UV-Vis Spectroscopy

CHAPTER 1

UV-VIS SPECTROSCOPY

General info:

Spectrometry is based on the absorption of photons by the analyte.

Spectrometric method: the sample solution absorbs electromagnetic radiation from an appropriate source and the amount absorbed is related to the concentration of the analyte in the solution.

E.g: copper solution is blue because it absorbs the yellow color from white light and transmits the remaining blue light. The higher[cu], the more yellow light is absorbed and the deeper the resulting blue color of the solution.

The electromagnetic spectrum- Review

A form of radiant energy that is propagated as a transverse wave

Vibrate perpendicular to the direction of propagation - a wave motion to the radiation

Continue…

The relationship between wavelength and frequency:

λ = wavelength in cm

ѵ = frequency in s-1 or Hz

c = velocity of light (3x1010 cm/s)

The wavelength varies from a few Å to several meters,

λ for UV & Vis – nanometer

λ for IR – micrometer, commonly used as wave number, ṽ in cm-1

c

v

Continue…

Electromagnetic radiation possesses a certain amount of energy called photon, related to the ν or λ by:

E = energy of the photon in ergs

h = Plank’s constant, 6.62 x 10-34 Jsec

Note: Shorter wavelength, greater energy

UV from the sun burns you!!!!

λ

hchvE

Working ranges of the UV-Vis and IR spectra

UV 200-380 nm

Vis 380-780 nm

Near IR 0.78-2.5 μm

Mid IR 2.5-15 μm

The most analytically region

How does matter absorb radiation Three basic processes by which a molecule can absorb radiation; all involve

raising the molecule to a higher internal energy level, the increase in energy being equal to the energy of the absorbed radiation, hν. Three type of internal energy are quantized;

Rotational transition: the molecule rotates about various axes, the energy of rotation being a definite energy levels, so the molecule may absorb radiation and be raised to a higher rotational energy level.

Vibrational transition: the atoms or groups of atoms within a molecule vibrate relative to each other, and the energy of this vibration occurs at definite quantized levels. The molecules may then absorb a discrete amount of energy and be raised to a higher vibrational energy level.

Electron transition: the electron of a molecule may be raised to higher electron energy. UV-VIS spectroscopy principal/theory

Energy level diagram illustrating energy changes associated with absorption of electromagnetic radiation

A = pure rotational changer (far IR) B = rotational + vibrational changes (near IR) C = rotational + vibrational + electronic transition (Vis + UV) Eo = electronic ground state E1 = first electronic exited state

Electronic Spectra and Molecule Structure

The electronic transition in the UV-VIS regions are due to the absorption of radiation by specific types of: Groups, bonds and functional groups within the molecule

Kinds of transitions: Electrons in a molecule can be classified into 4 different types: i. Closed shell electron – not involved in bonding ii. Covalent single bond electron, σ iii. Paired non bonding outer – shell electron (n electrons)

such as those on N, O, S, and halogens. These are less tightly held then electrons and can be excited by visible or UV radiation

iv. Electron in π orbitals, in = or ≡ bonds. These are the most readily excited and are responsible for a majority of electronic spectra in the UV-VIS regions.

* π and n electrons responsible for UV-VIS spectrum

Other electron remain/occupate in orbitals

A molecule also possesses unoccupied orbitals called antibonding orbitals; these correspond to excited- state energy levels (either σ* or π*). Therefore, absorption of radiation results in an electronic transition to antibonding orbitals.

The most common transition:

π π* or,

n π*

n σ*

Above 200 nm

Occur at very short λ

HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital

Example, molecule H2

Example of transition π → π* and n → π* in ketone (acetone)

The probability of π → π* is higher than n → π*

Molar absorptivities, ε

π → π* (1000-100,000)

n → π* (less than 1000)

π → π* n → π*

ABSORPTION BY ISOLATED CHROMOPHORES

The absorbing groups (the electrons responsible for the absorption) in a molecules are called chromophores

A molecule containing a chromophore – chromogen

The present of auxochrome enhance the absorption by a chromophore or shift the λ of absorption.

