NMR Flashcards
What does NMR stand for?
Nuclear Magnetic Resonance
What is the purpose of the NMR?
to determine the structure of the molecule in question
How does an NMR work?
a sample is dissolved and put into a NMR tube which is places in a conducting magnetic instrument. the probe zaps the NMR tube with radio waves and the sample absorbed the radio waves. they eventually emit them back and that is detected by the probe.
Can you add a radio frequency photon to a nuclei to raise it to a higher energy state? why or why not?
yes, only with an external magnetic field. the external magnetic field allows for the nuclei to be raised from a lower energy level (alpha state, +1/2) to a higher energy state (beta state, -1/2)
Are the nuclei, alpha and beta states, ever the same?
yes, only when they are not in an external magnetic field.
Can the energy state every change in size?
yes, the energy state can have a larger difference between the alpha and beta states based on if the magnet used externally is larger.
the stronger the magnet, the greater the distance between the alpha and beta states.
change of energy of spin is proportional to B external magnetic field
What happens when the nuclei returns from the beta state to the alpha state?
radio frequencies are emitted and are detected to create signals
How does NMR, nuclei absorption, help us determine structures?
not all nuclei absorb the same frequency, every nuclei absorbs at different frequencies and these help you determine the structure
Are all nuclei the same? active?
No. not all nuclei are the same and not all nuclei are active. in order for an NMR to be active, you have to have an odd number of protons. e.g. hydrogen, 13-carbon, fluorine (?)
Explain heterotopic and homotopic protons. (rule?)
electrons in different environments (chemical environments) absorb at different frequencies
when protons are in different chemical environments, you call them chemically distinct protons or heterotopic protons
when protons are in the same chemical environment, you call them chemical equivalent protons or homotopic protons
RULE: methyl hydrogens will always be equivalent.
What is the replacement test?
the replacement test takes a molecule and replaces 1 hydrogen with and X. then takes the other molecule and replaces the other hydrogen with an X. you then rearrange the molecules to determine the relationship with each other.
homotopic - equivalent, 1 signal; identical
enantiotopic - equivalent, 1 signal; rule: there are always equivalent in achiral environments, but if it is in a chiral environment, there will be 2 signals (ALWAYS ASSUME ACHIRAL ENVIRONMENT, UNLESS SAID OTHERWISE)
diastereotopic - non-equivalent, 2 signals
define diastereomers, enantiomers, meso compounds, chiral, and achiral.
Diastereomers - non-superimposable and non-mirror images
Enantiomers - non-superimposable, mirror images
Meso compounds - a chiral molecule with a plane of symmetry
Chiral - compounds with 4 different molecules attached to the carbon
Achiral - compounds without 4 different molecules attached to the carbon
Cyclohexane has 12 hydrogens and 1 signal even with axial and equatorial hydrogens. How is this possible?
has to do with timing. we know chair flips occur but they happen faster than the NMR takes a “picture” of the chair flip. therefore the NMR takes the average of the chair flips, giving 1 signal.
How does a group of methyl hydrogens only give off 1 signal even though some are close to oxygens for example in the molecule?
because the bond spins so fast and at such a high speed, the NMR just takes the average and gives 1 signal
What are deuterated solvents for NMR samples and why are they used?
Deuterated solvents are regular solvents like chloroform, DMSO, water, and methanol. But they have the hydrogens replaced with deuterium so the NMR doesn’t get overwhelmed with all the NMR active hydrogens. Deuterium is not active in NMR so they will not give off a signal and you can focus your attention to the hydrogen in question.
You will sometimes still get a small signal in a random place, this is because not 100% of the deuterated solvent becomes deuterated. You may still get trace amounts of whatever solvent you use with hydrogen and that gives off a signal.
What causes chemical shifts?
They are governed by the electrons surrounding the nucleus
What is the local magnetic field (Bloc)?
The local magnetic field is generated since the electrons are moving fast in an external magnetic field.
Local is always in opposition to external magnetic field
the local magnetic field shields the proton from the feeling the external magnetic field gives, doesn’t allow the full effect of the external magnetic field
more electron density means more Bloc
What is the effective magnetic field (Beff)?
the effective magnetic field is the magnetic field that the proton ACTUALLY feels
What is shielding?
when there is electron density around the proton, and it shields the proton from the external magentic field
why you can have protons resonating at different resonance frequencies, because not every proton has the same amount of electrons around it
shielded = more electron density
deshielded = less electron density
use it in comparison to each other
How do you calculate Beff?
