《有机化学》课程教学资源（教材文献，英文版）CHAPTER 13 SPECTROSCOPY.pdf_大学文库

As diverse as these techniques are, all of them are based on the absorption of energy by a molecule, and all measure how a molecule responds to that absorption. In describing these techniques our emphasis will be on their application to structure determination. We’ll start with a brief discussion of electromagnetic radiation, which is the source of the energy that a molecule absorbs in NMR, IR, and UV-VIS spectroscopy. 13.1 PRINCIPLES OF MOLECULAR SPECTROSCOPY: ELECTROMAGNETIC RADIATION Electromagnetic radiation, of which visible light is but one example, has the properties of both particles and waves. The particles are called photons, and each possesses an amount of energy referred to as a quantum. In 1900, the German physicist Max Planck proposed that the energy of a photon (E) is directly proportional to its frequency (). E hv The SI units of frequency are reciprocal seconds (s1 ), given the name hertz and the symbol Hz in honor of the nineteenth-century physicist Heinrich R. Hertz. The constant of proportionality h is called Planck’s constant and has the value h 6.63 1034 J s Electromagnetic radiation travels at the speed of light (c 3.0 108 m/s), which is equal to the product of its frequency and its wavelength : c v The range of photon energies is called the electromagnetic spectrum and is shown in Figure 13.1. Visible light occupies a very small region of the electromagnetic spectrum. It is characterized by wavelengths of 4 107 m (violet) to 8 107 m (red). 488 CHAPTER THIRTEEN Spectroscopy 100 Infrared Ultraviolet 10–2 102 104 106 108 1010 1012 1020 1018 108 106 104 Frequency (s–1) Wavelength (nm) 400 500 600 750 nm X-ray Microwave Radio frequency Gamma ray Ultraviolet 1016 Visible Infrared Visible region 1012 1014 1010 Highest energy Lowest energy 700 “Modern” physics dates from Planck’s proposal that energy is quantized, which set the stage for the development of quantum mechanics. Planck received the 1918 Nobel Prize in physics. FIGURE 13.1 The electromagnetic spectrum. (From M. Silberberg, Chemistry, 2d edition, WCB/McGraw-Hill, 2000, p. 260.) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

13.2 Principles of Molecular Spectroscopy: Quantized Energy State When examining Figure 13. 1 be sure to keep the following two relationships in mind 1. Frequency is inversely proportional to wavelength; the greater the frequency, the shorter the wavelength. 2. Energy is directly proportional to frequency; electromagnetic radiation of higher frequency possesses more energy than radiation of lower frequency Depending on its source, a photon can have a vast amount of energy; gamma rays and X-rays are streams of very high energy photons. Radio waves are of relatively lov energy. Ultraviolet radiation is of higher energy than the violet end of visible light Infrared radiation is of lower energy than the red end of visible light. When a molecule is exposed to electromagnetic radiation, it may absorb a photon, increasing its energy by an amount equal to the energy of the photon Molecules are highly selective with respect to the frequencies that they absorb. Only photons of certain specific frequencies are absorbed by a molecule. The particular photon energies absorbed by a molecule depend on molecular structure and can be measured with instruments called spectrometers. The data obtained are very sensitive indicators of molecular structure and have revolution ized the practice of chemical analysis. 13.2 PRINCIPLES OF MOLECULAR SPECTROSCOPY: QUANTIZED ENERGY STATES What determines whether or not a photon is absorbed by a molecule? The most impor tant requirement is that the energy of the photon must equal the energy difference between two states, such as two nuclear spin states, two vibrational states, or two elec- tronic states. In physics, the term for this is resonance--the transfer of energy between two objects that occurs when their frequencies are matched. In molecular spectroscopy. we are concerned with the transfer of energy from a photon to a molecule, but the idea is the same. Consider, for example, two energy states of a molecule designated El and E2 in Figure 13. 