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Nuclear magnetic resonance spectroscopy
Published in D. Campbell, R.A. Pethrick, J.R. White, Polymer Characterization, 2017
D. Campbell, R.A. Pethrick, J.R. White
Spin–spin coupling is normally only effective between nuclei separated by one or two covalent bonds. The magnitude of the spin–spin coupling decreases with increasing bond separation and is characterized by the coupling constant J. Coupling constants of nuclei which have very different chemical shifts are, by convention, expressed as JAX, whereas for nuclei with chemical shifts of the same magnitude the coupling constants are expressed by JAB. Coupling constants are expressed in units of frequency (Hz) and typical values of 1H–1H coupling in organic molecules lie in the range 2–20 Hz. The magnitude of spin–spin coupling constants depends on the chemical nature of the interacting nuclei and this sensitivity to structure is one of the principal advantages of NMR in that it provides a means of identifying particular chemical groups in molecules. Coupling constants are also sensitive to differences in conformations of molecules and depend on the rates of bond rotations and so may also be used to provide information on molecular motions in polymers.
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Published in Mara Cercignani, Nicholas G. Dowell, Paul S. Tofts, Quantitative MRI of the Brain: Principles of Physical Measurement, 2018
Some brain metabolites, such as glutamate, have resonances split into several small lines, making them appear as multiplets in the spectrum. This phenomenon is referred to as J-coupling (spin–spin coupling) and is caused by the interaction between electrons and the adjacent nucleus through a small number of chemical bonds. Unlike chemical shift, J-coupling is independent of the applied magnetic field strength and may make the quantification of 1H spectra more complex. The presence of multiplets may be valuable for identifying the signatures of particular chemical species and can be used in conjunction with more advanced spectral editing strategies to separate metabolites with overlapping peaks. This is discussed in Section 12.4.4.
Spectroscopy and Spectroscopic Imaging
Published in Christakis Constantinides, Magnetic Resonance Imaging, 2016
This phenomenon gives rise to the multiplet structure of spectra. Such a coupling arises from interactions between nuclei that cause energy level splittings, giving rise to several transitions instead of the single energy transition. Spin–spin coupling is an intramolecular effect (a through-bond effect). The coupling constant J (in Hz) denotes the frequency difference of the different multiplet peaks, and it is independent of the external magnetic field Bo. A first-order analysis can be used to predict the spectral multiplet pattern if Δf >> J and the nuclei (or groups of nuclei involved) are both chemically and magnetically equivalent.
Reactivity trends for mechanochemical reductive coupling of aryl iodides
Published in Green Chemistry Letters and Reviews, 2023
Courtney Carson, Joshua Hassing, Trinity Olguin, Karl P. Peterson, Rebecca A. Haley
Chemicals for this research were purchased from Matrix Scientific (4-iodobenzotrifluoride), Sigma Aldrich (1-iodo-3,4-dimethylbenzene, 1-iodo-3,5-dimethylbenzene, N,N-dimethylformamide, acetonitrile), Eastman Chemical Company (2,2′-bipyridine), TCI (4-iodotoluene), Acros Organics (iodobenzene, dimethyl carbonate), American Elements (Mn shot), LabGuard (NiCl2), and ESPI Metals (Ni powder) and used without further purification. Mechanochemical reactions were done in a Spex8000M Mixer Mill with 15-mL, stainless-steel Form-Tech Scientific jars. Ball bearings used were 3/16″ diameter and 316 stainless-steel alloy from Grainger. A ThermoFisher Scientific Sorvall ST 8 Centrifuge was used to separate the Mn powder created in the reaction. 1H and 13C nuclear magnetic resonance (NMR) spectra were taken on a Bruker Avance III 400 MHz spectrometer (400 and 100 MHz, respectively) using CDCl3 solvent from Sigma Aldrich. Chemical shifts are reported as δ (ppm) and referenced to TMS. Spin–spin coupling constants are recorded as J (Hz) values.
Nuclear spin-spin coupling constants prediction based on XGBoost and LightGBM algorithms
Published in Molecular Physics, 2020
Xin-xin Zhang, Tong Deng, Guo-zhu Jia
The measurement of spin–spin coupling constants is the key to using NMR to explore complex molecular structure information. In this paper, two excellent machine learning algorithms are used to predict the spin–spin coupling constant between two atoms in a molecule. The LightGBM (R2: 0.93) algorithm shows a better performance than XGBoost. Moreover, the predictive determinant of partial molecules has been improved. XGBoost has R2 of 0.99 and LightGBM is 1.00. This study enriches the calculation methods of spin–spin coupling constants. Machine learning algorithms can greatly reduce the complexity and high cost of quantum mechanical computation, thus helping chemists use NMR to obtain molecular structure information. However, the prediction fitting degree of many molecules in this experiment does not reach the accuracy of quantum mechanics. Next work, based on atomic coordinates, more algorithms will be used to predict some molecules in order to improve their calculation results.
Computations of the chirality-sensitive effect induced by an antisymmetric indirect spin–spin coupling
Published in Molecular Physics, 2018
The spin–spin coupling between two nuclear spins (I and S) can be separated into two contributions: the direct coupling (the dipole–dipole interaction) and the indirect coupling (the interaction via electrons of the molecule). The Hamiltonian of this spin system, , truncated to interactions only between spins I and S is [12] where h is Planck's constant, while and are spin operators.