What is tetramethylsilane (TMS) and why is it used as a reference compound in NMR?

TMS is Si(CH 3 ) 4 , used as the internal standard in NMR spectroscopy because it is non-toxic, does not react with the sample, has a low boiling point for easy removal and produces a single, sharp absorption peak defined as 0 ppm.

NMR spectroscopy can provide information about the positions of .......... and .......... atoms in a molecule. Chemical shifts are measured relative to TMS and reported in units of .......... .

NMR spectroscopy can provide information about the positions of 13 C and 1 H atoms in a molecule. Chemical shifts are measured relative to TMS and reported in units of parts per million (ppm) .

True or False? The TMS peak appears at 0 ppm and sample peaks appear as a chemical shift away from this reference signal.

True. TMS gives a single sharp peak at 0 ppm. The difference in position between a sample peak and this reference is the chemical shift (δ), measured in ppm.

State four properties that make TMS a suitable NMR reference compound.

Non-toxic Does not react with the sample Easily separated from the sample due to its low boiling point Produces one strong, sharp absorption peak at 0 ppm

In NMR, the chemical shift of a proton depends on its .......... environment. Protons in the same environment are described as chemically .......... .

In NMR, the chemical shift of a proton depends on its chemical environment. Protons in the same environment are described as chemically equivalent .

True or False? TMS has a high boiling point, which makes it difficult to separate from the NMR sample after analysis.

False. TMS has a low boiling point, which makes it easy to separate from the sample after NMR analysis.

The position of an NMR signal on the spectrum, measured in parts per million (ppm) relative to the reference compound TMS at 0 ppm. It reflects the electronic environment of the nucleus being measured.

What does the integration of a peak in a 1 H NMR spectrum represent?

An enantiomer is one of two non-superimposable mirror-image forms of a chiral molecule. Enantiomers have identical physical properties except for the direction in which they rotate plane-polarised light.

The n + 1 rule states that a proton with .......... equivalent protons on adjacent carbon atoms will be split into .......... peaks. A CH 2 group adjacent to a CH 3 group therefore appears as a .......... .

The n + 1 rule states that a proton with n equivalent protons on adjacent carbon atoms will be split into n + 1 peaks. A CH 2 group adjacent to a CH 3 group therefore appears as a quartet .

True or False? In a 1 H NMR spectrum, the height of a peak is used to determine the ratio of protons in different environments.

False. It is the area under each peak (the integration) that gives the ratio of protons — not the height. Peaks can vary in height but still represent the same number of protons.

What splitting pattern is produced by a CH 3 group adjacent only to a CH 2 group, and what are the relative intensities?

A triplet (n + 1 = 2 + 1 = 3). The relative intensities are 1 : 2 : 1.

The chemical shift range for an aldehyde proton (RCHO) is .......... ppm, and for a carboxylic acid proton (RCOOH) is .......... ppm.

The chemical shift range for an aldehyde proton (RCHO) is 9.0–10.0 ppm, and for a carboxylic acid proton (RCOOH) is 10.0–12.0 ppm.

True or False? A quartet and a triplet appearing together in a 1 H NMR spectrum typically indicate the presence of an ethyl group (CH 3 CH 2 –).

True. The CH 3 protons (n = 2 neighbours) appear as a triplet and the CH 2 protons (n = 3 neighbours) appear as a quartet . This pair is the diagnostic pattern for an ethyl group.

How many 1 H NMR peaks does ethanol (CH 3 CH 2 OH) show, and what is the integration ratio?

Three peaks — one for each distinct proton environment: CH 3 , CH 2 and OH. The integration ratio is 3 : 2 : 1 respectively.

In propane (CH 3 CH 2 CH 3 ), the central CH 2 group is adjacent to .......... equivalent protons in total, so its signal is split into a .......... ( .......... peaks).

In propane (CH 3 CH 2 CH 3 ), the central CH 2 group is adjacent to six equivalent protons in total, so its signal is split into a heptet ( seven peaks).

Why do carbon atoms appear in a 13 C NMR spectrum rather than the more abundant 12 C?

12 C has an even mass number and does not give an NMR signal. 13 C is a less abundant isotope with an odd mass number, so it responds to the applied magnetic field and produces signals on a 13 C NMR spectrum.

