Infrared Spectroscopy Lokanathan Arcot Department of Forest Products Technology School of Chemical Technology Aalto University

Basis of Infrared Spectroscopy Atoms Molecules Bond (e– density transfer) Dipole Moment 𝛿0 𝛿+ 𝛿– + – Non-Polar ’0’ Dipole moment Highly Polar ’High’ The two atoms shown in zoomed in version of molecule is in fact NaCl http://www.angelo.edu/faculty/kboudrea/general/shapes/00_lewis.htm Delta EN (difference in electronegativity) Measure of Polarity Dr. Lokanathan Arcot

Vibrational Spectroscopy: Infrared Examples of Molecular Vibrations Others Rocking Wagging Twisting In-plane Scissoring Asymmetric Stretching Symmetric Stretching Effect of Vibrations - Monopoles of dipole vibrate at a Freq. - Oscillating elec. Field of same Freq. Absorption of Light of wavelength λ or Frequency Absorption occurs if the incident light wave has the same frequency as oscillating electric field of a molecule vibrating in a ’non-zero’ dipole moment mode http://chemwiki.ucdavis.edu/Analytical_Chemistry/Analytical_Chemistry_2.0/10_Spectroscopic_Methods/10A%3A_Overview_of_Spectroscopy Dr. Lokanathan Arcot

Vibrational Properties of Bonds Vibrational frequencies of a bond between two atoms (1 and 2) Energy of Vibration Frequency of Vibration En is the energy of the nth vibrational level n is an integer h is Planck’s constant  is the frequency of the vibration k is the force constant of the bond µ is the reduced mass m1 and m2 the mass of the vibrating atoms 1&2 Reduced Mass Dr. Lokanathan Arcot

Vibrational Properties of Bonds Vibrational frequencies of a bond between two atoms (1 and 2) Energy of Vibration Frequency of Vibration Reduced Mass Main implications: Vibrational frequencies increase () with increasing bond strength (k) Vibrational frequencies increase with decreasing mass of the vibrating atoms Dr. Lokanathan Arcot

How a IR spectrum is recorded Source of Light A range of λ Intensity IO Sample Molecules Absorption Transmitted I Detector IO - I Dr. Lokanathan Arcot

What an IR spectrum looks like spectrum is plotted as a function of either absorbance or transmittance IR spectrum of clay 100 % 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20 0,22 0,24 0,26 0,28 500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1) 55 60 65 70 75 80 85 90 95 500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1) Absorbance % Transmittance I =Intensity measured with a sample in the beam Io= Intensity measured with no sample in the beam Course 3130, Dr. Lokanathan Arcot

Unit Wavenumber (cm –1) instead of nm or Hz A typical IR Spectrum Example: 2-pentanone Unit Wavenumber (cm –1) instead of nm or Hz Dr. Lokanathan Arcot

Why use Wavenumber (cm –1) instead of m or Hz Mid IR region is the most useful region for spectroscopy Microns – 2.5µm to 25µm Hertz – 120 THz to 12 THz Wavenumber – 4000 cm –1 to 400 cm –1 c = λ*   ∝ 1/ λ Example: 2.5µm = 2.5*10-4cm Wavenumber = 1/Wavelength (cm) For 2.5µm we get 4000 cm –1 Where C – velocity, λ – wavelength,  – Frequency of light Course 3130, Dr. Lokanathan Arcot

What causes the absorption? IR radiation induces vibrations in molecules and/or functional groups each vibration by a functional group is induced at a distinct wavelength (or wavenumber 1/) Example: CH2 -group Asymmetrical stretching (as CH2) ~ 2926 cm –1 Symmetrical stretching (s CH2) ~ 2853 cm –1 Course 3130, Dr. Lokanathan Arcot

What causes the absorption? Other fundamental vibrations in CH2 group, induced by IR radiation: In-plane bending or scissoring (δs CH2) ~ 1465 cm –1 Out-of-plane bending or wagging (ωs CH2) ~ 2926 cm –1 Out-of-plane bending or twisting (t CH2) ~ 1350-1150 cm –1 in-plane bending or rocking (r CH2) ~ 720 cm –1 Course 3130, Dr. Lokanathan Arcot

What causes the absorption? Example: IR spectrum of cyclohexane (contains only CH2 groups) Fundamental vibrations in CH2 induced by IR radiation Asymmetrical stretching (as CH2) ~ 2926 cm –1 Symmetrical stretching (s CH2) ~ 2853 cm –1 Course 3130, Dr. Lokanathan Arcot

