Opportunities in Chemistry Today and Tomorrow (1987) / Chapter Skim
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IV. Intellectual Frontiers in Chemistry
IV. Intellectual Frontiers in Chemistry
Pages 115-166
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From page 115... ...
The basis for this optimistic expectation is that this is a time of special opportunity for intellectual advances in chemistry. The opportunity comes from our developing ability to investigate the elemental steps of chemical change and the ability to deal with extreme molecular complexity.
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From page 116... ...
An alkyl C' ~ :~/C~1 1 benzene molecule camed along in such a stream loses all its vibrational energy, thus relaxing the molecular tail. Then, the cold molecules intersect a brief pulse of light with color that is absorbed by the benzene ring.
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From page 117... ...
Over the next three decades we will see advances in our understandings of chemical kinetics that will match the advances in molecular structures over the last three decacles. Fast Chemical Processes A chemical reaction begins with mixing reactants and ends with formation of final products.
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From page 118... ...
One of their unique capabilities is to provide short-duration light pulses with which to investigate chemical processes that occur in less than a millionth of a second all the way down to a millionth of a millionth of a second (i.e., down to a picosecond, 10- ~2 see)
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From page 119... ...
we can see how rapidly energy moves within the molecule and where it goes. Thus, we are beginning to map and understand the high-energy electronic states of molecules so that they can be used to open new reaction pathways.
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As tuned lasers became available they were used to excite particular vibrations in a molecule. Then, experiments were devised to permit us to watch this carefully placed energy move into other parts of the molecule or into another molecule if collisions occurred.
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From page 121... ...
The earliest efforts, based upon chemiluminescence, revealed a part of the picture: the energy distribution among the products. For example, when a gaseous hydrogen atom and a chlorine molecule react they form hydrogen chloride and a chlorine atom.
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From page 122... ...
These sophisticated instruments depend upon ultra-high vacuum equipment, high-intensity supersonic beam sources, sensitive mass spectrometers for detectors, and electronic timing circuitry for time-of-flight measurements. With this incredible control it has become possible to predetermine the energy state of each reactant molecule and then to measure both the probability of a certain reaction and the energy distribution in the products.
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From page 123... ...
Two-photon ionization has been used to detect specific molecules in difficult environments, like those found in explosions and in flames. Thus, nitric oxide, NO, which is an ingredient in smog, can be easily measured in a flame by counting the ions produced by a finely tuned laser probe.
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From page 124... ...
Because SF6 and UFO have identical molecular structures, they have similar vibrational patterns. Thus, multiple photon excitation might offer a new and simpler approach to isolation of the uranium isotopes that undergo nuclear fission.
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From page 125... ...
For example, fluonne, F2, and ethylene, C2H4, suspended in solid argon at lOK do not react until one of the vibrational motions of ethylene is excited with a resonantly tuned laser. Then it is found that the most efficient vibrational motions are those that distort the molecular plananty.
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From page 126... ...
Short-lived reaction intermediates, excited states, and even energy barners to reaction can now be understood, at least for small polyatomic molecules. Our increasing understanding and control of chemical reactivity is providing us with new reaction pathways in synthetic chemistry that are sure to lead to new products and new processes.
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From page 127... ...
With only 100-picogram (10-~° grams) amounts, mass spectrometry contributes by furnishing precise molecular weights up to 13,000 and, through the fragmentation patterns, providing revealing clues to substructures.
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. When right- and lefthanded molecular structures are possible, most chemical reactions will produce a mixture of the two.
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IV-A. CONTROL OF CHEMICAL FACTIONS The significance of these new frontiers of organic synthesis can be seen in health applications.
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The carbon monoxide molecule, and any groups that take their places, are called "ligands." Some or all of them can be replaced by other electron donors (Lewis bases) such as nitric oxide, NO; ammonia, NH3; halide ions, F-, Cl-, Br-; water, H2O; cyanide ion, CN-; and many more.
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matrices and under molecular beam conditions using supersonic jet cooling. These techniques have provided a wealth of information, including molecular ~eometrv.
