INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACH 1 .

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INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACHInterpretation of InfraredSpectra, A Practical ApproachJohn CoatesCoates Consulting, Newtown, USA1 Introduction2 The Origins of the Infrared Spectrum3 Spectral Interpretation by Application ofVibrational Group Frequencies3.1 The Hydrocarbon Species andMolecular Backbone3.2 Simple Functional Groups3.3 The Carbonyl Group3.4 Other Functional Groups Associatedwith Heteroatoms3.5 Simple Inorganics4 The Practical Situation – Obtaining theSpectrum and Interpreting the Results4.1 Sample History4.2 Physical Characteristics of theSample4.3 The Chemistry of the Sample4.4 The Infrared Sampling Method5 An Overview to Infrared SpectralInterpretation – Some Simple Rules andGuidelines5.1 A Quick Diagnostic Assessment ofan Infrared Spectrum1266912131415161717181920Abbreviations and Acronyms22Related ArticlesReferences2223The vibrational spectrum of a molecule is considered tobe a unique physical property and is characteristic of themolecule. As such, the infrared spectrum can be used asa fingerprint for identification by the comparison of thespectrum from an ‘‘unknown’’ with previously recordedreference spectra. This is the basis of computer-basedspectral searching. In the absence of a suitable referencedatabase, it is possible to effect a basic interpretation of thespectrum from first principles, leading to characterization,and possibly even identification of an unknown sample.This first principles approach is based on the fact thatstructural features of the molecule, whether they are thebackbone of the molecule or the functional groups attachedto the molecule, produce characteristic and reproducibleEncyclopedia of Analytical ChemistryR.A. Meyers (Ed.) Copyright John Wiley & Sons Ltd1absorptions in the spectrum. This information can indicatewhether there is backbone to the structure and, if so,whether the backbone consists of linear or branched chains.Next it is possible to determine if there is unsaturationand/or aromatic rings in the structure. Finally, it is possibleto deduce whether specific functional groups are present.If detected, one is also able to determine local orientationof the group and its local environment and/or location inthe structure. The origins of the sample, its prehistory, andthe manner in which the sample is handled all have impacton the final result. Basic rules of interpretation exist and,if followed, a simple, first-pass interpretation leading tomaterial characterization is possible. This article addressesthese issues in a simple, logical fashion. Practical examplesare included to help guide the reader through the basicconcepts of infrared spectral interpretation.1 INTRODUCTIONThe qualitative aspects of infrared spectroscopy are oneof the most powerful attributes of this diverse andversatile analytical technique. Over the years, much hasbeen published in terms of the fundamental absorptionfrequencies (also known as group frequencies) which arethe key to unlocking the structure – spectral relationshipsof the associated molecular vibrations. Applying thisknowledge at the practical routine level tends to bea mixture of art and science. While many purists willargue against this statement, this author believes that itis not possible to teach a person to become proficient asan interpretive spectroscopist by merely presenting theknown relationships between structure and the observedspectra. Instead, the practical approach, which has beenadopted in this text, is to help the reader appreciate thevisual aspects of the spectroscopy and how to interpretthese relative to the structure and chemistry of the sample.This is achieved by recognizing characteristic shapesand patterns within the spectrum, and by applying theinformation obtained from published group frequencydata, along with other chemical and physical data fromthe sample.Included in the text is a discussion of the interrelationships that exist between the practical side of acquiringthe spectrum, the chemistry and physics of the sampleunder study, the physical interactions of the sample withits environment, and the impact of the structure on thespectrum. In essence, the interpretation of infrared spectra is much more than simply assigning group frequencies.The spectrum is rich in information, and this article isintended to help the reader to extract the maximumusing the knowledge available for the sample and theacquired spectral data. One important factor to bear in

