FT two-dimensional NMR

Chapter 9 (Morris) critically compares the advantages of the principal two-dimensional FT-NMR experiments, and includes a comprehensive literature survey through mid-1980. Chapter 10 (Mims) gives a brief review of the somewhat parallel development of electron spin echo FT spectroscopy. Chapter 11 (Dalai) critically compares the... 

M. Ohuchi, M. Hosono, K. Matushita, and M. Imanari, A new pulse sequence in two-dimensional FT NMR. Solvent elimination using a Hahn echo, J. Magn. Resonance 43, 499 (1981). 

Two significant developments in NMR spectroscopy are the use of Fourier transform techniques, and the development of two-dimensional (2D) NMR spectroscopy. Two-dimensional spectra are obtained using a sequence of rf pulses that includes a variable delay or delays. A set of FIDs is acquired and stored. The variable delay is incremented by a small amount of time and a new set of FIDs are obtained and stored, and so on. At the end, the resulting matrix of FID data is Fourier transformed twice once with respect to the acquisition time (as in normal FT-NMR) and second with respect to the time of the variable delay in the pulse sequence. The resulting data represent a surface and are presented as a contour plot of that surface. 

The second development that has revolutionized the practice of NMR was the introduction of multidimensional spectroscopy. This was initialized by Jeener [2], who showed that, by introducing a second pulse and varying the time between them, a second time-axis could be constructed. A double Fourier transformation yields the familiar two-dimensional spectrum, nowadays known by everyone as the COSY spectrum. Ernst, already involved in the development of FT-NMR, showed that the concept was more generally applicable [3], and paved... 

It is interesting to note that several of the concepts for improving NMR technology, as listed by Levy and Craik, in 1988, already have been partially or fully achieved (1) two-dimensional Fourier transform (FT NMR) (2) high-resolution NMR in solids (3) new types of pulse sequences (4) chemically induced dynamic nuclear polarization (5) multiple quantum NMR and (6) NMR imaging (MRI). 

Figure 7.2.8 Two-dimensional NMR spectroscopy in supercritical fluids 1 FT 1 H-COSY of benzyl-n-butylphthalate, acquired in the stop-flow mode...

In a one-dimensional (1-D) NMR experiment, t, is kept constant, t2 varies and the FT is taken with respect to t2 only in two-dimensional (2-D) NMR experiments, both t, and t2 are varied and the FT is taken with respect to both time variables. 

As we saw in Section 3.4, quadrature phase detection discriminates between frequencies higher and lower than the pulse frequency, but it does not prevent foldover from frequencies higher than the Nyquist frequency. For a desired spectral width FT, there are two common methods for carrying out quadrature phase detection, as was indicated in Section 3.4. One method uses two detectors and samples each detector at FT points per second, thus acquiring 2 FT data in the form of FT complex numbers. The other (commonly called the Redfield method ) requires only a single detector and samples at 2 FT points per second while incrementing the phase of the receiver by 90° after each measurement. (In two-dimensional NMR studies, a variant of this method is usually called the rime-proportional phase incrementation, or TPPI, method.) Because these methods result in quite different treatment of folded resonances, we now consider these approaches in more detail. 

The power of FT NMR is that one is not confined to a single exciting pulse. One can have several pulses with various durations, delays and phases in order to edit a one-dimensional spectrum. Or one can have an array of pulses with a variable evolution time and then perform the Fourier transform with respect to both the evolution time and the decay of the FID, generating a two-dimensional spectrum whose output is a contour plot. With very powerful machines (> 600 MHz, H) it is even possible to perform the Fourier transform in three dimensions, with two evolution times. These pulse sequences are known by (usually arch) acronyms such as COSY, INADEQUATE, etc., and modern NMR machines are supplied with the hardware and software to perform the commoner experiments already installed. It is not necessary to understand fully the spin physics behind such sequences in order to use them, but the basic viewpoint used in their description is worth grasping. 

In complete analogy to NMR, FT EPR has been extended into two dimensions. Two-dimensional correlation spectroscopy (COSY) is essentially subject to the same restrictions regarding excitation bandwidth and detection deadtime as was described for one-dimensional FT EPR. In 2D-COSY EPR a second time dimension is added to the FID collection time by a preparatory pulse in front of the FID detection pulse and by variation of the evolution time between them (see figure B1.15.10(B)). The FID is recorded during the detection period of duration t, which begins with the second 7r/2-pulse. For each the FID is collected, then the phase of the first pulse is advanced by 90°, and a second set of FIDs is collected. The two sets of FIDs, whose amplitudes oscillate as functions of t, then undergo a two-dimensional complex Fourier transformation, generating a spectrum over the two frequency variables co and co,. 

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