Spectra L7 Flashcards
Describe chemical equivalence
If a plane of symmetry or a rotation axis renders two or more nuclei equivalent by symmetry they are chemically equivalent
What do chemically equivalent nuclei have
Chemically equivalent nuclei have the same chemical shift
Describe spectra of chemically equivalent nuclei
Chemically equivalent nuclei have one signal with varying intensities. They do not exhibit coupling to one another but do couple to other nuclei which are chemically not equivalent
Why is coupling not observed between 13C nuclei in NMR?
The probability of finding an 13C adjacent to another 13C in a molecule is very low. Even if two 13C nuclei are near each other, the coupling constant is usually too weak to be detected.
What is the problem with coupling between 13C coupling with 1H in 13C spectrum
coupling between 13C and 1H should be observed in 13C spectrum. Coupling reduces signal being of already weak peaks
What is the solution to the problem which is coupling between 13C coupling with 1H in 13C spectrum
Decouple 1H nuclei
Describe decoupling 1H nuclei in 13C nmr
All proton frequencies irradiates. This saturates spins of 1H nuclei. Therefore the is no C-H coupling and each 13C is a singlet in absence of coupling to another NMR active nucleus.
Describe the nuclear overhauser effect
The proton frequencies irradiated which makes the 1H spin 1/2 and -1/2 spin states equal. As a result the 1H spins are saturated so there are more 1H nuclei in the upper state, the 13C nuclei sense this and adjust so more 13C in the lower state. This means there is a greater than normal population difference for 13C nuclei so there is a stronger 13C signal
What does nOe stand for
Nuclear Overhauser effect
What is the consequence of nOe
Intensity of signals not related to number of 13C nuclei in chemical environments. Peak height is related to the number of hydrogens on that carbon
What are 13C shifts affected by
Nature of adjacent atoms
Hybridisation of C
Anisotropic effect
What does dept stand for
Distortionless enhancement by polarisation transfer
What does dept do
A technique used to enhance the
13C NMR spectrum by selectively detecting carbon-hydrogen bonds.
It helps distinguish between different types of carbons.
What does DEPT 90 show
Shows only CH signals
What does DEPT 135 show
CH and CH3 carbons appear positive, CH2 carbons appear negative.
Important note about DEPT experiment
DEPT experiments do no snow quaternary IPSP 13C signals
What does IPSO mean in NMR
Refers to a position on an aromatic ring that is directly bonded to a substituent.
Which nuclei have low natural abundance
13C
29Si
77Se
195Pt
207Pb
Describe the spectra of low abundance nuclei
For most of the elements the remaining nuclei have I=O and therefore are NMR inactive. The spectrum of low abundance nuclei are weak but observable. If other NMR active nuclei are present we will observe scalar coupling
Define satellites
satellites are small peaks that appear on either side of the main peak in an NMR spectrum. They are caused by the coupling of an NMR-active atom to a neighboring atom that is not 100% abundant
What does the intensity of satellites depend on
Number of adjacent nuclei
What is broadband decoupling indicated by
13C{1H}
Describe when the effect of nOe would be greater
the more 1H nuclei bonded to 13C the greater the effect
CH3>CH2>CH
describe nOe for quarternary or ipso atoms
no nOe for quarternary or ipso C atoms as they have no directly bonded hydrogens
What does nOe decrease by and what does this mean
nOe decreases as r^-6
R= internuclear separation
Therefore 1H on adjacent C atoms have little effect
Describe the difference in relaxation times for 13C nuclei
The more hydrogens on the carbon the shorter the relaxation time. If pulse delay too short then the the 13C nuclei may not fully relax and the signals may not be as strong
Describe the relaxation times for quarternary/ ipso C atoms
no bonded hydrogens so the longer the relaxation times. Quarternary C atoms usually have a relaxation times greater than 10 seconds. Signals are a lot weake
Which nuclei have a high natural abundance
19F
31P
103RH
