Unit-III: Spectroscopic Techniques

Spectroscopy: It is a general term for the science that deals with the interactions of various types of radiation with matter

Initially interest was laying on interaction of electromagnetic radiation with matter

But now-a-days, spectroscopy has been broaden to include interactions between matter and other forms of energy

Spectrometry & spectrometric methods refer to the measurements of the intensity of radiation with a photoelectric transducers or other type of electronic device

Most readily recognizable forms of energy being light and radiant heat

Molecular Spectroscopy Light has magnetic and electric components -oriented at 90

-can be polarized

General Terms

Frequency, , is the number of oscillations of the field that occurs per second

Wavelength (): The linear distance between any two equivalent points on successive waves (e.g. successive maxima or minima)

Velocity of propagation (v): frequency in cycles per second wavelength in meter per cycles

Velocity of radiation depends on the composition of the medium

Wavenumber (): Reciprocal of the wavelength in cm1, equal to k where k is proportionality constant

-

Electromagnetic Spectrum

X-ray: core electron excitation

UV: valance electronic excitation

IR: molecular vibrations

Radio waves: Nuclear spin states (in a magnetic field)

300 kJ/mol 170 kJ/mol

Regions of the Electromagnetic Spectrum

Quantum Transitions

Spectroscopic Techniques and Chemistry they Probe

UV-vis UV-vis region bonding electronsAtomic Absorption UV-vis region atomic transitions (val. e)FT-IR IR/Microwave vibrations, rotationsRaman IR/UV vibrationsFT-NMR Radio waves nuclear spin statesX-Ray Spectroscopy X-rays inner electrons, elemental X-ray Crystallography X-rays 3-D structure

Spectroscopic Techniques and Common Uses

UV-vis UV-vis regionQuantitative analysis/

Beers Law

Atomic Absorption UV-vis regionQuantitative analysis

Beers Law

FT-IR IR/Microwave Functional Group Analysis

Raman IR/UVFunctional Group Analysis/

quant

FT-NMR Radio waves Structure determinationX-Ray Spectroscopy X-rays Elemental Analysis X-ray Crystallography X-rays 3-D structure Anaylysis

Different Spectroscopies

UV-vis electronic states of valence electrons/ d-orbital transitions for solvated transition metals

Fluorescence emission of UV/vis by certain molecules

FT-IR vibrational transitions of molecules

FT-NMR nuclear spin transitions

X-Ray Spectroscopy electronic transitions of core electrons

Molecular Spectroscopy

UV and UV-Visible Spectroscopy

Absorption

Types of Transitions

In alkenes, only and bonds. So, only , and *, * Mos are present But amines, ketones, aldehydes & halides have N, O, X and lone pairs on it. So we need to consider them as non-bonding electrons (n) in the MOs.

* and * transitions: high-energy, accessible in vacuum UV (max

Example for to *

In alkenes, only and bonds. So, only , and *, * MOs are present

Conjugated Polynenes

Consideration of Non-bonding electrons (n)

n

n

*

n

**

*

*

*

* *

*

*;max=218=11,000

n*;max=320=100

Example for a simple enone (ketone + alkene):

Here conjugated ketone, We do to consider lone pairs on the O atom as non-bonding electrons (n) in the MOs.

HowDoUVspectrometerswork?

Matchedquartzcuve9es

Sampleinsolu@onatca.105M.

SystemprotectsPMtubefromstraylight

D2lampUV

TungstenlampVis

DoubleBeammakesitadierencetechnique

l (path through sample)

Transmi9ance:T=P/P0 P0

(powerin)P(powerout)

Absorbance:A=log10T=log10P0/P

TheBeerLambertLaworBeersLaw:A=clWheretheabsorbanceAhasnounits,sinceA=log10P0/P

isthemolarabsorb@vitywithunitsofLmol1cm1listhepathlengthofthesampleincm

cistheconcentra@onofthecompoundinsolu@on,expressedinmolL1(orM,molarity)

The Quantitative Picture

Organic compounds (many of them) have UV spectra

Uvscanbeverynonspecic

Itshardtointerpretexceptatacursorylevel,andtosaythatthespectrumisconsistentwiththestructure

EachbandcanbeasuperposiFonofmanytransiFons

GenerallywedontassigntheparFculartransiFons.

