Infra-Red and Uv/Vis Spectroscopy

Infrared Spectroscopy

The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibrations

We perceive this vibration as heat

When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms

For a simple diatomic molecule, this model is easy to visualize:

Infrared Spectroscopy

There are two types of bond vibration:Stretch Vibration or oscillation along the line of the bond

Bend Vibration or oscillation not along the line of the bond

scissorasymmetricsymmetricrocktwistwagin planeout of planeInfrared Spectroscopy

The IR Spectroscopic ProcessAs a covalent bond oscillates due to the oscillation of the dipole of the molecule a varying electromagnetic field is produced

The greater the dipole moment change through the vibration, the more intense the EM field that is generated

Infrared Spectroscopy

The IR Spectroscopic ProcessWhen a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed

The coupled wave now vibrates with twice the amplitude

IR beam from spectrometerEM oscillating wavefrom bond vibrationcoupled wave Infrared Spectroscopy

The IR Spectrum

Each stretching and bending vibration occurs with a characteristic frequency as the atoms and charges involved are different for different bondsInfrared Spectroscopy

The IR Spectrum

The x-axis of the IR spectrum is in units of wavenumbers, n, which is the number of waves per centimeter in units of cm-1 (Remember E = hn or E = hc/l)Infrared Spectroscopy

The IR SpectrumThis unit is used rather than wavelength (microns) because wavenumbers are directly proportional to the energy of transition being observed chemists like this, physicists hate it

High frequencies and high wavenumbers equate higher energyis quicker to understand thanShort wavelengths equate higher energy

This unit is used rather than frequency as the numbers are more real than the exponential units of frequency

IR spectra are observed for the mid-infrared: 600-4000 cm-1

The peaks are Gaussian distributions of the average energy of a transition

Infrared Spectroscopy

The IR SpectrumIn general:Lighter atoms will allow the oscillation to be faster higher energyThis is especially true of bonds to hydrogen C-H, N-H and O-H

Stronger bonds will have higher energy oscillationsTriple bonds > double bonds > single bonds in energyEnergy/n of oscillation

The IR Spectrum The detection of different bondsAs opposed to chromatography or other spectroscopic methods, the area of a IR band (or peak) is not directly proportional to concentration of the functional group producing the peak

The intensity of an IR band is affected by two primary factors:Whether the vibration is one of stretching or bending

Electronegativity difference of the atoms involved in the bond

For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak.

The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment

Typically, stretching will change dipole moment more than bending

The IR Spectrum The detection of different bondsIt is important to make note of peak intensities to show the effect of these factors:

Strong (s) peak is tall, transmittance is low (0-35 %)

Medium (m) peak is mid-height (75-35%)

Weak (w) peak is short, transmittance is high (90-75%)

* Broad (br) if the Gaussian distribution is abnormally broad(*this is more for describing a bond that spans many energies)Exact transmittance values are rarely recorded

General The primary use of the IR is to detect functional groups Because the IR looks at the interaction of the EM spectrum with actual bonds, it provides a unique qualitative probe into the functionality of a molecule, as functional groups are merely different configurations of different types of bonds

Since most types of bonds in covalent molecules have roughly the same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show up in similar regions of the IR spectrum

Remember all organic functional groups are made of multiple bonds and therefore show up as multiple IR bands (peaks)Infrared Group Analysis

II. Infrared Group AnalysisA. General The four primary regions of the IR spectrum4000 cm-12700 cm-12000 cm-11600 cm-1600 cm-1Bonds to HTriple bondsDouble bondsSingle Bonds

O-HN-HC-H

CCCN

C=OC=NC=C

Fingerprint Region

C-CC-NC-O

Effects on IR bandsConjugation by resonance, conjugation lowers the energy of a double or triple bond. The effect of this is readily observed in the IR spectrum:

Conjugation will lower the observed IR band for a carbonyl from 20-40 cm-1 provided conjugation gives a strong resonance contributor

Inductive effects are usually small, unless coupled with a resonance contributor (note CH3 and Cl above)Infrared Spectroscopy

Effects on IR bandsSteric effects usually not important in IR spectroscopy, unless they reduce the strength of a bond (usually p) by interfering with proper orbital overlap:

