NMR

Question 1

2 types of NMR

Answer 1

1. 13C NMR
2. 1H (proton) NMR

Question 2

number of signals = methylpropene

Answer 2

2

Question 3

number of signals in methylamine

Answer 3

2

Question 4

number of signals in propene

CH2=CH-CH3

Answer 4

3

Question 5

calibration

Answer 5

-small quantity of TMS (tetramethylsilane) is added to samples as it produces a single providing internal standard to which other peaks are compared

-only givies one single, non-toxic, inert, low boiling point

Question 6

NMR

Answer 6

-number of peaks in an NMR spectrum refers to Hydrogen atoms

-area under the peak = proportional to the number of hydrogen atoms

Chemical shift = closer the hydrogen is to an electronegative atom the greater the shift

Question 7

why compounds absorb radiowaves

Answer 7

-the nucleus of some atoms have a nuclear spins e.g 1H, 13C, 19F etc)

-a nucleus with spin generates a small magnetic field

-there is a small difference in energy ebtween these 2 alignments that corresponds to the energy of radiowaves

Question 8

how NMR is done

Answer 8

-the sample is usually dissolved in a solvent in a long thin tube and placed inside the machine where it is bombarded with radiowaves in a strong magnetic field

-the stronger the magnetic field the better the resolution of the signals

Question 9

solvents

Answer 9

-if the solvent contains any 1H atoms then these will appear in the spectrum

-common solvents used = CCl4

Question 10

chemical shifts

Answer 10

-the chemical shift depends on what other atoms/groups are near the H. The closer the H to electronegative atoms the greater the shift.

Question 11

scales

Answer 11

-horizontal scale = chemical shift

-vertical axis = intensity of the absorption

Question 12

signals

Answer 12

-same distance of CH3 = 1 signal. Difference = multiple signals

-in 1 H NMR the area of the signal is proportional to the number of hydrogen atoms is represents

-the height of the signal doesn’t matter but rather the area which the signal covers.

At A-level to indicate the relative intensity of the signals from which the simplest whole number ratio can be calculated:

e.g 1.2 : 1.2 : 1.8 = 2:2: 3

Question 13

calculating signals

Answer 13

1) Divide all values with the smallest number

2) Find whole number

Question 14

splitting patterns

Answer 14

-the number of lines in a signal is split into and gives information about neighbouring H atoms

-usually only H atoms on the neighbouring C atom cause splitting

-number of lines = 1 + number of H atoms on adjacent carbon atoms (n+1)

-H atom on OH group in alcohols rarely causes splitting or is split itself.

Doublet = 1:1

Triplet = 1:2:1

Quartllet = 1: 3: 3: 1