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					Ch. 13: NMR Spectroscopy

      Sections 13.1-13.4
     Chem 30B, Lecture 3
      Molecular Spectroscopy

• Nuclear magnetic resonance (NMR)
  spectroscopy: A spectroscopic technique that
  gives us information about the number and
  types of atoms in a molecule, for example,
  about the number and types of
   – hydrogen atoms using 1H-NMR
     spectroscopy.
   – carbon atoms using 13C-NMR
     spectroscopy.
   – phosphorus atoms using 31P-NMR
     spectroscopy.
          Nuclear Spin States
• An electron has a spin quantum number of
  1/2 with allowed values of +1/2 and -1/2.
  – This spinning charge has an associated magnetic
    field.
  – In effect, an electron behaves as if it is a tiny bar
    magnet and has what is called a magnetic
    moment.
• The same effect holds for certain atomic
  nuclei.
  – Any atomic nucleus that has an odd mass number,
    an odd atomic number, or both, also has a spin
    and a resulting nuclear magnetic moment.
  – The allowed nuclear spin states are determined by
    the spin quantum number, I, of the nucleus.
               Nuclear Spin States
 – A nucleus with spin quantum number I has
   2I + 1 spin states; if I = 1/2, there are two
   allowed spin states.
 – Spin quantum numbers and allowed
   nuclear spin states for atoms common to
   organic compounds.

                1    2   12   13    14       15    16       19   31   32
Element         H    H    C    C     N        N     O        F    P    S

Nuclear spin
quantum        1/2   1    0   1/2        1   1/2        0 1/2 1/2      0
number ( I )

Number of       2    3    1    2         3    2         1    2    2    1
spin states
       Nuclear Spins in B0
– Within a collection of 1H and 13C atoms,
  nuclear spins are completely random in
  orientation.
– When placed in a strong external magnetic
  field of strength B0, however, interaction
  between nuclear spins and the applied
  magnetic field is quantized. The result is
  that only certain orientations of nuclear
  magnetic moments are allowed.
      Nuclear Spins in B0
– for 1H and 13C, only two orientations are
  allowed.
        Nuclear Spins in B0
• In an applied field strength of 7.05T the
  difference in energy between nuclear spin
  states for
   – 1H is approximately 0.120 J (0.0286
     cal)/mol, which corresponds to a frequency
     of 300 MHz (300,000,000 Hz).
   – 13C is approximately 0.030 J (0.00715
     cal)/mol, which corresponds to a frequency
     of 75MHz (75,000,000 Hz).
           Nuclear Spin in B0
– The energy difference between allowed spin
  states increases linearly with applied field
  strength.
– Values shown here are for 1H nuclei.
   Nuclear Magnetic Resonance
– When nuclei with a spin quantum number of 1/2
  are placed in an applied field, a small majority of
  nuclear spins are aligned with the applied field
  in the lower energy state.
– The nucleus begins to precess and traces out a
  cone-shaped surface, in much the same way a
  spinning top or gyroscope traces out a cone-
  shaped surface as it precesses in the earth’s
  gravitational field.
Nuclear Magnetic Resonance
• If the precessing nucleus is irradiated
  with electromagnetic radiation of the
  same frequency as the rate of
  precession,
  – the two frequencies couple
  – energy is absorbed
  – the nuclear spin is flipped from spin state
    +1/2 (with the applied field) to -1/2 (against
    the applied field).
   Nuclear Magnetic Resonance
– (a) Precession and (b) after absorption of
  electromagnetic radiation.
     Nuclear Magnetic Resonance
• Resonance: In NMR spectroscopy, resonance is
  the absorption of energy by a precessing nucleus
  and the resulting “flip” of its nuclear spin from a
  lower energy state to a higher energy state.
• The precessing spins induce an oscillating
  magnetic field that is recorded as a signal by the
  instrument.
  – Signal: A recording in an NMR spectrum of a nuclear
    magnetic resonance.
Nuclear Magnetic Resonance
 – If we were dealing with 1H nuclei isolated from all
   other atoms and electrons, any combination of
   applied field and radiation that produces a signal
   for one 1H would produce a signal for all 1H. The
   same is true of 13C nuclei.
 – Hydrogens in organic molecules, however, are not
   isolated from all other atoms. They are surrounded
   by electrons, which are caused to circulate by the
   presence of the applied field.
 – The circulation of electrons around a nucleus in an
   applied field is called diamagnetic current and the
   nuclear shielding resulting from it is called
   diamagnetic shielding.
   Nuclear Magnetic Resonance
– The difference in resonance frequencies among
  the various hydrogen nuclei within a molecule
  due to shielding/deshielding is generally very
  small.
– The difference in resonance frequencies for
  hydrogens in CH3Cl compared to CH3F under an
  applied field of 7.05T is only 360 Hz, which is 1.2
  parts per million (ppm) compared with the
  irradiating frequency.
         360 Hz          1.2
               6
                       =   6      = 1.2 ppm
       300 x 10 Hz       10
Nuclear Magnetic Resonance
– Signals are measured relative to the signal of the
  reference compound tetramethylsilane (TMS).
                        CH3
                  CH3   Si CH3
                        CH3
               Tetrameth yls ilane (TMS)

