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Edit Comment Close Premium member Presentation Transcript Nuclear Magnetic Resonance Spectroscopy : Nuclear Magnetic Resonance Spectroscopy Introduction : Chapter 13 2 Introduction NMR is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1H 13C 15N 19F 31P => Nuclear Spin : Chapter 13 3 Nuclear Spin A nucleus with an odd atomic number or an odd mass number has a nuclear spin. The spinning charged nucleus generates a magnetic field. External Magnetic Field : Chapter 13 4 External Magnetic Field When placed in an external field, spinning protons act like bar magnets. => Two Energy States : Chapter 13 5 Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. => E and Magnet Strength : Chapter 13 6 E and Magnet Strength Energy difference is proportional to the magnetic field strength. E = h = h B0 2 Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for H). In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. Low energy, radio frequency. => Magnetic Shielding : Chapter 13 7 Magnetic Shielding If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. But protons are surrounded by electrons that shield them from the external field. Circulating electrons create an induced magnetic field that opposes the external magnetic field. => Shielded Protons : Chapter 13 8 Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. Protons in a Molecule : Chapter 13 9 Protons in a Molecule Depending on their chemical environment, protons in a molecule are shielded by different amounts. NMR Signals : Chapter 13 10 NMR Signals The number of signals shows how many different kinds of protons are present. The location of the signals shows how shielded or deshielded the proton is. The intensity of the signal shows the number of protons of that type. Signal splitting shows the number of protons on adjacent atoms. => The NMR Spectrometer : Chapter 13 11 The NMR Spectrometer => The NMR Graph : Chapter 13 12 The NMR Graph => Tetramethylsilane : Chapter 13 13 Tetramethylsilane TMS is added to the sample. Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. Organic protons absorb downfield (to the left) of the TMS signal. => Chemical Shift : Chapter 13 14 Chemical Shift Measured in parts per million. Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz). Same value for 60, 100, or 300 MHz machine. Called the delta scale. => Delta Scale : Chapter 13 15 Delta Scale => Location of Signals : Chapter 13 16 Location of Signals More electronegative atoms deshield more and give larger shift values. Effect decreases with distance. Additional electronegative atoms cause increase in chemical shift. => Typical Values : Chapter 13 17 Typical Values => Aromatic Protons, 7-8 : Chapter 13 18 Aromatic Protons, 7-8 => Vinyl Protons, 5-6 : Chapter 13 19 Vinyl Protons, 5-6 => Acetylenic Protons, 2.5 : Chapter 13 20 Acetylenic Protons, 2.5 => Aldehyde Proton, 9-10 : Chapter 13 21 Aldehyde Proton, 9-10 => Electronegative oxygen atom Slide 22: Chapter 13 22 . Thank you O-H and N-H Signals : Chapter 13 23 O-H and N-H Signals Chemical shift depends on concentration. Hydrogen bonding in concentrated solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H. Proton exchanges between the molecules broaden the peak. => Carboxylic Acid Proton, 10+ : Chapter 13 24 Carboxylic Acid Proton, 10+ => Number of Signals : Chapter 13 25 Number of Signals Equivalent hydrogens have the same chemical shift. => Intensity of Signals : Chapter 13 26 Intensity of Signals The area under each peak is proportional to the number of protons. Shown by integral trace. How Many Hydrogens? : Chapter 13 27 How Many Hydrogens? When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens. Spin-Spin Splitting : Chapter 13 28 Spin-Spin Splitting Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field. This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed. All possibilities exist, so signal is split. => 1,1,2-Tribromoethane : Chapter 13 29 1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. => Doublet: 1 Adjacent Proton : Chapter 13 30 Doublet: 1 Adjacent Proton => Triplet: 2 Adjacent Protons : Chapter 13 31 Triplet: 2 Adjacent Protons => The N + 1 Rule : Chapter 13 32 The N + 1 Rule If a signal is split by N equivalent protons, it is split into N + 1 peaks. => Range of Magnetic Coupling : Chapter 13 33 Range of Magnetic Coupling Equivalent protons do not split each other. Protons bonded to the same carbon will split each other only if they are not equivalent. Protons on adjacent carbons normally will couple. Protons separated by four or more bonds will not couple. => Splitting for Ethyl Groups : Chapter 13 34 Splitting for Ethyl Groups => Splitting for Isopropyl Groups : Chapter 13 35 Splitting for Isopropyl