1,4-Dehydrocubane: a tale of three suspects
1,4-Cubylane
1,4-Cubadiyl
Diene
an anion, dianion, radical, radical anion, triplet diradical, or diene. Rather, it appears to be a groundstate singlet 1,4-diradical, Michl concludes. "This may be the first 1,4-diradical that has been directly observed as a ground-state singlet," he says. Borden notes that 1,4-cubadiyl is a special kind of diradical because although C-l and C-4 are not directly bonded to each other, "there still is a strong enough interaction between them through the other bonds of the cubane skeleton such that the singlet state lies well below the triplet." In fact, both Borden's and Michl's calculations indicate that the diyl's singlet state is more stable than its triplet by more than 10 kcal per mole.
The nature of the intermediate is still not absolutely settled, but the three groups seem to be "fairly certain" that it is the cubadiyl. Whatever it is, it's incredibly unstable. When the Austin workers allow the frozen argon matrix to warm up to about 40 K, the infrared signature of the embedded intermediate disappears. At that temperature, Michl explains, molecular species diffuse quite readily in argon and react with each other. The final product is cubane, he says, presumably because 1,4-cubadiyl is a good hydrogen abstractor. Cubane is also formed in the gas phase if the intermediate is not frozen out in the argon matrix as soon as it forms. The chances of ever isolating 1,4-dehydrocubane neat under ordinary conditions appear nil. Even at -100 °C, Eaton's group had no luck in sighting it by nuclear magnetic resonance spectroscopy. Because 1,4-dehydrocubane currently is generated under "dirty" conditions—that is, in the presence of solvents, excess reactants, and byproducts—detailed study of the species is difficult, Eaton says. Therefore, his group is searching for other chemical precursors of the intermediate. It hopes such precursors will enable it to generate 1,4-dehydrocubane under cleaner conditions, perhaps photochemically. So far, he tells C&EN, attempts to do so have not been successful. Eaton remains concerned about the true nature of the intermediate. But that hasn't stopped him from using the species' reactivity to gain entry into new realms of cubane chemistry, such as the preparation of polycubyls, interesting rodlike molecules with a number of potential uses. The work of the three groups also illustrates the synergism between experiment and calculations. Experimental work led to calculations, which made predictions that led to more experiments, Borden says. The process continues, for his calculations predict that 1,3-dehydrocubane should be more stable than the 1,4 isomer and should be relatively easy to prepare. C-l and C-3 are located at opposite corners of a "cyclobutane" face of the cube, so it should be possible to connect them via a
diagonal bond, Borden notes. In 1988 he and Hrovat predicted that 1,2-dehydrocubane (cubene) should exist. And, sure enough, evidence for its formation and that of a homolog was obtained that same year, respectively, by Eaton and coworker Michèle Maggini, and by Borden and Hrovat and another group. Ron Dagani
PET bottles recycled into insulation foam As bottles of polyethylene terephthalate, or PET, continue to gain favor, the question of what to do with the empties arises. Two chemists in Switzerland believe they have come up with an economically viable answer: Convert the discarded plastic to thermal insulation foam. Their process now is being made available under license. Insulation board made from recycled PET has several advantages over conventional expanded polystyrene board, says H. Verity Smith, who with Edwin W. Trevitt, has patented a process for making it. (Smith and Trevitt are chemists at the Geneva-based recycling technology firm, Tisslam.) Foamed PET has a melting point some 100 °C higher than its polystyrene counterpart. It also has a lower degree of flammability. Besides, PET insulation board would make practical use of material that is increasingly becoming something of a social nuisance. "K, or thermal insulation, values of foamed PET over a range of densities are being determined," Smith notes. "We expect that this new foam will have insulation properties similar to expanded polystyrene. On a cost/performance basis, the PET product likely will be superior," he maintains. Until now it hasn't been possible to make a stable foam from molten PET because of the polymer's extremely low melt viscosity index, Smith explains. The foaming agent, usually a chlorofluorocarbon, simply bubbles through the liquid polymer and escapes. Molten polystyrene, on the other hand, has a much higher level of viscosity and holds the gas. Smith and Trevitt have overFebruary 12, 1990 C&EN 25
