nmr signals
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RSC Advances ◽  
2022 ◽  
Vol 12 (3) ◽  
pp. 1479-1485
Author(s):  
Ben. J. Tickner ◽  
Sanna Komulainen ◽  
Sanna Palosaari ◽  
Janne Heikkinen ◽  
Petri Lehenkari ◽  
...  

SABRE hyperpolarisation enhances NMR signals of low concentration nicotine and acrolein and allows quantitative detection in electronic cigarette aerosol solutions.


Colorants ◽  
2021 ◽  
Vol 1 (1) ◽  
pp. 20-37
Author(s):  
Anthony Pagan ◽  
Jong I. Lee ◽  
Jeonghee Kang

Flavylium ions (6a–6e) were synthesized using Aldol condensation to compare the spectroscopic properties caused by the different numbers and locations of hydroxy groups on the flavylium cations (AH+). Without the addition of acid, increasing the concentration of flavylium ions to 10 mM in ethanol led to the following observation. The flavylium ions with the least number of OH groups (6a and 6b) showed a change in peak at higher concentrations, whereas 6c and 6d did not show the same degree of change in their 1H-NMR signals. This suggests an equilibrium that shifts the isomers B, CE, and Cz more towards the flavylium ion with more OH groups even at low concentrations. One possibility for the cause of this shift is that the flavylium ions become more stable through aggregation. In addition to the NMR results, the UV measurements confirmed that isomers with fewer OH groups showed a more dramatic shift towards the flavylium ion at higher concentrations. Using modeling data at DFT//B3LYP/6311**G(d) level, the self-association was investigated to show that the hydrogen bonding of OH groups is the main player but cannot stabilize entirely without the presence of the chloride ion in the complex.


2021 ◽  
Author(s):  
◽  
Muhammad Ali Raza Anjum

<p>Nuclear Magnetic Resonance spectroscopy (NMR) is a powerful technique for rapid and efficient quantitation of compounds in chemical samples. NMR causes the nuclei in the molecules to resonate and various chemical arrangements appear as peaks in the Fourier spectrum of a free induction decay (FID). The spectral parameters elicited from the peaks serve as a fingerprint of the chemical components contained in the molecule. These fingerprints can be employed to understand the chemical structure.  Signal acquired from a NMR spectrometer is ideally modelled as a superposition of multiple damped complex exponentials (cisoids) in Additive White Gaussian Noise (AWGN). The number as well as the spectral parameters of the cisoids need to be estimated for characterisation of the underlying chemicals. The estimation, however, suffers from numerous difficulties in practice. These include: unknown number of cisoids, large signal length, large dynamic range, large peak density, and numerous distortions caused by experimental artefacts.  This thesis aims at the development of estimators that, in view of the above-mentioned practical features, are capable of rapid, high-resolution and apriori-information-free quantitation of NMR signals. Moreover, for the analytic evaluation of the performance of such estimators, the thesis aims to derive interpretable analytic results for the fundamental estimation theory tool for assessing the performance of an unbiased estimator: the Cramer Rao Lower Bound (CRLB). By such results, we mean those that analytically allow the determination, in terms of the CRLB, of the impact of the free model parameters on the estimator performance.  For the CRLB, we report analytic expressions on the variance of unbiased parameter estimates of damping factors, frequencies and complex amplitudes of an arbitrary number of damped cisoids embedded in AWGN. In addition to the CRLB, analytic expressions for the determinant and the condition number of the associated Fisher Information Matrix (FIM) are also reported. Further results, in similar order, are reported for two special cases of the damped cisosid model: the Magnetic Resonance Relaxometry model and the amplitude-only model (employed in quantitative NMR - qNMR). Some auxiliary results for the above-mentioned models are also presented, i.e., on the multiplicity of the eigenvalues and the factorisation of the characteristic polynomial associated with their respective FIMs.  These results have not been previously reported. The reported theoretical results successfully account for various physical and chemical phenomena observed in experimental NMR data, and quantify their impact on the accuracy of an unbiased estimator as a function of both model and experimental parameters, e.g., influence of prior knowledge, peak multiplicity, multiplet symmetry, solvent peak, carbon satellites, etc.  For rapid, high-resolution and apriori-information-free quantitation of NMR signals, a sub-band Steiglitz-McBride algorithm is reported. The developed algorithm directly converts the time-domain FID data into a table of estimated amplitudes, phases, frequencies and damping factors, without requiring any previous knowledge or pre-processing. A 2D sub-band Steiglitz-McBride algorithm, for the quantitation of 2D NMR data in a similar manner, is also reported. The performance of the developed algorithms is validated by their application to experimental data, which manifests that they outperform the state-of-the-art in terms of speed, resolution and apriori-information-free operation.</p>