Groups => Coupling Constants : Chapter 13 36 Coupling Constants Distance between the peaks of multiplet Measured in Hz Not dependent on strength of the external field Multiplets with the same coupling constants may come from adjacent groups of protons that split each other. => Values for Coupling Constants : Chapter 13 37 Values for Coupling Constants => Complex Splitting : Chapter 13 38 Complex Splitting Signals may be split by adjacent protons, different from each other, with different coupling constants. Example: Ha of styrene which is split by an adjacent H trans to it (J = 17 Hz) and an adjacent H cis to it (J = 11 Hz). => Splitting Tree : Chapter 13 39 Splitting Tree Spectrum for Styrene : Chapter 13 40 Spectrum for Styrene => Stereochemical Nonequivalence : Chapter 13 41 Stereochemical Nonequivalence Usually, two protons on the same C are equivalent and do not split each other. If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other. => Some Nonequivalent Protons : Chapter 13 42 Some Nonequivalent Protons Time Dependence : Chapter 13 43 Time Dependence Molecules are tumbling relative to the magnetic field, so NMR is an averaged spectrum of all the orientations. Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal. Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule. => Hydroxyl Proton : Chapter 13 44 Hydroxyl Proton Ultrapure samples of ethanol show splitting. Ethanol with a small amount of acidic or basic impurities will not show splitting. N-H Proton : Chapter 13 45 N-H Proton Moderate rate of exchange. Peak may be broad. Identifying the O-H or N-H Peak : Chapter 13 46 Identifying the O-H or N-H Peak Chemical shift will depend on concentration and solvent. To verify that a particular peak is due to O-H or N-H, shake the sample with D2O Deuterium will exchange with the O-H or N-H protons. On a second NMR spectrum the peak will be absent, or much less intense. => Carbon-13 : Chapter 13 47 Carbon-13 12C has no magnetic spin. 13C has a magnetic spin, but is only 1% of the carbon in a sample. The gyromagnetic ratio of 13C is one-fourth of that of 1H. Signals are weak, getting lost in noise. Hundreds of spectra are taken, averaged. => Fourier Transform NMR : Chapter 13 48 Fourier Transform NMR Nuclei in a magnetic field are given a radio-frequency pulse close to their resonance frequency. The nuclei absorb energy and precess (spin) like little tops. A complex signal is produced, then decays as the nuclei lose energy. Free induction decay is converted to spectrum. => Hydrogen and Carbon Chemical Shifts : Chapter 13 49 Hydrogen and Carbon Chemical Shifts Combined 13C and 1H Spectra : Chapter 13 50 Combined 13C and 1H Spectra => Differences in 13C Technique : Chapter 13 51 Differences in 13C Technique Resonance frequency is ~ one-fourth, 15.1 MHz instead of 60 MHz. Peak areas are not proportional to number of carbons. Carbon atoms with more hydrogens absorb more strongly. => Spin-Spin Splitting : Chapter 13 52 Spin-Spin Splitting It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible. 13C will magnetically couple with attached protons and adjacent protons. These complex splitting patterns are difficult to interpret. => Proton Spin Decoupling : Chapter 13 53 Proton Spin Decoupling To simplify the spectrum, protons are continuously irradiated with “noise,” so they are rapidly flipping. The carbon nuclei see an average of all the possible proton spin states. Thus, each different kind of carbon gives a single, unsplit peak. => Off-Resonance Decoupling : Chapter 13 54 Off-Resonance Decoupling 13C nuclei are split only by the protons attached directly to them. The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks. => Interpreting 13C NMR : Chapter 13 55 Interpreting 13C NMR The number of different signals indicates the number of different kinds of carbon. The location (chemical shift) indicates the type of functional group. The peak area indicates the numbers of carbons (if integrated). The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. => Two 13C NMR Spectra : Chapter 13 56 Two 13C NMR Spectra => MRI : Chapter 13 57 MRI Magnetic resonance imaging, noninvasive “Nuclear” is omitted because of public’s fear that it would be radioactive. Only protons in one plane can be in resonance at one time. Computer puts together “slices” to get 3D. Tumors readily detected. => Slide 58: Chapter 13 58 You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.
