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:<math>\frac{\nu_{^{16}O}}{\nu_{^{18}O}} = \sqrt{\frac{9}{8}} \approx \frac{832}{788}.</math>
== Fourier transform infrared spectroscopy ==
{{Main|Fourier transform spectroscopy}}
'''Fourier transform infrared (FTIR) spectroscopy''' is a measurement technique for collecting infrared spectra. Instead of recording the amount of energy absorbed when the frequency of the infra-red light is varied (monochromator), the IR light is guided through an [[interferometer]]. After passing through the sample, the measured signal is the interferogram. Performing a mathematical [[Fourier transform]] on this signal results in a spectrum identical to that from conventional (dispersive) infrared spectroscopy.
FTIR spectrometers are cheaper than conventional spectrometers because building of interferometers is easier than the fabrication of a monochromator. In addition, measurement of a single spectrum is faster for the FTIR technique because the information at all frequencies is collected simultaneously. This allows multiple samples to be collected and averaged together resulting in an improvement in sensitivity. Because of its various advantages, virtually all modern infrared spectrometers are FTIR instruments.
== Two-dimensional infrared spectroscopy ==
{{Main|Two-dimensional infrared spectroscopy correlation analysis}}
'''Two-dimensional infrared correlation spectroscopy analysis''' is the application of [[2D correlation analysis]] on infrared spectra. 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.
{{Main|Two-dimensional infrared spectroscopy}}
[[Image:2dir_pulse_sequence_newversion.png|thumb|300px|right|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 | author = P. Hamm, M. H. Lim, R. M. Hochstrasser | title = Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy
| journal = J. Phys. Chem. B | volume = 102 | pages = 6123 | year = 1998 | doi = 10.1021/jp9813286}}</ref><ref>{{cite journal | author = S. Mukamel | title = Multidimensional Fentosecond Correlation Spectroscopies of Electronic and Vibrational Excitations | journal = Annual Review of Physics and Chemistry | volume =51 | pages = 691 | year = 2000 | doi = 10.1146/annurev.physchem.51.1.691 }}</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 are applied to the sample. This is followed by a waiting time, where the system is allowed to relax. The waiting time typically 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 <math>\omega_1</math> that was excited by the initial pump pulses and the frequency <math>\omega_3</math> excited by the probe pulse after the waiting time. This allows the observation of coupling between different vibrational modes. Because of its extremely high 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.
Like in two-dimensional nuclear magnetic resonance (2DNMR) spectroscopy this technique spreads the spectrum in two dimensions and allow 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 involve 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#NOESY|NOESY]], are frequently used. The cross peaks in the first are related to the scalar coupling, while in the later 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 proteins.<ref>{{cite journal | author = N. Demirdöven, C. M. Cheatum, H. S. Chung, M. Khalil, J. Knoester, A. Tokmakoff | title= Two-dimensional infrared spectroscopy of antiparallel beta-sheet secondary structure | journal = [[Journal of the American Chemical Society]] | volume = 126 | pages = 7981 | year = 2004 | doi = 10.1021/ja049811j}}</ref>
==Daerah Identifikasi==
Vibrasi yang digunakan untuk identifikasi adalah vibrasi bengkokan, khususnya goyangan (rocking), yaitu yang berada di daerah bilangan gelombang 2000 – 400 cm-1. Karena di daerah antara 4000 – 2000 cm-1 merupakan daerah yang khusus yang berguna untuk identifkasi gugus fungsional. Daerah ini menunjukkan absorbsi yang disebabkan oleh vibrasi regangan. Sedangkan daerah antara 2000 – 400 cm-1 seringkali sangat rumit, karena vibrasi regangan maupun bengkokan mengakibatkan absorbsi pada daerah tersebut.
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