– For a 1H-NMR spectrum, signals are reported by
  their shift from the 12 H signal in TMS.
– For a 13C-NMR spectrum, signals are reported by
  their shift from the 4 C signal in TMS.
– Chemical shift (): The shift in ppm of an NMR
  signal from the signal of TMS.
        NMR Spectrometer
• Schematic diagram of a nuclear
  magnetic resonance spectrometer.
         NMR Spectrometer

• Essentials of an NMR spectrometer are a
  powerful magnet, a radio-frequency
  generator, and a radio-frequency detector.
• The sample is dissolved in a solvent, most
  commonly CDCl3 or D2O, and placed in a
  sample tube which is then suspended in the
  magnetic field and set spinning.
• Using a Fourier transform NMR (FT-NMR)
  spectrometer, a spectrum can be recorded in
  about 2 seconds.
             NMR Spectrum
• 1H-NMR spectrum of methyl acetate.




  – High frequency: The shift of an NMR signal to the
    left on the chart paper.
  – Low frequency: The shift of an NMR signal to the
    right on the chart paper.
Ch. 13: NMR Spectroscopy

      Sections 13.5-13.7
     Chem 30B, Lecture 4
              Equivalent Hydrogens
  • Equivalent hydrogens: Hydrogens that have
    the same chemical environment.
     – A molecule with 1 set of equivalent
       hydrogens gives 1 NMR signal.

    O                                          H3 C           CH3
CH3 CCH3      ClCH 2 CH2 Cl                           C   C
                                               H3 C           CH3
Propanone      1,2-Dichloro-   Cyclope ntane    2,3-Dime thyl-
 (Aceton e)       ethane                           2-bute ne
            Equivalent Hydrogens


     – A molecule with 2 or more sets of
       equivalent hydrogens gives a different
       NMR signal for each set.

       Cl                         Cl         CH3
 CH3 CHCl                  O           C C
                                    H      H
1,1-D ich loro-   Cyclop ent-     (Z)-1-Ch loro-   Cyclohexen e
    eth ane         an on e          prop ene       (3 signals)
  (2 signals )    (2 s ign als)     (3 signals)
               Signal Areas
– Relative areas of signals are proportional to the
  number of H giving rise to each signal, Modern
  NMR spectrometers electronically integrate and
  record the relative area of each signal.
     Chemical Shift - 1H-NMR