Science/Technology come the problem by using an additive that increases PET's melt viscosity level to the point where the bubbles are entrapped. In a typical operation, the waste PET containers are washed, shredded, ground to a 5-mm particle size, and thoroughly dried. "It's critical that all the water is removed," Smith cautions. "Even trace amounts of moisture can lead to hydrolysis of the polymer chain." The granules, mixed with the melt viscosity index enhancer and a nucleating agent, which acts as a foam stabilizer, go to a standard foam slab extruder unit. There they melt, and Freon 12 is injected into the molten plastic just before it emerges through the extruder at the base, in the same way that foamed polystyrene slabstock is made. The foamed PET that emerges from the die is amorphous. But it converts to a stable, rigid crystalline form within seconds as it cools, at the same time changing color from brown to white. Because PET foam melts at about 260 °C, laminates can be made from it using hot-melt adhesives without damaging its structure. Likewise, it can withstand hot bitumen, thereby allowing slabs of the foam to be used as roofing insulation. Another factor in PET foam's favor is that when it burns, it doesn't emit the dense black smoke that burning polystyrene does. This should be an important advantage in building applications, Smith points out. He acknowledges that use of chlorofluorocarbons for foaming is losing favor because of concern over their adverse effect on the ozone in the upper atmosphere. However, he is confident that when suitable alternatives are developed they will work equally well with recycled PET. There should be no shortage of feedstock. Smith points out that use of PET beverage bottles throughout Western Europe alone now exceeds 330 million lb a year. "Future growth rates are difficult to predict. They will vary from country to country reflecting emerging legislation, which largely is politically motivated," he adds. Dermot O'Sullivan 26
February 12, 1990 C&EN
SOFTWARE/DATABASEUPDATE
• DBMS-Interface Module to Ingres (Molecular Design Ltd., $3000 to $53,000, Circle 301) provides a link between the MACCS-II chemical information system and Ingres (Ingres Corp.), a relational database that runs on DEC VAX/VMS computers. Ingres is used at chemical and pharmaceutical research sites to store and retrieve large amounts of data, such as biological information obtained in safety and efficacy testing of new drugs and chemicals. The new interface module provides MACCS-II users with another relational database option—in addition to IBM's SQL/ DS and DB2 databases and Oracle Development Co.'s Oracle database—for design of integrated scientific information management systems. • Current Contents on Diskette/Engineering, Technology & Applied Sciences (Institute for Scientific Information, $345 annual subscription, Circle 302) is a database that provides access to the contents pages of more than 800 engineering and technology journals published worldwide. The system is updated weekly and is available in versions for IBMs and compatibles, Macintoshes, and NEC 9800s. • Units (MCA Consultants; $99 commercial, $49 academic; Circle 303) makes unit conversions and displays definitions of units and various constants. The program runs on IBM PCs, XTs, ATs, and PS/2s and compatibles. • ATHIAS (Stanford University; $280 commercial, $100 academic; Circle 304), a kinetics program, is a Lotus 1-2-3 worksheet that runs on IBM PCs, XTs, ATs, and compatibles. ATHIAS is an acronym for abiotic transformations of halogenated hydrocarbons in aqueous solutions. The program includes a database of kinetic data on reactions of haloaliphatic compounds in aqueous solution; macros to retrieve rate constants and half-lives of compounds; and
routines to evaluate rate constants and half-lives under different temperature and pH conditions. The current version includes data on 58 compounds; new data can easily be added by the user. • ESR* (Calleo Scientific Software Publishers, $225 to $295, Circle 305) is a first-order anisotropic electron spin resonance program for the Macintosh that simulates both powder and solution ESR spectra. Naturally occurring isotopes for 103 elements are supported. The program permits the user to define lineshape, number of integration steps for solid-angle surface integration, nuclear abundances, spectrometer frequency, linewidths, spectral width, spectral resolution, and other parameters. • CMC-3D (Molecular Design Ltd., $6000 per year, Circle 306) is an electronic three-dimensional database based on "Comprehensive Medicinal Chemistry Drug Compendium" (Pergamon Press), a six-volume work covering rational design, mechanistic study, and therapeutic applications of pharmaceutical compounds. The new database is searchable using MACCS-3D, a module of Molecular Design's MACCS-II system that stores, searches, and retrieves 3-D models and related data. Pharmaceutical researchers can use CMC-3D and MACCS-3D to develop pharmacophores—specific sets of structural, electronic, and geometric parameters that relate to bioactivity.
Literature • 1990 catalog (National Instruments, free, Circle 307) describes line of hardware and software products for scientific and engineering applications, including automated testing, instrument control, data acquisition, process monitoring and control, and lab automation. For information on these items or numbered ads, see Reader Service Card
