H-NMR

Question 1

What is the magnetogyric ratio?

Answer 1

is a constant for any given nucleus, indicates how “strongly magnetic” a nucleus is.

Question 2

What is considered an NMR active nuclei?

Answer 2

A nucleus that possesses either odd mass, odd atomic number, or both.
Example: 1H or 13C

Question 3

What is diamagnetic shielding?

Answer 3

Diamagnetic shielding occurs when protons are shielded by the electrons that surround them.

When a magnetic field is applied, the valence electrons circulate and oppose the field, producing their own magnetic field.

Question 4

How does shielding affect resonance frequency?

Answer 4

The greater the electron density around a proton, the greater the shielding, which results in a lower resonance frequency.
(aka peak will appear UPFIELD)

Question 5

How is the frequency of a proton measured?

Answer 5

The frequency of a proton is measured as a difference from the reference peak.

Question 6

What is the chemical shift of proton peaks measured in?

Answer 6

Chemical shift is measured in ppm

Question 7

How can ppm be calculated from resonance frequency of peaks?

Answer 7

shift in ppm is equal to the shift in Hz divided by the frequency of the spectrometer in MHz.

Question 8

Do shift value change between different spectrometers?

Answer 8

No. PPM stays the exact same for each proton

Question 9

How can one achieve a higher resolution of H-NMR peaks?

Answer 9

Increasing the strength of the magnetic field increases the resolution of H-NMR peaks.

Question 10

How does shielding affect frequency and chemical shift?

Answer 10

Deshielding causes higher frequency to occur which shifts peaks downfield (left)

More Shielding causes lower frequency to occur which shifts peaks upfield (right)

Question 11

What is the shielding constant?

Answer 11

The shielding constant signifies how shielded a proton is.
A positive constant means the nucleus is shielded
A negative constant means the nucleus is deshielded

Question 12

What is the electronegativity effect?

Answer 12

Chemical shift INCREASES (more downfield) as the electronegativity of the substituent increases.

(It is deshielding the nucleus)

Question 13

What can affect the electronegativity effect?

Answer 13

• The more electronegative substituents there are, the stronger the effect is.
• The farther the proton is from the electronegativity, the less affected they are by it.

Question 14

What are resonance effects?

Answer 14

Some molecules have resonance structures which can increase/decrease electron density around certain protons, thus increasing/decreasing chemical shift

Question 15

What is the anisotropic effect?

Answer 15

When a molecule is placed in a magnetic field, the electrons within that molecule circulate.
When a proton is outside of that circulation range it is shielded
When a proton is within that circulation range it is deshielded

Question 16

How does the anisotropic effect affect benzene rings

Answer 16

Similar to the regular anisotropic effect, when benzene is placed in a magnetic field, the pi electrons in the ring begin to circulate around the ring.
This circling of electrons around the ring cause electrons on the side to be deshielded while electrons above and below are shielded.

Question 17

How does the anisotropic effect affect pi systems

Answer 17

All groups that have pi electrons generate secondary anisotropic fields
The magnitude of these fields decrease with distance
Protons located within the “cones” on the ends are shielded
Protons located outside of the cones are deshielded

Question 18

How does diamagnetic anisotropy affect
C-C single bonds?

Answer 18

In rigid six membered rings (chair-hexane), the equatorial proton is deshielded, making it more downfield whereas the axial proton is shielded, making it more upfield

Question 19

What is the (n+1) rule?

Answer 19

A proton is able to “sense” the number of equivalent protons on the neighboring carbon. The resonance peak is split into (n+1) peaks with n being the number of equivalent protons.

Question 20

Do equivalent protons show coupling with eachother?

Answer 20

No. Equivalent protons will show up as only one peak

Question 21

What is the coupling constant (j)?

Answer 21

It is the distance between peaks in a simple multiplet. It is always measured in Hz.

Question 22

How do you calculate the j constant?

Answer 22

You multiply the chemical shift (in ppm) by the operating frequency of the spectrometer (MHz)

Question 23

Where is chemical shift measured in first-order spectra?

Answer 23

In first-order spectra, chemical shift is measured as the center of the peak.

Question 24

Does the coupling constant values change with different spectrometers?

Answer 24

No. Like chemical shift, the coupling constant stays the same in every spectrometer.

