NMR Lecture 2 Flashcards
Define chemical shifts. What does PPM mean?
Chemical shift: the difference in the resonance frequency of a nucleus in a magnetic field compared to a reference standard
ppm = parts per million
- in calculating chemical shift, v-v(ref) is super small, so we have to amplify by a million (ppm)
Why is a chemical shift tensor and magnetic field dependent? Why can we represent it using a value under aqueous condition?
The tensor has 3 components corresponding to the x, y, and z axes. The chemical shift tensor arises because the interaction between the nucleus and the external magnetic field is not uniform in all directions.
As the strength of the magnetic field changes, the interaction between the nuclear magnetic moments and the external field also changes, influencing the chemical shift.
Compounds don’t stay still, so by using a reference compound simplifies the interpretation of NMR spectra and facilitates the identification of the different chemical environments in a sample.
How does shielding effect impact the chemical shift? Know the different sources of chemical shift origins.
Electrons move to generate a secondary field (B’) that opposes the stronger field.
As shielding constant increases, so does the effect.
1) Electronic environment: different functional groups or molecular structures lead to distinct chemical shifts
2) Hybridization and bonding: sp3 exhibit lower chemical shifts than sp2 due to differences in electron density distribution.
3) Hydrogen bonding: leads to deshielding effects, causing downfield shifts in protein NMR spectra.
How does inductive effect impact chemical shift?
Electronegative elements pull electron density away from the hydrogen nucleus, decreasing the magnitude of the shielding field.
Ex. Chlorine will withdraw electron density from the central atom to deshield and causing a downfield shift in the NMR spectrum.
What is the “ring” effect? How does it affect the chemical shifts of interior and exterior protons respectively?
The circulation of electrons create a magnetic field that can reinforce or oppose the external magnetic field applied during NMR.
Interior protons: typically deshielded. It causes a downfield shift in the NMR spectrum and the nucleus experiences a higher effective magnetic field due to the deshielding effect.
Exterior protons: a magnetic field opposes the external magnetic field, leading to shielding of these exterior protons. This results in an upfield shift in the NMR spectrum, meaning that the nucleus experiences a lower effective magnetic field due to the shielding effect.
Which two sections does Fourier 1D NMR experiment consist of?
Pulse sequence: intense burst of radiofrequency radiation (B1) is applied ot the xy plane. It causes magnetic field to process toward the xy plane
Fourier Transform: Signals emitted by the sample are detected by the NMR spectrometer. These signals, (FIDs) contain information about the nuclear spin interactions. Fourier transformation converts the time-domain signals into frequency-domain spectra.
What’s the difference between B0 and B1 field?
B0: static magnetic field
B1: magnetic field generated by an RF pulse
What is FID? After Fourier transformation, what information does a FID give?
FID: Free induction Decay - obtained during NMR. When a sample is subjected to an RF pulse in a NMR experiment, the excited nuclear spins precess in the external magnetic field.
Fourier transformation (converts time-domain signal into frequency-domain spectrum) results in the NMR spectrum.
- chemical shifts
- coupling constants
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?
Unfolded protein: narrow range at 8.1-8.5 ppm for NH and 0.8-2 ppm for CH3
- broad peaks = increased conformational flexibility and dynamic motion
- lacks well-defined peaks = lack of stabilized structures
Folded protein: 8.7ppm or larger for NH and 0.5 ppm for CH3
- sharp and well-defined peaks in the NMR = more rigid and ordered structures
- broader chemical shift dispersion
- narrow line widths = reduced mobility and increased structural order
