read-CH 15 UV VIS

7/27/2019 read-CH 15 UV VIS

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X-ray:

core electronexcitation

UV:

valanceelectronic

excitation

IR:

molecularvibrations

Radio waves:

Nuclear spin states(in a magnetic field)

Electronic Excitation by UV/Vis Spectroscopy :


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The wavelength and amount of light that a compound absorbs depends on

its molecular structure and the concentration of the compound used.

The concentration dependence follows Beers Law.

A=ebc = log I/I0Where A is absorbance

e is the molar absorbtivity with units of L mol-1 cm-1b is the path length of the sample (typically in cm).

c is the concentration of the compound in solution, expressed in mol L-1


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UV Spectrum of Isoprene

=>


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- Single bonds are usually too high in excitation energy for most instruments (185 nm)

vacuum UV

most compounds of atmosphere absorb in this range, so difficult to work with.

- Types of electron transitions:

i) s, p, n electrons

Sigma (s) single bond electron

Low energy bonding orbi ta l High energy ant i -bonding orbi ta l


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Pi (p) double bond electron

Low energy bonding orbi ta l High energy ant i -bonding orbi ta l

Non-bonding electrons (n): dont take part in any bonds,

neutral energy level.

Example:Formaldehyde


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C C

p p

Example: ethylene absorbs at max = 165 nm e= 10,000

= hv=hc/

s

s

hv

p

p

s

s

p

p


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C O

pn

The n to p* transition is at even longer wavelengths but is notas strong as p to p* transitions. It is said to be forbidden.

Example:

Acetone: ns max = 188 nm ; e= 1860

np max = 279 nm ; e= 15

s

s

hv

p

p

n

s

s

p

p

n


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C C

C C

C O

C O

H

s s 135 nm

p p165 nm

n s 183 nm weak

p p 150 nm

n s 188 nmn p 279 nm weak

A

188 nm

279 nm

C O


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C C

HOMO

LUMO

Conjugated systems:

Preferred transition is between Highest Occupied Molecular Orbital(HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).

Additional conjugation (double bonds) lowers the

HOMO-LUMO energy gap:


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O

O

O

Similar structures have similar UV spectra:

max = 238, 305 nmmax = 240, 311 nm max = 173, 192 nm


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ss* transition in vacuum UV

ns* saturated compounds with non-bonding electrons

n ~ 150-250 nm

e ~ 100-3000 ( not strong)

np*, pp* requires unsaturated functional groups (eq. double bonds)

most commonly used, energy good range for UV/Vis

n ~ 200 - 700 nm

np* : e ~ 10-100

pp*: e ~ 1000 10,000

The valence electrons are the only ones whose energies permit them to be

excited by near UV/visible radiation.

s (bonding)

p (bonding)

n (non-bonding)

s (anti-bonding)

p (anti-bonding)Four types of transitions

pp*ns*np*

ss*


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ns*TransitionsStill rather high in energy. between 150 and 250 nm.

Not many molecules with ns* transitions in UV/vis region

max

emax

H2O 167 1480

CH3OH 184 150

CH3Cl 173 200

CH3I 258 365(CH3)2S 229 140

(CH3)2O 184 2520

CH3NH2 215 600

(CH3)3N 227 900


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np* andpp*TransitionsMost UV/vis spectra involve these transitions. pp* are

generally more intense than np*.

max emax type

C6H13CH=CH2 177 13000 pp*

C5H11CCCH3 178 10000 pp*

O

CH3CCH3 186 1000 ns*

O

CH3COH 204 41 np*

CH3NO2 280 22 np*

CH3N=NCH3 339 5 np*


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Ultraviolet Spectroscopy

200-400 nm photons excite electrons

from a p bonding orbital to a p*

antibonding orbital. Conjugated dienes have MOs that are

closer in energy.

