Spectroscopy Topic 3

Question 1

For nucleus to be NMR active it need to posses what?

Answer 1

Spin (the nuclear spin quantum number I must be non-zero)

Question 2

A nucleus with spin possesses what?

Answer 2

A magnetic dipole moment μ. This means it behaves like a tiny bar magnet.

Question 3

What two stable orientations the magnetic dipole moment μ of an NMR active nucleus has?

Answer 3

Pointing in the same direction as the external magnetic field (parallel)

Pointing in the opposite direction to the external magnetic field (antiparallel)

Question 4

Which orientation state is lower in energy?

Answer 4

The parallel (or α) state is lower in energy than the antiparallel (or β) state.

Question 5

How you calculate energy gap between states ΔE?

Answer 5

Where γ is the magnetogyric ratio, and is a constant for each type of nucleus. The frequency of absorption ν is equal to the precession frequency of the magnetic moment about B₀, also known as the Larmor frequency.

Question 6

What happens when a photon of EM radiation is absorbed?

Answer 6

The orientation of the nucleus switches from the α to the β state (by resonance excitation of the Larmor precession frequency) and an NMR signal detected.

Question 7

Why the precise frequency at which nuclei absorb magnetic field depends on the electron density around them?

Answer 7

Because nuclei do not exist in a naked state, but are surrounded by electrons. These partially shield the nucleus from the external magnetic field

Question 8

How you convert absorption frequency (MHz) to shemical shift in parts per million?

Answer 8

Where v_sample= absolute absorption frequency of sample signal, v_ref= absolute absorption frequency of tetramethylsilane reference, v_res= operating frequency of the NMR spectrometer

Question 9

What is used as a reference compound for both 1H and 13C NMR?

Answer 9

Tetramethylsilane (TMS), by definition has a value δ = 0.

Question 10

What is the advantage of using chemical shift?

Answer 10

Using the chemical shift scale produces consistent δ values for the same sample acquired on spectrometers with different magnetic fields.

Question 11

Why NMR scale is in ppm?

Answer 11

Because the difference between sample and reference frequencies (νsample - ν_ref ) is in Hz, whilst the operating frequency (ν_res ) is in MHz. This gives 1/10⁶, or ppm as the units.

Question 12

What is the use of a superconducting magnet in NMR?

Answer 12

It is used to provide a strong, homogeneous magnetic field.

Question 13

What are the effects of a broadband pulse, known as a chirp, in NMR?

Answer 13

It excites all of the ¹H or ¹³C environments at once. The pulse is switched off and the residual signal recorded over time to produce a free induction decay (FID) pattern.

Question 14

What is the typical range of ¹³C NMR?

Answer 14

0-220 ppm

Question 15

For ¹³C NMR what is the 0-80 ppm range?

Answer 15

Aliphatic C

Question 16

For 13C NMR what is the 80-120 ppm range?

Answer 16

Alkyne C

Question 17

For 13C NMR what is the 100-140 ppm range?

Answer 17

Alkene C

Question 18

For 13C NMR what is the 110-130 ppm range?

Answer 18

Nitrile C

Question 19

For 13C NMR what is the 110-160 ppm range?

Answer 19

Aromatic C

Question 20

For 13C NMR what is the 160-220 ppm range?

Answer 20

Carbonyl C

Question 21

What is the effect Electron releasing groups on δ?

Answer 21

Lower δ, as there are higher electron density that partially shield the nucleus from the external magnetic field

Question 22

What is the effect electron donating groups on δ?

Answer 22

Increase δ, as there are less electrons that partially shield the nucleus from the external magnetic field

Question 23

What are the effects of ¹H-¹³C coupling in ¹³C NMR?

Answer 23

It causes the signal to be split depending on how many protons are attached to the carbon atom.

Question 24

Considering CH, what is the effect of proton magnetic moment pointing in the same direction as magnetic field?

Answer 24

The local magnetic field that is felt by carbon nucleus is the sum of the external magnetic field and effect of proton magnetic field. This result in shift to the higher value.

Question 25

Considering CH, what is the effect of proton magnetic moment pointing in the opposide direction as magnetic field?

Answer 25

The local magnetic field that is felt by carbon nucleus is the strength of external magnetic field minus the effect of proton magnetic field. This result in shift to the lower value.

Question 26

Why considering ¹³C NMR of CH we see two peaks despite having just one C?

Answer 26

Because we get two signals, one is when H has positive spin and other when H has negatove spin.

Question 27

Why ration of NMR coupling of CH is 1:1?

Answer 27

Because the energy gap is so small that the number of nuclei in the anti-parallel state at any given time almost exactly equal to the number in the parallel state.

Question 28

What it means if a peak is twice as big as other in NMR?

Answer 28

That we have two statistical ways of having one spin parallel and one spin anti-parallel.

Question 29

In ¹³C NMR hoq many signals we het for each C?

Answer 29

For n H atoms we get n+1 signals.

Question 30

What is decoupling?

Answer 30

The appearance of a simple signal for each carbon atom after removing proton coupling from the ¹³C spectrum

Question 31

Why in ¹³C NMR we produce decoupled spectra?

Answer 31

As a result of sensitivity- NMR is intrinsically insensitive. If the signal is split by proton coupling the signal to poise ratio is reduced furter.

Also, the signal from different carbons overlap

Question 32

Why ¹³C NMR is intrinsically insensitive?

Answer 32

Due to the low abundance of the isotope and its relatively low magnetic ratio.

Question 33

What is the meaning of intrinsically?

Answer 33

In an essential or natural way.

Question 34

What causes signals from different carbons to everlap?

Answer 34

The ¹H-¹³C coupling constant, which is a measure of how much the signals are split, is large

Question 35

How do we remove coupling from ¹³C NMR?

Answer 35

During the acquisition of the ¹³C spectrum, broad band Radio Frequency is applied to excite all the protons and this prevents them from coupling to ¹³C

Question 36

What arethe names of spliting from 1 to 4?

Answer 36

Singlet, doublet, triplet, quartet

Question 37

How distortionless enhancement by polarisation transfer (DEPT) works?

Answer 37

It involves applying precise RF pulses to both the ¹H and ¹³C nuclei whilst measuring the ¹³C NMR spectrum. By comparing the original ¹³C NMR spectrum, and the DEPT 135 and DEPT 90 spectra we can unambiguously assign all types of carbon without the disadvantages of proton coupled spectra.

Question 38

How DEPT 135 effects ¹³C NMR?

Answer 38

Fully substituted C has signals=0, CH groups become positive, CH₂ becomes negative and CH₃ becomes positive

Question 39

How DEPT 90 affects ¹³C NMR?

Answer 39

CH becomes positive, other C has 0 value

Question 40

How DEPT 45 affects ¹³C NMR?

Answer 40

All of the signals remain except fully substituted C