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Structure and bonding in some simple hydrocarbons and in ethane and ethene analogues of the heavier Group 14 elements

Arne Haaland

in Molecules and Models: The molecular structures of main group element compounds

This chapter describes the molecular structures and CC bond energies in ethane, ethene, and ethyne, and the bonding in terms of the molecular orbital model. The delocalized π molecular orbitals ... More

This chapter describes the molecular structures and CC bond energies in ethane, ethene, and ethyne, and the bonding in terms of the molecular orbital model. The delocalized π molecular orbitals obtained by Hückel calculations on benzene are described. The molecular structures of disilene and other ethene analogues of the heavier Group 14 elements, R2EER2, E = Si, Ge, Sn, or Pb are discussed and their non-planar, trans-folded structures explained in terms of the molecular orbital model. In the final section, the totally unexpected structure of the ethyne analogue Si2H2 is discussed.Less

Introduction

Jenny Pickworth Glusker and Kenneth N. Trueblood

in Title Pages

Much of our present knowledge of the architecture of molecules has been obtained from studies of the diffraction of X rays or neutrons by crystals. X rays ... More

Much of our present knowledge of the architecture of molecules has been obtained from studies of the diffraction of X rays or neutrons by crystals. X rays are scattered by the electrons of atoms and ions, and the interference between the X rays scattered by the different atoms or ions. in a crystal can result in a diffraction pattern. Similarly, neutrons are scattered by the nuclei of atoms. Measurements on a crystal diffraction pattern can lead to information on the arrangement of atoms or ions within the crystal. This is the experimental technique to be described in this book. X-ray diffraction was first used to establish the three-dimensional arrangement of atoms in a crystal by William Lawrence Bragg in 1913 (Bragg, 1913), shortly after Wilhelm Conrad Röntgen had discovered X rays and Max von Laue had shown in 1912 that these X rays could be diffracted by crystals (Röntgen, 1895; Friedrich et al., 1912). Later, in 1927 and 1936 respectively, it was also shown that electrons and neutrons could be diffracted by crystals (Davisson and Germer, 1927; von Halban and Preiswerk, 1936; Mitchell and Powers, 1936). Bragg found from X-ray diffraction studies that, in crystals of sodium chloride, each sodium is surrounded by six equidistant chlorines and each chlorine by six equidistant sodiums. No discrete molecules of NaCl were found and therefore Bragg surmised that the crystal consisted of sodium ions and chloride ions rather than individual (noncharged) atoms (Bragg, 1913); this had been predicted earlier by William Barlow and William Jackson Pope (Barlow and Pope, 1907), but had not, prior to the research of the Braggs, been demonstrated experimentally. A decade and a half later, in 1928, Kathleen Lonsdale used X-ray diffraction methods to show that the benzene ring is a flat regular hexagon in which all carbon–carbon bonds are equal in length, rather than a ring structure that contains alternating single and double bonds (Lonsdale, 1928).Her experimental result, later confirmed by spectroscopic studies (Stoicheff, 1954), was of great significance in chemistry. Less

The derivation of trial structures. I. Analytical methods for direct phase determination

Jenny Pickworth Glusker and Kenneth N. Trueblood

in Crystal Structure Analysis: A Primer

As indicated at the start of Chapter 4, after the diffraction pattern has been recorded and measured, the next stage in a crystal structure determination ... More

As indicated at the start of Chapter 4, after the diffraction pattern has been recorded and measured, the next stage in a crystal structure determination is solving the structure—that is, finding a suitable “trial structure” that contains approximate positions for most of the atoms in the unit cell of known dimensions and space group. The term “trial structure” implies that the structure that has been found is only an approximation to the correct or “true” structure, while “suitable” implies that the trial structure is close enough to the true structure that it can be smoothly refined to give a good fit to the experimental data. Methods for finding suitable trial structures form the subject of this chapter and the next. In the early days of structure determination, trial and error methods were, of necessity, almost the only available way of solving structures. Structure factors for the suggested “trial structure” were calculated and compared with those that had been observed. When more productive methods for obtaining trial structures—the “Patterson function” and “direct methods”—were introduced, the manner of solving a crystal structure changed dramatically for the better. We begin with a discussion of so-called “direct methods.” These are analytical techniques for deriving an approximate set of phases from which a first approximation to the electron-density map can be calculated. Interpretation of this map may then give a suitable trial structure. Previous to direct methods, all phases were calculated (as described in Chapter 5) from a proposed trial structure. The search for other methods that did not require a trial structure led to these phaseprobability methods, that is, direct methods. A direct solution to the phase problem by algebraic methods began in the 1920s (Ott, 1927; Banerjee, 1933; Avrami, 1938) and progressed with work on inequalities by David Harker and John Kasper (Harker and Kasper, 1948). The latter authors used inequality relationships put forward by Augustin Louis Cauchy and Karl Hermann Amandus Schwarz that led to relations between the magnitudes of some structure factors. Less

