Electronic Spectroscopy Ultraviolet and visible
Where in the spectrum are these transitions?
Why should we learn this stuff Why should we learn this stuff? After all, nobody solves structures with UV any longer! Many organic molecules have chromophores that absorb UV UV absorbance is about 1000 x easier to detect per mole than NMR Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in biochemistry, enzymology. Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors One wavelength may not be the best for all compound in a mixture. Affects quantitative interpretation of HPLC peak heights
What are the nature of these absorptions? Example: * transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empirical excited-states methods (Gaussian 03W): h 170nm photon LUMO g antibonding molecular orbital HOMO u bonding molecular orbital
The UV Absorption process * and * transitions: high-energy, accessible in vacuum UV (max <150 nm). Not usually observed in molecular UV-Vis. n * and * transitions: non-bonding electrons (lone pairs), wavelength (max) in the 150-250 nm region. n * and * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm. Any of these require that incoming photons match in energy the gap corrresponding to a transition from ground to excited state. Energies correspond to a 1-photon of 300 nm light are ca. 95 kcal/mol
How Do UV spectrometers work? Rotates, to achieve scan Matched quartz cuvettes Sample in solution at ca. 10-5 M. System protects PM tube from stray light D2 lamp-UV Tungsten lamp-Vis Double Beam makes it a difference technique Two photomultiplier inputs, differential voltage drives amplifier.
Experimental details What compounds show UV spectra? Generally think of any unsaturated compounds as good candidates. Conjugated double bonds are strong absorbers Just heteroatoms are not enough but C=O are reliable Transition metal complexes, inorganics Solvent must be UV grade (great sensitivity to impurities with double bonds) The NIST databases have UV spectra for many compounds
An Electronic Spectrum Absorbance Wavelength, , generally in nanometers (nm) 0.0 400 800 1.0 200 maxwith certain extinction UV Visible
Solvents for UV (showing high energy cutoffs) Water 205 CH3CN 210 C6H12 210 Ether 210 EtOH 210 Hexane 210 MeOH 210 Dioxane 220 THF 220 CH2Cl2 235 CHCl3 245 CCl4 265 benzene 280 Acetone 300 Various buffers for HPLC, check before using.
Organic compounds (many of them) have UV spectra One thing is clear Uvs can be very non-specific Its hard to interpret except at a cursory level, and to say that the spectrum is consistent with the structure Each band can be a superposition of many transitions Generally we don’t assign the particular transitions.
Beer-Lambert Law Linear absorbance with increased concentration--directly proportional Makes UV useful for quantitative analysis and in HPLC detectors Above a certain concentration the linearity curves down, loses direct proportionality--Due to molecular associations at higher concentrations. Must demonstrate linearity in validating response in an analytical procedure.
The Quantitative Picture (power in) P (power out) Transmittance: T = P/P0 Absorbance: A = -log10 T = log10 P0/P B(path through sample) The Beer-Lambert Law (a.k.a. Beer’s Law): A = ebc Where the absorbance A has no units, since A = log10 P0 / P e is the molar absorbtivity with units of L mol-1 cm-1 b is the path length of the sample in cm c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)
Interpretation of UV-Visible Spectra Transition metal complexes; d, f electrons. Lanthanide complexes – sharp lines caused by “screening” of the f electrons by other orbitals One advantage of this is the use of holmium oxide filters (sharp lines) for wavelength calibration of UV spectrometers. See Shriver et al. Inorganic Chemistry, 2nd Ed. Ch. 14
UV of Benzene in heptane Group K band () B band() R band Alkyl 208(7800) 260(220) -- -OH 211(6200) 270(1450) -O- 236(9400) 287(2600) -OCH3 217(6400) 269(1500) NH2 230(8600) 280(1400) -F 204(6200) 254(900) -Cl 210(7500) 257(170) -Br -I 207(7000) 258/285(610/180) -NH3+ 203(7500) 254(160) -C=CH2 248(15000) 282(740) -CCH 248(17000) 278(6500 -C6H6 250(14000) -C(=O)H 242(14000) 328(55) -C(=O)R 238(13000) 276(800) 320(40) -CO2H 226(9800) 272(850) -CO2- 224(8700) 268(800) -CN 224(13000) 271(1000) -NO2 252(10000) 280(1000) 330(140) Benzenoid aromatics UV of Benzene in heptane From Crewes, Rodriguez, Jaspars, Organic Structure Analysis