CHMBD 449 Organic Spectral AnalysisFall 2005Chapter 2: IR SpectroscopySpectroscopic ProcessIR Absorption ProcessUses of IRCovalent bonds Vibrational Modes Absorption Trends

IR Spectroscopy

I. IntroductionThe IR Spectrum Factors that affect group frequencies

Remember, most interesting molecules are not diatomic, and mechanical or electronic factors in the rest of the structure may effect an IR bandFrom a molecular point of view (discounting phase, temperature or other experimental effects) there are 10 factors that contribute to the position, intensity and appearance of IR bandsSymmetryMechanical CouplingFermi ResonanceHydrogen BondingRing StrainElectronic EffectsConstitutional IsomerismStereoisomerismConformational IsomerismTautomerism (Dynamic Isomerism)

IR Spectroscopy

I. IntroductionThe IR Spectrum Factors that affect group frequenciesConstitutional IsomersObvious in most cases molecules with identical molecular formulas with different connectivity implies a different set of covalent bonds and interactions and therefore different IR

Important to note that permutations of the substitution pattern (i.e. ortho- meta- or para-) on an aromatic system are also constitutional isomers IR is a powerful tool for analysis

Variations of heterocyclic structures within the same molecular formula can also be discerned by IR

IR Spectroscopy

I. IntroductionThe IR Spectrum Factors that affect group frequenciesStereoisomerism - enantiomers and diastereomersEnantiomers (optical isomers) pure enantiomers do not differ in the arrangement of atoms and bonds relative to the rest of the molecule identical IR spectra

Diastereomers - (geometric isomers) will have different changes in dipole moment for given sets of vibration

Consider cis- and trans- alkenes; the stretching modes of dibromoethylene are given as an example:

IR Spectroscopy

I. IntroductionThe IR Spectrum Factors that affect group frequenciesConformational IsomerismIn open chain compounds the barrier to rotation is small, and most often IR are observed of the energetically favored form

Likewise in cyclic systems, the lowest energy conformer(s) will be the one observed

These conformers will change depending on solvent, temperature and phase, and can be observed in the IR

IR Spectroscopy

I. IntroductionThe IR Spectrum Factors that affect group frequenciesConformational IsomerismSometimes, there is a stabilizing interaction that may allow observation of a second band for an additional stable conformer

Example: a-halogenated ketones the lone e- pairs of the carbonyl oxygen are deflected strengthening the p bond; these structures are purported to be either cis- or gauche

Again for cyclic systems, bands may be observed for each energy minima

IR Spectroscopy

I. IntroductionThe IR Spectrum Factors that affect group frequenciesTautomerism (Dynamic Isomerism)Carbonyl compounds with a-hydrogens exist as an equilibrium between keto- and enol forms

In most cases, the keto form is favored; however in cases where the equilibrium has an appreciable fraction of enol (or the enol form is actually favored) those additional IR bands are observed

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformDispersive IR SpectrometersAll spectrometers consist of four basic parts that are coupled with all four parts of the spectroscopic process - irradiation, absorption-excitation, re-emission-relaxation and detection.

Irradiation: Spectrometer needs to generate photons hnhnhnDetection-reemission : Spectrometer needs to detect the photons emitted by the sample and ascertain their energy EnergyAbsorption-Excitation: Spectrometer needs to contain the samplehnRelaxation

rest staterest stateexcited state

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformDispersive IR SpectrometersThose four parts are:Source/MonochromatorSample cellDetector/AmplifierOutput

Dispersive IR spectrometers were the first IR instruments, however their simplicity and longevity allows them to continue in service for most routine organic analyses their speed and resolution is adequate

For the most part, their design is austere and relies on simple mechanics and optics to generate a spectrum, very similar to simply rotating a glass prism to see different bands of visible light

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformDispersive IR SpectrometersHere is a general schematic:

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformDispersive IR SpectrometersSource is a heated nichrome wire which produces a broad band continuum of IR light (as heat)The beam is directed through both the sample and a reference cellA rapidly rotating sector (beam chopper) continuously switches between directing the two beams to a diffraction grating

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformDispersive IR SpectrometersThe diffraction grating slowly rotates, such that only one narrow frequency band of IR light is at the proper angle to reach the detectorA simple circuit compares the light from the sample and reference and sends the difference to a chart recorder

