C NMR Spectroscopy - 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of...

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Transcript of C NMR Spectroscopy - 1 13 C NMR Spectroscopy 13 C NMR 12 C is the most abundant natural isotope of...

  • 1

    13C NMR Spectroscopy

    13C NMR

    12C is the most abundant natural isotope of carbon,

    but has a nuclear spin I = 0, rendering it

    unobservable by NMR.

    Limited to the observation of the 13C nucleus which

    constitutes only 1.1% of naturally occurring carbon.

    40.01.00251.719F

    75.07.05

    50.04.70

    10.71.0067.2813C

    6.51.0041.12H

    300.7.05

    200.4.70

    42.61.00267.531H

    Frequency νννν

    (MHz)

    Field

    strength B0

    (Tesla)

    γγγγ

    (106 rad/Tesla ×××× sec)

    Nucleus

    13C Transition Energy

    The magnetogyric ratio, γγγγ, for the 13C is 67.3 compared to 267.5 for 1H.

    Remember the resonance condition for a nucleus is given by:

    νννν = (γγγγ/2ππππ)B0

    If the gyromagnetic ratio is lowered, the ∆∆∆∆E is also lowered. Where a 1H spectrum using a 1.41 T magnet is observed at 60

    MHz, a 13C spectrum is observed at 15 MHz – roughly 4 times

    less energetic.

    Boltzmann: Nupper/Nlower = e -∆∆∆∆E/kT = e-hνννν/kT

    @ 298 K the ratio is 1,000,000 / 1,000,002

  • 2

    13C NMR

    The combined effects of smaller excess populations in the

    lower energy state, low natural abundance, and slow

    relaxation rates result in a 13C signal that is typically 6000

    times weaker than that observed for 1H.

    With FT instruments, this is not a problem – simply take

    more scans! (recall S/N increases as the square root of the

    number of scans).

    16 scans on a 5-10 mg sample will give a good 1H spectrum,

    512 scans on a 50 mg sample will give a good 13C spectrum.

    Fourier Transform NMR

    •Radio-frequency pulse given.

    •Nuclei absorb energy and precess (spin) like little tops.

    •A complex signal is produced, then decays as the nuclei lose energy.

    •Free induction decay is converted to spectrum.

    13C NMR

    • low 13C abundance

    • a single molecule will have at most only one 13C

    atom

    • however, we are sampling a very large number of

    molecules, even in a 50 mg sample!

    • thus our sampling will ‘see’ a 13C at every C

    position in the molecule!

    Chem 325

    TUTORIAL

    TONIGHT @ 7PM

  • 3

    13C Shielding

    13C spectra are typically recorded from 0 – 220 ppm;

    with the zero being the methyl carbon in TMS

    (much wider range than 1H spectra!)

    13C nuclei are shielded or deshielded (CHEMICAL

    SHIFT) due to the same factors as for 1H NMR.

    1. Electron withdrawing ability (by inductance or

    resonance) of nearby groups.

    2. Hybridization.

    3. Electron current effects.

    13C NMR Chemical Shifts

    Several functionalities appear directly on 13C NMR which are

    not ‘visible’ in 1H NMR:

    - Quaternary carbons

    - ipso carbons

    - Carbonyl carbons Si

    CH3

    H3C CH3CH3

    downfield δ (ppm) upfield deshielded shielded higher ∆E lower ∆E

    0.020406080100120140160180200220

    carbonyl carbons

    aromatic carbons

    alkene carbons

    alkyne carbons

    13C-EWG

    sp3 carbon

    Carbonyl Carbon Chemical Shifts

    110120130140150160170180190200210220

    ketones

    conj. ketones

    aldehydes

    carboxylic acids

    anhydrides nitriles

    esters

    acid chlorides

    amides

    Spin-Spin Coupling in 13C NMR

    Homonuclear coupling of 13C-13C is possible in theory.

    However, due to the low natural abundance of 13C, it is rare to

    find two 13C’s in the same molecule, let alone adjacent to one

    another.

    No need to consider 13C-13C coupling except for enrichment

    studies!

    Heteronuclear coupling between 13C and the 1H atoms attached

    to them is observed (1H abundance ~99%).

    Because the 1H atoms are directly attached, the coupling

    constants (1J)are large, typically 100-250 Hz.

    When such spectra are observed, they are referred to as proton

    coupled spectra (or non-decoupled spectra).

  • 4

    1H – 13C Splitting

    The splitting follows the simple N+1 rule:

    The multiplet analysis gives useful information, but there are

    two major limitations:

    1) If the 13C signal is weak (common) the outer peaks of the

    multiplet may be lost in the noise of the spectrum.

