Resonance offset

Selective experiments can also be performed by the tailored excitation method of Tomlinson and Hill. The selective pulse is frequency-modulated with a function designed to yield zero effective field at the resonance offset of the neighboring nuclei. Although this technique is especially promising for studies of more-complex spin systems, its use is as yet very limited, in part because the instrumentation needed is not yet commercially available. 

Fig. 11 (a) 2D NCO experiment with optimal control element inserted for 15N — 13C transfer. Transfer efficiencies for the ocNCO experiment optimized for 12 kHz spinning speed as function of (b) resonance offsets for 13C and 1SN and (c) rf inhomogeneity/adjustment in terms of scaling factors on the nominal rf field strengths for 13C and 15N. (d) Experimental ocNCO 2D spectrum of uniformly 13C,15N-labeled ubiquitin with the projections to the left comparing ocNCO experiment most intense) and DCP (less intense) based NCO experiments [reproduced with permission from [161] (a, d) and [164] (c)]... 

In the fast-spinning case Tics = isotropic chemical shift and the quadrupolar terms are given by (16) and (18). Thus, we obtain... 

A plot of the 2 magnetization (M /Moo) as a function of the irradiation resonance offset is given for an offset saturation experiment on a sample of degassed cyclohexane (CeHn) in CDCI3 at a temperature of 303 K. At this temperature, the cyclohexane resonance was noticeably narrow at 280 K, the linewidth of the cyclohexane resonance was approximately 1.2 Hz, compared to a width of 0.5 Hz for the internal TMS standard. From the resulting values of T and T2, and the measured low-temperature axial-equatorial chemical shift difference of 45.4 Hz at 100 MHz used in eq. (22), the rate at 293 K was determined to be 1.94 x 10 s Thus, rates on the order of 10 or 10 s are accessible in relatively simple systems using the offset-saturation method. 

A plot of the 2 magnetization (Mz/Moo) as a function of the irradiation resonance offset is given in fig. 10 for an offset saturation experiment performed on the aldehydic proton of furfural for a degassed sample of furfural in acetone-de at a temperature of 262.5 0.5 K. 

Figure 8.2.10 (A) Pulse sequence used for selective excitation of each of the four samples in turn. The RF pulses are frequency-selective and applied at different resonant offsets via phase modulation in the time domain. (B) Normal 1H spectrum of (a) the four samples, and the resulting sub-spectra of 0.5 M (b) 1-propanol, (c) 2-propanol, (d) acetic acid and (e) ethanol in D2O. Since the four spectra are acquired within the relaxation time 77 eff, which is typically set to be3Ti, there is an increase in efficiency by a factor of four. Reprinted with permission from Hou, T., Smith, J., MacNamara, E., Macnaughton, M. and Raftery, D., Anal. Chem., 73, 2541-2546 (2001). Copyright (2001) American Chemical Society...

Resonance offset effects on spin-echo pulse sequences... 

In NQR, composite pulses have been proposed for the compensation of resonance offset [107-111] and RF field inhomogeneity [107,108,112-114], Several reports on pulses of continuously variable shape for use in NQR have also appeared [115-117], Shaped pulses that compensate for RF field inhomogeneity also partially compensate for the powder average effect (Section 2.1.2.). Theoretically, shaped excitation pulses can enhance SNR by up to 15% in powder samples [117]. Combinations of shaped excitation and refocusing pulses may also provide better response from multi-echo pulse sequences [9],... 

Note that we use A v to refer to the rotating-frame frequency (sometimes called the resonance offset). This is the difference between the Larmor frequency and the reference frequency v0 - vr. The above equation shows that the same physical law expressed in the equation on the left-hand side (precession rate is proportional to y and to B0) is operating in the equation on the right-hand side (resonance offset is proportional to y and to fires) in the rotating frame of reference, as long as we introduce the pseudofield. In the NMR spectrum, A v is the distance from the center of the spectral window to the NMR peak (Fig. 6.2), also represented as 2 in units of radians per second. If the peak is in the downfield half (left half) of the spectrum, the Larmor frequency is greater than the reference frequency ( v0 > vr) and we have a positive resonance offset (A v > 0). This corresponds to the motion of the net magnetization... 

These simple product operators precess in the x -y plane of the rotating frame at a frequency corresponding to the chemical shift in hertz relative to the center of the spectral window (the resonance offset Av = v0 — vr). The chemical shift frequency Av can also be represented as the angular velocity 2 in units of rad/s ( 2 = 2ttAv). Using 2 allows us to skip all the 2tt terms. 

If we used a continuous-wave spin lock (ROESY mixing scheme) for TOCSY mixing, the only way efficient transfer could occur via 7 couplings within a spin system would be if the two protons have opposite resonance offsets (v0 - vr), where vr is the carrier or reference frequency. This is a special case of the Hartmann-Hahn match ... 

Cross-polarization in tilted frame has been proposed.275 In this scheme, the two effective fields matching the Hartmann-Hahn condition have a large offset comparable to two RF fields. This method was named SPECIFIC CP (spectrally induced filtering in combination with cross-polarization Fig. 20) because the matching depends on the resonance offset of the involved nuclei. This method is clearly useful for spectral simplification, as demonstrated in Fig. 21. Under fast spinning conditions, 7-coupling-mediated CP can be observed as shown by Verhoeven et al.216... 

This 2D correlation method uses a multiple-pulse sequence to suppress resonance offsets and promote polarisation transfer driven only by the /-coupling while simultaneously avoiding the recoupling of dipolar interactions. A much... 

To partially address the problem of resonance offset artifacts, we can first separate the nuclear spin phase evolution induced by the gradient from the evolution induced by all other sources of resonance offset. Nobel Prize winner Richard Ernst introduced a form of imaging that uses pulsed gradients, in analogy... 

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