ABSTRACT
The two-dimensional infrared (2D IR) method allows correlating the frequency distributions of different vibrators in a molecular system, thus providing structural constraints as well as important insight into molecular dynamics [1-6]. The applicability of the 2D IR method as an analytical technique will be decided by several factors, such as its sensitivity, availability of convenient spatially localized vibrational labels, and ability of collecting efciently a large number of structural constraints for any given molecular system. Dual-frequency 2D IR measurements using a threepulse noncollinear geometry with heterodyned detection provide the highest sensitivity and allow collecting a large number of cross peaks, while permitting a suppression of the diagonal peaks [7-9]. The details of such measurements are discussed in the next section focusing on sensitivity and phase stability issues. A recently proposed relaxation-assisted 2D IR (RA 2D IR) approach [10] permits enhancing the cross-peak amplitudes by over an order of magnitude as well as delivers additional structural constraints that can be linked to bond connectivity patterns and vibrational mode delocalization extents. The principles and examples illustrating the advantages of RA 2D IR spectroscopy are given in Section 11.3. RA 2D IR permits measuring energy transport on a molecular level; such transport molecular systems are discussed in Section 11.4. 2D IR measurements involving delocalized modes in the ngerprint region are discussed in Section 11.5.