Structural parameters: Analysis of results

Jenny Pickworth Glusker and Kenneth N. Trueblood

in Crystal Structure Analysis: A Primer

The results of an X-ray structure analysis are coordinates of the individual, chemically identified atoms in each unit cell, the space group (which gives ... More

The results of an X-ray structure analysis are coordinates of the individual, chemically identified atoms in each unit cell, the space group (which gives equivalent positions), and displacement parameters that may be interpreted as indicative of molecular motion and/or disorder. Such data obtained from crystal structure analyses may be incorporated into a CIF or mmCIF (Crystallographic Information File or Macromolecular Crystallographic Information File). These ensure that the results of crystal structure analyses are usefully archived. There are many checks that the crystallographer can make to ensure that the CIF or mmCIF file is correctly informative. For example, the automated validation program PLATON (Spek, 2003) checks that all data reported are up to the standards required for publication by the International Union of Crystallography. It does geometrical calculations on the structure, illustrates the results, finds if any symmetry has been missed, investigates any twinning, and checks if the structure has already been reported. We now review the ways in which these atomic parameters can be used to obtain a three-dimensional vision of the entire crystal structure. When molecules crystallize in an orthorhombic, tetragonal, or cubic unit cell it is reasonably easy to build a model using the unit-cell dimensions and fractional coordinates, because all the interaxial angles are 90◦. However, the situation is more complicated if the unit cell contains oblique axes and it is often simpler to convert the fractional crystal coordinates to orthogonal coordinates before calculating molecular geometry. The equations for doing this for bond lengths, interbond angles, and torsion angles are presented in Appendix 12. If the reader wishes to compute interatomic distances directly, this is also possible if one knows the cell dimensions (a, b, c, ∝ , β , γ ,), the fractional atomic coordinates (x, y, z for each atom), and the space group. Less

Introduction

C. G. Gray and K. E. Gubbins

in Title Pages

The application of statistical mechanics to the study of fluids over the past fifty years † or so has progressed through a series of problems of gradually increasing ... More

The application of statistical mechanics to the study of fluids over the past fifty years † or so has progressed through a series of problems of gradually increasing difficulty. The first and most elementary calculations were for the thermodynamic functions (heat capacities, entropies, free energies, etc.) of perfect gases. These properties are related to the molecular energy levels, which for perfect gases can be determined theoretically (by quantum calculations) or experimentally (by spectroscopic methods, for example). For simple molecules (CO2 , CH4 , etc.) the energy levels, and hence the thermodynamic properties, can be determined with great accuracy, and even for quite complex organic molecules it is now possible to obtain thermodynamic properties with satisfactory accuracy. With the advent of digital computers it became possible to calculate thermodynamic properties for a wide variety of substances and temperatures, and several useful tabulations of perfect gas properties now exist. Having successfully treated the perfect gas, it was natural to consider gases of moderate density, where intermolecular forces begin to have an effect, by expanding the thermodynamic functions in a power series (or virial series) in density. Although the mathematical basis for a theoretical treatment of this series was laid by Ursell in 1927, it was not exploited until ten years later, when Mayer re-examined the problem. Since that time a great deal of effort has been put into evaluating the virial coefficients that appear in the series for a variety of intermolecular force models. As the expressions for the virial coefficients are exact, they provide a very useful means of checking such force models by comparison of calculated and experimental coefficients. While the theory of dilute gases at equilibrium is essentially complete, this is far from being the case for all dense gases and liquids. The virial series cannot be applied directly to liquids. As an alternative to the ‘dense gas’ approach to liquids, there were early attempts to treat liquids as disordered solids by using cell or lattice theories; these were popular from the mid-1930s until the early 1960s. Less

