Chm 115 F13 Lect 20

B c 85 m 91 m 97 m 25 question 2 h the molecule

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Unformatted text preview: B. C. 8.5 m 9.1 m 9.7 m 25 Question 2 H • The molecule formaldehyde molecule formaldehyde modes. CO H has six vibrational has six vibrational • The hydrogen atoms have an asymmetrical and a symmetrical stretch with the following frequencies: 8.55 x 1013 s-1, 8.36 x 1013 s-1. • What wavelength corresponds to these vibrations? • Find the energy in J of each vibration. Which vibrational mode uses the most energy? 26 13 Using IR spectroscopy to Identify Functional Groups • Because IR frequencies provide information about bonds between atoms, it is very useful it for identifying functional groups: Functional Group Characteristic Vibration wavelength -OH -Cl C=O CN O-H stretch stretch C-Cl stretch C=O stretch C N stretch 2.7 14.3 5.8 4.4 The IR stretching frequency is often highly characteristic for a given functional group m m m m 27 Three C4 Compounds C2H5OC2H5 C4H9OH C2H5NHC2H5 increases Energy increases 28 14 Summary • Light interacts with matter – Bonds break, vibration, rotation • Uneven distribution of charge in molecule or distribution of charge in molecule or upon vibration allows for absorption of IR light – CO2, N2O, H2O, and chlorofluorocarbons (CFCs) are Greenhouse gases – Identification of functional groups – Absorption frequencies depend on mass of atoms bonded and bond strength 29 15...
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