2mind is that a successful interpretation is based not onlyon the presence of particular bands within the spectrum,but also the absence of other important bands. Completeclasses of compounds can be rapidly excluded during theinterpretation by the use of no-band information.It must be understood that this article addressesthe issue of infrared spectral interpretation from theperspective of the average operator of an infraredinstrument. It is not a detailed treatise on the theoryof infrared spectroscopy where the modes of vibrationare discussed in terms of group theory, and wheremathematical models are used to compare theoreticaland observed values for the fundamental vibrations of amolecule. There are many excellent texts that cover thissubject.1 – 4/ Instead, this article focuses on the day-today problems associated with characterizing a material orattempting to perform some form of identification. Oneof the main challenges in presenting a text on spectralinterpretation is to form a balance between the theorythat is needed to appreciate the links between molecularstructure and the observed spectrum and the practice.For this reason, a minimum amount of relevant theoryis included in the next section, which provides a basicunderstanding of why the spectrum exists, how it isformed, and what factors contribute to the complexityof observed spectra. It has been assumed that the readerhas a fundamental knowledge of molecular theory andbonding, and that there is an understanding of basicstructures, in particular for organic compounds.Infrared spectral interpretation may be applied toboth organic and inorganic compounds, and there aremany specialized texts dealing with these compounds, incombination and as individual specialized texts. Thereare too many to reference comprehensively, and thereader is directed to a publication that provides abibliography of the most important reference texts.5/However, the most informative general reference textsare included,.6 – 14/ with books by Socrates.10/ and LinVien.11/ being recommended for general organics, andby Nakamoto.13/ and Nyquist et al.14/ for inorganics(salts and coordination compounds). There are numerousspecialized texts dealing with specific classes of materials,and undoubtedly polymers and plastics form the largestindividual class.15 – 17/ In this particular case, texts byHummel and Scholl.16/ and Koenig.17/ provide a goodbasic understanding.The following comments are made relative to the conventions used within this article. The term frequencyis used for band/peak position throughout, and this isexpressed in the commonly used units of wavenumber (cm 1 ). The average modern infrared instrumentrecords spectra from an upper limit of around 4000 cm 1(by convention) down to 400 cm 1 as defined by theoptics of the instrument (commonly based on potassiumINFRARED SPECTROSCOPYbromide, KBr). For this reason, when a spectral region isquoted in the text, the higher value will be quoted first,consistent with the normal left-to-right (high to low cm 1 )representation of spectra. Also, the terms infrared band,peak and absorption will be used interchangeably withinthe text to refer to a characteristic spectral feature.The spectral group frequencies provided in this textwere obtained from various literature sources publishedover the past 30 years, and most of these are includedin the cited literature. Every attempt to ensure accuracyhas been taken; however, there will be instances whenindividual functional groups may fall outside the quotedranges. This is to be expected for several reasons: theinfluences of other functional groups within a molecule,the impact of preferred spatial orientations, and environmental effects (chemical and physical interactions) on themolecule.The preferred format for presenting spectral data forqualitative analysis is in the percentage transmittanceformat, which has a logarithmic relationship ( log10 ) withrespect to the linear concentration format (absorbance).This format, which is the natural output of mostinstruments (after background ratio), provides the bestdynamic range for both weak and intense bands. In thiscase, the peak maximum is actually represented as aminimum, and is the point of lowest transmittance for aparticular band.2 THE ORIGINS OF THE INFRAREDSPECTRUMIn the most basic terms, the infrared spectrum is formedas a consequence of the absorption of electromagneticradiation at frequencies that correlate to the vibration ofspecific sets of chemical bonds from within a molecule.First, it is important to reflect on the distribution of energypossessed by a molecule at any given moment, defined asthe sum of the contributing energy terms (Equation 1):Etotal D Eelectronic C Evibrational C Erotational C Etranslational.1/The translational energy relates to the displacement ofmolecules in space as a function of the normal thermalmotions of matter. Rotational energy, which gives riseto its own form of spectroscopy, is observed as thetumbling motion of a molecule, which is the result ofthe absorption of energy within the microwave region.The vibrational energy component is a higher energyterm and corresponds to the absorption of energy by amolecule as the component atoms vibrate about the meancenter of their chemical bonds. The electronic componentis linked to the energy transitions of electrons as they

3INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACHare distributed throughout the molecule, either localizedwithin specific bonds, or delocalized over structures, suchas an aromatic ring. In order to observe such electronictransitions, it is necessary to apply energy in the form ofvisible and ultraviolet radiation (Equation 2):E D hnfrequency/energy.2/The fundamental requirement for infrared activity, leading to absorption of infrared radiation, is that there mustbe a net change in dipole moment during the vibrationfor the molecule or the functional group under study.Another important form of vibrational spectroscopy isRaman spectroscopy, which is complementary to infraredspectroscopy. The selection rules for Raman spectroscopyare different to those for infrared spectroscopy, and inthis case a net change in bond polarizability must beobserved for a transition to be Raman active. The remaining theoretical discussion in this article will be limited to avery simple model for the infrared spectrum. The readeris encouraged to refer to more complete texts.2 – 4/ fordetailed discussion of the fundamentals.While it was stated that the fundamental infraredabsorption frequencies are not the only component to beevaluated in a spectral interpretation, they are the essenceand foundation of the art. For the most part, the basicmodel of the simple harmonic oscillator and its modification to account for anharmonicity suffice to explain theorigin of many of the characteristic frequencies that canbe assigned to particular combinations of atoms withina molecule. From a simple statement of Hooke’s law wecan express the fundamental vibrational frequency of amolecular ensemble according to Equation (3):r1knD.3/2pc µwhere n D fundamental vibration frequency, k D forceconstant, and µ D reduced mass. The reduced mass, µ Dm1 m2 /.m1 C m2 /, where m1 and m2 are the componentmasses for the chemical bond under consideration.This simple equation provides a link between thestrength (or springiness) of the covalent bond betweentwo atoms (or molecular fragments), the mass of the interacting atoms (molecular fragments) and the frequency ofvibration. Although simple in concept, there is a reasonably good fit between the bond stretching vibrationspredicted and the values observed for the fundamentals.This simple model does not account for repulsion andattraction of the electron cloud at the extremes of thevibration, and does not accommodate the concept ofbond dissociation at high levels of absorbed energy. Amodel incorporating anharmonicity terms is commonlyused to interpret the deviations from ideality and theoverall energy – spatial relationship during the vibrationof a bond between two atomic centers. The fundamental,which involves an energy transition between the groundstate and the first vibrational quantum level, is essentially unaffected by the anharmonicity terms. However,transitions that extend beyond the first quantum level(to the second, third, fourth, etc.), which give rise toweaker absorptions, known as overtones, are influencedby anharmonicity, which must be taken into accountwhen assessing the frequency of these higher frequencyvibrations.Having defined the basis for the simple vibration ofan atomic bond, it is necessary to look at the moleculeas a whole. It is very easy to imagine that there is aninfinite number of vibrations, which in reality wouldlead to a totally disorganized model for interpretation.Instead, we describe the model in terms of a minimumset of fundamental vibrations, based on a threefold set ofcoordinate axes, which are known as the normal modesof vibration. All the possible variants of the vibrationalmotions of the molecule can be reduced to this minimumset by projection on to the threefold axes. It can be shownthat the number of normal modes of vibration for a givenmolecule can be determined from Equations (4) and (5):number of normal modes D 3ND 3N6 (nonlinear) .4/5 (linear).5/where N is the number of component atoms in themolecule.In practice, apart from the simplest of compounds, mostmolecules have nonlinear structures, except where a specific functional group or groups generate a predominantlinear arrangement to the component atoms. If we calculate the number of modes for a simple hydrocarbon, suchas methane (nonlinear, tetrahedral structure), a value ofnine is obtained. This would imply that nine sets of absorption frequencies would be observed in the spectrum ofmethane gas. In reality, the number observed is far less,corresponding to the asymmetric and symmetric stretching and bending of the C H bonds about the centralcarbon atom. The reason for the smaller than expectednumber is that several of the vibrations are redundant ordegenerate, that is, the same amount of energy is requiredfor these vibrations. Note that although a small numberof vibrational modes is predicted, and in fact observed,the appearance of the methane spectrum at first glanceis far more complex than expected, especially at higherspectral resolutions ( 1 cm 1 ). At relatively high resolutions, a fine structure is superimposed, originating fromrotational bands, which involve significantly lower energytransitions. Each of the sets of vibrational – rotationalabsorptions manifest this superimposed fine structurefor low-molecular-weight gaseous compounds, methanebeing a good example. Several medium-molecular-weight

