•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 (Gamma) X-Ray

Ultraviolet Visible Infrared

Microwave Radio

lowhigh Frequency ()

Energy

X-RAY ULTRAVIOLET INFRARED MICRO- WAVE

RADIO FREQUENCY

Ultraviolet VisibleVibrationalinfrared

Nuclear magneticresonance

200 nm 400 nm 800 nm

2.5 m 15 m 1 m 5 m

short longWavelength ()

high low

THE ELECTROMAGNETIC SPECTRUMTHE ELECTROMAGNETIC SPECTRUM

BLUE RED

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 SpectrumType 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 nmouter electron molecular

vibrations

infrared 1013-1014 25 µm-2.5 µm molecular vibrations

microwaves 3x1011-1013 1 mm-25 µmmolecular 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

Energy

nAtomic orbitalAtomic orbital

Molecular orbitals

Occupied levels

Unoccupied levels

UV-Spectroscopy

Possible electronic transitions of , , and n electrons are;

                                                                                                                                    

Possible electronic transitions of p, s, and n

electrons are

UV Spectroscopy

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

Energy

n

n

n

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

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:-

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

2. 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

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

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

UV Spectroscopy

I. Chromophores B. Organic Chromophores

1. Alkanes – only posses -bonds and no lone pairs of electrons, so only the high energy * transition is observed in the far UV

This transition is destructive to the molecule, causing cleavage of the -bond

C C

C C

UV Spectroscopy

I. ChromophoresB. Organic Chromophores

2. Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic examples of these compounds the n * is the most often observed transition; like the alkane * it is most often at shorter than 200 nm

Note how this transition occurs from the HOMO to the LUMO

CN

CN

nN sp3C N

C N

C N

C N

anitbonding orbital

UV Spectroscopy

I. ChromophoresB. Organic Chromophores

3. Alkenes and Alkynes – in the case of isolated examples of these compounds the * is observed at 175 and 170 nm, respectively

Even though this transition is of lower energy than *, it is still in the far UV – however, the transition energy is sensitive to substitution

UV Spectroscopy

I. ChromophoresB. Organic Chromophores

4. Carbonyls – n * transitions (~285 nm); * (188 nm)

n

CO transitions omitted for clarity

O

O

C O

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

Chromophor Example Excitation λmax, nm ε Solvent

C=C Ethene π  __>   π* 171 15,000 hexane

C≡C 1-Hexyne π  __>   π* 180 10,000 hexane

C=O Ethanaln  __>  π*π  __>  π*

290180

1510,000

hexanehexane

N=ONitromethan

en  __>  π*π  __>  π*

275200

175,000

ethanolethanol

C-X   X=Br      X=I

Methyl bromideMethyl Iodide

n  __>  σ*n  __>  σ*

205255

200360

hexane hexane

UV Spectroscopy

I. Chromophore

C. Substituent Effects

General – from our brief study of these general chromophores, only the weak n * 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. ChromophoresC. Substituent Effects

General – Substituents may have any of four effects on a chromophore

i. Bathochromic shift (red shift) – a shift to longer ; lower energy

ii. Hypsochromic shift (blue shift) – shift to shorter ; higher energy

iii. Hyperchromic effect – an increase in intensity

iv. Hypochromic effect – a decrease in intensity

200 nm 700 nm

H

yp

oc

hro

mic

Hypsochromic

Hy

pe

rch

rom

ic

Bathochromic

UV Absorptions for some isolate chromophores

UV Spectroscopy

II. 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:

sam

ple

refe

ren

ce

det

ecto

r

I0

I0 I0

Ilog(I0/I) = A

200 700, nm

monochromator/beam splitter optics

UV-VIS sources

200 220 240 260 280 300 320

16

14

12

10

8

6

4

wavelength(mu)

Molecular Extinction

Coefficient(w)

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