• Average values of chemical shifts of
  representative types of hydrogens.
             Type of      Chemical             Type of      Chemical
             Hydrogen     Shift ()            Hydrogen     Shift ()
Chemical   ( CH3 ) 4 Si   0 (by defin ition)    O
Shifts      RCH3          0.8-1.0              RCOCH3       3.7-3.9
1H-NMR      RCH2 R        1.2-1.4               O
            R3 CH         1.4-1.7              RCOCH2 R     4.1-4.7
            R2 C= CRCH R2 1.6-2.6              RCH 2 I      3.1-3.3
            RC CH         2.0-3.0              RCH 2 Br     3.4-3.6
            A rCH3        2.2-2.5              RCH 2 Cl     3.6-3.8
            A rCH2 R      2.3-2.8              RCH 2 F      4.4-4.5
            ROH           0.5-6.0              A rOH        4.5-4.7
            RCH2 OH       3.4-4.0              R2 C= CH 2   4.6-5.0
            RCH2 OR       3.3-4.0              R2 C= CHR    5.0-5.7
            R2 NH         0.5-5.0              A rH         6.5-8.5
              O                                 O
            RCCH 3        2.1-2.3              RCH          9.5-10.1
              O                                 O
            RCCH 2 R      2.2-2.6              RCOH         10-13
                  Chemical Shift
• Chemical shift depends on the (1)
  electronegativity of nearby atoms, (2)
  hybridization of adjacent atoms, and (3)
  diamagnetic effects from adjacent pi bonds.
• Electronegativity              Electron eg- Chemical
                         CH3 -X        ativity of X   Shift ()
                         CH3 F             4.0          4.26
                         CH3 OH            3.5          3.47
                         CH3 Cl            3.1          3.05
                         CH3 Br            2.8          2.68
                         CH3 I             2.5          2.16
                         (CH3 ) 4 C        2.1          0.86
                         (CH3 ) 4 Si       1.8          0.00
              Chemical Shift
• Hybridization of adjacent atoms.

   Type of Hydrogen        N ame of      Chemical
   (R = alkyl)             Hydrogen      Sh ift ()
   RCH3 , R2 CH2 , R3 CH   Alk yl        0.8 - 1.7
   R2 C=C(R)CHR2           Allylic       1.6 - 2.6
   RC CH                   Acetylen ic   2.0 - 3.0
   R2 C=CHR, R2 C=CH2      Vin ylic      4.6 - 5.7
   RCHO                    Ald ehydic    9.5-10.1
                 Chemical Shift
• Diamagnetic effects of pi bonds
  – A carbon-carbon triple bond shields an
    acetylenic hydrogen and shifts its signal to lower
    frequency (to the right) to a smaller  value.
  – A carbon-carbon double bond deshields vinylic
    hydrogens and shifts their signal to higher
    frequency (to the left) to a larger  value.
                                  Chemical
           Type of H   N ame      Shift ()
           RCH3        Alk yl      0.8- 1.0
           RC CH       Acetylenic 2.0 - 3.0
           R2 C=CH2    Vin ylic    4.6 - 5.7
             Chemical Shift
– Magnetic induction in the p bonds of a carbon-
  carbon triple bond shields an acetylenic
  hydrogen and shifts its signal lower frequency.
             Chemical Shift
– Magnetic induction in the p bond of a carbon-
  carbon double bond deshields vinylic hydrogens
  and shifts their signal higher frequency.
              Chemical Shift
– The magnetic field induced by circulation of p
  electrons in an aromatic ring deshields the
  hydrogens on the ring and shifts their signal to
  higher frequency.
Ch. 13: NMR Spectroscopy

      Sections 13.8-13.9
          Lecture 5
 Signal Splitting; the (n + 1) Rule

• Peak: The units into which an NMR signal is
  split; doublet, triplet, quartet, multiplet, etc.
• Signal splitting: Splitting of an NMR signal
  into a set of peaks by the influence of
  neighboring nonequivalent hydrogens.
• (n + 1) rule: If a hydrogen has n hydrogens
  nonequivalent to it but equivalent among
  themselves on the same or adjacent atom(s),
  its 1H-NMR signal is split into (n + 1) peaks.
             Signal Splitting (n + 1)

      – 1H-NMR spectrum of 1,1-dichloroethane.




