NMR

Question 2

What does NMR spectroscopy stand for?

Answer 2

Nuclear Magnetic Resonance spectroscopy

Question 3

What particles are detected in NMR?

Answer 3

Certain atomic nuclei with an odd number of protons and/or neutrons, e.g. ¹H and ¹³C.

Question 4

What is the principle behind NMR?

Answer 4

Nuclei in a magnetic field absorb radiofrequency radiation, depending on their environment.

Question 5

What is a chemical shift measured in?

Answer 5

Parts per million (ppm)

Question 6

What reference compound is used in NMR?

Answer 6

Tetramethylsilane (TMS)

Question 7

Why is TMS used as a standard?

Answer 7

It has a single, sharp peak at 0 ppm and is inert, non-toxic, and volatile.

Question 8

What does each peak in a ¹³C NMR spectrum represent?

Answer 8

A different carbon environment in the molecule.

Question 9

Why do some compounds have fewer peaks than carbon atoms?

Answer 9

Symmetrical environments cause multiple carbons to produce the same signal.

Question 11

What does a peak around 160–220 ppm indicate?

Answer 11

A carbon in a carbonyl group (e.g. ketone, aldehyde, acid, ester).

Question 12

What does each peak in a ¹H NMR spectrum represent?

Answer 12

A different hydrogen (proton) environment in the molecule.

Question 13

What is meant by integration in ¹H NMR?

Answer 13

The relative area under each peak shows the number of hydrogens in that environment.

Question 14

What causes splitting in ¹H NMR?

Answer 14

Spin-spin coupling due to neighbouring non-equivalent protons (n+1 rule).

Question 15

What is the splitting pattern for 2 adjacent Hs?

Answer 15

Triplet (n+1 = 2+1 = 3)

Question 16

What is the splitting pattern for 3 adjacent Hs?

Answer 16

Quartet (n+1 = 3+1 = 4)

Question 17

Which types of protons do not split?

Answer 17

Exchangeable protons (OH or NH) often appear as broad singlets.

Question 18

What is meant by a ““chemical environment””?

Answer 18

The electronic surroundings of a nucleus, affected by nearby atoms and bonds.

Question 19

How does symmetry affect the number of environments?

Answer 19

Symmetrical atoms share the same environment and give a single peak.

Question 20

How can you deduce the structure of a compound using NMR?

Answer 20

Combine information about number of environments, chemical shifts, splitting, and integration.

Question 21

Why are deuterated solvents used in NMR?

Answer 21

They do not produce interfering ¹H signals (deuterium is ²H).

Question 22

Give an example of a deuterated solvent.

Answer 22

CDCl₃ (deuterated chloroform)

Question 23

A ¹³C NMR spectrum shows 3 peaks for a compound with 4 carbon atoms. What does this suggest?

Answer 23

Two carbon atoms are in the same environment (symmetrical structure).

Question 24

In a ¹H NMR spectrum

Answer 24

a peak integrates to 3H and appears as a singlet. What group is likely?

Question 25

In a ¹H NMR spectrum

Answer 25

a triplet integrating to 2H and a quartet integrating to 3H suggests what?

Question 26

What is the chemical shift range for protons in aldehydes?

Answer 26

~9–10 ppm

Question 27

What is the chemical shift range for protons in carboxylic acids?

Answer 27

~10–12 ppm (broad)

Question 28

What is the chemical shift range for protons on benzene rings (aromatic)?

Answer 28

~6.5–8.5 ppm

Question 29

What is the chemical shift range for OH/NH protons?

Answer 29

~1–5 ppm (broad, variable)

Question 30

What is the formula for working out the number of peaks in ¹H NMR from adjacent protons?

Answer 30

n + 1 rule (where n = number of adjacent non-equivalent protons)

Question 31

How do you calculate the ratio of protons from integration trace?

Answer 31

Use the relative areas under the peaks and simplify to smallest whole-number ratio.

Question 32

Explain why tetramethylsilane (TMS) is used as a standard in NMR. (2 marks)

Answer 32

It produces a single sharp peak at 0 ppm and is inert/non-reactive.

