14782_02_Ch2_p015-104.pp2.gxd1/25/0810:27Page25 2.5 The Infrared Spectrometer 25 le pm e ey celed since they are present in both beams. B.Fourier Transform Spectrometers rs)operate on a comple but the frequencies that make up the infraredsp trum.An in ogram is essenti ally a plot of intensity versus time(a tin ne-domain spectrum) mo a plot o ty versus fre ET rate the individual absorption frequencies from the interferogram.producing a spectrum virtu- ally identical to that obtained with a dispe sive spect neter.This type of instrument is known as ter,or FT-l 1g e pl of greater speed an A schematic diagram of an ft-ir is shown in figure 2 3h The ft-ir uses an interferometer to n splitter,a to oriented at a 9 angle.One beam.the one oriented at 9 in Figure 2.3b.goes to a stationary or "fixed"mirror and is retumned to the beam splitter.The undeflected beam goes to a moving mirror and is also retumed to the be n splitter.Th motion of the mirror caus pathlength differences (differing wavelength content)of the two beams cause both constructive and destructive interferences.The combined beam co taining these interference patters is called the wram.Thi nte ram contains all of the radiative energy coming from the source and The interferogram generated by combining the two beams is oriented toward the sample by the beam splitter.As it passes through the sample,the sample absorbs all of the wave are norma nd in its int ared spectrum.The modifi ogran ncy)The cor laser beam to have a standard of comparison.The final interferogram contains all of the information one time-domain a signal that cannot be read by a humar al process ca absorbed and o struct and plot whate Computer-interfaced FT-IR instruments operate in a single-beam mode.To obtain a spectrum of a compound,the che st first obtains san interferogram of the"background,"which consists of th The pr yA0 an FT-IR A271
2.5 The Infrared Spectrometer 25 the compound is placed in the sample beam, and nothing is inserted into the reference beam. When the spectrum of the liquid is obtained, the effects of the atmospheric gases are automatically canceled since they are present in both beams. B. Fourier Transform Spectrometers 1 The principles of interferometry and the operation of an FT-IR instrument are explained in two articles by W. D. Perkins: “Fourier Transform–Infrared Spectroscopy, Part 1: Instrumentation,” Journal of Chemical Education, 63 (January 1986): A5–A10, and “Fourier Transform–Infrared Spectroscopy, Part 2: Advantages of FT-IR,” Journal of Chemical Education, 64 (November 1987): A269–A271. The most modern infrared spectrometers (spectrophotometers) operate on a different principle. The design of the optical pathway produces a pattern called an interferogram. The interferogram is a complex signal, but its wave-like pattern contains all the frequencies that make up the infrared spectrum. An interferogram is essentially a plot of intensity versus time (a time-domain spectrum). However, a chemist is more interested in a spectrum that is a plot of intensity versus frequency (a frequency-domain spectrum). A mathematical operation known as a Fourier transform (FT) can separate the individual absorption frequencies from the interferogram, producing a spectrum virtually identical to that obtained with a dispersive spectrometer. This type of instrument is known as a Fourier transform infrared spectrometer, or FT-IR.1 The advantage of an FT-IR instrument is that it acquires the interferogram in less than a second. It is thus possible to collect dozens of interferograms of the same sample and accumulate them in the memory of a computer. When a Fourier transform is performed on the sum of the accumulated interferograms, a spectrum with a better signal-to-noise ratio can be plotted. An FT-IR instrument is therefore capable of greater speed and greater sensitivity than a dispersion instrument. A schematic diagram of an FT-IR is shown in Figure 2.3b. The FT-IR uses an interferometer to process the energy sent to the sample. In the interferometer, the source energy passes through a beam splitter, a mirror placed at a 45° angle to the incoming radiation, which allows the incoming radiation to pass through but separates it into two perpendicular beams, one undeflected, the other oriented at a 90° angle. One beam, the one oriented at 90° in Figure 2.3b, goes to a