NMR Spectroscopy Flashcards
What is NMR spectroscopy?
Uses radio-frequency electromagnetic radiation to cause transitions between the Zeeman energy levels of nuclei in a magnetic field.
What is spin?
A form of angular momentum possessed by all elementary particles, including protons + neutrons.
What type of spin do protons + neutrons possess?
Nuclear spin (I).
How do we expresses spin orientation?
mi
What does I determine?
Whether a nucleus will give an NMR signal.
What does I depend on?
The number of protons + neutrons. If I = 0 then the atom won’t give an NMR signal.
In the absence of a magnetic field, the two spin levels will have the same energy. What causes spin levels to split energy?
Application of a magnetic field (Bo);
If spin lines up WITH the direction of the magnetic field, it will go to lower energy + be more stable.
If spin lines up AGAINST the direction of the magnetic field, it will go to higher energy + be more unstable due to repulsion.
This is the Zeeman effect.
What would give bigger energy separations?
A bigger magnetic field.
State the equation linking energy separation and magnetic field strength.
∆E = h§Bo/2pi = hv
What does § indicate + what are its units?
§ = gamma
Represents magnetogyric ratio, indicates how magnetic a nucleus is.
Units = rad s^-1 T^-1
What does v indicate?
v = resonance frequency
Differs between nuclei therefore we can only look at one at a time i.e. C13 or H1
Frequency v will induce transitions between the two states.
How does NMR spectroscopy work?
A super conducting magnet maintained at liquid helium temperature by a surrounding vessel containing liquid helium.
This vessel is further surrounded by a vessel of liquid nitrogen to prevent helium evaporating.
What is sigma and what does it measure?
Sigma is the shielding constant, it measures how different the frequency is for each nucleus.
What causes the frequency of a nucleus to differ?
Nuclei resonate at different frequencies depending on their chemical environment i.e. what the surrounding atoms are.
What is shielding?
Orbiting electrons around the nucleus set up a magnetic field which opposes the applied magnetic field, creating shielding.
State the equation for shielding.
v = (§Bo)/2pi (1-sigma)
How do we obtain data using NMR? (3 bullet points).
- use a magnetic field (Bo) to separate energy levels by the Zeeman effect
- use electromagnetic radiation to induce transitions between states
- a transition will occur when the oscillating frequency of the electromagnetic radiation matches v (resonance frequency)
How are we able to compare spectra of different magnetic field strengths?
Vref (TMS), has very high shielding due to presence of many CH3 groups.
State the equation for the scale of chemical shift (§).
§ (ppm) = 10^6 (Vresonance - Vref) / Vref.
What happens when we increase the strength of Bo?
The separation energies and frequencies increase.
What gives the signal in C13 NMR?
The C nuclei.
What gives the signal in H1 NMR?
The protons.
In C13 NMR, what does increasing electronegativity cause?
Increases the frequency of resonance of C13 nucleus.
State the factors of shielding in C13 NMR (3).
- The more H atoms on a carbon, the greater the shielding.
- Adjacent electronegative atoms have a de-shielding effect.
- Unsaturation (C=C) has a de-shielding effect.
In H1 NMR, what has the lowest chemical shift?
CH3.
State the factors of shielding in H1 NMR (1).
- Adjacent electronegative atoms have a de-shielding effect.
In H1 NMR, what does the peak intensity tell us?
The area under each resonance signal is proportional to the number of H1 nuclei giving rise to the signal.
In H1 NMR, how can we use peak intensity?
To create an integer ratio of H atoms.
In C13 NMR, what is the effect of symmetry?
C atoms in exactly the same environment will give peaks at exactly the same chemical shift.
State what is mean by coupling (HINT: peak splitting).
When nuclear spin energy levels are perturbed by nearby nuclear spins, splitting NMR transitions into multiplets.
How does coupling occur in C13 NMR?
Between the C13 and H1 nuclei of hydrogens attached to the same carbon.
How does coupling occur in H1 NMR?
Between H1 nuclei on adjacent carbon atoms, NOT the same carbon atom.
State what is meant by the “n+1” rule in C13 NMR. Give an example.
A C13 atom bonded to n H atoms will give rise to n+1 lines. For CH3, n = 3 and the signal will be split into n+1 fragments = quartet.
State what is meant by the “n+1” rule in H1 NMR. Give an example.
The signal of a H1 atom adjacent to n equivalent hydrogen atoms will give n+1 lines. For CH - CH3, the CH signal will be split into n+1 fragments = quartet as the adjacent C atom has 3 hydrogens attached.