In a 13 C NMR spectrum, the number of peaks equals the number of .......... carbon environments. Unlike 1 H NMR, the height of each peak is .......... to the number of carbon atoms in that environment.

In a 13 C NMR spectrum, the number of peaks equals the number of non-equivalent carbon environments. Unlike 1 H NMR, the height of each peak is not proportional to the number of carbon atoms in that environment.

True or False? 13 C NMR spectra show complex splitting patterns similar to those seen in high-resolution 1 H NMR.

False. 13 C NMR displays sharp single signals with no complex splitting patterns, making the spectrum easier to interpret by simply counting the number of carbon environments.

How many 13 C NMR peaks does propanone (CH 3 COCH 3 ) show, and why?

Two peaks — one for the two equivalent CH 3 carbons and one for the carbonyl carbon (C=O). The two methyl groups are in identical environments, so they give a single combined signal.

In 13 C NMR, all chemical shifts are measured relative to .......... at 0 ppm. The chemical shift range for C=O in a ketone is approximately .......... ppm.

In 13 C NMR, all chemical shifts are measured relative to TMS at 0 ppm. The chemical shift range for C=O in a ketone is approximately 190–220 ppm.

True or False? Counting the number of 13 C resonances is useful for distinguishing structural isomers because isomers with different symmetry will show different numbers of peaks.

True. For example, the three isomers of dihydroxybenzene have different symmetry, so they produce different numbers of 13 C peaks. Counting peaks is a quick first step in spectral analysis.

How many 13 C NMR peaks does 1,3-dihydroxybenzene show?

Four peaks, corresponding to four distinct carbon environments: The carbon bearing OH, the carbon adjacent to both OH groups, and the two remaining distinct aromatic carbons, as identified by the molecule's symmetry.

The unique set of bonds, atoms, and electronic influences surrounding a particular nucleus in a molecule. Nuclei in the same chemical environment are chemically equivalent and give a single peak in an NMR spectrum.

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Nuclear Magnetic Resonance Spectroscopy (AQA A Level Chemistry): Flashcards

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What is tetramethylsilane (TMS) and why is it used as a reference compound in NMR?

