june 3 lecture _spectroscopy intro

7/31/2019 June 3 Lecture _Spectroscopy Intro

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Chapter17: Fundamentals of Spectrophotometry

HW: To be posted after class


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Electromagnetic Radiation

-rays 0.001 - 0.01 nm Nuclear transitions

X-rays .01 -10 nm Core e-s

Ultraviolet 10 - 400 nm Valence e-s

Visible 400 - 800 nm Valence e-s

Infrared 800 - 106 nm Molecular Vibrations

Microwave 106 - 108 nm Molecular Rotations

Radio 108 - 1011 nm Nuclear Spin


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The Visible Spectrum The visible spectrum

400 nm 750 nm

IRUV


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Energy/Photons

-rays 10-14 J/photonX-rays 10

-16J/photon

Ultraviolet 10-18 J/photonVisible 10-19 J/photon

Infrared 10-20

J/photon

Microwave 10-22

J/photon

Radio 10-24

J/photon


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Absorption Energy Levels

E

N

E

R

G

Y

Electronic level

Rotational level

Vibrational level


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E = UV/Vis photon

E = IR photon

E = wave photon

E

N

E

R

G

Y

Absorption Energy Levels


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What happens to excited molecule?

1. UV-Vis Absorption: from ground state (S0) toexcited state (S1)

10-15 seconds

S0 T1: Spin-Forbidden (low probability)

2. Vibrational Relaxation: excited molecule bumps intosolvent, drains some E (radiationless transition)

10-12 seconds

Cascade down manifold of excited vibrationallevels (not electronic)

3. Internal Conversion: radiationless transition betweenstates with the same spin quantum number (S1 to S0)


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Fluorescence/Phosphorescence

4. Fluorescence: the emission of radiation from excited

state to ground state (into range of vibrational states)

10-6 to 10-10 seconds

5. More internal conversion: non-radiant relaxation

(spreading out energy/heat)

6. Intersystem crossing: radiationless transition between

states with different spin quantum numbers (S1

to T1)

7. Phosphorescence: emission of radiation from T1 to S1)

> 10-6 seconds (very long: different spins!)


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Luminescence

For fluorescence the intensity is

I = KP02.3bc

Measure absolute number of photons

I is DEPENDENT on P0

K depends upon:

efficiency of fluorescence

light collection efficiency


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1. Significant overlap across

the entire spectrum.

2. Regions of no overlap

A

X

Y A

XY

]Y[b]X[bA YXm


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Know that absorbance of mixture of X and Y at

any l is

Prepare standard solutions of X and Y

]Y[b]X[bA YXm

SXX ]X[bA S

SYY ]Y[bA S

S

X

X

b[X]

A

S

S

Y

Yb[Y]

A

S

Case 1


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SX

Y

SX

m

]X[

]X[

A

A

]Y[

]Y[

A

A

S

S

S

b[Y]

b[Y]

Ab[X]

b[X]

AA

S

Y

S

X

mSS

S

Y

S

X

m[Y]

[Y]A

[X]

[X]AA SS


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SX

Y

SX

m

]X[

]X[

A

A

]Y[

]Y[

A

A

S

S

S

y = m x + b

Case 1


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How do we get enough data points to construct a line?

Case 1

SX

m

A

A

S

S

X

Y

A

AFind[Y]fr

omslope

Find [X] from y-intercept

SX

m

A

A

S

S

X

Y

A

AFind[Y]fr

omslope

Find [X] from y-intercept

SX

Y

SX

m

]X[

]X[

A

A

]Y[

]Y[

A

A

S

S

S


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i. measure absorbance at lmax for X

ii. measure absorbance at lmax for Y

A

X

Y

l l

]Y[b]X[bA YX

]Y[b]X[bA YX

Case 2


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Isosbestic points

Analytical useful if one analyte (X) can be converted toanother (Y) and/or vice versa.

Examples:

X = B (base) and Y = HB+ (conjugate acid)

X = HA (acid) and Y = A- (conjugate base)

Characteristic feature of mixture of X and Y Abs spectra

REQUIREMENT:

Absorbance spectra of pure X and pure Y must cross


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Absorbance spectra of separate solutions of

HA and A- prepared at same concentration

HA

A- ]HA[bA465HA

465HA

]A[bA 465A

465

A

465

A

465

HA


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Vary pH of a solution to vary principal species

pKa = 5.1

HA A- + H+

Will these peaks necessarily be equal?


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]A[b]HA[bA 465A

465HA

465m

465

A

465

HA

])A[]HA([bA465465

m

Mass balance: ([HA] + [A-]) = constant

The Absorbance at 465 is constant!


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Other analytical uses:

Monitor progress of titration (equivalence point) by

following absorbance.

Useful in situations where analyte and producthave very different values at a given l

Example: analyte has color but product does not,or vice versa.


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Due to large number of vibrational states,

IR spectra can be quite complex


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IR Absorption: real strength is ID

Functional Group Wavenumber(cm-1)

Wavelength( m)

C-H, aliphatic 3000-2850 3.3-3.5

C-H, aromatic 3150-3000 3.2-3.3

O-H 3600-3000 2.8-3.3

C=O,

aldehyde/ketone

1740-1660 5.7-6.0

-CH2Cl 1300-1200 7.6-8.2

C-C 850-890 13.2-14.0


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june 3 lecture _spectroscopy intro

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