NMR

Question 1

What are advantages of NMR?

Answer 1

*complete analysis and interpretation of entire spectrum
*non-destructive
*can analyse samples less than 1mg with modern instruments

Question 2

What is a disadvantage of nmr?

Answer 2

Need larger amounts of sample than mass spec

Question 3

In terms of atoms, what property allows NMR analysis?

Answer 3

The nuclear spin

Question 4

What different spins are there?

Answer 4

*odd mass nuclei= FRACTIONAL SPINS- H1, C13
*even mass nuclei with odd numbers of protons and neutrons= INTEGRAL SPINS- H2, N14
*even mass nuclei with even numbers of protons and neutrons= ZERO SPIN- C12, O16
NB** non zero spin nuclei have magnetic moments (u)

Question 5

What is a chemical shift?

Answer 5

The difference in ppm between the resonance frequency of observed proton and TMS hydrogen. It is expressed in delta.

Question 6

What type of solvents are used in proton NMR and why?

Answer 6

*non hydrogen containing solvents
*CCL4 or CDCL3
*hydrogen containing solvents would cause confusing hydrogen peaks

Question 7

What is a factor contributing to chemical shift differences in proton resonance?

Answer 7

The inductive effect

Question 8

What influences the intensity of proton (H1) NMR signals?

Answer 8

*molar concentration of sample
*number of protons

Question 9

What causes splitting patterns?

Answer 9

Compounds which have different sets of hydrogen atoms bonded to adjacent carbon atoms.

Question 10

What is a singlet?

Answer 10

No hydrogens bonded to adjacent carbon

Question 11

What is a doublet?

Answer 11

One hydrogen bonded to adjacent carbon

Question 12

What is a triplet?

Answer 12

Two hydrogens bonded to adjacent carbon

Question 13

What is a quartet?

Answer 13

Three hydrogens bonded to adjacent carbon

Question 14

What are the limitations with coupling?

Answer 14

*doesn’t occur between ‘same type’ hydrogens eg CH3-CH3 would observe a singlet
*doesn’t occur across heteroatoms such as O and N. Will show as singlet. H on OH would show as singlet, essentially ignore it when figuring out other H

Question 15

How is intensity ratio of splitting pattern lines predicted?

Answer 15

Pascal’s triangle

Question 16

What is the equation for degree of unsaturation (DoU)

Answer 16

DoU= (2C + 2 + N - X - H)/2

X=no of halogens I.e Cl, F, Br, I

Question 17

When would NMR be used?

Answer 17

*GLP and GMP- compliant nmr analysis of pharmaceuticals
*discovery of natural sources
*drug distribution (drug content of urine)
*metabolism- predicting drug pathway in body, drug-drug interactions

Question 18

What are considered ‘unsaturations’?

Answer 18

*benzene ring outside = 1
*1 double bond= 1 saturation

Question 19

If there are two symmetrical splitting patterns to the far left of the spectrum, what does this indicate?

Answer 19

*benzene ring
* symmetry= PARA substitution = above and below

Question 20

If you are told you have a 2.5 ppm peak, what does this mean?

Answer 20

There is an alcohol or amine present (look at chemical structure- no N means it’s an alcohol

Question 21

Why does the 2.5ppm peak not show in the spectrum?

Answer 21

Because it was exchanged with deuterium so protons missing so no peak.
IF YOU ARE TOLD ABOUT 2.5 OR D2O, THERE WILL BE AN ALCOHOL, ACID OR AMINE IN COMPOUND