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformDispersive IR SpectrometersOn the older instruments the motor in the chart recorder was synchronized (& calibrated) to the motor on the diffraction gratingBecause each spectrum is the result of the tabulation of the spectroscopic process at each frequency individually, it is said to record the spectrum in the frequency domain

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformDispersive IR SpectrometersAdvantages simple, easy to maintain last the life of the source and moving partsDisadvantages to cover the entire IR band of interest to chemists it is necessary to use two diffraction gratingsAt high q, the component frequencies are more spread out, so the resulting spectra appear to have various regions expanded or compressedThe limit to resolution is 2-4 cm-1

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformFourier Transform IR SpectrometersFT-IR is the modern state of the art for IR spectroscopyThe system is based on the Michelson interferometerLaser source IR light is separated by a beam splitter, one component going to a fixed mirror, the other to a moving one and are reflected back to the beam splitterThe beam splitter recombines the two to a pattern of constructive and destructive interferences known as an interferogram a complex signal, but contains all of the frequencies that make up the IR spectrum

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformFourier Transform IR SpectrometersThe resulting signal is essentially a plot of intensity vs. time Such information if plotted would look like the following:

This is meaningless to a chemist we need this to be in the frequency domain rather than time.

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformFourier Transform IR SpectrometersBy applying a mathematical transform on the signal a Fourier transform the resulting frequency domain spectrum can be observed

FT-IRs give three theoretical advantages:Fellgetts advantage every point in the interferogram is information all wavelenghts are representedJacquinots advantage the entire energy of the source is used increasing signal-to-noiseConnes advantage frequency precision Dispersive instruments can have errors in the ability to move slits and gratings reproducibly FTIR is internally referenced from its own beam

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Dispersive and Fourier TransformFourier Transform IR SpectrometersJustiks advantage does it give me what I needSingle-beam instrument collect a background (air has IR active molecules!)

Fast all frequencies are scanned simultaneously

No referencing!

Computer based scaling and editing of the spectrum to squeeze out the most data; spectra are proportional (no stretching or squeezing of regions), comparison with spectral libraries

Disadvantages expenisve relative to dispersive instruments, and the components take more expertise and service calls to replace

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Experimental aspectsSample size typically the size of the beam mms mgs

Non-destructive sample can be recovered with varying degrees of difficulty

Liquid samples the easiest IR spectra are those of neat liquid samplesSolid samples are too dense for good IR spectra inter-molecular coupling of vibrational states occurs and peaks are greatly broadened

In the liquid state full 3-D motion is available, and these effects are averaged out and diminished

The thickness of a sample can be decreased to reduce these effects further

Thin film liquid samples are best!

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Experimental aspectsLiquid samplesSample cell cannot possess covalent bonds (SiO2, or glass is out)

The most common cell is a pair of large transparent windows of inorganic salts

Most common:

NaCl cheap, transparent from 650 4000 cm-1, but fragileLess common AgCl, KBr, etc. if you need transparency below 650, limit is practically 400

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Experimental aspectsSolution samplesOne way solids can be handled is as a solutionKey is that the solvent picked will cover the least amount of the spectrum as possible, as it will also be presentCommon solvents typically are symmetrical, or have many halogenated bonds low cm-1: CCl4, CHCl3, CH2Cl2, etc.The cell in this case is two NaCl (or other) windows with a spacer, the sample is loaded via a syringe into the cell:

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Experimental aspectsSolution samplesA newer method involves the use of a polyethylene matrix, that will hold allow a solution sample to evaporate, leaving small portions of the sample embedded in the matrix

The samples are liquid-like

The only interference is that of hydrocarbon

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Experimental aspectsSolid SamplesThe most common treatment for solid samples is to mull them with thick mineral oil (high MW hydrocarbon) - Nujol

Just like with the polyethylene cards, the molecules of the sample are held in suspension within the oil matrix

Again, the interference is that of hydrocarbon

IR Spectroscopy

Instrumentation and Experimental AspectsThe IR Spectrometer Experimental aspectsSolid SamplesThe connoisseurs method (with no organic interference) is to press the solid with KBr into a pelletUnder high pressure the KBr liquefies and entraps individual molecules of the sample in the matrixThese spectra are the only spectra of solids that are as interference free as liquids