Pseudo-FID

There are actually two independent time periods involved, t and t. The time period ti after the application of the first pulse is incremented systematically, and separate FIDs are obtained at each value of t. The second time period, represents the detection period and it is kept constant. The first set of Fourier transformations (of rows) yields frequency-domain spectra, as in the ID experiment. When these frequency-domain spectra are stacked together (data transposition), a new data matrix, or pseudo-FID, is obtained, S(absorption-mode signals are modulated in amplitude as a function of t. It is therefore necessary to carry out second Fourier transformation to convert this pseudo FID to frequency domain spectra. The second set of Fourier transformations (across columns) on S (/j, F. produces a two-dimensional spectrum S F, F ). This represents a general procedure for obtaining 2D spectra. 

The second time variable, is the so called real-time variable, representing the time spent in data acquisition, while ti is the evolution interval between the two pulses during the experiment. The effect of can only be observed indirectly by noting its influence on It is therefore necessary to carry out F2 transformation first, in order to generate a series of spectra (rows of the matrix), which is then used as a pseudo-FID for F] transformation in the second step. 

The deuterium nucleus, used normally for the lock channel, can be employed as an internal NMR thermometer." Using two deuterated compounds, a power pseudo-FID can be constructed from either a conventional continuous wave (CW) deuterium sweep or a pulsed deuterium FID. The Fourier transform of this signal gives a Lorentzian line with double the natural... 

In a 2D experiments a series of FIDs are acquired using an incremented delay tl to build up a 2D data matrix, each row of the matrix corresponding to a specific value of tl. The effect of the incremented delay is to modulate the FIDs from the experiments, which can be interpreted and analysed as a pseudo FID with respect to the time delay tl. By convention the incremented delay is called dO and the increment inO. The detected frequency range in the fl dimension is given by ASW [Hz] = l/(inO ndO ). Normally there is only one time increment variable but in special pulse sequences based upon for instance the ACCORDION-principle, see section 5.9.3, additional increment variable ini, in2... can be defined. Usually for the first experiment dO = 3 us and for the second and subsequent experiments dO = 3u -i- inO (n -1). In a number of 2D experiments the... 

Figure 4 (A) Left Different elementary slices subjected to the decreasing RF field produced by the gradient coil. Right corresponding nutations assuming that nuclear magnetization was initially at thermal equilibrium. (B) Left The RF gradient pulse train with interleaved detection windows leading to the acquisition of a pseudo-FID. Right the Fourier transform of this pseudo-FID yields a profile representative of the object shape (see A).

With modern Bruker spectrometers the selective ID NOE experiment is usually performed in a pseudo 2D mode. The raw data is obtained as a 2D matrix with the individual rows (FIDs) corresponding to the different decoupler frequencies used for the selective perturbation plus one row where the decoupler frequency is set well away from any resonance line (reference FID). Consequently this 2D data matrix must first be decomposed into the individual 1D FIDs before the difference FIDs can be calculated. 

Load the raw data of the selective pseudo 2D H H NOE experiment D NMRDATA OLIGOSAC 1D H OHNO 2D 001001.SER. Decompose the data matrix into a series of 1D FIDs, calculate the difference FIDs and process the whole series following the procedures as outlined in previous chapters. Exploit the options for automatic and serial processing for this purpose. Use the multiple display option to compare the individual NOE spectra and plot the... 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

Tncludes d-orbitals for use m connection with MNDO, called MNDO/d. With reference -toAfae above description for MNDQ/AM1/PM3, it is clear that there are imme three new parameters Q-, Ud and fid (eqs. (3.82) and (3.83)). Of the 12 new one-centre two-electron integrals only one (Gdd) is taken as a freely varied parameter. The other 11 are calculated analytically based on pseudo-orbital exponents, which are assigned so that the analytical formulas regenerate Ggs, Gpp and Gdd-... 

NMR-SIM does not permit such spectrometer imperfections to be simulated so that the DC offset correction is redundant. Indeed this correction must never be applied if a resonance signal is placed directly on-resonance, i.e. a nuclear spin is defined with 5 = 01 ppm (the transmitter frequency). In ID WIN-NMR the DC Correction dialog box opens automatically before processing of the current FID data but for the processing of NMR data generated by NMR-SIM the No option should always be selected. The effect of the DC offset correction is shown in Check it 3.2.3.1 using a data set containing a pseudo DC offset. 

It has been known for some time that the FIDs of such systems often decay in a way that is well represented for the most part by a Weibullian [60,61]. Arguments based on consideration of correlation functions suggested that the FID of high molecular weight polydimethyl siloxane in the melt should decay for the most part with a Weibullian power of between 1.25 and 1.5 [62], and the existence of residual static dipolar interactions in these systems was confirmed by the existence of the pseudo-solid echo [63]. This reference forms part of a much larger body of work on such systems by Cohen-Addad and co-workers which it is beyond the scope of this chapter to cover in any detail, but interested readers are directed to literature such as [64] and [65]. 

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