Lineshape mixed phase

Mixed phase lineshapes, typical for the multistep RELAY spectra, sometimes make the identification of individual signals difficult. At the same time, for extensive networks of coupled spins, the TOCSY transfer proved... 

COSY-p Simple and robust. Magnitude processing well suited to automated operation. Simplification of crosspeak structures reduces peak overlap. Vicinal and geminal couplings can be distinguished in some cases from tilt of peaks. Usually requires magnitude-mode presentation as phase-sensitive variant has mixed-phase lineshapes. 

The double quantum filter eliminates or at least suppresses the strong signals from protons that do not experience J-coupling, e.g. the solvent signal, which would otherwise dominate the spectrum and possibly be a source of troublesome tl noise. Compared to a phase-sensitive but non-DQ-filtered COSY with pure absorption lineshapes for the cross peaks but mixed lineshapes for the diagonal peaks, the phase-sensitive, DQ-filtered COSY has pure absoiption lineshapes throughout. 

Prior to the advent of the above methods that allowed the presentation of phase-sensitive displays, 2D data sets were collected that were phase-modulated as a function of ti rather than amplitude-modulated. Phase-modulation arises when the sine and cosine modulated data sets collected for each ti increment are combined (added or subtracted) by the steps of the phase cycle, meaning each FID per tj increment contains a mixture of both parts. Here it is the sense of phase precession that allows the differentiation of positive and negative frequencies. This method is inferior to the phase-sensitive approach because of the unavoidable mixing of absorptive and dispersive lineshapes, so is generally only suitable for routine, low-resolution work. 

The peculiar temperature and composition dependence of the valence of chemically collapsed phases of SmS was first observed in the system Sm1 xGdxS (55). Recently a phase diagram in the (x, T)-plane has been proposed for this system (41) as well as for CeTh alloys (44). When cooled below about 200 K the chemically collapsed phases of SmS show a dramatic lattice expansion, in some cases with explosive character (55) and disintegration of the crystal into a black powder. An example of this transition towards a more divalent state of Sm on cooling in SmAs 18 S 82 and Sm 81 Y19S is shown in Fig. 22 and 23, where we show the data of Poliak et al (63). The temperature dependent configurational mixing of Sm ions which is directly visualized in the XPS data is the source of the anomalous temperature dependence of the lattice constant. No detailed analysis of the dependence of the Sm 4/lineshape on temperature... 

These could consist of a whole range of Mo(VI) and Mo(IV) mixed oxygen/ sulfur compounds. There is no evidence for the presence of MoOi, except in one case. Simulation of the static powder lineshapes allowed the deconvolution of the various components, thus yielding values of quadrupole parameters as well as relative intensity data. From the spikelet experiments it was ascertained that all species are present as both static adsorbed and dynamically active phases. The Mo spectrum of the used catalyst shows the presence of the tetrahedral molybdenum-oxo species, along with a much reduced MoSt resonance (relative to the fresh system), perhaps suggesting that MoSt is the active site in the... 

The greatest drawback with data collected with phase modulation is the inextricable mixing of absorption and dispersion-mode lineshapes. The resonances are said to possess a phase-twisted lineshape (Fig. 5.21a), which has two principal disadvantages. Firstly, the undesirable and complex mix of both positive and negative intensities and secondly, the presence of dispersive contributions and the associated broad tails that are unsuitable for high-resolution spectroscopy. To remove confusion from the mixed positive and negative intensities, spectra are routinely presented in absolute-value mode, usually after a magnitude calculation (Fig. 5.22). 

A more direct approach to the elimination of zero-quantum interference is via the use of swept-frequency inversion pulse applied in the presence of a field gradient, which destroys the zero-quantum contributions in a single transient. The application of this has already been described in Section 5.7.3 with a view to obtaining pure lineshapes in TOCSY spectra, and the operation of the filter is itself described in Section 10.6 and so will not be explored here. The implementation of this in the 2D NOESY sequence is illustrated in Fig. 8.37 and requires the incorporation of the filter within the usual mixing time, followed by a purging gradient. The effective suppression of zero-quantum interference is illustrated in the high-resolution NOESY spectra of Fig. 8.38 in which spectrum (a) shows clear anti-phase... 

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