Two-dimensional infrared spectroscopy

The initial idea of generating two-dimensional correlation spectra was introduced several decades ago in the field of NMR spectroscopy [13-16]. Since then, numerous successful applications of multidimensional resonance spectroscopy techniques have been reported, including many different types of studies of polymeric materials by 2D NMR [17 19]. Flowever, until now the propagation of this powerful concept of multidimensional spectroscopy in other areas of spectroscopy, especially vibrational spectroscopies such as IR and Raman, has been surprisingly slow. 

One of the reasons why the two-dimensional correlation approach as applied in NMR spectroscopy was not readily incorporated into the field of IR spectroscopy was the relatively short characteristic times (on the order of picoseconds) associated with typical molecular vibrations probed by IR. Such a time scale is many orders of magnitude shorter than the relaxation times usually encountered in NMR. Consequently, the standard approach used so successfully in 2D NMR, i.e., multiple-pulse excitations of a system followed by the detection and subsequent double Fourier transformation of a series of free induction decay signals, is not readily applicable to conventional IR experiments. A very different experimental approach, therefore, needed to be developed in order to produce 2D IR spectra useful for the characterization of polymers using an ordinary IR spectrometer. 

There are, of course, many different types of physical stimuli which could induce such dynamic variations in the spectral intensities of polymeric samples. Possible sources of perturbations include electrical, thermal, magnetic, acoustic, chemical, optical, and mechanical stimuli. The waveform or specific time signature of the perturbation may also vary from a simple step function or short pulse, to more complex ones, including highly multiplexed signals and even random noises. In this chapter, dynamic spectra generated by a simple sinusoidal mechanical perturbation applied to polymers will be discussed. 

MechanicaL elecirical, chemical, magneiic. optical, tfiermal. efc. 

The optical anisotropy, as characterized by the difference between the absorption of IR light polarized in the directions parallel and perpendicular to the reference axis (i.e., the direction of applied strain), is known as the IR linear dichroism of the system. For a uniaxially oriented polymer system [10, 28-30], the dichroic difference, A/4(v) = y4 (v) - Ax v), is proportional to the average orientation, i.e., the second moment of the orientation distribution function, of transition dipoles (or electric-dipole transition moments) associated with the molecular vibration occurring at frequency v. If the average orientation of the transition dipoles absorbing light at frequency is in the direction parallel to the applied strain, the dichroic difference AA takes a positive value on the other hand, the IR dichroism becomes negative if the transition dipoles are perpendicularly oriented. 

---

P. Hamm and R. M. Hochstrasser, Structure and dynamics of proteins and peptides Femto second two dimensional infrared spectroscopy, in Ultrafast Infrared and Raman Spectroscopy, Markel Dekker, New York, 2001, p. 273. 

Bredenbeck J, Ghosh A, Smits M, Bonn M (2008) Ultrafast two dimensional-infrared spectroscopy of a molecular monolayer. J Am Chem Soc 130 2152... 

Transient two-dimensional infrared spectroscopy - towards measuring ultrafast structural dynamics... 

Noda, I., Two-dimensional infrared spectroscopy Theory and application, Applied Spectroscopy, 44, 550 (1990). 

Structure and Dynamics of Proteins and Peptides Femtosecond Two-Dimensional Infrared Spectroscopy... 

Asplund MC, Zanni MT, Hochstrasser RM. Two-dimensional infrared spectroscopy of peptides by phase-controlled femtosecond vibrational photon echoes. Proc Natl Acad Sci USA 2000 97 8219-8224. 

DYNAMICAL MODELS FOR TWO-DIMENSIONAL INFRARED SPECTROSCOPY 13 circuit 5((b) is proportional to the absolute square of that sum ... 

DYNAMICAL MODELS FOR TWO-DIMENSIONAL INFRARED SPECTROSCOPY 17 Rephasing... 

---