Dr. Evi Goldfield
Wayne State University

Molecular Hydrogen Inside Carbon Nanotubes: Quantum Sieving, Rotational Structure and Extreme Confinement

The inside of single walled carbon nanotubes provides a unique environment for studying physical and chemical processes. This talk will focus on four studies that highlight different features of the effects of confinement within the intereriors of nanotubes. It will feature work done in the Goldfield and Schlegel research groups.

When molecules are confined within nanotubes, their translational and rotational motion becomes hindered and quantized. The dimensions of the nanotube can have a profound effect on these hindered degrees of freedom. We compute the quantum mechanical energy levels of for the hydrogen molecule and its homonuclear isotopes confined within carbon nanotubes of various sizes and structures using three different interaction potentials. Two translational and two rotational degrees of freedom are treated explicitly. We study the dependence on the interaction potential and the size of the nanotube of several features, including zero-pressure quantum sieving selectivities, ortho-para energy splittings and wavefunction characteristics. We show that large quantum sieving selectivities, as well as large deviations from gas phase ortho-para splittings, occur only under the condition of extreme two-dimensional confinement, when the characteristic length of the hydrogen-carbon interaction potential is nearly equal to the radius of the nanotube.

Mechanical confinement can also influence the energetics and the dynamics of a chemical reaction. We will discuss the test case of formaldehyde photodissociation, in which confinement alters the internal energy distribution of the products.

The polarizability of nanotubes can affect the energetics of reactions, especially if charged species are involved. The Menshutkin reaction,
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is studied inside (8,0) and (9,0) nanotubes. While the reaction has a high barrier in the gas phase, the Menshutkin reaction has a low barrier and is nearly thermoneutral inside the nanotube, as a result of the high polarizability of the nanotube stabilizing the charged products. Nanotubes have an even stronger effect on the ionization and electron affinities of molecules placed inside them. In some cases, the length of the nanotube segment determines whether the molecule or the nanotube is ionized.