Nuclear Magnetic Resonance Flashcards
What is the most powerful tool for organic structure determination
Nuclear magnetic resonance
What information does nuclear magnetic resonance provide?
A detailed blueprint of the carbon-hydrogen framework
What does nuclear magnetic resonance depend on to provide information?
The fact that both carbon and hydrogen nuclei behave as though as they are rapidly rotating around an axis
This spin along with the positive charge of nuclear protons, thus causes them to act as magnets and are affected by magnetic fields
What occurs to protons in the absence of an external magnetic field?
Magnetic vectors of the nuclei/ protons are randomly arranged and so cancel each other
How is alignment of protons affected by the presence of an external magnetic field?
This causes protons to line up in the direction of the magnetic field, some oppose the direction and others align with the direction.
Which protons have a higher energy in the presence of an external magnetic field. Opposing or aligned?
Protons that are aligned have a slightly lower energy than those that are opposing the external magnetic field
What happens when a large external magnet is applied to a chemical sample?
The individual nuclei within the sample align either with the field or in opposition to the field
What are the nuclei that align with the external magnetic field?
Low energy spin state nuclei- alpha spin
What are the nuclei that oppose the external magnetic field?
High energy spin state nuclei( beta spin)
How are radio waves used in NMR spectroscopy?
Application of radio waves causes nuclei in the low spin state to absorb this radiation and flip into a high state
This absorption of this spin flip energy is measured and recorded as an NMR spectrum
Nucleus which is flipped is said to be in resonance
What are the conditions for NMR spectroscopy?
Odd or even mass number
Odd atomic number
What are the two common isotopes used in O chem?
1H- protium
Carbon-13
Why is the absorption frequency not the same for all 1H and 13C nuclei in a molecule?
Because of shielding
When an external magnetic field is applied, what occurs to the electrons?
The moving electrons set up their own tiny local magnetic fields, this acts in opposition to the applied field which weakens the effect of the applied field on the nucleus (shielding)
Explain in detail, why absorption frequency is not the same for all 1H and 13C nuclei in a molecule?
Each specific nucleus in a molecule is in a slightly different electronic environment because each nucleus is shielded to a slightly different extent and thus the effective magnetic field felt by each is slightly different
How can the complete effect of an external magnetic field be calculated?
B effective= B applied- B local
When B is the external magnetic field
Chemically equivalent nuclei always show …
A single absorption
Why do the 3 hydrogens on the methyl group of methyl acetate show a single absorption?
They have the same chemical and magnetic environment, are shielded to the same extent and are said to be chemically equivalent
Methyl acetate shows 3 absorption’s, what does this indicate?
There must be 3 chemically distinct carbons in the molecule
Contrast hydroxyl protons and methyl protons in terms of shielding and absorption
Hydroxyl protons are not shielded as much as methyl protons so hydroxyl protons absorption at a lower field than methyl protons
Protons are somewhat de-shielded by the electronegative oxygen atom
How does shielding affect absorption?
The more the shielding, the more the absorption
Equivalent protons are such in a molecule are those that have… and such protons have…
Identical electronic environments
Identical locations in the 1H NMR
Nonequivalent protons will have…
Different locations in the 1H NMR
Protons can be considered equivalent if…
They can be interchanged by a free rotation or a symmetry operation
What are the parts to the original NMR spectrometer
- A stable magnet with a sensitive controller to produce a precise magnetic field
- A Radio Frequency transmitter emitting a precise wave( continuous wave/CW)
- A detector to measure the sample’s absorption of Radio Frequency
- A recorder to plot the output from the detector against the applied magnetic field
- The printer records a graph of absorption(y graph) as a function of the applied magnetic field ( X axis)
The more shielded methyl protons appear toward the _________ of the spectrum while the less shielded oxygen protons appears toward the ______ of the spectrum. Why is this?
Right
Left
More Shielded methyl protons have higher field strength e for resonance
Chemical shift is difficult to measure practically which is why…
An accurate method for expressing chemical shift is to determine chemical shift by determining the value relative to TMS with TMS being set to the zero point and other peaks occur left on the chart
Why is TMS used an internal standard?
Tetramethylsilane is used as silicon is less electronegative so it offers more shielding to the carbon. TMS protons are highly shielded
What does the number of signals show in NMR?
The number of signals shows how many different kinds of protons are present
What does the location show in NMR?
Shows how shielded or deshielded the proton is
What does the intensity show in NMR?
Shows the number of protons of that type
What does signal splitting show in NMR?
Shows the number of protons on adjacent atoms
How do electronegative atoms effect location of signals?
More electronegative atoms deshield more and give larger chemical shift values
- This effect decreases with distance
- additional electronegative atoms cause increase in chemical shift
Account for the chemical shifts greater than 10 in carboxylic acid protons
Because of the high polarity of the carboxylic O-H, the shifts for the acidic proton will be at shifts greater than 10
On tert-butyl acetoacetate, why is methylene group the most deshielded group?
Because it is between two carbonyl groups
What is spin-spin splitting?
Phenomenon when protons interact with other protons that are not equivalent to it due to the magnetic field effecting the proton is being influenced by surrounding protons
Explain the effect of distance on spin splitting
Spin splitting effect tends to fall off with distance but cam often extend over 2 adjacent carbons
This means nearby protons that are nonequivalent will cause a splitting in the observed 1HNMR signal
Explain how the degree of splitting is affected by the number of adjacent hydrogens
The number of adjacent hydrogens will cause a split of n+1 times where n is the number of neighboring protons
Use the splitting rule to give the number/ name of splits for the equivalent number of neighboring nonequivalent hydrogens
Number of neighboring protons. Splitting
- 1-singlet
- 2-doublet
- 3-triplet
- 4-quartet
- 5- quintet(or multiplet)
- 6- sextet(or multiplet)
What is long range coupling?
This is when spin-spin splitting is observed between hydrogen atoms 4 or more bonds apart
When hydrogen atoms are 4 or more bonds apart…
Spin-spin splitting is not USUALLY observed
How can 1H NMR be used for structural determination?
Most 1H NMR absorption’s occurs between 0 and 10 which can be divided into 5 regions
By remembering the position of these specific kinds of protons we can tell details about a molecule
Usually protons in a saturated sp3 hybridized molecule absorb at higher fields whereas molecules of sp2 hybridized absorb at lower fields
Protons bonded to electronegative atoms such as N, O or halogen also absorb as lower fields
In 2,2 dimethylproponoate, there are 2 peaks, one with a relative area of 3 and another at 1. What does this indicate?
For every hydrogen under the peak with a relative area of 1 there are 3 under the same environment with a relative environment of 3
Contrast and explain the chemical shift values of methane, chloromeyhane, dichloromethane, trichloromethane.
Methane- 0.2
Chloromethane- 3.0
Dichloromethane- 5.3
Trichloromethane- 7.2
More electronegative atoms present de-shield more and give larger shift values
Additional electronegative atoms cause increase in chemical shift
What is the usual chemical shift range for aromatic protons in 1H NMR?
About 6.5 to 8
What is the usual chemical shift range for vinyl if protons in 1H NMR?
About 4.5 to 6.5
What is the usual chemical shift range for protons near oxygen, nitrogen or a halogen?
About 2.5 to 4.5
What is the usual chemical shift range of protons in an allylic compound?
1.5 to 2.5
What is the chemical shift for protons in a saturated compound?
0 to 1.5
What is the exact position on the chart at which a nucleus absorbs?
Chemical shift
What can integration/electronic measuring the area under each peak be used for?
To measure the relative numbers of the different kinds of protons in a molecule
