Molecular Structure and Spectroscopy Laboratory

Dr. Dieter Klapstein, Department of Chemistry, St. Francis Xavier University.

The research in this laboratory is focused on the conformational behavior of unsaturated organic compounds. An example is the series of carbonyl isocyanates which have been found to exist as a mixture of two rotational isomers:

The experimental analysis has been carried out by gas-phase Fourier transform infrared spectroscopy (FT-IR). Such conformational behavior is currently being investigated in other carbonyl and aromatic species. Another powerful tool for providing insight into the factors affecting rotational isomerism is computational molecular modelling. A range of levels of theory is available here, from molecular mechanics through semi-empirical to ab initio quantum chemical calculations performed on PCs or workstations. Such molecular orbital calculations also provide clues to the electronic structure of molecules, that is, the orbital occupancies and energies. These determine the physical properties and chemical reactivities of molecules.

As an example of the visualization power of quantum chemical calculations, the following graphic shows the diphenyl ether molecule (tube structure) with a visualization of the calculated highest occupied molecular orbital (HOMO) with solid, colour-coded lobes, surrounded by a total electron density grid onto which the electrostatic potential has been mapped. Molecular modelling is also used extensively in some of our courses, Chem 421/422.

Click for full image

The orbital energies can also be probed experimentally in this laboratory by molecular photoelectron spectroscopy (PES). In this technique, high-energy and monochromatic photons (E = 21.22 eV, l = 58.4 nm) are used to photoionize molecules, and the resulting photoelectrons are analyzed in terms of their kinetic energies by passing them through an electric field. The photons have enough energy to cause ejection of electrons from any of the occupied molecular orbitals with binding energy less than the photon energy.

By knowing the photon energy and measuring the kinetic energies of the electrons, conservation of energy allows determination of the ionization energies (or orbital binding energies).

Thus a photoelectron spectrum is an energy mapping of the higher occupied molecular orbitals, each occupied MO producing a band in the PE spectrum.

Bands in a PE spectrum can be assigned to ionizations from specific MOs on the basis of:

-Characteristic band positions and shapes (functional groups).

-Comparison with the PE spectra of structurally related molecules.

-Comparison with predicted ionization energies based on MO eigenvalues determined by quantum chemical calculations.

Useful References on Molecular Photoelectron Spectroscopy:

-Principles of Ultraviolet Photoelectron Spectroscopy, J.W.

Rabalais, Wiley, New York, 1977; QC 454P48 R3.

-Handbook of HeI Photoelectron Spectra of Fundamental

Organic Molecules, K. Kimura, S. Katsumata,

Y. Achiba, T. Yamazaki, S. Iwata, Halsted, New York, 1981;

QC 462.5 H34.

Before any experimental investigation can be carried out, many of the compounds to be studied must be synthesized in this laboratory. Thus a variety of equipment and techniques are available for synthesis and purification of the compounds, most of which are extremely moisture-sensitive or subject to thermal decomposition. Besides FT-IR and PE spectroscopies, sample characterization also involves UV-visible absorption spectroscopy and nuclear magnetic resonance spectroscopy, all available in this department.

For some recent publications from this laboratory, go here.

feedback (dklapste@stfx.ca)