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Dr. Wolf's CHM 201 & 202 13- 1 Chapter 13 Spectroscopy Infrared spectroscopy Ultraviolet-Visible spectroscopy Nuclear magnetic resonance spectroscopy Mass.

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Presentation on theme: "Dr. Wolf's CHM 201 & 202 13- 1 Chapter 13 Spectroscopy Infrared spectroscopy Ultraviolet-Visible spectroscopy Nuclear magnetic resonance spectroscopy Mass."— Presentation transcript:

1 Dr. Wolf's CHM 201 & Chapter 13 Spectroscopy Infrared spectroscopy Ultraviolet-Visible spectroscopy Nuclear magnetic resonance spectroscopy Mass Spectrometry

2 Dr. Wolf's CHM 201 & Principles of Molecular Spectroscopy: Electromagnetic Radiation

3 Dr. Wolf's CHM 201 & is propagated at the speed of light has properties of particles and waves the energy of a photon is proportional to its frequency Electromagnetic Radiation

4 Dr. Wolf's CHM 201 & Figure 13.1: The Electromagnetic Spectrum 400 nm750 nm Visible Light Longer Wavelength ( )Shorter Wavelength ( ) Higher Frequency ( )Lower Frequency ( ) Higher Energy (E)Lower Energy (E)

5 Dr. Wolf's CHM 201 & Figure 13.1: The Electromagnetic Spectrum UltravioletInfrared Longer Wavelength ( ) Shorter Wavelength ( ) Higher Frequency ( ) Lower Frequency ( ) Higher Energy (E) Lower Energy (E)

6 Dr. Wolf's CHM 201 & Cosmic rays  Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Cosmic rays  Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Figure 13.1: The Electromagnetic Spectrum Energy

7 Dr. Wolf's CHM 201 & Principles of Molecular Spectroscopy: Quantized Energy States

8 Dr. Wolf's CHM 201 & Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states.  E = h  E = h

9 Dr. Wolf's CHM 201 & electronicvibrationalrotational nuclear spin UV-Visinfraredmicrowaveradiofrequency What Kind of States?

10 Dr. Wolf's CHM 201 & Infrared Spectroscopy Gives information about the functional groups in a molecule

11 Dr. Wolf's CHM 201 & region of infrared that is most useful lies between  m ( cm -1 ) depends on transitions between vibrational energy states stretchingbending Infrared Spectroscopy

12 Dr. Wolf's CHM 201 & Stretching Vibrations of a CH 2 Group SymmetricAntisymmetric

13 Dr. Wolf's CHM 201 & Bending Vibrations of a CH 2 Group In plane

14 Dr. Wolf's CHM 201 & Bending Vibrations of a CH 2 Group Out of plane

15 Dr. Wolf's CHM 201 & Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved Wave number, cm -1 Figure 13.31: Infrared Spectrum of Hexane CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 C—H stretching bending

16 Dr. Wolf's CHM 201 & Wave number, cm -1 Figure 13.32: Infrared Spectrum of 1-Hexene H 2 C=CHCH 2 CH 2 CH 2 CH 3 H—C C=C—H C=C H 2 C=C Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.

17 Dr. Wolf's CHM 201 & Structural unitFrequency, cm -1 Stretching vibrations (single bonds) sp C—H sp 2 C—H sp 3 C—H sp 2 C—O1200 sp 3 C—O Infrared Absorption Frequencies

18 Dr. Wolf's CHM 201 & Structural unitFrequency, cm -1 Stretching vibrations (multiple bonds) Infrared Absorption Frequencies C C —CN —CC—

19 Dr. Wolf's CHM 201 & Structural unitFrequency, cm -1 Stretching vibrations (carbonyl groups) Aldehydes and ketones Carboxylic acids Acid anhydrides and Esters Amides Infrared Absorption Frequencies C O

20 Dr. Wolf's CHM 201 & Structural unitFrequency, cm -1 Bending vibrations of alkenes Infrared Absorption Frequencies CH 2 RCH R2CR2CR2CR2C CHR' cis-RCH CHR' trans-RCH CHR' R2CR2CR2CR2C

21 Dr. Wolf's CHM 201 & Structural unitFrequency, cm -1 Bending vibrations of derivatives of benzene Monosubstituted and Ortho-disubstituted Meta-disubstituted and Para-disubstituted Infrared Absorption Frequencies

22 Dr. Wolf's CHM 201 & Wave number, cm -1 Figure 13.33: Infrared Spectrum of tert-butylbenzene H—C Ar—H Monsubstituted benzene C 6 H 5 C(CH 3 ) 3 Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.

