Uv Visible Spectroscopy

•MOLECULAR SPECTROSCOPY

Spectroscopy • Study of the Interaction of Electromagnetic Radiation (Energy) and Matter • When energy is applied to matter it can be absorbed, emitted, cause a chemical change (reaction), or be transmitted. • Electromagnetic Spectrum:-

Cosmic γ (Gamma) Ultraviolet Visible Microwave Radio

X-Ray Infrared

THE ELECTROMAGNETIC SPECTRUM Frequency (ν )

high high

X-RAY

Energy

INFRARED MICROWAVE

ULTRAVIOLET

Vibrational infrared

Visible

Ultraviolet

2.5 µ m 200 nm

400 nm BLUE

short

low low

15 µ m

RADIO

Nuclear magnetic resonance 1m

800 nm RED

Wavelength (λ )

FREQUENCY

long

5m

THE SOLAR SPECTRUM

VISIBLE SPECTRUM (wavelengths of various radiations)

• • • • • • •

Violet: 400 - 420 nm Indigo: 420 - 440 nm Blue: 440 - 490 nm Green: 490 - 570 nm Yellow: 570 - 585 nm Orange: 585 - 620 nm Red: 620 - 780 nm

Electromagnetic Spectrum Type of Radiation

Frequency Range (Hz)

Wavelength Range

Type of Transition

gamma-rays

1020-1024

<1 pm

nuclear

X-rays

1017-1020

1 nm-1 pm

inner electron

ultraviolet

1015-1017

400 nm-1 nm

outer electron

visible

4-7.5x1014

750 nm-400 nm

outer electron

near-infrared

1x1014-4x1014

2.5 µm-750 nm

outer electron molecular vibrations

infrared

1013-1014

25 µm-2.5 µm

molecular vibrations

microwaves

3x1011-1013

1 mm-25 µm

molecular rotations, electron spin flips*

radio waves

<3x1011

>1 mm

nuclear spin flips*

* for energy levels split by a magnetic field

A typical spectrophotometer (or) spectrometer

Spectroscopy Types: – Ultraviolet Spectroscopy (UV) – Electronic Energy States • Use –Conjugated Molecules; Carbonyl Group, Nitro Group – Infrared Spectroscopy (IR) – Vibrational Energy States • Use – Functional Groups; Compound Structure – Nuclear Magnetic Resonance (NMR) – Nuclear Spin States • Use – The number, type, and relative position of protons (Hydrogen nuclei) and Carbon-13 nuclei – Mass Spectrometry (MS) – Hi-Energy Electron Bombardment • Use – Molecular Weight, Presence of Nitrogen, Halogens

UV Spectroscopy Observed electronic transitions

σ∗

Unoccupied levels

π∗ Energy

Atomic orbital

n

Atomic orbital

Occupied levels

π σ Molecular orbitals

UV-Spectroscopy

electrons are

UV Spectroscopy From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy:

σ∗ π∗ Energy

n

π σ

σ

σ ∗ alkanes

σ

π ∗ carbonyls

π

π ∗ unsaturated cmpds.

n

σ ∗ O, N, S, halogens

n

π ∗ carbonyls

UV transition type • 1.σ→σ* transitions: for compounds with σ bond only, high ΔE, short λ (< 200 nm).

•

Appears in saturated hydrocarbons with σ orbital and transition to antibonding σ* or to molecular Rydberg orbital (higher valence shell orbitals, 3s, 3p, 4s, …), and involves large ΔE, and small λmax that appears in far-UV region.

• Eg:- cyclopropane λmax 190 nm. • cycloalkane λmax 135 nm. (vacum UV)

UV transition type • 2. n→π* transitions : the excitation of an electron on an nonbonding orbital, such as unshared pair e’s on O, N, S,..to an antibonding π*, usually in an double bond with hetero atoms, such as C=O, C=S, N=O, etc. A sym. forbidden and low intensity transition. • Ex:- saturated aldehydes and ketones : λmax at 185-300 nm.

UV transition type • 3.π→π* transitions : for compounds containing double, triple bonds, or aromatic rings; a π electron is excited to an antibonding π* orbital. This is usually a sym. allowed and high intensity transition. • Ethylene : absorbs at 162 nm (10000), in vacuum UV. Extended conjugation lowers, ΔE, and increase in λmax, if extended beyond 5 double bonds then getting into visible region.

UV transition type • 4. n→σ* transitions: excitation from nonbonding orbital to an antibonding σ* orbital. Ex:- CH3OH(vap.) 183 nm (ε 150) NEt3(vap.) 227 nm (900) MeI(hexane) 258 nm (380)

Terms describing UV absorptions 1.

