NMR: Chemical Shifts and Coupling

What is the typical range of chemical shift values for protons in an NMR spectrum?

• 0 to 10 ppm

• 0 to 20 ppm

• 0 to 5 ppm

• 0 to 15 ppm

Answer explanation

The typical range of chemical shift values for protons in an NMR spectrum is 0 to 10 ppm. This range encompasses most organic compounds, making it the correct choice among the options provided.

Which of the following factors does NOT affect the chemical shift in NMR spectroscopy?

• Electronegativity of neighbouring atoms

• Temperature

• Magnetic field strength

• Molecular weight

Answer explanation

Molecular weight does not influence chemical shift in NMR spectroscopy. Chemical shifts are primarily affected by factors like electronegativity of neighboring atoms, temperature, and magnetic field strength.

• 1 neighbouring proton

• 2 neighbouring protons

• 3 neighbouring protons

• 4 neighbouring protons

Answer explanation

In a 1H NMR spectrum, a triplet indicates that there are 2 neighbouring protons. This is based on the n+1 rule, where n is the number of adjacent protons. Therefore, 1 (the triplet) - 1 = 2 neighbouring protons.

• 0-2 ppm

• 2-4 ppm

• 6-8 ppm

• 8-10 ppm

Answer explanation

Aromatic protons typically resonate in the range of 6-8 ppm in 1H NMR due to their deshielding effects from the aromatic ring. This makes 6-8 ppm the correct choice for identifying aromatic protons.

• The proton has 3 neighbouring protons

• The proton has 4 neighbouring protons

• The proton has 2 neighbouring protons

• The proton has 1 neighbouring proton

Answer explanation

In a 1H NMR spectrum, a quartet indicates that the proton is coupled to 3 neighboring protons. This follows the n+1 rule, where n is the number of neighboring protons, leading to 3+1=4 peaks.

What is the effect of increasing electronegativity of a substituent on the chemical shift of a proton in NMR?

• It decreases the chemical shift

• It increases the chemical shift

• It has no effect

• It causes the peak to disappear

Answer explanation

Increasing electronegativity of a substituent pulls electron density away from nearby protons, causing them to resonate at a higher frequency. This results in an increased chemical shift in NMR, making the correct answer "It increases the chemical shift."

In spin-spin coupling, what is the term used to describe the splitting pattern of a proton with two equivalent neighbouring protons?

• Singlet

• Doublet

• Triplet

• Quartet

Answer explanation

In spin-spin coupling, a proton with two equivalent neighboring protons exhibits a splitting pattern known as a triplet. This occurs because the neighboring protons can be in two states, leading to three distinct resonance peaks.

• It is measured in ppm

• It is measured in Hz

• It is measured in metres

• It is measured in degrees Celsius

Answer explanation

The coupling constant, J, in NMR spectroscopy is a measure of the interaction between nuclear spins and is expressed in Hertz (Hz), indicating the frequency of the coupling. Therefore, the correct choice is 'It is measured in Hz'.

• 0-2 ppm

• 2-4 ppm

• 9-10 ppm

• 6-8 ppm

Answer explanation

Aldehyde protons typically resonate in the range of 9-10 ppm in 1H NMR due to the deshielding effect of the adjacent carbonyl group. This makes 9-10 ppm the correct choice for aldehyde protons.

• 0 neighbouring protons

• 1 neighbouring proton

• 2 neighbouring protons

• 3 neighbouring protons

Answer explanation

A singlet in a 1H NMR spectrum indicates that there are no neighboring protons affecting the signal. This means the proton responsible for the singlet has 0 neighboring protons.