NMR IMP nanlu Download Post to : URL : Related Presentations : Share Add to Flag Embed Email Send to Blogs and Networks Add to Channel Uploaded from authorPOINT lite Insert YouTube videos in PowerPont slides with aS Desktop Copy embed code: (To copy code, click on the text box) Embed: URL: Thumbnail: WordPress Embed Customize Embed The presentation is successfully added In Your Favorites. Views: 354 Category: Education License: All Rights Reserved Like it (1) Dislike it (0) Added: October 28, 2010 This Presentation is Public Favorites: 0 Presentation Description No description available. Comments Posting comment... By: vpgodbole (17 month(s) ago) Awesome..Thanx a lot.. It really helped me...awesome...awesome...awesome!!! Saving..... Post Reply Close Saving..... Edit Comment Close Premium member Presentation Transcript Nuclear Magnetic Resonance Spectroscopy : Nuclear Magnetic Resonance Spectroscopy Introduction : Chapter 13 2 Introduction NMR is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1H 13C 15N 19F 31P => Nuclear Spin : Chapter 13 3 Nuclear Spin A nucleus with an odd atomic number or an odd mass number has a nuclear spin. The spinning charged nucleus generates a magnetic field. External Magnetic Field : Chapter 13 4 External Magnetic Field When placed in an external field, spinning protons act like bar magnets. => Two Energy States : Chapter 13 5 Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. => E and Magnet Strength : Chapter 13 6 E and Magnet Strength Energy difference is proportional to the magnetic field strength. E = h = h B0 2 Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for H). In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. Low energy, radio frequency. => Magnetic Shielding : Chapter 13 7 Magnetic Shielding If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. But protons are surrounded by electrons that shield them from the external field. Circulating electrons create an induced magnetic field that opposes the external magnetic field. => Shielded Protons : Chapter 13 8 Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. Protons in a Molecule : Chapter 13 9 Protons in a Molecule Depending on their chemical environment, protons in a molecule are shielded by different amounts. NMR Signals : Chapter 13 10 NMR Signals The number of signals shows how many different kinds of protons are present. The location of the signals shows how shielded or deshielded the proton is. The intensity of the signal shows the number of protons of that type. Signal splitting shows the number of protons on adjacent atoms. => The NMR Spectrometer : Chapter 13 11 The NMR Spectrometer => The NMR Graph : Chapter 13 12 The NMR Graph => Tetramethylsilane : Chapter 13 13 Tetramethylsilane TMS is added to the sample. Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. Organic protons absorb downfield (to the left) of the TMS signal. => Chemical Shift : Chapter 13 14 Chemical Shift Measured in parts per million. Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz). Same value for 60, 100, or 300 MHz machine. Called the delta scale. => Delta Scale : Chapter 13 15 Delta Scale => Location of Signals : Chapter 13 16 Location of Signals More electronegative atoms deshield more and give larger shift values. Effect decreases with distance. Additional electronegative atoms cause increase in chemical shift. => Typical Values : Chapter 13 17 Typical Values => Aromatic Protons, 7-8 : Chapter 13 18 Aromatic Protons, 7-8 => Vinyl Protons, 5-6 : Chapter 13 19 Vinyl Protons, 5-6 => Acetylenic Protons, 2.5 : Chapter 13 20 Acetylenic Protons, 2.5 => Aldehyde Proton, 9-10 : Chapter 13 21 Aldehyde Proton, 9-10 => Electronegative oxygen atom Slide 22: Chapter 13 22 . Thank you O-H and N-H Signals : Chapter 13 23 O-H and N-H Signals Chemical shift depends on concentration. Hydrogen bonding in concentrated solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H. Proton exchanges between the molecules broaden the peak. => Carboxylic Acid Proton, 10+ : Chapter 13 24 Carboxylic Acid Proton, 10+ => Number of Signals : Chapter 13 25 Number of Signals Equivalent hydrogens have the same chemical shift. => Intensity of Signals : Chapter 13 26 Intensity of Signals The area under each peak is proportional to the number of protons. Shown by integral trace. How Many Hydrogens? : Chapter 13 27 How Many Hydrogens? When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens. Spin-Spin Splitting : Chapter 13 28 Spin-Spin Splitting Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field. This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed. All possibilities exist, so signal is split. => 1,1,2-Tribromoethane : Chapter 13 29 1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. => Doublet: 1 Adjacent Proton : Chapter 13 30 Doublet: 1 Adjacent Proton => Triplet: 2 Adjacent Protons : Chapter 13 31 Triplet: 2 Adjacent Protons => The N + 1 Rule : Chapter 13 32 The N + 1 Rule If a signal is split by N equivalent protons, it is split into N + 1 peaks. => Range of Magnetic Coupling : Chapter 13 33 Range of Magnetic Coupling