23 Dr. Wolf's CHM 201 & Structural unitFrequency, cm -1 Stretching vibrations (single bonds) O—H (alcohols) O—H (carboxylic acids) N—H Infrared Absorption Frequencies

24 Dr. Wolf's CHM 201 & Wave number, cm -1 Figure 13.34: Infrared Spectrum of 2-Hexanol H—C O—H OH CH 3 CH 2 CH 2 CH 2 CHCH 3 Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.

25 Dr. Wolf's CHM 201 & Wave number, cm -1 Figure 13.35: Infrared Spectrum of 2-Hexanone H—C C=O O CH 3 CH 2 CH 2 CH 2 CCH 3 Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.

26 Dr. Wolf's CHM 201 & Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated  electron systems

27 Dr. Wolf's CHM 201 & gaps between electron energy levels are greater than those between vibrational levels gap corresponds to wavelengths between 200 and 800 nm Transitions between electron energy states  E = h  E = h

28 Dr. Wolf's CHM 201 & X-axis is wavelength in nm (high energy at left, low energy at right) max is the wavelength of maximum absorption and is related to electronic makeup of molecule— especially  electron system max is the wavelength of maximum absorption and is related to electronic makeup of molecule— especially  electron system Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity (  ) Conventions in UV-VIS

29 Dr. Wolf's CHM 201 & Wavelength, nm max 230 nm max 230 nm  max 2630 Molar absorptivity (  ) UV Spectrum of cis,trans-1,3-cyclooctadiene

30 Dr. Wolf's CHM 201 & Most stable  -electron configuration  -Electron configuration of excited state          * Transition in cis,trans-1,3-cyclooctadiene HOMO LUMO  E = h  E = h

31 Dr. Wolf's CHM 201 &  * Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bond as HOMO-LUMO energy difference decreases (smaller  E), max shifts to longer wavelengths

32 Dr. Wolf's CHM 201 & Methyl groups on double bond cause max to shift to longer wavelengths C C H H H H C C H H CH 3 max 170 nm max 170 nm CH 3 max 188 nm max 188 nm

33 Dr. Wolf's CHM 201 & Extending conjugation has a larger effect on max ; shift is again to longer wavelengths C C H H H H C C H H max 170 nm max 170 nm max 217 nm max 217 nm H C C H H H

34 Dr. Wolf's CHM 201 & max 217 nm (conjugated diene) max 217 nm (conjugated diene) H C CHH C C H H H C C H CH 3 H H C C H3CH3CH3CH3CH C C H H max 263 nm conjugated triene plus two methyl groups max 263 nm conjugated triene plus two methyl groups

35 Dr. Wolf's CHM 201 & LycopeneLycopene max 505 nm max 505 nm orange-red pigment in tomatoes

36 Dr. Wolf's CHM 201 & Mass Spectrometry

37 Dr. Wolf's CHM 201 & Atom or molecule is hit by high-energy electron Principles of Electron-Impact Mass Spectrometry e–e–e–e–

38 Dr. Wolf's CHM 201 & Atom or molecule is hit by high-energy electron electron is deflected but transfers much of its energy to the molecule e–e–e–e– Principles of Electron-Impact Mass Spectrometry

39 Dr. Wolf's CHM 201 & Atom or molecule is hit by high-energy electron electron is deflected but transfers much of its energy to the molecule e–e–e–e– Principles of Electron-Impact Mass Spectrometry

40 Dr. Wolf's CHM 201 & This energy-rich species ejects an electron. Principles of Electron-Impact Mass Spectrometry

41 Dr. Wolf's CHM 201 & This energy-rich species ejects an electron. Principles of Electron-Impact Mass Spectrometry forming a positively charged, odd-electron species called the molecular ion e–e–e–e– +

42 Dr. Wolf's CHM 201 & Molecular ion passes between poles of a magnet and is deflected by magnetic field amount of deflection depends on mass-to-charge ratio highest m/z deflected least lowest m/z deflected most Principles of Electron-Impact Mass Spectrometry +

43 Dr. Wolf's CHM 201 & Principles of Electron-Impact Mass Spectrometry If the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose. However, the molecular ion often fragments to a mixture of species of lower m/z.