2. The energy difference between them is E2-El, or AE In nuclear magnetic resonance(NMR) spectroscopy these are two different spin states of an atomic nucleus; in infrared (IR)spectroscopy, they are two different vibrational energy states in ultraviolet-visible (UV-VIS) spectroscopy, they are two different electronic energy states. Unlike kinetic energy, which is continuous, meaning that all values of kinetic energy are available to a molecule, only certain energies are possible for electronic, vibra- tional, and nuclear spin states. These energy states are said to be quantized. More of the molecules exist in the lower energy state E than in the higher energy state E2. Exci tation of a molecule from a lower state to a higher one requires the addition of an incre- ment of energy equal to AE. Thus, when electromagnetic radiation is incident upon a molecule, only the frequency whose corresponding energy equals AE is absorbed. All other frequencies are transmitted Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment △E=E,-E1=hy containing the sample through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compared with that at the source. When the frequency is reached at which the sample absorbs radiation, the El detector senses a decrease in intensity. The relation between frequency and absorption is plotted on a strip chart and is called a spectrum. A spectrum consists of a series of peaks FIGURE 13.2 Two energy at particular frequencies; its interpretation can provide structural information. Each type states of a molecul of spectroscopy developed independently of the others, and so the format followed in tion of energy presenting the data is different for each one. An NMR spectrum looks different from an from its lower ener egplecul IR spectrum, and both look different from a UV-VIs spectrum to the next higher state Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

When examining Figure 13.1 be sure to keep the following two relationships in mind: 1. Frequency is inversely proportional to wavelength; the greater the frequency, the shorter the wavelength. 2. Energy is directly proportional to frequency; electromagnetic radiation of higher frequency possesses more energy than radiation of lower frequency. Depending on its source, a photon can have a vast amount of energy; gamma rays and X-rays are streams of very high energy photons. Radio waves are of relatively low energy. Ultraviolet radiation is of higher energy than the violet end of visible light. Infrared radiation is of lower energy than the red end of visible light. When a molecule is exposed to electromagnetic radiation, it may absorb a photon, increasing its energy by an amount equal to the energy of the photon. Molecules are highly selective with respect to the frequencies that they absorb. Only photons of certain specific frequencies are absorbed by a molecule. The particular photon energies absorbed by a molecule depend on molecular structure and can be measured with instruments called spectrometers. The data obtained are very sensitive indicators of molecular structure and have revolutionized the practice of chemical analysis. 13.2 PRINCIPLES OF MOLECULAR SPECTROSCOPY: QUANTIZED ENERGY STATES What determines whether or not a photon is absorbed by a molecule? The most important requirement is that the energy of the photon must equal the energy difference between two states, such as two nuclear spin states, two vibrational states, or two electronic states. In physics, the term for this is resonance—the transfer of energy between two objects that occurs when their frequencies are matched. In molecular spectroscopy, we are concerned with the transfer of energy from a photon to a molecule, but the idea is the same. Consider, for example, two energy states of a molecule designated E1 and E2 in Figure 13.2. The energy difference between them is E2 E1, or E. In nuclear magnetic resonance (NMR) spectroscopy these are two different spin states of an atomic nucleus; in infrared (IR) spectroscopy, they are two different vibrational energy states; in ultraviolet-visible (UV-VIS) spectroscopy, they are two different electronic energy states. Unlike kinetic energy, which is continuous, meaning that all values of kinetic energy are available to a molecule, only certain energies are possible for electronic, vibrational, and nuclear spin states. These energy states are said to be quantized. More of the molecules exist in the lower energy state E1 than in the higher energy state E2. Excitation of a molecule from a lower state to a higher one requires the addition of an increment of energy equal to E. Thus, when electromagnetic radiation is incident upon a molecule, only the frequency whose corresponding energy equals E is absorbed. All other frequencies are transmitted. Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment containing the sample through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compared with that at the source. When the frequency is reached at which the sample absorbs radiation, the detector senses a decrease in intensity. The relation between frequency and absorption is plotted on a strip chart and is called a spectrum. A spectrum consists of a series of peaks at particular frequencies; its interpretation can provide structural information. Each type of spectroscopy developed independently of the others, and so the format followed in presenting the data is different for each one. An NMR spectrum looks different from an IR spectrum, and both look different from a UV-VIS spectrum. 