stationary or “fixed” mirror and is returned to the beam splitter. The undeflected beam goes to a moving mirror and is also returned to the beam splitter. The motion of the mirror causes the pathlength that the second beam traverses to vary. When the two beams meet at the beam splitter, they recombine, but the pathlength differences (differing wavelength content) of the two beams cause both constructive and destructive interferences. The combined beam containing these interference patterns is called the interferogram. This interferogram contains all of the radiative energy coming from the source and has a wide range of wavelengths. The interferogram generated by combining the two beams is oriented toward the sample by the beam splitter. As it passes through the sample, the sample simultaneously absorbs all of the wavelengths (frequencies) that are normally found in its infrared spectrum. The modified interferogram signal that reaches the detector contains information about the amount of energy that was absorbed at every wavelength (frequency). The computer compares the modified interferogram to a reference laser beam to have a standard of comparison. The final interferogram contains all of the information in one time-domain signal, a signal that cannot be read by a human. A mathematical process called a Fourier transform must be implemented by computer to extract the individual frequencies that were absorbed and to reconstruct and plot what we recognize as a typical infrared spectrum. Computer-interfaced FT-IR instruments operate in a single-beam mode. To obtain a spectrum of a compound, the chemist first obtains an interferogram of the “background,” which consists of the infrared-active atmospheric gases, carbon dioxide and water vapor (oxygen and nitrogen are not infrared active). The interferogram is subjected to a Fourier transform, which yields the spectrum of 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 25
Infrared Spectroscop the background.Then the chemist places the con d (sample e)into the beam and obtains the The computer software automatically sub tracts the spectrum of the b ackground fom the sample spectrum,yielding the spectrum of the ng analyzed.The sive instrume ore detailed info about the background spectrum. 2.6 PREPARATION OF SAMPLES FOR INFRARED SPECTROSCOPY To dete ust place the und in ple holder throughout the infrared region of the spectrum.Cells must be constructed of ionic substances typically nide plates chloride plates are used widely e for their use in spectros copy extends from 4000to 650 cm-Sodiun n chloride b egins to absorb at 650 cmand any value will not be observed Since w rtant bands appea Liquids.A drop of a liquid organic compound is placed be n a pair of polished sodium chloride oridn um b e plates,referred to as salt plat e plate squ gently,a th ater nic ne pounds nayd byhs tehnique must b fr of water.The pair of plates is insered intoa holder Solids.The are at least thre n metheds for mnle for irt method involves mixing the finely gound solidredsum omid and gh pres nde pre re,th nd se neter The main disadvantage of this method is that may interfere with the spectrum that is obtained.If a good pellet is prepared,the spectrum obtained will have no inter cepotassium b nt down to 400 cm of the finely The thicks Nujol)t cr ures bands tha yzed co 1462.and1377cm(p.32 te o 4 the spectrumare obcured by bands inthe it is the spectrum by computer or instrumental techniques,the region around 785 cmis ofen obscured by the strong C Cl stretch that occurs there 2.7 WHAT TO LOOK FOR WHEN EXAMINING INFRARED SPECTRA e sizes of all the ab orptions,or peaks d region and plots t ce nsit
the background. Then the chemist places the compound (sample) into the beam and obtains the spectrum resulting from the Fourier transform of the interferogram. This spectrum contains absorption bands for both the compound and the background. The computer software automatically subtracts the spectrum of the background from the sample spectrum, yielding the spectrum of the compound being analyzed. The subtracted spectrum is essentially identical to that obtained from a traditional double-beam dispersive instrument. See Section 2.22 for more detailed information about the background spectrum. 