44 Dr. Wolf's CHM 201 & The molecular ion dissociates to a cation and a radical. Principles of Electron-Impact Mass Spectrometry +

45 Dr. Wolf's CHM 201 & The molecular ion dissociates to a cation and a radical. Principles of Electron-Impact Mass Spectrometry + Usually several fragmentation pathways are available and a mixture of ions is produced.

46 Dr. Wolf's CHM 201 & mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each ion of different mass in mixture separation of peaks depends on relative mass Principles of Electron-Impact Mass Spectrometry

47 Dr. Wolf's CHM 201 & mixture of ions of different mass gives separate peak for each m/z intensity of peak proportional to percentage of each atom of different mass in mixture separation of peaks depends on relative mass Principles of Electron-Impact Mass Spectrometry

48 Dr. Wolf's CHM 201 & m/z m/z = Relative intensity Some molecules undergo very little fragmentation Benzene is an example. The major peak corresponds to the molecular ion.

49 Dr. Wolf's CHM 201 & HHH HH H HHH HH H HHH HH H all H are 1 H and all C are 12 C one C is 13 C one H is 2 H Isotopic Clusters %6.5%0.1%

50 Dr. Wolf's CHM 201 & m/z Relative intensity Isotopic Clusters in Chlorobenzene visible in peaks for molecular ion 35 Cl 37 Cl

51 Dr. Wolf's CHM 201 & m/z Relative intensity 77 Isotopic Clusters in Chlorobenzene no m/z 77, 79 pair; therefore ion responsible for m/z 77 peak does not contain Cl H H H H H

52 Dr. Wolf's CHM 201 & Alkanes undergo extensive fragmentation m/z Decane CH 3 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 3 Relative intensity

53 Dr. Wolf's CHM 201 & Propylbenzene fragments mostly at the benzylic position m/z Relative intensity CH 2 —CH 2 CH

54 Dr. Wolf's CHM 201 & Molecular Formula as a Clue to Structure

55 Dr. Wolf's CHM 201 & Molecular Weights One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula. However, we can gain some information even from the molecular weight. Mass spectrometry makes it relatively easy to determine molecular weights.

56 Dr. Wolf's CHM 201 & The Nitrogen Rule A molecule with an odd number of nitrogens has an odd molecular weight. A molecule that contains only C, H, and O or which has an even number of nitrogens has an even molecular weight. NH2NH2NH2NH NH2NH2NH2NH2 O2NO2NO2NO2N 183 NH2NH2NH2NH2 O2NO2NO2NO2N NO2NO2NO2NO2

57 Dr. Wolf's CHM 201 & Exact Molecular Weights CH 3 (CH 2 ) 5 CH 3 Heptane CH 3 CO O Cyclopropyl acetate Molecular formula Molecular weight C 7 H 16 C5H8O2C5H8O2C5H8O2C5H8O Exact mass Mass spectrometry can measure exact masses. Therefore, mass spectrometry can give molecular formulas.

58 Dr. Wolf's CHM 201 & Molecular Formulas Knowing that the molecular formula of a substance is C 7 H 16 tells us immediately that is an alkane because it corresponds to C n H 2n+2 C 7 H 14 lacks two hydrogens of an alkane, therefore contains either a ring or a double bond

59 Dr. Wolf's CHM 201 & Index of Hydrogen Deficiency relates molecular formulas to multiple bonds and rings index of hydrogen deficiency = 1 2 (molecular formula of alkane – molecular formula of compound)

60 Dr. Wolf's CHM 201 & Example 1 index of hydrogen deficiency C 7 H (molecular formula of alkane – molecular formula of compound) = 12 (C 7 H 16 – C 7 H 14 ) = 12 (2) = 1 = Therefore, one ring or one double bond.

61 Dr. Wolf's CHM 201 & Example 2 C 7 H (C 7 H 16 – C 7 H 12 ) = 1 2 (4) = 2 = Therefore, two rings, one triple bond, two double bonds, or one double bond + one ring.

