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Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from [5] =, where ν sample is the absolute resonance frequency of the sample, and ν ref is the absolute resonance frequency of a standard reference compound, measured in the same applied magnetic field B 0.
Examples of electron withdrawing substituents are −OH, −OCOR, −OR, −NO 2 and halogens. These cause a downfield shift of approximately 2–4 ppm for H atoms on C α (an aliphatic C atom directly bonded to the substituent in question) and of less than 1–2 ppm for H atoms on C β (an aliphatic C atom bonded to C α).
The hyperpolarization creates a large signal intensity, and the wide range of chemical shifts allows for identifying when the 129 Xe associates with molecules like hemoglobin. 129 Xe is preferred over 131 Xe for MRI because 129 Xe has spin 1/2 (compared to 3/2 for 131 Xe), a longer T1, and 3.4 times larger gyromagnetic ratio (11.78 MHz/T). [3]
Fast spin echo (RARE, FAISE or FSE [10] [11] [12]), also called turbo spin echo (TSE) is an MRI sequence that results in fast scan times. In this sequence, several 180 refocusing radio-frequency pulses are delivered during each echo time (TR) interval, and the phase-encoding gradient is briefly switched on between echoes. [ 13 ]
Another approach uses the chemical shifts to generate angle restraints. Both methods use the fact that the geometry around the alpha carbon affects the coupling constants and chemical shifts, so given the coupling constants or the chemical shifts, a qualified guess can be made about the torsion angles.
In vivo magnetic resonance spectroscopy (MRS) is a specialized technique associated with magnetic resonance imaging (MRI). [1] [2]Magnetic resonance spectroscopy (MRS), also known as nuclear magnetic resonance (NMR) spectroscopy, is a non-invasive, ionizing-radiation-free analytical technique that has been used to study metabolic changes in brain tumors, strokes, seizure disorders, Alzheimer's ...
Example of chemical shift index. The chemical shift index or CSI is a widely employed technique in protein nuclear magnetic resonance spectroscopy that can be used to display and identify the location (i.e. start and end) as well as the type of protein secondary structure (beta strands, helices and random coil regions) found in proteins using only backbone chemical shift data [1] [2] The ...
Shoolery's rule, which is named after James Nelson Shoolery, is a good approximation of the chemical shift δ of methylene groups in proton nuclear magnetic resonance.We can calculate shift of the CH 2 protons in a A–CH 2 –B structure using the formula