deshielded

Meaning: Electrons circulating around a nucleus create a small field that opposes the applied field B₀. This is shielding. If electron density is pulled away, the opposition shrinks, the nucleus is deshielded, and it resonates at a higher frequency. On the spectrum it appears downfield, to the left, at larger δ. Common causes are nearby electronegative atoms (O, N, halogens), the ring current of an aromatic ring (benzene H near 7 ppm), the π bond of C=O or C=C (aldehyde H near 9–10 ppm), and hydrogen bonding (carboxylic acid O–H near 10–12 ppm).

Equation: δ = (ν − νref)/ν₀ × 10⁶, where δ is the chemical shift (ppm, dimensionless), ν is the sample nucleus's resonance frequency (Hz), νref is the reference (TMS) frequency (Hz), and ν₀ is the spectrometer operating frequency (Hz).

On the MCAT: You are given structures or a spectrum and asked to rank or assign protons. The more electronegative the nearby atom, and the closer the proton is to it, the more deshielded it is. In CH₃CH₂Cl, the CH₂ is downfield of the CH₃. Questions also combine shift with integration and splitting.

Common confusion: Deshielded does not mean "protected from the field." It means less protected, so downfield with a larger δ. TMS is set to 0 ppm because its protons are strongly shielded, so most signals fall downfield of it.