2D resolved spectroscopy

What is the difference between homo- and heteronuclear 2D /-resolved spectroscopy ... 

In the previous section we were concerned with 2D /-resolved spectroscopy i.e., chemical shifts of the nuclei were presented along one axis and their couplings along the other axis. The data thus obtained provided information about multiplicity. 

The pulse sequence used in homonuclear 2D /-resolved spectroscopy is shown in Fig. 5.18. Let us consider a proton. A, coupled to another proton, X. The 90° pulse bends the magnetization of proton A to the y -axis. During the first half of the evolution period, the two vectors Hf (faster vector) and Hv (slower vector) of proton A precess in the x y -plane with angular velocities of 0/2 + (J/2) and 0/2 - iJ/2), respectively. The 180° H pulse results in (a) the vectors adopting mirror image positions across the x -axis so that comes to lie ahead of Hf when viewed in a clockwise fashion. However, since the 180° H pulse was not selective, it also affects... 

Homonuclear NMR Structural elucidation becomes much more difficult as spectral complexity increases. Under these circumstances, proton NMR spectroscopy has benefited considerably from the use of 2D NMR techniques. For example, homonuclear correlation spectroscopy (COSY) identifies spin-coupled pairs of nuclei as well as spin-coupled networks of nuclei in a molecule, even without prior structural information. The 2D /-resolved spectroscopy method permits even highly overlapping resonances to be resolved into readily interpretable multiplets. This enables chemical shift assignments to be made in a very straightforward manner. Both of... 

Carbon-proton connectivities can be determined using several methods. The number of protons directly attached to the carbon in question will split the carbon resonance according to the 2nl + 1 rule seen in proton NMR. There tends to be, however, much overlap of the multiplets in fully proton-coupled carbon spectra, sometimes such that it is very difficult to distinguish between the various multiplets. Routine carbon spectra are therefore measured fully proton decoupled for simplicity. Information regarding the exact number of protons attached to the carbons can be acquired from APT, DEPT or INEPT experiments. In APT spectra, the carbons bearing an odd number of protons (CH, CH3) can be distinguished from carbons with no or two attached protons (quaternary C, CH2). DEPT and INEPT experiments can distinguish between all four types of carbons (primary, secondary, tertiary and quaternary). Heteronuclear 2D /-resolved spectroscopy can also be used to obtain the multiplicities of the carbons, as well as Vc-h ... 

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.

The pulse sequence used in homonuclear 2D y-resolved spectroscopy is shown in Fig. 5.18. Let us consider a proton, A, coupled to another proton, X. The 90° pulse bends the magnetization of proton A to the y -axis. During the first half of the evolution period, the two vectors (faster... 

Figure 5.18 (A) Pulse sequence for homonuclear 2D y-resolved spectroscopy. (B) Effect of 90° H and 180° H pulses on an H doublet. (C) In the absence of coupling, the vectors are refocused by the 180° H pulse after t. This serves to remove any field inhomogeneities or chemical shift differences.

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... 

Two-dimensional spectroscopy has two broad classes of experiments (a) 2D /-resolved spectra (Mtlller et ai, 1975 Aue et ai, 1976), in which no coherence transfer or mixing process normally occurs, and chemical shift and coupling constant frequencies are spread along two different axes. 

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

In recent years new NMR techniques offering broad applications in stereochemical analysis have come into use. A prominent example is 2D-NMR (both 2D-resolved and 2D-correlated spectroscopy), which has been extensively applied to biopolymers (149-151). Its use with synthetic polymers has, until now, been limited to but a few cases (152, 153). A further technique, cross-polarization magic-angle spinning spectroscopy (CP-MAS NMR) will be discussed in the section on conformational analysis of solid polymers. 

Cyclo(Pro-Gly) (fig. 3) is a convenient model for demonstration of various aspects of 2D exchange spectroscopy. It is small rigid molecule with 10 protons, of which 8 are spectroscopically well resolved. It is well dissolved in dimethyl sulfoxide (DMSO)Zwater mixtures and stable at a broad range of temperatures. We used a 10 mM solution of cyclo(Pro-Gly) in 70/30 volume/volume mixture of DMSO/water. This solvent mixture is suitable for the cross-relaxation studies because it is rather viscous even at room temperature and does not freeze down to 223 K [29, 30]. Thus, molecules dissolved in this mixture can be studied at a broad range of temperatures (correlation times). 

We have characterized (8e-i) this intermediate using solid-state 13C CP/MAS NMR, 2H NMR, and two dimensional (2D). /-resolved 3C NMR spectroscopy in the course of dehydration of isobutyl alcohol and /er/-butyl alcohol in HZSM-... 

The pulse methods rely on selective irradiation of a particular resonance line with a radio frequency (rf) and observation of the resulting effects in the rest of the spectrum. Among commonly employed methods are 2D correlated spectroscopy (COSY), 2D spin-echo correlated spectroscopy (SECSY), 2D nuclear Overhauser and exchange spectroscopy (NOESY), 2D J-resolved spectroscopy (2D-J), and relayed coherence-transfer spectroscopy (RELAYED-COSY) (Wutrich, 1986). 

Laurie was one of the first to apply two-dimensional (2D) NMR to carbohydrates. With students Subramaniam Sukumar and Michael Bernstein, and visiting scientist Gareth Morris, he demonstrated and extended the application of many of the directly observed 2D NMR techniques of the time. These included the homo- and hetero-nuclear 2D /-resolved techniques, delayed proton /-resolved NMR that allowed broad resonances to be suppressed, for example, those of dextran in the presence of methyl /Lxvlopyranoside. proton-proton chemical shift correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), proton-carbon chemical shift correlation (known later as HETCOR), and spin-echo correlated spectroscopy (SECSY). Trideuteriomethyl 2,3,4,6-tetrakis-<9-trideuterioacetyl-a-D-glucopyranoside served as a commonly used model compound for these studies. 

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