Subjects and lessons to explore.

Law

Organic Chemistry · Structure Determination: Nuclear Magnetic Resonance Spectroscopy

Chemical Shifts

8 min read
7 key termsSourcesReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

A chemical shift is the position of an NMR absorption along the spectrum — a fingerprint of the electronic environment around the nucleus. In the spectrometer's magnetic field, the electrons around each nucleus circulate and generate a small induced field of their own. In most molecules that induced field opposes the applied field at the nucleus, so the nucleus experiences a slightly weaker effective field than B0. This is : a shielded nucleus needs a slightly higher applied frequency to reach resonance, so its absorption appears "upfield" at a smaller chemical shift. Nearby electronegative atoms pull electron density away, deshield the nucleus, and move its absorption "downfield" to a larger shift.

Because every chemically distinct set of protons (and every distinct carbon) sits in a different electron cloud, each gives a peak at its own characteristic shift. Shifts are reported in parts per million (ppm) relative to tetramethylsilane, (CH3)4Si, assigned δ= 0. The ppm scale makes shifts independent of field strength: a proton at δ= 3.5 appears 1050 Hz from on a 300 MHz instrument but 1750 Hz at 500 MHz — yet its chemical shift is 3.5 ppm in both.

Why this matters

  • The chemical shift is the first structural information from any NMR spectrum: what kind of proton or carbon you are looking at (alkane, alkene, aromatic, aldehyde, and so on).
  • With integration (how many H) and splitting (which H are neighbors), shifts let chemists assemble an unknown molecule's structure without other data.
  • Pharmacists and QC chemists compare measured shifts with literature values to confirm a drug candidate is the intended compound, not an isomer or impurity.
  • For exams, shift prediction is a guaranteed question type: given a structure, assign δ values; given shifts, propose a structure.

The college version

Core Concepts

The reference standard: TMS

Tetramethylsilane, (CH3)4Si, is the universal zero of the shift scale (δ= 0): its 12 protons are all equivalent (one sharp line), silicon is less electronegative than carbon so the methyl protons are more shielded than almost any other organic proton (far upfield edge of the scale), and it is inert, non-toxic, and volatile (bp 27 °C) — easy to add a drop and evaporate away. It never overlaps the region where most organic protons absorb.

Shielding, deshielding, and the effective field

The electron cloud around a nucleus circulates in the applied field and produces an induced field σB0 (shielding constant σ) opposing B0 at the nucleus:

Beff = B0(1 - σ)

A more shielded nucleus (larger σ) experiences a smaller effective field and must be irradiated at a slightly higher frequency to satisfy ν= γBeff/2π — it absorbs upfield (lower δ). An electronegative substituent (O, N, Cl, Br, F) withdraws electron density through the σ bonds, lowers σ, deshields the nucleus, and moves absorption downfield (higher δ). The effect falls off with distance: chlorine on the same carbon deshields strongly, but two or three bonds away, much less.

The ppm scale and field independence

The chemical shift is the frequency offset from the reference divided by the spectrometer frequency:

δ (ppm) = νsample - νrefνspectrometer × 106

Dividing by the spectrometer frequency removes the field dependence, so the same proton has the same δ on any instrument. What changes with field is the Hz separation: 1 ppm equals 300 Hz at 300 MHz but 500 Hz at 500 MHz (Worked Example 1). This is also why a multiplet spanning 7 Hz (a typical coupling constant) looks "tighter" in ppm as field increases — an idea that returns in the spin–spin splitting topic.

What controls the magnitude of a shift

Four factors dominate (detailed in the next topic):

  • Electronegativity of nearby atoms — inductive withdrawal deshields (e.g., CH3-Cl protons δ ≈ 3.0 vs. alkane CH3 δ ≈ 0.9).
  • Hybridization — sp2-bound protons (alkene δ ≈ 5, aromatic δ ≈ 7) are more deshielded than sp3 protons because the more electronegative sp2 carbon holds less electron density on the hydrogen.
  • — circulating π electrons create direction-dependent induced fields; aromatic ring currents strongly deshield protons on the ring edge (δ 6.5–8.5).
  • Hydrogen bonding — OH and NH protons are deshielded by H-bonding, so their shifts vary with concentration, temperature, and solvent (alcohol OH commonly δ 0.5–5; carboxylic acid OH 10–12).

Common Confusions

Do Not ConfuseWithDifference
Chemical shift (ppm)Frequency offset (Hz)Shift is normalized by spectrometer frequency and field-independent; Hz offset scales with field
UpfieldDownfieldUpfield = smaller δ = more shielded; downfield = larger δ = more deshielded
Larger δ = more shieldedLarger δ = less shieldedLarger δ means LESS shielding — a common exam trap
Shielding by electronsDeshielding by electronegative atomsElectron density shields (upfield); withdrawal by O, N, halogens deshields (downfield)
TMS reference peakSolvent residual peakTMS is added at δ= 0; residual CHCl3 in deuterated chloroform appears near δ 7.26
Chemical shift of a nucleusIts coupling constantShift = position (ppm), from the electron environment; coupling = multiplet spacing (Hz), from neighbor spins
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine each proton is a tiny compass needle floating in a crowd of electrons. The electrons act like a little shield that blocks part of the big magnet's pull, so a well-shielded proton "feels" a weaker magnet and needs a stronger nudge to flip. Protons near greedy atoms like oxygen or chlorine lose their electron shield, feel more of the magnet, and flip at a different spot on the spectrum. That spot — the chemical shift — is like the proton's home address, telling you what kind of neighborhood (functional group) it lives in.

