Nuclear Magnetic Resonance Flashcards
Study of interactions between ___ and ___
Atomic nuclei
Radio waves
Most commonly used to study ___ nuclei
Hydrogen
This method applies to nuclei with odd ___ or ___ numbers because …
Atomic
Mass
Because both possess nuclear spin whereby a spinning proton generates a magnetic field that can be measured
What are the 2 ways the magnetic field created by the spinning proton can behave when exposed to an external magnetic field?
Can align with the field or against the field
Aligned with = alpha state, lower energy
Aligned against = beta state, higher energy
How does strength of the external magnetic field affect the difference in energy between alpha and beta states?
The stronger the magnetic field (external) the greater the difference in energy between these 2 states
What is NMR actually detecting?
When these nuclei release photons of light equal to the difference between alpha and beta states, a nuclei whose spin is aligned with the external field can flip to align against it. When the nuclei flips back to re align with the field, it emits electrical impulses that are detected by an NMR spectrometer.
What is held constant during NMR? What is varied? Why?
Magnetic field frequency is held constant, magnetic field strength is varied.
Protons within a compound experience different electron densities, and electron density near a proton can shield the proton from the magnetic field. Thus, protons within the same compound may absorb electromagnetic energy at the same frequency but at different magnetic field strengths.
On an NMR graph, the x acid represents field strength. In what direction does magnetic field increase?
What is the left side of the graph called? The right?
Left to right
Downfield (unshielded)= left
Upfield (shielded) = right
What does each peak represent?
One or a group of chemically equivalent hydrogens
What is the area under the peak proportional to?
The number of hydrogen atoms represented by that peak
What is splitting? What is the time?
Let’s say I have a hydrogen bound to the 1st carbon in a molecule. That carbon is then bound to 2 chlorine’s and carbon 2. Carbon 2 is then bound to 1 hydrogen and 1 chlorine and carbon 3.
The hydrogen attached to C1 is not chemically equivalent to the hydrogen bound to C2. Thus, the hydrogen bound to C1 is split into n+1 peaks, where n = the number of adjacent chemically inequivalent hydrogens (in this case n=1).
Ha - CCl2 - CClHb- …
How do electron withdrawing / donating groups contribute to the spectrum?
Electron withdrawing = makes the proton less shielded and moves the peak downstream (to the left)
Electron donating = makes the proton more shielded and moves the peak upstream (to the right)
