pronounced in molecular crystals, advances in low-temperature data collection have done much to alleviate this disadvantage. In recent years, subliquid-nitrogen cooling techniques have been increasingly applied. Among the most interesting aspects of molecular crystals are the influence of intermolecular interactions on the electronic structure. Physically meaningful Coulombic parameters pertinent to a molecule in a condensed environment can be obtained from the diffraction analysis, and can be used in the modeling of macromolecules. The enhancement of the electrostatic moments relative to those of the isolated species has been noted in chapter 7. But, beyond these considerations, molecular crystals are important in their own right. For example, crystals of aromatic molecules substituted with π-electron donor and acceptor groups are among the most strongly nonlinear optical solids known, considerably exceeding the nonlinearity of inorganic crystals such as potassium titanyl phosphate (KTP); while mixed-valence organic components of low-dimensional solids can become superconducting at low temperatures. The relation between such properties of molecular crystals and their charge distribution provides a continuing impetus for further study. The suitability of light-atom crystals for charge density analysis can be understood in terms of the relative importance of core electron scattering. As the perturbation of the core electrons by the chemical environment is beyond the reach of practically all experimental studies, the frozen-core approximation is routinely used. It assumes the intensity of the core electron scattering to be invariable, while the valence scattering is affected by the chemical environment, as discussed in chapter 3.
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Small molecules consisting of light-, few-electron atoms were the first species beyond atoms to yield to quantum-mechanical methods. Similarly, crystals of small light-atom molecules have served as most useful test cases of charge density mapping. The small number of core electrons in first-row atoms enhances the relative contribution of valence electron scattering to the diffraction pattern. Early studies, done just after automated diffractometers became widely available, were concerned with molecular crystals such as uracil (Stewart and Jensen 1967), s-triazine (Coppens 1967), oxalic acid dihydrate (Coppens et al. 1969), decaborane (Dietrich and Scheringer 1978), fumaramic acid (Hirshfeld 1971), glycine (Almlof et al. 1973), and tetraphenylbutatriene (Berkovitch-Yellin and Leiserowitz 1976). While thermal motion is often pronounced in molecular crystals, advances in low-temperature data collection have done much to alleviate this disadvantage. In recent years, subliquid-nitrogen cooling techniques have been increasingly applied. Among the most interesting aspects of molecular crystals are the influence of intermolecular interactions on the electronic structure. Physically meaningful Coulombic parameters pertinent to a molecule in a condensed environment can be obtained from the diffraction analysis, and can be used in the modeling of macromolecules. The enhancement of the electrostatic moments relative to those of the isolated species has been noted in chapter 7. But, beyond these considerations, molecular crystals are important in their own right. For example, crystals of aromatic molecules substituted with π-electron donor and acceptor groups are among the most strongly nonlinear optical solids known, considerably exceeding the nonlinearity of inorganic crystals such as potassium titanyl phosphate (KTP); while mixed-valence organic components of low-dimensional solids can become superconducting at low temperatures. The relation between such properties of molecular crystals and their charge distribution provides a continuing impetus for further study. The suitability of light-atom crystals for charge density analysis can be understood in terms of the relative importance of core electron scattering. As the perturbation of the core electrons by the chemical environment is beyond the reach of practically all experimental studies, the frozen-core approximation is routinely used. It assumes the intensity of the core electron scattering to be invariable, while the valence scattering is affected by the chemical environment, as discussed in chapter 3.
Claude Balny
- Published in print:
- 1996
- Published Online:
- November 2020
- ISBN:
- 9780195097221
- eISBN:
- 9780197560839
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780195097221.003.0017
- Subject:
- Chemistry, Organic Chemistry
In a detailed study of an enzyme reaction pathway, a measured composite rate constant, for example, kcat, can be interpreted in ways that lead to ambiguous ...
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In a detailed study of an enzyme reaction pathway, a measured composite rate constant, for example, kcat, can be interpreted in ways that lead to ambiguous conclusions. Two conditions must be met to solve this problem: (1) an elementary rate constant must be measured, and (2) a maximum number of physical-chemical parameters must be used to perturb the system under study. To gain access to elementary rate constants, cryobaroenzymology and/or transient methods, such as stopped-flow and flow-quench kinetics, can be used. Both perturbation and kinetics measurements performed under either high pressure or low temperatures can then be used to probe the thermodynamics of the interconversion of two successive intermediates to obtain parameters such as ΔG‡, ΔS‡, ΔH‡, and ΔV‡ The interdependence of the two major variables, namely temperature and pressure, is presented in this article, in which the role of organic cosolvents is considered as a third variable. During catalytic reactions, enzymes undergo a number of conformational changes related to their dynamic structural flexibility. This appears as a succession of different steps. A complete study of such processes, which generally are very rapid, consists of the exploration of the properties of these steps, including thermodynamic features obtained by the action of temperature and pressure. As long ago as 1950, Laidler (1950) formulated the first theoretical basis for explaining the responses of enzymes to high hydrostatic pressures. Chemists used this parameter extensively, and in the early stages of high-pressure kinetics they attempted to analyze the observed results on the basis of collision theory (Asano, 1991) or transition-state theory (Evans & Polanyi, 1935). These theories are still used to describe pressure effects on enzyme reactions. It is postulated that between two successive intermediates there is a labile transition state which governs the energetics of the reaction (Glastone et al., 1941). But we must remember that this theory was first applied only to simple homogeneous reactions in gases. For solutions, the treatment can require the introduction of other parameters such as the viscosity.