Question 25

What does the number infront of the coupling constant mean?

Answer 25

The number corresponds to the number of bonds seperating the two coupled protons

Question 26

What is 1-bond coupling?

Answer 26

Coupling that occurs mainly between C-H bonds. The favoured arrangement is when both nuclei have opposed spins. This is very rarely seen in H-NMR

Question 27

What is 2-bond coupling?

Answer 27

Known as Geminal Coupling.
Occurs between two Hydrogen atoms on the same Carbon.
J2 coupling is only observed when the two protons are magnetically inequivalent

Question 28

How does the bond angle affect 2-bond coupling?

Answer 28

As the bond angle increases, the j constant decreases.
Think of it as the protons trying to be as far apart from eachother as possible. The closer they are, the more energy is released.

Question 29

What is 3-bond coupling

Answer 29

Known as Vicinal Coupling
The two interacting nuclei are attached to adjacent Carbons

Question 30

What is the dihedral angle and how does it affect 3-bond coupling?

Answer 30

The dihedral angle is the angle between two connected substituent groups. Think back to Newman projections and the angle between substituent groups.
The coupling constant is greatest when the dihedral is 0 or 180 degrees
The coupling constant is lowest when the dihedral is 90 degrees
This is due to overlap of orbitals

Question 31

What is the Karplus relationship?

Answer 31

The Karplus relationship is the relationship between dihedral angle and coupling constant

Question 32

What is long-range coupling?

Answer 32

Coupling between protons that are separated by 4 or more bonds. Mostly occurs in allylic systems, aromatic rings, and rigid bicyclic systems.

Question 33

What is chemical equivalence?

Answer 33

chemical equivalence occurs when two or more nuclei are considered symmetrical via a plane of symmetry
Chemically equivalent nuclei have the same resonance frequency and do not couple with eachother

Question 34

What is magnetic equivalence?

Answer 34

magnetically equivalent nuclei do not have observable spin-spin splitting and give first-order spectra that can be analysed using the (n+1) rule

Question 35

What is magnetic inequivalence?

Answer 35

Magnetically inequivalent nuclei can give second-order spectra in which unexpected peaks may appear as multiplets

Question 36

What requirements does magnetic equivalence have?

Answer 36

1. Magnetically equivalent nuclei must have the same chemical shifts
2. magnetically equivalent nuclei must have equal coupling constants to ALL other nuclei in the molecule

Question 37

How does bond rotation affect magnetic equivalence?

Answer 37

Free bond rotations may lead to magnetic equivalence

Question 38

What are homotopic geminal groups?

Answer 38

Homotopic groups are always equivalent and give a single NMR absorption

Question 39

What are enantiotopic geminal groups?

Answer 39

Enantiotopic groups appear to be equivalent, however, when placed in a chiral environment, they will experience peak splitting

Question 40

What are diastereotopic geminal groups?

Answer 40

Diastereotopic groups are not equivalent and will have different chemical shifts. Diastereotopic hydrogens frequently split each other with geminal coupling

Question 41

What is a first order spectra?

Answer 41

A first-order spectra is when the chemical shift difference (v) between any coupled resonances is significantly larger than the coupling constant that relates them
(remember, coupled resonances have the same coupling constant)

Question 42

How are coupling constants measured in first-order spectra?

Answer 42

For simple multiplets, the j value can be found by multiplying the difference between peaks (in ppm) by the spectrometer frequency (in MHz)

Question 43

How can coupling constants be found in more complex multiplets like doublet of doublets?

Answer 43

First, make a tree diagram describing the multiplet. Then, measure the distance between peaks based on the coupling constant you are finding. J1 will be the largest distance with J2 being the second and so on.

Question 44

What is a second-order spectra?

Answer 44

A second-order spectra is one that cannot be explained by the (n+1) rule or graphical analysis (tree diagrams). Instead, advanced mathematical analysis done by a computer is needed to compute them

They are often observed when the shift difference (v) between coupled resonances equals the coupling constant (j) between groups

Question 45

What are labile protons?

Answer 45

Labile protons are protons that move so fast that they are not split and do not split other protons. They will appear as a small singlet

Question 46

What is double resonance? aka selective spin-spin decoupling?

Answer 46

Selective spin-spin decoupling is when you irradiate a proton with its specific resonance frequency, which will decouple it from any other protons.