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What is tetramethylsilane (TMS) and why is it used as a reference compound in NMR?
TMS is Si(CH₃)₄, used as the internal standard in NMR spectroscopy because it is non-toxic, does not react with the sample, has a low boiling point for easy removal and produces a single, sharp absorption peak defined as 0 ppm.
NMR spectroscopy can provide information about the positions of .......... and .......... atoms in a molecule. Chemical shifts are measured relative to TMS and reported in units of .......... .
NMR spectroscopy can provide information about the positions of ¹³C and ¹H atoms in a molecule. Chemical shifts are measured relative to TMS and reported in units of parts per million (ppm).
True or False?
The TMS peak appears at 0 ppm and sample peaks appear as a chemical shift away from this reference signal.
True.
TMS gives a single sharp peak at 0 ppm. The difference in position between a sample peak and this reference is the chemical shift (δ), measured in ppm.
State four properties that make TMS a suitable NMR reference compound.
1. Non-toxic
2. Does not react with the sample
3. Easily separated from the sample due to its low boiling point
4. Produces one strong, sharp absorption peak at 0 ppm
In NMR, the chemical shift of a proton depends on its .......... environment. Protons in the same environment are described as chemically ...........
In NMR, the chemical shift of a proton depends on its chemical environment. Protons in the same environment are described as chemically equivalent.
True or False?
TMS has a high boiling point, which makes it difficult to separate from the NMR sample after analysis.
False.
TMS has a low boiling point, which makes it easy to separate from the sample after NMR analysis.
Chemical shift
The position of an NMR signal on the spectrum, measured in parts per million (ppm) relative to the reference compound TMS at 0 ppm. It reflects the electronic environment of the nucleus being measured.
What does the integration of a peak in a ¹H NMR spectrum represent?
An enantiomer is one of two non-superimposable mirror-image forms of a chiral molecule. Enantiomers have identical physical properties except for the direction in which they rotate plane-polarised light.
The n + 1 rule states that a proton with .......... equivalent protons on adjacent carbon atoms will be split into .......... peaks. A CH₂ group adjacent to a CH₃ group therefore appears as a .......... .
The n + 1 rule states that a proton with n equivalent protons on adjacent carbon atoms will be split into n + 1 peaks. A CH₂ group adjacent to a CH₃ group therefore appears as a quartet.
True or False?
In a ¹H NMR spectrum, the height of a peak is used to determine the ratio of protons in different environments.
False.
It is the area under each peak (the integration) that gives the ratio of protons — not the height. Peaks can vary in height but still represent the same number of protons.
What splitting pattern is produced by a CH₃ group adjacent only to a CH₂ group, and what are the relative intensities?
A triplet (n + 1 = 2 + 1 = 3).
The relative intensities are 1 : 2 : 1.
The chemical shift range for an aldehyde proton (RCHO) is .......... ppm, and for a carboxylic acid proton (RCOOH) is .......... ppm.
The chemical shift range for an aldehyde proton (RCHO) is 9.0–10.0 ppm, and for a carboxylic acid proton (RCOOH) is 10.0–12.0 ppm.
True or False?
A quartet and a triplet appearing together in a ¹H NMR spectrum typically indicate the presence of an ethyl group (CH₃CH₂–).
True.
The CH₃ protons (n = 2 neighbours) appear as a triplet and the CH₂ protons (n = 3 neighbours) appear as a quartet. This pair is the diagnostic pattern for an ethyl group.
How many ¹H NMR peaks does ethanol (CH₃CH₂OH) show, and what is the integration ratio?
Three peaks — one for each distinct proton environment: CH₃, CH₂ and OH.
The integration ratio is 3 : 2 : 1 respectively.
In propane (CH₃CH₂CH₃), the central CH₂ group is adjacent to .......... equivalent protons in total, so its signal is split into a .......... (.......... peaks).
In propane (CH₃CH₂CH₃), the central CH₂ group is adjacent to six equivalent protons in total, so its signal is split into a heptet (seven peaks).
Why do carbon atoms appear in a ¹³C NMR spectrum rather than the more abundant ¹²C?
¹²C has an even mass number and does not give an NMR signal. ¹³C is a less abundant isotope with an odd mass number, so it responds to the applied magnetic field and produces signals on a ¹³C NMR spectrum.
In a ¹³C NMR spectrum, the number of peaks equals the number of .......... carbon environments. Unlike ¹H NMR, the height of each peak is .......... to the number of carbon atoms in that environment.
In a ¹³C NMR spectrum, the number of peaks equals the number of non-equivalent carbon environments. Unlike ¹H NMR, the height of each peak is not proportional to the number of carbon atoms in that environment.
True or False?
¹³C NMR spectra show complex splitting patterns similar to those seen in high-resolution ¹H NMR.
False.
¹³C NMR displays sharp single signals with no complex splitting patterns, making the spectrum easier to interpret by simply counting the number of carbon environments.
How many ¹³C NMR peaks does propanone (CH₃COCH₃) show, and why?
Two peaks — one for the two equivalent CH₃ carbons and one for the carbonyl carbon (C=O).
The two methyl groups are in identical environments, so they give a single combined signal.
In ¹³C NMR, all chemical shifts are measured relative to .......... at 0 ppm. The chemical shift range for C=O in a ketone is approximately .......... ppm.
In ¹³C NMR, all chemical shifts are measured relative to TMS at 0 ppm. The chemical shift range for C=O in a ketone is approximately 190–220 ppm.
True or False?
Counting the number of ¹³C resonances is useful for distinguishing structural isomers because isomers with different symmetry will show different numbers of peaks.
True.
For example, the three isomers of dihydroxybenzene have different symmetry, so they produce different numbers of ¹³C peaks. Counting peaks is a quick first step in spectral analysis.
How many ¹³C NMR peaks does 1,3-dihydroxybenzene show?
Four peaks, corresponding to four distinct carbon environments:
The carbon bearing OH, the carbon adjacent to both OH groups, and the two remaining distinct aromatic carbons, as identified by the molecule's symmetry.
Chemical environment
The unique set of bonds, atoms, and electronic influences surrounding a particular nucleus in a molecule. Nuclei in the same chemical environment are chemically equivalent and give a single peak in an NMR spectrum.

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