Selection rules can be relaxed due to:

vibronic coupling spin-orbit coupling geometry relaxation during transition

Laporte Selection Rule

Spin Selection Rule

Shifts in the UV-Visible Spectroscopy

(Red shift) (Blue shift)

Color Wheel

Why grass and leaves appear in Green color? Absorption Spectrum of Chlorophyll

particle in a box QM theory; bigger box

Substituents attached to a chromophore that cause a red shift are called auxochromes

Strain has an effect

max 253239 256 248

Generally, extending conjugation leads to red shift

Color of Transition Metal Complexes UV-Vis Absorption Spectra of [Ti(H2O)]3+

GS ES

20,300 cm1

The Color of [Ti(H2O)]3+

Reddish Violet Wavelength (nm) d-d Transition

UV-Vis Spectroscopy of Inorganic Complexes

Same Metal ion with different geometries!

Aqua Complexes of Different Metal ions

Co(III) Complexes shows the effects of Ligands

Co(III) Complexes shows the effects of Ligands

Different Colors!

Charge Transfer (CT) Transitions

Ligand Field Theory (LFT)

Transi@onmetalcomplexes;d,felectrons.

Lanthanidecomplexessharplinescausedbyscreeningofthefelectronsbyotherorbitals

Oneadvantageofthisistheuseofholmiumoxidelters(sharplines)forwavelengthcalibra@onofUV

spectrometers.

See Shriver et al. Inorganic Chemistry, 2nd Ed. Ch. 14

Interpretation of UV-Visible Spectra

IR Spectroscopy Basics

Basic Equation and Isotope Effect

IR Spectroscopy Basics

IR Spectroscopy Molecular Motions

All of the motions can be described in terms of two types of molecular vibrations. One type of vibration, a stretch, produces a change of bond length. A stretch is a rhythmic movement along the line between the atoms so that the interatomic distance is either increasing or decreasing

The second type of vibration, a bend, results in a change in bond angle. These are also sometimes called scissoring, rocking, or "wig wag" motions

Notethehighwavenumber(highenergy)requiredtoproducethesemo@ons.Thebendingmo@onsaresome@mesdescribedaswaggingorscissoringmo@ons

Each of these two main types of vibration can have variations. A stretch can be symmetric or asymmetric. Bending can occur in the plane of the molecule or out of plane; it can be scissoring, like blades of a pair of scissors, or rocking, where two atoms move in the same direction.

Different stretching and bending vibrations can be visualized by considering the CH2 group in hydrocarbons. The arrows indicate the direction of motion. The stretching motions require more energy than the bending ones.

You can see that the lower wavenumber values are consistent with lower energy to cause these vibrations. A molecule absorbs a unique set of IR light frequencies. Its IR spectrum is often likened to a person's fingerprints. These frequencies match the natural vibrational modes of the molecule. A molecule absorbs only those frequencies of IR light that match vibrations that cause a change in the dipole moment of the molecule. Bonds in symmetric N2 and H2 molecules do not absorb IR because stretching does not change the dipole moment, and bending cannot occur with only 2 atoms in the molecule. Any individual bond in an organic molecule with symmetric structures and identical groups at each end of the bond will not absorb in the IR range. For example, in ethane, the bond between the carbon atoms does not absorb IR because there is a methyl group at each end of the bond. The C-H bonds within the methyl groups do absorb.

Frequency Range:

IR Spectroscopy Basics

A typical IR Spectrum of an Org. Compound

O-H stretch appears at 3200-3600, C-H at 3000.