Boom Days in Appliance City

Chad Broughton

in Title Pages

Packing Insulation Was Mike Patrick’s first job at Midwest Manufacturing. He was one of 300 men, mostly young, hired in January 1959 to help Admiral, a Chicago-based ... More

Packing Insulation Was Mike Patrick’s first job at Midwest Manufacturing. He was one of 300 men, mostly young, hired in January 1959 to help Admiral, a Chicago-based company that owned the Galesburg factory, meet America’s seemingly insatiable postwar appetite for appliances. He had failed an eye test during the nurse’s exam at the factory and had to get glasses before he started. Patrick had suspected he needed glasses because he always had trouble see­ing the chalkboard from the back of the room in high school. But because he was an athlete, he didn’t want to tie glasses around his head during basketball games. New hires got the nastiest, most grueling jobs, and stuffing insulation— which was like prickly cotton candy—into bare metal cabinets was one of them. The cabinets came from the metal-cutting area of the factory known as the “black line,” because the steel, darkened with oil, hadn’t yet been painted. The black line was the birthplace of these early Admiral refrigerators. Flatbed semis unloaded massive rolls of thick steel from Chicago—the plant used 10 rolls a day, 50 million pounds a year—that cutters and folding machines would shape into five sides. Gun welders then joined what would become the back, the two sides, and the top and bottom of the refrigerator. They left the door for later. The fused steel cabinet dangled from an overhead conveyor as it rode to the paint shop to be cleaned of its oily residue and painted. It would continue on the conveyor to a cabinet bank, where the empty cabinets gathered until they were needed on the line. When the scheduler called for them, men would slide the cabinets to the line across a concrete floor, which had been treated with a smooth, protective coating to prevent damage. A young man then spread scalding, gooey tar into the corners and up and down the creases of the bare metal cabinets. He shot the tar out of a pistol-gripped nozzle attached to a long canvas hose that he snaked in and around the metal shell. Less

The Impact of Inorganic Trace Gases on Ozone in the Atmosphere

Jack G. Calvert, John J. Orlando, William R. Stockwell, and Timothy J. Wallington

in The Mechanisms of Reactions Influencing Atmospheric Ozone

A major focus of the previous six chapters has been on the chemistry and interactions of the HOx, NOx, and volatile organic compound (VOC) families. Details of the ... More

A major focus of the previous six chapters has been on the chemistry and interactions of the HOx, NOx, and volatile organic compound (VOC) families. Details of the reactions of O3 NO3, and HO that act to initiate VOC oxidation have been presented, as has the ensuing chemistry involving organic peroxy and alkoxy radicals and their interactions with NOx. In this chapter, we complete our discussion of thermal chemical reactions that impact tropospheric ozone. The chapter begins with a discussion of the budgets of two simple (inorganic) carbon-containing species not yet discussed, carbon dioxide (CO2) and carbon monoxide (CO). Although CO2 is not directly involved in ozone-related tropospheric chemistry, it is of course the species most critical to discussions of global climate change, and thus a very brief overview of its concentrations, sources, and sinks is presented. CO is a ubiquitous global pollutant, and its reaction with HO is an essential part of the tropospheric background chemistry. This is followed by a presentation of the tropospheric chemistry of halogen species, beginning with a discussion of inorganic halogen cycles that impact (in particular) the ozone chemistry of the marine boundary layer (MBL) and concluding with a detailed presentation of the reactions of Cl atoms and Br atoms with VOC species. The chapter concludes with an overview of tropospheric sulfur chemistry. The reactions leading to the oxidation of inorganic (SO2 and SO3) as well as organic sulfur compounds (e.g., DMS, CH3SCH3) are detailed, and a brief discussion of the effects of the oxidation of sulfur species on aerosol production in the troposphere and stratosphere is also given. The abundance of CO2 in the atmosphere has obviously received a great deal of attention in recent decades due to the influence of this gas on Earth’s climate system. Indeed, changes in the atmospheric CO2 concentration represent the single largest contributor to changes in radiative forcing since preindustrial times (c. 1750). The atmospheric burden of CO2 is controlled by the processes that make up the global carbon cycle—the exchanges of carbon (mostly in the form of CO2) between various “reservoirs,” including the atmosphere, land (vegetation and soil), the surface ocean, the intermediate and deep ocean, sediment on the ocean floor, and the fossil fuel reservoir (IPCC, 2007). Less

