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2.5: Chemical Shifts in ¹H NMR Spectroscopy

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    469361
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    As mentioned previously, differences in chemical shifts are caused by the small local magnetic field of electrons surrounding different nuclei. Nuclei that are more strongly shielded by electrons require a higher applied field to bring them into resonance so they absorb on the right side of the NMR chart. Nuclei that are less strongly shielded need a lower applied field for resonance so they absorb on the left of the NMR chart.

    Most 1H chemical shifts fall within the range 0 to 10 δ, which can be divided into the five regions shown in Figure \(\PageIndex{1}\). By remembering the positions of these regions, it’s often possible to tell at a glance what kinds of protons a molecule contains.

    H N M R shows shifts of hydrogens(from downfield to upfield): aromatic, vinylic, on carbon bonded to O, N, or halogen, allylic, and saturated.
    Figure \(\PageIndex{1}\): Regions of the 1H NMR Spectrum

    Figure \(\PageIndex{2}\) shows the correlation of 1H chemical shift with electronic environment in more detail. In general, protons bonded to saturated, sp3-hybridized carbons absorb at higher fields, whereas protons bonded to sp2-hybridized carbons absorb at lower fields. Protons on carbons that are bonded to electronegative atoms, such as N, O, or halogen, also absorb at lower fields.

    Figure \(\PageIndex{2}\): Correlation of 1H Chemical Shifts with Environment
    Type of hydrogen Structure Chemical shift (δ)
    Reference Si(CH3)4 0
    Alkyl (primary) —CH3 0.7–1.3
    Alkyl (secondary) —CH2 1.2–1.6
    Alkyl (tertiary) A C H group with three open single bonds. The hydrogen atom is highlighted. 1.4–1.8
    Allylic A 3-carbon chain with a double bond between C 1 and C 2. C 3 with two open single bonds is bonded to a highlighted hydrogen atom. 1.6–2.2
    Methyl ketone A carbonyl group with an open single bond is bonded to a methyl group, in which the hydrogen atoms are highlighted. 2.0–2.4
    Aromatic methyl Ar—CH3 2.4–2.7
    Alkynyl CCHCCH 2.5–3.0
    Alkyl halide A carbon with two open single bonds is bonded to a highlighted hydrogen and halogen. 2.5–4.0
    Alcohol A carbon with three open single bonds is bonded to an oxygen atom, which is bonded to a highlighted hydrogen atom. 2.5–5.0
    Alcohol, ether A carbon with two open single bonds is bonded to a highlighted hydrogen and oxygen atom that has an open single bond. 3.3–4.5
    Vinylic A carbon with two open bonds is double bonded to another carbon that has an open single bond and is bonded to highlighted hydrogen. 4.5–6.5
    Aryl Ar—H 6.5–8.0
    Aldehyde A carbonyl group with an open single bond is bonded to a highlighted hydrogen atom. 9.7–10.0
    Carboxylic acid A carbonyl group with an open single bond is bonded to an oxygen atom, which is bonded to a highlighted hydrogen atom. 11.0–12.0
    Worked Example \(\PageIndex{1}\): Predicting Chemical Shifts in 1H NMR Spectra

    Methyl 2,2-dimethylpropanoate (CH3)2CCO2CH3 has two peaks in its 1H NMR spectrum. What are their approximate chemical shifts?

    Strategy

    Identify the types of hydrogens in the molecule, and note whether each is alkyl, vinylic, or next to an electronegative atom. Then predict where each absorbs, using Figure \(\PageIndex{2}\) if necessary.

    Solution

    The –OCH3 protons absorb around 3.5 to 4.0 δ because they are on carbon bonded to oxygen. The (CH3)3C– protons absorb near 1.0 δ because they are typical alkane-like protons.

    Exercise \(\PageIndex{1}\)

    Each of the following compounds has a single 1H NMR peak. Approximately where would you expect each compound to absorb?

    1. a=
    2. A chemical structure of acetone. 
    3. A chemical structure of benzene. 
    4. The structure of dichloromethane whose condensed structural formula reads, C H 2 C l 2. 
    5. The structure of ethane-1, 2-dial has a 2-carbon chain. Each carbon is single-bonded to a hydrogen atom and double-bonded to an oxygen atom. 
    6. The structure of N,N-dimethylmethanamine in which nitrogen atom is bonded to three methyl groups.
    Answer
    1. 1.43 δ
    2. 2.17 δ
    3. 7.37 δ
    4. 5.30 δ
    5. 9.70 δ
    6. 2.12 δ
    Exercise \(\PageIndex{2}\)

    Identify the different types of protons in the following molecule, and tell where you would expect each to absorb:

    Two carbon atoms with a trans double bond in-between. C 1 is bonded to benzene with a methoxy group at C 4. C 2 is bonded to an ethyl group.

    Answer

    There are seven kinds of protons labeled. The types and expected range of absorption of each follow. a: ether, 3.5–4.5 δb: aryl, 6.5–8.0 δc: aryl, 6.5–8.0; d: vinylic, 4.5–6.5 δe: vinylic, 4.5–6.5 δf: alkyl (secondary), 1.2–1.6 δg: alkyl (primary), 0.7–1.3 δ.

    Structure of 1-[(E)-but-1-enyl]-4-methoxybenzene. Hydrogens labeled a (methoxy), b  and c (ortho and meta to methoxy), d (benzylic), e (trans to d), f (methylene), and g (methyl) .


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