DQF-COSY spectra

Artifactual peaks are even more dangerous than noise since they may not always be immediately recognized and may lead to erroneous assignments. An important source of artifacts is instability in the steady-state condition, e.g. if the relaxation delay is set too short. A commonly encountered example is presented by peaks which occur at the double quantum frequencies in DQF-COSY spectra. For detailed treatments of aspects of noise and artifacts see [4, 5]. 

The sensitivity of the DQF COSY experiment could be increased by employing magnetic field gradient filtration instead of the phase cycling required in the above sequence. The authors obtained usable DQF COSY spectra from equimolar (or 0.37 M) aqueous solutions of siloxanes in 12 hours (10 mm NMR tubes at 59.595 MHz), illustrating the... 

A simple variant of the COSY experiment is COSY-35 (sometimes called COSY-45), in which the second 90° pulse is reduced from a 90° pulse to a 35° or 45° pulse (Fig. B.3). The result is that the fine structure of crosspeaks is simplified, with half the number of peaks within the crosspeak. This makes it much easier to sort out the coupling patterns in both dimensions and to measure couplings (active and passive) from the crosspeak fine structure. A more important variant of the COSY experiment is the DQF (double-quantum filtered)— COSY (Fig. B.4), which adds a short delay and a third 90° pulse. The INEPT transfer is divided into two steps antiphase I spin SQC to I,S DQC, and I,S DQC to antiphase S spin SQC. The filter enforces the DQC state during the short delay between the second and third pulses either by phase cycling or with gradients. DQF-COSY spectra have better phase characteristics and weaker diagonal peaks than a simple COSY, so this has become the standard COSY experiment. 

Two-dimensional exchange spectroscopy might also be a valuable tool in the study of the metal skeleton. No example has been found in the literature. Preferably for nuclei with 100% natural abundance, homonuclear COSY or double-quantmn filtered (DQF)-COSY spectra can give the connectivity pattern of the different metal atoms in a cluster. This holds even, in some cases, for quadnipolar nuclei such as Co [21]. 

The construction of proton spin-coupling networks is achieved by a variety of COSY experiments (Section 7-7). Standard COSY and DQF-COSY spectra of the 15-mg sample of the unknown compound are shown in Figure 7-1 la, and the data from these contour plots are summarized in Table 8-3. Coupling constants, which have been measured in the NMR spectrum, correspond to cross peaks, and are given in parentheses in the table. 

The COSY spectra are DQF-COSY spectra. The labeled frequency for all 2-D NMR spectra is that of the acquired signal (F2). 

Unless otherwise labeled, the NMR spectra were obtained at 300 MHz for protons, 75.5 MHz for 13C CDC13 was the solvent unless otherwise labeled. The IR spectra were obtained neat (i.e., no solvent) unless otherwise labeled. The mass spectra were obtained by GC/MS. CH4 was used to obtain the chemical ionization mass spectra. The COSY spectra are DQF-COSY spectra. The labeled frequency axis for all 2-D NMR spectra is that of the acquired signal (F2). The FI axis may also be labeled. 

Reynolds et addressed the question as to why the powerful LP methods remain of so little use years after their potential was first demonstrated. They tried to convince HSQC users to apply linear prediction with their detailed study of natural products. They showed that resolution can be improved by a factor of up to 16 in the first spectral dimension when the S/N ratio is favorable. Similarly, treatment of DQF-COSY spectra makes it possible to obtain a factor of two resolution improvement which means a factor two in acquisition time. 

COSY and DQF-COSY spectra were the first to be used for the construction of H spin networks. In that context, Eich solved a particular problem... 

Back to the standard correlation spectra, and in the context of the search for methods to obtain one-dimensional spectra devoid of coupling structures, Woodley and Freeman - considered scanning DQF-COSY spectra and searching for cross-peaks only at the chemical shift where protons are known to be located according to the results of processing a 7-spectrum. Application to low S/N ratio (e.g. >3) is given in a separate paper." - ... 

All 2D NOE spectra (Xm = 50, 150, 200 and 400 ms) in D2O were acquired in the hypercomplex mode (44) at 25 °C, using a spectral width of 5999 Hz in both dimensions with the carrier frequency set to the HDO resonance frequency. A total of 400 t] values were recorded for each fid. 32 scans with a repetition time of 2.5 s including the t2-acquisition time were recorded for each tj increment with 2K data points in f2. DQF COSY spectra were acquired under the same conditions. All 2D NOE experiments in H2O were collected at 10 °C or 15 °C using the SS-NOESY pulse sequence (45) with a spectral width of 12999 Hz using a symmetrically-shifted S-pulse with pulse width of 88.8 ps for water suppression. The excitation maximum occurs at 7576 Hz from the carrier frequency. 

H chemical shifts and scalar coupling constants can be measured directly from one-dimensional spectra if the peaks are well resolved, or, if spectra are too complex, they may be measured from DQF-COSY spectra crosspeaks. However, such measurements are often inaccurate and so are used as a basis of simulating the observed ID spectrum to obtain more accurate values. The measurements are used as a starting point and are systematically altered until the stimulated spectrum best matches the observed spectrum. H- H scalar coupling constants are especially useful in providing information on the dihedral angle within a HC-CH system and are thus one... 

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