Magnetic Vibrational Circular Dichroism

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. Early studies (Devine and 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 (Pawlikowski) indicated that to obtain the splitting needed to fit the observed intensity involved mixing ground and excited state potential surfaces by means of the asymmetric vibration. This model even works reasonably well for C60 fullerene spectra (Pawlikowski, Tam).

More recently we (Croatto, B. Wang, Tam) 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. 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 and with standard quantum chemical techniques estimate it from the paramagnetic susceptibility computed.

--Research Topics--MVCD--Biopolymers--Instrumentation--Small Molecules--Other Interests--Keiderling Home--