13.4 Chemical Shifts in 1H NMR Spectroscopy

Chemical Shifts in 1H NMR Spectroscopy

Chemical shift interpretation for hydrogens

The chemical shift ($\delta$) is measured in parts per million (ppm) relative to a reference compound called tetramethylsilane (TMS), which is assigned a chemical shift of 0 ppm. TMS works well as a reference because all 12 of its hydrogens are equivalent and highly shielded, placing its signal far upfield from most organic protons.

A proton's chemical shift depends on its electronic environment. The key idea is shielding vs. deshielding:

• Electron-withdrawing groups (EWGs) pull electron density away from a proton, reducing the shielding around it. This causes a downfield shift (higher ppm). Carboxylic acid protons are an extreme example, appearing at 10–13 ppm.
• Electron-donating groups (EDGs) increase electron density around a proton, adding shielding. This causes an upfield shift (lower ppm). Simple alkyl groups like $-CH_3$ on saturated carbons appear near 0.8–1.2 ppm.

Typical chemical shift ranges for broad categories of protons:

• Aromatic protons: 6–8 ppm (e.g., benzene at ~7.3 ppm)
• Alkene (vinyl) protons: 4.5–6.5 ppm (e.g., ethylene at ~5.3 ppm)
• Protons on carbons adjacent to heteroatoms ($-OH$, $-NH$, halogens): ~2.5–4.5 ppm
• Aliphatic protons (saturated C–H with no nearby EWGs): 0.5–2.5 ppm

Chemical shift ranges of functional groups

Protons attached to $sp^3$ hybridized carbons:

• Methyl ($-CH_3$): 0.8–1.2 ppm. A tert-butyl group, for instance, shows a tall singlet near 0.9 ppm because all nine hydrogens are equivalent.
• Methylene ($-CH_2-$): 1.2–1.4 ppm in simple alkyl chains. These shift significantly downfield when adjacent to electronegative atoms.
• Methine ($-CH<$): 1.4–1.7 ppm in simple alkyl environments (e.g., the central C–H in isopropane).

Protons attached to $sp^2$ hybridized carbons:

• Alkene protons: 4.5–6.5 ppm. The vinyl hydrogens of 1-butene appear around 5.0–5.8 ppm.
• Aromatic protons: 6–8 ppm. Toluene's ring protons appear near 7.1–7.3 ppm.
• Aldehyde protons ($-CHO$): 9–10 ppm. These are strongly deshielded by both the carbonyl and the $sp^2$ carbon.

Protons attached to heteroatoms:

• Alcohols ($-OH$): 1–5 ppm, highly variable due to hydrogen bonding and exchange. In dilute $CDCl_3$ solution, ethanol's $-OH$ might appear near 2–3 ppm, but in concentrated or protic conditions it shifts downfield.
• Amines ($-NH_2$, $-NH-$): 1–3 ppm, also variable and often broad.
• Thiols ($-SH$): 1–2 ppm (e.g., ethanethiol near 1.3 ppm).
• Carboxylic acids ($-COOH$): 10–13 ppm. Acetic acid appears near 11.4 ppm. This extreme downfield position reflects strong deshielding from the carbonyl plus extensive hydrogen bonding.

Protons on carbons adjacent to EWGs:

• Adjacent to halogens (e.g., $-CH_2Cl$): 2–4 ppm. In chloromethane, the protons appear near 3.0 ppm. The more electronegative the halogen and the closer the proton, the further downfield the shift.
• Adjacent to carbonyls (e.g., $-CH_2C{=}O$): 2–3 ppm. The methyl protons in acetone appear at about 2.1 ppm.

Factors affecting proton chemical shifts

Electronegativity of neighboring atoms

This is the most straightforward factor. Electronegative atoms (O, N, F, Cl) withdraw electron density through sigma bonds, deshielding nearby protons and shifting them downfield. The effect is cumulative and decreases with distance. Compare: $CH_3F$ (~4.3 ppm), $CH_2F_2$ (~5.4 ppm), $CHF_3$ (~6.4 ppm). Each additional fluorine pulls more electron density away.

Hybridization of the attached carbon

Protons on $sp^3$ carbons resonate at the lowest ppm values (most shielded), while protons on $sp^2$ carbons appear further downfield. This is partly because $sp^2$ carbons have more s-character (33% vs. 25%), holding bonding electrons closer to the carbon and away from the hydrogen.

One important exception: alkyne protons ($sp$ hybridized, 50% s-character) appear near 2–3 ppm, which is upfield from alkene protons. This seems contradictory, but it's explained by magnetic anisotropy (see below). The cylindrical electron cloud around the triple bond shields the terminal proton.

Magnetic anisotropy

When placed in an external magnetic field, circulating $\pi$ electrons generate their own local magnetic fields. Depending on a proton's position relative to the $\pi$ system, it can be shielded or deshielded.

• Aromatic ring currents strongly deshield ring protons (6–8 ppm). The circulating $\pi$ electrons create a magnetic field that reinforces the external field at the edges of the ring where the protons sit.
• Carbonyl groups deshield the aldehyde proton (9–10 ppm) for similar reasons.
• Alkyne triple bonds shield terminal protons because those protons sit along the axis of the cylindrical $\pi$ system, where the induced field opposes the external field.

Hydrogen bonding

Protons involved in hydrogen bonding ($-OH$, $-NH$, $-COOH$) show variable chemical shifts. Stronger hydrogen bonding pulls electron density away from the proton, causing a downfield shift. This is why $-OH$ chemical shifts depend heavily on concentration, solvent, and temperature. Carboxylic acid $-OH$ protons, which form strong hydrogen bonds, consistently appear far downfield (10–13 ppm).

Inductive effects through multiple bonds

The deshielding effect of an EWG drops off with distance. A proton directly on a carbon bearing a chlorine is shifted much further downfield than a proton two or three carbons away. As a rough guide, the inductive effect is roughly halved for each additional bond separating the proton from the electronegative group.

Nuclear Magnetic Resonance Principles

Nuclear magnetic resonance (NMR) works because certain nuclei (including $^1H$) possess a property called spin. When placed in a strong external magnetic field, these nuclei can adopt different energy states. Absorbing radiofrequency radiation causes transitions between these states, and the specific frequency absorbed depends on the local electronic environment around each proton. That frequency difference, measured relative to TMS, is the chemical shift.

Spin-spin coupling occurs when nearby non-equivalent protons influence each other's local magnetic field, causing signal splitting. This splitting pattern follows the n+1 rule: a proton with n equivalent neighboring protons splits into n+1 peaks. Coupling provides connectivity information but is a separate topic from chemical shift.

Relaxation is the process by which excited nuclei return to their lower energy state after absorbing RF energy. Relaxation rates affect peak width and signal intensity. Protons that exchange rapidly (like $-OH$ and $-NH$) often give broad peaks because their relaxation behavior differs from C–H protons.