241section3-10-6

241section3-10-6 - Chem 241 - Summer 2010 Spectrophotometry...

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Chem 241 - Summer 2010 Spectroscopy 1 Spectrophotometry Spectrophotometry is the use of “light” in chemical measurements “Light” = electromagnetic radiation Spectrophotometry Electromagnetic radiation consists of perpendicular oscillating electric and magnetic fields 2 Described by wavelength and frequency Spectrophotometry = wavelength distance Spectrophotometry Units of Wavelength » nanometer (nm) = 10 -9 m » Angstrom (Å) = 10 -8 cm = 10 -10 m 1 »reciprocal centimeter (cm -1 ) is called the wavenumber = 1/ Spectrophotometry T T = period time Spectrophotometry 1/T = = frequency » cycles/second = Hertz (Hz) » radians/second - 1 cycle = 360°
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Chem 241 - Summer 2010 Spectroscopy 2 Spectrophotometry Spectrophotometry  = c = the speed of light in vacuum c = 3.00 x 10 8 m/s in other material c/n where n is the refractive index n 1; light travels slower in any media than in vacuum Spectrophotometry The wavelength of light determines how it interacts with matter. We use these interactions as a probe to obtain chemical information about to obtain chemical information about samples. Spectrophotometry Spectrophotometry Spectrophotometry The visible spectrum 400 nm 750 nm IR UV
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Chem 241 - Summer 2010 Spectroscopy 3 Spectrophotometry IR typically referred to by wavenumber (10-12,500 cm -1 ) nm 4000 1 7 2500 10 4000 1 ~ 1 cm cm nm nm Spectrophotometry micro and radio waves usually referred to by frequency » e.g. radio station G105 = 105 MHz  c c m s s m 86 . 2 10 105 / 10 3 1 6 8 Spectrophotometry Light as a particle = photon E = h = h c/ » h = Planck’s constant » 6.626 x 10 -34 Joule seconds (J-s) » shorter wavelength = higher energy Spectrophotometry Spectrophotometry Absorption Absorption of light increases the energy of a molecule or atom lowest energy state = ground state
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Chem 241 - Summer 2010 Spectroscopy 4 Absorption Absorption of a photon promotes a ground state atom or molecule to an excited state states are quantized i e discrete states are quantized, i.e. discrete levels, not continuous Absorption - UV He atom absorbs a UV photon, promotes electron to next level in shell » 1 2 + h 1 1 2 1 » 1s + 1s 2s Absorption - IR HCl absorbs an IR photon, increases vibrational energy HC l + h l * Absorption - microwave H 2 O absorbs a microwave photon, increases rotational energy H 2 O + h H 2 O* Absorption - Energy levels E Rotational level N E R G Y Electronic level Vibrational level Absorption - Energy levels E E = IR photon E = wave photon N E R G Y E = UV/Vis photon
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Chem 241 - Summer 2010 Spectroscopy 5 Absorption Electronic states are described by electron spin (+½ and -½) Most molecules in ground state have no net spin singlet (S) stat no net spin, singlet (S) state S 1 - first excited state singlet T 1 - first excited state triplet (T), net electron spin = 1 Absorption - Energy levels S 2 S 0 S 1 Absorption - Energy levels S 2 S 0 S 1
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This note was uploaded on 03/14/2011 for the course CHEM 241 taught by Professor Tiani during the Spring '08 term at UNC.

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241section3-10-6 - Chem 241 - Summer 2010 Spectrophotometry...

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