Shapes and Structures 1 - C-13 NMR

Question 1

A nucleus with a spin I has how many different energy levels?

Answer 1

2I + 1 energy levels

Question 2

What is the spin of nuclei with odd masses?

Answer 2

half-integral spin

Question 3

What is the spin of nuclei with odd numbers of protons and neutrons?

Answer 3

Integral spin

Question 4

What is the spin of nuclei with even numbers of protons and neutrons?

Answer 4

Zero spin

Question 5

Why does a C-13 resonance frequency increase as the atoms attached become more electronegative?

Answer 5

Electronegative elements withdraw electrons, deshielding the C atom
Electrons cause the nucleus to experience a weaker magnetic field overall, so lower electron density means a larger energy difference between spin states and larger frequency absorbed

Question 6

Describe the peak given by the solvent CDCl3

Answer 6

Triplet at 75-80 ppm

Question 7

How many peaks will be shown for a molecule in C-13?

Answer 7

Number of carbon environments

Question 8

What should initially be looked for when predicting the spectrum of a molecule?

Answer 8

Planes/axes of symmetry

Question 9

How is the chemical shift in ppm worked out?

Answer 9

10^6 x (frequency of resonance - frequency of reference)/(frequency of reference)

Question 10

Outline the reference compound and its properties

Answer 10

Tetramethylsilane - TMS - Si(CH3)4

The chemical shift of 0 ppm by definition

Question 11

In which range of ppm are sp3 carbons?

Give two examples of sp3 carbons

Answer 11

0 - 50 ppm

C-C, C-N

Question 12

In which range of ppm are sp3 carbons with very electron-withdrawing groups?
Give examples

Answer 12

50 - 100 ppm
C-O, C-F
C≡C also in this range

Question 13

In which range of ppm are sp2 carbons?

Give examples

Answer 13

100 - 150 ppm
C=C, benzene
C≡N also in this range

Question 14

In which range of ppm are sp2 carbons with very electron-withdrawing groups?
Give examples

Answer 14

150 - 200 ppm

C=O

Question 15

With respect to the carbonyl region, how can aldehydes and ketones be distinguished?

Answer 15

Ketones typically just over 200 ppm

Aldehydes typically just under 200 ppm

Question 16

At what range do carbonyl carbons for acid derivatives resonate at? What are the groups in this range?

Answer 16

160 - 170 ppm

Carboxylic acid, ester, acyl chloride, amide, acid anhydride

Question 17

What does the difference between the shift of aldehydes and ketones with that of acid derivatives indicate?

Answer 17

Carbon atoms in acid derivatives are more shielded

Heteroatoms (O, N, Cl) must be donating electron density

Question 18

Why would a CH3F molecule result in a doublet?

Answer 18

C and F nuclei couple
F has spin 1/2 so two energy states
C - F spin up, increased field, increased shift
C - F spin down, decreases field, smaller shift

Question 19

Why is C-H coupling not usually seen?

Answer 19

Usually proton decoupled

Question 20

Why is C-C coupling not usually seen?

Answer 20

A low abundance of C-13 in naturally occurring carbon

Question 21

If a C atom couples to two nuclei that give different coupling constants, what peak type is shown?

Answer 21

Doublet of doublets

Question 22

If a C atom couples to two equivalent nuclei, what peak type is shown?

Answer 22

Triplet

Question 23

What produces a quartet?

Answer 23

A C atom couples to three equivalent nuclei

Question 24

What produces a triplet of doublets?

Answer 24

A C atom couples to two equivalent nuclei and one different nucleus

Question 25

What is the term for C-13 spectra that do not show coupling to protons?

Answer 25

Proton decoupled

Question 26

Explain broadband proton decoupling

Answer 26

While the C-13 NMR spectrum is being recorded
Protons irradiated such that nuclear spins move rapidly between up and down
Fast enough the couplings average to zero and splittings disappear

Question 27

With broadband proton decoupling, what is the significance of “broadband”?

Answer 27

Irradiating over a range of frequencies so that all protons are rapidly moving between spin states

Question 28

What are the advantages and disadvantages of proton coupling?

Answer 28

Advantages:
Easy to see whether carbons are CH3, CH2, CH or quaternary since they appear as q, t, d and s respectively
Disadvantages:
Signal strength weaker so signals can be lost in the background
Most compounds multiplets will overlap and sometimes can’t distinguish C peaks

Question 29

Outline the Attached Proton Test, APT

Answer 29

Carbons with 0 or 2 protons attached point one way
Carbons with 1 or 3 protons attached point the other
You can tell which way by how CDCl3 points as it has 0 protons

Question 30

Where would a methyl group (CH3) peak show?

Answer 30

10-20 ppm

Question 31

Where would an alkyl group (CH2) peak show?

Answer 31

20-30 ppm

Question 32

Where would an alkyne carbon peak show?

Answer 32

70 ppm

Question 33

Where would an aromatic carbon attached to an electronegative atom peak show?

Answer 33

150 ppm

Question 34

Where would an aromatic carbon peak show?

Answer 34

130 ppm

Question 35

With the solvent CDCl3, knowing that D has I=1, and Cl-35 and Cl-37 have I=3/2, why is a triplet shown?

Answer 35

I=1 for D so spin states = 2x1 + 1 = 3

Cl has no observed coupling because spin states relax too quickly