Organic NMR Spec

Question 1

NMR looks at transitions between nuclear energy levels. What determines this property?

Answer 1

Nuclear spin, l, which can take values of 0,1/2,1,3/2 etc.

Question 2

What is l dependent on? What do the values of l mean?

Answer 2

l is dependent on the specific nucleus. If l=0 then it is spin silent, if l > 0 then it is spin ctive and NMR spectrum can be obtained

Question 3

What is the Zeeman effect?

Answer 3

degeneracy of energy levels is lifted, and they split into many states labelled Ml.

Question 4

What is the formula for how many nuclear energy levels there will be based on its spin value

Answer 4

Will have 2l+1 levels, so if l=1/2 there will be 2 dfferent states, Ml= +1/2, Ml=-1/2

Question 5

What we do we actually measure in NMR spectroscopy?

Answer 5

We measure the energy required to flip to the lowest energy state, spin aligned, from the highest energy state, spin opposed, using light

Question 6

What is the selection rule for NMR?

Answer 6

(the selection rule for NMR spectroscopy and nuclear energy level transitions is ΔmI = ± 1 )

Question 7

What is the energy required to bring about this transition? What is the resonant frequency of nucleus, the Lamor Frequency?

Answer 7

ΔE = hν, where
𝑣 = 𝛾𝐵 / 2pi
𝛾 is the gyromagnetic ratio, how susceptible that specific nucleus is to the magnetic field, B is the magnetic field strength

Question 8

What is shielding?

Answer 8

Shielding is where there is a high electron density surrounding a nucleus that ‘screens’ the applied magnetic field, basically lowering its effect on the nucleus and lowering its resonant frequency

Question 9

What is deshielding?

Answer 9

Deshielding is where there is less of a electron density around the nucleus making it more susceptible to the magnetic field so it will feel it stronger and will have a higher resonant frequency

Question 10

What effects deshielding and shielding?

Answer 10

Electronegativity of surrounding groups and the number of them, the nature of bonding, sp2 vs sp3 etc. and ring currents

Question 11

What does chemically equivalent mean? What determines the number of signals in a spectrum?

Answer 11

Chemical equivalent means two molecules that have the same chemical environment in a molecule, eg. C-H3 in a methyl group, the Hydrogens would be chemically equivalent. The number of signals is determined by the number of different chemical environments in the molecule

Question 12

Why do we use chemical shift and not actual resonant frequency?

Answer 12

Because people use different magnetic fields on different machines all around the world which will change the frequencies observed for the same molecule so they use chemical shift which is relative to a known standard

Question 13

What is the equation for chemical shift in ppm

Answer 13

𝛿 (𝑝𝑝𝑚) =
(𝑣𝑠𝑎𝑚𝑝𝑙𝑒−𝑣𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒(𝐻𝑧)) ///𝑣𝑠𝑝𝑒𝑐𝑡𝑟𝑜𝑚𝑒𝑡𝑒𝑟(𝑀𝐻𝑧)

Question 14

What does the integral of a signal show?

Answer 14

The integration of a signal in the NMR spectrum tells you the relative number of nuclei responsible for that signal

Question 15

Why does coupling occur?

Answer 15

Because nuclei A spin can be influenced by the spin on nuclei B as it can align with and against it so the magnetic field experienced by A could be slightly stronger or weaker
depending on the alignment of the spin on B, and hence A will be seen to resonate at two different frequencies / chemical shifts. The signal for A will be split into two signals

Question 16

What is the rule for the number of lines observed in a multiplet? What are the relative intensities of each line in the multiplet determined by?

Answer 16

In general, the number of lines seen in a ‘multiplet’ for nucleus A coupling to n × B nuclei,
is given by 2nI + 1 (where n is the number of nuclei being coupled to, and I is the nuclear spin
of the nucleus being coupled to).
The number of lines observed in a multiplet tells you the number of nearby equivalent
nuclei that are coupling to the nucleus producing that signal. The different multiplets (number of lines) have different names and relative intensities of
the lines produced, as determined by Pascal’s Triangle

Question 17

Is coupling mutual?

Answer 17

Yes, if A couples to B then B must also couple to A

Question 18

How do you measure the strength of coupling between nuclei?

