Lecture_15 - Energy ("light") + Matter...

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Unformatted text preview: Energy ("light") + Matter (molecules) region of spectrum gamma UVC Wavelength range pm nm 200280 nm molecular effects ionizing break bonds ...and this means... damage DNA Highly damaging; totally absorbed by O2 and O3 in upper atmosphere Sunburn and skin cancer. Absorbed by O3 in stratosphere. A lot reaches surface of earth through atmosphere; can result in eye damage; tanning Results in color spectroscopy to look at structures of molecules Microwave ovens UVB 280320 nm Break bonds UVA 320400 break bonds/ electron transitions Electron transitions Bond vibrations bond rotations Visible IR (near) Microwave 400700 nm 7005000 nm 1 mm 1 m An example of a Photochemical reaction with visible light Cl2 + hv 2 Cl 2 Cl + H2 2 HCl (net) first step initiated by blue/violet light (visible) Demo Photchemistry in the Stratosphere: UV and ozone O3 + h(2 8 0 - 2 0 nm) O2 + O 3 O2 + O O3 Ozone in upper atmosphere absorbs in the UVB range. It is then spontaneously regenerated from products. Ozone protects us from damaging radiation. How do we see stuff that is colored? White Light We don't see wavelengths that are absorbed We see the wavelengths that get reflected (or transmitted) The eye as a visible light detector How does the eye detect visible light? Retinal Absorption of light by retinal in rhodopsin triggers a cis to trans geometry change Absorption spectra for retinal in different areas in rhodopsin The wavelength where retinal absorbs depends on the environment in which it is in, which generates different "cones." Together, the rod and cones absorb over the entire visible region of the electromagnetic spectrum The Amazing Eye! Just because most organic molecules don't absorb VISIBLE doesn't mean they don't absorb anything We may not be able to "see" UV or IR light with our eyes, but we can detect it with our bodies IR radiation is detected as heat UV radiation is detected as ... sunburn Absorption depends on concentration Absorbance = x l x C = molar absorptivity l = path length C = concentration The next two laboratory experiments deal with absorption of visible light and determination of concentration. Not everything that is clear and colorless is the same: how can we tell them apart? Although we can't always see differences in organic materials with our eyes, we can characterize them if we look for absorption in the IR IR absorption: due to vibrations of bonds in molecules; best suited for distinguishing/identifying functional groups Detecting an IR Spectrum CH4 IR Source CH4 Molecular vibrations: IR absorption http://icn2.umeche.maine.edu/genchemlabs/IR/ Atoms bonded together are not stationary, they vibrate. Bonds Stretch and Bend Simple model: two atoms connected by a spring Adding Energy (via IR absorption) makes the atoms vibrate with more energy; the vibrational energies are quantized (just like N levels in atoms) Interpreting IR stretching frequencies IR absorption frequencies depend on _____________________________________ ______________________________________ For the same bond order, _________ mass atoms vibrate more slowly; the energy required to excite the vibration is lower The CD stretching frequency is lower than the CH stretching frequency Pairs with higher bond energies will generally vibrate faster (have shorter wavelength IR absorption (c = * )) trend in vibrational frequency HF > HCl > HBr > HI Bond energies ____ _____ ____ ______ C C=C, & C C C, CH3CH2CH2CH2CH2CH3 C=C stretch CH3CH2CH2CH2CH=CH2 CH3CH2CH2CH2C CH C(triple bond)C stretch s C-C stretches & H-C-H bends increases C-H stretches Energy increases A. CC B. CH C. CO D. CCl 9.1 m 3.4 m 9.7 m 14.3 m CPS #2 - buddy Which molecule has the lowest XH stretching frequency? A. Using IR spectroscopy to Identify Functional Groups Because IR frequencies provide information about bonds between atoms, it is very useful for identifying functional groups: Characteristic Vibration OH stretch CCl stretch C=O stretch CN stretch Frequency Functional Group OH CL C=O CN 2.7 m 14.3 m 5.8 m 4.4 m The IR stretching frequency is often highly characteristic for a given functional group Three C4 Compounds C2H5OC2H5 ________________ ________________ ________________ increases Energy increases Detection of light by biological molecules (not including the eye) Your skin contains a wide variety of organic molecules, including those that contain double bonds. Thus, you can detect Sunlight (UV), and Heat (IR) with your eyes closed! ...
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This note was uploaded on 04/07/2008 for the course CHEM 115 taught by Professor L during the Fall '02 term at Purdue.

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