Dr. S. M. Condren Chapter 16 An Introduction to Infrared Spectroscopy

Dr. S. M. Condren Table 16-1, pg. 381 Infrared Spectral Regions

Dr. S. M. Condren Infrared Spectrum Fig. 16-1, pg. 382 “Infrared absorption spectrum of a thin polystyrene film recorded with a modern infrared spectrometer. Note that the abscissa scale changes at 2000 cm -1.”

Dr. S. M. Condren Two Conditions Necessary for Infrared Spectroscopy 1. for absorption, vibration frequency equals incident radiation frequency 2. oscillating dipole moment

Dr. S. M. Condren IR Absorption IR radiation is of too low an energy to excite electronic transitions Absorption is limited to vibration and rotational levels For liquids and solids, molecular rotation is often limited so the major type of interaction is vibrational.

Dr. S. M. Condren IR Absorption The types of vibrations available to a molecule are determined by the: Number of atoms Types of Atoms Type of bonding between the atoms As a result, IR absorption spectroscopy is a powerful tool in characterizing pure organic and inorganic compounds.

Dr. S. M. Condren

Vibration Modes Fig. 16-2, pg. 383 “Types of molecular vibrations. Note: + indicates motion from the page toward the reader; - indicates motion away from the reader.”

Dr. S. M. Condren Harmonic Oscillators m 1 m 2  = m 1 + m 2 where  => reduced mass m 1 => mass of atom 1 m 2 => mass of atom 2

Dr. S. M. Condren Harmonic Oscillators F = m a = - k y whereF => force m => mass a => acceleration k => force constant y => distance of displacement

Dr. S. M. Condren Harmonic Oscillators second derivative of y with time (acceleration) d 2 y a = dt 2 wheret => time

Dr. S. M. Condren Harmonic Oscillators second derivative of y with time (acceleration) thus d 2 y F = m = - k y dt 2

Dr. S. M. Condren Harmonic Oscillators one solution of d 2 y k ---- = dt 2 is y = A sin((k/m) 1/2 *t) y m

Dr. S. M. Condren Harmonic Oscillators one solution of d 2 y k ---- = dt 2 is y = A sin((k/m) 1/2 *t) y m y = A sin 2  t thus (k/m) 1/2 *t = 2  t

Dr. S. M. Condren Harmonic Oscillators one solution of m = (1/2  )*(k/m) 1/2 for one mass m = (1/2  )*(k/  ) 1/2 for two masses 1 m = * ((k(m 1 + m 2 ))/(m 1 m 2 )) 1/2 2 

Dr. S. M. Condren Potential Energy Diagram Fig. 16-3, pg. 384 “Potential energy diagrams. Curve 1, harmonic oscillator. Curve 2, anharmonic oscillator.”

Dr. S. M. Condren EXAMPLE: Calculate the fundamental frequency expected in the infrared absorption spectrum for the C - O stretching frequency. The value of the force constant is 5.0 X 10 5 dynes/cm. wave number =  1  = * ((k(m 1 + m 2 ))/(m 1 m 2 )) 1/2 2  c 1  = cm -1 2(3.14)(3.0E10) * ((5.0E5(12+16)(6.02E23))/(12*16)) 1/2

Dr. S. M. Condren EXAMPLE: Calculate the fundamental frequency expected in the infrared absorption spectrum for the C - O stretching frequency. The value of the force constant is 5.0 X 10 5 dynes/cm. 1  = cm -1 2(3.14)(3.0E10) * ((5.0E5(12+16)(6.02E23))/(12*16)) 1/2  = 1112 cm -1

Dr. S. M. Condren Vibration Modes stretch –symmetrical –unsymmetrical bend –symmetrical –unsymmetrical

Dr. S. M. Condren Number of Fundamental Vibrations Linear molecules 3N-5 Non-Linear Molecules 3N-6

Dr. S. M. Condren CO 2 Linear molecule3N-5 N = 3 3N-5 = 3(3) - 5 = 4 4 fundamental vibrations

Dr. S. M. Condren Stretching Vibration in CO 2 2 fundamental vibrations

Dr. S. M. Condren Bending Vibration in CO 2 2 fundamental vibrations, because the molecule is linear, these two bending vibrations are degenerate

