NMR Splitting

NMR signals are split into multiple peaks by neighboring hydrogens. The n+1 rule says: a signal is split into (n+1) peaks, where n is the number of equivalent H’s on the neighboring (adjacent) carbon(s). This splitting pattern reveals the hydrogen connectivity in the molecule.

The n+1 Rule

For a hydrogen with:

• 0 neighbors: singlet (1 peak).
• 1 neighbor: doublet (2 peaks).
• 2 neighbors: triplet (3 peaks).
• 3 neighbors: quartet (4 peaks).
• 4 neighbors: pentet (5 peaks).
• 5 neighbors: sextet (6 peaks).
• 6 neighbors: septet (7 peaks).

“Neighbors” means H’s on an adjacent carbon, not H’s on the same carbon (those are equivalent and do not split each other).

Classic Ethyl Pattern: Quartet and Triplet

In ethanol (CH₃-CH₂-OH), the CH₃ has 2 neighbors on the adjacent CH₂ carbon → triplet (3 peaks). The CH₂ has 3 neighbors on the adjacent CH₃ carbon → quartet (4 peaks). Seeing a triplet + quartet together is almost always diagnostic of an ethyl group (-CH₂CH₃).

Isopropyl Pattern: Septet and Doublet

In isopropyl (-CH(CH₃)₂), the central CH has 6 equivalent neighbors (two methyl groups with 3 H’s each, all equivalent) → septet (7 peaks). The two CH₃ groups each have 1 neighbor (the central CH) → doublet (2 peaks). Seeing a septet + 6H-doublet is diagnostic for isopropyl.

Why the n+1 Rule Works

Each neighboring hydrogen’s spin (up or down) slightly alters the magnetic field felt by the observed hydrogen. If n equivalent neighbors exist, there are n+1 possible combinations of their spin sums (0 up, 1 up, 2 up, …, n up), each giving a distinct effective field. The signal splits into n+1 peaks, with intensities proportional to the binomial distribution (Pascal’s triangle):

• Doublet (n=1): 1:1.
• Triplet (n=2): 1:2:1.
• Quartet (n=3): 1:3:3:1.
• Pentet (n=4): 1:4:6:4:1.

The Distance Rule

Splitting usually comes from protons THREE BONDS AWAY (one bond from the observed H to its C, one C-C bond, one bond from that C to its H). Longer-range couplings are usually too small to resolve on typical NMR.

Exceptions: W-coupling (four-bond in certain geometries), aromatic meta/para couplings (small but resolvable).

Coupling Constants

The space between adjacent peaks in a multiplet is called the coupling constant (J), measured in Hz. J values are diagnostic:

• Axial-axial on cyclohexane: J = 9-12 Hz (large).
• Axial-equatorial or eq-eq: J = 2-3 Hz (small).
• Vicinal H’s on freely-rotating alkyl chain: J = 6-8 Hz (typical).
• Cis H’s on alkene: J = 6-12 Hz.
• Trans H’s on alkene: J = 12-18 Hz.

Coupling constant magnitudes help distinguish E vs. Z alkenes, axial vs. equatorial cyclohexane stereochemistry, and more subtle structural features.

When Splitting Does NOT Occur

Protons on the SAME carbon do not split each other (they are equivalent - their signals overlap with zero spacing). Equivalent protons on different carbons (related by symmetry) also do not split each other.

Example: p-xylene (benzene with two para methyls) has 4 aromatic H’s, all equivalent by symmetry → single 4H singlet, not a quartet or any more complex pattern.

Exchangeable Protons: Broad and No Coupling

Protons on O or N (alcohol OH, amine NH, carboxylic acid OH) often appear as a broad singlet because fast exchange with other acidic/basic species averages out any coupling. Do not expect splitting from exchangeable protons unless the sample is very dry and cold.

Summary of MCAT-Level Patterns