Chromophores: functional groups that give electronic transitions.

2.  Auxochromes: substituents with unshared pair e's like OH, NH, SH ..., when attached to π chromophore they generally move the absorption max. to longer λ. 3. Bathochromic shift: shift to longer λ, also called red shift. 4. Hysochromic shift: shift to shorter λ, also called blue shift. 5. Hyperchromism: increase in ε of a band. 6. Hypochromism: decrease in ε of a band.

UV Spectroscopy

Chromophores A. Definition:•

Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N

•

Since similar functional groups will have electrons capable of discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves

•

A functional group capable of having characteristic electronic transitions is called a chromophore (color loving)

•

Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions

UV Spectroscopy I. Chromophores A. Organic Chromophores • Alkanes – only posses σ-bonds and no lone pairs of electrons, so only the high energy σ  σ* transition is observed in the far UV This transition is destructive to the molecule, causing cleavage of the σ-bond C

σ∗

σ

C

C

C

UV Spectroscopy I. Chromophores A. Organic Chromophores • Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic examples of these compounds the n  σ* is the most often observed transition; like the alkane σ  σ* it is most often at shorter λ than 200 nm Note how this transition occurs from the HOMO to the LUMO

σ∗ CN

C

N

C

nN sp

C

3

σ CN

C

N anitbonding orbital

N

N

UV Spectroscopy I. Chromophores A. Organic Chromophores • Alkenes and Alkynes – in the case of isolated examples of these compounds the π  π* is observed at 175 and 170 nm, respectively Even though this transition is of lower energy than σ  σ*, it is still in the far UV – however, the transition energy is sensitive to substitution

π∗

π

UV Spectroscopy I. Chromophores A. Organic Chromophores • Carbonyls – n  π* transitions (~285 nm); π  π* (188 nm)

O

π∗

n

π σCO transitions omitted for clarity

C

O

O

It has been determined from spectral studies, that carbonyl oxygen more approximates sp rather than sp2 !

λmax, nm

ε

Solvent

> π*

171

15,000

hexane

> π*

180

10,000

hexane

__

> π* __ > π*

290 180

15 10,000

hexane hexane

__

> π* __ > π*

275 200

17 5,000

ethanol ethanol

> σ* __ > σ*

205 255

200 360

hexane hexane

Chromophor

Example

Excitation

C=C

Ethene

π

__

C≡C

1-Hexyne

π

__

C=O

Ethanal

n π

N=O

Nitromethan e

n π

C-X X=Br X=I

Methyl bromide Methyl Iodide

n n

__

UV Spectroscopy I. Chromophore A.

Substituent Effects General – from our brief study of these general chromophores, only the weak n  π* transition occurs in the routinely observed UV The attachment of substituent groups (other than H) can shift the energy of the transition Substituents that increase the intensity and often wavelength of an absorption are called

auxochromes

Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens

I. Chromophores A. Substituent Effects General – Substituents may have any of four effects on a chromophore • Bathochromic shift (red shift) – a shift to longer λ; lower energy Hypsochromic shift (blue shift) – shift to shorter λ; higher energy

•

Hyperchromic effect – an increase in intensity

•

Hypochromic effect – a decrease in intensity Hyperchromic

•

ε Hypsochromic

Hypochromic

200 nm

Bathochromic

700 nm

UV Absorptions for some isolate chromophores

UV Spectroscopy

III. Instrumentation and Spectra A. Instrumentation :1. The construction of a traditional UV-VIS spectrometer is very similar to an IR, as similar functions – sample handling, irradiation, detection and output are required 2. Here is a simple schematic that covers most modern UV spectrometers:

I0

I

I0

detector

monochromator/ beam splitter optics

I0

sample

UV-VIS sources

reference

log(I0/I) = A

200

λ, nm

700

16 14

Molecular 12 Extinction 10 Coefficient(w) 8 6 4

200 220

240

260

280

wavelength(mu)

300

320

APPLICATIONS OF UV AND VISIBLE SPECTROSCOPY • UV/Vis spectroscopy is routinely used in the quantitative determination of solutions of transition metal ions and highly conjugated organic compounds. • Organic compounds, especially those with a high degree of conjugation, also absorb light in the UV or visible regions of the electromagnetic spectrum. • Thus UV/VIS spectroscopy can be used to determine the concentration of a solution.

• Applications • Quantitative determination of chromophores concentrations in solution • Impurity determination by spectrum subtraction • Determination of reaction kinetics