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XV–74
UV-vis (Electronic) Spectra-2014 -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
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:
ge g
exe ex
vibronic overlap often unresolved Born-Oppenheimer, separate integrals: elec(r) and nucl(R)
Intensity (A or F) ~ De-g = ex* g d 2
(Dipole strength) = (exe ex
* ge g
d 2
=(exe*g
e dr
2(ex
*g dR
2
integrated distribution over vib intensity F-C factor-vertical trans
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.459-60)
Top, left: Vibration Spacing reflect: A excited, F ground state Bottom: bigger potential shift, more distribution,
eventually get continuum (right, structureless—dissociate)
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 I2 electronic states- Many states, not all transitions seen – selection rules Plus each has own vibration energies
XV–78
Absorbance A = -log10 I/I0 = b c {b – path, c – conc.
– molecular property relate to dipole strength D
QM link: Intensity - A ~ D10 = 1* 0 d 2
Electronic Spectra – Broad - vibrations couple electronic
Spectra reflect: h = E a) change electronic energies
Eel = E1 – E0
b) change of vibration (note: frequencies differ)
Evib = (e+½)he – (g+½)hg
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 - IF ~ D01 same probability as absorbance
vibronic pattern differ – spacing g linear: measure IF ~ Iexcite (if excite by absorption) but measure fluor. signal against null background
extremely sensitive / can even do single molecule [Problem – other relaxation limit quantum yield]
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 gu 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
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.: H2O , MeOH -- closed shells, saturated CnH2n+2 , CnH2n-m Fm+2 -- light atoms LiF , CaF2 -- salts He, Ne, Ar – rare gas
b. UV (ultraviolet) (: 200-400 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 (: 400-700 nm , = 25,000-14,000 cm-1
)
need very delocalized system (-electron)
N +
N
N
N
N
porphyrin
retinal(off a bit)
dyes are like this-aromatic
or open shells – radicals N O_
transition metal Fe(CN)6-3
, CuII(SH)2(NH3)2 etc.
complexes : red blue
d. near-IR (: 700-2500 nm , = 14,000-4,000 cm-1
) mostly transition metals (d-d), open shells, NO,
1O2
XV–81
Benzene electronic spectra – * -displaced surfaces
vibronic progressions, vi = ±1, ±2,… totally sym. modes,
for first trans. forbidden, build on four asym modes vj = ±1
allowed transition A1gE1u at <200nm (intense, ~105),
Triplet trans. at ~330 nm, S=1 forbidden (very weak, ~10-3
)
XV–83
Comparison of porphyrin and hemoglobin absorb. with O2 & CO
Rhodopsin visible absorbance in dark and changes after exposure and adding 11-cis-retinal