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ion's rigidity can be ascribed to con­jugation between the cation center and the π systems of the adjacent ferrocenes. This conjugation is an important way of stabilizing a-fer-rocenyl carbonium ions in general. But in this bis-ferrocenyl carbonium ion, this effect is more important since the molecule's rings are forced into a nearly coplanar orientation.

The ferrocenophane carbonium ion further stabilizes itself by un­loading most of the positive charge onto the ferrocene units. Data from Môssbauer spectroscopy reveal that the carbonium ion center carries only one third of the charge; the remain­der is distributed equally on the two iron atoms.

This efficient charge delocalization may explain why hydride exchange (analogous to proton exchange in the carbanion) does not occur. Accord­ing to the San Jose scientists, such hydride shifts have been observed in a number of carbonium ion sys­tems "whose geometry appears to be much less favorable for such an exchange" than that of the bis-ferrocenyl carbonium ion. But as they suggest in a forthcoming report in Angewandte Chemie's interna­tional edition, the carbocation cen­ter apparently hasn't enough attrac­tive potential to entice the nearby endo hydrogen to surmount the bar­rier to hydride shift.

The carbonium ion center also ap­pears to gain some stability by bond­ing directly to the iron atoms. A crystal structure has revealed that the cyclopentadiene rings in the fer­rocene units are not coplanar, but tilted about 7°, opening toward the center of the cation. Moreover, the carbocation center leans toward the metal atoms. This allows the carbon's empty ρ orbital to overlap more with the d orbitals of the iron atoms. In this way, the carbonium ion seeks to further stabilize itself, though "at the price of some skeletal deforma­tion." These results support earlier work by others that suggested the importance of direct interaction be­tween the carbocation center and the iron atoms in the stabilization of α-ferrocenyl carbonium ions.

Despite the interesting nuggets the IBM group has stumbled across in these chemical side trips, the main thrust of their work still involves using the unique chemistry of the ferrocenophane system to help har­ness solar energy. Yet, even though management may not always encour­age such digressions, Mueller-Wes-terhoff believes that they sometimes can pay off. D

Microscope images unstained organics Unstained, isolated organic mole­cules on a metal surface now can be visualized using a new kind of mi­croscope developed at Sandia Na­tional Laboratories, Albuquerque, N.M. The field-desorption tomo­graphic microscope uses an electric field to produce up to 20 contour-slice images of a molecule. A com­puter superimposes these images to give a three-dimensional picture, which it then enhances.

The new microscope may be use­ful in studying the shape and struc­ture of organic molecules and their interaction with a metal surface. "Al­though several techniques exist to determine the shape of a molecule," says microscope developer John Panitz, "there has been no way to observe isolated organic molecules deposited on a metallic substrate."

In their early work with the new technique, Panitz and his coworkers have focused on ferritin, a protein that stores iron in the liver and spleen. The protein's spherical mol­

ecules have been magnified more than 1 million times and the images resolved to better than 15 Â. These figures compare favorably with those that can be obtained by transmis­sion electron microscopy of nonbio-logical samples, Sandia says.

In the field-desorption microscope, organic molecules are deposited on the hemispherical tip of a wire, which then is cooled to cryogenic tempera­tures. Next, benzene is condensed onto the tip, blanketing the mole­cules of interest in an immobile fro­zen layer. Then an electric field is used to ionize the benzene blanket, causing benzene ions to be ejected radially from the tip. The ions im­pinge on a nearby detector, which produces an image that can be photographed. As the benzene blan­ket slowly evaporates, the molecules of interest are uncovered. This causes dark regions to appear in the image that accurately depict the outline of the molecules. The computer pro­duces the final picture by superim­posing up to 20 individual contour-slices of the imaged molecule. This image is processed digitally to add highlights and shadow. D

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March 1, 1982 C&EN 27