4INFRARED 0018002000240028003200360004000Transmittance (%)compounds may also show evidence of some fine structure when studied in the vapor state. For example, it iscommon to observe the sharp feature (or spike) assignedto the Q-branch of the vibrational – rotational spectrum,as indicated by the vapor spectrum of acetone (Figure 1).If we proceed up the homologous series from methane(CH4 ) to n-hexane (C6 H14 ), there are 20 componentatoms, which would imply 54 normal modes. In this casethe picture is slightly more complex. Methane is a uniquemolecule, and only contains one type of C H group – noother types of bond exist in this molecule. In hexane thereare several types of bond and functionality. For reference,a simple two-dimensional representation of the structureis provided in Figure 2(a).As we can see, there are two terminal methyl groups(CH3 ) and four connecting methylene groups (CH2 ).Each of these groups has its corresponding C Hstretching and bending vibrations (see later text for theactual absorption frequencies). Also, the methyl groupsare linked to a neighboring methylene group, which isin turn linked to neighboring methylene groups, and soon. These linkages feature carbon– carbon bonds. Forinterpretation, we view the C H groups as functionalgroups, giving rise to the common group frequencies,and the C C linkages as the backbone, producing theskeletal vibrations. As a rule, a group frequency maybe applied generally to most compounds featuring thecorresponding functional group. In contrast, the skeletalvibrations are unique to a specific molecule. The groupfrequencies help to characterize a compound, and theWavenumber (cm 1)Figure 1 Vapor spectrum of acetone with characteristicQ-branch slitting, denoted by Q. Copyright Coates Consulting.HH H HHH C C C HH C C C HH H H HH(a)HHH CH CHH(b)CH HHC HCC HHHHFigure 2 Structures for hexane isomers: (a) n-hexane and(b) isohexane (2-methylpentane). Copyright Coates Consulting.combination of the bands associated with these groupfrequencies and the skeletal frequencies are used toidentify a specific compound. The latter forms the basis ofthe use of reference spectra for spectral matching by visualcomparison or by computer-based searching, for theidentification of an unknown from its infrared spectrum.The group frequencies may be viewed quantitatively,as well as qualitatively. A given absorption band assignedto a functional group increases proportionately with thenumber times that functional group occurs within themolecule. For example, in the spectrum of n-hexane, theintensities measured for the group frequency absorptionsassigned to methyl and methylene correspond to fourmethylene groups and two methyl groups on a relative basis, when compared with other hydrocarboncompounds within a homologous series. For example,if we examine the C H stretching (or bending) bandintensities for CH3 and CH2 , we will observe that therelative intensities of CH3 to CH2 decrease with increasein chain length. Restated, there is less methyl contribution and more methylene contribution with increase inchain length/molecular weight. The reverse holds true ifwe examine the spectra of linear hydrocarbons with chainlengths shorter than that of hexane.If we apply these ideas to a different hexane isomer, such as isohexane (2-methylpentane), we wouldsee significant differences in the spectrum. These can beexplained by evaluating the structure (Figure 2b), whichcontains three methyl groups, two methylene groups, anda group that contains a single hydrogen attached to carbon (the methyne group). This adds a new complexityto the spectrum: the main absorptions show differencesin appearance, caused by the changes in relative bandintensities, splittings of absorptions occur (originatingfrom spatial/mechanical interaction of adjacent methylgroups), and changes are observed in the distributions ofthe C C skeletal vibrations, in part due to the splitting bythe methyl side chain. Further discussions concerning theimpact of chain branching are covered later in this article.Comparison of Figures 3 and 4 provides a graphical representation of the aspects discussed for the hexanes of structurally similar compounds, i.e. n-heptane and isooctane.From a first-order perspective, the idea of the quantitative aspects of the group frequencies carries throughfor most functional groups, and the overall spectrum isessentially a composite of the group frequencies, withband intensities in part related to the contribution of eachfunctional group in the molecule. This assumes that thefunctional group does give rise to infrared absorptionfrequencies (most do), and it is understood that eachgroup has its own unique contribution based on it

The fundamental requirement for infrared activity, lead-ing to absorption of infrared radiation, is that there must be a net change in dipole moment during the vibration for the molecule or the functional group under study. Another important form of vibrational spectroscopy is Raman spectroscopy, which is complementary to infrared

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