Observation and Calculation of Vibrational Circular Birefringence: A New Form of Vibrational Optical Activity

We report the first mid-infrared observation of vibrational circular birefringence (VCB) arising from individual chiral molecules. VCB can also be called vibrational optical rotatory dispersion (VORD) and is the Kramers-Kronig transform of vibrational circular dichroism (VCD). The method of measurement involves a simple change in the optical set-up and electronic processing of a VCD spectrometer such that the VCB spectrum appears at twice the polarization modulation frequency as a pseudo vibrational linear dichroism (VLD) spectrum. VCB spectra are also calculated with density function theory (DFT) for the first time using a commercially available program for rotational strengths where the calculated intensities are convolved with the real, dispersive part of a normalized complex Lorentzian lineshape rather than the imaginary, absorptive part, normally used for IR and VCD intensity calculations. Comparison of the measured and calculated VCB, VCD, and IR spectra of (+)-R-limonene and (-)-S-alpha-pinene show close agreement and confirm the validity of the new VCB measurements. Chirality 21:E277-E286, 2009. (c) 2009 Wiley-Liss, Inc.