Beff = Bext + Bloc
Bext and Bloc will always be opposite signs (+ and -)
What does a greater Bext do to the Alpha and Beta states?
if you feel more external magnetic field, you will have a larger gap between Alpha and Beta states
the beta state will then require a higher energy photon of light to make the nuclei resonate, so it will make a higher chemical shift
What are two ways that chemical shifts happen?
Inductive Effects and Magnetic Anisotropy
What happens to the chemical shifts as the electronegativity of an atom gets larger?
the chemical shift becomes larger because you are deshielding to a greater extent as you use more electronegative molecules
What does distance from the electronegative atom do to the protons on the molecule?
the proton (H atom) that is farthest from the electronegative atom, or the magnetic anisotropy, will feel the chemical shift less than the one that is closer to the electronegative atom
it plays a role in which proton is more downfield than others
What is magnetic anisotropy?
magnetic anisotropy is a situation where in benzene, alkenes, or alkynes, the ring structure or the pi electrons create a local magnetic field that causes the electrons to move in a circular fashion
when an external magnetic field is added to the molecules, the external and local fields on the outside of the molecule are pointed in the same direction. which makes the two additive.
this situation explains the big chemical shifts in molecules that only have carbons and hydrogens. no highly electronegative atoms.
Why do aldehydes feel massive deshielding?
magnetic anisotropy deshields hydrogen nuclei associated with simple alkenes and aldehydes
the proton is very distinct (chemical shift around 9-10 ppm)
What are shielding cones and how does it affect chemical shifts?
alkynes have pi bonds but the chemical shifts are only 2.4, which is low compared to the other molecules. this is because of the cylindrical shape and the symmetry in the molecule.
a shield cone becomes generated by the local magnetic field lines and the shield cone lowers the chemical shift in the protons.
whereas in alkenes, the shape is not as cylindrical, therefore there are some hydrogens that are not in the shielding cone = higher chemical shift
Is chemical shifts additive?
yes, the more electronegative atoms you add to a molecule, the more larger chemical shift you will see in the proton.
When looking at splitting factors of a hydrogen, when do you add all adjacent carbons and when do you not?
When adjacent carbons have equal protons, you add all the protons together for the splitting factor. If they are not equal protons, they you have either or in the frame of:
triplet of quartet/quartet of triplet
If you have two protons that are different signals but on the same carbon, how does that affect proton splitting?
the different protons would add to the splitting
e.g. quartet of a doublet doublet
What is a complex splitting factor?
That is when the proton signaling has carbons adjacent to it with different protons that have different signals. So you write them both together instead of choosing one.
How does a stronger magnet affect NMR spectrums?
The stronger magnets cause a higher resolution of the NMR spectrum allowing you to see signals more clearly
What is the J coupling constant? What can it be used for?
It is the frequency difference between the peaks on a split signal, it remains unchanged no matter the power of magnet used
if 2 protons are coupled, they will have the same J coupling constant, so you can use that on the NMR spectrum to see if protons are coupled or not
REMEMBER: j coupling constants - trans = 13-18; cis = 7-12; geminal = 1-3
Why does 13C NMR Spectroscopy work?
because 13 is an odd number and that it has 1/2 (+/-) spin rates just like hydrogen
the abundance of carbon is 12C (98.9%), therefore the 13C has a low abundance and majority of carbon is not active in an NMR spectrum. This causes 13C NMR to take much longer than regular NMR spectroscopy with hydrogen
How is the noise cleaned up in 13C NMR? any extra signals?
13C NMR has a lot of noise, in order for it to be cleaned up, it performs signal averaging which gives a clearer view of the NMR spectrum
you will have a solvent and TMS signal that you don’t account for
Do you see splitting in 13C NMR?
Nope. Only singlets
Explain 13C NMR chemical shifts.
The shifts are much larger than regular NMR.
methyl - 10-25
methylene - 20-45
methyne - 25-45
quat - 30-35
carbonyls - 190-220
Does 13C NMR have any integration?
no, the height of the signals tell us nothing about how many carbons there are.
What is DEPT Spectroscopy?
DEPT only shows signals if carbons have a hydrogen attached (carbonyl in ketone will not show)
they also signals on two different sides facing different ways. Therefore the top half signals are methyne or methyl.
