Feb 17, 2025
Direct-Detect HETCOR
This protocol is a draft, published without a DOI.
- Alexander L. Paterson1
- 1National Magnetic Resonance Facility at Madison (NMRFAM), University of Wisconsin-Madison, Madison, WI, United States
Protocol Citation: Alexander L. Paterson 2025. Direct-Detect HETCOR. protocols.io https://protocols.io/view/direct-detect-hetcor-dfpk3mkw
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: In development
We are still developing and optimizing this protocol, but it should be functional. We hope to solicit feedback primarily on clarity and usability. We intend to publish it in June 2025.
Created: June 14, 2024
Last Modified: February 17, 2025
Protocol Integer ID: 101836
Keywords: Materials Spin-1/2 HETCOR
Funders Acknowledgements:
National Science Foundation
Grant ID: 1946970
Abstract
Purpose
To collect a 2D through-space correlation spectrum between 1H and a spin-1/2 nucleus.
Scope
This protocol is focused on the basic Frequency Switched Lee Goldberg (FSLG) heteronuclear correlation (HETCOR) experiment. There are more advanced modifications and experiments which can provide various benefits, but the basic sequence is generally adequate, robust, reliable, straightforward to set up, and can be run on a wide variety of hardware configurations.
Guidelines
As the experiment is CP-based, correlations are through-space. Setting a very short CP contact time will de-emphasize, but not necessarily eliminate, remote couplings transferred via 1H homonuclear coupling. Setting a long CP contact time will allow for long-range correlations to be observed.
This protocol assumes that the user has substantially optimized a cross-polarization transfer prior to beginning. CP optimization is best performed using a 1D sequence.
HETCOR has similar sensitivity to CPMAS experiments. It may be preferable to perform significant optimization on an isotopically enriched set-up sample if available. The 1H, Hartmann-Hahn CP condition, and FSLG parameters are all largely transferrable from sample to sample if the probe configuration is unchanged.
However, if there is a serious difference in the tuning and matching between samples, re-optimization may be required.
Samples with very poor CP transfer efficiency (e.g., highly dynamic environments) or small heteronuclear dipolar couplings will not perform well using this protocol.
Materials
Definitions:
- CP: Cross polarization
- MAS: Magic Angle Spinning
- rf: Radiofrequency
- HH: Hartmann-Hahn
- FSLG: Frequency Switched Lee Goldberg
- t1: Evolution time
Safety warnings
Unless low-power proton decoupling is used, user must ensure that the total high-power time is less than 50 ms or the limit of the probe, whichever is less.
Before start
User should be familiar with the power, duty cycle, and decoupling limits of the probe.
Unless low-power proton decoupling is used, user must ensure that the total high-power time is less than 50 ms or the limit of the probe, whichever is less.
Expected amount of time SOP will use: 1 hour to 1 day, depending on sample sensitivity.
Procedure
Procedure
Begin with an optimized 1H pulse width and power and CP condition.
Load the lghetfq pulse program. It will need to be immediately converted to a 2D experiment before proceeding.
Set the following initial parameters. These will likely not require significant additional optimization.
aq: Set to 50 ms or less. This is required for the safety of the probe.
d1: Set to 1.3 × T1(1H). Ensure that the time is at least 20 times longer than aq; for example, if aq is 50 ms, d1 should be at least 1 s.
Note
If significant t1 noise is encountered, d1 can be reduced from 1.3 × T1(1H) as long as it remains 20 times longer than aq.
o1: Set the X transmitter frequency to the desired center of expected X chemical shift range.
o2: Set the 1H transmitter frequency to the low-frequency edge of the 1H chemical shift range.
p15: Set to the previously optimized CP contact pulse length. This will likely be longer than optimal for probing short-range correlations, but will allow for greater sensitivity during the optimization process.
plw1: Set to the previously optimized X power for CP.