Question 33

A compound gives two ¹³C NMR peaks and three ¹H NMR peaks. What does this suggest about the structure? (2–3 marks)

Answer 33

Two carbon environments (likely some symmetry) and three distinct hydrogen environments.

Question 34

A ¹H NMR spectrum shows: 1H singlet at 9.8 ppm

Answer 34

2H quartet at 4.2 ppm

Question 35

What is the typical chemical shift for a carbon in a carboxylic acid group?

Answer 35

~170–185 ppm

Question 36

What is the typical chemical shift range for carbon atoms in benzene rings (aromatic)?

Answer 36

~110–160 ppm

Question 37

Why do some carbon atoms give weaker peaks in ¹³C NMR?

Answer 37

Because ¹³C is only ~1% naturally abundant, and quaternary carbons (no H) give weaker signals.

Question 38

What does a broad singlet in the range 1–5 ppm often suggest?

Answer 38

An OH or NH proton

Question 39

Why do OH/NH protons often not follow splitting patterns?

Answer 39

They rapidly exchange with water and are not coupled to nearby protons.

Question 40

What is the typical chemical shift range for -CH₂OH (primary alcohol)?

Answer 40

3.3–4.0 ppm for the CH₂, and ~1–5 ppm for the OH

Question 41

A ¹H NMR spectrum shows a 1H singlet at 11 ppm. What is the likely functional group?

Answer 41

Carboxylic acid proton (COOH)

Question 42

How can symmetry be used to simplify NMR analysis?

Answer 42

If parts of the molecule are symmetrical, identical environments will produce fewer peaks.

Question 43

What combination of NMR techniques helps confirm a compound’s structure?

Answer 43

Use both ¹H and ¹³C NMR spectra, along with IR and mass spectrometry.

Question 44

How do you identify an ethyl group using ¹H NMR?

Answer 44

Look for a triplet (CH₃) and a quartet (CH₂) with a 3:2 integration ratio.

Question 45

A compound shows a ¹H NMR singlet integrating to 3H at ~2.1 ppm. What could this be?

Answer 45

A methyl group adjacent to a carbonyl (e.g., CH₃CO−)

Question 46

Suggest the structure of a compound with molecular formula C₄H₈O₂

Answer 46

and the following NMR data: ¹H NMR: 1H singlet (12 ppm)

Question 47

A compound has the formula C₄H₈O and shows the following: ¹H NMR: 2H quartet (4.1 ppm)

Answer 47

3H triplet (1.3 ppm); ¹³C NMR: 4 peaks Suggest the structure.

Question 48

A compound shows only 2 peaks in its ¹³C NMR but has 4 carbon atoms. What does this suggest?

Answer 48

High symmetry — possibly 2 pairs of equivalent carbons, e.g., butan-2-one (CH₃COCH₂CH₃)

Question 49

How can NMR help distinguish between isomers with the same molecular formula?

Answer 49

Isomers produce different numbers and types of NMR signals (due to different environments).

Question 50

What extra information can IR give when interpreting NMR spectra?

Answer 50

Functional groups (e.g., O-H, C=O) confirmed via IR to support NMR data.

Question 51

what does NMR spectroscopy stand for?

Answer 51

nuclear magnetic resonance (NMR) spectroscopy

Question 52

what is NMR spectroscopy used for?

Answer 52

it is used to help determine the structure of a molecule

Question 53

what does 13C NMR tell us?

Answer 53

tells us information on how carbon atoms are arranged

Question 54

what does 1H NMR tell us?

Answer 54

tells us information on how hydrogen atoms are arranged

Question 55

how does an atomic nucleus gain a nuclear spin?

Answer 55

if it has an odd number of nucleons (protons and neutrons)

Question 56

how does NMR work?

Answer 56

the nuclear spin creates a weak magnetic field NMR detects how these magnetic fields are affected by a larger external magnetic field NMR fires out radio waves At a specific frequency, the nuclei that are aligned with the magnetic field absorb the energy and flip to the higher level Those with higher energy can also drop to lower energy and emit radio waves. NMR measures the amount of energy absorbed