2021 ◽  
Author(s):  
◽  
Muhammad Ali Raza Anjum

<p>Nuclear Magnetic Resonance spectroscopy (NMR) is a powerful technique for rapid and efficient quantitation of compounds in chemical samples. NMR causes the nuclei in the molecules to resonate and various chemical arrangements appear as peaks in the Fourier spectrum of a free induction decay (FID). The spectral parameters elicited from the peaks serve as a fingerprint of the chemical components contained in the molecule. These fingerprints can be employed to understand the chemical structure.  Signal acquired from a NMR spectrometer is ideally modelled as a superposition of multiple damped complex exponentials (cisoids) in Additive White Gaussian Noise (AWGN). The number as well as the spectral parameters of the cisoids need to be estimated for characterisation of the underlying chemicals. The estimation, however, suffers from numerous difficulties in practice. These include: unknown number of cisoids, large signal length, large dynamic range, large peak density, and numerous distortions caused by experimental artefacts.  This thesis aims at the development of estimators that, in view of the above-mentioned practical features, are capable of rapid, high-resolution and apriori-information-free quantitation of NMR signals. Moreover, for the analytic evaluation of the performance of such estimators, the thesis aims to derive interpretable analytic results for the fundamental estimation theory tool for assessing the performance of an unbiased estimator: the Cramer Rao Lower Bound (CRLB). By such results, we mean those that analytically allow the determination, in terms of the CRLB, of the impact of the free model parameters on the estimator performance.  For the CRLB, we report analytic expressions on the variance of unbiased parameter estimates of damping factors, frequencies and complex amplitudes of an arbitrary number of damped cisoids embedded in AWGN. In addition to the CRLB, analytic expressions for the determinant and the condition number of the associated Fisher Information Matrix (FIM) are also reported. Further results, in similar order, are reported for two special cases of the damped cisosid model: the Magnetic Resonance Relaxometry model and the amplitude-only model (employed in quantitative NMR - qNMR). Some auxiliary results for the above-mentioned models are also presented, i.e., on the multiplicity of the eigenvalues and the factorisation of the characteristic polynomial associated with their respective FIMs.  These results have not been previously reported. The reported theoretical results successfully account for various physical and chemical phenomena observed in experimental NMR data, and quantify their impact on the accuracy of an unbiased estimator as a function of both model and experimental parameters, e.g., influence of prior knowledge, peak multiplicity, multiplet symmetry, solvent peak, carbon satellites, etc.  For rapid, high-resolution and apriori-information-free quantitation of NMR signals, a sub-band Steiglitz-McBride algorithm is reported. The developed algorithm directly converts the time-domain FID data into a table of estimated amplitudes, phases, frequencies and damping factors, without requiring any previous knowledge or pre-processing. A 2D sub-band Steiglitz-McBride algorithm, for the quantitation of 2D NMR data in a similar manner, is also reported. The performance of the developed algorithms is validated by their application to experimental data, which manifests that they outperform the state-of-the-art in terms of speed, resolution and apriori-information-free operation.</p>