More Chemistry

Eric Scerri

in The Periodic Table: Its Story and Its Significance

The trends within rows and columns of the periodic table are quite well known and are not repeated here. Instead, I concentrate on a number of other chemical trends, ... More

The trends within rows and columns of the periodic table are quite well known and are not repeated here. Instead, I concentrate on a number of other chemical trends, some of which challenge the form of reductionism that attempts to provide explanations based on electronic configurations alone. In the case of one particular trend described here, the knight’s move, the chemical behavior defies any theoretical understanding whatsoever, at least at the present time. As is well known to students of inorganic chemistry, a small number of elements display what is termed diagonal behavior where, in apparent violation of group trends, two elements from adjacent groups show greater similarity than is observed between these elements and the members of their own respective groups. Of these three classic examples of diagonal behavior, let us concentrate on the first one to the left in the periodic table, that between lithium and magnesium. The similarities between these two elements are as follows:1. Whereas the alkali metals form peroxides and superoxides, lithium behaves like a typical alkaline earth in forming only a normal oxide with formula Li2O. 2.Unlike the other alkali metals, lithium forms a nitride, Li3N, as do the alkaline earths. 3.Although the salts of most alkali metals are soluble, the carbonate, sulfate, and fluorides of lithium are insoluble, as in the case of the alkaline earth elements. 4.Lithium and magnesium both form organometallic compounds that act as useful reagents in organic chemistry. Lithium typically forms such compounds as Li(CH3)3, while magnesium forms such compounds as CH3MgBr, a typical Grignard reagent that is used in nucleophilic addition reactions. Organolithium and organomagnesium compounds are very strong bases that react with water to form alkanes. 5.Lithium salts display considerable covalent character, unlike their alkali metal homologues but in common with many alkaline earth salts. 6.Whereas the carbonates of the alkali metals do not decompose on heating, that of lithium behaves like the carbonates of the alkaline earths in forming the oxide and carbon dioxide gas. 7.Lithium is a considerably harder metal than other alkali metals and similar in hardness to the alkaline earths. Less

Kekulé's “Dreams”

in Image and Reality: Kekule, Kopp, and the Scientific Imagination

At a celebration held in his honor in Berlin on March 11, 1890, August Kekulé told his “dream stories.” According to Arthur Koestler, Kekulé's benzene story was “probably the most important dream in ... More

At a celebration held in his honor in Berlin on March 11, 1890, August Kekulé told his “dream stories.” According to Arthur Koestler, Kekulé's benzene story was “probably the most important dream in history since Joseph's seven fat and seven lean cows.” Whether Kekulé was sincere in telling these tales has been the subject of considerable debate in recent years. This chapter, which examines the context in which Kekulé told his stories, begins by providing an overview of his sixtieth birthday party in September 1889, when Adolf Baeyer secretly proposed the idea of a birthday party for the benzene theory. It then looks at another famous story of private moments of discovery, told by the great French mathematician Henri Poincaré while speaking to an audience of psychologists in Paris in May 1908.Less

Aromatic Apparitions

in Image and Reality: Kekule, Kopp, and the Scientific Imagination

Benzene, the prototype of what chemists call “aromatic” substances, was first isolated from coal gas by Michael Faraday in 1825. In 1832, Justus von Liebig and Friedrich Wöhler collaborated on a ... More

Benzene, the prototype of what chemists call “aromatic” substances, was first isolated from coal gas by Michael Faraday in 1825. In 1832, Justus von Liebig and Friedrich Wöhler collaborated on a study of the oil of bitter almonds (benzaldehyde), during which they prepared a dozen related compounds and arrived at the conclusion that all of them contained a “benzoyl” radical. After William Perkin discovered mauve, the first coal tar dye, aromatic compounds became the object of intense commercial interest, even though the structural details of their molecules were still unknown. Aromatic chemistry became a recognized subfield of organic chemistry in the late 1850s, which coincided with the rise of the coal tar dye industry. The development of the structural theory of aromatic substances is a classic example of the heuristic importance of visual symbols and mental images in the pursuit of chemical science.Less

