Assumptions About What Holds Molecular Complexes Together Have Been Based On Faulty Measures

As scientists create molecular complexes to perform increasingly minute operations -- such as molecular level switches or memory devices -- it is critical that the association forces that hold the molecular components together be accurately understood.

But measurements of association constants are often not accurate, according to an article by Virginia Tech Ph.D. student Jason Jones and chemistry professor Harry W. Gibson, published in May 15, 2003 online issue of the Journal of the American Chemical Society ("Ion Pairing and Host-Guest Complexation in Low Dielectrict Constant Solvents," scheduled for print on June 25, 2003).

Designing molecules that switch on and off or that attach to a material to transport it and then release it at some external signal, requires a complexation (or connection) process that is reversible. Fine control of reversible complexation processes requires precise understanding of the attractive forces that hold two or more molecules together in the supramolecular assembly.

Gibson and his students began to take a closer look at how association constants (Ka) are determined in the molecular complexes they were building when Ka measurements did not match those in the literature.

Using a simple host-guest system known as a pseudorotaxane, large cyclic host molecules were allowed to interact with linear guest molecules to create a supramolecular threaded complex. The host and guest are bound by electrostatic forces, primarily hydrogen bonding. Since the guest systems are salts, in low-polarity solvents "The attraction between positive and negative ions mean that the salt species are predominantly intimately ion-paired but not entirely," says Gibson.

In Chemistry 101, A+B=C. "If you know how much of each species -- hosts (H) and guests (G) -- you have, then you can predict how much of the complex (C) you will get -- if you also know Ka -- the association constant," says Gibson.

Ka = C divided by (A)(B) where A = the H you started with minus C (H₀-C) and B = the G you started with minus C (G₀-C). [Ka = C / (H ₀-C) (G₀-C)]

However, that only works if the guest molecules and the complex are ion-paired to the same extent, says Gibson. And that does not happen.

The researchers developed equations for determining Ka when the extents of ion-pairing were different and tested them. "It requires more work and more data," says Gibson. "But in order to make functional materials using supramolecular chemistry, you have to have as high a Ka as possible. Now that we know more about the individual steps and factors involved, we have been able to build better supramolecular arrangements."

Jones has been able to increase Ka 50-fold in recent work (presented to the American Chemical Society in March 2003). An increased understanding of bonding dynamics will allow the development of innovative molecular level materials, such as molecular motors and molecular memory devices.

Contact for more information: Harry W. Gibson, 540-231-5902, hwgibson@vt.edu More about Dr. Gibson lab is at www.chemistry.vt.edu/chem-dept/gibson/Gibson/gibson.htm

The article will be online for subscribes of the ACS journal. For a copy of the article, contact Beverly Hassell of the ACS at 202 872 4065 or newsroom@acs.org