I. Absorbing Species

• Absorption of light is a two step process:

Absorption M + h M*

Relaxation M* M + heat

• The heat evolved (very minute) does not affect the system temperature.

• There are other modes of relaxation that cause deviations in Beer’s Law.

A) Photodecomposition

B) Fluorescence

C) Phosphorescence

Applications of UV/Vi Molecular Absorption Spectrometry

Absorbing species containing , , and n electrons

(organic compounds).

Antibonding FormaldehydeBonding

Observed Electronic Transitions• * and n * are high energy, short

wavelength transitions. - * < 185 nm (Vacuum UV)

- n * = 150 – 250 nm (mainly vac. UV) - Very difficult to measure

• n * and *

200 – 700 nm - The most important and useful

transitions in molecular UV

spectroscopy.

- Molar absorptivities ():

n * 10 – 100 L cm-1 mol-1

* 1000 – 10,000

ethylene absorbs at longer wavelengths: max = 165 nm, = 10,000

hv

C C

C O

n

hv

n

nExample: Acetone

n-max =188 nm ; = 1860

nmax = 279 nm ; = 15

The n to π* transition is at even lower wavelengths but is not as strong as π to π* transitions. It is said to be “forbidden.”

1. Solvent Effect

As polarity increases, λ ↓ for

n π*

(shift to shorter λ, Blue

shift)

As polarity increases, λ ↑ for

π π*

(Shift to longer λ, Red shift)

As polarity increases, fine

structure ↓

(Fine structure due to vibrational

modes)

Factors that change transition energies

Absorption spectra for tetrazine

UV-VIS spectra of 4-methyl-3-penten-2-one in methanol (left) and

heptane (right). The ~320 nm absorption is the n π* transition,

the ~240 nm is mainly π π*

*n *n

In methanol In heptane

*

2. Organic ChromophoresMolecules having unsaturated bonds or free nonbonding

electrons that can absorb radiation of relatively low energy are

called chromophores. Examples include alkenes, alkynes,

ketones, aldehydes, phenyl and other aromatic species, etc.

2.a. Effect of Conjugation of ChromophoresAs conjugation is increased in a molecule, more delocalization

(stability) of the π electrons results. The effect of this

delocalization is to decrease the π * molecular orbital. The result

is a decrease in transition energy from π - π * and thus a red or

bathochromic shift. The molar absorptivity will increase in

this case and better quantitative analysis will be achieved.

• Conjugation causes delocalization of p electrons stabilizing p*, therefore shifting absorbance to longer wavelength (lower

energy).

Highly conjugated molecules are colored

Lycopene

β-Carotene

Absorption characteristics of some common chromophores