13.2 Principles of Molecular Spectroscopy: Quantized Energy States 489 E2 E1 E E2 E1 h FIGURE 13.2 Two energy states of a molecule. Absorption of energy equal to E2 E1 excites a molecule from its lower energy state to the next higher state. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER THIRTEEN Spectroscopy with this as background, we will now discuss spectroscopic techniques individu ally. NMR, IR, and UV-VIS spectroscopy provide complementary information, and all are useful. Among them, NMR provides the information that is most directly related to molecular structure and is the one we shall examine first 13.3 INTRODUCTION TO H NMR SPECTROSCOPY Nuclear magnetic resonance spectroscopy depends on the absorption of energy when the nucleus of an atom is excited from its lowest energy spin state to the next higher one. We should first point out that many elements are difficult to study by NMr, and some can't be studied at all. Fortunately though, the two elements that are the most common in organic molecules(carbon and hydrogen) have isotopes(H andC) capable of giv g NMR spectra that are rich in structural information. A proton nuclear magnetic res f protons was first detected nance(H NMR) spectrum tells us about the environments of the various hydrogens in a molecule; a carbon-13 nuclear magnetic resonance (C NMR)spectrum does the same (Stanford). Purcell and Bloch for the carbon atoms. Separately and together H andC NMR take us a long way shared the 1952 Nobel Prize toward determining a substance's molecular structure. We'll develop most of the general principles of NMr by discussing H NMR, then extend them toC NMR. The C NMR discussion is shorter, not because it is less important than H NMR, but because many of the same principles apply to both techniques. Like an electron, a proton has two spin states with quantum numbers of + and -2. There is no difference in energy between these two nuclear spin states; a proton is just as likely to have a spin of +3 as -=. Absorption of electromagnetic radiation can only occur when the two spin states have different energies. A way to make them different is to place the sample in a magnetic field. A proton behaves like a tiny bar mag net and has a magnetic moment associated with it(Figure 13.3). In the presence of an external magnetic field o, the state in which the magnetic moment of the nucleus is aligned with o is lower in energy than the one in which it opposes il ￠|< (a) No external magnetic field (b) Apply external magnetic field o FIGURE 13.3(a)In the absence of an external magnetic field, the nuclear spins of the protons re randomly oriented. (b)In the presence of an external magnetic field o, the nuclear spins are oriented so that the resulting nuclear magnetic moments are aligned either parallel or antiparallel to o. The lower energy orientation is the one parallel to o and there are more nuclei that have this orientation Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

With this as background, we will now discuss spectroscopic techniques individually. NMR, IR, and UV-VIS spectroscopy provide complementary information, and all are useful. Among them, NMR provides the information that is most directly related to molecular structure and is the one we shall examine first. 13.3 INTRODUCTION TO 1 H NMR SPECTROSCOPY Nuclear magnetic resonance spectroscopy depends on the absorption of energy when the nucleus of an atom is excited from its lowest energy spin state to the next higher one. We should first point out that many elements are difficult to study by NMR, and some can’t be studied at all. Fortunately though, the two elements that are the most common in organic molecules (carbon and hydrogen) have isotopes (1 H and 13C) capable of giving NMR spectra that are rich in structural information. A proton nuclear magnetic resonance (1 H NMR) spectrum tells us about the environments of the various hydrogens in a molecule; a carbon-13 nuclear magnetic resonance (13C NMR) spectrum does the same for the carbon atoms. Separately and together 1 H and 13C NMR take us a long way toward determining a substance’s molecular structure. We’ll develop most of the general principles of NMR by discussing 1 H NMR, then extend them to 13C NMR. The 13C NMR discussion is shorter, not because it is less important than 1 H NMR, but because many of the same principles apply to both techniques. Like an electron, a proton has two spin states with quantum numbers of and . There is no difference in energy between these two nuclear spin states; a proton is just as likely to have a spin of as . Absorption of electromagnetic radiation can only occur when the two spin states have different energies. A way to make them different is to place the sample in a magnetic field. A proton behaves like a tiny bar magnet and has a magnetic moment associated with it (Figure 13.3). In the presence of an external magnetic field 0, the state in which the magnetic moment of the nucleus is aligned with 0 is lower in energy than the one in which it opposes 0. 