What causes the absorption? Example 2: Water Fundamental vibrations symmetrical stretching at 3652 cm-1 has no change in dipole moment IR requires the vibration be such that it changes the dipole moment hydrogen bonding shifts the absorption to lower wavenumbers Course 3130, Dr. Lokanathan Arcot

What causes the absorption? Example 2: Water Fundamental vibrations although water has only 2 IR bands in IR spectrum, they are very broad  water usually disturbs the IR spectrum (samples are measured without water) s – symmetric as - asymmetric r – scissoring w - wagging Course 3130, Dr. Lokanathan Arcot

IR active vibrations Course 3130, Dr. Lokanathan Arcot

Carbon Dioxide Bending Dipole moment change Symmetric Stretching Asymmetric Stretching No Dipole moment change Dipole moment change Course 3130, Dr. Lokanathan Arcot

Carbon Dioxide – IR Spectrum Course 3130, Dr. Lokanathan Arcot

CHARACTERISTIC GROUP ABSORPTIONS OF ORGANIC MOLECULES Course 3130, Dr. Lokanathan Arcot

Example: dodecane CH3(CH2)10CH3 s – symmetric as - asymmetric C-H bend: s CH2: 1467 cm-1 asCH3: 1450 cm-1 s CH3: 1378 cm-1 C-H stretch: as CH3: 2962 cm-1 s CH3: 2872 cm-1 as CH2: 2924 cm-1 s CH2: 2853 cm-1 CH2 rock:  CH2: 721 cm-1 s – symmetric as - asymmetric r – scissoring w - wagging Course 3130, Dr. Lokanathan Arcot

Example: tert-butyl alcohol 1040 cm-1 C-O stretch C-H bend C-H stretch O-H stretch Neat sample Course 3130, Dr. Lokanathan Arcot

Example: phenol Neat sample Overtone – multiple of given frequency 1224 cm-1 C-O stretch 810 cm-1 752 cm-1 Out-of-plane C-H bend 1360 cm-1 In-plane O-H bend C=C ring stretch Overtone or combination bands 3008 cm-1 Aromatic C-H stretch O-H stretch 690 cm-1 Out-of-plane C=C bend Neat sample Overtone – multiple of given frequency Course 3130, Dr. Lokanathan Arcot

Example: 2-pentanone 1366 cm-1 s CH3 of CH3O unit C-H bend 1171 cm-1 C-CO-C stretch and bend 1717 cm-1 C=O stretch for ketones C-H stretch

Example: hexanoic acid 1413 cm-1 C-O-H in-plane bend 1285 cm-1 C-O stretch with C-O-H interaction 939 cm-1 O-H out-of-plane bend 3300-2500 cm-1 Broad O-H stretch 1711 cm-1 C=O stretch for carboxylic acids C-H stretch Neat sample

Example: octylamine Diluted sample 1073 cm-1 C-N stretch 1617 cm-1 N-H bend (scissoring) 1467 cm-1  CH2 of (scissoring) ~780 cm-1 N-H wag C-H stretch Diluted sample

Note: effect of hydrogen bonding IR spectra of alcohols, carboxylic acids, amines etc. are severely affected by their surrounding medium during the measurement. In gas phase or diluted in a solvent Narrow O-H stretch Neat sample Broad O-H stretch

Absorption regions of some organic functional groups Course 3130, Dr. Lokanathan Arcot

Example - Cellulose although a relatively simple molecule, cellulose is more complex than cyclohexane or water IR spectrum of cellulose comprises of ~ 60 different bands qualitative analysis based only on IR is difficult often IR is used as complementary technique (especially with NMR) Course 3130, Dr. Lokanathan Arcot

Example - Cellulose IR is reliable with pure compounds when spectral libraries are used IR is also a handy tool for quick detection of certain functional groups Course 3130, Dr. Lokanathan Arcot

cellulose vs. trimethylsilyl cellulose (TMSC) Example – Modified Cellulose cellulose vs. trimethylsilyl cellulose (TMSC) Course 3130, Dr. Lokanathan Arcot

Example of Nanoparticle Characterization using IR Spectroscopy Cationic How do we follow this reaction ? STEP 1: Look at all the bonds in reactants STEP 2: Reactant specific bonds Difference between Cellulose (CNC) and cationic molecule (C18) CH2 groups – 1 in each glucose molecule 16 in each cationic molecule C-N group – only in cationic From Thesis: http://www.diva-portal.org/smash/get/diva2:506963/FULLTEXT02 Course 3130, Dr. Lokanathan Arcot

