Imaging fractional differences in chemical bond order

In their simplest form, chemical bonds between pairs of atoms come in single, double, and triple varieties, depending on how many electron pairs the atoms share. But when electron pairs spread—sometimes unevenly—among many atoms, bonds can take on fractional order, intermediate between single and double. For example, in the soccer-ball-shaped fullerene C₆₀, theory predicts that a bond fusing two hexagonal faces should have 0.16 more electron pairs than a bond fusing a hexagon with a pentagon. Now Leo Gross, Gerhard Meyer, and colleagues at IBM Research in Zürich have used atomic force microscopy (AFM) to experimentally measure that subtle difference in bond order. The key to getting such exquisite subatomic resolution was to attach a single carbon monoxide molecule to the copper AFM tip. Shown in the figure is an image they obtained of a C₆₀ molecule, in which the topmost face appears as a bright hexagon. Because AFM is sensitive to electron density, the researchers expected—and saw—a slight difference in brightness between the electron-rich hexagon–hexagon bonds (at 3, 7, and 11 o’clock) and the electron-poor hexagon–pentagon bonds. More surprising was the difference they observed in the bond lengths. Higher-order bonds are shorter than lower-order bonds, but in C₆₀ the difference is just 5%—at the very limit of the AFM resolution—whereas the image shows a difference 10 times greater. Calculations revealed that the images were being distorted by the bending of the CO tip. Images of other organic molecules suggest that the distortion is well behaved, so it provides a second contrast mechanism from which bond order can be inferred. (L. Gross et al., Science 337, 1326, 2012.)—Johanna Miller