Interpreting Infrared Spectra

When a molecule absorbs IR radiation, the molecular vibration with a frequency matching that of the radiation increases in amplitude. In other words, the spring connecting the two atoms stretches and compresses a bit further. Since each frequency absorbed by a molecule corresponds to a specific molecular motion, we can see what kinds of motions a molecule has by measuring its IR spectrum. By then interpreting those motions, we can find out what kinds of bonds (functional groups) are present in the molecule. 

IR spectrum— What molecular motions — What functional groups  

Problem 12.8 Because IR absorptions can be expressed either in micrometers or in wavenumbers, it s useful to be able to interconvert between units. Do the following conversions  

TABLE 12.1 Characteristic IR Absorptions of Some Functional Croups 1  

Functional group class Band position (cm ) absorption 

Problem 12.6 Because IR absorptions can be expressed either in micrometers or in wavenumbers, 

Infrared spectra of (a) hexane, (b) 1-hexene, and (c) 1-hexyne. Spectra like these a easily obtained on milligram amounts of material in a few minutes using commercially available instruments. 

IR spectrum What molecular motions What functional groups  

Functional group Absorption (cm Intensity Functional group Absorption (cm ) Intensity 

FIGURE 10.15 The four regions of the infrared spectrum single bonds to hydrogen, triple bonds, double bonds, and fingerprint. 

WORKED EXAMPLE 10.4 Distinguishing Isomeric Compounds by IR Spectroscopy 

Acetone (CH3COCH3) and prop-2-en-l-ol (H2C=CHCH20H) are isomers. How could you distinguish them by IR spectroscopy  

Thomson Click Organic Interactive to learn to utilize infrared spectrometry to deduce molecular structures. 

Look at the IR spectra of hexane, l-hexene, and l-hexyne in I igure 12.14 to see an example of how IR spectroscopy can be used. Although all three IR spectra contain many peaks, there are characteri.stic absorptions of the C=C and C=C functional groups that allow the three compounds to be distinguished. Thus, 1-he.xene shows a characteristic C=C absorption at 1660 cm and a vinylic =C- H absorption at 3100 cm , whereas l-hexyne has a C=C absorption at 2100 cm and a terminal alkyne =C—H absorption at 3300 cm . 

I The region from 4000 to 2500 cm corresponds to absorptions caused by N-H, C-H, and 0-11 single-bond stretching motions. N-Fl and U-11 bonus absorb in the 3300 to 3600 an range C-H bond-stretching occurs near 3000 cm . 

I The region from 2500 to 2000 cm is where triple-bond stretching occurs. Both C=N and C=C bonds absorb here. 

424 CHAPTER 12 Structure Determination Mass Spectrometry and Infrared Spectroscopy 

The complete interpretation of an IR spectrum is difficult because most organic molecules have dozens of different bond stretching and bending motions, and thus have dozens of absorptions. On the one hand, this complexity is a problem because it generally limits the laboratory use of IR spectroscopy to pure samples of fairly small molecules— little can be learned from IR spectroscopy 

C-CI C-Br 600-800 500-600 Strong Strong Carboxylic acid Strong, broad 

Identify the functional groups in each molecule, and refer to Table 12.1. 

---

B-67MI50503 H. A. Szymanski Interpreted Infrared Spectra , Plenum, New York, 1967, vol. 

Szymanski, H.A. (1964-1967). Interpreted Infrared Spectra, Vols. I—III. New York Plenum Press. 

Neural networks have been applied to infrared spectrum interpreting systems in many variations and applications. Anand introduced a neural network approach to analyze the presence of amino acids in protein molecules with a reliability of nearly 90% [37]. Robb used a linear neural network model for interpreting infrared spectra in routine analysis purposes with a similar performance [38]. Ehrentreich et al. used a counterpropagation (CPG) network based on a strategy of Novic and Zupan to model the correlation of structures and infrared spectra [39]. Penchev and colleagues compared three types of spectral features derived from infrared peak tables for their ability to be used in automatic classification of infrared spectra [40]. 

The weak absorption at 304 0 cm in the infrared spectrum would be indicative of the N-H stretch of secondary amines. The absorption at 1380 would be indicative of an ethyl, methyl, isopropyl or t-butyl group. The broad absorption at 1120 would be indicative of a C-N stretch, while the broad absorptions at 750 cm and 700 cm would be indicative of a monosubstituted benzene ring. Unfortunately one can only interpret infrared spectra if one is familiar with the various group characteristic peaks. There are no "easy" ways of interpreting ir spectra. 

Problems in Interpreting Infrared Spectra and Mass Spectra... 

---