Dimensional Molecules

in Image and Reality: Kekule, Kopp, and the Scientific Imagination

Every perceptive atomist recognizes the truism that atoms exist in a three-dimensional space. This chapter traces the evolution of certain theories from John Dalton to Jacobus Henricus van't Hoff ... More

Every perceptive atomist recognizes the truism that atoms exist in a three-dimensional space. This chapter traces the evolution of certain theories from John Dalton to Jacobus Henricus van't Hoff that eventually constituted a new field called “stereochemistry,” or the chemistry of three-dimensional molecules. It first looks at how Dalton developed his chemical theory and then turns to Van't Hoff's theory in which he offered an explanation for many hitherto mysterious cases of “absolute isomerism,” including that of the two acids from wine tartar and the two lactic acids. The chapter also examines August Kekulé's generalization of his phenakistoscopic theory of the benzene ring before concluding with an analysis of the works of physicists and the incipient kinetic theory of gases.Less

Kopp's World

in Image and Reality: Kekule, Kopp, and the Scientific Imagination

Hermann Kopp (1817–1892) made a number of important scientific and historical contributions, especially in the field of chemistry, and wrote a book entitled Aus der Molecular-Welt (From the Molecular ... More

Hermann Kopp (1817–1892) made a number of important scientific and historical contributions, especially in the field of chemistry, and wrote a book entitled Aus der Molecular-Welt (From the Molecular World) in 1882, a fanciful ground-level exploration of atoms and molecules. His scientific work addressed the relationship between macroscopic physical and chemical properties of substances and the invisible microworld of the atoms and molecules that compose them. Kopp also performed experiments on atoms of variable “handedness” (valence) as well as molecular compounds. This chapter describes his contributions in chemistry and examines Aus der Molecular-Welt. It also explores a chemical parody that has some parallels to Kopp's fantasy. This lampoon includes a paper by “F. W. Findig,” who argues that zoology can be of the greatest assistance to organic chemistry. There is also a long poem entitled “Disputation (Frei nach Heine)” by Otto Witt, who looks at the battles that had erupted between August Kekulé's hexagon and Albert Ladenburg's prism formula over the formula for benzene.Less

Chemical Hazards

Michael Gochfeld and Robert Laumbach

in Occupational and Environmental Health

Building on the principles of toxicology, this chapter describes chemicals by structure, source, use, mechanism of action, environmental properties, and target organ. Major advances in toxic effects ... More

Building on the principles of toxicology, this chapter describes chemicals by structure, source, use, mechanism of action, environmental properties, and target organ. Major advances in toxic effects include more detailed understanding of the mechanisms by which toxic chemicals damage receptors at the subcellular, cellular, and organ level. The chapter describes properties of various types of inorganic and organic chemicals and their adverse health effects. It discusses asphyxiants, such as carbon monoxide and hydrogen sulfide; heavy metals, such as lead, mercury, and cadmium; organic solvents, such as benzene and trichlorethylene; pesticides, including chlorinated hydrocarbons and organophosphates; and a variety of other toxic chemicals to which people are exposed in the home, community, or workplace environment. Several cases are presented to illustrate various concepts concerning chemical hazards in occupational and environmental health.Less

Boll Weevils, Worms, and Moths: A Hundred-Year War

D. Clayton Brown

in King Cotton in Modern America: A Cultural, Political, and Economic History since 1945

This chapter describes the struggle against boll weevils. The Mexican boll weevil (A. grandis) became the most dreaded insect pest of the South, with its capacity to wipe out entire fields of cotton. ... More

This chapter describes the struggle against boll weevils. The Mexican boll weevil (A. grandis) became the most dreaded insect pest of the South, with its capacity to wipe out entire fields of cotton. In 1923 President Warren G. Harding urged the cotton-growing states to “unite with the national government in the war on the boll weevil.” Triumph over the boll weevil finally came through the efforts of agricultural entomologists, the NNC, along with dependable government support. The struggle lasted about a century, with the real progress beginning around 1945, when a new era of insect control began with the introduction of DDT, benzene hexachloride (BHC), and toxaphene.Less