How do we follow a reaction ? STEP 1: Look at all the bonds in reactants STEP 2: Reactant specific bonds (CH2 , C-N) STEP 3: Check if the bonds are IR or Raman active Raman and IR comparison http://www.horiba.com/fileadmin/uploads/Scientific/Documents/Raman/bands.pdf IR bands http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html We need a IR/Raman database Course 3130, Dr. Lokanathan Arcot

Example of Nanoparticle Characterization using IR Spectroscopy SymCH2 assymCH2 C=O Course 3130, Dr. Lokanathan Arcot

Example of Polymer Characterization using IR Spectroscopy Alternating Co-polymer Random Co-polymer Ethylene Propene What is the difference between the two polymers? Course 3130, Dr. Lokanathan Arcot

Polyethylene-propylene co-polymer + Ethylene (C 2) Propene (C 3) Random Copolymer Course 3130, Dr. Lokanathan Arcot

IR spectroscopy of mixture PE, PP X- axis on top is in wavenumber units and blow it is microns PE- Polyethylene, PP- Polypropylene Course 3130, Dr. Lokanathan Arcot

Pyrolysis IR spectroscopy Out of plane C-H olefinic (C=C) bending of mixture PE, PP 450 °C Pyrolysis PE 909 cm-1 Vinyl groups Out of plane C-H olefinic (C=C) bending 450 °C Pyrolysis PP 889 cm-1 Vinylidene groups PE- Polyethylene, PP- Polypropylene Course 3130, Dr. Lokanathan Arcot

Pyrolysis IR spectroscopy of mixture PE, PP Different IR spectra after pyrolysis of mixtures of PP and PE Increasing PP % Note: Polyethylene (PE) - Vinyl group – 909 cm-1 Polypropylene (C3) Vinylidene – 889 cm-1 Course 3130, Dr. Lokanathan Arcot

Pyrolysis IR spectroscopy of mixture PE, PP Increasing PP % Note: Polyethylene (PE) - Vinyl group – 909 cm-1 Polypropylene (C3) Vinylidene – 889 cm-1 Peak ratio at 909cm-1 and 889 cm-1 gives a quantitative estimate of relative amount of Propylene-Ethylene copolymer ratio Course 3130, Dr. Lokanathan Arcot

Summary of Part I Basics of IR spectroscopy Conditions for IR absorption Frequency of vibration – bond characteristics A typical IR spectrum Vibrational modes IR active- inactive (Water and Carbon dioxide) Simple examples – Alkane, OH, COOH, NH, H-bond General molecular bond-IR absorption bands Cellulose and silylated cellulose Example I - Nanoparticle Cellulose Nanocrystal Example II – Polymer – Pyrolysis IR spectroscopy Polyethylene-porpylene copolymer Course 3130, Dr. Lokanathan Arcot

Short Break Course 3130, Dr. Lokanathan Arcot

INSTRUMENTATION Course 3130, Dr. Lokanathan Arcot

IR instrument Radiation source: Globar thermal radiators or Nernst rod thermal radiators provide high intensity in IR region Course 3130, Dr. Lokanathan Arcot

IR instrument Detectors Photoelectric detectors: based on changes in electrical conductivity caused by radiation in semiconductors Thermal detectors: based on the changes in material upon thermal energy of radiation Course 3130, Dr. Lokanathan Arcot

Dispersive Vs Fourier Transform Spectra Apparatus Dispersive Vs Fourier Transform Dispersive : One wavelength of IR radiation at a time FTIR : Simultaneously measuring several wavelengths Course 3130, Dr. Lokanathan Arcot

FT-IR instrument all modern commercial IR instruments are Fourier Transform Infrared (FT-IR) Spectroscopes two beams: one fixed length, the other variable length the beam length is varied by moving mirror occasionally the difference in wavelengths hits an integer  constructive interference occasionally, the difference in wavelengths hits an odd integer of one quarter of the wavelength  destructive interference Course 3130, Dr. Lokanathan Arcot

FT-IR instrument Result: INTERFEROGRAM IR SPECTRUM Fourier transformation http://see.stanford.edu/materials/lsoftaee261/book-fall-07.pdf http://mmrc.caltech.edu/FTIR/FTIRintro.pdf Course 3130, Dr. Lokanathan Arcot

FT-IR instrument FT-IR allows the detection of all the wavenumbers simultaneously before FT-IR, each wavenumber had to be measured separately  measuring one spectrum took several hours, maybe days introduction of FT-IR in 1960s revolutionised the IR technique advances in computer technology in 1980s (PC) made FT-IR a common instrument in all chemical laboratories Course 3130, Dr. Lokanathan Arcot

Types of Samples (Physical) SOLIDS LIQUIDS Powder Solid Surface Rough Smooth Course 3130, Dr. Lokanathan Arcot

