Generalized 2D correlation spectroscopy

The advantages of generalized 2D correlation spectroscopy he in the following points (13, 15). 

The general experimental approach used in 2D correlation spectroscopy is based on the detection of dynamic variations of spectroscopic signals induced by an external perturbation (Figure 7.43). Various molecular-level excitations may be induced by electrical, thermal, magnetic, chemical, acoustic, or mechanical stimulations. The effect of perturbation-induced changes in the local molecular environment may be manifested by time-dependent fluctuations of various spectra representing the system. Such transient fluctuations of spectra are referred to as dynamic spectra of the system. Apart from time, other physical variables in a generalised 2D correlation analysis may be temperature, pressure, age, composition, or even concentration. 

General symmetry principles for rotor-synchronized pulse sequences in MAS solid-state NMR have been presented. The synunetry theory has been extended to the case of generalized Hartmann-Hahn sequences, in which rotor-synchronized r.f. irradiation is applied simultaneously to two isotopic spin species. The symmetry theory has been used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin-spin interactions, and which also suppress CSAs. Experimental demonstrations of heteronuclear 2D correlation spectroscopy, heteronuclear MQ spectroscopy, and the estimation of intemuclear dipolar couplings have been given. 

For simple systems such as stocks obtained from plant tissues, NMR spectra include a wealth of peaks. Generally, peak assignment of IH NMR spectra can be done by addition of pure compounds to solutions, by comparison of published data and further confirmation can be obtained throu 2D (correlation spectroscopy, COSY, and total correlation spectroscopy, TOCSY) experiments. Thus, each major carbohydrate was identified by peak assignment of ID H NMR spectra and as many peaks overlapped in the amino acids and organic acid region (between 1 and 3.5 ppm), identification was performed using 2D IH NMR spectra (COSY). 

Figure 1-2. A generalized experimental scheme for 2D correlation spectroscopy based on perturbation-induced dynamic spectral signals [8. ...

Figure 7.43 General conceptual scheme to obtain a 2D correlation spectrum by inducing selective time-dependent spectral variations with an external perturbation (mechanical, electrical, chemical, magnetic, optical, thermal, etc.). After Noda [1006]. From I. Noda, Applied Spectroscopy, 44, 550-561 (1990). Reproduced by permission of the Society for Applied Spectroscopy...

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... 

With regard to the wealth of pulse schemes for X, "Y correlation, some consideration has to be put in the selection of the best-suited method for a certain application. In H,"X correlation spectroscopy, the high receptivity of protons makes inverse detection schemes generally the most effective protocol from the point of sensitivity. 2D experiments with detection are... 

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

More generally, note that the application of almost any multiple pulse sequence, where at least two pulses are separated by a time comparable to the reciprocal of the coupling constants present, will lead to exchanges of intensity between multiplets. These exchanges are the physical method by which coupled spins are correlated in 2D NMR methods such as correlation spectroscopy (COSY) [21]. 

Generalized 2D NIR correlation spectroscopy has been appUed to study, for example, temperature-dependent spectral variations of various compounds such as A-methylacetamide (NMA) (34) and nylon 12 (29), concentration-dependent spectral changes in milk (18) and protein solntions at various temperatures (35, 36), composition-dependent spectral changes in polymer blends (37), and depth-dependent spectral variations of a polymer film (38). Examples of heterospectral correlation are 2D NlR-mid IR heterospectral correlation analysis of nylon 11 (39) and 2D NIR-Raman correlation analysis of polymer blends (40). 

---