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Analytical Chemistry

Infrared Spectroscopy Gets A Resonance Raman Analog

New technique reveals peaks that can’t be seen by FTIR or resonance Raman

by Celia Henry Arnaud
April 28, 2014 | APPEARED IN VOLUME 92, ISSUE 17

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Credit: J. Phys. Chem. A
This region of the resonance IR spectrum of a copper complex (shown) differs from both the resonance Raman and the FTIR spectra.
09217-scicon-spectracxd.jpg
Credit: J. Phys. Chem. A
This region of the resonance IR spectrum of a copper complex (shown) differs from both the resonance Raman and the FTIR spectra.
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Copper phthalocyanine tetrasulfonate.
09217-scicon-spectra.jpg
Copper phthalocyanine tetrasulfonate.

In resonance Raman spectroscopy, matching the excitation frequency to an absorption frequency in the sample boosts the resulting Raman signal. But not all molecular vibrational modes are Raman active, and there has been no infrared version of resonance Raman. John C. Wright and coworkers at the University of Wisconsin, Madison, now report that a spectroscopic technique called triply resonant sum frequency (TRSF) spectroscopy provides that missing IR analog (J. Phys. Chem. A 2014, DOI: 10.1021/jp5018554). In TRSF spectroscopy, excitation pulses from three lasers interact with sample molecules to generate signals involving two vibrational states and an electronic state. When the laser pulses are in resonance with those coupled vibrational and electronic states, the signal from IR-active vibrational modes is enhanced. Wright and coworkers demonstrated the method with copper phthalocyanine tetrasulfonate in deuterated water. The symmetry of the copper complex means that its vibrational modes are either Raman active or IR active, but not both. The researchers observed features in the resonance IR spectrum that were not observable in either the FTIR spectrum or the resonance Raman spectrum, making the technique potentially useful for studying biomolecules such as hemes, as well as synthetic transition-metal complexes.

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