Cavity Ring-Down Polarimetry (CRDP):  A New Scheme for Probing Circular Birefringence and Circular Dichroism in the Gas Phase

Abstract

Cavity ring-down polarimetry (CRDP), a new, ultrasensitive method for probing circular birefringence and circular dichroism, has been developed by extending the well-established technique of pulsed cavity ring-down spectroscopy (CRDS). The concurrent incorporation of polarization elements into the stable resonator, injection optics, and detection train of a conventional CRDS apparatus is found to permit the quantitative measurement of optical rotation and differential absorption induced by the presence of chiral compounds. The sensitivity of this novel scheme is sufficient to allow (low-pressure) gas-phase species to be interrogated under ambient conditions, a fact highlighted by the direct determination of specific rotation at 355 nm for gaseous samples of α-pinene, β-pinene, cis -pinane, limonene, fenchone, and propylene oxide. Although usually precluded by the signal discrimination limits imposed upon traditional polarimeters, such gas-phase studies of nonresonant optical activity in the visible and near-ultraviolet regions of the electromagnetic spectrum can serve to calibrate ab initio theoretical predictions and to examine the roles of solvent−solute interactions. Comparison of the CRDP-measured specific rotation angles for isolated (gaseous) chiral molecules with analogous solution-phase results reveals that solvent effects can be significant and nonintuitive, often leading to solvent-mediated values that differ significantly from their gas-phase counterparts in both magnitude and sign.