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Spektroskopi Molekul Organik (SMO): Nuclear Magnetic Resonance (NMR) Spectroscopy All is adopted from McMurry’s Organic Chemistry 2 The Use of NMR Spectroscopy Used to determine relative location of atoms within a molecule Most helpful spectroscopic technique in organic chemistry Maps carbon-hydrogen framework of molecules Depends on very strong magnetic fields (imagine the strongest electromagnet you can and the imagine it on steroids)

Spektroskopi Molekul Organik (SMO): Nuclear … Molekul Organik (SMO): Nuclear Magnetic Resonance (NMR) Spectroscopy All is adopted from McMurry’s Organic Chemistry 2 The Use of

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  • Spektroskopi Molekul Organik (SMO): Nuclear Magnetic Resonance

    (NMR) Spectroscopy

    All is adopted from McMurrys Organic Chemistry

    2

    The Use of NMR Spectroscopy

    Used to determine relative location of atoms within a moleculeMost helpful spectroscopic technique in organic chemistryMaps carbon-hydrogen framework of moleculesDepends on very strong magnetic fields(imagine the strongest electromagnet you can and the imagine it on steroids)

  • 3

    Why This Chapter?

    NMR is the most valuable spectroscopic technique used for structure determination

    More advanced NMR techniques are used in biological chemistry to study protein structure and folding

    4

    Nuclear Magnetic Resonance Spectroscopy

    1H or 13C nucleus spins and the internal magnetic field aligns parallel to or against an aligned external magnetic fieldParallel orientation is lower in energy making this spin state more populatedRadio energy of exactly correct frequency (resonance) causes nuclei to flip into anti-parallel state Energy needed is related to molecular

    environment (proportional to field strength, B)

  • The spin state of a nucleus is affected by an appliedmagnetic field

    The energy difference between the two spin statesdepends on the strength of the magnetic field (that the atom feels)

  • absorb E

    -spin states -spin states

    release E

    Signals detected by NMR

    FID

    The electrons surrounding a nucleus affect the effectivemagnetic field sensed by the nucleus

    Electron poor environment

    Electron rich environment

  • Shielded nuclei do not sense as large a magnetic field as deshieldednuclei do. As a result, the energy difference between the - and -spin states is much lower in energy for shielded nuclei and resonate at a lower frequency.

    Deshielded nuclei have a much higher energy difference between the - and -spin states and these resonate at a much higher frequency.

    10

    The Nature of NMR AbsorptionsElectrons in bonds shield nuclei from magnetic fieldDifferent signals appear for nuclei in different environments

  • 11

    The NMR Measurement

    The sample is dissolved in a solvent that does not have a signal itself* and placed in a long thin tubeThe tube is placed within the gap of a magnet and spunRadio frequency (Rf) energy is transmitted and absorption is detectedSpecies that interconvert give an averaged signal that can be analyzed to find the rate of conversionCan be used to measure rates and activation energies of very fast processes

  • 13

    Chemical Shifts

    The relative energy of resonance of a particular nucleus resulting from its local environment is called chemical shift NMR spectra show applied field strength increasing from left to rightLeft part is downfield the right is upfieldNuclei that absorb on upfield side are strongly shieldedChart calibrated versus a reference point, set as 0, tetramethylsilane [TMS]

    14

    Measuring Chemical ShiftNumeric value of chemical shift: difference between strength of magnetic field at which the observed nucleus resonates and field strength for resonance of a reference

    Difference is very small but can be accurately measuredTaken as a ratio to the total field and multiplied by 106 so the shift is in parts per million (ppm)

    Absorptions normally occur downfield of TMS, to the left on the chartCalibrated on relative scale in delta () scale

    Independent of instruments field strength

  • 15

    16

    13C NMR Spectroscopy: Signal Averaging and FT-NMR

    Carbon-13: only carbon isotope with a nuclear spinNatural abundance 1.1% of Cs in moleculesSample is thus very dilute in this isotope

    Sample is measured using repeated accumulation of data and averaging of signals, incorporating pulse and the operation of Fourier transform (FT-NMR)All signals are obtained simultaneously using a broad pulse of energy and resonance recordedFrequent repeated pulses give many sets of data that are averaged to eliminate noise Fourier-transform of averaged pulsed data gives spectrum (see Figure 13-6)

  • 1 scan of conc. sample

    200 scans of same sample

  • 19

    Characteristics of 13C NMR Spectroscopy

    Provides a count of the different types of environments of carbon atoms in a molecule13C resonances are 0 to 220 ppm downfield from TMSChemical shift affected by electronegativity of nearby atoms

    O, N, halogen decrease electron density and shielding (deshield), moving signal downfield.

    sp3 C signal is at 0 to 9; sp2 C: 110 to 220C(=O) at low field, 160 to 220

  • 13C NMR

    1H NMR

    Low Field High Field

    Deshielding ShieldingDown field Up field

    22

    Spectrum of 2-butanone is illustrative- signal for C=O carbons on left edge

  • 24

    DEPT 13C NMR Spectroscopy

    Improved pulsing and computational methods give additional informationDEPT-NMR (distortionless enhancement by polarization transfer)Normal spectrum shows all Cs then:

    Obtain spectrum of all Cs except quaternary (broad band decoupled)Change pulses to obtain separate information for CH2, CHSubtraction reveals each type (See Figure 13-10)

