Spin trimming

Fig. 9. ID TOCSY-ROESY. (a) H spectrum of oligosaccharide 3 (5 mg/0.5 ml D2O). (b) ID TOCSY spectrum acquired using the pulse sequence of fig. 7(b) and a selective excitation of H-lc by a 49.2 ms 270° Gaussian pulse. Duration of the spin lock was 132.7 ms including two 2.5 ms trim pulses. 32 scans were accumulated, (c) ID TOCSY-ROESY spectrum acquired using the pulse sequence of fig. 7(d) with initial selective TOCSY transfer from H-lc and selective ROESY transfer from H-4c. Parameters for the TOCSY part were the same as in (b). A 49.2 ms 270° Gaussian pulse was used at the beginning of the ROESY transfer. A 500 ms ROESY spin-lock pulse ( yBi/2 K = 2.8 kHz) was applied 1000 Hz downfield from the H-4c resonance. The time used for the frequency change was 3 ms. 128 scans were accumulated. A partial structure of 3 is given in the inset. Solid and dotted lines represent TOCSY and ROESY transfers, respectively.

Friction welding is the most common solid-state process. One type, direct-drive friction welding, is accomplished by spinning one piece rapidly against a second, stationary piece to produce frictional heating. When the metal is at a forging temperature below the solidus, rotation is stopped and axial force is applied. Heated metal, together with oxide and surface contamination, is squeezed out of the interface between the two pieces in the form of a metal flash, which can be trimmed to form a smooth surface. 

Trim pulses are short spin-locking periods without compensation for rf inhomogeneity. Magnetization components orthogonal to the spin-lock axis are dephased. The minimum duration of a trim pulse for elimination of the unwanted magnetization components depends on the rf amplitude and can be estimated by... 

In addition to the selected magnetization component (e.g., 7 ), several terms in the density operator survive the application of trim pulses (or z filters). For example, if a trim pulse is applied along the x axis of the rotating frame, all terms of the density operator that commute with remain unaffected, that is, in addition to the in-phase operators and (x magnetization), antiphase combinations like (lyS - I Sy) or (I SyTy + also survive the trim pulses. In the effective field frame, these terms represent operators with coherence order p = 0. Modified z filters and spin-lock pulses that are able to suppress these zero-quantum-type terms will be discussed in Section XII.B. 

Fig. 4. Special pulse schemes for 2D- X, "Y H) correlations. The same notation as before is used A denotes a fixed delay of length ( /( H,Y(X)) l (a) HMQC sequence for indirect detection of spin-1 nuclei. (b) INEPT-HMQC. (c) INEPT-HETCOR. (d) HMQC-TOCSY si denotes an MLEV spinlock sequence of duration t which is framed by trim pulses. ...

There are essentially two approaches based on composite-pulse methods in widespread use for the practical implementation of the TOCSY experiment (Fig. 5.68). The first of these [51] (Fig. 5.68a) is based on the so-called MLEV-17 spin-lock, in which an even number of cycles through the MLEV-17 sequence are used to produce the desired total mixing period. To ensure the collection of absorption-mode data, only magnetisation along a single axis should be retained, so it is necessary to eliminate magnetisation not parallel to this before or after the transfer sequence. In this implementation, this is achieved by the use of trim-pulses applied for 2-3 ms along the chosen axis. 

Figure 5.68. Two practical schemes for implementing TOCSY based on (a) the MLEV-17 mixing scheme and (b) the DIPST2 isotropic mixing scheme. The MLEV sequence is bracketed by short, continuous-wave, spin-lock trim pulses (SL) to provide pure-phase data. In scheme (b) this can be achieved by phase-cycling the 90° z-filter pulses that surround the mixing scheme. This demands the independent inversion of each bracketing 90° pulse with coincident receiver inversion, thus (p =x, —X, X, —x (j) = X, X, —X, —X and (j)r = x, —X, —X, X. The S periods allow for the necessary power switching.

In Check it 5.4.2.1 the ID selective COSY, ID selective relayed COSY without and with z-filter and a ID selective TOCSY spectrum are simulated for the same spin system and the results compared. As already mentioned the spinlock for isotropic mixing can be generated in different ways and this has lead to the development of improvements and elements being added to the spinlock sequence. Of these improvements the trim pulse and z-filter, adapted to the spinlock sequence [5.154], are the most popular. 

In the MLEV-17 sequence a 180° pulse (or a 60° pulse) is appended to the MLEV-16 sequence. The additional 180° pulse inverts the magnetization that is not perfectly aligned with a particular axis of the rotating frame so that after an even number of MLEV17 cycles the magnetization is perfectly aligned. Any residual magnetization which is not perfectly parallel to the selected axis to which the spins are locked are defocused by the two trim pulses. 

Slater determinants enforce the Pauli exclusion principle, which forbids any two electrons in a system to have all quantum numbers the same. This is readily seen for an atom if the three quantum numbers n, I and trim of 4/ (x, y, z) (section 4.2.6) and the spin quantumnumber Wj of a or /8 were all the same for any electron, two rows (or columns, in the alternative formulation) would be identical and the determinant, hence the wavefunction, would vanish (section 4.3.3). 

The tantalum and niobium tubes are readily trimmed and opened after use with the spin-cutter of the sort used for cutting copper pipe. A tapered flaring tool is also helpful during the construction of apparatus. The tantalum tubing can often be reused if contamination from contents or outgassing of the jacket is not serious. Ductility or softness (low yield stress) is a good indication of purity. 

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