A compound that has a longer chain ofconjugated double bonds absorbs light

at a longer wavelength. =>


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Chromophore Example Solvent max (nm) emax Type oftransition

Alkene n-Heptane 177 13,000 pp*Alkyne n-Heptane 178

196

225

10,000

2,000

160

pp*_

_

Carbonyl n-Hexane

n-Hexane

186

280

180

293

1,000

16

Large

12

ns*np*

ns*np*

Carboxyl Ethanol 204 41 np*

Amido Water 214 60 np*

Azo Ethanol 339 5 np*Nitro CH3NO2 Isooctane 280 22 np*Nitroso C4H9NO Ethyl ether 300

665

100

20

_

np*Nitrate C2H5ONO2 Dioxane 270 12 np*

C6H13HC CH2

C5H11C C CH3

CH3CCH3

O

CH3CH

O

CH3COH

O

CH3CNH2

O

H3CN NCH3

Absorption Characteristics of Some Common Chromophores


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Most organic spectra are complex

superimposedelectronic and vibration transitions

absorption bands usually broad

detailed theoretical analysis not possible,

effects of solvent & molecular details complicate

comparison


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- For Compounds with Multiple Chromophores:

If isolated (more than one single bond apart)

- e are additive

- constant

CH3CH2CH2CH=CH2 max= 184 emax = ~10,000

CH2=CHCH2CH2CH=CH2 max=185 emax = ~20,000

Ifconjugated - shifts to highers (red shift)

1,3 butadiene: max= 217 nm ; emax= 21,000

1,3,5-hexatriene max= 258 nm ; emax= 35,000


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- For Compounds with Multiple Chromophores:


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UV Spectral Nomenclature

Red Shift (Bathochromic) Peaks shift to longer wavelength.

Blue Shift (Hypsochromic) Peaks shift to shorter wavelength.


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Solvent Effects - IntensitySolvents can induce significant changes in the intensity of

peaks.

HyperchromicIncrease in absorption intensity.

HypochromicDecrease in absorption intensity.

Solvent max emax

Hexane 260 2000

Chloroform 263 4500

Ethanol 260 4000

Water 260 4000

Ethanol - HCl (1:1) 262 5200

Absorption characteristics of 2-methylpyridine


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Solvent effects

-> * transitions leads to more polar excitedstate that is more easily stabilized by polar

solvent associations (H-bonds). The *

state is more polar and stabilized more inpolar solvent relative to nonpolar one, thus

in going from nonpolar to polar solvent

there is a red shift or bathochromic shift(increase in max, decrease in E).


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Solvent effectsFor n -> * transition, the n state is much more

easily stabilized by polar solvent effects (H-bonds and association), so in going from

nonpolar to polar solvent there is a blue shift

or hypsochromic shift (decrease in max,increase in E).


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heptanemethanol

Hypsochromic shiftO

np*


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Solvent Effects


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AuxochromeSubstitutent groups which are not themselves optically active in this energy range, but

which do interact with other chromophores to shift both intensity and wavelength.

Derivative max emax

Pyridine 257 2750

2-CH3 262 3560

3-CH3 263 3110

4-CH3 255 2100

2-F 257 3350

2-Cl 263 3650

2-I 272 400

2-OH 230 10000

Absorption Characteristics of Pyridine Derivatives


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UVA and UVB

UVA 320nm to 400nm (indirect interaction) both tans and burns the skin

suppressing the immune system

immediate sunburn reactive oxygen species

UVB 290nm to 320nm (direct interaction) Skin cancer

Aging Delayed sunburn


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Tanning is based on the control of a complex series of natural

chemical reactions. When exposed to ultraviolet radiation

certain molecules in skin undergo rearrangement. Thisrearrangement leads to

formation of Vitamin D from cholesterol,

coloring of skin through the formation of melanins, and

burning or cancer.


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Tanning involves the formation of melanin polymers in our

skin. Melanin monomers are already present in the outer layer of

the skin, but in a reduced state. When oxidized upon exposure

to UV, the melanin polymer forms and absorbs light in the visibleand ultraviolet region.


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MED = smallest dose (J/m2) of UVB that

produces a delayed sunburn

SPF 34 should protect you from burning for

thirty-four times the time of unprotected skin.

Sun Protection Factor is defined as the ratio of delayed

sunburn on protected skin to unprotected skin, where the

protected skin is covered by 2mg/cm2 of sunscreen.

SPF is based on the physiological response

in the wearer and not based on a direct

comparison of the chemical properties or

dosages of the compounds being used


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Active Ingredients

Aminobenzoic acid 15% Octyl salicylate 5%

Avobenzone 3% Oxybenzone 6%

Cinoxate 3% Padimate O 8%

Dioxybenzone 3% Phenylbenzimidazole sulfonic acid 4%

Homosalate 15% Sulisobenzone 10%

Menthyl anthranilate 5% Titanium dioxide 25%Octocrylene 10% Trolamine salicylate 12%

Octyl methoxycinnamate 5% Zinc oxide 25%

(FDA, 1999, p27687)


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read-CH 15 UV VIS