A NEW DEVICE FOR RAMAN DIFFERENCE SPECTROSCOPY AND ITS APPLICATION TO OBSERVE FREQUENCY-SHIFTS DUE TO ISOTOPE MIXING

A new double beam device was developed for Raman difference spectroscopy (RDS). The device is intended to measure Raman spectra of two samples simultaneously by using branched bundles of optical fibers and a multichannel detector. This technique avoids the time drift of Raman band positions on the diode array detector. The accuracy of the frequency shift ( Δv) of a Raman band observed upon mixing of two liquids is increased to 0.01 cm-1 from 0.05 cm-1 of the previous measurements. The device was applied to explore the frequency shifts due to isotopic dilution of C6H6/C6D6 and CH3CN/CD3CN pairs as a function of concentration. The symmetric C-H and C-D stretching modes exhibited a linear dependence on the mole fraction of the probe species (ΧA) except for a very dilute solutions of C6D6 in C6H6 (ΧA < 0.1 ). The Δv-ΧA curves for some other bands show a saturation effect. The results are discussed on the basis of the vibrational resonance coupling and the polarizability and volume differences between the parent and isotope molecules. © 1990 American Chemical Society.