NMR3 Flashcards
(17 cards)
Consider the keto-enol tautomerism below. The hydrogen atom shown in red has a chemical shift of 3.41 ppm in the keto form, and 4.98 ppm in the enol form.
Imagine an exchange rate of 100 s-1: How would the resonance of the red hydrogen atom appear at a field strength of 14.1 T?
two separate signals
one average signal
coalescense (broad signal)
two separate signals, due to the slow rate
If you have a signal of exchanging hydrogen atoms at coalescense, but you need to know the chemical shifts of both the hydrogen atoms, what could you do (more than one answer possible):
lower the temperature
raise temperature
go to a lower magnetic field strength
go to a higher magnetic field strength
lower the temperature and go to a higher magnetic field strength
What is dynamic NMR?
It is the observation of a chemical shift of an atom while it is changing. This can occur due to chemical procceses in the sample.
What is a coalescense temperature?
The coalescence temperature is the temperature/point at which two distinct NMR signals merge into one because the rate of exchange (e.g., conformational change, proton transfer, ring flip) becomes fast on the NMR timescale.
What are the typical ¹J, ²J, and ³J C–H coupling constants in NMR?
¹J(C–H) = 140–180 Hz
²J(C–H) = 1–3 Hz
³J(C–H) = 3–8 Hz
What is de-coupling?
Decoupling is a technique used in NMR spectroscopy to eliminate scalar (J) coupling between certain nuclei, usually to simplify the spectrum by collapsing multiplets into singlets. Mostly used for removing ¹H–¹³C coupling, so each carbon appears as a single sharp peak, rather than split into multiplets.
How can you collaps two signals into one in dynamic NMR?
If two environments are in chemical exchange, and the exchange rate becomes faster than the frequency difference (Δν) between them, the NMR signals will collapse into one. This can be achieved by increasing the temperature or irradiating with 180° pulses to speed up the exchange.
Why does it take much longer time to record a 13C NMR spectrum than a 1H spectrum?
- 13C has a lower gyromagnetic ratio than 1H
- 13C has a nuclear spin of 3/2
- The natural abundance of 13C is only 1.1%
- There are alot more hydrogen than carbon atoms in the molecule
13C has a lower gyromagnetic ratio than 1H
The natural abundance of 13C is only 1.1%
What could you do to increase the intensity/ shorten the measurement time of a 13C spectrum?
- Increase the amount of compound
- Order the compound with enriched 13C
- Use a larger magnet
- Change the nuclear spin of 13C
- Increase the amount of compound
- Order the compound with enriched 13C
- Use a larger magnet
13C will have coupling to 13C and 1H, the latter is often undesirable. What can be done about 1H coupling?
- Increase the amount of compound
- Use a larger magnet
- Invert the 1H magnetization with 180 degree pulses faster than the coupling constant
- Change the nuclear spin of 13C
Invert the 1H magnetization with 180 degree pulses faster than the coupling constant
What is DEPT-spektra used for?
To keep the information about the multiplicies when using de-coupling.
Which signals are shown in a dept-45 spectra?
CH, CH_2, CH_3
Which signals are shown in a dept-90 spectra?
CH
Which signals are shown in a dept-135 spectra?
CH_3, CH positive, CH_2 negative
Can quatinary carbon (CH_4) in a dept-spectra?
no, instead APT is used
What is a homo-decouping?
It is 1H decoupling in a 1H spectra
Which of the following gives signals in a APT experiment?
C (negative),CH (positive),CH2 (negative), CH3 (positive)