OH CH

CH3CH2OH

NH

CH

N-H stretch appears at 3200-3500, C-H at 3000

CH

CC

CCstretchappearsat2250,CHat3000

Me Me

CN

CH

CN stretch appears at 2250, C-H at 3000

CH3CN

C=O

CH

C=O stretch appears at 1650-1800, C-H at 3000

C=C

CH

C=C stretch appears at 1650, C-H at 3000

Alkanes will have only C-H Stretching Frequency

WhatIRpeakswouldyouexpectforthetwomoleculeswiththeformulaC2H6O?

Ethanol Dimethylether

OH32003400cm1(broad)

CH3000cm1

CO10601150cm1

CH3000cm1

CO10601150cm1

The spectrum for the aldehyde, octanal (CH3(CH2)6CHO), is shown here. The most important features of the spectrum are carbonyl CO stretch near 1700 cm-1 and the CH stretch at about 3000 cm-1. If you see an IR spectrum with an intense strong band near 1700 cm-1 and the compound contains oxygen, the molecule most likely contains a carbonyl group,

IR Spectrum of Octanal

IR spectra can be used to identify molecules by recording the spectrum for an unknown and comparing this to a library or data base of spectra of known compounds. Computerized spectra data bases and digitized spectra are used routinely in this way in research, medicine, criminology, and a number of other fields. In this exercise you will try to identify the outstanding bands characteristic of certain bonds and functional groups in the spectra you examine. You are certainly not expected to identify all the absorption bands in each IR spectrum at this point in your work. When you analyze the spectra, it is easier if you follow a series of steps in examining each spectrum. Look first for the carbonyl C=O band. Look for a strong band at 1820-1660 cm-1. This band is usually the most intense absorption band in a spectrum. It will have a medium width. If you see the carbonyl band, look for other bands associated with functional groups that contain the carbonyl by going to step 2. If no C=O band is present, check for alcohols and go to step 3.

ACID Lookforindica@onsthatanOHisalsopresent.Ithasabroadabsorp@onnear33002500cm1.ThisactuallywilloverlaptheCHstretch.TherewillalsobeaCOsinglebondbandnear11001300cm1.Lookforthecarbonylbandnear17251700cm1.

ESTER LookforCOabsorp@onofmediumintensitynear1200cm1.TherewillbenoOHband..Lookforthecarbonylbandnear17351750cm1

ALDEHYDE LookforaldehydetypeCHabsorp@onbands.Thesearetwoweakabsorp@onstotherightoftheCHstretchnear2850cm1and2750cm1andarecausedbytheCHbondthatispartoftheCHOaldehydefunc@onalgroup.Lookforthecarbonylbandaround17401720cm1.

KETONE TheweakaldehydeCHabsorpFonbandswillbeabsent.LookforthecarbonylCObandaround17251705cm1.

If a C=O is present you want to determine if it is part of an acid, an ester, or an aldehyde or ketone. At this time you may not be able to distinguish aldehyde from ketone and you will not be asked to do so.

ALKENE Lookforweakabsorp@onnear1650cm1foraC=C.TherewillbeaCHstretchbandnear3000cm1.

AROMATIC Lookforthebenzene,C=C,doublebondswhichappearasmediumtostrongabsorp@onsintheregion16501450cm1.TheCHstretchbandismuchweakerthaninalkenes.

If no carbonyl band appears in the spectrum, look for an alcohol O-H band.

ALCOHOL LookforthebroadOHbandnear36003300cm1andaCOabsorp@onbandnear13001000cm1.

IfnocarbonylbandsandnoOHbandsareinthespectrum,checkfordoublebonds,C::C,fromanaroma@coranalkene.

Ifnoneofthepreviousgroupscanbeiden@ed,youmayhaveanalkane.

ALKANEThemainabsorp@onwillbetheCHstretchnear3000cm1.Thespectrumwillbesimplewithanotherbandnear1450cm1.

The Possible Modes of Interactions of CO with M

CO Stretching Frequency Depends on e density of M

Structural Assignments using CO Stretching Frequency