NMR Package Flashcards
What does the spin of a Proton create?
A magnetic moment
Why does a spinning proton create a magnetic moment?
Because protons carry a positive charge
What happens to a proton in the presence of an external magnetic field?
Two orientations become possible for the proton. An alpha or beta spin state
What are the Alpha and Beta spin state?
An alpha spin state is when a protons magnetic field is aligned with the external magnetic field. A beta spin state is when a protons magnetic field is against an external magnetic field
Which spin state are most protons in?
An alpha spin state
Which spin state of protons has the higher energy level?
Beta spin state
What is Resonance?
The energy needed to flip an alpha spin state to a beta spin state
What nuclei are responsive to NMR?
Protons and neutrons are both odd
Protons OR neutrons are odd, giving an odd mass number
Which nuclei are not Responsive to NMR?
When protons and neutrons are both even
What region is the frequency of electromagnetic radiation?
Radio wave frequency
What is Shielding?
When the surrounding electrons shield the nucleus so that the effective magnetic field felt by the nucleus is lower than the applied field. Because the electrons create a field that oppose the applied field
What is Deshielding?
When the induced field can reinforce the applied magnetic field because the electron density around nucleus is decreased
What is the electron density around the proton in Shielding?
Electron density is increased producing a stronger induced magnetic field
What is the Magnetic field like around the proton in Shielding?
Proton experiences less external magnetic field and a stronger magnetic field is needed for resonance
What is the effect of Shielding on the NMR spectrum?
Signal goes upfield (to the right of the NMR spectrum)
What is the electron density around the proton in Deshielding?
Electron density is decreased and a weaker magnetic field is generated
What is the Magnetic field like around the proton in Deshielding?
A proton experiences a stronger magnetic field (A weaker magnetic field is needed for resonance)
What is the effect of Deshielding on the NMR spectrum?
The signal goes downfield (to the left of the NMR spectrum)
What is the reference compound in NMR?
Tetramethylsilane (TMS)
Why is TMS used as a reference in NMR?
All the methyl groups are equivalent giving a single peak
It is soluble in most organic solvents
It absorbs at the highest field
What is Chemical Shift?
The difference in absorption of a particular priority from the absorption position of the reference proton
How does Magnetic Field strength affect NMR?
The stronger the magnetic field, the broader the NMR scale and a better resolution
What are the characteristics of the ideal NMR solvent?
Must not have any protons so as not to produce peaks
Low boiling point
Inert
What are the 3 important feature of NMR?
The chemical shifts
Peak area
Spin splittings
How does increased electronegativity of an element affect the H NMR?
Increased electronegativity of nearby elements pulls electrons from the H increasing the deshielding effect causing the signal to move downfield
What is the chemical shift like around acetylene?
The protons are strongly shielded by the pi electrons and therefore move upfield
What is the chemical shift of H in vinyl?
The chemical shift is higher because the protons are deshielded by the induced field of pi electrons
What does the position of chemical shift depend on?
The electronegativity of the substituents near the proton
Induced magnetic field of the pi electrons
What is the Chemical Shift of protons on aromatic compounds?
Aromatic protons are deshielded by the induced ring current causing the signal to move downfield. Around 7.27
What is the Chemical shift of Aldehydic protons?
They are strongly deshielded by the magnetic field of the double bond and the electronegativity of the oxygen. Around 10
What is the Chemical shift Carboxylic protons?
Because they are strongly desheilded by the double bond and the oxygen it is around 10-12
What does the area under the peak depict?
The ratio of H’s in a compound
What is spin spin coupling?
When instead of showing up as single peaks, thay appear as multiplets in NMR because the coupling effect of protons. Adjacent protons affect protons nearby
What is N+1 rule for spin spin coupling?
The nearby protons that are not identical to the +1 determines the amount of peaks that a proton will have
When does Complex splitting occur?
Complex splitting occurs when the neighboring protons do not have all the same signal
What is different about -OH and -NH in NMR?
Due to hydrogen bonding, their position on NMR depends on concentration. Concentrated solutions move downfield due to hydrogen bonding that increases Deshielding
How does D2O determine the presence of hydrogen bonds?
If D2O is shaken with a compound with hydrogen bonds, the signal will disappear because it will undergo a proton exchange
