Correlation spectrum

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

SWi, which in turn is related to the homonuclear or heteronuclear coupling constants. In homonuclear 2D spectra, the transmitter offset frequency is kept at the center of (i.e., at = 0) and F domains. In heteronuclear-shift-correlated spectra, the decoupler offset frequency is kept at the center (Fi = 0) of thei i domain, with the domain corresponding to the invisible or decoupled nucleus. 

Heteronuclear-shift-correlation spectra, which are usually presented in the absolute-value mode, normally contain long dispersive tails that are suppressed by applying a Gaussian or sine-bell function in the F domain. In the El dimension, the choice of a weighting function is less critical. If a better signal-to-noise ratio is wanted, then an exponential broadening multiplication may be employed. If better resolution is needed, then a resolution-enhancing function can be used. 

One-dimensional spectra obtained by projecting 2D spectra along a suitable direction often contain information that cannot be obtained directly from a conventional ID spectrum. They therefore provide chemical shift information of individual multiplets that may overlap with other multiplets in the corresponding ID spectra. The main difference between the projection spectrum and the ID spectrum in shift-correlated spectra is that the projection spectrum contains only the signals that are coupled with each other, whereas the ID H-NMR spectrum will display signals for all protons present in the molecule. 

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

Titman, J. J., Keeler, J. Measurement of homonuclear coupling constants from NMR correlation spectra. J. Magn. Reson. 1990, 89, 640-646. 

A simple NMR technique, and arguably the most widely used and effective for hit validation, is the chemical shift perturbation method. In this approach, a reference spectrum of isotopically labeled target is recorded in absence and presence of a given test ligand (or a mixture of test ligands). Commonly, differences in chemical shift between free and bound protein target are observed in 2D [15N, 1H and/or 2D [13C, H] correlation spectra of a protein (or nucleic acid) upon titration of a ligand... 

Synchronous 2D correlation spectra represent coupled or related changes of spectral intensities, while asynchronous correlation spectra represent independent or separate variations [1007]. The 2D cross-correlation analysis enhances similarities and differences of the variations of individual spectral intensities, providing spectral information not readily accessible from ID spectra. 

Although the idea of generating 2D correlation spectra was introduced several decades ago in the field of NMR [1008], extension to other areas of spectroscopy has been slow. This is essentially on account of the time-scale. Characteristic times associated with typical molecular vibrations probed by IR are of the order of picoseconds, which is many orders of magnitude shorter than the relaxation times in NMR. Consequently, the standard approach used successfully in 2D NMR, i.e. multiple-pulse excitations of a system, followed by detection and subsequent double Fourier transformation of a series of free-induction decay signals [1009], is not readily applicable to conventional IR experiments. A very different experimental approach is therefore required. The approach for generation of 2D IR spectra defined by two independent wavenumbers is based on the detection of various relaxation processes, which are much slower than vibrational relaxations but are closely associated with molecular-scale phenomena. These slower relaxation processes can be studied with a conventional... 

Figure 7.44 shows the 2D UV chromatogram (RPLC-UV/VIS (DAD)) for a five-compound test mixture of polymer additives [662]. Any spectral data collected during hyphenated chromatography-spectroscopy measurements can be readily transformed into 2D correlation spectra. 

In Problem 40 we have recorded two long-range correlation spectra using delays set for two different long-range J values, 8 and 2 Hz. The differences between the spectra give us even more structurally-relevant information. 

MHz, solvent CDC13 CH-and CH-long range correlation spectra... 

In 2003, Sprang and Bigler have developed a pulse sequence, HMBC-RELAY, subsequently improved in 2004, that yields two simultaneously detected types of long-range correlation spectra.60,61 One spectrum shows all 7ch connectivities while the other shows exclusively 2/ch connectivities. Their method uses homonuclear 3/hh couplings between the protons of adjacent carbons, as already been exploited for the XCORFE... 

Generalized Two-Dimensional (2D) correlation analysis is a powerful tool applicable to data obtained from a very broad range of measurements, such as chromatography or infrared spectroscopy. Relationships among systematic variations in infrared spectra are obtained as a function of spectroscopic frequencies. In this paper, the variation is induced by the introduction of small doses of CO in the catalytic cell, inducing a pressure change and a modification of adsorbed CO concentration. The correlation intensities are displayed in the form of 2D maps, usually referred to as 2D correlation spectra. 2D correlation analysis can help us to solve the complexity of the spectra... 

In 1993, Noda has introduced generalized two-dimensional spectroscopy that can be applied to a lot of spectroscopic measurements [1], A simple and efficient computational technique to obtain the 2D correlation spectra is to use a form of discrete... 

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