DATA PROCESSING IN 2D NMR

What are dummy scans, and why are a number of these scans acquired before actual data acquisition  

At the end of the 2D experiment, we will have acquired a set of Ni FIDs composed of quadrature data points, with Ni/2 points from channel 

A and N2/2 points from channel B, acquired with sequential (alternate) sampling. How the data are processed is critical for a successful outcome. The data processing involves (a) dc (direct current) correction (performed automatically by the instrument software), (b) apodization (window multiplication) of the 2 time-domain data, (c) Fourier transformation and phase correction, (d) window multiplication of the t] domain data and phase correction (unless it is a magnitude or a power-mode spectrum, in which case phase correction is not required), (e) complex Fourier transformation in F, (f) coaddition of real and imaginary data (if phase-sensitive representation is required) to give a magnitude (M) or a power-mode (P) spectrum. Additional steps may be tilting, symmetrization, and calculation of projections. A schematic representation of the steps involved is presented in Fig. 3.5. 

Fourier transformation in 2 (Fti), Ni spectra are obtained with real (P) and imaginary (/) data jx)ints. For detection in the quadrature mode with simultaneous sampling, a complex Fourier transformation is performed, with a phase correction being applied in P2. (c) A normal phase-sensitive transform R- RR and I— RI. (d) Complex FT is applied to pairs of columns, which produces four quadrants, of which only the RR quadrant is plotted. 

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The use of sine or cosine functions in FID data processing is an essential tool in 2D NMR. 

The volumes in the series of Spectroscopic Techniques An Interactive Course are delivered with special versions of ID WIN-NMR and 2D WIN-NMR. They are a supplement for this course to be installed on a stand-alone PC and to be used exclusively for processing the experimental data supplied in the NMR data base. They cannot be used to process the users personal NMR data. The full version of ID WIN-NMR and 2D WIN-NMR software must be installed for this purpose and a special copy protection dongle (a WIBU key for the single user mode, or a Net-HASP key for the multi-user/network mode) must be used. Note also that for 2D WIN-NMR a standard 16-bit and a more powerful 32-bit version exist. Please refer to the description in the corresponding Bruker manuals [2.1, 2.2]. 

We will see that in 2D NMR, the sampling in the second dimension can also be done either way, except that this choice is up to the user and is not hard wired. The alternate ( Bruker-like ) sampling method is called TPPI (time proportional phase incrementation), and the simultaneous ( Varian-like ) method is called States or States-Haberkorn (after the originators of the technique). The consequences for processing and interpretation of the data are the same in the second dimension of 2D spectra as they are in ID NMR. 

The second part of this section examines the processing of 2D NMR data using 2D WIN-NMR. By necessity the description of 2D data processing is very brief and the raw 2D data is processed in a single step rather than the stepwise approach used for ID data. Table 3.5 at the end of this section summarizes the recommended processing parameters for a number of the more common 2D experiments. 

The basic processing steps for ID NMR data can also be applied to the processing of 2D NMR data with similar effects. Of particular importance for the processing of 2D data matrices are zero filling and apodization. Usually 2D experiments are recorded with a relatively small number of time domain data points TD2, compared with a ID experiment, and small number of increments TDl in order to minimize data acquisition times. Typical time domain values are 512, Ik or 2k words. Small values of TD2 and TDl and the correspondingly short acquisition times cause poor spectral resolution and... 

As an alternative to the specific solutions that are available in ID NMR, the general approach adopted in 2D NMR is to apply a series of RF pulses to the sample such that there are two independent variable time intervals in the pulse sequence. One of these is the acquisition time, denoted by t, and the other is some incremental delay denoted /i. If an NMR FID is acquired for a period for each of a set of t values, the digital NMR signal intensity (S) will be a function of both t and giving a matrix of data S(ti, /i). If Fourier transformation is carried out with respect to both ti and tz a matrix of NMR intensity as a function of two frequencies will result. This is now a 2D NMR spectrum as it represents signal intensity as a function of two frequency axes. The delay ti and the actual nature of the pulse sequence will, as may be seen later, define exactly what the two frequency axes will represent. As a consequence of this process, there are two immediate benefits increased signal dispersion and hence less overlap, and if the experiment is conducted in certain ways, it is possible to evaluate connectivities between the various NMR resonances. On the other hand, the 2D approach usually requires increased experiment time and increased computer data storage requirements. 

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. 

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