MC 13.1 Spectroscopy, Pt I 1 Spectrocopy  Nuclear Magnetic Resonance (NMR)spectroscopy  Infrared (IR) Spectroscopy  Ultraviolet-Visible (UV-VIS) Spectroscopy.

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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…..

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…..

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….

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…..

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…..

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…..

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…..

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…..

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 + +

10 The distribution of nuclear spins is random in the absence of an external magnetic field continue…..

11 An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field H0H0 continue…..

MC 13.1 Spectroscopy, Pt I 12 There is a slight excess of nuclear magnetic moments aligned parallel to the applied field H0H0 continue…..

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 + + EE  E ' Increasing Field Strength No Magnetic Field continue…..

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…..

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…..

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…..

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…..

MC 13.1 Spectroscopy, Pt I 18 Introduction to 1 H NMR Spectroscopy (cont) The Proton NMR Chemical Shift Scale: Chemical shift ( , ppm) - measured relative to TMS Downfield Decreased shielding Upfield Increased shielding (CH 3 ) 4 Si (TMS) continue…..

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…..

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…..

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…..

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

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…..

MC 13.1 Spectroscopy, Pt I Chemical shift ( , ppm) (TMS) Example: continue….. Interpreting Proton NMR Spectra (cont) 4 lines; quartet 2 lines; doublet Cl 2 CH CH 3

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) 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…..