Worked example

Example 1: Converting Hz to ppm, and ppm back to Hz at a different field

A proton absorbs 1050 Hz downfield of TMS on a 300 MHz instrument. (a) What is its chemical shift in ppm? (b) How many Hz downfield of TMS will it appear on a 500 MHz instrument?

Part (a) — Write the defining formula:

δ (ppm) = νsample - νrefνspectrometer × 106

Substitute (300 MHz = 300 × 106 Hz):

δ= 1050 Hz300 × 106 Hz × 106 = 3.50 ppm

Part (b) — Rearrange to solve for the Hz offset:

νsample - νref = δ × νspectrometer

νsample - νref = 3.50 × 500 MHz = 1750 Hz

Interpretation: the shift stayed 3.50 ppm, but the Hz offset grew from 1050 to 1750 Hz — in proportion to the field. Dimensional check: ppm × MHz = 10-6 × 106 Hz = Hz. ✓

Example 2: Assigning shifts by shielding arguments

Rank the methyl protons of CH4, CH3-Cl, and CH3-O-CH3 in order of expected chemical shift, and explain with shielding.

Step 1 — State the trend. Electronegativity of the atom attached to the methyl carbon: O (3.4) > Cl (3.2) > H (2.2). More electronegative substituent → more electron density withdrawn from the C–H bonds → more deshielding → larger δ.

Step 2 — Apply it: CH3-O-CH3 (largest, δ ≈ 3.3) > CH3-Cl (δ ≈ 3.0) > CH4 (smallest, δ ≈ 0.2).

Step 3 — Sanity check: methane absorbs near δ 0.23, chloromethane near 3.05, dimethyl ether near 3.3 — matching the prediction.

Example 3: How close are two peaks — Hz separation at a given field

Two signals sit at δ 2.1 and δ 3.6. How far apart (in Hz) are they on a 400 MHz instrument? Would the same peaks be easier to resolve at 600 MHz?

Formula first:

Δν (Hz) = Δδ (ppm) × νspectrometer (MHz)

Substitute:

Δν= (3.6 - 2.1) × 400 = 1.5 × 400 = 600 Hz

Compare fields: at 600 MHz, Δν= 1.5 × 600 = 900 Hz. The peaks stay 1.5 ppm apart, but the Hz gap — actual resolution — grows with field, which is why chemists pay for stronger magnets.

Key takeaways

  • Chemical shift δ = position of an absorption relative to TMS (δ= 0), reported in ppm.
  • δ= νsample - νrefνspectrometer × 106 — field-independent because the offset is normalized by the spectrometer frequency.
  • Shielding (electron density) → upfield, smaller δ; deshielding (electronegative neighbors) → downfield, larger δ.
  • Beff = B0(1 - σ); resonance requires ν= γBeff2π.
  • TMS: 12 equivalent, highly shielded protons; inert and volatile — the universal reference.
  • 1 ppm = 300 Hz at 300 MHz and 500 Hz at 500 MHz: Hz offsets scale with field; ppm does not.
  • Four main shift controllers: electronegativity, hybridization, π-system anisotropy, hydrogen bonding.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

  1. Define chemical shift and explain why the ppm scale is independent of spectrometer field strength.

    Show answer

    δ (ppm) = νsample - νrefνspectrometer × 106. Dividing the Hz offset by the spectrometer frequency cancels the field dependence.

  2. Why is TMS assigned δ= 0, and what properties make it a good reference?

    Show answer

    TMS has 12 equivalent, highly shielded protons (Si is less electronegative than C), giving one sharp line at the upfield edge of the scale; it is inert, non-toxic, and volatile (bp 27 °C).

  3. A signal is 780 Hz downfield of TMS on a 300 MHz instrument. What is its δ in ppm, and where (in Hz) would it appear on a 600 MHz instrument?

    Show answer

    δ= 780300 = 2.60 ppm; at 600 MHz the same proton appears at 2.60 × 600 = 1560 Hz downfield.

  4. Rank the expected shifts of the methyl protons in CH3-CH3, CH3-NH2, and CH3-F, and justify with shielding.

    Show answer

    CH3-F (largest,  ≈ 4.3) > CH3-NH2 ( ≈ 2.3) > CH3-CH3 ( ≈ 0.9): F withdraws electron density most strongly, deshielding the methyl protons most.

  5. A proton at δ 9.8 is typical of which functional group, and why is it so deshielded?

    Show answer

    Aldehyde protons (R–CHO) absorb near δ 9–10: the carbonyl's electronegative oxygen plus π-system anisotropy deshield the proton strongly.

  6. Two peaks are 0.35 ppm apart. How many Hz separate them at 500 MHz?

    Show answer

    Δν= 0.35 × 500 = 175 Hz.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Chemical shift (δ)
Position of an NMR absorption relative to TMS, in ppm
Shielding
Reduction of the effective field at a nucleus by its electrons
Deshielding
Loss of electron density at a nucleus from electron-withdrawing groups
TMS
Tetramethylsilane, (CH3)4Si, the NMR reference at δ= 0
Effective field (Beff)
The actual field a nucleus experiences: B0(1 - σ)
Upfield / downfield
Smaller δ (shielded) / larger δ (deshielded)
Magnetic anisotropy
Direction-dependent shielding from circulating π electrons

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.