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In a detailed study of an enzyme reaction pathway, a measured composite rate constant, for example, kcat, can be interpreted in ways that lead to ambiguous conclusions. Two conditions must be met to solve this problem: (1) an elementary rate constant must be measured, and (2) a maximum number of physical-chemical parameters must be used to perturb the system under study. To gain access to elementary rate constants, cryobaroenzymology and/or transient methods, such as stopped-flow and flow-quench kinetics, can be used. Both perturbation and kinetics measurements performed under either high pressure or low temperatures can then be used to probe the thermodynamics of the interconversion of two successive intermediates to obtain parameters such as ΔG‡, ΔS‡, ΔH‡, and ΔV‡ The interdependence of the two major variables, namely temperature and pressure, is presented in this article, in which the role of organic cosolvents is considered as a third variable. During catalytic reactions, enzymes undergo a number of conformational changes related to their dynamic structural flexibility. This appears as a succession of different steps. A complete study of such processes, which generally are very rapid, consists of the exploration of the properties of these steps, including thermodynamic features obtained by the action of temperature and pressure. As long ago as 1950, Laidler (1950) formulated the first theoretical basis for explaining the responses of enzymes to high hydrostatic pressures. Chemists used this parameter extensively, and in the early stages of high-pressure kinetics they attempted to analyze the observed results on the basis of collision theory (Asano, 1991) or transition-state theory (Evans & Polanyi, 1935). These theories are still used to describe pressure effects on enzyme reactions. It is postulated that between two successive intermediates there is a labile transition state which governs the energetics of the reaction (Glastone et al., 1941). But we must remember that this theory was first applied only to simple homogeneous reactions in gases. For solutions, the treatment can require the introduction of other parameters such as the viscosity.
H. G. Drickamer
- Published in print:
- 1996
- Published Online:
- November 2020
- ISBN:
- 9780195097221
- eISBN:
- 9780197560839
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780195097221.003.0005
- Subject:
- Chemistry, Organic Chemistry
Pressure-tuning spectroscopy is a powerful tool for investigating molecular interactions. These interactions may involve organic or inorganic materials in liquid, ...
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Pressure-tuning spectroscopy is a powerful tool for investigating molecular interactions. These interactions may involve organic or inorganic materials in liquid, polymeric, or crystalline media. In this article we confine our attention to organic molecules, largely in dilute solution in polymers or liquids. We demonstrate the use of high-pressure luminescence to study the effect of the environment on π* →π, π* →n and charge-transfer excitations, as well as the interaction between singlet and triplet states. In addition, we provide tests of the energy gap law for non-radiative dissipation of excitation, the role of viscosity in luminescent efficiency, and the internal consistency of various means of predicting and correlating energy transfer. Over the past 40 years, it has been amply demonstrated that high pressure is a powerful tool for studying electronic phenomena in condensed phases. The basic concept is as follows. The optical, electrical, magnetic, and chemical properties—collectively the electronic properties—of condensed phases depend on the interactions of the outer electrons on the atoms, molecules, or ions that make up the phase. Different kinds of electronic orbitals have different spatial characteristics—different radial extent, different shape (orbital angular momentum), and different diffuseness; therefore, pressure perturbs the energies associated with these orbitals in different degrees. This relative perturbation we call “pressure tuning,” and the measurement and explanation of the tuning is “pressure-tuning spectroscopy.” Pressure-tuning spectroscopy of the vibrational and rotational excitations of atoms in molecular and in crystal lattices is also an active and important field, but in this article we arc concerned mainly with electronic phenomena. We further limit this discussion primarily to organic molecules in solid polymers or liquid solutions, as these have the greatest relevance to biologically active systems. A variety of probes are used for studying electronic phenomena under high pressure, but the emphasis here is on luminescence. The presentation consists of a series of examples of various types of excitations on interactions where high pressure has been an effective tool. Only references directly relevant to each example are included. Two general references to pressure studies of molecular luminescence have been published (Drickamer, 1982, 1990).
