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Synthesis and Spectroscopy > Spectroscopy Topic 4 > Flashcards

Spectroscopy Topic 4 Flashcards

1
Q

What is peak integration?

A

The relative number of protons in the signal

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2
Q

What is the typical range of 1H NMR?

A

0-12 ppm

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3
Q

What is shown in the range 0-2 ppm in 1H NMR?

A

Aliphatic H

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4
Q

What is shown in the range 2-4.5 ppm in 1H NMR?

A

Aliphatic H near strong electron withdrawing groups

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5
Q

What is shown in the range 5-6 ppm in 1H NMR?

A

Olefinic H

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6
Q

What is shown in the range 6.6-8.5 ppm in 1H NMR?

A

Aromatic H

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7
Q

What is shown in the range 9-11 ppm in 1H NMR?

A

CHO

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8
Q

What is shown in the range 9-12 ppm in 1H NMR?

A

COOH

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9
Q

What is inductive effect?

A

The chemical shift of aliphatic protons on carbon atoms adjacent to electron withdrawing groups (EWGs) is moved to a higher value.

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10
Q

What is olefinic?

A

Any of a class of unsaturated open-chain hydrocarbons having at least one double bond.

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11
Q

How resonance effects of olefinic or aromatic protons can lower chemical shift?

A

Electron donating group can push lone pair electrons from O atom through the molecule causing dhielding and lowering chemical shift.

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12
Q

How resonance effects of olefinic or aromatic protons can increase chemical shift?

A

Electrons move to electron withdrawing groups by deshielding molecules and increasing chemical shift.

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13
Q

What happens when molecules containing π -systems are placed in the magnetic field of an NMR spectrometer?

A

The electrons circulate within the π -orbitals and generate a secondary magnetic field which can influence the local magnetic field experienced by 1H

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14
Q

What are anisotropic effects?

A

In some regions of the molecule, the induced magnetic field is oriented with the direction of the external magnetic field, whereas in other regions it opposes it. This will increase or decrease the chemical shift of nuclei in those regions, respectively.

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15
Q

Why in aromatic molecule 1H NMR protons that are in the centre have lower chemical shift?

A

It is because π bonds create magnetic field that goes around the bond. In the centre of the aromatic ring magnetic field moves in opposide direction to the external magnetic field. The shemical shift will be equal to external magnetic field minus magnetic field made my π bond.

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16
Q

Why protons of alkyne have lower shemical shift than expected?

A

It is because π electrons move in NMR and form a magnetic field that will move in opposide direction through alkyne tripple bond than external magnetic field, lowering shemical shift.

17
Q

What is shown in the range 0.5-5.0 ppm in 1H NMR?

A

ROH groups

18
Q

What is shown in the range 0.5-4.5 ppm in 1H NMR?

A

RNH groups

19
Q

What is shown in the range 5.0-11 ppm in 1H NMR?

A

PhOH groups

20
Q

What is shown in the range 9-12 ppm in 1H NMR?

A

COOH group

21
Q

Why addition of D2O to a sample causes the disapperance of signals from protons on heteroatoms?

A

This is because the protons exchange with deuterium, which doesn’t give a signal in the proton chemical shift range.

22
Q

What is deuterium?

A

A stable isotope of hydrogen with a mass approximately twice that of the usual isotope.

23
Q

What causes splitting in 1H NMR?

A

The presence of 1Hs on adjacent carbon atoms.

24
Q

Why we do not prefer to remove coupling for 1H compared to 13C?

A

Signal overlap is less of a problem, and 1H NMR is much more sensitive than 13C NMR

25
Q

What causes coupling?

A

Coupling arises when NMR active nuclei interact with one another by altering the local magnetic field.

26
Q

What are two mechanisms nuclei couple?

A

Dipolar coupling and scalar coupling

27
Q

When does dipolar coupling occurs?

A

Dipolar coupling occurs through space, and is only seen when protons are held in fixed orientation with respect to each other (e.g. on double bonds and in some ring systems). In all other circumstances it averages out to zero as the positions of the nuclei move through rotation of the bonds between them.

28
Q

When does scalar coupling occurs?

A

Scalar coupling is mediated by electrons in bonds between nuclei. Since electrons have spin, they can transmit coupling between nuclei. This doesn’t average to zero through bond rotation.

29
Q

What is coupling constant, J?

A

The amount by which a signal is split. It is independent of the external magnetic field strength, as it is a property of the nuclei themselves, and can thus be quoted in Hz

30
Q

What prefix and suffic of J indicates?

A

A prefix is used to indicate how many bonds the coupling is over, whilst a suffix indicates the nuclei that are coupling.

31
Q

How to convert signal splitting in ppm to J in Hz?

A

J (Hz)= Δσ (ppm) x vres

Where σ is separation of signals in multiplet,

vres is operating frequency of NMR spectrometer

32
Q

In order to couple to each other, 1Hs need to be what?

A

Non-equivalent and close enough in the molecule for coupling to be efficient ytansmitted by electrons in the bonds between them

33
Q

What are the equivalent Hs?

A

They have identical chemical shifts, they are on the same carbon atom.

34
Q

Jow J is affected with increasing number of bonds between protons?

A

It decreases very rapidly

35
Q

What are the names of cuplings from 1 to 7?

A

Singlet, doublet, triplet, quarter, quintet, sextet, septet.

36
Q

What is Pascal triangle?

A
37
Q

What is peak integration?

A

This is a measurement of the area under the peak of an NMR signal. For 1H NMR this is proportional to the relative number of protons in the signal

38
Q

How to analyze peak integration?

A

The difference in height between the upper and lower horizontal parts of the sigmoid is the relative peak area value.

Synthesis and Spectroscopy (12 decks)

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