Dr. S. M. Condren H2OH2O Non-Linear Molecules3N-6 N = 3 3N - 6 = 3(3) - 6 = 3 3 fundamental vibrations

Dr. S. M. Condren H 2 O Vibrations

Dr. S. M. Condren Infrared Sources Most Common IR Sources Nernst glower –cylinder of rare-earth oxides glowbar –silicon carbide rod –50mm long by 5mm diameter incandescent wire –nichrome wire

Dr. S. M. Condren Infrared Sources Special Application IR Sources mercury arc –far-infrared tungsten filament –near-infrared carbon dioide laser –tunable –used to monitor atmospheric conditions

Dr. S. M. Condren Infrared Detectors thermocouples pyroelectrics

Dr. S. M. Condren P-E Model 1600 FTIR

Dr. S. M. Condren PE Spectrum One FTIR

Dr. S. M. Condren Components of FTIR Instruments Drive mechanism –constant velocity mirror of known location plus a fixed position planar mirror Beam Splitters Michelson Interferometer

Dr. S. M. Condren Block Diagram of the Major Components of an FTIR

Dr. S. M. Condren Michelson Interferometer Fig. 16-6, pg. 394 “Interferometeres in an infrared Fourier transform spectrometer. Subcripts 1 define radiation path in the infrared interferometer; subscripts 2 and 3 refer to the laser and white light interferometers, respectively.”

Dr. S. M. Condren Interferogram from a Typical Infrared Glower

Dr. S. M. Condren Interferogram "The interferogram of a single frequency source is a cosine function with a periodicity that varies with the frequency of the emiiting source (a and b). The interferogram of a two-frequency source may be calculated by geometrically adding the cosine function corresponding to each of the individual lines in the source (c).”

Dr. S. M. Condren Interferogram of a Two- Frequency Source

Dr. S. M. Condren Interferogram 2V m f = where => wavelength of incident radiation f => frequency of interferogram V m => constant velocity of moveable mirror

Dr. S. M. Condren Interferogram f = 2 V m  where  => wavenumber value

Dr. S. M. Condren Interferogram f = 2 V m where => optical frequency c => speed of light

Dr. S. M. Condren Interferogram "The generation of a spectrum by an FTIR spectrometer. An interferogram of the source (background) is scanned, transformed into a single-beam spectrum, and stored in computer memory (a). The sample, indene in an mm cell, is placed in the beam and the process repeated (b). The two single-beam spectra, in computer memory, are ratioed to produce the more conventional "double-beam" presentation (c).”

Dr. S. M. Condren Interferogram vs. Spectrum

Dr. S. M. Condren Fig. 16-8, pg. 395 P-E Model 1600 FTIR

Dr. S. M. Condren Spectrum One Optical Layout

Dr. S. M. Condren FTIR Sources and Detectors Sources usual IR sources Detectors triglycine sulfate pyroelectric (NH 2 CH 2 COOH) 3   SO 4

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 1.Interferometer uses all wavelengths at the same time, Fellgett's advantage S/N enhancement = (( )/  ) 1/2 where => limits of regions scanned  => resolution

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 2.Jacquinot advantage greater throughput of radiation than for concentional monochromator for comparable sized instruments, may favor dispersion – (grating area beam splitter area) smaller advantage than Fellgett's advantage

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 3.Higher S/N for same time, same S/N for less time (faster scanning)

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 4.Applications: GC-FTIRpossible GC-IRimpractical

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 5.Hard to do samples - always require signal averaging => faster scanning allows greater # signals to average in same time

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 6.Better wavenumber accuracy

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 7.Constant Resolution w/ variable noise level vs. variable resolution w/ constant noise for Dispersive

Dr. S. M. Condren Advantages of FTIR over Dispersion IR 8.No stepping at "filter changes" for various order changes of grating

Dr. S. M. Condren Instrument Dispersive Instruments Fig , pg. 398 "Schematic of a double-beam spectrophotometer."