Light emitting polymers

William Barford

in Electronic and Optical Properties of Conjugated Polymers

An experimental survey of the optical properties of the phenyl-based light emitting polymers (poly (para-phenylene) and poly (para-phenylene vinylene)) is given. These are then explained by ... More

An experimental survey of the optical properties of the phenyl-based light emitting polymers (poly (para-phenylene) and poly (para-phenylene vinylene)) is given. These are then explained by developing a theoretical model of their excited states via a discussion of the excited states of benzene, biphenyl, stilbene and oligophenylenes. Results of DMRG and CI-S calculations of the Pariser-Parr-Pople model are shown to reliably explain the experimental data. Vibrational relaxation of the photo-excited states and the formation of exciton-polarons are described.Less

Electronic Warfare: Electrophilic Substitution

Peter Atkins

in Reactions: The private life of atoms

Benzene, 1, is a hard nut to crack. The hexagonal ring of carbon atoms each with one hydrogen atom attached has a much greater stability than its electronic structure, with an alternation of double ... More

Benzene, 1, is a hard nut to crack. The hexagonal ring of carbon atoms each with one hydrogen atom attached has a much greater stability than its electronic structure, with an alternation of double and single carbon–carbon bonds, might suggest. But for reasons fully understood by chemists, that very alternation, corresponding to a continuous stabilizing cloud of electrons all around the ring, endows the hexagon with great stability and the ring persists unchanged through many reactions. The groups of atoms attached to the ring, though, may come and go, and the reaction type responsible for replacing them is commonly ‘electrophilic substitution’. Whereas the missiles of Reaction 15 sniff out nuclei by responding to their positive electric charge shining through depleted regions of electron clouds, electrophiles, electron lovers, are missiles that do the opposite. They sniff out the denser regions of electron clouds by responding to their negative charge. Let’s suppose you want to make, for purposes you are perhaps unwilling to reveal, some TNT; the initials denote trinitrotoluene. You could start with the common material toluene, which is a benzene ring with a methyl group (–CH3) in place of one H atom, 2. Your task is to replace three of the remaining ring H atoms with nitro groups, –NO2, to achieve 3. And not just any of the H atoms: you need the molecule to have a symmetrical array of these groups because other arrangements are less stable and therefore dangerous. It is known that a mixture of concentrated nitric and sulfuric acids contains the species called the ‘nitronium ion’, NO2+, 4, and this is the reagent you will use. Before we watch the reaction itself, it is instructive to see what happens when concentrated sulfuric acid and nitric acid are mixed. If we stand, suitably protected, in the mixture, we see a sulfuric acid molecule, H2SO4, thrust a proton onto a neighbouring nitric acid molecule, HNO3. (Funnily enough, according to the discussion in Reaction 2, nitric ‘acid’ is now acting as a base, a proton acceptor! I warned you of strange fish in deep waters.) The initial outcome of this transfer is unstable; it spits out an H2O molecule which wanders off into the crowd. We see the result: the formation of a nitronium ion, the agent of nitration and the species that carries out the reaction for you. Less

Carbon in atomic, molecular and crystalline (3D and 2D) forms

E. L. Wolf

in Graphene: A New Paradigm in Condensed Matter and Device Physics

The carbon atom is discussed in the Bohr and Schrödinger pictures. Radii and energies of core and valence states are estimated. Ionization levels of carbon are used to estimate properties at variable ... More

The carbon atom is discussed in the Bohr and Schrödinger pictures. Radii and energies of core and valence states are estimated. Ionization levels of carbon are used to estimate properties at variable principal quantum numbers, n. Linear combinations of 2s and 2p wavefunctions leading to bonds are described. Two electron wavefunctions are discussed in connection with covalent and hybrid bonds. The benzene molecule is discussed in terms of sp2 bonding. The number of free electrons supporting diamagnetism in the benzene ring is estimated using the diamagnetic susceptibility and NMR chemical shifts. The bonding of benzene in modern chemical terms is described. Graphane and fluorographene are described. Natural and Kish graphite are compared with highly oriented pyrolitic graphite (HOPG). Bernal stacking and the phenomenon of superlubricity are described.Less

We’re on the road to nowhere: An exhibition of molecules to transport us

John Emsley

in Molecules at an Exhibition: Portraits of Intriguing Materials in Everyday Life