1 2 1 2 1 2 1 2 490 CHAPTER THIRTEEN Spectroscopy (a) No external magnetic field (b) Apply external magnetic field 0 0 FIGURE 13.3 (a) In the absence of an external magnetic field, the nuclear spins of the protons are randomly oriented. (b) In the presence of an external magnetic field 0, the nuclear spins are oriented so that the resulting nuclear magnetic moments are aligned either parallel or antiparallel to 0. The lower energy orientation is the one parallel to 0 and there are more nuclei that have this orientation. Nuclear magnetic resonance of protons was first detected in 1946 by Edward Purcell (Harvard) and by Felix Bloch (Stanford). Purcell and Bloch shared the 1952 Nobel Prize in physics. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

13.3 Introduction to ' H NMR Spectroscopy As shown in Figure 13. 4, the energy difference between the two states is directly proportional to the strength of the applied field. Net absorption of electromagnetic radi- The SI unit for magnetic field ation requires that the lower state be more highly populated than the higher one, and strength is the tesla (m) detectable signal. A magnetic field of 4.7T. which is about 100,000 times stronger than contemporary of Thomas o quite strong magnetic fields are required to achieve the separation necessary to give a named after Nikola Tesla, earth's magnetic field, for example, separates the two spin states of ' h by only 8X 10-5 Edison and who, like Edison as an inventor of electrical kJ/mol(1.9 10 kcal/mol). From Planck's equation AE= hv, this energy gap cor- devices responds to radiation having a frequency of 2 X 10 Hz(200 MHz) which lies in the radio frequency(rf) region of the electromagnetic spectrum(see Figure 13.1) Frequency of Energy difference electromagnetic is proportional to. between nuclear is proportional to Magnetic field adiation spin states (s or Hz) (kJ/mol or kcal/mol) PROBLEM 13.1 Most of the Nmr spectra in this text were recorded on a spec rometer having a field strength of 4.7 T(200 MHz for H). The first generation of widely used NMR spectrometers were 60-MHz instruments What was the mag netic field strength of these earlier spectrometers? The response of an atom to the strength of the external magnetic field is different for different elements, and for different isotopes of the same element. The resonance fre quencies of most nuclei are sufficiently different that an NMR experiment is sensitive only to a particular isotope of a single element. The frequency for H is 200 MHz at 4.7 T, but that of C is 50.4 MHz. Thus, when recording the NMr spectrum of an organic compound, we see signals only for H orC, but not both; H andC NMR petra are recorded in separate experiments with different instrument settings PROBLEM 13.2 What will be the C frequency setting of an NMR spectrome ter that operates at 100 MHz for protons? The essential features of an NMR spectrometer, shown in Figure 13.5, are not hard to understand. They consist of a magnet to align the nuclear spins, a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one, a receiver to detect the absorption of rf radiation, and a recorder to print ut the spectrum. Nuclear magnetic moment antiparallel △E moment par FIGURE 13. 4 An external magnetic field causes the two nuclear spin states to in absence of extemal have different ies. th magnetic field e In energy△Eis Increasing strength of proportional to the streng Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

As shown in Figure 13.4, the energy difference between the two states is directly proportional to the strength of the applied field. Net absorption of electromagnetic radiation requires that the lower state be more highly populated than the higher one, and quite strong magnetic fields are required to achieve the separation necessary to give a detectable signal. A magnetic field of 4.7 T, which is about 100,000 times stronger than earth’s magnetic field, for example, separates the two spin states of 1 H by only 8 105 kJ/mol (1.9 105 kcal/mol). From Planck’s equation E h, this energy gap corresponds to radiation having a frequency of 2 108 Hz (200 MHz) which lies in the radio frequency (rf) region of the electromagnetic spectrum (see Figure 13.1). PROBLEM 13.1 Most of the NMR spectra in this text were recorded on a spectrometer having a field strength of 4.7 T (200 MHz for 1 H). The first generation of widely used NMR spectrometers were 60-MHz instruments. What was the magnetic field strength of these earlier spectrometers? The response of an atom to the strength of the external magnetic field is different for different elements, and for different isotopes