RAS question
Nuclear Magnetic Resonance (NMR) spectroscopy is based on the property of:
Correct answer: (C) Nuclear spin in a magnetic field.
Nuclear Magnetic Resonance spectroscopy is based on nuclear spin in a magnetic field, where nuclei with non-zero spin respond to resonant radiofrequency excitation.
Explanation
NMR works because certain nuclei with non-zero spin behave differently when placed in a strong external magnetic field. The verified PMC source describes nuclear magnetic resonance as the oscillatory response of non-zero-spin nuclei in a magnetic field to resonant radiofrequency irradiation. This is why the key property is not ordinary electronic or vibrational excitation but nuclear spin behaviour under a magnetic field. The PMC source also explains that NMR frequencies can report chemical structure because they are sensitive to the electronic environment around a nucleus. This supports the standard use of NMR in chemistry for molecular structure determination, while MRI is the medical application built on the same NMR principle.
Why the other options are wrong
- (A) Electron excitation is the basis of UV-Visible spectroscopy, whereas NMR depends on nuclear-spin states responding in a magnetic field.
- (B) Molecular vibration belongs to infrared spectroscopy, not to NMR, which observes radiofrequency responses of nuclei with non-zero spin.
- (D) Electron spin is associated with ESR spectroscopy, while NMR specifically concerns nuclear spin rather than electron spin.
Concept
This tests the spectroscopy portion of Science and Technology: matching each analytical technique with the physical property it measures. It recurs in RAS because NMR links basic physics with chemistry and medical imaging through MRI.