Abstract
We present precise optical rotation measurements of gaseous chiral samples using near-IR continuous-wave cavity-enhanced polarimetry. Optical rotation is determined by comparing cavity ring-down signals for two counter-propagating beams of orthogonal polarisation which are subject to polarisation rotation by the presence of both an optically active sample and a magneto-optic crystal. A broadband RF noise source applied to the laser drive current is used to tune the laser linewidth and optimise the polarimeter, and this noise-induced laser linewidth is quantified using self-heterodyne beat-note detection. We demonstrate the optical rotation measurement of gas phase samples of enantiomers of α-pinene and limonene with an optimum detection precision of 10 µdeg per cavity pass and an uncertainty in the specific rotation of ∼0.1 deg dm-1 (g/ml)-1 and determine the specific rotation parameters at 730 nm, for (+)- and (-)-α-pinene to be 32.10 ± 0.13 and -32.21 ± 0.11 deg dm-1 (g/ml)-1, respectively. Measurements of both a pure R-(+)-limonene sample and a non-racemic mixture of limonene of unknown enantiomeric excess are also presented, illustrating the utility of the technique.
Highlights
Chirality is crucial in many areas of science, from fundamental physics, where chiral molecules have been identified as candidates for the investigation of parity violation [1,2], to pharmacology, where two molecular enantiomers can show striking differences in cellular toxicity [3], and yet possess nominally identical physico-chemical properties
Multi-exponential decays can occur because of excitation of multiple cavity modes and the resultant mode interference-induced intensity fluctuations can lead to a reduction in the sensitivity. These issues can be addressed by using cw lasers and recently we have reported on the development of cw diode laser based cavity ring-down polarimetry (CRDP) [16] within a bow-tie cavity for interrogating gaseous and liquid phase samples
We have presented precise gas-phase optical rotation measurements using continuous-wave cavity-enhanced polarimetry at 730 nm
Summary
Chirality is crucial in many areas of science, from fundamental physics, where chiral molecules have been identified as candidates for the investigation of parity violation [1,2], to pharmacology, where two molecular enantiomers can show striking differences in cellular toxicity [3], and yet possess nominally identical physico-chemical properties. The precise analysis of chirality is essential in drug synthesis and biomolecular recognition. It is not surprising that several optical techniques have been developed for determining molecular chirality. Examples include: optical rotation, electronic and vibrational circular dichroism (CD), microwave, femtosecond time-resolved CD and superchiral light spectroscopies, and ionization imaging [4,5,6,7,8,9]. Optical rotation, i.e. the measurement of the change in polarization of a linearly polarized light beam passing through a chiral sample, is one of the most facile methods with which to ascertain the chirality of liquid-phase samples. Modern commercial polarimeters are ubiquitous and have a typical precision of ∼1 mdeg, mainly limited by the effect of birefringence
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