Chapter 13 Chapter 13 - - Spectroscopy Spectroscopy YSU 400 MHz Nuclear Magnetic Resonance Spectrometers Techniques used to find structures of organic molecules Techniques used to find structures of organic molecules NMR spectroscopy: Based on the response of magnetic nuclei to an external magnetic field and an energy source (Radio frequency) IR spectroscopy: Response of bonds within organic molecules to externally applied Infra Red light UV/Vis spectroscopy: Response of electrons within bonds to externally applied UV or Visible light Mass spectrometry: Response of molecules to being bombarded with high energy particles such as electrons 13.1 The Electromagnetic spectrum Figure 13.1 13.1 The Electromagnetic spectrum 1
13.2 Two (quantized) energy states Figure 13.2 13.2 Two (quantized) energy states 13.2 Physics Concepts 13.2 Physics Concepts E = h ν i.e. Energy of the radiation is directly proportional to its frequency ( ν = Planck’s constant) ν = c/ λ i.e. Frequency of the radiation is inversely proportional to its wavelength (c = speed of light) E = hc/ λ i.e. Energy of the radiation is inversely proportional to its wavelength Take home : Longer wavelength, lower energy Higher frequency, higher energy 1 H NMR 13.3 Introduction to 1 13.3 Introduction to H NMR – – Nuclear Spin Nuclear Spin Nuclear spins of protons ( 1 H nucleus) Figure 13.3 2
Energy difference between states increases with field strength Energy difference between states increases with field strength (Fig. 13.4) (Fig. 13.4) Schematic diagram of a Schematic diagram of a n nuclear uclear m magnetic agnetic r resonance spectrometer esonance spectrometer Basic operation of a Fourier Transform (FT) NMR Instrument ( Fig. 13.5) Basic operation of a Fourier Transform (FT) NMR Instrument ( Fig. 13.5) 3
13.4 NMR Spectrum Characteristics – – Chemical Shift Chemical Shift 13.4 NMR Spectrum Characteristics Position of signal is the chemical shift downfield upfield downfield upfield 13.4 NMR Spectrum Characteristics – – Chemical Shift Chemical Shift 13.4 NMR Spectrum Characteristics Chemical shift ( δ ) = position of signal – position of TMS peak x 10 6 spectrometer frequency Enables us to use same scale for different size spectrometers (60 MHz, 400 MHz, 850 MHz, etc.) TMS = (CH 3 ) 4 Si, signal appears at 0 Hz on spectrum, therefore used as reference Chemical shifts are reported as ppm (parts per million) relative to TMS and usually occur in the 0-12 ppm range for 1 H spectra 1 H Chemical Shift 13.5 Effect of molecular structure on 13.5 Effect of molecular structure on 1 H Chemical Shift CH 3 F CH 3 OCH 3 (CH 3 ) 3 N CH 3 CH 3 4.3 3.2 2.2 0.9 i.e. electronegativity of other atoms plays a role in shift CH 3 CH 3 ~0.9 ppm 2 1 0 PPM 4
CH 3 N H 3 C CH 3 ~2.2 ppm 2 1 0 PPM H 3 C O CH 3 ~3.2 ppm 3 2 1 0 PPM CH 3 F ~4.3 ppm 5
13.5 Effect of structure on 1 1 H Chemical Shift H Chemical Shift 13.5 Effect of structure on H H H H H CH 3 CH 3 H H H H H 7.3 5.3 0.9 Pi electrons reinforce external field and signals show downfield CH 3 CH 3 ~0.9 ppm “R 3 C- H – alkyl” 2 1 0 PPM H H ~5.3 ppm “C=C- H alkene” H H 5 4 3 2 1 0 PPM H H H H H ~7.3 ppm “Ar- H benzene” H 7 6 5 4 3 2 1 0 PPM 1 H Chemical Shift 13.5 Effect of structure on 1 13.5 Effect of structure on H Chemical Shift CH 3 H 3 C CH 3 H 3 C CH 3 N CH 3 O 7 6 5 4 3 2 1 0 PPM Spectra typically have multiple signals the number depending on the number of unique types of protons 6
Table 13.1 – – Chemical Shift Values Chemical Shift Values Table 13.1 Table 13.1 – – Chemical Shift Values Chemical Shift Values Table 13.1 1 H NMR Spectra 13.5 Typical 13.5 Typical 1 H NMR Spectra 2 1 0 PPM Simple alkane protons – R 2 CH 2 From spectroscopy sheet – chemical shift ~ 0.9-1.8 ppm 7
13.5 Typical 1 1 H NMR Spectra H NMR Spectra 13.5 Typical H 3 C O CH 3 3 2 1 0 PPM Ether protons -O-C-H From spectroscopy sheet – chemical shift ~ 3.3-3.7 ppm 13.5 Typical 1 1 H NMR Spectra H NMR Spectra 13.5 Typical H 3 C O C O CH 3 H 2 5 4 3 2 1 0 PPM Two types of ether protons -O-C-H From spectroscopy sheet – chemical shift ~ 3.3-3.7 ppm CH 2 further downfield (two neighbouring O atoms) 1 H NMR Spectra 13.5 Typical 13.5 Typical 1 H NMR Spectra O H 10 8 6 4 2 0 PPM Aldehyde proton -CHO From spectroscopy sheet – chemical shift ~ 9-10 ppm 3 types of Ar-H proton – chemical shift ~ 6.5-8.5 ppm 8
