Stretching IR spectroscopy to an extra dimension enables experimental visualization of ultrafast chemical exchange processes

Abstract

In a conventional infrared (IR) spectrum, the peak positions of the stretching modes are used for molecular characterization while the widths relate to dynamics. With the advance of ultrafast femtosecond lasers, non-linear IR spectroscopic methods have evolved to probe ultrafast molecular dynamics. Multiple radiation field-matter interactions allow two dimensional infrared (2D-IR) spectroscopy to study structural fluctuations and corformational interchanges in biomolecules. Most importantly, stretching IR spectroscopy to an extra dimension provides direct experimental signatures of (sub)-picosecond chemical exchange processes. In this paper, we describe how steady state IR and NMR spectroscopic techniques, when used in tandem, can qualitatively predict the existence of ultrafast conformational interchanges in limited chromophores but fails to obtain a quantitative result. Analysis of the time evolution of the cross peaks in the phase and frequency resolved 2D-IR spectra, however, provides a direct quantitative estimate of the exchange timescale. We present results of a cosolvent mediated chemical exchange processes to explicitly elaborate on the advantage of using 2D-IR spectroscopy.