Functional Groups

IR SpectroscopyUnit 2 Module 2Section 6.1 6.5Mellony S. Manning

2Recall that...A molecule of any substance has an internal energy (U) which can be considered as the sum of the energy of its electrons, the energy of vibration between its constituent atoms and the energy associated with rotation of the molecule. U Eelec + Erot + Evib

Types of Excitation

34Recall that...

Eelec > Evib > Erot5Recall that...Electronic energy levels are widely separated the absorption of a high energy photon (short wavelength) can excite a molecule from one energy level to anotherIn complex molecules the energy levels are more closely spaced than in simple molecules; near ultraviolet and visible light can cause electronic transitionsThese substances will absorb light in some areas of the near ultraviolet and visible regions

6.1 Explain the origin of absorption in IR Spectroscopy6Electromagnetic Spectrum

78Infra-red SpectroscopyThe vibrational energy states of the various parts of a molecule are much closer together than the electronic energy levelsRadiation of lower energy (longer wavelength) are sufficient to bring about vibrational changes

9Infra-red SpectroscopyLight absorption which is due only to vibrational changes occurs in the infrared regionRotational energy states of molecules are so closely spaced that light in the far infrared and microwave regions of the electromagnetic spectrum has enough energy to cause these small changes

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11Infra-red SpectroscopyBonds are vibrating all the time, but if you shine exactly the right amount of energy on a bond, you can kick it into a higher state of vibrationThe amount of energy needed to do this will vary from bond to bond; each different bond will absorb a different frequency (and hence energy) of infra-red radiationhttp://www.chemguide.co.uk/analysis/ir/background.html#top12IR Spectroscopy - PrincipleShine a range of infra-red frequencies through an organic sample and some frequencies get absorbed by the compoundSome frequencies pass through the compound with almost no loss, but other frequencies are strongly absorbedHow much of a particular frequency gets through the compound is measured as percentage transmittance13

15Infra-red SpectroscopyThe horizontal axis is in wavenumbers:

Energies in infra-red radiation correspond to the energies involved in bond vibrations

***For a molecule to absorb IR, the vibrations within a molecule must cause a net change in the dipole moment of the molecule

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/irspec1.htm16VibrationsFor a molecule to absorb infrared radiation it must undergo a net change in dipole moment as a result of vibrational or rotational motion.Vibrations can be subdivided into two classes, depending on whether the bond length or angle is changing:Stretching: Change in inter-atomic distance along bond axisTwo types of stretch: Symmetric, Asymmetric

Bending: Change in angle between two bonds. There are four types of bend: Rocking, Scissoring, Wagging, Twisting 17Vibrations - Bond StretchingThe two nuclei can vibrate backwards and forwards - towards and away from each other - around an average position.Insert animation18Vibrations Stretching (Cont'd)The energy involved in this vibration depends on the length of the bond and the mass of the atoms at either endSmaller mass higher frequency of vibrationStronger bonds higher frequency of vibration

What is the relationship between bond length and bond strength?19Vibrations - Bending

Different bonds will absorb a different frequency of infra-red radiation in order to jump from one vibration state to a higher one20IR Active MoleculesThe fundamental vibrational modes (where n is # atoms) for a molecule is given by:Linear molecules = 3n 5Non-linear molecules = 3n 6Some modes are not IR active. Why?

Degree of FreedomDegree of freedom is the number of variables required to describe the motion of a particle completely. For an atom moving in 3-dimensional space, three coordinates are adequate so its degree of freedom is three. Its motion is purely translational. If we have a molecule made of N atoms (or ions), the degree of freedom becomes 3N, because each atom has 3 degrees of freedom. Furthermore, since these atoms are bonded together, all motions are not translational; some become rotational, some others vibration. For non-linear molecules, all rotational motions can be described in terms of rotations around 3 axes, the rotational degree of freedom is 3 and the remaining 3N-6 degrees of freedom constitute vibrational motion. For a linear molecule however, rotation around its own axis is no rotation because it leave the molecule unchanged. So there are only 2 rotational degrees of freedom for any linear molecule leaving 3N-5 degrees of freedom for vibration.

