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MC 13.1 Spectroscopy, Pt I 1 Spectrocopy Nuclear Magnetic Resonance (NMR)spectroscopy Infrared (IR) Spectroscopy Ultraviolet-Visible (UV-VIS) Spectroscopy Mass Spectrometry (MS) continue…..
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MC 13.1 Spectroscopy, Pt I 2 Principles of Molecular Spectroscopy: Electromagnetic Radiation Electromagnetic Radiation travels at the speed of light Has both particle and wave properties A single packet of energy is a photon The energy of a photon is proportional to its frequency continue…..
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MC 13.1 Spectroscopy, Pt I 3 The Visible Electromagnetic Spectrum 400 nm750 nm Visible Light Longer Wavelength ( )Shorter Wavelength ( ) Higher Frequency ( )Lower Frequency ( ) Higher Energy (E)Lower Energy (E) continue….
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MC 13.1 Spectroscopy, Pt I 4 The Electromagnetic Spectrum (cont) UltravioletInfrared Longer Wavelength ( )Shorter Wavelength ( ) Higher Frequency ( )Lower Frequency ( ) Higher Energy (E)Lower Energy (E) continue…..
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MC 13.1 Spectroscopy, Pt I 5 Cosmic rays Rays X-rays Ultraviolet light Visible light Infrared radiation Microwaves Radio waves Energy The Electromagnetic Spectrum (cont) continue…..
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MC 13.1 Spectroscopy, Pt I 6 Quantized Energy States Electromagnetic radiation is absorbed when the energy of the photon corresponds to difference in energy between two states E = h State #1 State #2 continue…..
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MC 13.1 Spectroscopy, Pt I 7 Quantized Energy States (cont) What Kind of States are of Interest? Electronic Nuclear spin Vibrational Rotational UV-Vis Radiofrequency Infrared Microwave StateSpectral RegionType of Spectroscopy Ultraviolet-Visible Nuclear Magnetic Resonance Infrared Raman continue…..
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MC 13.1 Spectroscopy, Pt I 8 Introduction to 1 H NMR Spectroscopy The nuclei that are most useful to organic chemists are: 1 H and 13 C Both have spin = + or - 1/2 1 H is 99% natural abundance 13 C is 1.1% natural abundance continue…..
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MC 13.1 Spectroscopy, Pt I 9 continue….. Introduction to 1 H NMR Spectroscopy (cont) 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 + +
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10 The distribution of nuclear spins is random in the absence of an external magnetic field + + + + + continue…..
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11 An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field + + + + + H0H0 continue…..
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MC 13.1 Spectroscopy, Pt I 12 There is a slight excess of nuclear magnetic moments aligned parallel to the applied field + + + + + H0H0 continue…..
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13 Introduction to 1 H NMR Spectroscopy (cont) Energy Differences Between Nuclear Spin States: There is no difference in the absence of a magnetic field The difference is proportional to the strength of the external magnetic field + + EE E ' Increasing Field Strength No Magnetic Field continue…..
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MC 13.1 Spectroscopy, Pt I 14 Introduction to 1 H NMR Spectroscopy (cont) Some important relationships in NMR: The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1H) depends on its molecular environment This is why NMR is such a useful tool for structure determination continue…..
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MC 13.1 Spectroscopy, Pt I 15 Introduction to 1 H NMR Spectroscopy (cont) Nuclear Shielding and 1 H Chemical Shifts: What do we mean by "shielding?“ What do we mean by "chemical shift?" 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 0 continue…..
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MC 13.1 Spectroscopy, Pt I 16 Introduction to 1 H NMR Spectroscopy (cont) Chemical Shift: What do we mean by "shielding?“ What do we mean by "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 0 continue…..
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MC 13.1 Spectroscopy, Pt I 17 Introduction to 1 H NMR Spectroscopy (cont) Chemical Shift (cont): Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard Si CH 3 H3CH3C = Position of Signal - Position of TMS Signal Spectrometer Frequency x 10 6 Calculating Chemical Shift Numbers: continue…..
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MC 13.1 Spectroscopy, Pt I 18 Introduction to 1 H NMR Spectroscopy (cont) The Proton NMR Chemical Shift Scale: 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) - measured relative to TMS Downfield Decreased shielding Upfield Increased shielding (CH 3 ) 4 Si (TMS) continue…..
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19 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) (CH 3 ) 4 Si (TMS) Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts: Protons in different environments experience different degrees of shielding and have different chemical shifts 7.28 ppm H C Cl continue…..
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MC 13.1 Spectroscopy, Pt I 20 Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts (cont): Protons in different environments experience different degrees of shielding and have different chemical shifts Electronegative substituents decrease the shielding of hydrogen atoms on methyl groups Decreased ShieldingIncreased shielding 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 continue…..
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MC 13.1 Spectroscopy, Pt I 21 Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts (cont): Electronegative substituents decrease shielding H 3 C — CH 2 — CH 3 0.9 1.3 O 2 N — CH 2 — CH 2 — CH 3 1.0 4.3 2.0 continue…..
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MC 13.1 Spectroscopy, Pt I 22 Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts (cont): Electronegative substituents decrease shielding The electronegativity effect is cumulative continue….. CHCl 3 7.3 CH 2 Cl 2 5.3 CH 3 Cl 3.1
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MC 13.1 Spectroscopy, Pt I 23 Methyl ( CH 3 ), Methylene ( CH 2 ), and Methine ( CH ), : Introduction to 1 H NMR Spectroscopy (cont) Effects of Molecular Structure on 1 H Chemical Shifts (cont): CH 3 more shielded than CH 2 CH 2 more shielded than CH H3CH3C C CH 3 H 0.9 1.6 0.8 0.9 H3CH3C C CH 3 CH 2 CH 3 1.2 continue…..
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MC 13.1 Spectroscopy, Pt I 24 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) (TMS) Example: continue….. Interpreting Proton NMR Spectra (cont) 4 lines; quartet 2 lines; doublet Cl 2 CH CH 3
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25 Interpreting Proton NMR Spectra (cont) CH 3 CH 2 X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet) 01.02.03.04.05.06.07.08.09.010.0 Chemical shift ( , ppm) (TMS) 3 lines; triplet CH 3 BrCH 2 CH 3 4 lines; quartet CH 2 Splitting Patterns: The Ethyl Group BrCH 2 CH 3 continue…..
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