Note
Note that the sequence inherently uses a square pulse for the X channel CP contact pulse. If your CP condition was previously optimized using a shaped pulse on the X channel, you will need to reoptimize your condition.
cpdprg2: Set to an appropriate decoupling sequence such as SPINAL64.
pcpd2: Set the decoupling pulse length as appropriate for the chosen decoupling sequence. This pulse uses plw12 for power.
p3: Set to a previously optimized 1H excitation pulse width at plw12.
plw12: Set to a previously optimized 1H pulse power for the excitation pulse p3 and decoupling pulse pcpd2.
spnam0: Set to the previously optimized 1H CP contact pulse shape.
spw0: Set to the previously optimized 1H CP contact pulse power.
Set the following initial FSLG parameters.
cnst24: Set to 0. This parameter is used to shift the 1H axis away from the transmitter frequency to avoid artefacts and can be adjusted later.
plw13: Set to a previously optimized 1H pulse power. This can be the same value as the 1H excitation pulse power plw12.
cnst20: Set to the 1H rf field strength at plw13, as expressed in Hz. This value is used to calculate the FSLG pulse widths and frequency offsets. Starting with a well-calibrated 1H pulse power will minimize the amount of optimization of cnst20 that is required. Higher power usually provides better resolution, particularly when run on higher field magnets.
Set the following 2D parameters using eda and edp:
1 sw: Set an initial value of sw to ensure that the F1 sweep width is large enough to observe the expected 1H chemical shift range. Note that because o2 is set to the low-frequency edge of the 1H spectrum, peaks will only appear in half of the spectral width. 30 to 40 ppm is a reasonable default value. Note the value of in_f for later reference.
1 td: set to a short value for initial calibration of the sequence, e.g., 4.
FnMode: set to States-TPPI.
mc2: Set to States-TPPI.
Run the ased command to compile the pulse sequence. Change the value of l3 such that dwellf1 is close to the desired value of in_f. Set in_f to the same value as dwellf1.
Note
l3 adjusts the number of FLSG pulse pairs per t1 increment, effectively setting the F1 spectral width. The value of dwellf1 adjusts the F1 chemical shift axis for the 0.578 scaling factor inherent to FSLG chemical shifts, which should assist with assigning 1H resonances.
With a minimal number of scans and 1 td points, acquire a 2D spectrum. This should only take a few seconds.
Ensure that the F1 spectral width encompasses the desired chemical shift range. Adjust o2p and l3 as needed.
If there is a significant artefact in F1 along the 1H transmitter o2 which is overlapping with peaks of interest, set cnst24 to -1000 or -2000. This adds an offset to the LG frequencies which should reduce the
overlap.
If sensitivity permits, cnst20 can be optimized to maximize resolution:
Increase 1 td to a sufficiently high value that 1H resolution is not limited by the digitization.
Create multiple experiment environments. In each experiment, change cnst20 slightly. An error of 0.05 µs in the 1H power calibration corresponds to about 2 kHz in cnst20.
Run the experiments in series. Compare the F1 resolution and select the value of cnst20 with the best performance.
If the 1H power is substantially mis-set, recalibration via a 1H 1D or X{1H} CPMAS experiment may be more efficient than via cnst20.
Once FLSG performance is satisfactory, prepare to run the actual experiment:
Increase 1 td to minimize truncation artefacts in sharp peaks.
Reduce p15 to a short value, typically between 50 µs to 250 µs. Shorter values will reduce signal originating from distant protons.
Increase ns to best make use of experiment time, recognizing that the reduction in p15 will reduce overall sensitivity.
Acquire the experiment. Reference the 1H axis by comparison to a well-resolved peak in a 1H 1D experiment.
Protocol references
van Rossum, B. J.; Förster, H.; de Groot, H. J. M. High-Field and High-Speed CP-MAS 13C NMR Heteronuclear Dipolar-Correlation Spectroscopy of Solids with Frequency-Switched Lee–Goldburg Homonuclear Decoupling. Journal of Magnetic Resonance 1997, 124 (2), 516-519. DOI: 10.1006/jmre.1996.1089.
Bruker Solid State NMR User Manual Z4D10641B, Section 8
Protocol
NAME
CPMAS
CREATED BY
NMRFAM Facility