Here the methyl group in the structure at the right causes the carbonyl group to be slightly out of plane, interfering with resonance

Strain effects changes in bond angle forced by the constraints of a ring will cause a slight change in hybridization, and therefore, bond strength

As bond angle decreases, carbon becomes more electronegative, as well as less sp2 hybridized (bond angle < 120)Infrared Spectroscopy

Effects on IR bandsHydrogen bondingHydrogen bonding causes a broadening in the band due to the creation of a continuum of bond energies associated with itIn the solution phase these effects are readily apparent; in the gas phase where these effects disappear or in lieu of steric effects, the band appears as sharp as all other IR bands:

Gas phase spectrum of1-butanol

Steric hindrance to H-bondingin a di-tert-butylphenol

H-bonding can interact with other functional groups to lower frequenciesInfrared Spectroscopy

Thermal Occupation of Molecular Energy LevelsIntensity of a transitions depends on strength and on population ratioof initial and final states. Probability that energy level Ei occupied isgiven by the Boltzmann distribution:where kB is the Boltzmann constant [1.381023 J K1] and g(Ei) isthe degeneracy of Ei (number of quantum states corresponding to Ei).Population ratio for two energy levels Ei and Ef isAt room temperature, kBT = 41021 J (or 2.5 kJ mol1), and is largecompared with rotational energy spacings (hence, many rot. states populated at room temperature), smaller than vibrational energy spacings, very much smaller then electronic energy spacings.

Vibrations and Rotations of Molecules: Infrared & Microwave Spectroscopy Three types of nuclear motion: translation, rotation, and vibration.

Translation corresponds to particle-in-a-box motion in a macroscopicbox not relevant to molecular spectroscopy.

We consider rotational motion (spectroscopy in MW region) andvibrational motion (spectroscopy in IR region).

Rotations of MoleculesRotational motion corresponds to tumbling of molecule in space(change of angular coordinates relative to fixed, laboratory axes.

Example: diatomic molecule

We can assume that the bond length does notchange. Molecule can be consideredrigid as it rotates. xyz

Rotations of Moleculeswhere J = 0,1,2,Degeneracy g(EJ) = 2J + 1Diatomic moleculeEnergy depends on moment of inertia:Only certain quantized energies

Rotations of MoleculesSelection rules: Molecule must have permanent dipole moment (homonuclear diatomics have no pure rotational spectrum), allowed changes in J: J = +1.

Rotational Spectrumof a Diatomic MoleculeEmission spectrum from rotationally hot HF molecules

RadioastronomyMicrowave emission spectrum of Orion nebula, showing the presence of diatomic and polyatomic molecules in the interstellar cloud.

Vibrations of MoleculesWe now consider distortions to the bond lengths. Consider potentialenergy curve of a diatomic molecule.

Vibrational motion about equilibriumbond length. For small change R Re,restoring force isk is the force constant, measuresstiffness of bond, related to strength.Units of k: J m2, or kg s2.

This force describes a harmonicoscillator.

Vibrations of MoleculesIn classical mechanics, atoms will oscillate about equilibrium separation,at frequencyOnly certain energies in quantummechanics:Note ground state (v=0) does not have zero energy. Residual energy (1/2)h is called zero-point energy, consequence of uncertainty principle.R Reenergyv=0 v=1 V(R Re) = (1/2)k(R Re)2

Vibrations of MoleculesVibrational transitions occur in the infrared spectral region. Strongabsorption observed for v = +1 changes in vibrational quantum number.Harmonic oscillator only approximatedescription of molecular vibration. True PE curve of molecule showsdissociation for a diatomic molecule. PE curve is anharmonic. Can determine dissociation energyby summing vibrational spacings.

Vibrations of MoleculesAbsorption spectrum of diatomic molecules domonated by the v=0 v=1 transition. Example: carbon monoxide

Vibrations of MoleculesIn polyatomic molecules, several types of vibrational motion possible.

Nonlinear triatomic molecule: Nvib = 3N 6 = 9 6 = 3 modes.Example: water (H2O) molecule

Vibrations of MoleculesAbsorption spectrum of methane:How many vibrational degreesof freedom for methane?Absorption spectra of gases show rotational fine-structure built upon thevibrational transitionsstretchbend

Vibrations of MoleculesSpectra of larger polyatomic molecules can be useful in identificationof compounds.