13.5 Typical 1 1 H NMR Spectra H NMR Spectra 13.5 Typical O OH 10 8 6 4 2 0 PPM Carboxylic acid proton -CO 2 H From spectroscopy sheet – chemical shift ~ 10-13 ppm 3 types of Ar-H proton – chemical shift ~ 6.5-8.5 ppm 13.6 Integration – – Ratio of different types of H Ratio of different types of H 13.6 Integration O OH 5 1 10 8 6 4 2 0 PPM Lines on spectra are curves Areas underneath each curve give a reliable ratio of the different numbers of each type of proton 13.6 Integration 13.6 Integration – – Ratio of different types of H Ratio of different types of H CH 3 CH 2 OCH 3 3 3 2 3 2 1 0 PPM Areas are given as a ratio , not an absolute number 9
13.6 Integration – – Ratio of different types of H Ratio of different types of H 13.6 Integration OCH 2 CH 3 3 3 O CH 3 2 2 2 7 6 5 4 3 2 1 0 PPM 13.7 Spin- -Spin Splitting Spin Splitting – – Effect of Effect of neighbouring neighbouring H on shape H on shape 13.7 Spin H H H C C Cl H Cl 13.7 Spin- 13.7 Spin -Spin Splitting Spin Splitting – – Effect of Effect of neighbouring neighbouring H on shape H on shape H H H C C H H Br 10
13.7 Spin- -Spin Splitting Spin Splitting – – Effect of Effect of neighbouring neighbouring H on shape H on shape 13.7 Spin H H H H C C C H H Br H 13.7 Spin- -Spin Splitting Spin Splitting – – Effect of Effect of neighbouring neighbouring H on shape H on shape 13.7 Spin General rule for splitting patterns For simple cases, multiplicity for H = n + 1 Where n = number of neighbours i.e 1 neighbour, signal appears as a doublet 2 neighbours, signal appears as a triplet 3 neighbours, signal appears as a quartet 4 neigbours, signal appears as a quintet , etc. Complex splitting patterns are referred to as multiplets 13.7 13.7- -13.10 Basis of Splitting Patterns 13.10 Basis of Splitting Patterns H H Cl C C Br Cl Br H o For red H : neighbouring H (blue) has two possible alignments, either with, or against, the external field (Ho). This effects the local magnetic environment around the red H and thus there are two slightly different frequencies (and thus chemical shifts) at which the red H resonates. Same applies to the blue H. 11
13.7- -13.10 Basis of Splitting Patterns 13.10 Basis of Splitting Patterns 13.7 H H Cl C C H Cl Br H o Red H will be a triplet H H Cl C C H Cl Br H o Blue H’s will be a doublet 13.7- -13.10 Basis of Splitting Patterns 13.10 Basis of Splitting Patterns 13.7 H H Cl C C H Cl H H o Red H will be split into a quartet, blue H’s will be split into a doublet 13.7 13.7- -13.10 Basis of Splitting Patterns 13.10 Basis of Splitting Patterns - - Coupling Constants Gaps between lines (in Hz) will be the same for adjacent protons (here ~7.4 Hz). This is the coupling constant. 12
Using Coupling Constants CH 3 CH 2 but which one? H H H H H H H CH 3 CH 2 O H H O H O H H H H 7 6 5 4 3 2 1 0 PPM Find J and match signals Coupling Constants – Nonequivalent Neighbours If nonequivalent neighbours have same J value then n+1 applies for signal H H H H H Cl CH 3 CH 2 H CH 3 CH 2 H H H CH 3 CH 2 Cl CH 3 CH 2 CH 2 3 2 1 0 PPM 13.11 Complex Splitting Patterns 13.11 Complex Splitting Patterns When nonequivalent neighbours have different J values then n+1 does not apply for signal Figure 13.20 Generally for alkene protons: J trans > J cis 13
13.11 Complex Splitting Patterns 13.11 Complex Splitting Patterns H H OAc H H O AcO N 3 AcO H AcO H H 13.12 1 1 H NMR Spectra of Alcohols H NMR Spectra of Alcohols 13.12 Figure 13.21 Acidic protons exchange with any H 2 O in sample Glycosyl Glycosyl amide structure from NMR amide structure from NMR - - NOESY NOESY H H H N-H O N H O H H N-H YSU YSU 14
Glycosyl amide structure from NMR amide structure from NMR - - COSY COSY Glycosyl H-2 H 4 H-4 H-3 H 2 H-5 H N-H O N O H 5 H 1 H 3 H1, H2, H3, and H4 hard to distinguish just from coupling constants (all t, J ~9 Hz) David Temelkoff David Temelkoff YSU YSU 13 C NMR Spectroscopy 13.14 13.14 13 C NMR Spectroscopy Figure 13.23 13 C NMR Spectroscopy 13.14 13.14 13 C NMR Spectroscopy • Carbon 13 isotope and not 12 C is observed in NMR • 13 C very low abundance (<1%), integration not useful • Spectra usually “decoupled” and signals are singlets • Number of distinct signals indicates distinct carbons • Same ideas about shielding/deshielding apply • Spectra often measured in CDCl 3 and referenced to either the C in TMS (0 ppm) or the C in CDCl 3 , which shows as a triplet at 77.0 ppm 15
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