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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 RbNO3. In (b), spectrawere acquired using the pulse sequence in Figure 1, with different pulses for the conversion of triple- to single-quantumcoherences. The single pulse, SPAM, FAM-I, FAM-II, and DFS pulseswere optimized experimentally (Supporting Information). The FAM-N pulse was generated for a single 87Rb spinat 14.1 T with ω1/2π = 114 kHz, ωR/2π = 12.5 kHz, CQ = 1.9MHz, and ηQ = 0. For full details of the experimentalparameters used, see the Supporting Information.
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fig4: 87Rb (14.1 T, 12.5 kHz) (a)MAS and (b) isotropic projectionsof triple-quantum MAS NMR spectra of RbNO3. In (b), spectrawere acquired using the pulse sequence in Figure 1, with different pulses for the conversion of triple- to single-quantumcoherences. The single pulse, SPAM, FAM-I, FAM-II, and DFS pulseswere optimized experimentally (Supporting Information). The FAM-N pulse was generated for a single 87Rb spinat 14.1 T with ω1/2π = 114 kHz, ωR/2π = 12.5 kHz, CQ = 1.9MHz, and ηQ = 0. For full details of the experimentalparameters used, see the Supporting Information.

Mentions: Figure 4a shows a 87RbMAS spectrumof RbNO3, where the quadrupolar line shapes of the threedistinct Rb sites (all of which exhibit similar CQ values, between 1.65 and 2.0 MHz),25 are overlapped. Three distinct resonances can be seen inthe isotropic projections shown in Figure 4b, obtained from 87Rb two-dimensional triple-quantum MASNMR experiments, acquired using the phase-modulated split-t1 shifted-echo pulse sequence shown in Figure 1. The conversion of single- to triple-quantum coherenceswas carried out using different approaches in the six different spectrashown. The single pulse, SPAM, FAM-I, FAM-II, and DFS pulses wereoptimized experimentally (resulting in the pulses described in the Supporting Information). For FAM-N, the pulsewas produced using the computational optimization approach describedabove, using ω1/2π = 114 kHz, ωR/2π = 12.5 kHz, CQ = 1.9 MHz, andηQ = 0. It is possible to perform an additional experimentaloptimization step by varying the B1 fieldstrength to ensure maximum signal is obtained (e.g., to account forany inaccuracies in estimation or calibration of the rf nutation rate).In this particular case experimental optimization did not result inany improvement. Figure 4 shows that for allthree sites FAM-N has much greater efficiency than the single-pulseconversion (∼105% more signal) and greater also than FAM-II(by ∼20%). The efficiency is slightly higher than that achievedusing DFS, although it is not clear whether this result arises fromthe more challenging experimental optimization for DFS or from aninherent difference in efficiency. Both FAM-N and SPAM require littleor no experimental reoptimization, ensuring they are easy to implement.The additional optimization time required increases for FAM-II, FAM-I,and DFS experiments, respectively.


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 RbNO3. In (b), spectrawere acquired using the pulse sequence in Figure 1, with different pulses for the conversion of triple- to single-quantumcoherences. The single pulse, SPAM, FAM-I, FAM-II, and DFS pulseswere optimized experimentally (Supporting Information). The FAM-N pulse was generated for a single 87Rb spinat 14.1 T with ω1/2π = 114 kHz, ωR/2π = 12.5 kHz, CQ = 1.9MHz, and ηQ = 0. For full details of the experimentalparameters used, see the Supporting Information.
© Copyright Policy
Related In: Results  -  Collection

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

fig4: 87Rb (14.1 T, 12.5 kHz) (a)MAS and (b) isotropic projectionsof triple-quantum MAS NMR spectra of RbNO3. In (b), spectrawere acquired using the pulse sequence in Figure 1, with different pulses for the conversion of triple- to single-quantumcoherences. The single pulse, SPAM, FAM-I, FAM-II, and DFS pulseswere optimized experimentally (Supporting Information). The FAM-N pulse was generated for a single 87Rb spinat 14.1 T with ω1/2π = 114 kHz, ωR/2π = 12.5 kHz, CQ = 1.9MHz, and ηQ = 0. For full details of the experimentalparameters used, see the Supporting Information.
Mentions: Figure 4a shows a 87RbMAS spectrumof RbNO3, where the quadrupolar line shapes of the threedistinct Rb sites (all of which exhibit similar CQ values, between 1.65 and 2.0 MHz),25 are overlapped. Three distinct resonances can be seen inthe isotropic projections shown in Figure 4b, obtained from 87Rb two-dimensional triple-quantum MASNMR experiments, acquired using the phase-modulated split-t1 shifted-echo pulse sequence shown in Figure 1. The conversion of single- to triple-quantum coherenceswas carried out using different approaches in the six different spectrashown. The single pulse, SPAM, FAM-I, FAM-II, and DFS pulses wereoptimized experimentally (resulting in the pulses described in the Supporting Information). For FAM-N, the pulsewas produced using the computational optimization approach describedabove, using ω1/2π = 114 kHz, ωR/2π = 12.5 kHz, CQ = 1.9 MHz, andηQ = 0. It is possible to perform an additional experimentaloptimization step by varying the B1 fieldstrength to ensure maximum signal is obtained (e.g., to account forany inaccuracies in estimation or calibration of the rf nutation rate).In this particular case experimental optimization did not result inany improvement. Figure 4 shows that for allthree sites FAM-N has much greater efficiency than the single-pulseconversion (∼105% more signal) and greater also than FAM-II(by ∼20%). The efficiency is slightly higher than that achievedusing DFS, although it is not clear whether this result arises fromthe more challenging experimental optimization for DFS or from aninherent difference in efficiency. Both FAM-N and SPAM require littleor no experimental reoptimization, ensuring they are easy to implement.The additional optimization time required increases for FAM-II, FAM-I,and DFS experiments, respectively.

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.