    2) Due to the large J-constants, the multiplets quickly begin to

    overlap and become congested.

    C 13

    C

    H

    13 C

    H

    H 13

    C

    H

    H

    H 13

    quaternary

    singlet

    methine

    doublet

    methylene

    triplet

    methyl

    quartet

    13C NMR Spectrum

    Proton-Coupled

    Effect of Coupling

    Coupling can cause 13C NMR spectra to become very complicated (convoluted) quite easily.

    1H Coupled

    Three equal intensity lines

    at 77 ppm

    CDCl3 solvent

    13C- 2D coupling

    1H Decoupling

    To simplify the 13C spectrum, and to increase the intensity of

    the observed signals, a decoupler is used to remove the spin

    effects of the 1H nucleus.

    A second RF generator irradiates at the 1H resonance

    frequency causing the saturation – effectively averaging all

    their spin states to zero.

    1H channel-

    13C channel

    13C νννν pulse

    13C FID

  • 5

    13C Proton Decoupled Spectrum

    13C{1H}

    Effect of Decoupling

    1H Coupled

    1H Decoupled

    13C NMR Spectra

    Due to signal enhancement and spectral simplification, 13C

    spectra are usually reported as 1H decoupled.

    Each chemically unique carbon in the molecule gives rise to a

    single peak.

    Of course chemically equivalent carbons contribute to the

    same peak!

    The number of different signals (peaks) indicates the number

    of different kinds of carbon.

    The location (chemical shift) indicates the type of functional

    group.

    13C NMR Intensities

    Peak areas (~heights) are NOT proportional to number of carbons.

    Carbon atoms with more hydrogens give stronger signals, due to more efficient relaxation (transfer of spin to the hydrogens).

    However, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls)

  • 6

    Example: Ethanol

    CH2 CH3

    OH

    Example: 1-bromohexane

    CH2

    CH2

    CH2

    CH2

    CH2

    CH3Br

    Example: cyclohexane Example: cyclohexene

  • 7

    Example: 1,3-cyclohexadiene Example: 1,4-cyclohexadiene

    Example: m-nitrotoluene

    CH3O2N 1 2

    3

    4 6

    7

    5

    3

    1

    2

    6 5

    7

    4

    28

    13C Chemical Shift Predictions

    Examining a large set of chemical shift data has allowed the

    development of ‘empirical’ rules or substituent parameters to

    allow chemical shift predictions for most commonly

    encountered situations.

    Example: the carbon atoms of a substituted benzene ring.

    Benzene itself → single peak at 128.7 ppm

    Add to this value substituent increments which depend on the

    chemical nature of the substituent and where it is on the ring

    relative to the carbon whose shift is being predicted.

  • 8

    29

    13C Aromatic Substituent Parameters

    CH3O2N 1 2

    3

    4 6

    7

    5

    C1 = 128.7 + (CH3)ipso + (NO2)meta = 128.7 + 8.9 + 0.8 = 138.9 ppm

    C2 = 128.7 + (CH3)ortho + (NO2)ortho = 128.7 + 0.7 + (-5.3) = 124.1 ppm

    31

    Example: m-nitrotoluene CH3O2N 1

    2 3

    4 6

    7

    5

    3

    1

    2

    6 5

    7

    4

    135.4135.46

    129.2129.45

    120.6120.54

    148.4148.23

    123.8124.12

    139.9138.41

    Obs’dCalc’dC

    32

    Example: p-Hydroxyacetophenone

    C O CH3

    OH

    1

    6

    5

    4

    3

    2

    131.4130.26

    115.8115.85

    162.1158.54

    115.8115.83

    131.4130.22

    129.2128.71

    Obs’dCalc’dC

    4

    3

    5

    2

    6 1

  • 9

    33

    13C Shift Predictions – Alkyls

    Can also make predictions for alkyl groups

    Base value: use

    unsubstituted

    hydrocarbon 34

    Example: bromocyclopentane

    Br

    23.3233

    37.9362

    53.5511

    Obs’dCalc’dC 1 2

    3

    1

    2

    3

    35

    13C NMR Intensities

    Peak areas (~heights) are NOT proportional to number of carbons.

    Carbon atoms with more hydrogens give stronger signals, due to more efficient relaxation (transfer of spin to the hydrogens).

    However, peak areas (~heights) can be compared within the same type of carbons (e.g. methyls)

    Nuclear Overhauser Enhancement (NOE)

    A phenomenon observed with proton-decoupled 13C-NMR is

    that the intensity of the signal for a given 13C increases versus

    the proton-coupled spectrum roughly proportional to the

    number of protons attached.

    The degree of this signal enhan