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Chapter 13 Spectroscopy Infrared spectroscopy Ultraviolet-visible spectroscopy Nuclear magnetic resonance spectroscopy Mass spectrometry Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13.1 Principles of Molecular Spectroscopy: Electromagnetic Radiation Electromagnetic Radiation Is propagated at the speed of light Has properties of particles and waves The energy of a photon is proportional to its frequency Figure 13.1: The Electromagnetic Spectrum Shorter Wavelength () 400 nm Longer Wavelength () 750 nm Visible Light Higher Frequency () Higher Energy (E) Lower Frequency () Lower Energy (E) Figure 13.1: The Electromagnetic Spectrum Shorter Wavelength () Ultraviolet Higher Frequency () Higher Energy (E) Longer Wavelength () Infrared Lower Frequency () Lower Energy (E) Figure 13.1: The Electromagnetic Spectrum Cosmic rays Rays X-rays Energy Ultraviolet light Visible light Infrared radiation Microwaves Radio waves 13.2 Principles of Molecular Spectroscopy: Quantized Energy States E = h Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states. What Kind of States? electronic UV-Vis vibrational infrared rotational microwave nuclear spin radiofrequency 13.3 Introduction to 1H NMR Spectroscopy The Nuclei that are Most Useful to Organic Chemists are: 1H and 13C both have spin = ±1/2 1H is 99% at natural abundance 13C is 1.1% at natural abundance Nuclear Spin + + A spinning charge, such as the nucleus of 1H or 13C, 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. The distribution of nuclear spins is random in the absence of an external magnetic field. + + + + + An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + + + B0 + + There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. + + + B0 + + Energy Differences Between Nuclear Spin States + E E ' + increasing field strength No difference in absence of magnetic field Proportional to strength of external magnetic field Some Important Relationships in NMR Units 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. Hz kJ/mol (kcal/mol) tesla (T) 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: 1H absorbs radiation having a frequency of 200 MHz (200 x 106 s-1) 13C absorbs radiation having a frequency of 50.4 MHz (50.4 x 106 s-1) 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. 13.4 Nuclear Shielding and 1H Chemical Shifts What do we mean by "shielding"? What do we mean by "chemical shift"? Shielding 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 B0 Shielding 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 B0 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 B0 Chemical Shift Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard. d = CH3 H3C Si CH3 position of signal - position of TMS peak spectrometer frequency CH3 x 106 Downfield Decreased shielding Upfield Increased shielding (CH3)4Si (TMS) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 Chemical shift (d, ppm) measured relative to TMS 1.0 0 Chemical Shift Example: The signal for the proton in chloroform (HCCl3) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz. d = d = position of signal - position of TMS peak spectrometer frequency 1456 Hz - 0 Hz 200 x 106 Hz d = 7.28 x 106 x 106 Cl d 7.28 ppm H C Cl Cl 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 Chemical shift (d, ppm) 2.0 1.0 0 13.5 Effects of Molecular Structure on 1H Chemical Shifts Protons in different environments experience different degrees of shielding and have different chemical shifts. Electronegative Substituents Decrease the Shielding of Methyl Groups least shielded H CH3F CH3OCH3 d 4.3 d 3.2 most shielded H (CH3)3N d 2.2 CH3CH3 (CH3)4Si d 0.9 d 0.0 Electronegative Substituents Decrease Shielding d 0.9 d 1.3 d 0.9 H3C—CH2—CH3 d 4.3 d 2.0 d 1.0 O2N—CH2—CH2—CH3 Effect is Cumulative CHCl3 CH2Cl2 CH3Cl d 7.3 d 5.3 d 3.1 Methyl, Methylene, and Methine CH3 more shielded than CH2 ; CH2 more shielded than CH d 0.9 CH3 H3C C CH3 H d 1.6 d 0.9 CH3 H3 C C CH3 d 1.2 CH2 d 0.8 CH3 Protons Attached to sp2 Hybridized Carbon are Less Shielded than those Attached to sp3 Hybridized Carbon H H H H H C H H CH3CH3 C H H H d 7.3 d 5.3 d 0.9 But Protons Attached to sp Hybridized Carbon are More Shielded than those Attached to sp2 Hybridized Carbon d 5.3 H H C H C H d 2.4 H C C CH2OCH3 Protons Attached to Benzylic and Allylic Carbons are Somewhat Less Shielded than Usual H3C CH3 d 0.8 d 1.5 d 0.9 d 1.3 d 0.9 H3C—CH2—CH3 d 1.2 H3C d 2.6 CH2 Proton Attached to C=O of Aldehyde is Most Deshielded C—H d 2.4 H H3C C O C CH3 d 1.1 H d 9.7 Table 13.1 Type of proton H H C C R C Chemical shift (d), Type of proton ppm 0.9-1.8 C C H C C H C Ar N 2.1-2.3 C 1.5-2.6 O H Chemical shift (d), ppm 2.0-2.5 2.3-2.8 Table 13.1 Type of proton H C NR Chemical shift (d), Type of proton ppm 2.2-2.9 H C H H C C Cl Br Chemical shift (d), ppm C 4.5-6.5 3.1-4.1 2.7-4.1 H Ar 6.5-8.5 O H C O 3.3-3.7 H C 9-10 Table 13.1 Type of proton Chemical shift (d), ppm H NR 1-3 H OR 0.5-5 H OAr 6-8 O HO C 10-13