Chem 115 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 transition bond vibrations bond rotations ___Visible____ 400700 nm _ IR (near)_ Microwave 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 = * l * 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 Atoms bonded together are not stationary, they vibrate. _Bonds stretch and angles 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 mass of atoms bonded together_ Strength of the interaction between the atoms_ For the same bond order, larger 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 565 427 363 295 Note that this means that the HI absorbs at the largest wavelength C C=C, & C C C, CH3CH2CH2CH2CH2CH3 CH3CH2CH2CH2CH=CH2 CH3CH2CH2CH2C CH s increases Energy increases Which molecule has the strongest CC bond? Absorption Wavelength A. CC B. CH C. CO D. CCl 9.1 m 3.4 m 9.7 m 14.3 m Smallest wavelength equals strongest bond CPS #2 - buddy Which molecule has the lowest XH stretching frequency? Heavier atoms will have a lower frequency A. B. C. A H2O B NH3 C PH3 C Answer= 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 2.7 m 14.3 m 5.8 m 4.4 m Functional Group __OH_____ ____CL___ ____C=O___ ____C O_ The IR stretching frequency is often highly characteristic for a given functional group Three C4 Compounds _____C2H5_OC2H5__ ____C4H9OH____ ______C2H5NHC2H5 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 Spring '02 term at Purdue University-West Lafayette.

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