Topic 32: NMR

Question 1

Spin

Answer 1

Nuclei with either odd mass no. or odd atomic no. possess spin.
Angular momentum

Question 2

Information from C13 nmr

Answer 2

• No. different signals in spectrum
• Position of the signals (chemical shift)
  PEAK HEIGHT MEANS NOTHING

Question 3

Information from H1 NMR

Answer 3

• No. different signals in spectrum
• Position of the signals (chemical shift)
• Intensity of different signals
• Splitting pattern of signals

Question 4

Solvents used to dissolve solids in NMR

Answer 4

• CCl4 non-polar so for non-polar samples
• CDCl3 polar so for polar samples - deuterated chloroform

neither affect the spectrum

Question 5

TMS

Answer 5

Tetramethylsilane (CH3)4Si
Reference for NMR
- non toxic
- inert
- low bp - distilled and reused
- H+ ions chemically equivalent with large peak
- doesn’t obscure other signals as peak is far from others at 0

Question 6

δ in NMR

Answer 6

Chemical shift which shows the environment - what is attached to and what is attached to that (not just adjacent)

Question 7

Desheilded

Answer 7

The more desheilded the greater the chemical shift, more to the left of the graph

Question 8

Point of NMR

Answer 8

Determines structure of molecule by showing how many “unique” carbon/ hydrogens there are in it

Question 9

Sheliding in NMR

Answer 9

By being bonded to certain atoms changes the bond environment and so cause a chemical shift causing a different peak (oversimplifying stuff to do with electronegativity and sheilding)

Question 10

Downfield on NMR

Answer 10

Left of the specta cause by more deshielding

Question 11

Splitting for Carbon NMR

Answer 11

NOPE
One peak per carbon

Question 12

Area under peak of 1H NMR

Answer 12

Proportional to ratio of no. hydrogens
How many of each type there are

Question 13

Chemical shift def

Answer 13

Difference between the field strength at which it absorbs and the field strength at which TMS protons absorb

Question 14

Temp/solvent effect (NMR)

Answer 14

No difference to proton chemical shift - constant

Question 15

Shifting H downfield

Answer 15

Nearer to electronegative species shift downfield to higher δ values

Question 16

Low resolution NMR

Answer 16

1 peak for each environmentally different group of protons

Question 17

High resolution NMR

Answer 17

Gives more complex signals including doublets, triplets…

Question 18

Number of peaks on 1H NMR (high res)

Answer 18

Number of Hs on adjacent atoms + 1

Question 19

Doublet

Answer 19

2 peaks, 1 neighbouring H

Question 20

Triplet

Answer 20

3 peaks, 2 neighbouring H

Question 21

Quartet

Answer 21

4 peaks, 3 neighbouring H

Question 22

Quintet

Answer 22

5 peaks, 4 neighbouring H

Question 23

OH signal on 1H NMR

Answer 23

Unaffected by hydrogens on adjacent atoms. Always a singlet

Question 24

Integration on 1H NMR

Answer 24

Determines RATIO of number of Hs in each environment not actual number

Question 25

How to use integration in 1H NMR

Answer 25

Continuous integration line Measure integration line rise between signals Compare values and work out simple ratio

Question 26

Why H on OH different? (NMR)

Answer 26

The H on OH rapidly exchanges with protons on other molecules and is not attached to any oxygen long enough to register splitting signal

Question 27

Equivalent H atoms (NMR)

Answer 27

H attached to same C are equivalent Can also be neighbouring as just have to be the exact same environment Cause no splitting to each other Cl-CH2-CH2-Cl 4 equivalent H One peak no spliting