The rays of the Sun, and the motions of the Moon and Earth, provide energy in abundance. Light from the Sun is absorbed by plants on land and algae in the sea and is used to convert carbon dioxide ... More

The rays of the Sun, and the motions of the Moon and Earth, provide energy in abundance. Light from the Sun is absorbed by plants on land and algae in the sea and is used to convert carbon dioxide into high energy carbohydrates, which in turn become oils. Together these provide most of the food energy for animals like ourselves. We can also harvest plants and trees and burn them to release this energy as heat. The sunlight which falls on barren terrain, or on the roofs of buildings, we can also gather by using solar panels to heat water or to make electricity. The sunlight which falls on the oceans leads to evaporation of water which is precipitated on land, and this too we can use to generate hydroelectricity. The Earth itself is a vast reservoir of heat below the crust, but this is not so easily tapped—although in parts of the world, such as New Zealand, hydrothermal heat is an important source of power. We can extract energy from the effects of the Earth’s daily rotation, partly through the weather systems this produces, by using windmills, and possibly through the rise and fall of sea levels, by using tidal barriers and wave power. These sources of clean energy should be able to provide all the fuel and electricity for a sustainable human population of several billion, provided we did most of our travelling on foot or by bicycle. How much these natural renewable sources could really provide is debatable, but we have the means to utilize them so they could supply enough food and energy for a world population of two or three billion, and at a level which allows for most of the high-tech living that we now take for granted. It might even be possible for most families to run a car, provided they were content to travel only a couple of thousand miles a year in it. The trouble is that there are already six billion of us, and forecasts are that this will reach ten billion by the middle of the next century. Most of these people will no doubt aspire to owning a car. Less

Approximate Molecular Orbital Theory: The Hückel/Tight-binding Model

Jochen Autschbach

in Quantum Theory for Chemical Applications: From Basic Concepts to Advanced Topics

Huckel molecular orbital (HMO) theory is a simple approximate parameterized molecular orbital (MO) theory that has been very successful in organic chemistry and other fields. This chapter introduces ... More

Huckel molecular orbital (HMO) theory is a simple approximate parameterized molecular orbital (MO) theory that has been very successful in organic chemistry and other fields. This chapter introduces the approximations made in HMO theory, and then treats as examples ethane, hetratriene and other linear polyenes, and benzene and other cyclic polyenes. The pi binding energy of benzene is particularly large according to HMO theory, rationalizing the special ‘aromatic’ behaviour of benzene. But there is a lot more to benzene than that. It is shown that the pi bond framework of benzene would rather prefer a structure with alternating single and double C-C bonds, rather than the actually observed 6-fold symmetric structure where all C-C bonds are equivalent. The observed benzene structure is a result of a delicate balance between the tendencies of the pi framework to create bond length alternation, and the sigma framework to resist bond length alternation.Less

Leukemias

Eve Roman, Alexandra Smith, and Lorelei Mucci

in Textbook of Cancer Epidemiology

Leukemias are a diverse group of acute and chronic haematological malignancies, that account for 2% to 3% of cancers globally. Recent advances in molecular biology and therapy have transformed the ... More

Leukemias are a diverse group of acute and chronic haematological malignancies, that account for 2% to 3% of cancers globally. Recent advances in molecular biology and therapy have transformed the landscape for several leukemia subtypes changing some, but by no means all, from rapidly fatal diseases to treatable conditions with a good prognosis. In general, however, this progress has not been matched by new aetiological insights. Albeit accounting for a relatively small proportion, genetic predisposition syndromes such as neurofibromatosis, Li-Fraumeni and Trisomy 21, have the biggest impact in children and young adults. At older ages, established chemical, physical and biological risk factors, which explain only a small proportion of the total disease burden, include chemotherapy for a preceding cancer, ionizing radiation, and the viral infections human T-cell lymphotropic virus type 1 (HTLV-1) which causes the rare adult T-cell leukemia/lymphoma (ATLL) and the human immunodeficiency virus (HIV) which is associated with an increased risk of acute lymphoid leukaemias. Workplace exposures to potential carcinogens such as benzene, butadiene, and styrene have also been linked to increased risk of leukemia.Less