For these hydrogens, n = 1;                 For this hydrogen, n = 3;
their signal is split into     CH3 - CH- Cl its signal is split into
(1 + 1) = 2 peaks; a doublet         Cl     (3 + 1) = 4 peaks; a quartet
           Signal Splitting (n + 1)
    Problem: Predict the number of 1H-NMR
    signals and the splitting pattern of each.

        O
(a) CH3 CCH2 CH 3

            O
(b) CH3 CH2 CCH2 CH3

        O
(c) CH3 CCH( CH3 ) 2
      Origins of Signal Splitting

• Signal coupling: An interaction in which the
  nuclear spins of adjacent atoms influence
  each other and lead to the splitting of NMR
  signals.
• Coupling constant (J): The separation on an
  NMR spectrum (in hertz) between adjacent
  peaks in a multiplet.
   – A quantitative measure of the spin-spin
     coupling with adjacent nuclei.
      Origins of Signal Splitting
• Illustration of spin-spin coupling that gives rise
  to signal splitting in 1H-NMR spectra.
      Origins of Signal Splitting
– The quartet-triplet 1H-NMR signals of 3-
  pentanone with the original trace and an
  expansion to show the signal splitting clearly.
                      Coupling Constants
• Coupling constant (J): The distance between peaks
  in a split signal, expressed in hertz.
   – The value is a quantitative measure of the
     magnetic interaction of nuclei with coupled spins.
                                   Ha            Ha
        Ha Hb
                                                                 H
        C C                                      Hb            Hb a

                               Hb
        6-8 Hz             8-14 Hz      0-5 Hz        0-5 Hz

   Ha                 Ha           Hb           Ha         Ha
        C   C              C   C        C   C
                 Hb                             Hb         Hb
   11-18 Hz                5-10 Hz      0-5 Hz        8-11 Hz
         Origins of Signal Splitting
• The origins of signal splitting patterns. Each arrow
  represents an Hb nuclear spin orientation.
           Signal Splitting
• Pascal’s triangle.
  – As illustrated by the
    highlighted entries,
    each entry is the sum of
    the values immediately
    above it to the left and
    the right.
      Physical Basis for (n + 1) Rule
• Coupling of nuclear spins is mediated through
  intervening bonds.
   – H atoms with more than three bonds between
     them generally do not exhibit coupling.
   – For H atoms three bonds apart, the coupling is
     called vicinal coupling.
  Physical Basis for (n + 1) Rule

• Coupling that arises when Hb is split by
  two different nonequivalent H atoms, Ha
  and Hc.
           Coupling Constants
– An important factor in vicinal coupling is the angle
  a between the C-H sigma bonds and whether or
  not it is fixed.
– Coupling is a maximum when a is 0° and 180°;
  it is a minimum when a is 90°.
More Complex Splitting Patterns

 – Complex coupling that arises when Hb is
   split by Ha and two equivalent atoms Hc.
 More Complex Splitting Patterns
– Since the angle between C-H bond determines the extent
  of coupling, bond rotation is a key parameter.
– In molecules with free rotation about C-C sigma bonds, H
  atoms bonded to the same carbon in CH3 and CH2
  groups are equivalent.
– If there is restricted rotation, as in alkenes and cyclic
  structures, H atoms bonded to the same carbon may not
  be equivalent.
– Nonequivalent H on the same carbon will couple and
  cause signal splitting.
– This type of coupling is called geminal coupling.
More Complex Splitting Patterns