62 Dr. Wolf's CHM 201 & Oxygen has no effect CH 3 (CH 2 ) 5 CH 2 OH (1-heptanol, C 7 H 16 O) has same number of H atoms as heptane index of hydrogen deficiency = 1 2 (C 7 H 16 – C 7 H 16 O) = 0 = 0 no rings or double bonds

63 Dr. Wolf's CHM 201 & Oxygen has no effect index of hydrogen deficiency = 1 2 (C 5 H 12 – C 5 H 8 O 2 ) = 2 = 2 one ring plus one double bond CH 3 CO O Cyclopropyl acetate

64 Dr. Wolf's CHM 201 & If halogen is present Treat a halogen as if it were hydrogen. C C CH 3 Cl H H C 3 H 5 Cl same index of hydrogen deficiency as for C 3 H 6

65 Dr. Wolf's CHM 201 & Rings versus Multiple Bonds Index of hydrogen deficiency tells us the sum of rings plus multiple bonds. Catalytic hydrogenation tells us how many multiple bonds there are.

66 Dr. Wolf's CHM 201 & Introduction to 1 H NMR Spectroscopy

67 Dr. Wolf's CHM 201 & H and 13 C both have spin = ±1/2 1 H is 99% at natural abundance 13 C is 1.1% at natural abundance The nuclei that are most useful to organic chemists are:

68 Dr. Wolf's CHM 201 & Nuclear Spin A spinning charge, such as the nucleus of 1 H or 13 C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2. + +

69 Dr. Wolf's CHM 201 & The distribution of nuclear spins is random in the absence of an external magnetic field.

70 Dr. Wolf's CHM 201 & An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. H0H0H0H0

71 Dr. Wolf's CHM 201 & There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. H0H0H0H0

72 Dr. Wolf's CHM 201 & no difference in absence of magnetic field proportional to strength of external magnetic field Energy Differences Between Nuclear Spin States + + EEEE  E ' increasing field strength

73 Dr. Wolf's CHM 201 & Some important relationships in NMR The frequency of absorbed electromagnetic radiation is proportional to the energy difference between two nuclear spin states which is proportional to the applied magnetic field UnitsHz kJ/mol (kcal/mol) tesla (T)

74 Dr. Wolf's CHM 201 & Some important relationships in NMR The frequency of absorbed electromagnetic radiation is different for different elements, and for different isotopes of the same element. For a field strength of 4.7 T: 1 H absorbs radiation having a frequency of 200 MHz (200 x 10 6 s -1 ) 13 C absorbs radiation having a frequency of 50.4 MHz (50.4 x 10 6 s -1 )

75 Dr. Wolf's CHM 201 & Some important relationships in NMR The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1 H) depends on its molecular environment. This is why NMR is such a useful tool for structure determination.

76 Dr. Wolf's CHM 201 & Nuclear Shielding and 1 H Chemical Shifts What do we mean by "shielding?" What do we mean by "chemical shift?"

77 Dr. Wolf's CHM 201 & ShieldingShielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. C H H 0H 0H 0H 0

78 Dr. Wolf's CHM 201 & ShieldingShielding The induced field shields the nuclei (in this case, C and H) from the applied field. A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. C H H 0H 0H 0H 0

79 Dr. Wolf's CHM 201 & Chemical Shift Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. C H H 0H 0H 0H 0

80 Dr. Wolf's CHM 201 & Chemical Shift Chemical shifts (  ) are measured relative to the protons in tetramethylsilane (TMS) as a standard. Si CH 3 H3CH3CH3CH3C  = position of signal - position of TMS peak spectrometer frequency x 10 6

81 Dr. Wolf's CHM 201 & Chemical shift ( , ppm) measured relative to TMS Upfield Increased shielding Downfield Decreased shielding (CH 3 ) 4 Si (TMS)

82 Dr. Wolf's CHM 201 & Chemical Shift Example: The signal for the proton in chloroform (HCCl 3 ) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz.  = position of signal - position of TMS peak spectrometer frequency x 10 6  = 1456 Hz - 0 Hz 200 x 10 6 Hx x 10 6  = 7.28

83 Dr. Wolf's CHM 201 & Chemical shift ( , ppm)  7.28 ppm H C Cl ClCl

84 Dr. Wolf's CHM 201 & Effects of Molecular Structure on 1 H Chemical Shifts protons in different environments experience different degrees of shielding and have different chemical shifts