Absorption spectrum of ethanol:

Excited Electronic States:Electronic Spectroscopy of MoleculesExcited molecular electronic states can be described within framework ofmolecular orbital theory.

Example: ethylene

Conjugated SystemsConjugated -bonded system alternating double and single bonds.

Lowest-energy orbital extends over full geometry, resembles standingwave over full length of molecule. As length of the conjugated system is increased, wavelengths become longer, and energies shift lower.Absorption of Light by Molecules with Conjugated Electron Systems

MoleculeNumber of C=C Bonds Wavelength of Max. Abs.(nm) C2H4 1 162 C4H6 2 217 C6H8 3 251 C8H10 4 304

Conjugated Systems: -Carotene and IndigoIndigo Beta-Carotene Absorption spectra oftwo dyes:carotene and indigo

The Fate of Excited Electronic StatesWhat happens to molecules after they absorb radiation?

Some emit photon and return to ground electronic state (but in differentvib.-rot. level). This process is called fluorescence time scale 109 s.Second type of emission occurs much more slowly calledphosphorescence.

Difference in these processes due to spins of the electrons in excitedstate.

The Fate of Excited Electronic StatesLight absorption does not change spins of electrons remainantiparallel (called singlet state). Another possible state has spinsparallel (triplet state). Small probability that excited singlet can crossover to excited triplet. Triplet emits very slowly because spin mustflip back.

The Fate of Excited Electronic StatesRadiative and non-radiativedecay pathways forelectronically excited molecules.

Sensitive Detection of MoleculesBeer-Lambert law for absorption is not the most sensitive way to detect molecules at low concentration. Concentration depends ondetermining ratio of IS and IR accurately. Akin to measuring weightof captain by measuring captain+ship and ship, and taking difference.

One way to improve detection sensitivity is to use fluorescence.Signal comes on a zero background. However, method notapplicable to all molecules, only those that fluoresce.

LasersSpontaneous and stimulated emissionLaser (light amplification by stimulated emission of radiation))first demonstrated with visible light in 1960 (T. Maiman)unique properties of laser light:monochromaticcollimatedcoherentRequires a pumping mechanismpump photonselectric dischargechemical reaction

Examples of LasersFixed wavelengthNd:YAG laserHeNe laserVariable wavelengthDye laserDiode laser(used in CDs.DVDs, andlaser pointers)YAG = yttrium aluminum garnet

Introduction to Atmospheric PhotochemistryComposition of air at earths surfaceN278.11%O220.95Ar0.93H2O 07%Structure of atmosphere layered.Density is low in outer layers. Sunsradiation causes extensive ionization.Troposphere dynamically unstablesince cooler, denser air above warmerair at surface.

Photochemistry in the StratosphereIf molecule absorbs light of photon energy greater than bond energy,it can dissociate. The fragments are usually highly reactive and leadto chemical reactions photochemistry.

Consider photodissociation ofoxygen molecule. Absorptionspectrum shows transitions tovibrational levels of excitedelectronic states, as well ascontinuous absorption. The latteris transition to repulsiveexcited electronic state.192 nm250 nmwavelength (nm)

Photochemistry in the StratospherePotential energy curves of the electronic states of O2.UV solar radiation O2 is photodissociated

Photochemistry in the StratosphereFormation and destruction of ozone in the stratosphereO3* = energized molecule M = another molecule (usually N2 or O2)Ozone in the stratosphere absorbs 200-350 nm solar radiation, protectslife on earths surface from damaging UV radiation.Catalytic destruction of ozone by chlorofluorocarbons (CFCs), e.g. CCl2F2.CFCs inert in troposphere but photodissociated in the stratosphere:

The Greenhouse EffectSun illuminates earth with visible radiation. Earth radiates IR radiationback into space. Earths temperature results from balance betweenenergy input and loss.Atmospheric gases(e.g. CO2, CH4)absorb IR radiationand reduce amountof energy radiatedback into space.

The Greenhouse EffectIncrease in concentration of atmospheric gases, especially CO2 fromfossil fuel burning, is leading to increase in global temperature.Monthly CO2 concentrations at Mauna Loa, Hawaii

*****Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*Intro Chemistry II 030.102 Chapter 20Spring, 2012*