Answer 18

Coupling occurs through bonds; the stronger the bonds and the closer the nuclei, the stronger the interaction between the spin states and the stronger the coupling is

Question 19

What effect does the strength of coupling have on spectra

Answer 19

The stronger the coupling, the bigger the separation between peaks in a multiplet. The difference between the peaks is called the coupling constant and is given the label
nJAB, where n is the number of bonds separating the two nuclei, A and B, and J indicates this is scalar coupling (coupling through bonds)

Question 20

Equation for the coupling constant

Answer 20

Coupling constants are calculated using the expression:
J (Hz) = Δδ (ppm) × νspectrometer (MHz)
The strength of coupling is greater for nuclei that are closer to each other, hence 1J > 2J > 3J

Question 21

Can different spin active nuclei couple together?

Answer 21

Yes eg. F and H can couple to one another

Question 22

What are the 6 main factors affecting chemical shift from an ‘organic perspective’.

Answer 22

Electronegativity
Effects of hybridisation
Aromatic/circulating electrons
Resonance (Mesomeric) effects
Hydrogen bonding
Solvent, temperature, and concentration effects

Question 23

How does electronegativity effect chemical shifts?

Answer 23

A greater electronegativity of the nearby group to a nucleus will cause its
signal to appear at higher chemical shifts. Larger electronegativity will pull more electron density away from the nucleus
(deshield), expose it the external magnetic field, and so will resonate at higher
frequencies and the signal will appear at higher chemical shifts

Question 24

How does the number of electronegative groups affect chemical shifts?

Answer 24

A greater number of electronegative groups that are nearby a nucleus will
cause its signal to appear at higher chemical shifts.
More electronegative groups will deshield the nucleus to a greater extent,
and so the signal will appear at higher chemical shifts.

Question 25

How does distance away from electronegative groups affect chemical shift?

Answer 25

At greater distances from electronegative groups (more bonds away), the
signal for the nucleus will appear at lower chemical shifts.
The influence of electronegative group weakens the further away it is from a
nucleus (as other electrons and groups get in the way!)

Question 26

What are the effects of hybridisation on chemical shift in terms of sp3 hybridised carbons?

Answer 26

In general, the 1H signal for H atoms in CH groups appear at higher chemical
shifts than for CH2 groups, which in turn appear at higher chemical shifts than
for CH3 groups.
δ(1H) CH > CH2 > CH3 for H atoms bonded to saturated C atoms (sp3) This is because C is very slightly more electronegative than H

Question 27

What are the effects of hybridisation on chemical shift?(when there is at least 1 pi bond present)

Answer 27

Hybridisation deshields the H atoms (and shields the C atoms) leading to lower chemical shifts. This is because the bonding is unsaturated and sp2 meaning there is an unhybridised p orbital in each molecule that can overlap side on and form a pi bond. This leads to extra electron density above and below the plane meaning less electron density within the plane, where the H atoms are. δ(1H) sp2 > sp > sp3
for H atoms bonded to C atoms

Question 28

What are the effects of aromatic and circulating electrons

Answer 28

Take for example benzene – there are 6 × sp2 hybridised C atoms, each with one unhybridized
p orbital that can overlap with the two p orbitals either side to form a continuous π orbital
where the electrons are delocalised.
This not only produces extra electron density above and below the plane of the molecule
(much like in alkenes), but the delocalised electrons, when in the magnetic field, also create
a ‘ring current’ (a continuous movement of charged electrons around the ring) that produces
another local magnetic field that deshields nuclei in the plane of the molecule. chemical shift for H atoms in benzene even more deshielded than in alkenes

Question 29

Whats the effect of substituents on aromatic ring on chemical shift, depending on if they are electron withdrawing or electron donating?

Answer 29

EWGs withdraw electrons from the ring – specifically from the ortho and para
positions (relative to the group). 1H nuclei in these positions therefore appear
at slightly higher chemical shiftsin the aromatic region of the NMR spectrum.

EDGs donate electrons to the ring – specifically onto the ortho and para
positions (relative to the group). 1H nuclei in these positions therefore appear
at slightly lower chemical shifts in the aromatic region of the NMR spectrum.

Meta positions are only marginally affected

Question 30

What is the effect of Hydrogen bonding on chemical shift? Why is it difficult to be accurate with how much the effect is?

Answer 30

H atoms involved in hydrogen bonding tend to be deshielded (appear at
higher chemical shifts).

The positions of the signals for 1H nuclei in –OH and –NH involved in hydrogen
bonding are unpredictable, as the amount of shielding/deshielding is
dependant on the amount of hydrogen bonding present.

The amount of hydrogen bonding present is itself dependant on the solvent,
concentration and temperature of the sample.

Question 31

What is the effect of concentration on chemical shift in relation to hydrogen bonding?

Answer 31

Higher concentrations, larger probability of
forming intermolecular R–H
…OH bonds, more deshielded / higher chemical shift

Question 32

What are common solvents for 1H NMR?

