Chemistry

Chiral Inference from Interference

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Science  30 Jun 2006:
Vol. 312, Issue 5782, pp. 1848-1849
DOI: 10.1126/science.312.5782.1848d

When light impinges on a molecular sample that lacks inversion symmetry, two photons can mix to generate a third photon bearing the sum of their respective frequencies. This sum frequency generation (SFG) process is often used to characterize surfaces spectroscopically, because if the bulk environment below is centrosymmetric, the signal depends exclusively on the surface properties. SFG also arises from the irradiation of bulk chiral liquids and could, in principle, offer greater sensitivity for their characterization than the commonly applied circular dichroism or optical rotatory dispersion techniques. However, because SFG intensity scales quadratically with chiral susceptibility, it fails to reveal absolute stereochemical configurations; one enantiomer gives rise to the same signal as the other.

Ji et al. address this limitation by detecting the interference between the SFG response of a chiral liquid and the response of a quartz crystal below it. Samples of (R)- and (S)-binaphthol could be differentiated clearly from the ultraviolet spectra emergent upon irradiation with overlapped near-infrared and visible laser pulses. Because the response is highly sensitive to the orientation of the quartz crystal, the authors propose using the binaphthol spectrum to calibrate alignments in future experiments. — JSY

J. Am. Chem. Soc. 128, 10.1021/ja060888c (2006).

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