The Keiderling Group

Molecular Zeeman spectroscopy is a field opened a couple decades ago when high resolution instrumentation be came available for microwave and, eventually, IR spectroscopy, such that ever smaller perturbations might be studied. Zeeman studied spitting of sharp atomic lines from samples placed in a magnetic field. MCD developed as a way of accessing this information in the broader electronic transitions found in molecules. MCD essentially is a transducer of the frequency splitting caused by the Zeeman effect into an intensity, in this case a differential intensity for two polarization states of the light. MCD is typically characterized by A, B and C terms which offer information on splittings of degenerate states, off-diagonal mixing of excited states, and splitting of degenerate ground states, respectively.

Since we had developed sensitive VCD instrumentation to measure CD effects in the IR, since FTIR resolution enhancement is straightforward and since magnetic effects on vibrations were expected to be very small, it seemed natural to attempt to measure MVCD of degenerate vibrational modes. Some theoretical predictions on classical model systems suggested the phenomenon should just be measureable. Early MVCD studies at UIC (with Tim Devine and Paul Croatto) dealt with a variety of small condensed phase molecules and showed that spectra were indeed measurable, but were most intense in highly symmetric molecules like tri-substituted benzenes, porphyrins and metal hexacarbonyls. Analysis with a parameterized two-state model (developed by Marek Pawlikowski) indicated that the Zeeman splittings that would be needed to fit the observed intensities require a mechanism involving the mixing of ground and excited state potential surfaces by means of the asymmetric vibration. Essentially this is Herzberg-Teller coupling, but between the ground and a degenerate excited state, whereby the electronic magnetic moment of the excited state is "borrowed" or coupled into the ground state. This model even works reasonably well for C₆₀ fullerene spectra (with Pawlikowski and Cheok Tam).

Subsequently we (Croatto, Tam and Baoliang Wang) have pushed the resolution limit to 0.1 cm^-1 and have measured rotationally resolved MVCD of a number of gas phase small molecules such as CO, HCl, CH₄, NH₃, CH₃X and acetylene. Here the rotational Zeeman effect is dominant, the observed magnetic moment arising from the rotation dipole moment and changing with rotational J-level. These spectra, where resolved, are reasonably simulated using standard Hamiltonian formalisms and conventional polarization selection rules. The latest studies have let us definitively observe the sign of the rotational g-value and determine for the first time a vibrational g value for acetylene which can be related to the paramagnetic susceptibility computed with standard quantum chemical techniques.

To learn more about our work in MVCD, please check out our publications in this field.