85 Dr. Wolf's CHM 201 & Electronegative substituents decrease the shielding of methyl groups least shielded Hmost shielded H CH 3 FCH 3 OCH 3 (CH 3 ) 3 NCH 3 (CH 3 ) 4 Si  4.3  3.2  2.2  0.9  0.0

86 Dr. Wolf's CHM 201 & Electronegative substituents decrease shielding H 3 C—CH 2 —CH 3 O 2 N—CH 2 —CH 2 —CH 3  0.9  1.3  1.0  4.3  2.0

87 Dr. Wolf's CHM 201 & Effect is cumulative CHCl 3  7.3 CH 2 Cl 2  5.3 CH 3 Cl  3.1

88 Dr. Wolf's CHM 201 & Methyl, Methylene, and Methine CH 3 more shielded than CH 2 ; CH 2 more shielded than CH H3CH3CH3CH3C C CH3CH3CH3CH3 CH 3 H  0.9  1.6  0.8 H3CH3CH3CH3C C CH3CH3CH3CH3 CH 3 CH2CH2CH2CH2  0.9 CH 3  1.2

89 Dr. Wolf's CHM 201 & Protons attached to sp 2 hybridized carbon are less shielded than those attached to sp 3 hybridized carbon HH HH HH C CHHHH CH 3 CH 3  7.3  5.3  0.9

90 Dr. Wolf's CHM 201 & But protons attached to sp hybridized carbon are more shielded than those attached to sp 2 hybridized carbon C C HH HH  5.3  2.4 CH 2 OCH 3 C C H

91 Dr. Wolf's CHM 201 & Protons attached to benzylic and allylic carbons are somewhat less shielded than usual  1.5  0.8 H3CH3CH3CH3C CH 3  1.2 H3CH3CH3CH3C CH 2  2.6 H 3 C—CH 2 —CH 3  0.9  1.3

92 Dr. Wolf's CHM 201 & Proton attached to C=O of aldehyde is most deshielded C—H  2.4  9.7  1.1 CC O H H CH 3 H3CH3CH3CH3C

93 Dr. Wolf's CHM 201 & Type of proton Chemical shift (  ), ppm Type of proton Chemical shift (  ), ppm C HR C H CC C H CO C H NC C HAr C H CC

94 Dr. Wolf's CHM 201 & Type of proton Chemical shift (  ), ppm Type of proton Chemical shift (  ), ppm C HBr COH C HNR C HCl HAr C CH C H O

95 Dr. Wolf's CHM 201 & Type of proton Chemical shift (  ), ppm 1-3HNR0.5-5HOR6-8HOAr10-13 CO HOHOHOHO

96 Dr. Wolf's CHM 201 & Interpreting Proton NMR Spectra

97 Dr. Wolf's CHM 201 & number of signals 2. their intensity (as measured by area under peak) 3. splitting pattern (multiplicity) Information contained in an NMR spectrum includes:

98 Dr. Wolf's CHM 201 & Number of Signals protons that have different chemical shifts are chemically nonequivalent exist in different molecular environment

99 Dr. Wolf's CHM 201 & Chemical shift ( , ppm) CCH 2 OCH 3 N OCH 3 NCCH 2 O

100 Dr. Wolf's CHM 201 & are in identical environments have same chemical shift replacement test: replacement by some arbitrary "test group" generates same compound H 3 CCH 2 CH 3 chemically equivalent Chemically equivalent protons

101 Dr. Wolf's CHM 201 & H 3 CCH 2 CH 3 chemically equivalent CH 3 CH 2 CH 2 Cl ClCH 2 CH 2 CH 3 Chemically equivalent protons Replacing protons at C-1 and C-3 gives same compound (1-chloropropane) C-1 and C-3 protons are chemically equivalent and have the same chemical shift

102 Dr. Wolf's CHM 201 & replacement by some arbitrary test group generates diastereomers diastereotopic protons can have different chemical shifts Diastereotopic protons C CBr H3CH3CH3CH3C H H  5.3 ppm  5.5 ppm

103 Dr. Wolf's CHM 201 & not all peaks are singlets signals can be split by coupling of nuclear spins Spin-Spin Splitting in NMR Spectroscopy

