Chapter 14 Flashcards
What does the position of a signal mean in a H NMR spectroscopy?
Shielding and deshielding?
The chemical shift determines the position of a signal. The chemical shift is effected by shielding and deshielding.
Shielding shifts signal upfield.
Deshielding shifts signal downfield.
Do electronegative atoms deshield or shield a proton?
They withdraw electron density from proton and deshield nucleus. (Downfield)
Do double and triple bonds deshield or shield a proton?
Double bonds deshield protons.
Triple bonds shield protons.
What does the area under an NMR signal signify?
Signal is promotional to number of absorbing protons.
What is spin-spin splitting?
Tells us about nearby/adjacent nonequivalent protons.
Equivalent protons do not split!
Proton splits signal by n+1.
(N=number of protons on nearby carbon)
Do OH and NH protons split other protons?
No. They have their own signal.
Determine splitting when two sets of nonequivalent protons are nearby?
- Two sets of nonequivalent protons are equivalent to each other. Add all protons together=N. Then N+1
- Two sets of nonequivalent protons are not equivalent to each other. (N+1)(m+1)
What do the number of peaks mean in a C NMR spectroscopy?
Number of peaks if equal to the number of different protons. (Carbon signals to not split, all peaks are single)
What does the position of a signal mean on a C NMR spectroscopy?
Determined by shielding and deshielding. C=sp3 are shielded, carbons in Alkenes and benzene rings absorb downfield.
*carbonyl absorb even further downfield)
What do the number of peaks mean in a H NMR spectroscopy?
Number of peaks is equal to the number of different types of protons
Which nucleus spin has a higher and lower energy state with a applied field/magnet?
Lower energy state when the nucleus aligns with applied field.
Higher energy state when nucleus aligns opposite of applied field.
What is resonance?
When a nucleus absorbs RF radiation and spin flips to a higher energy state. (Flips to oppose the applied field)
How do you calculate chemical shifts?
Observed chemical shift from TMS/ frequency of NMR spectrometer
What are enantiotopic protons?
When substitution of a H with a ax forms enantiomers, the two H atoms are equivalent and give single NMR.
Usually no stereogenic centers, but a new one is made with substitution
What are diastereotopic protons?
When substitution of a H atom by Z forms diastereomers, the 2 H atoms are not equivalent and give two NMR signals.
Usually there is a stereogenic center already in the molecule and another one is formed with substitution
How does alkyl substitution affect chemical shift?
As alkyl substitutions increase so does chemical shifts
How does a benzene/double bonds effect a molecules chemical shift?
Benzene has double bonds that deshield causing downfield/higher chemical shift
How does a triple bond effect a molecules chemical shift?
Triple bonds shield electrons. Causing chemical shift to be lower/upfield
What compounds have a chemical shift higher than 6?
Benzene rings.(higher shift than regular double bond)
Then aldehydes and carboxylic acids. (Higher chemical shift then benzene)
How do you calculate the number of peaks when two sets of adjacent protons are different?
(N+1)(M+1)