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132 INTELLECTUAL FRONTIERS IN CHEMISTRY ~b~, ADA M ANTANE ADAMANTADINE LAB CURIOSITY ANTIVIRAL AGENT together in proper orientation (regioselectivity) , and the desired three-dimensional spatial relations must be obtained (stereoselectivity)
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B Woodward, who won the Nobel Prize for synthesizing molecules as complex as quinine and Vitamin By.
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molecular structures. "Strained" molecules are those with uncomfortable or unusual bond angles.
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9, 1982. Science "Molecular Beam Studies of Elementary Chemical Processes" (Nobel Prize Address)
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Prom this they were able to grow a single tiny crystal, so that a chemical crystallographer could analyze the molecular structure with x-ray d~ract~on. Just as X-rays penetrate an arm to reveal broken bones, they penetrate a crystal and reveal the geometrical arrangement of the atoms in brassinolide.
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From page 137... ...
As was emphasized in the discussion of biotechnology, our understanding of macromolecules has provided new insights into their function in biological systems. These new insights have come from structural studies, synthetic alterations, increased understanding of the relationship between molecular structure and function, and the techniques of molecular genetics.
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From page 138... ...
138 INTELLECTUAL FRONTIERS IN CHEMISTRY Streptomyces cinnamonensis, is perhaps the best-known example from among a group of about 50 naturally occurring polyether antibiotics. Three polyether antibiotics (monensin, lasalocid, and salinomycin)
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From page 139... ...
In this manner scientists can map reaction pathways. Extensive chemical degradations are no longer needed to locate sites of isotopic labeling because this task has been revolutionized by the development of stable isotope 139
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Although these amines are called "bases," in fact, each one couples the ability to form hydrogen bonds acting as an electron donor (a "base,') with the ability to form hydrogen bonds acting as an electron acceptor (a "proton donor" or "acidly.
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From page 141... ...
STRUCTURES OF MACROMOLECULES The structures of the giant molecules of living systems- the proteins and nucleic acids—offer challenges just like those encountered for smaller natural products. We must first know which atoms are bonded to which In order to descnbe the covalent molecular structure.
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Enzymes have clefts called "active sites" that bring reactants together and permit the formation of new chemical bonds between them. Thus, proteins have definite conformations that are at the heart of their biological roles.
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IV-B. DEALING WITH MOLECULE COMP~XI~ mechanisms, devise new diagnostic tests, and develop novel drugs and therapeutic strategies.
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, have been determined. In addition, these enzyme structures have been examined as they appear when various mole\\ \\Wl~; it:' by' ~ \\ \~o_~09 X-RAY CRYSTALLOGRAPHY CAN REVEAL COMPLEX MOLECULAR STRUCTURES cures are bound to them.
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IV-B. DEALING WITH MO~F:CUI~R COMPLEXITY secondary interactions, they may change in response to their environment, and two or more conformers can be present at once in dynamic equilibrium.
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146 INTELLECTUAL PRONTlERS lN CHEMISTRY I' Z Di\A LEFT-HANDED Mc M i nor _ G roove 3 DNA RIGHT-HANDED NATURE DOESN 'T ALWAYS TURN RIGHT Structure and Function in Biochemistry Structure determines properties and properties determine function. Thus, from the simplest molecules like ethyl alcohol, to molecules with the exquisite and vaned architectures of proteins, their molecular structure is inextricably related to their function as drugs, antibodies, biological catalysts, hormones, transport agents, cell surface receptors, structural elements, or muscles that convert chemical energy into work.
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from the amino end of a protein. Use of tandem mass spectrometry or other novel approaches might allow the complete sequence to be established for a protein of several hundred residues by automated techniques.
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From page 148... ...
The process of aging is poorly understood; it is possible that some of the more destructive aspects of this process are controlled by the activity of a few gerle products so that identification of the functions of these genes may lead to better treatments for aging patients. SUPPLEMENTARY READING Chemical & Engineering News "Experts Probe Issues, Chemistry of LightActivated Pesticides" by R.L.