 – In ethyl propenoate, an unsymmetrical
   terminal alkene, the three vinylic
   hydrogens are nonequivalent.
More Complex Splitting Patterns
– Tree diagram for the complex coupling seen for
  the three alkenyl H atoms in ethyl propenoate.
 More Complex Splitting Patterns
– Cyclic structures often have restricted rotation
  about their C-C bonds and have constrained
  conformations.
– As a result, two H atoms on a CH2 group can be
  nonequivalent, leading to complex splitting.
More Complex Splitting Patterns
– A tree diagram for the complex coupling seen
  for the vinyl group and the oxirane ring H atoms
  of 2-methyl-2-vinyloxirane.
   More Complex Splitting Patterns
• Complex coupling in flexible molecules.
  – Coupling in molecules with unrestricted bond
    rotation often gives only m + n + I peaks.
  – That is, the number of peaks for a signal is the
    number of adjacent hydrogens + 1, no matter
    how many different sets of equivalent H atoms
    that represents.
  – The explanation is that bond rotation averages
    the coupling constants throughout molecules
    with freely rotation bonds and tends to make
    them similar; for example in the 6- to 8-Hz range
    for H atoms on freely rotating sp3 hybridized C
    atoms.
More Complex Splitting Patterns
– Simplification of signal splitting occurs when
  coupling constants are the same.
 More Complex Splitting Patterns
– Peak overlap occurs in the spectrum of 1-chloro-
  3-iodopropane.
– Hc should show 9 peaks, but because Jab and Jbc
  are so similar, only 4 + 1 = 5 peaks are
  distinguishable.
Ch. 13: NMR Spectroscopy

     Sections 13.10-13.11
     Chem 30B, Lecture 6
            Stereochemistry & Topicity
    • Homotopic atoms or groups

H              Subs titute   H
        Cl                          Cl Subs titution doe s not
    C          one H by D       C      produce a ste reocente r;
        Cl                          Cl th erefore hydrogens
H                            D         are homotopic.
Dichloro-                     Achiral
methane
(achiral)

        – Homotopic atoms or groups have identical
          chemical shifts under all conditions.
         Stereochemistry & Topicity
• Enantiotopic groups

 H              Subs titute              Subs titution produce s a
         Cl                   H
     C
                one H by D            Cl stereocenter;
                                 C       th erefore, h ydrogens are
         F
 H                                     F enan tiotopic. Both
                              D          hydrogen s are prochiral;
Chlorofluoro-                   Chiral   one is pro-R-chiral, the
  methane                                other is pro-S-chiral.
  (achiral)

   – Enantiotopic atoms or groups have identical
     chemical shifts in achiral environments.
   – They have different chemical shifts in chiral
     environments.
          Stereochemistry & Topicity
• Diastereotopic groups
  – H atoms on C-3 of 2-butanol are diastereotopic.
  – Substitution by deuterium creates a chiral center.
  – Because there is already a chiral center in the
    molecule, diastereomers are now possible.
      H     OH      Subs titute one    H    OH
                    H on CH 2 by D


         H     H                           D    H
       2-Butanol                           Chiral
        (chiral)
  – Diastereotopic hydrogens have different chemical
    shifts under all conditions.
       Stereochemistry & Topicity
• The methyl groups on carbon 3 of 3-methyl-2-
  butanol are diastereotopic.
   – If a methyl hydrogen of carbon 4 is substituted by
     deuterium, a new chiral center is created.
   – Because there is already one chiral center,
     diastereomers are now possible.
                          OH



                  3-Methyl-2-butanol
  – Protons of the methyl groups on carbon 3 have
    different chemical shifts.
      Stereochemistry and Topicity
• 1H-NMR spectrum of 3-methyl-2-butanol.
   – The methyl groups on carbon 3 are
     diastereotopic and appear as two doublets.
          13C-NMR      Spectroscopy
• Each nonequivalent 13C gives a different signal
   – A 13C signal is split by the 1H bonded to it
     according to the (n + 1) rule.
   – Coupling constants of 100-250 Hz are common,
     which means that there is often significant
     overlap between signals, and splitting patterns
     can be very difficult to determine.
• The most common mode of operation of a 13C-NMR
  spectrometer is a proton-decoupled mode.
       13C-NMR         Spectroscopy

