Pure absorptivity

Although this eliminates negative contributions, since the imaginary part of the spectrum is also incorporated in the absolute-value mode, it produces broad dispersive components. This leads to the broadening of the base of the peaks ( tailing ), so lines recorded in the absolute-value mode are usually broader and show more tailing than those recorded in the pure absorption mode. 

Dispersion mode A Lorentzian line shape that arises from a phase-sensitive detector (which is 90 out of phase with one that gives a pure-absorption-mode line). Dispersion-mode signals are dipolar in shape and produce long tails. They are not readily integrable, and we need to avoid them in a 2D spectrum. 

Phasing A process of phase correction that is carried out by a linear combination of the real and imaginary sections of a 1D spectrum to produce signals with pure absorption-mode peak shapes. 

Phase-sensitive data acquisition NMR data are acquired in this manner so that peaks are recorded with pure absorption-mode or pure dispersionmode line shapes. 

For several technical reasons, it is not possible to acquire NMR data with perfect phase. One reason is the inability to detect XY magnetisation correctly another is the fact that we are unable to collect the data as soon as the spins are excited. These limitations mean that we have to phase correct our spectrum so that we end up with a pure absorption spectrum. What we don t want is a dispersion signal (see Spectrum 4.2). 

Phase The representation of an NMR signal with respect to the distribution of its intensity. We aim to produce a pure absorption spectrum (one where all the signal intensity is positive). 

The signals S+ and S are now amplitude-modulated as a function of tp, therefore, a double hypercomplex Fourier transformation of these data, following for instance the States-Haberkom-Ruben procedure, yields a pure-absorption 2D spectrum with sign discrimination in the 12 j dimension [169]. 

Atoms, ions and molecules present in the stars provide additional opacity at wavelengths corresponding to specific atomic transitions these give rise to comparatively narrow absorption lines (see Fig. 3.2) with intensities related to the abundances of the relevant elements (and much else). Despite the name, processes other than pure absorption (e.g. scattering and fluorescence) are involved in the production of these lines and, while they are often treated in LTE, this is now only a simplifying approximation which often works fairly well, but needs to be checked by more detailed calculations for each particular case. (In some cases, there are even emission lines or emission components, e.g. the solar Ca+ H and K lines in the near UV, which are so strong that the chromosphere affects their central parts.)... 

Mueller, K.T., Wooten, E.W., and Pines, A. (1991) Pure-absorption-phase dynamic-angle spinning. /. Magn. 

Figure 3. Contour plot of 500 MHz pure absorption NOESY spectrum of globoside at 303 K. The labeled cross peaks correspond to the interresidue and intraresidue connectivities for globoside. (Reproduced from ref. 40. Copyright 1986 American Chemical Society.)...

In order to overcome this problem, the half-absorption mode has been proposed by Bax [14] to take advantage of the phase-mode data processing. In this method, of Sx F, F2) and 7c,s of Sy F, F2) prepared from the data acquired by the States method are exchanged, and the final t2 axis data containing dispersion component sine) are presented in the power-mode, and the t axis data in the pure absorption-mode as follows ... 

HSQC rather than HMQC-based transfer schemes have recently in particular been employed in various indirectly detected two- and three-dimensional 111/X/Y correlation experiments involving multi-step coherence-transfer in either direction.38 40 43 44 The application of PFG s appears to be essential to obtain a sufficiently clean spectrum that is free of artefacts, and in many cases the pulse sequence shows only a satisfactory performance if composite pulses, with a larger excitation bandwidth than normal ones, are employed.21,38,39,43 The pulse schemes yield generally phase-sensitive spectra with pure absorptive lines and do not suffer from splitting or broadening of the cross peaks as a consequence of the undesired evolution... 

The shape of the resonance signals in this experiment are no longer in pure absorption. To minimise these unwanted effects z-filter -TOCSY experiments are applied. 

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. 

In contrast to the basic "C detected experiment, and as a consequence of the final H detection, the 2D spectra obtained with HMQC or HSQC have a projection onto the F2 axis which corresponds to the normal H spectrum and includes all chemical shifts and all Jfi, couplings. The latter may give rise to rather broad cross peaks for extensively coupled protons. The projection onto the Fl axis corresponds to a normal C spectrum but with the quaternary carbons missing. With HMQC, but not with HSQC, cross peaks are additionally split in Fl by "J couplings. The HMQC and the HSQC experiment are usually performed in phase-sensitive mode, which, after proper phasing in both dimensions, allow peaks to be displayed in pure absorption. 

Assuming your ID NMR data has already been processed, i.e. the Fourier transformation has been performed, the phases of the signals have been correctly adjusted to pure absorption and your ID spectrum is stored on your PC s disk, there are still a few final processing steps to be performed, before the final layout is completed and the data can be plotted. 

In reality the individual lines obtained after the Fourier transformation are composed of both absorptive A(f) and dispersive D(f) components. This non-ideality arises because of a phase shift between the phase of the radiofrequency pulses and the phase of the receiver, PHCO, and because signal detection is not started immediately after the excitation pulse but after a short delay period A. Whereas the effect of the former is the same for all lines in a spectrum and can be corrected by a zero-order phase correction PHCO, the latter depends linearly on the line frequency and can be compensated for by a first-order phase correction PHCl. Both corrections use the separately stored real and imaginary parts of the spectrum to recalculate a pure absorptive spectrum. 

Load the raw data of the ID H experiment measured for peracetylated glucose D NMRDATA GLUCOSE 1D H GH 001001.FID and perform a Fourier transformation. Inspect the spectrum, showing the entire proton range, and adjust the phases manually to get pure absorption lines throughout. Try out the Automatic button in the Phase button panel for this purpose. Do not forget to store the spectrum (... 001001.1R). 

H/ H-double quantum filtered COSY Note that with this experiment both the diagonal and the cross peaks may be phased to pure absorption. Therefore it is best to select diagonal peaks at the extremes and in the center of the spectrum for phase adjustment. The cros.s peaks when correctly phased consist of positive and negative peaks, which are anti-phase with respect to the active, and are in-phase with respect to the passive coupling(s). 

Note that with a non-DQ-filtered, phase sensitive COSY experiment the cross peaks are again purely absorptive while diagonal peaks irrespective of the phase correction will have both absorptive and dispersive character. Unlike most other 2D spectra, it is therefore best to phase correct a non-DQ-filtered phase sensitive COSY spectrum while examining the cross rather than the diagonal peaks. 

The applied electric field perturbs the orientational distribution function of the dipolar molecules. Dielectric relaxation due to classical molecular reorientational motions is a form of pure absorption spectroscopy whose frequency range of interest for materials, including polymers, is between 10 6 and 1011 Hz. 

It is quite important for the spectra measured to represent the pure absorption mode with neither saturation nor transient effects. They should be measured in a homogenous external magnetic field B0 in order to obtain purely Lorentzian lineshapes for individual spectral transitions. The signal-to-noise ratio should be as high as possible and experimental... 

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