of the same element. The resonance frequencies of most nuclei are sufficiently different that an NMR experiment is sensitive only to a particular isotope of a single element. The frequency for 1 H is 200 MHz at 4.7 T, but that of 13C is 50.4 MHz. Thus, when recording the NMR spectrum of an organic compound, we see signals only for 1 H or 13C, but not both; 1 H and 13C NMR spectra are recorded in separate experiments with different instrument settings. PROBLEM 13.2 What will be the 13C frequency setting of an NMR spectrometer that operates at 100 MHz for protons? The essential features of an NMR spectrometer, shown in Figure 13.5, are not hard to understand. They consist of a magnet to align the nuclear spins, a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one, a receiver to detect the absorption of rf radiation, and a recorder to print out the spectrum. Frequency of electromagnetic radiation (s1 or Hz) Magnetic field (T) Energy difference between nuclear spin states (kJ/mol or kcal/mol) is proportional to is proportional to 13.3 Introduction to 1 H NMR Spectroscopy 491 0 0 ' E1 E1 ' E2 E2 ' No energy difference in nuclear spin states in absence of external magnetic field Nuclear magnetic moment antiparallel to 0 Nuclear magnetic moment parallel to 0 Increasing strength of external magnetic field ∆E ∆E' FIGURE 13.4 An external magnetic field causes the two nuclear spin states to have different energies. The difference in energy E is proportional to the strength of the applied field. The Sl unit for magnetic field strength is the tesla (T), named after Nikola Tesla, a contemporary of Thomas Edison and who, like Edison, was an inventor of electrical devices. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER THIRTEEN Spectroscopy ample rt oscillator NMR Spectrum 13.5 Diagram of a nuclear magnetic resonance spectrometer(From S. H. Pine, J.B. son, D J Cram, and G S Hammond, Organic Chemistry, 4th edition, McGraw-Hill, New 980p.136) It turns out though that there are several possible variations on this general theme. We could, for example, keep the magnetic field constant and continuously vary the radiofrequency until it matched the energy difference between the nuclear spin states. Or we could keep the rf constant and adjust the energy levels by varying the magnetic field strength. Both methods work, and the instruments based on them are called continuous wave(CW) spectrometers. Many of the terms we use in NMR spectroscopy have their origin in the way CW instruments operate, but Cw instruments are rarely used anymore CW-NMR spectrometers have been replaced by a new generation of instruments called pulsed Fourier-transform nuclear magnetic resonance(FT-NMR) spectrometers FT-NMR spectrometers are far more versatile than CW instruments and are more com- plicated. Most of the visible differences between them lie in computerized data acquisi- tion and analysis components that are fundamental to FT-NMR spectroscopy. But there is an important difference in how a pulsed FT-NMR experiment is carried out as well Rather than sweeping through a range of frequencies(or magnetic field strengths ), the sample is irradiated with a short, intense burst of radiofrequency radiation(the pulse that excites all of the protons in the molecule. The magnetic field associated with the new orientation of nuclear spins induces an electrical signal in the receiver that decreases with time as the nuclei return to their original orientation. The resulting free-induction decay(FID) is a composite of the decay patterns of all of the protons in the molecule The free-induction decay pattern is stored in a computer and converted into a spectrum Richard r. ernst of the Swiss by a mathematical process known as a Fourier transform. The pulse-relaxation sequence Federal Institute of Technol- takes only about a second, but usually gives signals too weak to distinguish from back ground noise. The signal-to-noise ratio is enhanced by repeating the sequence many Prize in chemistry for devis- ing pulse-relaxation NMR times, then averaging the data. Noise is random and averaging causes it to vanish; sig nals always appear at the same place and accumulate. All of the operationsthe inter val between pulses, collecting, storing, and averaging the data and converting it to a pectrum by a Fourier transformare under computer control, which makes the actual taking of an FT-NMR spectrum a fairly routine operation Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

