Nuclear Magnetic Resonance (NMR) – MCQs with Explanation
Question 1.
The principle of NMR spectroscopy is based on the interaction of:
A. Electrons with infrared radiation
B. Atomic nuclei with magnetic field
C. Molecules with UV radiation
D. Ions with electric field
✅ Answer: B
Explanation:
NMR works on the principle that certain atomic nuclei possess nuclear spin. When placed in a strong magnetic field, these nuclei interact with the field and absorb radiofrequency radiation, producing resonance.
Question 2.
NMR spectroscopy was discovered by:
A. Robert Bunsen
B. Felix Bloch and Edward Purcell
C. Linus Pauling
D. James Watson
✅ Answer:
Felix Bloch and Edward Mills Purcell
Explanation:
In 1946, Bloch and Purcell independently discovered NMR. Their work established that nuclei in a magnetic field absorb radiofrequency radiation, forming the basis of modern NMR spectroscopy.
Question 3.
Which nucleus is most commonly studied in NMR spectroscopy?
A. Carbon-12
B. Hydrogen-1
C. Oxygen-16
D. Nitrogen-14
✅ Answer: B
Explanation:
¹H (Proton NMR) is most common because hydrogen is present in most organic compounds and produces strong signals.
Question 4.
Chemical shift in NMR spectroscopy is measured in:
A. Hertz
B. ppm
C. Joules
D. Tesla
✅ Answer: B
Explanation:
Chemical shift indicates the position of signals in the NMR spectrum and is measured in parts per million (ppm) relative to a reference compound.
Question 5.
The splitting of peaks in NMR spectra is known as:
A. Chemical shift
B. Coupling
C. Spin–spin splitting
D. Integration
✅ Answer: C
Explanation:
Spin–spin interaction between neighboring protons causes peak splitting, resulting in patterns such as singlet, doublet, triplet, etc.
Question 6.
The number of peaks in an NMR spectrum corresponds to:
A. Number of atoms in molecule
B. Number of different proton environments
C. Molecular weight
D. Number of bonds
✅ Answer: B
Explanation:
Each chemically different hydrogen environment produces a separate signal in the NMR spectrum.
Question 7.
Which solvent is commonly used in NMR analysis?
A. Water
B. Ethanol
C. Deuterated solvent
D. Methanol
✅ Answer: C
Explanation:
Deuterated solvents such as CDCl₃ or D₂O are used so that solvent hydrogen atoms do not interfere with the proton NMR signals.
Question 8.
The instrument used to detect signals in NMR spectroscopy is called:
A. Detector
B. Photomultiplier
C. Cathode tube
D. Spectrograph
✅ Answer: A
Explanation:
The RF detector/receiver detects the energy released when nuclei return to the lower energy state.
Question 9.
Which of the following nuclei is not normally active in NMR?
A. ¹H
B. ¹³C
C. ¹⁹F
D. ¹²C
✅ Answer: D
Explanation:
¹²C has no nuclear spin, so it does not produce NMR signals. Only nuclei with non-zero spin are NMR active.
Question 10.
The unit of coupling constant (J) is:
A. ppm
B. Tesla
C. Hertz
D. Joules
✅ Answer: C
Explanation:
The coupling constant represents the spacing between split peaks and is measured in Hertz (Hz).
Question 11.
Integration in an NMR spectrum gives information about:
A. Molecular weight
B. Number of protons
C. Carbon atoms
D. Functional groups
✅ Answer: B
Explanation:
The area under each signal indicates the relative number of hydrogen atoms responsible for that signal.
Question 12.
The reference compound commonly used in NMR spectroscopy is:
A. Benzene
B. Tetramethylsilane
C. Methanol
D. Acetone
✅ Answer: Tetramethylsilane (TMS)
Explanation:
TMS produces a single sharp signal at 0 ppm, making it ideal as a reference standard for chemical shift measurement.
Question 13.
Which technique is often used along with NMR in forensic drug identification?
A. IR spectroscopy
B. UV spectroscopy
C. GC-MS
D. X-ray diffraction
✅ Answer:
Gas Chromatography–Mass Spectrometry
Explanation:
GC-MS helps identify compounds based on mass fragmentation, while NMR provides structural information. Together they confirm the identity of drugs.
Question 14.
The strong magnetic field in an NMR instrument is produced by:
A. Electromagnet
B. Permanent magnet
C. Superconducting magnet
D. Radiofrequency generator
✅ Answer: C
Explanation:
Modern NMR spectrometers use superconducting magnets cooled with liquid helium to generate very strong magnetic fields.
Question 15.
Which of the following provides information about the carbon skeleton of molecules?
A. ¹H NMR
B. ¹³C NMR
C. IR spectroscopy
D. UV spectroscopy
✅ Answer: B
Explanation:
¹³C NMR provides signals for carbon atoms, revealing the framework of organic molecules.
Question 16.
Resonance in NMR occurs when nuclei:
A. Emit light
B. Absorb radiofrequency energy
C. Emit electrons
D. Release heat
✅ Answer: B
Explanation:
When nuclei absorb radiofrequency radiation, they transition from a lower energy state to a higher energy state, causing resonance.
Question 17.
The position of peaks in an NMR spectrum depends mainly on:
A. Temperature
B. Chemical environment of nuclei
C. Pressure
D. Molecular weight
✅ Answer: B
Explanation:
Electrons surrounding a nucleus affect the local magnetic field, resulting in different chemical shifts depending on the chemical environment.
Question 18.
In forensic science, NMR is mainly used for:
A. Fingerprint analysis
B. Drug identification
C. Ballistic examination
D. Blood grouping
✅ Answer: B
Explanation:
NMR helps determine chemical structures of drugs, poisons, and unknown compounds recovered from crime scenes.
Question 19.
Which of the following is an advantage of NMR spectroscopy?
A. It destroys the sample
B. It requires extremely small samples only
C. It provides detailed molecular structure
D. It cannot analyze mixtures
✅ Answer: C
Explanation:
NMR gives detailed structural information, including molecular framework and functional groups, without destroying the sample.
Question 20.
Which radiation is used in NMR spectroscopy?
A. X-rays
B. Infrared radiation
C. Radiofrequency radiation
D. Ultraviolet radiation
✅ Answer: C
Explanation:
NMR uses radiofrequency (RF) radiation, typically in the MHz range, to excite atomic nuclei in a magnetic field.
 – MCQs with Explanation | Forensic Science){kind=link}
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