Carey - Organic Chemistry - chapt13

Carey - Organic Chemistry - chapt13

(Parte 1 de 8)

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:

•Nuclearmagnetic resonance (NMR) spectroscopytells us about the carbon skeleton and the environments of the hydrogens attached to it.

•Infrared (IR) spectroscopyreveals the presence or absence of key functional groups.

•Ultraviolet-visible (UV-VIS) spectroscopyprobes 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.

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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’l start with a brief discussion of electromagnetic radiation, which is the source of the energy that a molecule absorbs in NMR, IR, and UV-VISspectroscopy.

13.1PRINCIPLES 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 ( ).

The SI units of frequency are reciprocal seconds (s 1), given the name hertzand the symbol Hz in honor of the nineteenth-century physicist Heinrich R. Hertz. The constant of proportionality his called Planck’s constantand has the value

The range of photon energies is called the electromagnetic spectrumand is shown in Figure 13.1. Visible light occupies a very small region of the electromagnetic spectrum. It is characterized by wavelengths of 4 10 7m (violet) to 8 10 7m (red).

488 CHAPTER THIRTEEN Spectroscopy

InfraredUltraviolet

Wavelength (nm)

X-ray Microwave Radio frequencyGammaray Ultra- violet

1016 Visible Infrared

Visible region

“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.)

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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.

13.2PRINCIPLES 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 E1and

E2in 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 E1than in the higher energy state E2. Excitation of a molecule from a lower state to a higher one requires the addition of an incre- ment of energy equal to E.Thus, when electromagnetic radiation is incident upon a molecule, only the frequency whose corresponding energy equals Eis 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.Aspectrum 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-VISspectrum.

FIGURE 13.2 Two energy states of a molecule. Absorption of energy equal to

to the next higher state.

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With this as background, we will now discuss spectroscopic techniques individually. NMR, IR, and UV-VISspectroscopy 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.

13.3INTRODUCTION TO 1H 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 (1H and 13C) capable of giving NMR spectra that are rich in structural information. Aproton nuclear magnetic resonance (1H 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 1H and 13C NMR take us a long way toward determining a substance’s molecular structure. We’l develop most of the general principles of NMR by discussing 1H NMR, then extend them to 13C NMR. The 13C NMR discussion is shorter, not because it is less important than 1H 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. Away to make them different is to place the sample in a magnetic field. Aproton 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 0is lower in energy than the one in which it opposes 0.

490 CHAPTER THIRTEEN Spectroscopy

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 0and 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.

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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. Amagnetic field of 4.7 T, which is about 100,0 times stronger than earth’s magnetic field, for example, separates the two spin states of 1H by only 8 10 5 kJ/mol (1.9 10 5kcal/mol). From Planck’s equation E h , this energy gap corresponds to radiation having a frequency of 2 108Hz (200 MHz) which lies in the radio frequency (rf) region of the electromagnetic spectrum (see Figure 13.1).

PROBLEM 13.1Most of the NMR spectra in this text were recorded on a spectrometer having a field strength of 4.7 T (200 MHz for 1H). 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 1H 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 1H or 13C, but not both; 1H and 13C NMR spectra are recorded in separate experiments with different instrument settings.

PROBLEM 13.2What 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 (s 1 or Hz)

Magnetic field (T)

Energy difference between nuclear spin states (kJ/mol or kcal/mol) is proportional tois proportional to

No energy difference in nuclear spin states in absence of external magnetic field

Nuclear magnetic moment antiparallel

Nuclear magnetic moment parallel

Increasing strength of external magnetic field

FIGURE 13.4 An external magnetic field causes the two nuclear spin states to have different energies. The difference in energy Eis 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.

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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 CWinstruments operate, but CWinstruments are rarely used anymore.

CW-NMR spectrometers have been replaced by a new generation of instruments called pulsed Fourier-transformnuclear magnetic resonance (FT-NMR) spectrometers. FT-NMR spectrometers are far more versatile than CWinstruments 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.

(Parte 1 de 8)

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