Two-dimensional infrared spectroscopy

[1][2] A frequency correlation spectrum can offer structural information such as vibrational mode coupling, anharmonicities, along with chemical dynamics such as energy transfer rates and molecular dynamics with femtosecond time resolution.

2DIR experiments have only become possible with the development of ultrafast lasers and the ability to generate femtosecond infrared pulses.

Among the many systems studied with infrared spectroscopy are water, metal carbonyls, short polypeptides, proteins, perovskite solar cells, and DNA oligomers.

[3][4] There are two main approaches to two-dimensional spectroscopy, the Fourier-transform method, in which the data is collected in the time-domain and then Fourier-transformed to obtain a frequency-frequency 2D correlation spectrum, and the frequency domain approach in which all the data is collected directly in the frequency domain.

After a determined waiting time, ranging from a zero to a few hundred picoseconds, an interaction with a third pulse again creates a coherence, which, due to an oscillating dipole, radiates an infrared signal.

Recently, pulse shaping approaches were developed to simplify overcoming this challenge.

The true power of 2DIR spectroscopy is that it allows following dynamical processes such as chemical exchange, motional narrowing, vibrational population transfer, and molecular reorientation on the sub-picosecond time scale.

It has for example been used successfully to study hydrogen bond forming and breaking and to determine the transition state geometry of a structural rearrangement in an iron carbonyl compound.

The consideration of the solvent effect has been shown to be crucial [12][13] in order to effectively describe the vibrational coupling in solution, since the solvent modify both vibrational frequencies, transition probabilities [14] and couplings.

[15][16] Computer simulations can reveal the spectral signatures arising from solvent degrees of freedom and their change upon water reorganization.

Pulse sequence used to obtain a two-dimensional Fourier transform infrared spectrum: is the coherence time, is the waiting time. The Fourier transform with respect to provides the excitation spectrum (frequency ).
Schematic of a 2D IR spectrum. The red circles correspond to bleaching of the ground state. The blue circles correspond to absorption of the excited state. The smaller off-diagonal circles to coupling between the two states. The linear absorption (FTIR) spectrum is indicated above the 2D IR spectrum. The two peaks in the 1D spectrum reveal no information on coupling between the two states.