Helen Leung's Research Interests

My research aims to understand intermolecular interactions due to van der Waals forces between nonchemically bonded molecules. Although van der Waals forces are much weaker than chemical forces, the immense number of pairwise interactions due to these intermolecular forces is responsible for the structures and functions of chemical and biological systems. Despite the ubiquity of these forces and their importance, however, they are still not completely understood, and the work in my group addresses this gap.

Because van der Waals forces are usually masked by chemical forces, they must be isolated before they can be effectively studied. This can be done by molecular beam methods, which generate molecular complexes that owe their very existence to van der Waals forces. The intermolecular forces determine the structures and affect the electronic environments of the complexes. Thus, the characterization of molecular properties, which can be efficiently accomplished using spectroscopic techniques, provides valuable information on the operative forces. My group utilizes a high resolution, pulsed molecular beam, Fourier transform microwave spectrometer to obtain the rotational spectrum of a complex that can then be analyzed to yield its structure, from which we can deduce the nature of the intermolecular forces.

My group has studied a series of nitrous oxide-containing complexes to examine how van der Waals forces perturb the electronic distribution of the subunits. More recently, in collaboration with Professor Mark Marshall, we seek to understand the effects of electron density on complex geometry, and we have chosen as our systems haloethylene-containing complexes.  With the presence of both electron withdrawing and electron donating functionalities in the ethylenes, the manner in which protic acids (such as HF, HCl, HCCH, each with an electropositive hydrogen and an electron rich region) bind to them reveals not only the delicate balance between attractive and repulsive forces, but also the nature of these forces.  This feature article published in the Journal of Physical Chemistry gives an overarching description of our work and findings on these haloethylene-protic acid systems.

Our two main current projects are (1) extending the work of haloethylene-containing complexes to halopropene-containing complexes and (2) forging a new direction to advance chiral analysis with other leading microwave laboratories across the world.  Because halopropenes provide more potenitial binding sites to a protic acid, we should be able to uncover a wealth of information about the competition among electrostatic interactions, dispersion forces, and steric factors.  The second project utilizes our expertise in determining the structures of non-covalently bound complexes.  Many pharmaceuticals are small, chiral molecules.  It is, therefore, important to develop a precise method to determine qualitatively the absolute stereochemistry (not just the connectivity among atoms) of a chiral compound as well as determine quantitatively the purity of a mixture of this compound and its enantiomer.  Current commercial implementations of these analyses are either difficult or inefficient.  In our lab, we tackle these problems by using an enantiopure complexing agent (“tag”) to convert enantiomers into diastereomers, which are structurally distinct chemical species and thus have different rotational spectra.  From the signal strength of these spectra, we can figure out the enantiomeric excess of the original sample, as we have illustrated in this article.