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310NSp0801162008

# 310NSp0801162008 - CH 310N MWF 8am Lecture 2 W Reading...

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CH 310N – MWF 8am – Lecture 2 – W 1/16/2008 Reading Assignment : Sections 13.1–13.4 Last Time : Overview of course Introduction to spectroscopy Textbook Problems : 12.5 through 12.11 Graded Homework : HW#01 will be posted later this week (due 1/28) http://courses.cm.utexas.edu/bbocknack/ch310n/ (address is case-sensitive) Course Web Site : In particular, be sure to take a careful look at the course syllabus (copies are also available at the Lower Division Office) Today : Infrared spectroscopy (Chapter 12)

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Example : Absorption spectrum of 2-butanol exposed to infrared radiation Horizontal axis records wavelength (frequency) Vertical axis records % transmittance Baseline (at top) = no absorption Downward spike = energy absorption at that wavelength Spectroscopic techniques employed by organic chemists: Infrared spectroscopy (IR radiation) Functional groups? Nuclear magnetic resonance (NMR) (radio waves) UV/Vis spectroscopy (UV/visible light) electron system? Mass spectrometry (bombard with high energy electrons) Mass and formula?
Infrared (IR) Spectroscopy The IR region of the electromagnetic spectrum falls between the visible region and microwaves. ranges from 7.8 x 10 –7 m up to about 10 –4 m Only the middle of the IR region ( = 2.5 x 10 –6 m to 2.5 x 10 –5 m) is typically of interest to organic chemists, however. Some conventions : is frequently reported in units of micrometers ( m). 1 m = 10 –6 m IR frequencies are expressed in wavenumbers ( ) rather than in Hz ~ ~ = 1 (in cm) (units = cm –1 ) Wavenumber represents the number of wavelengths in 1 cm The useful IR region spans from 4000 cm –1 to 400 cm –1 Why does an organic molecule absorb some wavelengths of IR radiation but not others? Molecules are not static entities. The atoms within a molecule are in constant motion, because all molecules have energy distributed throughout their structure. Some of this energy causes bonds to stretch and contract, atoms to wag back and forth, etc. In other words, the absorption of IR radiation is associated with molecular vibrations .

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Consider a typical O–H bond (like the one in 2-butanol). The bond length is not fixed, but rather expands and contracts (stretches) over time. It’s like the atoms are connected by a spring which vibrates with a characteristic
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310NSp0801162008 - CH 310N MWF 8am Lecture 2 W Reading...

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