Editing Infrared spectroscopy (section)

==Two-dimensional IR==
'''Two-dimensional infrared correlation spectroscopy analysis''' combines multiple samples of infrared spectra to reveal more complex properties. By extending the spectral information of a perturbed sample, spectral analysis is simplified and resolution is enhanced. The 2D synchronous and 2D asynchronous spectra represent a graphical overview of the spectral changes due to a perturbation (such as a changing concentration or changing temperature) as well as the relationship between the spectral changes at two different wavenumbers.{{citation needed|date=February 2024}}

{{Main|Two-dimensional infrared spectroscopy}}

[[File:2dir pulse sequence newversion.png|thumb|upright=1.25|Pulse Sequence used to obtain a two-dimensional Fourier transform infrared spectrum. The time period <math>\tau_1</math> is usually referred to as the coherence time and the second time period <math>\tau_2</math> is known as the waiting time. The excitation frequency is obtained by Fourier transforming along the <math>\tau_1</math> axis.]]

'''Nonlinear two-dimensional infrared spectroscopy'''<ref>{{cite journal| vauthors = Hamm P, Lim MH, Hochstrasser RM |title = Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy| journal = J. Phys. Chem. B|volume = 102|issue = 31|pages = 6123–6138|date = 1998|doi = 10.1021/jp9813286}}</ref><ref>{{cite journal | vauthors = Mukamel S | title = Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations | journal = Annual Review of Physical Chemistry | volume = 51 | issue = 1 | pages = 691–729 | date = 2000 | pmid = 11031297 | doi = 10.1146/annurev.physchem.51.1.691 | bibcode = 2000ARPC...51..691M | s2cid = 31230696 }}</ref> is the infrared version of [[correlation spectroscopy]]. Nonlinear two-dimensional infrared spectroscopy is a technique that has become available with the development of [[femtosecond]] infrared laser pulses. In this experiment, first a set of pump pulses is applied to the sample. This is followed by a waiting time during which the system is allowed to relax. The typical waiting time lasts from zero to several picoseconds, and the duration can be controlled with a resolution of tens of femtoseconds. A probe pulse is then applied, resulting in the emission of a signal from the sample. The nonlinear two-dimensional infrared spectrum is a two-dimensional correlation plot of the frequency ω<sub>1</sub> that was excited by the initial pump pulses and the frequency ω<sub>3</sub> excited by the probe pulse after the waiting time. This allows the observation of coupling between different vibrational modes; because of its extremely fine time resolution, it can be used to monitor molecular dynamics on a picosecond timescale. It is still a largely unexplored technique and is becoming increasingly popular for fundamental research.

As with two-dimensional nuclear magnetic resonance ([[2DNMR]]) spectroscopy, this technique spreads the spectrum in two dimensions and allows for the observation of cross peaks that contain information on the coupling between different modes. In contrast to 2DNMR, nonlinear two-dimensional infrared spectroscopy also involves the excitation to overtones. These excitations result in excited state absorption peaks located below the diagonal and cross peaks. In 2DNMR, two distinct techniques, [[Correlation spectroscopy#COSY|COSY]] and [[Correlation spectroscopy#Nuclear Overhauser effect spectroscopy (NOESY)|NOESY]], are frequently used. The cross peaks in the first are related to the scalar coupling, while in the latter they are related to the spin transfer between different nuclei. In nonlinear two-dimensional infrared spectroscopy, analogs have been drawn to these 2DNMR techniques. Nonlinear two-dimensional infrared spectroscopy with zero waiting time corresponds to COSY, and nonlinear two-dimensional infrared spectroscopy with finite waiting time allowing vibrational population transfer corresponds to NOESY. The COSY variant of nonlinear two-dimensional infrared spectroscopy has been used for determination of the secondary structure content of proteins.<ref>{{cite journal | vauthors = Demirdöven N, Cheatum CM, Chung HS, Khalil M, Knoester J, Tokmakoff A | title = Two-dimensional infrared spectroscopy of antiparallel beta-sheet secondary structure | journal = Journal of the American Chemical Society | volume = 126 | issue = 25 | pages = 7981–90 | date = June 2004 | pmid = 15212548 | doi = 10.1021/ja049811j | url = https://pure.rug.nl/ws/files/6680272/2004JAmChemSocDemirdoven.pdf }}</ref>