26 Infrared Spectroscopy 2.6 PREPARATION OF SAMPLES FOR INFRARED SPECTROSCOPY To determine the infrared spectrum of a compound, one must place the compound in a sample holder, or cell. In infrared spectroscopy, this immediately poses a problem. Glass and plastics absorb strongly throughout the infrared region of the spectrum. Cells must be constructed of ionic substances— typically sodium chloride or potassium bromide. Potassium bromide plates are more expensive than sodium chloride plates but have the advantage of usefulness in the range of 4000 to 400 cm−1 . Sodium chloride plates are used widely because of their relatively low cost. The practical range for their use in spectroscopy extends from 4000 to 650 cm−1 . Sodium chloride begins to absorb at 650 cm−1 , and any bands with frequencies less than this value will not be observed. Since few important bands appear below 650 cm−1 , sodium chloride plates are in most common use for routine infrared spectroscopy. Liquids. A drop of a liquid organic compound is placed between a pair of polished sodium chloride or potassium bromide plates, referred to as salt plates. When the plates are squeezed gently, a thin liquid film forms between them. A spectrum determined by this method is referred to as a neat spectrum since no solvent is used. Salt plates break easily and are water soluble. Organic compounds analyzed by this technique must be free of water. The pair of plates is inserted into a holder that fits into the spectrometer. Solids. There are at least three common methods for preparing a solid sample for spectroscopy. The first method involves mixing the finely ground solid sample with powdered potassium bromide and pressing the mixture under high pressure. Under pressure, the potassium bromide melts and seals the compound into a matrix. The result is a KBr pellet that can be inserted into a holder in the spectrometer. The main disadvantage of this method is that potassium bromide absorbs water, which may interfere with the spectrum that is obtained. If a good pellet is prepared, the spectrum obtained will have no interfering bands since potassium bromide is transparent down to 400 cm−1 . The second method, a Nujol mull, involves grinding the compound with mineral oil (Nujol) to create a suspension of the finely ground sample dispersed in the mineral oil. The thick suspension is placed between salt plates. The main disadvantage of this method is that the mineral oil obscures bands that may be present in the analyzed compound. Nujol bands appear at 2924, 1462, and 1377 cm−1 (p. 32). The third common method used with solids is to dissolve the organic compound in a solvent, most commonly carbon tetrachloride (CCl4). Again, as was the case with mineral oil, some regions of the spectrum are obscured by bands in the solvent. Although it is possible to cancel out the solvent from the spectrum by computer or instrumental techniques, the region around 785 cm−1 is often obscured by the strong CICl stretch that occurs there. 2.7 WHAT TO LOOK FOR WHEN EXAMINING INFRARED SPECTRA An infrared spectrometer determines the positions and relative sizes of all the absorptions, or peaks, in the infrared region and plots them on a piece of paper. This plot of absorption intensity versus wavenumber (or sometimes wavelength) is referred to as the infrared spectrum of the compound. 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 26
14782_02_Ch2p015-104.Pp2.gxd1/25/0810:27ge”⊕ 2.7 What to Look for When Examining Infrared Spectra 27 Figure 2.4 shows a typical infrared spectrum,that of 3-methyl-2-butanone.The sp ectrum exhibits at least two strongly absorbing peaks at about 3000 and 1715 cmfor the C-H and C=O stretching also unique to the C-bond.This is true for almost every type of absorption peak;both shape and ance,to some aracteristics often enable the c-0 1850-1630cm C-C 1680-1620cm- However,the C-O bond is a strong absorber,whereas the C-C bond generally absorbs only weakly (Fig.25).Hence,trained observers would not interpret a strong peak at 1670 cm to be a lthough the N-H and O-H regions overlap. O-H 3650-3200cm- N-H 3500-3300cm CH-C-CH-CH 820 FIGURE 2.4 The infra of 3-methyl-2-bu (peat liguid.KBr plates 1400 FI G U RE 2.5 A comparison of the intensities of the C-O and C-C absorption bands
