Correlation experiments

B1.12.4.6 DISTANCE MEASUREMENTS, DIPOLAR SEQUENCES AND CORRELATION EXPERIMENTS... 

What compound C gH2oO(, can be identified from the CH correlation experiments 40 and the H NMR spectra shown above ... 

In homonuclear-shift-correlated experiments, the Ft domain corresponds to the nucleus under observation in heteronuclear-shift-correlated experiments. Ft relates to the unobserved or decoupled nucleus. It is therefore necessary to set the spectral width SW, after considering the ID spectrum of the nucleus corresponding to the Ft domain. In 2D /-resolved spectra, the value of SW depends on the magnitude of the coupling constants and the type of experiment. In both homonuclear and heteronuclear experiments, the size of the largest multiplet structure, in hertz, determines... 

The matrix obtained after the F Fourier transformation and rearrangement of the data set contains a number of spectra. If we look down the columns of these spectra parallel to h, we can see the variation of signal intensities with different evolution periods. Subdivision of the data matrix parallel to gives columns of data containing both the real and the imaginary parts of each spectrum. An equal number of zeros is now added and the data sets subjected to Fourier transformation along I,. This Fourier transformation may be either a Redfield transform, if the h data are acquired alternately (as on the Bruker instruments), or a complex Fourier transform, if the <2 data are collected as simultaneous A and B quadrature pairs (as on the Varian instruments). Window multiplication for may be with the same function as that employed for (e.g., in COSY), or it may be with a different function (e.g., in 2D /-resolved or heteronuclear-shift-correlation experiments). 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

Another 2D homonuclear shift-correlation experiment that provides the coupling information in a different format is known as SECSY (spin-echo correlation spectroscopy). It is of particular use when the coupled nuclei lie in a narrow chemical shift range and nuclei with large chemical shift differences are not coupled to one another. The experiment differs... 

In homonudear shift-correlation experiments like COSY we were concerned with the correlation of chemical shifts between nuclei of the same nuclear species, e.g., H with H. In heteronuclear shift-correlation experiments, however, the chemical shifts of nuclei belonging to different nuclear species are determined (e.g., H with C). These may be one-bond chemical shift correlations, e.g., between directly bound H and C nuclei, or they may be long-range chemical shift correlations, in which the interactions... 

Figure 5.40 (A) Pulse sequence for the 2D heteronuclear shift-correlation experiment. (B) Effect of the pulse sequence in (A) on H magnetization vectors of CH.

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

This is a variation of the proton-detected shift-correlation experiment via long-range couplings proposed by Bax and Summers (Bax and Summers, 1986), with the difference that the first C pulse is substituted by a frequency selective pulse (Fig. 7.14) (Bermel et al., 1989 Kessler et al., 1989b,1990). This significantly increases resolution in the F dimension. For example, this can be used to remove the overlap between the cross-peaks of the carbonyl resonances of peptide bonds in proteins that all occur within a... 

Inverse experiments Heteronuclear shift-correlation experiments in which magnetization of the less sensitive heteronucleus (e.g., C) is detected through the more sensitive magnetization (e.g., H). 

Ralph, J. Ede, R. M. NMR of lignin model quinone methides. Corrected carbon-13 NMR assignments via carbon-proton correlation experiments. Holzforschung 1988,42,3 37-338. 

Our approach is to use the two-dimensional relaxation and diffusion correlation experiments to further enhance the resolution of different components. It is important to note that the correlation experiment, e.g., the Ti-T2 experiment, is different from two experiments of and T2 separately. For instance, the separate Ti and T2 experiment, in general, cannot determine the T1(/T2 ratio for each component. On the contrary, a component with a particular Tj and T2 will appear as a peak in the 2D 7i-T2 and the Ti/T2 ratio can be obtained directly. For example, small molecules often exhibit rapid rotation and diffusion in a solution and Ti/T2 ratio tends to be close to 1. On the other hand, the rotational dynamics of larger molecules such as proteins can be significantly slow compared with the Larmor frequency and resulting in a Ti/T2 ratio significantly larger than 1. 

In early 2004, Hurlimann studied several cheese samples using D-T2 correlation experiments. The D-T2 spectrum shows predominantly two signals, one with a diffusion coefficient close to that of bulk water, and the other with a D about a factor of 100 lower. The fast diffusing component is identified as water and the other as fat globules. Two components of cheese in the D-T2 map has also been observed by Callaghan and Godefroy [65]. Recently, Hurlimann et al. have performed a systematic 2D NMR study of milk, cream, cheeses and yogurts [66], Some of the preliminary results are discussed here. 

But we have a puzzle here since rotation around the aryl-CHP2 bond should be relatively unhindered, why does CP not couple to both aP and bP We will return to this question when we discuss the 2D phosphorus-phosphorus correlation experiment. 

The important experiments for our purposes are the correlation experiments, where both axes are chemical shift axes. Certainly the most useful of these is the proton-proton correlation experiment, initially known as COSY (for Correlated SpectroscopY) and now, to make things more precise, as H,H COSY. This experiment is important, as it provides direct information on which proton nuclei couple with which. 

In principle it is possible with many modern spectrometers to carry out correlation experiments using any two NMR-active nuclei, and we shall demonstrate this below by discussing P,C and P,P correlations. 

A P,C correlation experiment also requires that we use a predefined coupling constant value to determine the mixing time. A brief look at the proton decoupled carbon-13 spectrum (Fig.14) shows that is very large (around 200 Hz), while the long-range JPC values are much smaller (around 5-10 Hz). 

We mentioned above that it is possible to carry out carbon-carbon correlation experiments using the 2D INADEQUATE procedure. There, as you may remember from the discussion of one-dimensional INADEQUATE, we have a very difficult problem to solve carbon-13 represents only 1.1% of the total carbon nuclei present in a sample. And in the INADEQUATE experiment we need to detect only those molecules containing two carbon nuclei which couple with one another, i.e. about 10"4 of the nuclei present at the same time we have to get rid of the signal coming from those molecules containing only one carbon-13 (the great majority) ... 

In principle it is possible (with a suitably configured spectrometer) to carry out correlation experiments between any pair of NMR-active nuclei. How-... 

The experiment took 11 min, and the spectrum shows quite clearly the correlation signals for the acetal 5 cross peaks between the methyl and methylene signals from the ethyl group and between the magnetically non-equivalent protons of the ethylene bridge. CH correlation experiments can easily also be carried out, even though in the case of the two acetals 4 and 5 they require between two and three hours ... 

Methyl and methine protons naturally phase at 180° relative to the methylene carbons and the spectra are usually plotted with methyls and methines positive. (Note that should you encounter a signal that you cannot confidently assign to either a methyl or methine carbon, the DEPT 90 sequence may well be of use as it differentiates these carbons - methines appear positive and methyls are edited out of the spectrum but this technique can be considered obsolete if you have access to any of the 2-D proton-carbon correlated experiments discussed in Section 9.2.)... 

If interpreting the single-bond correlation experiments is easy, the multiple bond experiment (HMBC) can be considerably less so... 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

---