NMR Flashcards
What are advantages of NMR?
*complete analysis and interpretation of entire spectrum
*non-destructive
*can analyse samples less than 1mg with modern instruments
What is a disadvantage of nmr?
Need larger amounts of sample than mass spec
In terms of atoms, what property allows NMR analysis?
The nuclear spin
What different spins are there?
*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)
What is a chemical shift?
The difference in ppm between the resonance frequency of observed proton and TMS hydrogen. It is expressed in delta.
What type of solvents are used in proton NMR and why?
*non hydrogen containing solvents
*CCL4 or CDCL3
*hydrogen containing solvents would cause confusing hydrogen peaks
What is a factor contributing to chemical shift differences in proton resonance?
The inductive effect
What influences the intensity of proton (H1) NMR signals?
*molar concentration of sample
*number of protons
What causes splitting patterns?
Compounds which have different sets of hydrogen atoms bonded to adjacent carbon atoms.
What is a singlet?
No hydrogens bonded to adjacent carbon
What is a doublet?
One hydrogen bonded to adjacent carbon
What is a triplet?
Two hydrogens bonded to adjacent carbon
What is a quartet?
Three hydrogens bonded to adjacent carbon
What are the limitations with coupling?
*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
How is intensity ratio of splitting pattern lines predicted?
Pascal’s triangle
What is the equation for degree of unsaturation (DoU)
DoU= (2C + 2 + N - X - H)/2
X=no of halogens I.e Cl, F, Br, I
When would NMR be used?
*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
What are considered ‘unsaturations’?
*benzene ring outside = 1
*1 double bond= 1 saturation
If there are two symmetrical splitting patterns to the far left of the spectrum, what does this indicate?
*benzene ring
* symmetry= PARA substitution = above and below
If you are told you have a 2.5 ppm peak, what does this mean?
There is an alcohol or amine present (look at chemical structure- no N means it’s an alcohol
Why does the 2.5ppm peak not show in the spectrum?
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
