Spectra L7

Question 1

Describe chemical equivalence

Answer 1

If a plane of symmetry or a rotation axis renders two or more nuclei equivalent by symmetry they are chemically equivalent

Question 2

What do chemically equivalent nuclei have

Answer 2

Chemically equivalent nuclei have the same chemical shift

Question 3

Describe spectra of chemically equivalent nuclei

Answer 3

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

Question 4

Why is coupling not observed between 13C nuclei in NMR?

Answer 4

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.

Question 5

What is the problem with coupling between 13C coupling with 1H in 13C spectrum

Answer 5

coupling between 13C and 1H should be observed in 13C spectrum. Coupling reduces signal being of already weak peaks

Question 6

What is the solution to the problem which is coupling between 13C coupling with 1H in 13C spectrum

Answer 6

Decouple 1H nuclei

Question 7

Describe decoupling 1H nuclei in 13C nmr

Answer 7

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.

Question 8

Describe the nuclear overhauser effect

Answer 8

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

Question 9

What does nOe stand for

Answer 9

Nuclear Overhauser effect

Question 10

What is the consequence of nOe

Answer 10

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

Question 11

What are 13C shifts affected by

Answer 11

Nature of adjacent atoms
Hybridisation of C
Anisotropic effect

Question 12

What does dept stand for

Answer 12

Distortionless enhancement by polarisation transfer

Question 13

What does dept do

Answer 13

A technique used to enhance the
13C NMR spectrum by selectively detecting carbon-hydrogen bonds.
It helps distinguish between different types of carbons.

Question 14

What does DEPT 90 show

Answer 14

Shows only CH signals

Question 15

What does DEPT 135 show

Answer 15

CH and CH3 carbons appear positive, CH2 carbons appear negative.

Question 16

Important note about DEPT experiment

Answer 16

DEPT experiments do no snow quaternary IPSP 13C signals

Question 17

What does IPSO mean in NMR

Answer 17

Refers to a position on an aromatic ring that is directly bonded to a substituent.

Question 18

Which nuclei have low natural abundance

Answer 18

13C
29Si
77Se
195Pt
207Pb

Question 19

Describe the spectra of low abundance nuclei

Answer 19

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

Question 20

Define satellites

Answer 20

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

Question 21

What does the intensity of satellites depend on

Answer 21

Number of adjacent nuclei

Question 22

What is broadband decoupling indicated by

Answer 22

13C{1H}

Question 23

Describe when the effect of nOe would be greater

Answer 23

the more 1H nuclei bonded to 13C the greater the effect
CH3>CH2>CH

Question 24

describe nOe for quarternary or ipso atoms

Answer 24

no nOe for quarternary or ipso C atoms as they have no directly bonded hydrogens

Question 25

What does nOe decrease by and what does this mean

Answer 25

nOe decreases as r^-6
R= internuclear separation
Therefore 1H on adjacent C atoms have little effect

Question 26

Describe the difference in relaxation times for 13C nuclei

Answer 26

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

Question 27

Describe the relaxation times for quarternary/ ipso C atoms

Answer 27

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

Question 28

Which nuclei have a high natural abundance

Answer 28

19F
31P
103RH