Unformatted text preview: Not everything that is clear and
colorless is the same substance:
How can we tell them apart?
DEMO: colored water, pure water, water with salt, tonic water;
Conductivity setup, black (UV) light source 1 Energy (“light”) + Matter (molecules)
region of spectrum wavelength molecular effects
range …and this means… pm – nm ionizing damage DNA UVC 200280 nm break bonds Highly damaging; totally absorbed by O2 UVB 280320 nm break bonds Sunburn and skin cancer. UVA 320400 A lot reaches surface of earth through atmosphere; can result in eye damage; tanning Visible 400700 nm IR (near) 7005000 nm break bonds/ electron transitions
bond vibrations Microwave 1 mm – 1 m bond rotations gamma and O3 in upper atmosphere Absorbed by O3 in stratosphere. Results in color
spectroscopy to look at structures of molecules
Microwave ovens An example of a “photochemical
reaction” with visible light
Cl2 + hν 2 Cl∙
2 Cl∙ + H2 2 HCl (net)
first step initiated by blue/violet light (visible) 3 Photchemistry in the Stratosphere:
UV and ozone
O3 + hν(2 8 0 −3 2 0 nm) O2 + O∙
nm) 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. 4 How do we see stuff that is colored? White Light
Wavelengths we don’t see because they are absorbed We see the wavelengths that get reflected (or transmitted)
demo 5 The eye as a visible light detector Although there is a large range in the wavelength and frequency of electromagnetic radiation, the eye can only detect in the visible region. How does the eye detect visible light?
Absorption of light by retinal in rhodopsin triggers a cis to trans geometry change 6 Absorption spectra for retinal in different
areas in rhodopsin
areas 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
7 The Amazing Eye! Just because 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 8 Spectroscopy
Different molecules can absorb light of different colors (energies). Demo 9 Absorption depends on
concentration 10 Absorption depends on concentration Absorbance = ε × l × C ε = molar absorptivity
l = path length
C = concentation
The next two laboratory experiments deal with
absorption of visible light and determination of
concentration. 11 Not everything that is clear and colorless
is the same: how can we tell them apart?
is Although we can’t always see differences in organic materials with our eyes, we can characterize them if we look for absorption in theIR IR absorption: happens due to vibrations of bonds in molecules; best suited for distinguishing/identifying functional groups 12 Detecting an IR Spectrum
CH4 13 Molecular vibrations: IR absorption
http://icn2.umeche.maine.edu/genchemlabs/IR/ 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)
14 Interpreting IR stretching
frequencies IR absorption frequencies depend on Mass of atoms bonded together
Strength of the interaction between the atoms 15 Interpreting IR stretching
frequencies For the same elements, larger mass atoms will vibrate at a lower frequency (more slowly) (spring demo) The CD stretching frequency is lower than the CH stretching frequency. So, what would that indicate about the relative wavelengths? CD would absorb at a longer wavelength than CH. 16 Interpreting IR stretching
frequencies Pairs with higher bond energies will generally vibrate at a higher frequency and have shorter wavelength IR absorption, c = ν*λ Trend in vibrational frequency, largest to smallest. HF > HCl > HBr > HI Bond 565 427 363 295
*Note that this means that HI absorbs at the largest wavelength. 17 C C=C, & C≡ C
CH3CH2CH2CH2CH2CH3 C=C stretch =CH2 stretch C≡ C stretch C-C Stretches &
H-C-H bends CH3CH2CH2CH2CH=CH2
CH3CH2CH2CH2C≡ CH C-H Stretches λ increases Energy increases
≡ C-H stretch 18 CPS question #1
Which pair of atoms in the table below has the weakest bond?
Pair Absorption Wavelength A. CC
D. CCl 9.1 μm
19 CPS #2 Which molecule has the lowest H – X stretching frequency?
B. NH3 C. PH3 20 Using IR spectroscopy to Identify
Functional Because IR frequencies provide information about bonds between atoms, it is very useful for identifying functional groups: Functional Group Characteristic Vibration wavelength OH
C≡N OH stretch
C≡N stretch 2.7 μm
4.4 μm The IR stretching frequency is often highly characteristic for a given functional group 21 Three C4 Compounds
C2H5OC2H5 C4H9OH C2H5NHC2H5
λ increases Energy increases 22 Detection of light by biological
molecules (not including the eye)
molecules 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!
23 Lecture 16 & 17 Reading
Sections 9.8 (part),
Section14.1-14.2, pp. 408-412;
pp. 24 ...
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- Spring '08
- Mole, Visible spectrum, Infrared, IR absorption