2021 ◽  
Author(s):  
◽  
Sally Jane Davenport

<p>Carbon-13 CP/MAS NMR was used to study a selection of fifty-seven New Zealand coals and ten Australian coals. The coal rank varied from lignite to semianthracite. A qualitative survey of the plant origins of NMR signals was followed by an EPR study of the unpaired spin-species in coal. The quantitative reliability of the NMR response of coal was analysed in relaxation and "visibility" studies. Different approaches to the problem of accounting for intensity in spinning-side-band (SSB) signals were assessed. The most successful approach was found to be the complete computer simulation of the spectrum from combinations of SSB intensity patterns broadened by a mixture of Lorentzian and Gaussian lineshapes. This method of analysis produced oxygen contents that showed a good correlation with oxygen contents (by difference) from Ultimate analysis. The resultant carbon, hydrogen and oxygen functional group analyses allowed considerable insight into the depositional influences on, and alteration of, the coal structure with increasing degree of coalification.</p>


2021 ◽  
Author(s):  
◽  
Sally Jane Davenport

<p>Carbon-13 CP/MAS NMR was used to study a selection of fifty-seven New Zealand coals and ten Australian coals. The coal rank varied from lignite to semianthracite. A qualitative survey of the plant origins of NMR signals was followed by an EPR study of the unpaired spin-species in coal. The quantitative reliability of the NMR response of coal was analysed in relaxation and "visibility" studies. Different approaches to the problem of accounting for intensity in spinning-side-band (SSB) signals were assessed. The most successful approach was found to be the complete computer simulation of the spectrum from combinations of SSB intensity patterns broadened by a mixture of Lorentzian and Gaussian lineshapes. This method of analysis produced oxygen contents that showed a good correlation with oxygen contents (by difference) from Ultimate analysis. The resultant carbon, hydrogen and oxygen functional group analyses allowed considerable insight into the depositional influences on, and alteration of, the coal structure with increasing degree of coalification.</p>


2021 ◽  
Author(s):  
◽  
Sally Jane Davenport

<p>Carbon-13 CP/MAS NMR was used to study a selection of fifty-seven New Zealand coals and ten Australian coals. The coal rank varied from lignite to semianthracite. A qualitative survey of the plant origins of NMR signals was followed by an EPR study of the unpaired spin-species in coal. The quantitative reliability of the NMR response of coal was analysed in relaxation and "visibility" studies. Different approaches to the problem of accounting for intensity in spinning-side-band (SSB) signals were assessed. The most successful approach was found to be the complete computer simulation of the spectrum from combinations of SSB intensity patterns broadened by a mixture of Lorentzian and Gaussian lineshapes. This method of analysis produced oxygen contents that showed a good correlation with oxygen contents (by difference) from Ultimate analysis. The resultant carbon, hydrogen and oxygen functional group analyses allowed considerable insight into the depositional influences on, and alteration of, the coal structure with increasing degree of coalification.</p>


2021 ◽  
Author(s):  
◽  
Sally Jane Davenport

<p>Carbon-13 CP/MAS NMR was used to study a selection of fifty-seven New Zealand coals and ten Australian coals. The coal rank varied from lignite to semianthracite. A qualitative survey of the plant origins of NMR signals was followed by an EPR study of the unpaired spin-species in coal. The quantitative reliability of the NMR response of coal was analysed in relaxation and "visibility" studies. Different approaches to the problem of accounting for intensity in spinning-side-band (SSB) signals were assessed. The most successful approach was found to be the complete computer simulation of the spectrum from combinations of SSB intensity patterns broadened by a mixture of Lorentzian and Gaussian lineshapes. This method of analysis produced oxygen contents that showed a good correlation with oxygen contents (by difference) from Ultimate analysis. The resultant carbon, hydrogen and oxygen functional group analyses allowed considerable insight into the depositional influences on, and alteration of, the coal structure with increasing degree of coalification.</p>


Author(s):  
Kouqi Liu ◽  
Zhijun Jin ◽  
Lianbo Zeng ◽  
Yujie Yuan ◽  
Mehdi Ostadhassan

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