CHEM3440Lec8F06 - CHEM*3440 Chemical Instrumentation Topic 8

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Unformatted text preview: CHEM*3440 Chemical Instrumentation Topic 8 Ultraviolet/Visible Absorption Spectroscopy UV-Visible Electronic Transitions This technique is mainly a study of molecules and their electronic transitions. Molar absorptivity ( " ) ranges from 0 to 10 5 . Transitions with " < 10 3 are considered to be of low intensity. In organic molecules, most bonds are excited by # < 185 nm (vacuum UV). Most functional groups have lone pairs whose energies place them in the near UV and visible range. These groups are called chromophores. Valence Electronic Structure The valence electrons are the only ones whose energies permit them to be excited by near UV/Visible radiation. " * (anti-bonding) # * (anti-bonding) n (non-bonding) # (bonding) " (bonding) Four types of transitions Too high energy (VUV) High energy but accessible Most common transitions in UV/Visible range n ! ! * Transitions Still quite high in energy; ! is between 150 and 250 nm. Not many molecules with n ! ! * transitions in UV/Visible region. " max # max H 2 O 167 1480 CH 3 OH 184 150 CH 3 Cl 173 200 CH 3 I 258 365 (CH 2 ) 2 S 229 140 (CH 2 ) 2 O 184 2520 CH 3 NH 2 215 600 (CH 3 ) 3 N 227 900 n ! ! * and ! ! ! * Transitions Most UV/Visible spectra involve these transitions. # ! # * are generally more intense than n ! # *. " max # max Type C 6 H 13 CH=CH 2 177 13000 # ! # * C 5 H 11 C C-CH 3 178 10000 # ! # * (CH 3 ) 2 C=O 186 1000 n ! " * CH 3 COOH 204 41 n ! # * CH 3 NO 2 280 22 n ! # * CH 3 N=NCH 3 339 5 n ! # * Solvent Effects - Position Solvents can interact with analyte molecules and shift the position and intensity of their absorbance peaks. Red Shift (Bathochromic) - Peaks are shifted to longer wavelength. Blue Shift (Hypsochromic) - Peaks are shifted to shorter wavelength. n ! # * transitions are generally blue shifted; arises from the solvation of and hydrogen bonding to the lone pair of electrons making the non-bonding level more stable). Shifts are large (up to 30 nm). Both n ! # * and # ! # * are red shifted; arises from attractive polarization forces and it increases with increasing solvent polarity. Shifts are small (less than 5 nm). Solvent Effects - Intensity Solvents can also induce signiFcant changes in the intensity of peaks. Hyperchromic - Increase in absorption intensity Hypochromic - Decrease in absorption intensity tion Characteristics of 2-methylpyridine " max # max 260 2000 263 4500 Auxochrome Substituent groups which are not themselves optically active in this energy range but who interact with other chromophores and shifts both intensity and position. Absorption Characteristics of Pyridine Derivatives Derivative " max # max pyridine 257 2750 2-CH 3 262 3560 3-CH 3 263 3110 4-CH 3 255 2100 2-F 257 3350 2-Cl 263 3650 2-I 272 400 2-OH 230 10000 Typical Organic UV/Vis Spectra Note how the same molecule displays markedly different spectral features depending upon the environment in which it Fnds itself....
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CHEM3440Lec8F06 - CHEM*3440 Chemical Instrumentation Topic 8

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