Answer 32

Common solvents are CDCl3, C6D6, D2O, CD3CN etc.

If acquiring a 1H NMR spectrum, the solvent needs to have no H atoms,
otherwise this would swamp the spectrum (this is why deuterated solvents
are used).

Question 33

How do you get a doublet of doublets?

Answer 33

When an atom is coupled to 2 atoms in different chemical environments. It will first couple to one of them, getting a doublet, and then those doublets will further split into doublets as it couples to the other atom in a different chemical environment.

Question 34

What are the factors affecting the size of a coupling constant?

Answer 34

Factors affecting the size of coupling will be those that affect how well those
nuclear spins can interact with each other:
* Number of bonds between coupling nuclei
* Orbital overlap (between nuclei and bonds between those nuclei)
* Aromatic/conjugated coupling (long range)

Question 35

How do sp3 and sp2 carbons affect J2 coupling between 2 1H atoms

Answer 35

J coupling between two 1H that are attached to an
sp2 hybridised C atom are less than for those attached to sp3 hybridised C
atoms. Differences in J2 coupling is mainly due to hybridisation of the C they are attached to.

Question 36

Why is coupling better than chemical shift when deducing information about a molecule?

Answer 36

Because unlike chemical shift, coupling constants are independent of external factors like solvent, magnetic field strength etc.

Question 37

What is the dihedral angle?

Answer 37

The angle between two 1H nuclei in a Newman projection

Question 38

Why does dihedral angle matter?

Answer 38

If the dihedral angle was 0 degrees, this is when the nuclei are totally ecli[se each other and align. If the angle is 180 degrees this is when the nuclei are trans to each other and are unaligned. When 0 degrees, there is good side on orbital overlap and so good communication between the nuclei so stronger couplong and larger 3J constant. If completely out of phase at 180 degrees, then good communication again and large 3J constant. However it is at around 90 degrees where the orbitals become poorly overlapped and small J constant.

Question 39

What is Karplus curve?

Answer 39

J constant on the Y and dihedral angle on the X, showing maxima at 0 and 180 and minima at 90

Question 40

How does isomersation of alkenes affect the 3J coupling constant of 1H nuclei

Answer 40

Around a double bond, atoms are fixed and so dihedral angle will not change. However, in a trans isomer there is better overlap between C-H bond sigma orbitals and so better communication with the 1H nuclei so larger J coupling constant. However in cis isomers, poorer overlap equals smaller J constant

Question 41

When will long range coupling occur?

Answer 41

Long range coupling usually wont be observed because two many atoms and bonds between the 2 nuclei so little interaction, however if can be observed sometimes in conjugated pi systems

Question 42

Whats roofing?

Answer 42

Its the explanation to when your multiplets dont have the correct intensities you may expect. If two signals for coupling nuclei become ‘too close’ in the NMR spectrum, the appearance of the multiplets becomes ‘skewed’. The peaks of the multiplet begin to ‘roof’ and point towards each other (‘roof’, because they become sloped, much like the roof of a house!).

Question 43

Magnetically inequivalent vs chemically inequivalent

Answer 43

Chemically inequivalent are where two nuclei are in different chemical environments however if they are in the same environment they can still be magnetically inequivalent. This is where two chemically equivalent nuclei can couple to something in two different ways, creating second order coupling. This effect makes the base of the peaks slightly skewed and messy, and means there will be two different J values

Question 44

What can NMR spec tell us about enantiomers and diasteroisomers

Answer 44

NMR spectroscopy can NOT distinguish enantiomers by chemical shift
alone*, but CAN distinguish diastereoisomers by chemical shift differences.
Whilst it is possible to tell diastereoisomers apart by differences in chemical
shifts, we still do not know what the absolute stereochemistry is (R/S).

Question 45

Can you integrate C13 signals?

Answer 45

No

Question 46

what are the differences in C13 spectra?

Answer 46

13C NMR spectra are recorded at much lower frequencies on the
spectrometer.
NMR spectra can NOT be integrated to determine the no. of nuclei in that
chemical environment.
Coupling between two 13C nuclei is not observed
Coupling between 13C and 1H is often not observed, because of the way we
choose to run the NMR experiment
We normally obtain a 13C{1H} NMR spectrum – the {1H} means coupling to
1H is removed.

Question 47

How does s character affect coupling?

Answer 47

More s character in the bonding, promotes better interactions so stronger coupling so larger J constant

Question 48

DEPT Spectra??

Answer 48

Type of spectra to simplify spectra aswell as help us determine the molecules.