Dr. S. M. Condren Beckman Acculab IR

Dr. S. M. Condren Buck M500 IR

Dr. S. M. Condren Buck M500 IR

Dr. S. M. Condren Chapter 17 Applications of Infrared Spectrometry

Dr. S. M. Condren Table 17-1, pg. 405 Major Applications of Infrared Spectrometry

Dr. S. M. Condren Table 17-1, pg. 405 Major Applications of Infrared Spectrometry

Dr. S. M. Condren Table 17-1, pg. 405 Major Applications of Infrared Spectrometry

Dr. S. M. Condren Table 17-1, pg. 405 Major Applications of Infrared Spectrometry

Dr. S. M. Condren Sample Techniques film smear sample cell gas cell KBr pellet Nujol mull internal reflectance apparatus

Dr. S. M. Condren Sample Cell Fig. 17-2, pg. 406 “Expanded view of a demountable infrared cell for liquid samples. Teflon spacers ranging in thickness from to 1 mm are available.”

Dr. S. M. Condren Internal Reflectance Apparatus Fig , pg. 421 “Internal reflectance apparatus. (a) Sample mounted on reflection plates (b) internal reflection adapter.”

Dr. S. M. Condren Interference patterns N b = (  1 -  2 ) whereb => cell thickness N => number of interference peaks  1 &  2 => 2 frequencies in the range of cm -1

Dr. S. M. Condren EXAMPLE: Use the spectra shown to calculate the thickness of the polystyrene film. N b = (  1 -  2 )

Dr. S. M. Condren Polystyrene Spectra

Dr. S. M. Condren EXAMPLE: Use the spectra shown to calculate the thickness of the polystyrene film. N = b = = 8.7 X cm 2( )cm -1

Dr. S. M. Condren Important Spectral Regions in the Infrared Hydrogen stretching region 3700 to 2700 cm-1 Triple bond region 2700 to 1850 Double bond region 1950 to 1550 Finger-print region (single bonds) 1500 to 700

Dr. S. M. Condren Qualitative Analysis Fig. 17-4, pg. 408 Group frequency and fingerprint regions of the mid-infrared spectrum.

Dr. S. M. Condren Qualitative Analysis Fig. 17-4, pg. 408 Group frequency and fingerprint regions of the mid-infrared spectrum.

Dr. S. M. Condren Qualitative Analysis Fig. 17-4, pg. 408 Group frequency and fingerprint regions of the mid-infrared spectrum.

Dr. S. M. Condren Qualitative Analysis Fig. 17-4, pg. 408 Group frequency and fingerprint regions of the mid-infrared spectrum.

Dr. S. M. Condren Table 17-2 pg. 410 Abbreviated Table of Group Frequencies for Organic Groups

Dr. S. M. Condren Quantitative Applications Disadvantages & Limitations

Dr. S. M. Condren Fig. 17-7, pg. 416 Baseline method for determination of absorbance

Dr. S. M. Condren Fig. 17-8, pg. 417 Spectra of C 8 H 10 isomers in cyclohexane

Dr. S. M. Condren Transmission Spectra BaF 2 Cell "Transmission spectra run in a mm thick barium fluoride cell. Scan A is liquid water; scan B is an aqueous solution of a water soluble aspirin tablet; scan C is the ordinate expanded difference spectrum, that is solution minus water."

Dr. S. M. Condren BaF 2 Cell

Dr. S. M. Condren Transmission Spectra Internal Reflection Cell Zinc Selenide Element "Spectra run in a circular internal reflection cell with a zinc selenide internal reflection element. The two lower spectra are of water (a) and of a water concentrate of a commerical soil fumigant (b). The upper spectrum (c) is the difference spectrum (b) minus (a) presented in an ordinate expanded absorbance format."

Dr. S. M. Condren Internal Reflection Cell Zinc Selenide Element

Dr. S. M. Condren Suspected Drug Spectra "Spectrum of a single grain of a suspected drug scanned through an infrared microscope using a 200- X 400-  m viewing aperture." "Search 'hit list' showing ranked best matches between suspected drug spectrum and a reference library of drug spectra. Comparison of spectra confirmed that the sample was cocaine hydrochloride."

Dr. S. M. Condren Spectrum of a Single Grain of a Suspected Drug

Dr. S. M. Condren IR Spectrum of Cocaine in Vapor Phase

Dr. S. M. Condren Library Search Match with Cocaine-HCl

Dr. S. M. Condren Infrared Microscope