104 Dr. Wolf's CHM 201 & Chemical shift ( , ppm) Cl 2 CHCH 3 Figure (page 536) 4 lines; quartet 2 lines; doublet CH3CH3CH3CH3 CHCHCHCH

105 Dr. Wolf's CHM 201 & Two-bond and three-bond coupling C C H H C C HH protons separated by two bonds (geminal relationship) protons separated by three bonds (vicinal relationship)

106 Dr. Wolf's CHM 201 & in order to observe splitting, protons cannot have same chemical shift coupling constant ( 2 J or 3 J) is independent of field strength Two-bond and three-bond coupling C C H H C C HH

107 Chemical shift ( , ppm) Cl 2 CHCH 3 Figure (page 536) 4 lines; quartet 2 lines; doublet CH3CH3CH3CH3 CHCHCHCH coupled protons are vicinal (three-bond coupling) CH splits CH 3 into a doublet, CH 3 splits CH into a quartet

108 Dr. Wolf's CHM 201 & Why do the methyl protons of 1,1-dichloroethane appear as a doublet? C C HH Cl Cl HH signal for methyl protons is split into a doublet To explain the splitting of the protons at C-2, we first focus on the two possible spin orientations of the proton at C-1

109 Dr. Wolf's CHM 201 & Why do the methyl protons of 1,1-dichloroethane appear as a doublet? C C HH Cl Cl HH signal for methyl protons is split into a doublet There are two orientations of the nuclear spin for the proton at C-1. One orientation shields the protons at C-2; the other deshields the C- 2 protons.

110 Dr. Wolf's CHM 201 & Why do the methyl protons of 1,1-dichloroethane appear as a doublet? C C HH Cl Cl HH signal for methyl protons is split into a doublet The protons at C-2 "feel" the effect of both the applied magnetic field and the local field resulting from the spin of the C-1 proton.

111 Dr. Wolf's CHM 201 & Why do the methyl protons of 1,1-dichloroethane appear as a doublet? C C HH Cl Cl HH "true" chemical shift of methyl protons (no coupling) this line corresponds to molecules in which the nuclear spin of the proton at C-1 reinforces the applied field this line corresponds to molecules in which the nuclear spin of the proton at C-1 opposes the applied field

112 Dr. Wolf's CHM 201 & Why does the methine proton of 1,1-dichloroethane appear as a quartet? C C HH Cl Cl HH signal for methine proton is split into a quartet The proton at C-1 "feels" the effect of the applied magnetic field and the local fields resulting from the spin states of the three methyl protons. The possible combinations are shown on the next slide.

113 Dr. Wolf's CHM 201 & C C HH Cl Cl HH There are eight combinations of nuclear spins for the three methyl protons. These 8 combinations split the signal into a 1:3:3:1 quartet. Why does the methine proton of 1,1-dichloroethane appear as a quartet?

114 Dr. Wolf's CHM 201 & For simple cases, the multiplicity of a signal for a particular proton is equal to the number of equivalent vicinal protons + 1. The splitting rule for 1 H NMR

115 Dr. Wolf's CHM 201 & Splitting Patterns: The Ethyl Group CH 3 CH 2 X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet)

116 Dr. Wolf's CHM 201 & Chemical shift ( , ppm) BrCH 2 CH 3 4 lines; quartet 3 lines; triplet CH3CH3CH3CH3 CH2CH2CH2CH2

117 Dr. Wolf's CHM 201 & Splitting Patterns of Common Multiplets Number of equivalentAppearanceIntensities of lines protons to which H of multipletin multiplet is coupled 1Doublet1:1 2Triplet1:2:1 3Quartet1:3:3:1 4Pentet1:4:6:4:1 5Sextet1:5:10:10:5:1 6Septet1:6:15:20:15:6:1 Table 13.2 (page 540)

118 Dr. Wolf's CHM 201 & Splitting Patterns: The Isopropyl Group (CH 3 ) 2 CHX is characterized by a doublet- septet pattern (septet at lower field than the doublet)

