Spin lock axis

The simplest double tuned filter can be constructed by a concatenation of two X-half filters and removal of redundant 180° pulse pairs (Fig. 17.4d) [22]. Alternatively, it can also be realized by keeping the 180° pulse pairs and adding short spin-lock periods (to dephase the 1H-13C magnetization which is orthogonal to the spin-lock axis, Fig. 17.4e) [23], or it is based on the gradient-purging scheme of Fig. 17.4b, resulting in the double filter shown in Fig. 17.4f [18]. 

In a ID TOCSY-NOESY experiment [39], the proton magnetization is aligned along the spin-lock axis after the initial selective TOCSY step. The... 

Concatenation of two TOCSY steps in a ID TOCSY-TOCSY experiment [72] is a straightforward matter (fig. 10(a)). After the initial TOCSY transfer, the magnetization is returned to the 2 axis by a nonselective 90° pulse applied perpendicularly to the spin-lock axis. The carrier frequency is changed and the second 90° selective pulse applied to a different proton followed by the second TOCSY spin-lock period. 

What does this mean for the effect of the spin lock on sample magnetization If the sample magnetization starts on the / axis, for example, the tilted spin-lock axis will destroy the component that is perpendicular to the spin-lock axis and retain the component that is on the spin-lock axis. This preserved component is locked because it is on the axis of the effective field and has no reason to precess around the z axis. So even if the spin is off-resonance, its magnetization does not precess around the z axis during the spin-lock period. Instead, the component that is not on the tilted spin-lock axis precesses around the spin-lock axis until it is destroyed by Bi inhomogeneity, and the component that is on the spin-lock axis is retained. 

In both ROESY mixing and TOCSY mixing, the transfer is from in-phase net magnetization on the spin-lock axis to in-phase net magnetization on the spin-lock axis for example, ly ->1 (TOCSY) or ly - - Iy (ROESY). This is analogous to the NOESY transfer, It -> — for small molecules and in contrast to the INEPT transfer 21 -> 21 1 , which is antiphase to antiphase. 

ROESY (rotating frame Overhauser effect spectroscopy) is a variant of NOESY, in which the transfer of magnetization occurs on the spin-lock axis in the x-y plane rather than on the z axis (Fig. B.9). A continuous low-power radio frequency pulse provides the mixing by effectively reducing the field strength (Bq in the laboratory frame on the z axis to B in... 

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... 

The third complicating factor specific to ROESY is the attenuation of cross-peak intensities as a function of resonance offset from the transmitter frequency [69]. Off-resonance spins experience a spin-lock axis that is tipped out of the x-y plane (Section 3.2.1) resulting in a reduction in observable transverse signal in addition to a reduction in cross-relaxation rates. This is more of a problem for quantitative measurements, although fortunately mid-sized molecules show the weakest dependence of ROE cross-relaxation rates on offset. The so-called compensated ROESY sequence [69] eliminates these frequency-dependent losses should quantitative data be required. 

---