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UV-vis (Electronic) Spectra-2012 -Ch.12 Atkins, Ch.19 Engel

Most broadly used analytical tech / especially bio-applic.

inexpensive optics / solvent usually not problem

intense transitions sensitive, low concentrations

broader transitions – involve vibrational excitation

Optical Spectroscopy Processes diagram

But some molecules “don’t absorb” in UV-region >200nm

all absorb in vac. UV (

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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, first molecule relaxes to new equilibrium structure, then fluoresce Vibrational energy to solvent “vibrational relaxation” Mirror image spectra A – absorbance F – fluorescence Each broad band many component

“Vibronic” transitions: ge g

exe ex

vibronic overlap often unresolved Born-Oppenheimer idea, separate integrals: r and 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-vert. trans

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F-C allowed transitions Vertical excitation of electrons, nuclei stay near minimum of originating surface. Favor vibration with turning point over minimum of other state. Multiple vibrations excited, relative intensity given by overlap of vibrational functions, initial and final states F-C envelop

Potential energy surfaces shapes Engel (p.459-60) Top: Vibration Spacing: A excited, F ground state Bottom: bigger potential shift, more distribution,

eventually get continuum (structureless—dissociate)

Shift of potential surfaces reflected in F-C bandshape Gap - Absorb and Fluor shift, different geometry vibs closer, bond strength

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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

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

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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]

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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, 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

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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 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 …

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 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

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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

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Comparison of porphyrin and hemoglobin absorbance

with O2 & CO Rhodopsin visible absor-bance in dark and changes after exposure and adding 11-cis-retinal

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Transition metal complexes – open shells, visible absorb

dd transitions are weak because l ~ 0

Mn

+2 - d

5, ground state -

6A1g (

6S related) –

absorbance very weak, S≠0