119 Dr. Wolf's CHM 201 & Chemical shift ( , ppm) BrCH(CH 3 ) 2 7 lines; septet 2 lines; doublet CH3CH3CH3CH3 CHCHCHCH

120 Dr. Wolf's CHM 201 & C NMR Spectroscopy

121 Dr. Wolf's CHM 201 & H and 13 C NMR compared: both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons) both give us information about the environment of the nuclei (hybridization state, attached atoms, etc.) it is convenient to use FT-NMR techniques for 1 H; it is standard practice for 13 C NMR

122 Dr. Wolf's CHM 201 & H and 13 C NMR compared: 13 C requires FT-NMR because the signal for a carbon atom is times weaker than the signal for a hydrogen atom a signal for a 13 C nucleus is only about 1% as intense as that for 1 H because of the magnetic properties of the nuclei, and at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13 C (most are 12 C)

123 Dr. Wolf's CHM 201 & H and 13 C NMR compared: 13 C signals are spread over a much wider range than 1 H signals making it easier to identify and count individual nuclei Figure (a) shows the 1 H NMR spectrum of 1-chloropentane; Figure (b) shows the 13 C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13 C spectrum than by its 1 H spectrum.

124 Dr. Wolf's CHM 201 & Chemical shift ( , ppm) ClCH 2 CH3CH3CH3CH3 ClCH 2 CH 2 CH 2 CH 2 CH 3 1H1H1H1H

125 Dr. Wolf's CHM 201 & Chemical shift ( , ppm) ClCH 2 CH 2 CH 2 CH 2 CH C CDCl 3 a separate, distinct peak appears for each of the 5 carbons

126 Dr. Wolf's CHM 201 & C Chemical Shifts are measured in ppm (  ) from the carbons of TMS

127 Dr. Wolf's CHM 201 & C Chemical shifts are most affected by: electronegativity of groups attached to carbon hybridization state of carbon

128 Dr. Wolf's CHM 201 & Electronegativity Effects Electronegativity has an even greater effect on 13 C chemical shifts than it does on 1 H chemical shifts.

129 Dr. Wolf's CHM 201 & Types of Carbons (CH 3 ) 3 CH CH4CH4CH4CH4 CH3CH3CH3CH3CH3CH3CH3CH3 CH 3 CH 2 CH 3 (CH 3 ) 4 C primarysecondary tertiary quaternary Classification Chemical shift,  1H1H1H1H 13 C Replacing H by C (more electronegative) deshields C to which it is attached.

130 Dr. Wolf's CHM 201 & Electronegativity effects on CH 3 CH3FCH3FCH3FCH3F CH4CH4CH4CH4 CH 3 NH 2 CH 3 OH Chemical shift,  1H1H1H1H C

131 Dr. Wolf's CHM 201 & Electronegativity effects and chain length Chemical shift,  Cl CH 2 CH Deshielding effect of Cl decreases as number of bonds between Cl and C increases.

132 Dr. Wolf's CHM 201 & C Chemical shifts are most affected by: electronegativity of groups attached to carbon hybridization state of carbon

133 Dr. Wolf's CHM 201 & Hybridization effects sp 3 hybridized carbon is more shielded than sp sp hybridized carbon is more shielded than sp 2, but less shielded than sp 3 CH 3 HCC CH

134 Dr. Wolf's CHM 201 & Carbonyl carbons are especially deshielded O CH 2 C O CH

135 Dr. Wolf's CHM 201 & Table 13.3 (p 549) Type of carbon Chemical shift (  ), ppm Type of carbon Chemical shift (  ), ppm RCH3RCH3RCH3RCH30-35 CR2CR2CR2CR2 R2CR2CR2CR2C65-90 CRCRCRCR RCRCRCRC R2CH2R2CH2R2CH2R2CH R3CHR3CHR3CHR3CH25-50 R4CR4CR4CR4C

136 Dr. Wolf's CHM 201 & Table 13.3 (p 549) Type of carbon Chemical shift (  ), ppm Type of carbon Chemical shift (  ), ppm RCH 2 Br RCH 2 Cl RCH 2 NH RCH 2 OH RCH 2 OR RCOR O RCRRCRRCRRCRO RCRCRCRCN

137 Dr. Wolf's CHM 201 & C NMR and Peak Intensities Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive.

138 Dr. Wolf's CHM 201 & CH 3 OH Figure (page 551) Chemical shift ( , ppm) carbons give 7 signals, but intensities are not equal

139 Dr. Wolf's CHM 201 & End of Chapter 13


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