1 features of an nmr spectrum: shape spin coupling: neighboring nuclei “split” nmr signals...

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Features of an NMR spectrum: SHAPESpin Coupling: Neighboring nuclei “split” NMR signals

Usually n neighbors splits the signal into n+1 peaksMultiplicity = n+1

Which Isomer of C5H10O gives rise to this spectrum?

Quartet means the nuclei responsible for this signal have 3 H’s

“next door”

Triplet => 2 neighbors

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Origin of Spin Coupling:Nearby nuclei effect the magnetic field E

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When a neighbor adds to H0 the E is larger, and so is the chemical shift

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Pascals Triangle predicts pattern

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Multiplets have predictable shapesDoublets: 1:1Triplets: 1:2:1

Quartets: 1:3:3:1Quintet: 1:4:6:4:1

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The extent to which neighboring nuclei effect the E changes depends on the geometry of the molecule. It does not depend on

the magnet strength.

Coupling is MUTUAL, so extent of splitting is equal for both signals.The distance between the individual peaks making up the doublet is called the “coupling constant” (J). Here J is 7 Hz.

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Coupling constants are dependant primarily on conformation/ orientation between coupled signals

They are independent of magnet strength

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Generally coupling is only observed when neighbors are ≥ 3 bonds away from eachother

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Putting the features together allows for structure elucidation. The following spectrum is for C2H4Cl2

The Area of signal a is 3 times larger than b (CH3?)It is a doublet (ie it has only one neighbor)

Therefore a is a methyl next to a methine: CH3-CH

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It is important to note that non-equivalent nuclei split each other.

A split in one requires a split in the other. In addition, the coupling constants will be the same for each type of nuclei.

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Draw the structure of C3H7I consistent with this 1H NMR spectrum

Complications in 1H NMRHb is split into more than 5 peaks.

Coupling to “non-equivalent” neighbors may modify the simple n+1 rule: Max splitting is (n+1)(m+1)

where n and m are the numbers of neighbors on each side

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With 3 neighbors on left and 1 neighbor on rightmax multiplicity = (3+1)(1+1)= 8

Despite this complication, the splitting may still simplify to n+1 even with non-equivalent neighbors

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Multiplicity = 6(predicted as

(3+1)(2+1) = 12

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Signals from coupled protons that are close together (in Hz) can show distorted patterns.

When ν >> J, the spectra is said to be first-order.

Non-first-order spectra assume more complex shapes than Pascal’s triangle predicts and can only be analyzed with the help of

computers.

>>J

>J

≈J

<<J

J

Complications in 1H NMR:Signal distortion in non-first-order spectra