Femtosecond Pulse Shaping Enables Rapid Two-Dimensional Infrared Spectroscopy

Jul 01, 2013
Volume 28, Issue 7

Martin Zanni and his group in the Department of Chemistry at the University of Wisconsin-Madison (Madison, Wisconsin) are specialists in a new class of infrared spectroscopy: two-dimensional (2D) infrared spectroscopy. To collect a 2D IR spectrum, one uses a series of infrared femtosecond laser pulses to pump and then probe the response of the system. Using this technique, it is possible to probe the structures and dynamics of molecules. In this interview, Zanni explains the technique and how it is enabled by specialized laser methods. He also discusses current applications of the technique, such as solar cell research and the study of the kinetics of protein aggregation in type 2 diabetes.

What is 2D IR spectroscopy?

Zanni: Everyone knows that infrared spectroscopy is one of the most used analytical and research techniques in the world. It is often the first tool used to assess the chemical composition of a substance or the success of a reaction, because functional groups have characteristic vibrational frequencies. Students learn to interpret Fourier transform infrared (FT-IR) spectra in their undergraduate organic chemistry class. While frequencies are useful, vibrational motions of molecules contain an enormous wealth of information that goes far beyond what can be measured in an FT-IR spectrum. Vibrational coupling tells us if two molecules are bound to one another. Vibrational dynamics reveal solvation. Vibrational energy flow provides bond proximity. Vibrational transition dipoles give bond angles. Two-dimensional IR spectroscopy provides the means to extract these quantities, which are otherwise not available from an FT-IR spectrum.

Two-dimensional IR spectroscopy is the multidimensional analog of FT-IR spectroscopy. An FT-IR spectrometer measures the vibrational spectrum of a sample by probing it with an infrared pulse. In 2D IR spectroscopy, we first provide an infrared pump pulse that vibrationally excites the molecules in the sample, and then we probe the sample again, some time later. By varying the frequency of the pump pulse, we can create a 2D spectrum that correlates the vibrations that were pumped with those that were probed. The correlation occurs because the molecules are vibrationally coupled. Vibrational coupling provides information about structure, binding, and dynamics.

Figure 1: Experimental FT-IR and 2D IR spectra for a mixture of W(CO)6 and a rhodium dicarbonyl (RDC). For each peak in the FT-IR spectrum, the 2D IR spectrum exhibits a pair of diagonal peaks. The cross peaks in the 2D IR spectrum reveal that the two higher frequency peaks are coupled to one another, which is because peaks 2 and 3 are from a rhodium dicarbonyl (RDC) whereas peak 1 is from W(CO)6. W(CO)6 and RDC do not have cross peaks between them because the mixture is too dilute. (Data collected by Tianqi Zhang.)
As an example, consider the FT-IR spectrum shown in Figure 1, which is collected for a dilute mixture of two molecules. The FT-IR spectrum contains three absorption bands. We know from the frequency range that these bands must correspond to functional groups with triple-bond character, but which functional group belongs to which molecule? The measured 2D IR spectrum for the same mixture is also shown. Notice that each of the absorption bands in the FT-IR spectrum produces a pair of peaks along the diagonal in the 2D IR spectrum. A diagonal slice along a 2D IR spectrum contains essentially the same information as an FT-IR spectrum. In addition, there are pairs of cross peaks between peaks 2 and 3, but not between peaks 1 and 2 or between 1 and 3. Thus, peaks 2 and 3 are vibrationally coupled, but neither is coupled to peak 1. Therefore, from the peak pattern we can quickly and confidently assign peak 1 to one molecule and peaks 2 and 3 to the other. In fact, the solution is a mixture of W(CO)6 (peak 1) with a rhodium metal dicarbonyl (peaks 2 and 3). The peaks in the rhodium dicarbonyl exhibit very strong cross peaks because they share a common metal atom, but do not couple to the modes of W(CO)6 because the two molecules do not bind to one another. Thus, the cross peaks show us connectivity between absorption bands, and this information in turn teaches us about structure. Of course, mixtures like this can be disentangled using just FT-IR spectroscopy, such as by peak fitting with a spectral library or changing the concentration. But in many cases, the additional information from 2D IR spectra provides information not easily obtainable by other means. I give a few examples below in answers to other questions.

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