  • DEPT 13C NMR distinguish among CH3, CH2, and CHGroups (Distortionless Enhancement by Polarization Transfer

    26

    Uses of 13C NMR Spectroscopy

    Provides details of structureExample: product orientation in elimination from 1-chloro-methyl cyclohexaneDifference in symmetry of products is directly observed in the spectrum1-chloro-methylcyclohexane has five sp3 resonances ( 20-50) and two sp2 resonances 100-150

  • 27

    1H NMR Spectroscopy and Proton EquivalenceProton NMR is much more sensitive than 13C and the active nucleus (1H) is nearly 100 % of the natural abundanceShows how many kinds of nonequivalent hydrogens are in a compoundTheoretical equivalence can be predicted by seeing if replacing each H with X gives the same or different outcomeEquivalent Hs have the same signal while nonequivalent are different and as such may cause additional splitting (diastereotopic effect)

    There are degrees of nonequivalence

    28

    Nonequivalent HsReplacement of each H with X gives a different constitutional isomerThen the Hs are in constitutionally heterotopic environments and will have different chemical shifts they are nonequivalent under all circumstances

  • 29

    Equivalent HsTwo Hs that are in identical environments (homotopic) have the same NMR signalTest by replacing each with X

    if they give the identical result, they are equivalentProtons are considered homotopic

    30

    Enantiotopic DistinctionsIf Hs are in environments that are mirror images of each other, theyare enantiotopicReplacement of each H with X produces a set of enantiomersThe Hs have the same NMR signal (in the absence of chiralmaterials)

  • 31

    Diastereotopic DistinctionsIn a chiral molecule, paired hydrogens can have different environ-ments and different shiftsReplacement of a pro-R hydrogen with X gives a different diastereomer than replacement of the pro-S hydrogen Diastereotopic hydrogens are distinct chemically and spectrocopically

    32

    Chemical Shifts in 1H NMR Spectroscopy

    Proton signals range from 0 to 10Lower field signals are Hs attached to sp3 CHigher field signals are Hs attached to sp2 CElectronegative atoms attached to adjacent C cause downfield shift

  • Chemical Shifts in 1H NMR Spectroscopy

    34

    Integration of 1H NMR Absorptions: Proton Counting

    The relative intensity of a signal (integrated area) is proportional to the number of protons causing the signalThis information is used to deduce the structureFor example in ethanol (CH3CH2OH), the signals have the integrated ratio 3:2:1For narrow peaks, the heights are the same as the areas and can be measured with a ruler

  • 35

    Spin-Spin Splitting in 1H NMR Spectra

    Peaks are often split into multiple peaks due to interactions between nonequivalent protons on adjacent carbons, called spin-spin splitting

    The splitting is into one more peak than the number of Hs on the adjacent carbon (n+1 rule)

    The relative intensities are in proportion of a binomial distribution and are due to interactions between nuclear spins that can have two possible alignments with respect to the magnetic field

    The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet)

    36

    Simple Spin-Spin Splitting

    An adjacent CH3 group can have four different spin alignments as 1:3:3:1This gives peaks in ratio of the adjacent H signalAn adjacent CH2 gives a ratio of 1:2:1The separation of peaks in a multiplet is measured is a constant, in Hz

    J (coupling constant)

  • 37

    Rules for Spin-Spin Splitting

    Equivalent protons do not split each otherThe signal of a proton with n equivalent neighboring Hs is split into n + 1 peaksProtons that are farther than two carbon atoms apart do not split each other

    01.02.03.04.05.06.07.08.09.010.0

    Chemical shift (, ppm)

    BrCH2CH3

    4 lines;

    quartet

    3 lines;

    triplet

    CH3

    CH2

  • 01.02.03.04.05.06.07.08.09.010.0

    Chemical shift (Chemical shift (, , ppmppm))

    BrCH(CH3)2

    7 lines;7 lines;

    septetseptet

    2 lines;

    doublet

    CH3

    CH

    Protons Bonded to Oxygen and Nitrogen

    These protons can undergo proton exchange

    They always appear as broad signals

    The greater the extent of the hydrogen bond, the greaterthe chemical shift

  • dry ethanol

    ethanol with acid

    To observe well-defined splitting patterns, the difference in the chemical shifts (in Hz) must be 10 times the coupling constant values

  • 1H NMR Spectra of 2-sec-butylphenol at Different Field Strengths

    60 MHz

    300 MHz

    44

    More Complex Spin-Spin Splitting Patterns

    Spectra can be more complex due to overlapping signals, multiple nonequivalenceExample: trans-cinnamaldehyde

  • 45

    Uses of 1H NMR Spectroscopy

    The technique is used to identify likely products in the laboratory quickly and easilyExample: regiochemistry of hydroboration/oxidation of methylenecyclohexaneOnly that for cyclohexylmethanol is observed

    Peaks in a 13C NMR spectrum are typicallysinglets

    13C13C splitting is not seen because the probability of two 13C nuclei being in the same molecule is very small.

    13C1H splitting is not normally seen because spectrum is measured under conditions that suppress this splitting (broadband decoupling).

  • 1H Decoupled and Coupled 13C Spectra of 2-butanol

    coupled

    decoupled