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Analytical Chemistry

Molecules Vanish In Nanotubes

Molecular dipoles induce mirror-image nanotube dipole that screen molecules from IR light

by Mitch Jacoby
May 23, 2011 | APPEARED IN VOLUME 89, ISSUE 21

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Credit: Eric Borguet/Temple U
Mirror Image An electric dipole (green and red at center) associated with a molecule encapsulated in a carbon nanotube (grey) induces an image dipole (green and red on the nanotube walls). The two cancel out, effectively making the molecule invisible to IR radiation.
Credit: Eric Borguet/Temple U
Mirror Image An electric dipole (green and red at center) associated with a molecule encapsulated in a carbon nanotube (grey) induces an image dipole (green and red on the nanotube walls). The two cancel out, effectively making the molecule invisible to IR radiation.

Encapsulating molecules in single-walled carbon nanotubes (SWNTs) can cause the molecules to become nearly invisible as far as their vibrational signatures are concerned, according to a study led by researchers at Temple University (J. Am. Chem. Soc., DOI: 10.1021/ja108903u). The results indicate that infrared spectroscopy cannot be used to interrogate these types of confined molecules in a straightforward manner. Carbon nanotube “peapod” systems are being studied for use in numerous applications, including asymmetric surface catalysis and safe handling of propellants. Encapsulated compounds are routinely analyzed via microscopy and spectroscopy. Yet according to Temple’s Dmitry V. Kazachkin, Eric Borguet, and coworkers, the intensity of the nanotubes’ screening effect has not been studied in detail. Through experimental and computational analysis of acetone, diethyl ether, and n-heptane adsorbed inside SWNTs, the team found that the molecules’ electric dipoles induce an electronic rearrangement in the nanotube walls that mirrors and effectively cancels the molecular dipole. This screening action reduces the adsorbed molecules’ IR absorption intensity by more than a factor of 10 relative to molecules that are adsorbed externally, they say.

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