FT-NMR excitation – In Fourier-transform NMR experiments, how are nuclear spins typically excited to generate a broadband free induction decay (FID)?

Introduction / Context:
Fourier-transform NMR revolutionized spectroscopy by replacing slow frequency-sweep excitation with pulsed methods. A single short, intense radio-frequency pulse excites a broad swath of resonances simultaneously. The decaying signal captured afterward in the time domain (FID) is Fourier transformed to yield the frequency-domain spectrum containing all excited resonances at once.

Given Data / Assumptions:

• The pulse duration determines excitation bandwidth (shorter pulses excite wider bandwidths).
• The spectrometer digitizes the FID at high sampling rates over a defined spectral width.
• Phase cycling and pulse sequences tailor coherence pathways for multidimensional NMR as needed.

Concept / Approach:
An intense, brief RF pulse generates transverse magnetization from the equilibrium longitudinal magnetization. Because the pulse has a broad frequency content, many chemical environments (chemical shifts) are excited at once. After the pulse, spins precess at their characteristic Larmor frequencies, producing the FID that encodes all resonance information for subsequent Fourier transformation.

Step-by-Step Solution:

Verification / Alternative check:
Compare with frequency-swept continuous-wave (CW) NMR: FT-NMR provides higher sensitivity via signal averaging (Fellgett advantage) and multiplex detection, confirming why pulsed, broadband excitation is standard.

Why Other Options Are Wrong:

• Slow frequency sweeps describe CW NMR, not modern FT-NMR.
• Mechanical pressure or rapid heating are unrelated to coherent RF excitation of nuclear spins.
• “None” is incorrect because the pulse method is well established.

Common Pitfalls:
Equating “wide range of frequencies” with noise; the pulse is precisely timed and phase-controlled to create coherent excitation across the desired bandwidth.

Final Answer:
By a short, intense RF pulse that contains a wide range of frequencies