What is 2D NMR Spectroscopy

The term 2D NMR, which stands for two-dimensional NMR, is something of a misnomer. All the NMR spectra we have discussed so far in this book are two dimensional in the sense that they are plots of signal intensity versus frequency (or its Fourier equivalent, signal intensity versus time). By contrast, 2D NMR refers to spectroscopic data that are collected as a function of two time scales, tx (evolution and mixing) and t2 (detection). The resulting data set is then subjected to separate Fourier transformations of each time domain to give a frequency-domain 2D NMR spectrum of signal intensity versus two frequencies, Fx (the Fourier transform of the t time domain) and F2 (the Fourier transform of the t2 time domain). Thus, a 2D NMR spectrum is actually a three-dimensional data set  

The F2 frequency normally corresponds to a chemical shift axis, for example, H or l3C. When the Fx axis also corresponds to a chemical shift scale, we describe the result as a 

2D shift-correlated spectrum. When the Ft axis corresponds to a coupling constant scale (e.g., H- H or H- - C), we generate a 2D 7-resolved spectrum. 

A typical 2D NMR spectrum is actually composed of a large number of spectra (often several hundred) collected automatically, each with a uniformly incremented value of t. For example, consider an unedited DEPT (Section 12.12) experiment on 2-chlorobutane  

As with the advanced one-dimensional techniques discussed in Chapter 12, there is a large and growing family of 2D NMR pulse sequences.1-6 In this chapter we will examine some of the most widely used ones, focusing our attention on the information contained in each type of 2D spectrum. How- 

---

When performing 2D-NMR experiments one must keep in mind that the second frequency dimension (Fx) is digitized by the number of tx increments. Therefore, it is important to consider the amount of spectral resolution that is needed to resolve the correlations of interest. In the first dimension (F2), the resolution is independent of time relative to F. The only requirement for F2 is that the necessary number of scans is obtained to allow appropriate signal averaging to obtain the desired S/N. These two parameters, the number of scans acquired per tx increment and the total number of tx increments, are what dictate the amount of time required to acquire the full 2D-data matrix. 2D-homo-nuclear spectroscopy can be summarized by three different interactions, namely scalar coupling, dipolar coupling and exchange processes. 

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

In conclusion, what we have summarized in this article can be viewed as a very first step in the direction of a novel structural analysis method, which opens a vast new field of experimental research where much work remains to be done. Any 2D-IR spectroscopy certainly will be limited to molecular sizes smaller than are tractable by 2D NMR and x-ray spectroscopy because of the intrinsically poor spectral resolution of vibrational transitions. However, the one tremendous prospect that makes this research worthwhile is the inherent high time resolution of IR spectroscopy, which covers all biologically relevant time scales, starting from the subpicosecond time regime. The IR photon echo experiments have proven experimentally for the first time that the peptide backbone itself indeed fluctuates on these... 

Two-dimensional NMR spectroscopy has been the topic of numerous monographs [14—17, 23—27, 29—31]. It is the intent here to provide the reader with a brief introduction and the means of accessing key aspects of what has become a voluminous literature on the subject. Briefly, 2D NMR experiments are comprised of several fundamental segments or building blocks. Three periods are obligatory in a 2D NMR experiment. These consist of a preparation period, the evolution period, ti, which corresponds to what will be the indirectly digitized time domain, and a... 

---