NMR part 2 - practical applications

Question 1

process of interpreting an NMR spectroscopy

Answer 1

1- identify the number of nuclei environments.
2 - identify the relative intensities of the nuclei environments
3 - identify chemical shift properties of molecule
4 - identify j coupling properties

Question 2

number of nuclei environments

Answer 2

the number of different signals on the spectroscopy

Question 3

number of nuclei within nuclei environment.

Answer 3

look at the ratios of the integrals - they will show you the ratios of the hydrogen environments.

Question 4

high chemical shift groups

Answer 4

heteroatoms
olefinic molecules
aromatic molecules

Question 5

hybridisation chemical shift order

Answer 5

sp2>sp>sp3

Question 6

j coupling reason for occurrence

Answer 6

nuclei which are less than 3 bonds away will interact with each other due to the overlap of their local magnetic fields, which will result in a series of signals being produced instead of 1.

Question 7

rule for J coupling of equivalent carbons

Answer 7

multiplet = n+1
where n = the number of nuclei on adjacent carbons

Question 8

rule for j coupling of non equivalent carbons

Answer 8

multiplet = 2^n
where n = the number of nuclei on adjacent carbons

Question 9

ratios of equivalent nuclei

Answer 9

corresponds to pascals triangle,

Question 9

important property of 2^n rule

Answer 9

the 2^n rule must take into account the ratio of the different multiplets, which means it can be used instead of the n+1 rule

Question 10

proving nuclei equivalence methods

Answer 10

1. use 2^n rule to identify the number of nuclei on adjacent carbons.
2. check that the intensity ratios
   if they match pascals triangle then they are all equivalent.
   if they dont match pascals triangle move onto step 3.
3. use formulas
   Jax = p2 -p1
   jbx = p3 -p1
   jcx = p5 - p1
4. use equalities to determine the number of non equivalent carbons.

Question 11

normal chemical shift of proton NMR

Answer 11

1 - 12 ppm

Question 12

normal chemical shift range of carbon 13 NMR

Answer 12

0 - 220 ppm

Question 13

differences between proton and carbon 13 NMR

Answer 13

different chemical shifts - 0 - 12 ppm for proton and 0 - 220 ppm for carbon

proton NMR will have multiplets, whereas carbon NMR will have no multiplets.

proton NMR will have higher natural abundance of target species due to only 1H having only one isotope. whereas carbon NMR will only have 1% abundance within the sample.

Question 14

reason for no multiplets in 13C NMR

Answer 14

The carbon nuclei will be will J couple to many more different nuclei than a proton in proton NMR due to the carbon being fewer bonds away from other nuclei, this would make a 13C NMR too complicated to interpret due to huge multiplet splitting. this then results in chemists producing a spectroscopy technique which will remove the J coupling so no multiplets are formed on 13C NMR

Question 15

reason for low relative abundance of sample species in 13C NMR

Answer 15

13C is in low abundance because it has a very high abundance isotope - 12C which cannot be used for NMR because it isn’t NMR active