21Vibrational Modes

Consider water:http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes#Degree_of_FreedomDegree of FreedomDegree of freedom is the number of variables required to describe the motion of a particle completely. For an atom moving in 3-dimensional space, three coordinates are adequate so its degree of freedom is three. Its motion is purely translational. If we have a molecule made of N atoms (or ions), the degree of freedom becomes 3N, because each atom has 3 degrees of freedom. Furthermore, since these atoms are bonded together, all motions are not translational; some become rotational, some others vibration. For non-linear molecules, all rotational motions can be described in terms of rotations around 3 axes, the rotational degree of freedom is 3 and the remaining 3N-6 degrees of freedom constitute vibrational motion. For a linear molecule however, rotation around its own axis is no rotation because it leave the molecule unchanged. So there are only 2 rotational degrees of freedom for any linear molecule leaving 3N-5 degrees of freedom for vibration.

22Vibrational ModesConsider carbon dioxide:

http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes#Degree_of_FreedomDegree of FreedomDegree of freedom is the number of variables required to describe the motion of a particle completely. For an atom moving in 3-dimensional space, three coordinates are adequate so its degree of freedom is three. Its motion is purely translational. If we have a molecule made of N atoms (or ions), the degree of freedom becomes 3N, because each atom has 3 degrees of freedom. Furthermore, since these atoms are bonded together, all motions are not translational; some become rotational, some others vibration. For non-linear molecules, all rotational motions can be described in terms of rotations around 3 axes, the rotational degree of freedom is 3 and the remaining 3N-6 degrees of freedom constitute vibrational motion. For a linear molecule however, rotation around its own axis is no rotation because it leave the molecule unchanged. So there are only 2 rotational degrees of freedom for any linear molecule leaving 3N-5 degrees of freedom for vibration.

23Vibrational ModesConsider the methylene group:http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes#Degree_of_Freedom

Degree of FreedomDegree of freedom is the number of variables required to describe the motion of a particle completely. For an atom moving in 3-dimensional space, three coordinates are adequate so its degree of freedom is three. Its motion is purely translational. If we have a molecule made of N atoms (or ions), the degree of freedom becomes 3N, because each atom has 3 degrees of freedom. Furthermore, since these atoms are bonded together, all motions are not translational; some become rotational, some others vibration. For non-linear molecules, all rotational motions can be described in terms of rotations around 3 axes, the rotational degree of freedom is 3 and the remaining 3N-6 degrees of freedom constitute vibrational motion. For a linear molecule however, rotation around its own axis is no rotation because it leave the molecule unchanged. So there are only 2 rotational degrees of freedom for any linear molecule leaving 3N-5 degrees of freedom for vibration.

24Spectrum of propan-1-ol (CH3CH2CH2OH)

6.2 Describe the basic steps involved in analysing samples by IR spectroscopy2526The Infrared Spectrometer

27Infrared radiation is produced by electrically heating a filament which is divided by mirrors into 2 beams, a reference beam and a sample beam.In the sampling area, a segmented rotating disk allows each beam to pass through alternately.The reference beam and the sample beam are combined into a beam of alternating segments.The detector measures the heat energy and the recorder records the results as a plot of percent absorption (or transmittance) as a function of wavenumber (cm-1) or wavelength (m).28Liquid SamplesSamples sandwiched between two plates of a high purity salt (eg. KBr, CaF2)The plates are transparent to the infrared light and will not introduce any lines onto the spectraSome salt plates are highly soluble in water, so the sample and washing reagents must be without water.29Solid SamplesSolid samples can be prepared in four major ways: Crush the sample with a mulling agent (eg. Nujol). A thin film of the mull is applied onto salt plates and measured

Grind a quantity of the sample with a specially purified salt (eg. KBr) finely (to remove scattering effects from large crystals). This powder mixture is then crushed in a mechanical to form a translucent pellet through which the beam of the spectrometer can pass30Solid Samples (Cont'd)(3) Sample is first dissolved in a suitable, non-hygroscopic solvent. A drop of this solution is deposited on surface of KBr or NaCl cell