2.7 What to Look for When Examining Infrared Spectra 27 Figure 2.4 shows a typical infrared spectrum, that of 3-methyl-2-butanone. The spectrum exhibits at least two strongly absorbing peaks at about 3000 and 1715 cm−1 for the CIH and CJO stretching frequencies, respectively. The strong absorption at 1715 cm−1 that corresponds to the carbonyl group (CJO) is quite intense. In addition to the characteristic position of absorption, the shape and intensity of this peak are also unique to the CJO bond. This is true for almost every type of absorption peak; both shape and intensity characteristics can be described, and these characteristics often enable the chemist to distinguish the peak in potentially confusing situations. For instance, to some extent CJO and CJC bonds absorb in the same region of the infrared spectrum: CJO 1850–1630 cm−1 CJC 1680–1620 cm−1 However, the CJO bond is a strong absorber, whereas the CJC bond generally absorbs only weakly (Fig. 2.5). Hence, trained observers would not interpret a strong peak at 1670 cm−1 to be a CJC double bond or a weak absorption at this frequency to be due to a carbonyl group. The shape and fine structure of a peak often give clues to its identity as well. Thus, although the NIH and OIH regions overlap, OIH 3650–3200 cm−1 NIH 3500–3300 cm−1 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 WAVENUMBERS (CM–1) 2.5 3 4 5 6 7 8 9 10 11 12 13 14 15 16 19 25 100 90 80 70 60 50 40 30 20 10 0 MICRONS % TRANSMITTANCE CH3–C–CH–CH3 CH3 O – –– sp3 C–H stretch C O –– stretch FIGURE 2.4 The infrared spectrum of 3-methyl-2-butanone (neat liquid, KBr plates). MICRONS 100 90 80 70 60 50 40 30 20 10 0 % TRANSMITTANCE WAVENUMBERS (CM–1) 2000 1800 1600 1400 1200 1000 5 6 7 8 9 10 C O –– C –– C FIGURE 2.5 A comparison of the intensities of the CJO and CJC absorption bands. 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 27
Infrared Spectroscop 3500 F I G UR E 2.6 A comparison of the shapes of the absorption bands for the O-H and N-H groups shar alcohols as pure liquids give only one (Fig.26).Figure so shows typic or the stretch intensities.They are as important as the frequency at which an absorption occurs,and the eye must be trained to recognize these features.Often.when reading the literature of organic chemistry.you will find absorptions ref convey some te pe 2.8 CORRELATION CHARTS AND TABLES mation from inf as is known about where the varic ous functional gr at the end of this chapter r contain exten e series of correlatic the a ation iled ch 8pm ity with and abilit rpret s d T s most easily b atypical absorption alue a single number that can be used as a pivotal yalue -for each of the p ic ketone has a onyl at 15 ption.Then.more slowly.familiarize yourself with the extent of the catbonyl nd the visual pattern shov ing where the diffe kinds throug ce,for ins ction 14 affect the base values (i.e.in which dire ection the values are shifted)learn the trends alwav keeping the memorized base value(1715 cm)in mind As a beginning,it might prove useful orize the base values for this approach given in Table 2.4. otice that there are only eigh
28 Infrared Spectroscopy the NIH absorption usually has one or two sharp absorption bands of lower intensity, whereas OIH, when it is in the NIH region, usually gives a broad absorption peak. Also, primary amines give two absorptions in this region, whereas alcohols as pure liquids give only one (Fig. 2.6). Figure 2.6 also shows typical patterns for the CIH stretching frequencies at about 3000 cm−1 . Therefore, while you study the sample spectra in the pages that follow, take notice of shapes and intensities. They are as important as the frequency at which an absorption occurs, and the eye must be trained to recognize these features. Often, when reading the literature of organic chemistry, you will find absorptions referred to as strong (s), medium (m), weak (w), broad, or sharp. The author is trying to convey some idea of what the peak looks like without actually drawing the spectrum. WAVENUMBERS (CM–1) MICRONS 100 90 80 70 60 50 40 30 20 10 0 % TRANSMITTANCE 4000 3600 3200 2800 2400 2.5 3 4 C–H O–H WAVENUMBERS (CM–1) MICRONS 100 90 80 70 60 50 40 30 20 10 0 % TRANSMITTANCE 4000 3600 3200 2800 2400 2.5 3 4 NH2 C–H FIGURE 2.6 A comparison of the shapes of the absorption bands for the OIH and NIH groups. To extract structural information from infrared spectra, you must be familiar with the frequencies at which various functional groups absorb. You may consult infrared correlation tables, which provide as much