UV-vis (Electronic) Spectra Ch.13 Atkins, Ch.19 Engel

Transcription

1 XV 74 UV-vis (Electronic) Spectra Ch.13 Atkins, Ch.19 Engel Most broadly used analytical tech / especially bio-applic. inexpensive optics / solvent & cell usually not problem intense transitions sensitive, low concentrations broader transitions mix in vibrational excitation / low res. Optical Spectroscopy Processes diagram But some molecules don t absorb in UV-region >200nm all absorb in vac. UV (<200nm) e.g. salts, ions, saturated molecules: hydrocarbons, sugars, alcohols, etc. UV - -systems, open shells, broad less detail structure many do not fluoresce, compete paths for energy transfer

2 XV 75 Basic idea excite electrons to a new state Thus - new potential surface, i.e. vibrations will differ Franck-Condon Principle vertical transitions Nuclear motion slow compared to transition time effectively frozen nuclei In excited state, molecule first relaxes to new equilibrium structure, then fluoresces Vibrational energy goes to solvent vibrational relaxation Mirror image spectra A absorbance F fluorescence broad bands many component Vibronic transitions: g e g ex e ex vibronic overlap often unresolved Born-Oppenheimer, separate integrals: elec(r) and nucl(r) Intensity (A or F) ~ D e-g = ex * g d 2 (Dipole strength) = ( ex e ex * g e g d 2 = ( e ex * e g d r 2 ( ex * g d R 2 integrated distribution over vib intensity F-C factor-vertical trans

3 XV 76 F-C allowed transitions Vertical excitation of electrons, means nuclei stay near minimum of originating surface. Favor vibrations at turning point reference to minimum of other state. Multiple vibrations get excited but with different frequencies, relative intensity given by square of overlap of vibrational functions, initial and final states F-C envelop Potential energy surfaces shapes Atkins above, Engel (p ) Top, left: Vibration Spacing reflect: A excited, F ground state Bottom: bigger potential shift, more distribution, eventually get continuum (right, structureless dissociate)

4 XV 77 Shift of potential surfaces reflected in F-C bandshape, excitation to continuum, broad structureless, dissociating state Gap - Absorb and Fluor shift, different geometry vibs closer, bond strength Molecule - electronic energies change with nuclear positions, and gives rise to different vibrational levels Ex. Potential energies of I 2 electronic states- Many states, not all transitions seen selection rules Plus each has own vibration energies

5 XV 78 Absorbance A = -log 10 I/I 0 = b c {b path, c conc. molecular property relate to dipole strength D QM link: Intensity - A ~ D 10 = 1 * 0 d 2 Electronic Spectra Broad - vibrations couple electronic Spectra reflect: h = E a) change electronic energies E el = E 1 E 0 b) change of vibration (note: frequencies differ) E vib = ( e +½)h e ( g +½)h g initial state typically g = 0 but small g or high T hot band absorb from g 0 most probable vertical transition (Franck-Condon) Fluorescence if relax to e = 0 then can emit photon Can be mirror image of Absorption, but fluorescence Vibrational progression reflects lower state Intensity - I F ~ D 01 same probability as absorbance vibronic pattern differ spacing g linear: measure I F ~ I excite (if excite by absorption) but measure fluor. signal against null background extremely sensitive / can even do single molecule [Problem other relaxation limit quantum yield]

6 XV 79 Ex. absorption/fluorescence spectra vertical surface Selection rules less simple than for rotations and vibrations a. Molecule must change dipole moment, normally change electronic states where charge is dislocated (if center of symmetry g u allowed, polyatomic use symmetry) b. Spin not affected by E-field (light) S = 0 c. Between states, vibrations change - v = 0, ±1, ±2,.. But rotations restricted: J = 0, ±1

7 XV 80 What kind of molecules have measurable Absorbance? a. All absorb vacuum UV ( < 200 nm, > 50,000 cm -1 ) everything eventually (shorter ) absorbs Closed shell, saturated, light atoms only at higher (vacuv) e.g.: H 2 O, MeOH -- closed shells, saturated C n H 2n+2, C n H 2n-m F m+2 -- light atoms LiF, CaF 2 -- salts He, Ne, Ar rare gas b. UV (ultraviolet) ( : nm, = 50-25,000 cm -1 ) big contribution are -systems aromatics, polyenes, conjugated hetero atom: O O O O N H + lone pair delocalize plus heavier atom systems S S C I (also Cl -, Br -...) c. in Visible ( : nm, = 25,000-14,000 cm -1 ) need very delocalized system ( -electron) N N + N N retinal (off a bit) N porphyrin N O _ dyes are like this-aromatic or open shells radicals transition metal Fe(CN) -3 6, Cu II (SH) 2 (NH 3 ) 2 etc. complexes : red blue d. near-ir ( : nm, = 14,000-4,000 cm -1 ) mostly transition metals (d-d), open shells, NO, 1 O 2

8 XV 81 Benzene electronic spectra * -displaced surfaces vibronic progressions, v i = ±1, ±2, totally sym. modes, for first trans. forbidden, build on four asym modes v j = ±1 allowed transition A 1g E 1u at <200nm (intense, ~10 5 ), Triplet trans. at ~330 nm, S=1 forbidden (very weak, ~10-3 )

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10 XV 83 Comparison of porphyrin and hemoglobin absorb. with O 2 & CO Rhodopsin visible absorbance in dark and changes after exposure and adding 11-cis-retinal

11 XV 84 Transition metal complexes open shells, visible absorb d d transitions are weak because l ~ 0 Mn +2 - d 5, ground state - 6 A 1g ( 6 S related) absorbance very weak, S 0