CHAPTER 13 SPECTROSCOPY ntil the second half of the twentieth century, the structure of a substance-a newly discovered natural product, for example-was determined using information obtained from chemical reactions This information included the identification of functional groups by chemical tests, along with the results of experiments in which the substance was broken down into smaller, more readily identifiable fragments. Typical of his approach is the demonstration of the presence of a double bond in an alkene by cat- alytic hydrogenation and subsequent determination of its location by ozonolysis. After considering all the available chemical evidence, the chemist proposed a candidate struc- ture (or structures) consistent with the observations. Proof of structure was provided either by converting the substance to some already known compound or by an indepen- dent synthesis Qualitative tests and chemical degradation have been supplemented and to a large degree replaced by instrumental methods of structure determination. The most prominent methods and the structural clues they provide are: Nuclear magnetic resonance (NMR)spectroscopy tells us about the carbon skeleton and the of the hydrogens attached to it. Infrared (R) spectroscopy reveals the presence or absence of key functional Ultraviolet-visible (UV-VIs) spectroscopy probes the electron distribution, espe- cially in molecules that have conjugated T electron systems. Mass spectrometry (MS)gives the molecular weight and formula, both of the molecule itself and various structural units within it Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
487 CHAPTER 13 SPECTROSCOPY Until the second half of the twentieth century, the structure of a substance—a newly discovered natural product, for example—was determined using information obtained from chemical reactions. This information included the identification of functional groups by chemical tests, along with the results of experiments in which the substance was broken down into smaller, more readily identifiable fragments. Typical of this approach is the demonstration of the presence of a double bond in an alkene by catalytic hydrogenation and subsequent determination of its location by ozonolysis. After considering all the available chemical evidence, the chemist proposed a candidate structure (or structures) consistent with the observations. Proof of structure was provided either by converting the substance to some already known compound or by an independent synthesis. Qualitative tests and chemical degradation have been supplemented and to a large degree replaced by instrumental methods of structure determination. The most prominent methods and the structural clues they provide are: • Nuclear magnetic resonance (NMR) spectroscopy tells us about the carbon skeleton and the environments of the hydrogens attached to it. • Infrared (IR) spectroscopy reveals the presence or absence of key functional groups. • Ultraviolet-visible (UV-VIS) spectroscopy probes the electron distribution, especially in molecules that have conjugated electron systems. • Mass spectrometry (MS) gives the molecular weight and formula, both of the molecule itself and various structural units within it. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER THIRTEEN Spectroscopy 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 " Modern"physics dates from 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(v) the stage for the develop- Planck received the 1918 No- The SI units of frequency are reciprocal seconds(s ) given the name hertz and the bel Prize in physics symbol Hz in honor of the nineteenth-century physicist Heinrich R. Hertz. The constant of proportionality h is called Plancks constant and has the value h=6.63×10-34J.s Electromagnetic radiation travels at the speed of light(c=3.0 X 10 m/s), which is equal to the product of its frequency v and its wavelength A 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 spec- trum. It is characterized by wavelengths of 4 X 10 m(violet)to 8 x 10 m(red) Lowest energy Wavelength(nm) Gamma y X-ray violet nfrared Radio frequency FIGURE 13.1 The electromagnetic spectrum. From M. Silberberg, Chemistry, 2d edition WCB/McGraw-Hill, 2000, p. 260) Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
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