Equivalent protons do not split each other. Protons bonded to the same carbon will split each other only if they are not equivalent. Protons on adjacent carbons normally will couple. Protons separated by four or more bonds will not couple. => Splitting for Ethyl Groups : Chapter 13 34 Splitting for Ethyl Groups => Splitting for Isopropyl Groups : Chapter 13 35 Splitting for Isopropyl Groups => Coupling Constants : Chapter 13 36 Coupling Constants Distance between the peaks of multiplet Measured in Hz Not dependent on strength of the external field Multiplets with the same coupling constants may come from adjacent groups of protons that split each other. => Values for Coupling Constants : Chapter 13 37 Values for Coupling Constants => Complex Splitting : Chapter 13 38 Complex Splitting Signals may be split by adjacent protons, different from each other, with different coupling constants. Example: Ha of styrene which is split by an adjacent H trans to it (J = 17 Hz) and an adjacent H cis to it (J = 11 Hz). => Splitting Tree : Chapter 13 39 Splitting Tree Spectrum for Styrene : Chapter 13 40 Spectrum for Styrene => Stereochemical Nonequivalence : Chapter 13 41 Stereochemical Nonequivalence Usually, two protons on the same C are equivalent and do not split each other. If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other. => Some Nonequivalent Protons : Chapter 13 42 Some Nonequivalent Protons Time Dependence : Chapter 13 43 Time Dependence Molecules are tumbling relative to the magnetic field, so NMR is an averaged spectrum of all the orientations. Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal. Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule. => Hydroxyl Proton : Chapter 13 44 Hydroxyl Proton Ultrapure samples of ethanol show splitting. Ethanol with a small amount of acidic or basic impurities will not show splitting. N-H Proton : Chapter 13 45 N-H Proton Moderate rate of exchange. Peak may be broad. Identifying the O-H or N-H Peak : Chapter 13 46 Identifying the O-H or N-H Peak Chemical shift will depend on concentration and solvent. To verify that a particular peak is due to O-H or N-H, shake the sample with D2O Deuterium will exchange with the O-H or N-H protons. On a second NMR spectrum the peak will be absent, or much less intense. => Carbon-13 : Chapter 13 47 Carbon-13 12C has no magnetic spin. 13C has a magnetic spin, but is only 1% of the carbon in a sample. The gyromagnetic ratio of 13C is one-fourth of that of 1H. Signals are weak, getting lost in noise. Hundreds of spectra are taken, averaged. => Fourier Transform NMR : Chapter 13 48 Fourier Transform NMR Nuclei in a magnetic field are given a radio-frequency pulse close to their resonance frequency. The nuclei absorb energy and precess (spin) like little tops. A complex signal is produced, then decays as the nuclei lose energy. Free induction decay is converted to spectrum. => Hydrogen and Carbon Chemical Shifts : Chapter 13 49 Hydrogen and Carbon Chemical Shifts Combined 13C and 1H Spectra : Chapter 13 50 Combined 13C and 1H Spectra => Differences in 13C Technique : Chapter 13 51 Differences in 13C Technique Resonance frequency is ~ one-fourth, 15.1 MHz instead of 60 MHz. Peak areas are not proportional to number of carbons. Carbon atoms with more hydrogens absorb more strongly. => Spin-Spin Splitting : Chapter 13 52 Spin-Spin Splitting It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible. 13C will magnetically couple with attached protons and adjacent protons. These complex splitting patterns are difficult to interpret. => Proton Spin Decoupling : Chapter 13 53 Proton Spin Decoupling To simplify the spectrum, protons are continuously irradiated with “noise,” so they are rapidly flipping. The carbon nuclei see an average of all the possible proton spin states. Thus, each different kind of carbon gives a single, unsplit peak. => Off-Resonance Decoupling : Chapter 13 54 Off-Resonance Decoupling 13C nuclei are split only by the protons attached directly to them. The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks. => Interpreting 13C NMR : Chapter 13 55 Interpreting 13C NMR The number of different signals indicates the number of different kinds of carbon. The location (chemical shift) indicates the type of functional group. The peak area indicates the numbers of carbons (if integrated). The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. => Two 13C NMR Spectra : Chapter 13 56 Two 13C NMR Spectra => MRI : Chapter 13 57 MRI Magnetic resonance imaging, noninvasive “Nuclear” is omitted because of public’s fear that it would be radioactive. Only protons in one plane can be in resonance at one time. Computer puts together “slices” to get 3D. Tumors readily detected. => Slide 58: Chapter 13 58