7: FT-IR Spectroscopy (Experiment)
- Page ID
- 431944
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Pre-lab Questions
- What is the frequency range of infrared (IR) spectroscopy?
- When a molecule absorbs the frequencies in this range, what internal motion of the molecule would be excited?
- Which vibrational mode has a higher frequency: \(\ce{C-H}\) stretch, \(\ce{C-O}\) stretch, or \(\ce{H-C-H}\) bending?
Introduction
FTIR stands for Fourier Transform Infrared Spectroscopy. In this technique, the absorption of electromagnetic radiation by organic molecules in the infrared region is used to carry out quantitative and qualitative analysis. Every polyatomic molecule can absorb energy via vibrations. The quanta associated with this energy correspond to infrared radiation. Over time, chemists have learnt that the vibrational motion of certain groups in organic molecules has a characteristic frequency. This means that when absorption occurs at that frequency, one has qualitative evidence that a particular group is present in the molecules in the sample.
In traditional spectroscopy, the absorbance is measured as a function of wavelength or frequency of the electromagnetic radiation. In mid-range infrared spectroscopy, a typical frequency range is from 500 cm\(\ce{^{-1}}\) to 4000 cm\(\ce{^{-1}}\). In FT spectroscopy the electromagnetic radiation is presented to the sample as a burst of electromagnetic energy lasting over a very short period of time. This burst contains radiation with frequencies covering the whole range of the experiment. By the process of Fourier transformation, the data are converted to a spectrum of absorbance against frequency. Because a computer is connected to the spectrometer, this process can be repeated many times, and a very precise spectrum obtained. For this reason, FT instruments are very popular and give far superior spectra than the traditional instruments in which absorbance is directly measured as a function of frequency.
The present experiment will examine infrared spectra of common organic solvents.
Spectra of Common Organic Solvents
Organic solvents may be classified as polar or non-polar. Examples of polar organic solvents are methanol, acetone, and acetonitrile. Electrolytes may be dissolved in these media to a limited extent depending on the solvent’s specific properties. Non-polar solvents are those in which electrolytes do not dissolve. Examples are hexane, carbon tetrachloride and benzene. We will examine the spectra of solvents from both groups.
An example of polar solvent is acetone, a fairly simple ketone with the formula \(\ce{(CH3)2CO}\). The molecule is non-linear with 10 atoms and 24 vibrational modes. The most prominent feature in the infrared spectrum is the \(\ce{C=O}\) stretching absorption at 1716 cm\(\ce{^{-1}}\). This is also the polar bond in the molecule. Other features include the deformation modes of the \(\ce{CH3}\) groups at 1420 and 1364 cm\(\ce{^{-1}}\), and the \(\ce{C=O}\) deformation mode at 530 cm\(\ce{^{-1}}\) (see Figure 1). Use the information in Table 1 to identify other features of the vibrational spectrum of acetone.
| Bond | Type of Vibration | Frequency/cm-1 | Intensity |
| \(\ce{C-H}\) | Alkanes (stretch) | 3000 - 2850 | S |
| \(\ce{-CH3}\) (bend) | 1450, 1375 | M | |
| \(\ce{-CH2-}\) (bend) | 1465 | M | |
|
Alkanes (stretch) (out-of-plane-bend) |
3100 - 3000 100 - 650 |
M S |
|
|
Aromatics (stretch) (out-of-plane-bend) |
3150 - 3050 900 - 690 |
S S |
|
| Alkyne (stretch) | ~ 3300 | S | |
| Aldehyde | 2900 - 2800 | W | |
| \( \ce{C=C}\) | Alkene | 1680 - 1600 | m-w |
| Aromatic | 1600 - 1475 | m-w | |
| \(\ce{C≡C}\) | Alkyne | 225 - 2100 | m-w |
| \(\ce{C=O}\) | Aldehyde | 1740 - 1720 | S |
| Ketone | 1725 - 1705 | S | |
| Carboxylic acid | 1725 - 1700 | S | |
| Ester | 1750 - 1730 | S | |
| Amide | 1670 - 1640 | S | |
| Anhydride | 1810, 1760 | S | |
| Acid chloride | 1800 | S | |
| \(\ce{C-O}\) | Alcohols, ethers, esters, anhydrides, carboxylic acids | 1300 - 1000 | S |
| \(\ce{O-H}\) | Carboxylic acids | 3400 - 2400 | M |
|
Alcohols, phenols (free) (Hydrogen bonded) |
3650 - 3600 3500 - 3200 |
M M |
|
| \(\ce{N-H}\) |
1° & 2° amines/amides (stretch) (bend) |
3500 - 3100 1640 - 1550 |
M m-s |
| \(\ce{C-N}\) | Amines | 1350 - 1000 | m-s |
| \(\ce{C≡N}\) | Nitriles (cyanide) | 2260 - 2240 | M |
| \(\ce{N=O}\) | Nitro (\(\ce{NO2})\) | 1550 - 1350 | S |
| \(\ce{C-X}\) |
Fluoride Chloride |
1400 - 1000 800 - 600 |
S S |
Table \(\PageIndex{1}\): Principal Infrared Bands. S = strong; M = medium; W = weak
Experimental
Part I.
Infrared spectra for six common organic solvents will be obtained. The principal bands will be identified. It will be necessary to photocopy these spectra so that each student has a copy.
A. Acetone
Compare your spectrum with the one given in Figure 1. Identify each of the important peaks.
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B. Acetonitrile
Identify the \(\ce{C≡N}\) stretching frequency at 2254 cm\(\ce{^{-1}}\). Compare the spectral features around 1400 cm\(\ce{^{-1}}\) in acetone and acetonitrile. What are the bands around 3000 cm\(\ce{^{-1}}\) in these molecules?
C. 1-Butanol
The broad band at 3300 cm\(\ce{^{-1}}\) is due to the \(\ce{-OH}\) stretch. There is also a \(\ce{-OH}\) bending mode at 1450 cm\(\ce{^{-1}}\). Can you identify the features of the spectrum due to the \(\ce{CH3}\)-group? How does the spectrum of methanol compare with that of 1-butanol? What is the role of hydrogen bonding in these spectra?
D. Cyclohexane \(\ce{C6H10} \)
Identify the \(\ce{C=C}\) stretching band in the spectrum. Compare this spectrum with that for 1-butanol.
E. Chloroform
Chloroform has a very simple spectrum with bands at 750 and 1220 cm\(\ce{^{-1}}\). Identify these bands.
F. Nitromethane
The spectrum of nitromethane is complex. The major bands in the 1400-1600 cm\(\ce{^{-1}}\) region are due to the nitro group. Identify these bands and the other important ones.
Part II. Spectra of Common Polymers
In this experiment, the functional groups in a polymer are identified using FTIR spectroscopy. An example of polystyrene shown in Figure 2.
Strong bands are seen near 3000 cm\(\ce{^{-1}}\), then close to 1600 cm\(\ce{^{-1}}\), and finally below 1000 cm\(\ce{^{-1}}\). Identify these bands using the information in Table 1. What is the structure of the styrene monomer?
We will examine two common polymers found in most households, namely, the polymer in sandwich bags, and saran wrap. Each group should bring a third polymer to class. Discuss this with your classmates so that a wide variety of polymers can be studied by the class.
Your report will contain copies of at least three different spectra with the main bands identified.
In order to write a good report, you will have to consult a table of infrared bands which is available in most organic chemistry textbooks. A short summary of the relevant information is given in Table 1.