Sample Preparation Remember: Water interferes with IR spectroscopy Samples should be dry Compare this with Sample Preparation for Raman Spectroscopy Dispersion of sample inside a IR transparent matrix Pellet Mull Neat II. Direct measurement (no sample preparation required) Attenuation Total Reflectance Diffuse Reflectance Photoacoustic Spectroscopy Course 3130, Dr. Lokanathan Arcot

Dispersion of sample inside a IR transparent matrix Pellet - for powders Mix KBr with sample Alkali Halides become IR transparent pellets upon applying high pressure Example: KBr Load it up in the pressing machine 5000-10000 psi Absorbance measurement The formed Pellet can be used for IR absorption spectroscopy Course 3130, Dr. Lokanathan Arcot

Sample between two IR transparent plates Mull - for powders: Sample is mixed with a liquid and placed in-between two NaCl plates for IR absorbance meaurement. (Why not KBr? ) Nujol is brand of mineral oil most commonly used to make mull ( Nujol Mull) The oil or liquid used must be transparent in the IR region of interest, non-volatile Mix sample with liquid (mineral oil) Spread it over one of the pair of NaCl plates Make a sandwitch of sample between two plates KBr is more hygroscopic than NaCl But it is easier to polish Course 3130, Dr. Lokanathan Arcot

Sample inbetween two IR transparent plates Neat - for liquid sample: Liquid sample is placed in-between two NaCl plates for IR absorbance measurement. Place liquid on one of the pair of NaCl plates Make a sandwitch of sample between two plates Absorbance measurement Course 3130, Dr. Lokanathan Arcot

IR Transmittance after Sample preparation KBr Pellet / NaCl plates I sample mixed with KBr Detector Disadvantages: pressing KBr pellets is laborius KBr is hygroscopic  water interferes the analysis What about IR spectroscopy without sample preparation? Direct measurement (no sample preparation required) Attenuation Total Reflectance Diffuse Reflectance Photoacoustic Spectroscopy

What is Total Internal Reflection ? Attenuated total reflectance (ATR-IR) What is Total Internal Reflection ? Course 3130, Dr. Lokanathan Arcot

Total Internal Reflection creates an Evanescent Wave Attenuated total reflectance (ATR-IR) Total Internal Reflection creates an Evanescent Wave Upon internal reflection the electric and magnetic field of incident light partially propogate into the upper lower refractive index medium Right image from: http://www4.uwm.edu/Dept/celtic/ekeltoi/volumes/vol5/5_3/martin_5_3.html Left Image from: Quote from : http://arxiv.org/ftp/arxiv/papers/1306/1306.5493.pdf ‘The simulation shows that an evanescent wave is reflected from the structure at the interface between a high index dielectric material and a low index material. The reflected evanescent wave couples into the upper medium and radiates its energy forming a retro-reflected wave, which appears as a sharp peak near the edge of the structure when imaging the structure in hyper-numerical-aperture solid immersion microscopy. ‘ More Explanation: http://physics.stackexchange.com/questions/80115/difference-between-propagating-and-evanescent-waves When you have ordinary TIR going on at the boundary between a higher refractive index medium, and a lower index medium; with the beam incident from the high index side of the boundary, classical EM theory says there is no energy propagation in the low index medium; but there is an EM "field" in the low index medium, that drops exponentially with distance from the interface, in a space of the order of the wavelength. There isn't supposed to be any propagating wave, as a result of this evanescent field. Course 3130, Dr. Lokanathan Arcot

Total Internal Reflection creates an Evanescent Wave Attenuated total reflectance (ATR-IR) Total Internal Reflection creates an Evanescent Wave Z – distance from surface I – intensity of field d – arbitrary distance The intensity of field decays exponentially as a function of distance Evanescent means vanishing Right image from: http://www4.uwm.edu/Dept/celtic/ekeltoi/volumes/vol5/5_3/martin_5_3.html Left Image from: Quote from : http://arxiv.org/ftp/arxiv/papers/1306/1306.5493.pdf ‘The simulation shows that an evanescent wave is reflected from the structure at the interface between a high index dielectric material and a low index material. The reflected evanescent wave couples into the upper medium and radiates its energy forming a retro-reflected wave, which appears as a sharp peak near the edge of the structure when imaging the structure in hyper-numerical-aperture solid immersion microscopy. ‘ More Explanation: http://physics.stackexchange.com/questions/80115/difference-between-propagating-and-evanescent-waves When you have ordinary TIR going on at the boundary between a higher refractive index medium, and a lower index medium; with the beam incident from the high index side of the boundary, classical EM theory says there is no energy propagation in the low index medium; but there is an EM "field" in the low index medium, that drops exponentially with distance from the interface, in a space of the order of the wavelength. There isn't supposed to be any propagating wave, as a result of this evanescent field. Course 3130, Dr. Lokanathan Arcot