information as is known about where the various functional groups absorb. The references listed at the end of this chapter contain extensive series of correlation tables. Sometimes, the absorption information is presented in the form of a chart called a correlation chart. Table 2.3 is a simplified correlation table; a more detailed chart appears in Appendix 1. The volume of data in Table 2.3 looks as though it may be difficult to assimilate. However, it is really quite easy if you start simply and then slowly increase your familiarity with and ability to interpret the finer details of an infrared spectrum. You can do this most easily by first establishing the broad visual patterns of Figure 2.2 quite firmly in mind. Then, as a second step, memorize a “typical absorption value”—a single number that can be used as a pivotal value—for each of the functional groups in this pattern. For example, start with a simple aliphatic ketone as a model for all typical carbonyl compounds. The typical aliphatic ketone has a carbonyl absorption of about 1715 ± 10 cm−1 . Without worrying about the variation, memorize 1715 cm−1 as the base value for carbonyl absorption. Then, more slowly, familiarize yourself with the extent of the carbonyl range and the visual pattern showing where the different kinds of carbonyl groups appear throughout this region. See, for instance, Section 2.14 (p. 52), which gives typical values for the various types of carbonyl compounds. Also, learn how factors such as ring strain and conjugation affect the base values (i.e., in which direction the values are shifted). Learn the trends, always keeping the memorized base value (1715 cm−1 ) in mind. As a beginning, it might prove useful to memorize the base values for this approach given in Table 2.4. Notice that there are only eight of them. 2.8 CORRELATION CHARTS AND TABLES 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 28
14782_02_ch2p015-104.Pp2.gxd1/25/0810:27age29⊕ A品。 CORRELATION CHART Type of Vibration Fy Intensity C-H AIkane (stretch) 3000-2850 -c4 (bend) 1450and1375 -CH2-(bend) 1465 Alkenes (stretch) 310-3000 33 (out-of-plane bend) 1000-650 Aromatics (stretch) 3150-3050 43 (out-of-plane bend) 900-690 Alkyne ((stretch)） ca.3300 35 Aldehyde 2900-2800 56 2800-2700 c-c Alkane Not interpretatively useful C=c Alkene 1680-1600 m-4 33 Aromatic 1600and1475 m- 4 C=c Alkyne 2250-2100 m-w 3 c-0 Aldchvde 1740-1720 1725-1705 Carboxylic acid 1725-1700 6 Ester 1750-1730 Amide 1680-1630 70 ⊕ c-0 Acohokchcn,cstcrscaboyicacid,amlhydhidc 300 100 and 73 0-H Alcohols,phenols Free 3650-3600 m 47 H-bonded 3400-3200 Carboxvlic acids 3400-2400 m N-H Primary and secondary amines and amides (stretch) 3500-3100 74 (bend) 1640-1550 C-N Amine线 1350-1000 m 14 Imines and oximes 16g0-1640 C=N Nitriles 2260-2240 Allenes ketenes.isoc 2270-1940 Nitro(R-NO2 1550and1350 79 、e 2550 Sulfoxides 05 C-X Fluoride 1400-1000 85 Chloride 785-540 85 Bromide,iodide <667 85
TABLE 2.3 A SIMPLIFIED CORRELATION CHART Type of Vibration Frequency Intensity Page (cm–1) Reference CIH Alkanes (stretch) 3000–2850 s 31 ICH3 (bend) 1450 and 1375 m ICH2I (bend) 1465 m Alkenes (stretch) 3100–3000 m 33 (out-of-plane bend) 1000–650 s Aromatics (stretch) 3150–3050 s 43 (out-of-plane bend) 900–690 s Alkyne (stretch) ca. 3300 s 35 Aldehyde 2900–2800 w 56 2800–2700 w CIC Alkane Not interpretatively useful CJC Alkene 1680–1600 m–w 33 Aromatic 1600 and 1475 m–w 43 CKC Alkyne 2250–2100 m–w 35 CJO Aldehyde 1740–1720 s 56 Ketone 1725–1705 s 58 Carboxylic acid 1725–1700 s 62 Ester 1750–1730 s 64 Amide 1680–1630 s 70 Anhydride 1810 and 1760 s 73 Acid chloride 1800 s 72 CIO Alcohols, ethers, esters, carboxylic acids, anhydrides 1300–1000 s 47, 50, 62, 64, and 73 OIH Alcohols, phenols Free 3650–3600 m 47 H-bonded 3400–3200 m 47 Carboxylic acids 3400–2400 m 62 NIH Primary and secondary amines and amides (stretch) 3500–3100 m 74 (bend) 1640–1550 m–s 74 CIN Amines 1350–1000 m–s 74 CJN Imines and oximes 1690–1640 w–s 77 CKN Nitriles 2260–2240 m 77 XJCJY Allenes, ketenes, isocyanates, isothiocyanates 2270–1940 m–s 77 NJO Nitro (RINO2) 1550 and 1350 s 79 SIH Mercaptans 2550 w 81 SJO Sulfoxides 1050 s 81 Sulfones, sulfonyl chlorides, sulfates, sulfonamides 1375–1300 and s 82 1350–1140 CIX Fluoride 1400–1000 s 85 Chloride 785–540 s 85 Bromide, iodide < 667 s 85 14782_02_Ch2_p015-104.pp2.qxd 1/25/08 10:27 AM Page 29