Fad - Nmr

Front 1

Name four applications of NMR

Back 1

1. Structure determination

2. Metaboloics / mixture analysis

3. MRI

4. Small molecules

Front 2

What property does a nucleus with an odd atomic number possess?

Back 2

Nuclear Spin

Front 3

What does a spinning charged nucleus generate?

Back 3

A Magnetic Field

Front 4

How does an external magnetic field affect the magnetic moments created by spinning charged nuclei?

Back 4

Causes moments to align

Front 5

What is the result of there being more parallel than anti-parallel moments?

Back 5

Produces net magnetisation vector

Front 6

What is the name of the state in which the spinning nucleus is aligned with the external magnetic field

Back 6

α-state

Front 7

What is the name of the state in which the spinning nucleus is aligned against the external magnetic field?

Back 7

β-state

Front 8

What can cause a spinning nucleus to flip between α-state and β-state?

Back 8

A photon with the right amount of energy

Front 9

What does the energy difference between the two spin states depend on?

Back 9

The strength of the magnetic field

Front 10

Name 8 'NMR' nuclei

Back 10

1. 1H

2. 2H

3. 13C

4. 14N

5. 15N

6. 17O

7. 19F

8. 31P

Front 11

What is the name of the mathematical process applied to signals from NMR in order to produce an NMR spectrum?

Back 11

Fourier transformation

Front 12

What causes shielding in NMR?

Back 12

Protons are surrounded by electrons that shield them from the external field. Circulating electrons create an induced magnetic field that opposes the external magnetic field.

Front 13

What does the 'NLIS' initialism stand for?

Back 13

1. Number - different kinds of protons present

2. Location - shows how shielded the proton is

3. Intensity - shows number of protons of type

4. Splitting - no. of protons on adjacent atoms

Front 14

Why is TMS added to the sample during NMR analysis?

Back 14

- Silicon is less electronegative than carbon, TMS protons are highly shielded

- Signal from TMS is defined as 'zero'

- Organic protons absorb downfield of the TMS signal

Front 15

How is the shift from TMS in Hz converted into a field-independent value?

Back 15

chemical shift = shift in Hz / spectrometer frequenzy in MHz = ppm

Front 16

What is the approximate chemical shift of a -CH₃ proton?

Back 16

0.9

Front 17

What is the approximate chemical shift of a -CH₂- proton?

Back 17

1.3

Front 18

What is the approximate chemical shift of a -CH- proton?

Back 18

1.4

Front 19

What is the approximate chemical shift of a -CO-CH₃ proton?

Back 19

2.1

Front 20

What is the approximate chemical shift of a -C≡CH- proton?

Back 20

2.5

Front 21

What is the approximate chemical shift of a R-CH₂-X proton, where X = halogen, O?

Back 21

3-4

Front 22

What is the approximate chemical shift of a -C=CH- proton?

Back 22

5-6

Front 23

What is the approximate chemical shift of a C=C-CH₃ proton?

Back 23

1.7

Front 24

What is the approximate chemical shift of a Ph-H proton?

Back 24

7.2

Front 25

What is the approximate chemical shift of a Ph-CH₃ proton?

Back 25

2.3

Front 26

What is the approximate chemical shift of a R-CHO proton?

Back 26

9-10

Front 27

What is the approximate chemical shift of a R-COOH proton?

Back 27

10-12

Front 28

What is the approximate chemical shift of a R-OH proton?

Back 28

variable, about 2-5

Front 29

What is the approximate chemical shift of a Ar-OH proton?

Back 29

variable, about 4-7

Front 30

What is the approximate chemical shift of a R-NH₂ proton?

Back 30

variable, about 1.5-4

Front 31

State two types of spin-spin coupling

Back 31

1. Scalar coupling

2. Dipolar coupling

Front 32

Briefly outline scalar coupling

Back 32

- Relatively weak (1-20Hz)

- Through bond (i.e. via electron spins)

- Not scaled with field

Front 33

Briefly outline dipolar coupling

Back 33

- Relatively large (1-20KHz)

- Through space

- Related to orientation of internuclear vector with respect to field

- Generally not observed in solution due to rapid motions

Front 34

How does scalar coupling arise?

Back 34

Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field. This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced, and slightly upfield when it is opposed. All possibilities exist, so signal is split.

Front 35

Do equivalent protons split each other?

Back 35

No

Front 36

Protons separated by ______ or more bonds will not couple

Back 36

four

Front 37

Protons bonded to the same carbon will split each other only if they are ______________

Back 37

not equivalent

Front 38

What are coupling constants? Briefly describe

Back 38

- Distance between the peaks of multiplet

- Measured in Hz

- Not dependent on strength of external field

- Multiplets with same coupling constants may come from adjacent groups of protons that split each other

Front 39

What is the approximate value of the coupling constant for a 'free rotation' couple of protons (CH-CH)?

Back 39

7 Hz

Front 40

What is the approximate value of the coupling constant for a 'cis' couple of protons (HC=CH cis)?

Back 40

10 Hz

Front 41

What is the approximate value of the coupling constant for a 'trans' couple of protons (HC=CH trans)?

Back 41

15 Hz

Front 42

What is the approximate value of the coupling constant for a 'free rotation' couple of protons (R₂C=CH₂)?

Back 42

2 Hz

Front 43

What is the approximate value of the coupling constant for a 'ortho' couple of protons (Ar-H₂ ortho)?

Back 43

8 Hz

Front 44

What is the approximate value of the coupling constant for a 'meta' couple of protons (Ar-H₂ meta)?

Back 44

2 Hz

Front 45

What is the approximate value of the coupling constant for a 'allylic' couple of protons (R₂C=CH-CH)?

Back 45

6 Hz

Front 46

What is spin-spin decoupling by exchange?

Back 46

In alcohols coupling between the O-H hydrogen and those on adjacent carbon atoms is not usually see. Also holds for many amines.

Front 47

Why does spin-spin decoupling by exchange arise?

Back 47

Due to rapid exchange of OH (or NH) hydrogens between the various alcohol (or solvent) molecules in the solution. In ultra pure alcohol, coupling will still be sometimes seen.

Front 48

What is COSY 2D NMR?

Back 48

Correlation Spectroscopy. Maps out correlations between protons that are coupled (up to 3 bonds). This technique provides a clear representation of which groups are connected and is easier to interpret than splittings observed in 1D spectra, particularly for large spin systems.

Front 49

What is NOESY 2D NMR?

Back 49

Nuclear Overhauser Effect Spectroscopy. Correlates nuclei through space ≤5A and is arguably the most useful technique for determining structure. Cross peak intensities are related to internuclear distance and distances are input for structural models.

Front 50

What is TOCSY 2D NMR?

Back 50

Total Correlation Spectroscopy. Maps out correlations between all protons in a spin system, not just those that are directly coupled. Proton spin networks that are interrupted by e.g. a carbonyl are separate and hence separate groups of correlations are shown for each amino acid in a protein or each subunit of a polysaccharide.

Front 51

What is HSQC 2D NMR?

Back 51

Heteronuclear Single Quantum Correlation. Allows correlations between protons and (usually) ¹³C for small molecule research or ¹⁵N for proteins.