Attenuated total reflectance (ATR-IR) a beam of radiation is reflected on the interface of two materials with different refractive indices (n2n1)  an evanescent wave appears in the material with lower refractive index (n2) Evanescent wave penetratation depth: is the wavelength of IR radiation is the incident angle of IR radiation n21 is the ratio of refractive indices (n2/n1) Course 3130, Dr. Lokanathan Arcot

Attenuated total reflectance (ATR-IR) penetration depth usually in the order of 1-5 µm evanescent wave is absorbed selectively by the sample  IR spectrum of the surface of the sample  ATR-IR ATR crystal (internal reflection element) is very small  easy to select a location on  mapping Course 3130, Dr. Lokanathan Arcot

Attenuated total reflectance (ATR-IR) Advantages: minimal sample preparation fast analysis selecting locations enable mapping Disadvantages: problems in reproducibility: the contact between the sample and ATR-IR crystal (internal reflection element) is not always reproducible Sample types Powder Solids (Pulp, paper) Course 3130, Dr. Lokanathan Arcot

IR microscope Sample viewing with visible light surface spectra with ATR-objective Photo and instrumentation: VTT Course 3130, Dr. Lokanathan Arcot

Photo and instrumentation: VTT IR microscope IR microscope enables analysis of visually intriguing spots on the sample for instance, a dirt speckle on paper can be selected by the microscope and subjected to ATR-IR analysis Photo and instrumentation: VTT

Photoacoustic detection or photoacoustic spectroscopy (PAS) absorption of IR radiation generates heat in the sample  heat waves reach the sample surface  heat is released to the inert gas above the sample  pressure changes in the gas  pressure changes are detected with a sensitive microphone Sample types Powder Solids (Pulp, paper) microphone Course 3130, Dr. Lokanathan Arcot

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) Specular reflection Diffuse reflection When incident light penetrating a surface is scattered in all directions, the phenomenon is called diffuse reflectance. Applicable to: - powders - rough surfaces Course 3130, Dr. Lokanathan Arcot

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) Transmission-reflectance: When light hits a particle, it can pass through or reflect When passing through, the particle absorbs IR radiation Transmission-reflectance event can occur many times Finally, the outcoming IR beam is collected by a spherical mirror Course 3130, Dr. Lokanathan Arcot

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) Three main ways to prepare a sample for DRIFTS measurement: Powderize the sample Scratch the sample surface and collect the detached pieces on an abrasive paper (3) Disperse particles (like colloids) in a volatile solvent and allow the solvent to evaporate by placing a few drops on a substrate  rough surface Course 3130, Dr. Lokanathan Arcot

Reflection absorption infrared spectroscopy (RAIRS) IR beam is projected on a reflective surface (substrate) which supports an ultrathin film at grazing (very small) angle Only those vibrations, which are perpendicular (P-polarized) to the surface, are IR active and give rise to an observable absorption band Course 3130, Dr. Lokanathan Arcot

Reflection absorption infrared spectroscopy (RAIRS) Film roughness and thickness affect the spectrum Measurements require an ultrahigh vacuum (UHV) Restricted to ultrathin films on solid supports which reflect IR light Course 3130, Dr. Lokanathan Arcot

Example – Alkane thiol self assembly on Au and Silver surface Reflection absorption infrared spectroscopy (RAIRS) Alkane thiol (HS) 10° 30° CH2-stretching 2855 cm-1 sym 2925 cm-1 asym Au Silver J. Phys. Chem. B, 1998, 102 (2), 426-436 Course 3130, Dr. Lokanathan Arcot

Good handbooks on IR spectroscopy Silvestein, Bassler, Morrill Spectrometric identification of organic compounds, Wiley Williams, Fleming Spectroscopic methods in organic chemistry, McGraw-Hill Koenig Spectroscopy of polymers, Elsevier (Several editions available from all titles)

Summary of Part II Instrumentation IR sources, Detectors Dispersive Vs FITR Sample preparation KBr Pellet Mull Nujol Neat Direct Measurement Attenuated Total Internal Reflection Photoacoustic Spectroscopy Diffuse Reflectance Reflection Absorption Course 3130, Dr. Lokanathan Arcot

Have a nice weekend Next week Monday – Surface Plasmon Resonance Wednesday – Quarz Crystal Microbalance Friday – Atomic Force Microscopy Course 3130, Dr. Lokanathan Arcot