Lecture 2: Basic Concepts and 1D NMR Spectroscopy Flashcards
Know the definition of chemical shifts. In chemical shift unit, what does ppm mean?
In NMR, the chemical shift is the resonance frequency of a nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule.
Know that chemical shift is a tensor and is magnetic field dependent. Why can we represent it using a value under aqueous condition?
-In solution NMR, the anisotropy of the chemical shift is averaged out by fast molecular tumbling and only a single isotropic chemical shift value is observed.
-THE CHEMICAL SHIFT TENSOR:
~the electronic environment around the nucleus is generally anisotropic–it is not spherical. For this reason, the chemical shift is also anisotropic and changes as the orientation changes of a molecule with respect to magnetic field changes.
How does shielding impact the chemical shift? Know the different sources of chemical shift origins.
-When an external magnetic field is applied to a molecule, it causes the electrons to circulate throughout their atomic orbitals which induces a small magnetic field that opposes the applied field–thus shielding the nucleus from the external field.
How does the inductive effect impact chemical shift?
The effect on electron density in one portion of a molecule due to electron-withdrawing or electron-donating groups elsewhere in the molecule. This ultimately causes a decrease in the magnitude of shielding.
What is the ring effect, how does it affect the chemical shifts of interior and exterior protons respectively?
-Molecules within the inside of the ring induce a magnetic field that opposes the external magnetic field (and so are shielded), while those nuclei that are exterior of the ring will induce a magnetic field that is parallel to the external field (and so become less shielded, and more downfield).
Know the definition of the B1 field
B1 labels the field produced by the radio frequency coil. The B1 field is often conceived of two vectors rotating in opposite directions, usually in a plane transverse to B0
What is FID? After Fourier transformation, what information does a FID signal give?
If the magnetization vector has a nonzero component in the XY plane, then the precessing magnetization will produce an oscillating voltage in the coil surrounding the sample. As the M in the XY plane decreases, so does the NMR signal; that is, a free induction decay.
In the 1D 1H NMR spectrum of a protein, what are the features for an unfolded protein? What are the features for a folded protein?
- A FOLDED protein generally gives NMR signals of 8.7 ppm or larger for backbone NH in water, and signals of less than 0.5 ppm for CH3.
- An UNFOLDED protein is featured with narrow range of backbone NH (8.1-8.5 ppm) in water and CH3 (0.8-2 ppm) chemical shifts.
