NMR 2

Question 1

The nuclei of many atoms have a quantity described as a spin. TRUE OR FALSE?

Answer 1

TRUE

Question 2

Carbon has a spin value of 0. TRUE OR FALSE?

Answer 2

TRUE

Question 3

Proton NMR is one of the most common nuclei in organic molecules and is NMR active. TFRUE OR FALSE?

Answer 3

TRUE

Question 4

Why does proton NMR require small sample?

Answer 4

Because it is the most abundant

Question 5

Carbon NMR usually requires more time. TRUE OR FALSE?

Answer 5

TRUE

Question 6

NMR spectrometers are tuned to a certain frequency in radio wave region. TRUE OR FALSE?

Answer 6

TRUE

Question 7

When is a signal seen in NMR spectrometers?

Answer 7

-Whenever a transition between the lower and higher energy states is induced

Question 8

Any spinning charged particle generates a magnetic field. TRUE OR FALSE?

Answer 8

TRUE

Question 9

Which is more favoured by an external magnetic field; alignment with field or alignment against field?

Answer 9

alignment with field

Question 10

What type of hydrogen is obtained when hydrogens align against the applied magnetic field?

Answer 10

• B hydrogen

Question 11

What type of hydrogen is obtained when hydrogens align with the applied magnetic field?

Answer 11

• a hydrogen

Question 12

There is only a slight excess of a H because the difference between the two orientations (spin state) is small. TRUE OR FALSE?

Answer 12

TRUE

Question 13

Aligned with the field = High energy
Opposed to the field = Low energy
TRUE OR FALSE?

Answer 13

FALSE

Question 14

What does the strength of B0 determine?

Answer 14

The energy differences between the 2 spin states

Question 15

In the absence of an applied magnetic field B0 = 0 and the nuclear spin states are of equal energy . TRUE OR FALSE?

Answer 15

TRUE

Question 16

What is chemical shift?

Answer 16

Term used for signal positions

Question 17

How is Bi formed?

Answer 17

when applying an external magnetic field such as B0 , will cause the electrons in an atom or molecule to circulate within its regions of space so as to create an induced magnetic field Bi

Question 18

Bi can oppose or add to the applied field B0 . TRUE OR FALSE?

Answer 18

TRUE

Question 19

Deshielded hydrogens are those that require B0 to be reduced due to the field induced by the electrons Bi. TRUE OR FALSE?

Answer 19

TRUE

Question 20

Why are deshielded Hydrogens deshielded?

Answer 20

Because Bi is adding onto B0

Question 21

Where do deshielded Hydrogens appear on the ppm scale?

Answer 21

Appears down field, so have higher chemical shift value

Question 22

Shielded hydrogens are occasionally seen, so therefore appear up field. TRUE OR FALSE?

Answer 22

TRUE

Question 23

What happens to B0 in shielded hydrogens?

Answer 23

B0 is opposed by the induced field

Question 24

In shielded hydrogens, we need to apply more B0 to flip the nuclear spin. TRUE OR FALSE?

Answer 24

TRUE

Question 25

What is TMS and what is it used for?

Answer 25

Tetramethylsilane, used as a standard

Question 26

TMS is the ideal reference as it is distant from most other H’s in the organic molecule. TRUE OR FALSE?

Answer 26

TRUE

Question 27

What is the chemical shift value of TMS?

Answer 27

0ppm

Question 28

Where do most alkanes and simple cycloalkanes appear?

Answer 28

Appear up field - to lower chemical value

Question 29

What does integration allow us to do?

Answer 29

To measure the relative intensity of the signals in the spectrum

Question 30

What will EWG’s do to the neighbouring H’s?

Answer 30

Reduce the electron density at the neighbouring H’s

Question 31

When EWG’s reduce the electron density of neighbouring H’s what does this cause in terms of shielding and chemical shift of the neighbouring H’s?

Answer 31

• Reduces the ability of nearby electrons to shield the nucleus, therefore increasing the chemical shift so signals appear at lower fields. (true for alkanes)

Question 32

The more electronegative groups in the neighbourhood of H, the greater the electron withdrawing effect, therefore the more deshielded and the further down field the resonance position shifts. TRUE OR FALSE?

Answer 32

TRUE

Question 33

The more electronegative groups in the neighbourhood of H, the greater the electron withdrawing effect, therefore the more deshielded and the further down field the resonance position shifts. Is this true for halogenated and oxygenated compounds?

Answer 33

YES