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IRtheory

# IRtheory - Online edition for students of organic chemistry...

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The Electromagnetic Spectrum15 Infrared Spectroscopy: Theory 155 u Online edition for students of organic chemistry lab courses at the University of Colorado, Boulder, Dept of Chem and Biochem. (2002) Chapter 15 Infrared Spectroscopy: Theory An important tool of the organic chemist is Infrared Spectroscopy , or IR. IR spectra are acquired on a special instrument, called an IR spectrometer. IR is used both to gather information about the structure of a compound and as an analytical tool to assess the purity of a compound. IR spectra are quick and easy to run, and since the teaching labs are fortunate enough to have three of these instruments, students are encouraged to acquire spectra of compounds made in the laboratory throughout the Frst and second semesters of the lab course. 15.1 The Electromagnetic Spectrum Infrared refers to that part of the electromagnetic spectrum between the visible and microwave regions. Electromagnetic spectrum refers to the seemingly diverse col- lection of radiant energy, from cosmic rays to X-rays to visible light to microwaves, each of which can be considered as a wave or particle traveling at the speed of light. These waves differ from each other in the length and frequency, as illustrated in ±igure 15.1. ±requency, ν (nu), is the number of wave cycles that pass through a point in one sec- ond. It is measured in Hz, where 1 Hz = 1 cycle/sec. Wavelength, λ (lambda), is the length of one complete wave cycle. It is often measured in cm (centimeters). Wave- length and frequency are inversely related: and where c is the speed of light, 3 x 10 10 cm/sec Energy is related to wavelength and frequency by the following for- mulas: where h = Planck’s constant, 6.6 x 10 –34 joules-sec Note that energy is directly proportional to frequency and inversely proportional to wavelength. The IR region is divided into three regions: the near, mid, and far IR (see ±igure 15.2). The mid IR region is of greatest practical use to the organic chemist. This is the region of wavelengths between 3 x 10 –4 and 3 x 10 –3 cm. Chemists prefer to work with numbers which are easy to write; therefore IR spectra are sometimes reported in μ m, although another unit, ν (nu bar or wavenumber ), is currently pre- ferred. Eq. 1 ν c λ -- = c = Eq. 2 Eh hc ----- ==

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Online edition for students of organic chemistry lab courses at the University of Colorado, Boulder, Dept of Chem and Biochem. (2002) The Electromagnetic Spectrum15 Infrared Spectroscopy: Theory 156 A wavenumber is the inverse of the wavelength in cm: where ν is in units of cm –1 , λ is in units of cm and now: In wavenumbers, the mid IR range is 4000–400 cm –1 . An increase in wavenumber corresponds to an increase in energy. As you will learn later, this is a convenient relationship for the organic chemist. Infrared radiation is absorbed by organic molecules and converted into energy of molecular vibration. In IR spectroscopy, an organic molecule is exposed to infrared radiation. When the radiant energy matches the energy of a speciFc molecular vibra- tion, absorption occurs. A typical IR spectrum is shown below. The wavenumber,
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IRtheory - Online edition for students of organic chemistry...

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