Carbene Spectroscopy

Overview

Free radicals are intermediates in almost all chemical reactions. The study of their structure and chemistry is difficult because of their inherent reactivity. We have developed several ways of creating radicals and keeping them long enough to study their structure and energetics using laser-based spectroscopic techniques.

Objectives

To obtain high quality spectroscopic data on free radicals that allow their structure and energetics to be determined, and to provide data that challenge the most recent theoretical methods. Where possible, the radicals we choose to study will be of importance to relevant chemical problems (e.g. ozone depletion), or of fundamental chemical interest (e.g. to learn about novel classes of radicals).

How the Experiment is Done

Radicals are created in three different ways: photolysis (breaking the precursor using a laser); pyrolysis (by heat); or electric discharge. A photo of a discharge nozzle is shown at right. The discharge makes ions and electrons as well as radicals of interest. The experiment is performed in a vacuum chamber so that the radicals cannot further react. Lasers intersect the beam of radicals in the middle of the plume to excite the radicals and measure their spectrum.

An Example: Structure and Reactivity of CFBr

CFBr is an example of a carbene - a radical with two non-bonding electrons that may be paired in the same orbital (a carbene state), or in different orbitals (a biradical state). We have discovered that CFBr is formed when some halons (e.g. CFBr₃) absorb light in the stratosphere. Analysis of the laser induced fluorescence spectrum (right) tells us that CFBr has ~120º angle in the ground state and that the ground state is the carbene configuration. The first excited singlet state is a biradical with the bond angle now open to 104º.

The spectra at right also show some interesting chemistry. The laser induced fluorescence (LIF) spectrum stops for wavelengths shorter than 412 nm (23,900 cm^-1). If, instead of monitoring fluorescence, we probe for the presence of CF (phofex spectrum), the spectrum continues after the fluorescence spectrum stopped. This is very clear evidence that for l<412 nm, CFBr undergoes further chemistry in the atmosphere, forming two new radicals, CF and Br.

click on the thumbnail to view the figure

There is also evidence of "mode specific" chemistry in this spectrum. If you excite two different vibrational modes - one containing the CF stretch and the other without - then the molecule containing the CF-stretching energy will dissociate, while the molecule without CF excitation does not.

Selected References

Electronic spectroscopy of jet-cooled CFCl: Laser induced fluorescence, dispersed fluorescence, lifetimes and C-Cl dissociation barrier, J.S. Guss, O. Votava and S.H. Kable, J. Chem. Phys., 115, 11118, (2001).
Characterisation of the (¹A") state of HCF by laser induced fluorescence spectroscopy, T.W. Schmidt, G.B. Bacskay and S.H. Kable, J. Chem. Phys., 110, 11277, (1999).
Electronic spectroscopy and ab initio quantum chemical study of the (¹A") - (¹A') transition of CFBr, P.T. Knepp, C.K. Scalley, G.B. Bacskay, and S.H. Kable, J. Chem. Phys., 109, 2220, (1998).