Applications of NMR Spectroscopy in Organic Chem Flashcards
Give the expected values of 2J splitting.
10-18 Hz for most fixed position hydrogens 0-3 Hz for alkene hydrogens on one side
Give the 3J coupling values typical for alkenes and 6-membered rings
14-16 Hz trans alkene 8-10 Hz cis alkene 9-12 Hz trans diaxial 2-6 Hz ax-eq and eq-eq
What are typical values of 4J coupling and when does it occur?
0-3 Hz when a W shape is formed, such as in the chair conformation, benzene and in alkenes
How does the nuclei environment affect its position in the spectrum? How do different groups affect each other?
A higher number is a more deshielded nuclei, the effect is additive. Conjugation can increase or decrease the shift depending on where the conjugation goes to.
How do you determine how many lines a nuclei splits a signal into?
1 + 2I for each nuclei
How can DEPT experiments be used to distinguish carbon groups?
The pulse angle can change intensity and phase of certain signals. For θ = 45, all protonated carbons have positive signals, quaternary signals do not appear θ = 90, only CH signals appear and is positive θ = 135, CH3 point up, CH2 point down and CH point up
How can identifying one signal in a proton spectra lead to other identifications?
You can match the J values of the splitting between protons that split each other.
For a signal split by 2 protons with different J values (dd), how do you measure each J value?
Lines 1 to 2 gives one J value, lines 1 to 3 gives the other J value.
What happens when a pair of non-equivalent protons with the same J value (7) split a signal?
An apparent triplet is formed, labelled as dd J = 7, 7 Hz
How do you check if your measured splitting values are correct?
Add up all the values, it should be the same as the distance between the 2 furthest apart peaks.
If a peak is double as intense, what may have happened?
The splitting pattern has coincided to form the doubly intense peak. This may be due to 2 protons in the same environment, 2 of the same coupling constants by chance or where different coupling constants add up to another (couplings =5, 7, 2).
When and why does roofing occur?
When 2 signals that split each other have a very similar chemical shift, they will angle towards each other (in a doublet, the side close to the other signal will increase in height while the other decreases). In general, roofing occurs if |Ha - Hb| < 10 x J value
Give typical 2J spitting values, the limits of the range and when this would occur, and the situations where the splitting won’t be in the range
Typical range is 10-18 Hz When a carbon is adjacent to an electronegative atom the splitting will be reduced to ~10.5 Hz Adjacent pi systems (such as benzene and carbonyls) increase the splitting up to 18 Hz In terminal alkenes the coupling is either not seen, or in the range of 0-3 Hz
Give examples when geminal CH2 couplings are observed
In ring compounds, in molecules with one or more chiral centre and in terminal alkenes. Generally where there is 2 different environments between the protons.
What determines the magnitude of 3J coupling? Give the range of typical values.
A typical range is 0-12 Hz, it depends on the angle between the C-H bonds.
Give the common splitting values in cyclohexane rings with a brief description of why each has its magnitude.
3J ax-ax is approximately 9-12 Hz since the CH bonds are 180° to one another 3J ax-eq and eq-eq are approximately 2-6 Hz since the CH bonds are 60° to one another 2J coupling will be approximately 12 Hz
What are the typical values for alkene 3J splitting? Explain the origin of each value.
Trans hydrogens are held at 180° to each other and have good orbital overlap so they have a typical range of 14-16 Hz Cis hydrogens are held at 0° so have a typical range of 8-10 Hz
What is the trend of 3J values for decreasing cycloalkene ring size?
3J values decreases from 10 Hz for cyclohexene to 1 Hz for cyclopropene.
When do 4J couplings occur? What is a typical range and how should they be treated?
4J couplings occur between protons that have a W shape and a pi system have the best overlap. Example systems are benzene, alkene systems and cyclohexane. All have a range of 0-3 Hz. Never look for 4J coupling, just remember it when assigning coupling in the range of 0-3 Hz.
Give the general approach for assigning an NMR spectrum.
- Look at the chemical shift values, assign obvious signals 2. Look at integration values, label signals for clues in conjunction with other data 3. Look at multiplicity, start with protons with few splittings, then match couplings
When are protons diastereotopic?
If structures are diastereoisomers, then protons are diastereotopic. This means if there is one chiral centre, the protons on a CH2 group being different produce 2 diastereoisomers, hence they have different chemical shifts. Often the CH2 protons cannot be distinguished and are assigned as a pair of 2 signals.
How are OH and NH signals treated in NMR? Why is this the case?
They have a large possible range of chemical shift values and sometimes show coupling to other protons, this depends on the hydrogen bonding which is highly dependant on the environments. A D2O shake is used and the spectrum re-recorded to remove the OH and NH signals since they rapidly exchange with the deuterium faster than the timescale of the experiment. These peaks will be labelled as such in data.
How can OH signals be specifically observed and coupled to other signals?
By slowing down the exchange of protons. This can be achieved 3 ways: 1. Using an extremely dry solvent, removing water with a molecular sieve 2. Using a lower concentration of sample, the exchange is an intermolecular process, hence the rate will be lowered 3. Using a hydrogen bonding solvent such as DMSO which will slow down the rate of exchange
What is the nuclear overhauser effect and how is it used?
NOE is the change in intensity of the NMR signal due to dipole-dipole interactions between saturated and non-saturated nuclei. They occur through space and are not a coupling. They tell you when nuclei are close in space to one another which allows for further identification of nuclei.
The NOE spectrum is a difference spectrum observed when a certain group of the molecule is saturated with RF pulses (will not show as no relaxtion) which increases the intensity of adjacent groups (dipole-dipole interactions).
