The Execution of Solid-State NMR Experiments on Automation Systems

The technique in this article explores applied radio frequency fields B₁ expressed and employed as nutation frequencies for nucleus i,ƒ_(1,i)=1/2π |1/2 γ_i B_r,i|, vital constraints in solid-state nuclear magnetic resonance (NMR) studies as decoupling fields or spinlock fields in cross-polarization (CP).

The term nutation frequency pertains to ω_1,i=|1/2γ_i Br_f,i|, with ƒ_1,i=1/2π ω_1,i. These rf-fields ƒ₁ are frequently associated with the mechanical sample rotation frequency around the magic angle ƒ_MAS.

An AU-program within the parameter set reads the magic angle spinning (MAS) frequency automatically and records the ƒ_MAS value to the parameter cnst31. The parameter cnst31 is traditionally employed as the MAS frequency in pulse programs. It allows any study to be performed under numerous MAS spinning conditions under a straightforward standard operating procedure (SOP).

The SOP includes loading the sample, spinning it up to the required MAS rate, loading the parameter set in TopSpin, and inspecting the PROSOL parameters. PROSOL parameters are read into the parameter set by employing the solvent button illustrated in Figure 1 or by entering “getprosol” into the command line.

Figure 1. The solvent button in TopSpin can be used to read PROSOL parameters into the parameter set. Image Credit: Bruker BioSpin - NMR, EPR and Imaging

The study can begin with the command xaua, inputted into the command line of TopSpin, after adjusting and coordinating the NMR probe to the appointed rf-frequencies. The command xaua performs an au-program as part of the acquisition parameter set.

The AU-program scans the programmed MAS frequency from the MAS controller and inputs its value into cnst31=ƒ_MAS=MASR. The parameter MASR is generally only employed when the MASRSET is administered.

The parameter MASR within the acquisition status parameter set reports the MAS frequency employed when the study was performed. The status parameter can be found utilizing the letter “s” before inputting the parameter name into the command line, e.g. 1s MASR or s MASR.

Automated testing can be executed in IconNMR, Bruker’s computerization program that loads studies. IconNMR performs all required steps, sample loading (if a sample changer is offered), tuning and matching, loading the proper shim file, and setting the required MAS frequency when utilizing a CPMAS iProbe.

Other CPMAS probes demand manual tuning and matching after sample loading before initiating the study in IconNMR.

TopSpin Implementation

The pulse programs with the viewpoints detailed in the article by C. Johann et al.¹ were initially applied in TopSpin 4.4. These pulse programs utilize the nutation frequency/B₁ field and facilitate correction terms in frequency units of Hz for rf-power/rf-field correction of the different pulse sequence components in a study.

A well-setup spectrometer does not necessitate these correction terms but can be used at the spectroscopist’s discretion.

The studies consider that MAS defines many foundations of solid-state NMR experiments, such as the rf-amplitude of the spinlock pulse for the zero quantum CP matching condition.

The Hartmann Hahn matching condition² under MAS for nuclei i and s under MAS relies on the MAS frequency^1,2,3 ƒ_1,i - ƒ_1,s = nƒ_MAS.

Simply put, the s-nucleus is any X-nucleus from ³¹P to lower frequency nuclei but the i-nucleus is predominantly ¹H and sometimes ¹⁹F. The nucleus i is usually the nucleus from which the magnetization is transferred, while the nucleus s receives the magnetization in a CP study (Figure 2).

The pulse program fills the specified maximum spin nutation frequency f_1,s=1/2π |1/2 γ_s B_rf,s |, for the low gamma i-nucleus and evaluates from ƒ_MAS with n=1 the rf-power needed for ƒ_1,i=nƒ_MAS+ƒ_1,s.

