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

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 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. Breoadband decoupling indicated by 13C{1H}

Question 8

What effect does 1H decoupling have on the intensities of peaks

Answer 8

Irradiation of 1H nuclei leads to saturation of 1H spin states so there are more 1H nuclei than usual in higher energy spin states. 13C nuclei sense this and adjust their population so more 13C nuclei ads in lower spin states. This means there is a greater than nirmal population difference for 13C nuclei so the stronger the 13C signal

Question 9

What does nOe stand for

Answer 9

Nuclear Overhauser effect

Question 10

What is the nuclear overhauser effect

Answer 10

A through space effect where the signal intensity of one nucleus is affected by the presence of another nearby nucleus. It occurs when two nuclei are close to each other, affecting their relaxation times and
leading to changes in their signals.
Decreases as r^-6 where r is internuclear separation

Question 11

What is the consequence of nOe

Answer 11

Intensity of signals not related to number of 13C nuclei in chemical environments

Question 12

What are 13C shifts affected by

Answer 12

Nature of adjacent atoms
Hybridisation of C
Anisotropic effect

Question 13

What does dept stand for

Answer 13

Distortionless enhancement by polarisation transfer

Question 14

What does dept do

Answer 14

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

Question 15

What does DEPT 90 show

Answer 15

Shows only CH signals

Question 16

What does DEPT 135 show

Answer 16

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

Question 17

Important note about DEPT experiment

Answer 17

DEPT experiments do no snow quaternary IPSP 13C signals

Question 18

What does IPSO mean in NMR

Answer 18

Refers to a position on an aromatic ring that is directly bonded to a substituent. The ipso position is where a substitution occurs on the ring, often seen in aryl or aromatic compounds

Question 19

Which nuclei have low natural abundance

Answer 19

13C
29Si
77Se
195Pt
207Pb

Question 20

Describe the spectra of low abundance nuclei

Answer 20

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 21

Define satellites

Answer 21

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 22

What does the intensity of satellites depend on

Answer 22

Number of adjacent nuclei