Bathochromic shift hypsochromic shift the uv

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“bathochromic” shift “hypsochromic” shift
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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
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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): HOMO u bonding molecular orbital LUMO g antibonding molecular orbital h 170nm photon Example for a simple enone π π n π π n π* π π n π* π* π* π* π* π* π* π* - *; max =218 =11,000 n- *; max =320 =100
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How Do UV spectrometers work? Two photomultiplier inputs, differential voltage drives amplifier. 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 Rotates, to achieve scan
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Diode Array Detectors Diode array alternative puts grating, array of photosens. Semiconductors after the light goes through the sample. Advantage, speed, sensitivity, The Multiplex advantage Disadvantage, resolution is 1 nm, vs 0.1 nm for normal UV Model from Agilent literature. Imagine replacing “cell” with a microflow cell for HPLC!
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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 Most compounds have “end absorbance” at lower frequency. Unfortunately solvent cutoffs preclude observation. You will find molar absorbtivities in L•cm/mol, tabulated. 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
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An Electronic Spectrum Absorbance Wavelength, , generally in nanometers (nm) 0.0 400 800 1.0 200 UV Visible max with certain extinction Make solution of concentration low enough that A≤ 1 (Ensures Linear Beer’s law behavior) Even though a dual beam goes through a solvent blank, choose solvents that are UV transparent. Can extract the value if conc. (M ) and b (cm) are known UV bands are much broader than the photonic transition event. This is because vibration levels are superimposed on UV.
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Solvents for UV (showing high energy cutoffs) Water 205 CH 3 C N210 C 6 H 12 210 Ether 210 EtOH 210 Hexane 210 MeOH 210 Dioxane 220 THF 220 CH 2 Cl 2 235 CHCl 3 245 CCl 4 265 benzene 280 Acetone 300 Various buffers for HPLC, check before using.
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Organic compounds (many of them) have UV spectra From Skoog and West et al. Ch 14 One thing is clear
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