Figure 2. CPMAS experiment with nucleus i being the ¹H channel, typically and s any lower gamma nucleus. The contact pulse with the ramped amplitude is generally on the high gamma nucleus but can be used on the low gamma nucleus as well. Often, tangential amplitude modulated pulses are employed promising better magnetization yield ^{3, 4}. Image Credit: Bruker BioSpin - NMR, EPR and Imaging

A 10 kHz reference field in the PROSOL table is used to determine and adjust the accurate rf-power.¹ The rf-fields for the spinlock pulse of the i-nucleus are derived from the PROSOL table, which includes probe specifications and should be configured during installation by a Bruker engineer or an NMR manager.

By default, the CP pulse for the X-nucleus is a constant amplitude rf-pulse. For ¹H, the default is a tangentially modified pulse shape⁵ with an rf-field exceeding the X-nucleus B₁-field by f_MAS.^1,5

The maximum rf-fields for the s-spins, the highest decoupling field for the i-spin channel, and the reference rf-fields and rf-power values for both nuclei are retrieved from the PROSOL table via the relations file, determining the connection between PROSOL table entries and the study constraints demanded by the pulse program.

This setup enables automation by using an experiment parameter set that includes the relevant pulse program. Table 1 lists current parameter sets for the s-nuclei ³¹P, ¹³C, ²⁹Si, ¹⁵N, ²⁰⁹Pb, 79Br and ²⁷Al. Additional experiments can be provided upon request by Bruker’s solid-state applications team.

Table 1. Solid-state NMR Experiments which make use of the method described in this application note. The extension *.dp stands for direct polarization while *.cp stands for cross polarization experiment. Source: Bruker BioSpin - NMR, EPR and Imaging

Practical Examples

Figures 3 and 4 illustrate results from experiment runs. The only necessary optimization is adjusting the recycle delay D1 by determining the ¹H T₁-relaxation employing 1HSATRECT1 or SATRECT1_echo.

SATRECT1_echo utilizes a Hahn-echo for ¹H detection, which can lower a probe's ¹H background. The first method is quicker, requiring only one scan, while the second takes eight scans.

The recycle delay D1 is calculated by multiplying T₁ with a factor k, D₁=kT₁,k≥1.4. The factor varies according to study type.

Figure 3. ¹³C CPMAS project on tyrosine with an automation run at 7 kHz MAS using IconNMR. CPMAS is shown in blue, CPPI multiplicity editing ² in red, CPTOSS in green and TOSS NQS in purple. Image Credit: Bruker BioSpin - NMR, EPR and Imaging

Figure 4. ¹³C HETCOR experiment on fMLF tripeptide run in full automation. ⁵ Image Credit: Bruker BioSpin - NMR, EPR and Imaging

Solid-State NMR Experiments with IconNMR

These experiments can be performed with or without a sample changer in IconNMR. The IconNMR setting is simple and enables data acquisition for single or multiple samples.⁶

After completing the ¹H T₁ experiment, the experiments can be queued in IconNMR, setting D1 correctly and selecting the required MAS rate, which could differ for the various experiments.

IconNMR will vary the MAS rate as requested and run the queued experiments sequentially. The spectra in Figures 3 and 4 were acquired utilizing IconNMR.

References

1. Hartmann, S.R. and Hahn, E.L. (1962). Nuclear Double Resonance in the Rotating Frame. Physical Review, 128(5), pp.2042–2053. https://doi.org/10.1103/physrev.128.2042.
2. Hediger, S., Meier, B.H. and Ernst, R.R. (2000). Adiabatic passage Hartmann-Hahn cross polarization in NMR under magic angle sample spinning. Chemical Physics Letters, [online] 240(5-6), pp.449–456. https://doi.org/10.1016/0009-2614(95)00505-X.
3. Johann, C. et al. (2023). Automation in solid state NMR, Journal of Magnetic Resonance, 355, p. 107554. https://doi.org/10.1016/j.jmr.2023.107554.
4. Meier, B.H., (1992). Polarization transfer experiments in solid-state NMR. Chemical Physics Letters, 188, pp.201–207. https://doi.org/10.1016/0009-2614(92)90009-c

This information has been sourced, reviewed and adapted from materials provided by Bruker BioSpin - NMR, EPR and Imaging.

For more information on this source, please visit Bruker BioSpin - NMR, EPR and Imaging.