• In a proton-decoupled mode, a sample is
  irradiated with two different radiofrequencies,
   – one to excite all 13C nuclei.
   – a second broad spectrum of frequencies to cause
     all protons in the molecule to undergo rapid
     transitions between their nuclear spin states.
• On the time scale of a 13C-NMR spectrum,
  each proton is in an average or effectively
  constant nuclear spin state, with the result
  that 1H-13C spin-spin interactions are not
  observed; they are decoupled.
    13C-NMR     Spectroscopy

– Proton-decoupled 13C-NMR spectrum of 1-
  bromobutane.
      Chemical Shift - 13C-NMR
13C-NMR   chemical shifts of representative groups
    Chemical Shift - 13C-NMR
Typ e of   Chemical     Type of    Chemical
Carbon     S hift ()   Carb on    Sh ift ()
RCH3         10-40
RCH2 R       15-55          C R     110-160
R3 CH        20-60
                         O
RCH2 I       0-40       RCOR       160 - 180
RCH2 Br      25-65
                         O
RCH2 Cl      35-80      RCNR2       165 - 180
R3 COH       40-80       O
R3 COR       40-80      RCCOH      165 - 185
RC CR        65-85       O    O
R2 C=CR2    100-150     RCH, RCR   180 - 215
Ch. 13: NMR Spectroscopy

        Section 13.12
     Chem 30B, Lecture 7
         Interpreting NMR Spectra
• Alkanes
  – 1H-NMR signals appear in the range of  0.8-1.7.
  – 13C-NMR signals appear in the considerably
    wider range of  10-60.
• Alkenes
  – 1H-NMR signals appear in the range  4.6-5.7.
  – 1H-NMR coupling constants are generally larger
    for trans-vinylic hydrogens (J= 11-18 Hz)
    compared with cis-vinylic hydrogens (J= 5-10
    Hz).
  – 13C-NMR signals for sp2 hybridized carbons
    appear in the range  100-160, which is to higher
    frequency from the signals of sp3 hybridized
    carbons.
  Interpreting NMR Spectra


– 1H-NMR spectrum of vinyl acetate.
        Interpreting NMR Spectra
• Alcohols
• 1H-NMR O-H chemical shift often appears in the
  range  3.0-4.0, but may be as low as  0.5.
   – 1H-NMR chemical shifts of hydrogens on the
     carbon bearing the -OH group are deshielded
     by the electron-withdrawing inductive effect of
     the oxygen and appear in the range  3.0-4.0.
• Ethers
   – A distinctive feature in the 1H-NMR spectra of
     ethers is the chemical shift,  3.3-4.0, of
     hydrogens on the carbons bonded to the ether
     oxygen.
Interpreting NMR Spectra
– 1H-NMR spectrum of 1-propanol.
     Interpreting NMR Spectra
• Aldehydes and ketones
  – 1H-NMR: aldehyde hydrogens appear at 
    9.5-10.1.
  – 1H-NMR: a-hydrogens of aldehydes and
    ketones appear at  2.2-2.6.
  – 13C-NMR: carbonyl carbons appear at 
    180-215.
• Amines
  – 1H-NMR: amine hydrogens appear at  0.5-
    5.0 depending on conditions.
        Interpreting NMR Spectra
• Carboxylic acids
  – 1H-NMR: carboxyl hydrogens appear at  10-13,
    higher than most other types of hydrogens.
  – 13C-NMR: carboxyl carbons in acids and esters
    appear at  160-180.
    Interpreting NMR Spectra

• Spectral Problem 1; molecular formula
  C5H10O.
    Interpreting NMR Spectra

• Spectral Problem 2; molecular formula
  C7H14O.

				
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Description: NMR spectroscopic technique that gives us information about the number and types of atoms in a molecule