(4) Microtomy - cut a thin film from a solid sample. The integrity of the solid is preserved

31Gaseous SamplesGaseous samples require little preparation beyond purification, but a sample cell with a long path length (typically 5-10cm) is normally needed, as gases show relatively weak absorbances6.3 Comment on the limitations associated with the use of IR Spectroscopy3233Limitations of IR SpectroscopyIR Spectroscopy can only determine the functional groups present (Qualitative test)IR does not tell you the way the groups are attachedIR does not tell you the amounts of each groups presentIn order to fully elucidate the structure, other techniques are required such as:- Mass Spectroscopy and NMR (Nuclear Magnetic Resonance) Spectroscopy6.4. Deduce the functional groups present in Organic Compounds from IR spectra3435

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38The Fingerprint RegionThis is the region to the right-hand side of the diagram (~ 1500 to 500 cm-1) usually contains a very complicated series of absorptionsThese are mainly due to all manner of bending vibrations within the molecule39IR Spectrum for CH3CH2CH2CH2CH2CH3

Peaks: C-H stretches (2850-3300 cm-1)40IR Spectrum for H2C=CHCH2CH2CH2CH3

Peaks: C-H stretches (2850-3300 cm-1) C=C stretches (1610-1680 cm-1)41Ethanol - IR SpectrumPeaks: C-H stretches (2850-3300 cm-1) O-H (alcohol) stretches (3200-3550 cm-1)

42Octanal - IR SpectrumPeaks: C-H stretches (2850-3300 cm-1 C=O stretches (1670-1750 cm-1)

432-pentanone - IR SpectrumPeaks: C-H stretches (2850-3300 cm-1 C=O stretches (1670-1750 cm-1)

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45Homework

6.5. Cite examples of the use of IR spectroscopy in the monitoring of air pollutants

***Use SO2 and CO2 as examples***CO2SO2

Type of moleculelinearNon-linear

Polar vs non-polarNon-polarpolar

Vibrational mode (symmetric stretch)Non IR activeIR active

Vibrational mode (asymmetric stretch)IR activeIR active

Vibrational mode(bending)IR activeIR active

CHARACTERISTIC INFRARED ABSORPTION FREQUENCIES

BondCompound TypeFrequency range, cm-1

C-HAlkanes2960-2850(s) stretch

1470-1350(v) scissoring and bending

CH3 Umbrella Deformation1380(m-w) - Doublet - isopropyl, t-butyl

C-HAlkenes3080-3020(m) stretch

1000-675(s) bend

C-HAromatic Rings 3100-3000(m) stretch

Phenyl Ring Substitution Bands870-675(s) bend

Phenyl Ring Substitution Overtones2000-1600(w) - fingerprint region

C-HAlkynes3333-3267(s) stretch

700-610(b) bend

CHARACTERISTIC INFRARED ABSORPTION FREQUENCIES

BondCompound TypeFrequency range, cm-1

C=CAlkenes 1680-1640(m,w)) stretch

CC Alkynes2260-2100(w,sh) stretch

C=CAromatic Rings 1600, 1500(w) stretch

C-O Alcohols, Ethers, Carboxylic acids, Esters 1260-1000(s) stretch

C=O Aldehydes, Ketones, Carboxylic acids, Esters 1760-1670(s) stretch

O-H Monomeric -- Alcohols, Phenols 3640-3160(s,br) stretch

Hydrogen-bonded -- Alcohols, Phenols 3600-3200(b) stretch

Carboxylic acids 3000-2500(b) stretch

v - variable, m - medium, s - strong, br - broad, w - weakCHARACTERISTIC INFRARED ABSORPTION FREQUENCIES

BondCompound TypeFrequency range, cm-1

N-H Amines 3500-3300(m) stretch

1650-1580 (m) bend

C-NAmines 1340-1020(m) stretch

CNNitriles 2260-2220(v) stretch

NO2Nitro Compounds1660-1500(s) asymmetrical stretch

1390-1260(s) symmetrical stretch

v - variable, m - medium, s - strong, br - broad, w - weak