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Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR.

Colaux H, Dawson DM, Ashbrook SE - J Phys Chem A (2014)

Bottom Line: This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency.The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation.Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, EaStCHEM and Centre for Magnetic Resonance, University of St. Andrews , North Haugh, St. Andrews KY16 9ST, U.K.

ABSTRACT
The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed "too challenging".

No MeSH data available.


87Rb (14.1 T, 12.5 kHz) (a) MAS and (b) isotropic projectionsof triple-quantum MAS NMR spectra of Rb2SO4.In (b, c) spectra were acquired using the pulse sequence in Figure 1, using either a single pulse or FAM-N for the conversionof triple- to single-quantum coherences. Two FAM-N pulses were generated(i) using CQ = 2.52 MHz and ηQ = 1.0 and (ii) using CQ = 5.3MHz and ηQ = 0.11, each with ω1/2π= 123 kHz. For full details of the experimental parameters, see the Supporting Information.
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fig7: 87Rb (14.1 T, 12.5 kHz) (a) MAS and (b) isotropic projectionsof triple-quantum MAS NMR spectra of Rb2SO4.In (b, c) spectra were acquired using the pulse sequence in Figure 1, using either a single pulse or FAM-N for the conversionof triple- to single-quantum coherences. Two FAM-N pulses were generated(i) using CQ = 2.52 MHz and ηQ = 1.0 and (ii) using CQ = 5.3MHz and ηQ = 0.11, each with ω1/2π= 123 kHz. For full details of the experimental parameters, see the Supporting Information.

Mentions: Figure 6b shows a plot consideringhow theefficiency of a specific FAM-N pulse (optimized at the CQ value shown) varies as CQ changes. Note the logarithmic scale on the x axis.Maximum enhancements are observed for CQ values close to those for which the pulse was initially generated,but a significant enhancement is seen over the signal obtained usinga single pulse over a range of CQ values.This range is wider at lower values of CQ (even accounting for the log scale), where the optimized FAM-N pulseconsists of only a small number of individual pulses. Although therange of CQ values where maximum efficiencyis obtained drops as CQ increases, themagnitude of the enhancement achieved at higher CQ is greater. Figure 6b suggeststhat, although a prior estimate of the magnitude of the quadrupolarinteraction is useful, significant enhancements may still be achievedfor different CQ values. The presenceof multiple sites with differing CQ valueswithin a sample presents a challenge for any sensitivity enhancementtechnique but is a common situation in reality. Figure 7 shows both a MAS spectrum and isotropic projections obtainedfrom 87Rb two-dimensional triple-quantum MAS NMR experimentson Rb2SO4, acquired using the phase-modulatedsplit-t1 shifted-echo pulse sequence shownin Figure 1. The conversion of triple- to single-quantumcoherences has been carried out either with a single (high-power)pulse or with FAM-N. There are two distinct Rb sites in Rb2SO4 (as can be seen in the MAS spectrum, shown in Figure 7a), with CQ values of∼2.5 and ∼5.3 MHz, respectively.26 Two FAM-N pulses were used for conversion: one generatedfrom an optimization using CQ = 2.52 MHzand ηQ = 1.0 and a second using CQ = 5.30 MHz and ηQ = 0.11. In both cases,values of ω1/2π = 123 kHz and ωR/2π = 12.5 kHz were used in the computer-based optimization.For Rb1 (CQ = 2.52 MHz), as shown in Figure 7b, the maximum enhancement (of ∼105% overa single pulse) is seen with the pulse generated using that CQ value, but only 12% of the signal is lostwhen the alternative FAM-N conversion pulse is used. A similar resultis also observed for Rb2 (Figure 7c), withmaximum enhancement obtained using the FAM-N pulse generated froman optimization using CQ = 5.30 MHz, andthe signal enhancement dropping from 160% to 135% (both compared tothe signal obtained using a single pulse) when the FAM-N pulse ischanged.


Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR.

Colaux H, Dawson DM, Ashbrook SE - J Phys Chem A (2014)

87Rb (14.1 T, 12.5 kHz) (a) MAS and (b) isotropic projectionsof triple-quantum MAS NMR spectra of Rb2SO4.In (b, c) spectra were acquired using the pulse sequence in Figure 1, using either a single pulse or FAM-N for the conversionof triple- to single-quantum coherences. Two FAM-N pulses were generated(i) using CQ = 2.52 MHz and ηQ = 1.0 and (ii) using CQ = 5.3MHz and ηQ = 0.11, each with ω1/2π= 123 kHz. For full details of the experimental parameters, see the Supporting Information.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126738&req=5

fig7: 87Rb (14.1 T, 12.5 kHz) (a) MAS and (b) isotropic projectionsof triple-quantum MAS NMR spectra of Rb2SO4.In (b, c) spectra were acquired using the pulse sequence in Figure 1, using either a single pulse or FAM-N for the conversionof triple- to single-quantum coherences. Two FAM-N pulses were generated(i) using CQ = 2.52 MHz and ηQ = 1.0 and (ii) using CQ = 5.3MHz and ηQ = 0.11, each with ω1/2π= 123 kHz. For full details of the experimental parameters, see the Supporting Information.
Mentions: Figure 6b shows a plot consideringhow theefficiency of a specific FAM-N pulse (optimized at the CQ value shown) varies as CQ changes. Note the logarithmic scale on the x axis.Maximum enhancements are observed for CQ values close to those for which the pulse was initially generated,but a significant enhancement is seen over the signal obtained usinga single pulse over a range of CQ values.This range is wider at lower values of CQ (even accounting for the log scale), where the optimized FAM-N pulseconsists of only a small number of individual pulses. Although therange of CQ values where maximum efficiencyis obtained drops as CQ increases, themagnitude of the enhancement achieved at higher CQ is greater. Figure 6b suggeststhat, although a prior estimate of the magnitude of the quadrupolarinteraction is useful, significant enhancements may still be achievedfor different CQ values. The presenceof multiple sites with differing CQ valueswithin a sample presents a challenge for any sensitivity enhancementtechnique but is a common situation in reality. Figure 7 shows both a MAS spectrum and isotropic projections obtainedfrom 87Rb two-dimensional triple-quantum MAS NMR experimentson Rb2SO4, acquired using the phase-modulatedsplit-t1 shifted-echo pulse sequence shownin Figure 1. The conversion of triple- to single-quantumcoherences has been carried out either with a single (high-power)pulse or with FAM-N. There are two distinct Rb sites in Rb2SO4 (as can be seen in the MAS spectrum, shown in Figure 7a), with CQ values of∼2.5 and ∼5.3 MHz, respectively.26 Two FAM-N pulses were used for conversion: one generatedfrom an optimization using CQ = 2.52 MHzand ηQ = 1.0 and a second using CQ = 5.30 MHz and ηQ = 0.11. In both cases,values of ω1/2π = 123 kHz and ωR/2π = 12.5 kHz were used in the computer-based optimization.For Rb1 (CQ = 2.52 MHz), as shown in Figure 7b, the maximum enhancement (of ∼105% overa single pulse) is seen with the pulse generated using that CQ value, but only 12% of the signal is lostwhen the alternative FAM-N conversion pulse is used. A similar resultis also observed for Rb2 (Figure 7c), withmaximum enhancement obtained using the FAM-N pulse generated froman optimization using CQ = 5.30 MHz, andthe signal enhancement dropping from 160% to 135% (both compared tothe signal obtained using a single pulse) when the FAM-N pulse ischanged.

Bottom Line: This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency.The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation.Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, EaStCHEM and Centre for Magnetic Resonance, University of St. Andrews , North Haugh, St. Andrews KY16 9ST, U.K.

ABSTRACT
The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed "too challenging".

No MeSH data available.