It turns out though that there are several possible variations on this general theme. We could, for example, keep the magnetic field constant and continuously vary the radiofrequency until it matched the energy difference between the nuclear spin states. Or, we could keep the rf constant and adjust the energy levels by varying the magnetic field strength. Both methods work, and the instruments based on them are called continuous wave (CW) spectrometers. Many of the terms we use in NMR spectroscopy have their origin in the way CW instruments operate, but CW instruments are rarely used anymore. CW-NMR spectrometers have been replaced by a new generation of instruments called pulsed Fourier-transform nuclear magnetic resonance (FT-NMR) spectrometers. FT-NMR spectrometers are far more versatile than CW instruments and are more complicated. Most of the visible differences between them lie in computerized data acquisition and analysis components that are fundamental to FT-NMR spectroscopy. But there is an important difference in how a pulsed FT-NMR experiment is carried out as well. Rather than sweeping through a range of frequencies (or magnetic field strengths), the sample is irradiated with a short, intense burst of radiofrequency radiation (the pulse) that excites all of the protons in the molecule. The magnetic field associated with the new orientation of nuclear spins induces an electrical signal in the receiver that decreases with time as the nuclei return to their original orientation. The resulting free-induction decay (FID) is a composite of the decay patterns of all of the protons in the molecule. The free-induction decay pattern is stored in a computer and converted into a spectrum by a mathematical process known as a Fourier transform. The pulse-relaxation sequence takes only about a second, but usually gives signals too weak to distinguish from background noise. The signal-to-noise ratio is enhanced by repeating the sequence many times, then averaging the data. Noise is random and averaging causes it to vanish; signals always appear at the same place and accumulate. All of the operations—the interval between pulses, collecting, storing, and averaging the data and converting it to a spectrum by a Fourier transform—are under computer control, which makes the actual taking of an FT-NMR spectrum a fairly routine operation. 492 CHAPTER THIRTEEN Spectroscopy Magnet rf input oscillator rf output signal amplifier NMR Spectrum rf output receiver rf input coil Sample tube 0 FIGURE 13.5 Diagram of a nuclear magnetic resonance spectrometer. (From S. H. Pine, J. B. Hendrickson, D. J. Cram, and G. S. Hammond, Organic Chemistry, 4th edition, McGraw-Hill, New York, 1980, p. 136.) Richard R. Ernst of the Swiss Federal Institute of Technology won the 1991 Nobel Prize in chemistry for devising pulse-relaxation NMR techniques. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

13.4 Nuclear Shielding and H Chemical Shift Not only is pulsed FT-NMR the best method for obtaining proton spectra, it is the only practical method for many other nuclei, including C. It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy 13.4 NUCLEAR SHIELDING AND TH CHEMICAL SHIFTS Our discussion so far has concerned H nuclei in general without regard for the envi- other atoms--carbon, oxygen, nitrogen, and so on-by covalent bonds. The electrons in magnetic field of the elec the protons. Alone, a proton would feel the full strength of the external field, but a pro- gen bond opposes the - hydro these bonds, indeed all the electrons in a molecule, affect the magnetic environment of trons in the carbo ton in an organic molecule responds to both the external field plus any local fields within resulting magnetic field the molecule. An external magnetic field affects the motion of the electrons in a mole- experienced by the proton and the carbon is slightly cule, inducing local fields characterized by lines of force that circulate in the opposite less than se direction from the applied field(Figure 13. 6). Thus, the net field felt by a proton in a ess than %o- molecule will always be less than the applied field, and the proton is said to be shielded All of the protons of a molecule are shielded from the applied field by the electrons, but some are less shielded than others Sometimes the term"deshielded is used to describ this decreased shielding of one proton relative to another. The more shielded a proton is, the greater must be the strength of the applied field in order to achieve resonance and produce a signal. A more shielded proton absorbs rf radiation at higher field strength (upfield) compared with one at lower field strength (downfield). Different protons give signals at different field strengths. The dependence of the resonance position of a nucleus that results from its molecular environment is called its chemical shift. This is where the real power of NMR lies. The chemical shifts of various protons in a molecule can be different and are characteristic of particular struc tural features the igure 13.7 shows the H NMR spectrum of chloroform(CHCl3)to illustrate how ology just developed applies to a real Instead of measuring chemical shifts in absolute terms, we measure