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IV-B. DEALING WITH MOLECULAR COMPLEXITY Proteins" by S.L.
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Anyone who feels that electrical bills are too high can be cheered} about 20 percent of the electrical energy moved around the country is wasted in the copper transmission lines. That's enough energy to light up the whole West Coast.
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Analytical chemistry is the cornerstone upon which our monitoring and management of the environment is built. Finally, nuclear chemistry was nurtured in the World War IT Manhattan Project, and its influence continues to be of prime importance, since the worId's energy needs may involve nuclear reactors (despite ChernobyI)
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52 INTEL' ACTUAL FRONTIERS IN CHEMISTRY to a real science. The intellectual challenge to understand the chemical behavior of molecules on a surface has propelled surface science into the mainstream of fundamental research in most departments of chemistry and chemical engineering.
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From page 153... ...
Other substances are much more tightly held by the surface and can be removed only by heating to much higher temperatures perhaps 200 to 600°C. Here, covalent bonding to the surface is involved and the molecular structure of the adsorbed substance is probably different from what it was before adsorption.
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These frequencies provide a fingerprint that is readily interpreted by a chemist experienced in relating infrared spectra to molecular structures (see Section V-C)
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Since different types of orbitals have different spatial characteristics, they are affected to different degrees. This "pressure tuning" makes pressure a powerful tool for characterizing electronic states and discovering electronic transitions to new states with different physical and chemical properties.
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On a picosecond time scale (10- ~2 seconds) , we can sense the freedom of movement of a solute molecule held in its solvent cage.
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Within the last decade, the study of meteorites has been dramatically advanced by the recognition that if these projectiles from outer space land on the Antarctic ice sheet, they are immediately entombed in an inert environment and permanently refrigerated, stopping chemical changes. The question, of course, is how does one find these meteorites in the wide and forbidding spaces of this hostile region?
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158 . METEORITES: AN ANTARCTIC TREASURE TROVE I7vTEr r ~.cTuAL FRONTIERS IN CHEMISTRY collected (in the last decade)
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From page 159... ...
Optical Spectroscopy The intellectual opportunities in this field, which introduce a variety of valuable analytical techniques, can be illustrated by two notable achievements of the last decade: the incorporation of computers as an essential part of most instrumentation, and the detection of single atoms and molecules. "Smart" commercial instruments now include microcomputers preprogrammed to carry out a wide variety off expenmental procedures and sophisticated data analyses.
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In an alternate approach, molecules can be embedded in a cryogenic solid, such as solid argon, at temperatures near that of liquid helium (a process ceded matrix isolation)
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From page 161... ...
In the celebrated Viking Mars Probe, mass spectrometry was the basis for both the upper atmosphere analysis and the search for organic material in the planetary soil 30 million miles from home. Such sensitive soil sniffing to detect hydrocarbons might become a fast method for of!
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From page 162... ...
. Electroanalytical methods are also useful in such difficult environments as flowing rivers, nonaqueous chemical process streams, molten salts, and nuclear reactor core fluids.
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From page 163... ...
Chemical separations are vitally important in the nuclear fuel cycle, beginning at the uranium mill where low-grade uranium ores (typically only 0.1 to 0.3 percent U3Os) are treated in selective chemical processes to produce a concentrate of more than 80 percent U3Os.
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Nuclear techniques also played an important role in the chemical analyses performed by Soviet unmanned missions to the Moon, and in experiments designed to seek life on the surface of Mars by the U.S. Viking missions.
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From page 165... ...
This has a clear relationship to the possibility of long-term confinement of radioactive waste products in geologic formations. Nuclear Chemistry in Medicine Nearly 20 million nuclear medicine procedures are performed annually in the United States (radioactive iodine thyroid treatment is one example)
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From page 166... ...
SUPPLEMENTARY READING Chemical & Engineering News "Vibrational Optical Activity Expands Bounds of Spectroscopy" by S.C. Stinson (C.& E.N.
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