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Pressure-tuning spectroscopy is a powerful tool for investigating molecular interactions. These interactions may involve organic or inorganic materials in liquid, polymeric, or crystalline media. In this article we confine our attention to organic molecules, largely in dilute solution in polymers or liquids. We demonstrate the use of high-pressure luminescence to study the effect of the environment on π* →π, π* →n and charge-transfer excitations, as well as the interaction between singlet and triplet states. In addition, we provide tests of the energy gap law for non-radiative dissipation of excitation, the role of viscosity in luminescent efficiency, and the internal consistency of various means of predicting and correlating energy transfer. Over the past 40 years, it has been amply demonstrated that high pressure is a powerful tool for studying electronic phenomena in condensed phases. The basic concept is as follows. The optical, electrical, magnetic, and chemical properties—collectively the electronic properties—of condensed phases depend on the interactions of the outer electrons on the atoms, molecules, or ions that make up the phase. Different kinds of electronic orbitals have different spatial characteristics—different radial extent, different shape (orbital angular momentum), and different diffuseness; therefore, pressure perturbs the energies associated with these orbitals in different degrees. This relative perturbation we call “pressure tuning,” and the measurement and explanation of the tuning is “pressure-tuning spectroscopy.” Pressure-tuning spectroscopy of the vibrational and rotational excitations of atoms in molecular and in crystal lattices is also an active and important field, but in this article we arc concerned mainly with electronic phenomena. We further limit this discussion primarily to organic molecules in solid polymers or liquid solutions, as these have the greatest relevance to biologically active systems. A variety of probes are used for studying electronic phenomena under high pressure, but the emphasis here is on luminescence. The presentation consists of a series of examples of various types of excitations on interactions where high pressure has been an effective tool. Only references directly relevant to each example are included. Two general references to pressure studies of molecular luminescence have been published (Drickamer, 1982, 1990).
Brian G. Cox
- Published in print:
- 2013
- Published Online:
- May 2013
- ISBN:
- 9780199670512
- eISBN:
- 9780199670512
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199670512.003.0005
- Subject:
- Physics, Condensed Matter Physics / Materials
Protic solvents have hydrogen bound directly to electronegative atoms, such as oxygen or nitrogen. They are characterized by their ability to form strong hydrogen bonds with suitable acceptors, ...
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Protic solvents have hydrogen bound directly to electronegative atoms, such as oxygen or nitrogen. They are characterized by their ability to form strong hydrogen bonds with suitable acceptors, particularly simple anions. They include alcohols, formamide and other primary and secondary amides, and formic acid. In methanol, dissociation constants of carboxylic acids, phenols, and protonated nitrogen bases show excellent correlations with corresponding values in water. The largest differences occur for carboxylic acids, which are typically 5 pK-units weaker than in water. Acids become increasingly weak in the higher alcohols, especially t-butanol, because of poorer ion solvation. Homohydrogen-bond formation is generally weak, but ion-pair formation becomes progressively stronger as the solvent polarity decreases. Formamide contains a polar carbonyl group in addition to the ability to hydrogen-bond to anions, and displays pKa-values close to those in water. Formic acid hydrogen-bonds strongly with anions, but poor solvation of the proton, which inhibits the dissociation of acids, normally prevails.Less
Protic solvents have hydrogen bound directly to electronegative atoms, such as oxygen or nitrogen. They are characterized by their ability to form strong hydrogen bonds with suitable acceptors, particularly simple anions. They include alcohols, formamide and other primary and secondary amides, and formic acid. In methanol, dissociation constants of carboxylic acids, phenols, and protonated nitrogen bases show excellent correlations with corresponding values in water. The largest differences occur for carboxylic acids, which are typically 5 pK-units weaker than in water. Acids become increasingly weak in the higher alcohols, especially t-butanol, because of poorer ion solvation. Homohydrogen-bond formation is generally weak, but ion-pair formation becomes progressively stronger as the solvent polarity decreases. Formamide contains a polar carbonyl group in addition to the ability to hydrogen-bond to anions, and displays pKa-values close to those in water. Formic acid hydrogen-bonds strongly with anions, but poor solvation of the proton, which inhibits the dissociation of acids, normally prevails.
Brian G. Cox
- Published in print:
- 2013
- Published Online:
- May 2013
- ISBN:
- 9780199670512
- eISBN:
- 9780199670512
- Item type:
- chapter
- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199670512.003.0009
- Subject:
- Physics, Condensed Matter Physics / Materials
Tables of dissociation constants are given for a wide range of substrates in the most extensively studied non-aqueous solvents, including methanol, dimethylsulphoxide, dimethylformamide, ...
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Tables of dissociation constants are given for a wide range of substrates in the most extensively studied non-aqueous solvents, including methanol, dimethylsulphoxide, dimethylformamide, acetonitrile, and tetrahydrofuran. Equations representing correlations among the solvents and with water are also included.Less
Tables of dissociation constants are given for a wide range of substrates in the most extensively studied non-aqueous solvents, including methanol, dimethylsulphoxide, dimethylformamide, acetonitrile, and tetrahydrofuran. Equations representing correlations among the solvents and with water are also included.