them with respect to a standard--tetramethylsilane( CH3)4 Si, abbreviated TMS. The protons of Tms are more shielded than those of most organic compounds, so all of the signals in a sam- ple ordinarily appear at lower field than those of the TMS reference. When measured this chapter is an ele using a 100-MHz instrument, the signal for the proton in chloroform(CHCI,), for exam- pane tearing ay modeling con. pIe, appears 728 Hz downfield from the TMS signal. But since frequency is proportional tains models of (CHa) Si and to magnetic field strength, the same signal would appear 1456 Hz downfield from TMS (CHa)aC in which the greate on a 200-MHz instrument. We simplify the reporting of chemical shifts by converting and hydrogens of TMS is appar need not actually be present in the sample, nor even appear in the spectrum in order to tential and in the calculated o them to parts per million(ppm) from TMS, which is assigned a value of 0. The TMs ent be serve as a reference Chemical shift(8)= position of signal- position of TMS peak x 106 spectrometer frequency Thus, the chemical shift for the proton in chloroform is: 1456Hz-0Hz 200×10°Hz ×10=728pm When chemical shifts are reported this way, they are identified by the symbol 8 and are independent of the field strength. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

Not only is pulsed FT-NMR the best method for obtaining proton spectra, it is the only practical method for many other nuclei, including 13C. It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy. 13.4 NUCLEAR SHIELDING AND 1 H CHEMICAL SHIFTS Our discussion so far has concerned 1 H nuclei in general without regard for the environments of individual protons in a molecule. Protons in a molecule are connected to other atoms—carbon, oxygen, nitrogen, and so on—by covalent bonds. The electrons in these bonds, indeed all the electrons in a molecule, affect the magnetic environment of the protons. Alone, a proton would feel the full strength of the external field, but a proton in an organic molecule responds to both the external field plus any local fields within the molecule. An external magnetic field affects the motion of the electrons in a molecule, inducing local fields characterized by lines of force that circulate in the opposite direction from the applied field (Figure 13.6). Thus, the net field felt by a proton in a molecule will always be less than the applied field, and the proton is said to be shielded. All of the protons of a molecule are shielded from the applied field by the electrons, but some are less shielded than others. Sometimes the term “deshielded,” is used to describe this decreased shielding of one proton relative to another. The more shielded a proton is, the greater must be the strength of the applied field in order to achieve resonance and produce a signal. A more shielded proton absorbs rf radiation at higher field strength (upfield) compared with one at lower field strength (downfield). Different protons give signals at different field strengths. The dependence of the resonance position of a nucleus that results from its molecular environment is called its chemical shift. This is where the real power of NMR lies. The chemical shifts of various protons in a molecule can be different and are characteristic of particular structural features. Figure 13.7 shows the 1 H NMR spectrum of chloroform (CHCl3) to illustrate how the terminology just developed applies to a real spectrum. Instead of measuring chemical shifts in absolute terms, we measure them with respect to a standard—tetramethylsilane (CH3)4Si, abbreviated TMS. The protons of TMS are more shielded than those of most organic compounds, so all of the signals in a sample ordinarily appear at lower field than those of the TMS reference. When measured using a 100-MHz instrument, the signal for the proton in chloroform (CHCl3), for example, appears 728 Hz downfield from the TMS signal. But since frequency is proportional to magnetic field strength, the same signal would appear 1456 Hz downfield from TMS on a 200-MHz instrument. We simplify the reporting of chemical shifts by converting them to parts per million (ppm) from TMS, which is assigned a value of 0. The TMS need not actually be present in the sample, nor even appear in the spectrum in order to serve as a reference. Chemical shift () 106 Thus, the chemical shift for the proton in chloroform is: 106 7.28 ppm When chemical shifts are reported this way, they are identified by the symbol and are independent of the field strength. 1456 Hz 0 Hz 200 106 Hz position of signal position of TMS peak spectrometer frequency 13.4 Nuclear Shielding and 1 H Chemical Shifts 493 0 C H FIGURE 13.6 The induced magnetic field of the electrons in the carbon–hydrogen bond opposes the external magnetic field. The resulting magnetic field experienced by the proton and the carbon is slightly less than 0. The graphic that begins this chapter is an electrostatic potential map of tetramethylsilane. Learning By Modeling contains models of (CH3)4Si and (CH3)4C in which the greater electron density at the carbons and hydrogens of TMS is apparent both in the electrostatic potential and in the calculated atomic charges. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