Example: hydroxyl (OH), amine (-NH2), halogens (X)

posses n electrons – interact with π electrons in the chromophores (n → π) conjugation.

Spectra changes can be classed as:

Bathochromic shift (red shift) – λmax shifted to shorter wavelength. It may caused by removal of conjugation.

Hypsochromic shift (Blue shift) - λmax shifted to shorter wavelength. It may caused by removal of conjugation.

Eg. The conjugation of the lone pair electrons on the n atom on aniline with the π bond system of the benzene ring is removed on protonation

Neutral aniline absorbs at λmax = 230 nm

ε = 8600

But in acid solution (protonated),

λmax = 203 nm

ε = 7500

Spectra changes can be classed as: continue….

Hyperchromic effect

– increased absorption intensity (ε↑)

Hypochromic effect

– decreased absorption intensity (ε↓)

Note: λmax = the wavelength of the absorption maximum

= Absorption [ ] of a 1% solution in a cell with 1 cm pathlength- used when MW of a compound is not known

)(log 010 I

I1%

1cmε

BEER’S LAW-ABSORPTION LAW

Fraction of radiant energy transmitted decays exponentially with pathlength.

Transmittance in exponential form:

Putting in logarithmic form,

0I Ic

b

Transmitted radiation

Incident radiation

Pathlength concentration

kb

I

IT 10

0

kbI

IT

0

loglog (Eq 2)

k is a constant (Eq 1)

BEER’S LAW-ABSORPTION LAW…continue

Similar law holds for the dependence of T on the concentration,

Combining (Eq 1) and (Eq 3), describes the dependence of T on the pathlength and concentration.

Omit the –ve sign, and rearrange…

Hence,

cck

I

IT '

0

10'k is a new constant (Eq 3)

ckI

IT 'loglog

0

(Eq 4)

abc

I

IT 10

0

a is combined constant of k and k’ (Eq 5)

abcI

IT

0

loglog

abcI

I

TTA 0log

1loglog

abcA

Absorbance Absorptivity Pathlength in cm

Conc in g/L

BEER’S LAW-ABSORPTION LAW…continue

Note: in absorption spectrum, the absorbance varies with wavelength in direct proportion to a (b & c are held constant). The product of the absorptivity and the molecular weight of the absorbing species is called the molar absorptivity,

Thus bcA

)( 11 Lmolcm

Example

Amines, RNH2 react with picric acid to form amine picrates, which absorb strongly at 359 nm (ε=1.25 x 104 ). An unknown amine (0.1155 g) is dissolved in water and dilute to 100 mL. A 1 ml aliquot of this is diluted to 250 ml for measurement. If this final solution exhibit an absorbance of 0.454 at 359 nm using a 1.00 cm cell, what is the formula weight of the amine? What is a probable formula?

Solution:

in original flask

bcA

ccmLmolcm 00.11025.1454.0 114

Lmolc /1063.3 5

mL

mLLLmol

00.1

100)250.0)(/1065.3( 5

mol41008.9

1

4127

1008.9

1155.0

gmol

mol

g

Measurement of the UV Spectrum:

Using a dilute solution: 1 mg of compound with MW

(100-400 g/mol) is dissolved in 100 mL volumetric

flask. Hexane is used as a solvent. Portion of the

solution is transferred to a cuvett quartz of 1 cm

(Styrene, MW = 104 g/mol

max=250 nm

Solvent Choice of solvent Solvent consideration: cheap, good solvent transparent down

to about 210 nm Most commonly used – 95% ethanol

Solvent effect Electronic transition, π → π*: atom do not move, however

electrons of the atom and solvent molecules may reorganise Transition state is more polar than ground state due to dipole-

dipole interaction with solvents, thus lower the energy of the excited state.

E.g.: Styrene in ethanol (λmax = 270 nm) Styrene in hexane (λmax = 250 nm) about 20 nm λ shift (red shift, longer wavelength)

Electronic transition, n → π* (the weak transition of the O lone pair in ketones.