2D spectral window

In most cases ID spectra have already been measured before starting the 2D experiment and the spectral windows for the acquisition of the 2D spectrum are adjusted to encompass only the signals of the compound under investigation. As a consequence the signals of the reference compound, e.g. TMS, are usually outside of the spectral window and, are missing in the 2D spectrum. 

Although at first glance all FIDs look similar, the shape of FIDs differs and depends on the sample amount, kind of experiment (ID, 2D), the nuclei detected ( H, C) and on acquisition parameters such as the length of the acquisition time, the spectral window and the number of time domain data points TD. Subsequent processing must take this into account and be tailored to these different shapes. 

The same phenomenon applies to aliasing in the second dimension of a 2D spectrum Alternating (TPPI) acquisition in the second dimension will lead to aliasing on the same side of the spectral window ( folding ) simultaneous (States) acquisition will lead to aliasing from the opposite edge of the spectral window ( aliasing ). 

Because digital filtering can produce a brick wall frequency response, any peak that falls outside the spectral window is removed completely and will not alias. This can be a problem if you set the spectral window too narrow You will never be aware of the peaks you miss. If you accidentally set the spectral window to include nothing but noise, you will get just that in the spectrum nothing but noise The good news is that if we are only interested in a small part of the ID spectrum, we can cut out the rest of the spectrum using the digital filter. For example, in a 2D N-1 HSQC spectrum of a protein, we are only... 

One of the fundamental approaches to acquiring heteronuclear 2D NMR data for unknown molecules in many laboratories is to acquire data using survey spectrum conditions. For survey conditions, pre-established spectral windows are routinely used in both frequency domains. In the case of HSQC and HMBC experi-... 

HSQC) or heteronuclear multiple quantum correlation (HMQC). The combined experiments such as 2D HSQC(HMQC)-TOCSY experiments are powerful tools for the assignment of the 13C and 11 resonances belonging to the same sugar residue providing enhanced dispersion of TOCSY correlations in the carbon dimension. More recendy, different carbon multiplicity editing methods, for example, DEPT (distortionless enhanced polarization transfer)-HMQC and E-HSQC, have been developed to reduce the complexity of proton-carbon correlation spectra and to enhance the resolution by narrowing the applied spectral window.11... 

In 2D-NMR experiments, an additional evolution delay, tl (and in some cases an optional mixing delay), is inserted between the end of the preparation period and the beginning of the detection period (f2) (Figure 3b). A series of Fib s are collected with the tl period incremented by an amount related to the inverse of the desired spectral window in the /I dimension (Figure 4a). In nD-NMR, the convention is to sequentially label the evolution delays tl, t2,...t(n-l) in the order th are executed (Figure 3c), and the detection period is identified as tn. Sequential FT with respect to t2 (Figure 4b)... 

For both 2D experiments, H and C 90° pulses of 8.8 and 13.0 ps respectively, were used 8000 Hz and 32000 Hz spectral windows were used for the H (f2) and C (/ ) chemical shift dimensions, respectively 16 transients were averaged for each of 1024 real L increments. 2D data were processed with sinebell weighting zero filling was used so that 2D FT was performed on an 8192 x 8192 matrix and the spectra were displayed in the magnitude-mode in both dimensions. 

All the 2D-NMR experiments have the same basic format which can be divided into four units preparation, evolution, mixing, and detection periods. For an A-B correlated 2D-NMR experiment, the preparation period usually starts with a delay during which the spins are allowed to return to equilibrium this is followed by a pulse or a cluster of pulses and delays that transfers the magnetization from B to A nuclei, called coherence transfer, just before the evolution period. The evolution period (tj) is a variable time delay, increased in a stepwise manner from an initial value of zero to a final value ni x lAw (related with spectral window swl in the indirectly detected dimension of nucleus A), provides the key to the generation of the second dimension. The mixing period serves to transfer the magnetization back from A to B nuclei. Then the detection ((2) period is needed to collect the FID (Free Induction Decay) data. It produces a spectrum similar to the one obtained from a ID experiment of the detected nucleus. Each value in the 2D sequence is repeated nt times and np data points (FID s) are stored (np is related to the acquisition time at and sw is the direct detection dimension spectral window of nucleus B). 

A few alternative COSY methods have been developed to improve the quality of spectra with large numbers of overlapping correlations or with large spectral windows. Selective COSY utilizes long-shaped pulses to excite a small window within the field of correlations in a spectrum. The smaller spectral window of the selective COSY experiment enhances digital resolution and reduces experiment time due to the reduced number of 2D increments required to obtain high digital resolution. The... 

D. Jeannerat, High-precision heteronuclear 2D NMR experiments using 10-ppm spectral window to resolve carbon overlap, Chem. Commun. (2009) 950—952. 

Bmch et in this same paper also performed extensive F ID- and 2D-NMR smdies on a commercial sample of PVF with a significant number of stmctures from inverse monomer additions. They were able to detect and assign many resonances from H-H and T-T stmcmres as well as fine stmcture from stereosequence effects. However, they noted that a number of experted peaks were not detected. This was most likely due to the combined effects of the much larger spectral window required to encompass all of the F resonances in this more complicated polymer stmcture, the poorer digital resolution in the 2D-NMR spectra, and the resulting cancellation of antiphase multiplet components of the 2D-NMR crosspeak patterns. 

Pt), ID-NMR detection of these nuclei can also be useful. Because the H spectral window is small, severe overlap in the H ID-NMR speamm can be a problem if the dendrimer building block is complex. In such situations, ID-, 2D-, and even 3D-NMR experiments might be needed to obtain unambiguous stmctural determination and identification of stmcture defects. 

---