13.5 Effects of molecular Structure on 'H Chemical Shifts much as the shielding is the electrons, it isnt surprising to find that the chemi- cal shift depends on the de to which X draws electrons away from the methyl group Increased shielding of methyl protons Decreasing electronegativity of attached atom CH3OCH3 (CH3)3N CH3CH Methyl Dimethyl Trimethylamine Ethane fluoride Chemical shift of methyl protons (6),ppm: A similar trend is seen in the methyl halides, in which the protons in CH3 F are the least shielded(8 4.3 ppm) and those of CH3I (8 2. 2 ppm) are the most The deshielding effects of electronegative substituents are cumulative, as the chem- ical shifts for various chlorinated derivatives of methane indicate CH,CI CH3CI Chloroform Methylene chloride Methyl chloride Chemical shift (6),ppm: 7.3 PROBLEM 13. 4 There is a difference of 4.6 ppm in the 'H chemical shifts CHCI3 and CH3CCl3 What is the chemical shift for the protons in CH3 CCl3? Exp your reasonIng inyl protons in alkenes and aryl protons in arenes are substantially less shielded Ethylene Chemical shift 7.3 One reason for the decreased shielding of vinyl and aryl protons is related to the directional properties of the induced magnetic field of the TT electrons. As Figure 13.8 shows, the induced magnetic field due to the T electrons is just like that due to elec trons in o bonds; it opposes the applied magnetic field. However, all magnetic fields close upon themselves, and protons attached to a carbon-carbon double bond or ar FIGURE 13. 8 The induced matic ring lie in a region where the induced field reinforces the applied field, which magnetic field of the elec- decreases the shielding of vinyl and aryl protons trons of (a) an alkene and A similar, although much smaller, effect of T electron systems is seen in the chem- (b)an arene reinforces the ical shifts of benzylic and allylic hydrogens. The methyl hydrogens in hexamethylben- where vinyl and aryl protons zene and in 2, 3-dimethyl-2-butene are less shielded than those in ethane are located Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

much as the shielding is due to the electrons, it isn’t surprising to find that the chemical shift depends on the degree to which X draws electrons away from the methyl group. A similar trend is seen in the methyl halides, in which the protons in CH3F are the least shielded ( 4.3 ppm) and those of CH3I ( 2.2 ppm) are the most. The deshielding effects of electronegative substituents are cumulative, as the chemical shifts for various chlorinated derivatives of methane indicate: PROBLEM 13.4 There is a difference of 4.6 ppm in the 1 H chemical shifts of CHCl3 and CH3CCl3. What is the chemical shift for the protons in CH3CCl3? Explain your reasoning. Vinyl protons in alkenes and aryl protons in arenes are substantially less shielded than protons in alkanes: One reason for the decreased shielding of vinyl and aryl protons is related to the directional properties of the induced magnetic field of the electrons. As Figure 13.8 shows, the induced magnetic field due to the electrons is just like that due to electrons in bonds; it opposes the applied magnetic field. However, all magnetic fields close upon themselves, and protons attached to a carbon–carbon double bond or an aromatic ring lie in a region where the induced field reinforces the applied field, which decreases the shielding of vinyl and aryl protons. A similar, although much smaller, effect of electron systems is seen in the chemical shifts of benzylic and allylic hydrogens. The methyl hydrogens in hexamethylbenzene and in 2,3-dimethyl-2-butene are less shielded than those in ethane. Chemical shift (), ppm: H H H H H H Benzene 7.3 C H H H H C Ethylene 5.3 CH3CH3 Ethane 0.9 Chemical shift (), ppm: CHCl3 Chloroform (trichloromethane) 7.3 CH2Cl2 Methylene chloride (dichloromethane) 5.3 CH3Cl Methyl chloride (chloromethane) 3.1 Increased shielding of methyl protons Decreasing electronegativity of attached atom Chemical shift of methyl protons (), ppm: CH3F Methyl fluoride 4.3 CH3OCH3 Dimethyl ether 3.2 (CH3)3N Trimethylamine 2.2 CH3CH3 Ethane 0.9 13.5 Effects of Molecular Structure on 1 H Chemical Shifts 495 C C H H H H H H H H H H 0 (a) (b) 0 FIGURE 13.8 The induced magnetic field of the electrons of (a) an alkene and (b) an arene reinforces the applied fields in the regions where vinyl and aryl protons are located. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website