红外（IR）光谱实用解析课件.pdf

Interpretation of Infrared Spectra,A Practical ApproachJohn CoatesinEncyclopedia of Analytical ChemistryR.A.Meyers(Ed.)pp.1081510837 John Wiley&Sons Ltd,Chichester,2000INTERPRETATION OF INFRARED SPECTRA,A PRACTICAL APPROACH1Interpretation of InfraredSpectra,A Practical ApproachJohn CoatesCoates Consulting,Newtown,USA1Introduction12The Origins of the Infrared Spectrum23Spectral Interpretation by Application ofVibrational Group Frequencies63.1The Hydrocarbon Species andMolecular Backbone63.2Simple Functional Groups93.3The Carbonyl Group123.4Other Functional Groups Associatedwith Heteroatoms133.5Simple Inorganics144The Practical Situation Obtaining theSpectrum and Interpreting the Results154.1Sample History164.2Physical Characteristics of theSample174.3The Chemistry of the Sample174.4The Infrared Sampling Method185An Overview to Infrared SpectralInterpretation Some Simple Rules andGuidelines195.1A Quick Diagnostic Assessment ofan Infrared Spectrum20Abbreviations and Acronyms22Related Articles22References23The 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 thebackboneofthemoleculeorthefunctionalgroupsattachedto the molecule,produce characteristic and reproducibleabsorptions in the spectrum.This information can indicatewhether there is backbone to the structure and,if so,whetherthebackboneconsistsoflinearorbranchedchains.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 structurespectral 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 interprettheserelativetothestructureandchemistryofthesample.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 interrelation-ships 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 spec-traismuchmorethansimplyassigninggroupfrequencies.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 inEncyclopedia of Analytical ChemistryR.A.Meyers(Ed.)Copyright John Wiley&Sons Ltd2INFRARED SPECTROSCOPYmind 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.14/Instead,this article focuses on the day-to-day 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,.614/with books by Socrates.10/and Lin-Vien.11/being recommended for general organics,andby Nakamoto.13/and Nyquist et al.14/for inorganics(saltsandcoordination compounds).Therearenumerousspecialized texts dealing with specific classes of materials,and undoubtedly polymers and plastics form the largestindividual class.1517/In this particular case,texts byHummel and Scholl.16/and Koenig.17/provide a goodbasic understanding.The following comments are made relative to the con-ventions used within this article.The term frequencyis used for band/peak position throughout,and this isexpressed in the commonly used units of wavenum-ber(cm?1).The average modern infrared instrumentrecords spectra from an upper limit of around 4000cm?1(by convention)down to 400cm?1as defined by theoptics of the instrument(commonly based on potassiumbromide,KBr).For this reason,when a spectral region isquoted in the text,the higher value will be quoted first,consistentwiththenormalleft-to-right(hightolowcm?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 30years,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 environ-mental effects(chemical and physical interactions)on themolecule.The preferred format for presenting spectral data forqualitative analysis is in the percentage transmittanceformat,whichhasalogarithmicrelationship(?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,itisimportanttoreflectonthedistributionofenergypossessed by a molecule at any given moment,defined asthe sum of the contributing energy terms(Equation 1):EtotalD EelectronicC EvibrationalC ErotationalC 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 amoleculeasthecomponent atomsvibrateaboutthemeancenteroftheirchemicalbonds.Theelectroniccomponentis linked to the energy transitions of electrons as theyINTERPRETATION OF INFRARED SPECTRA,A PRACTICAL APPROACH3are 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,lead-ing 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,whichiscomplementary toinfraredspectroscopy.TheselectionrulesforRamanspectroscopyare different to those for infrared spectroscopy,and inthis case a net change in bond polarizability must beobservedforatransitiontobeRamanactive.Theremain-ing theoretical discussion in this articlewill be limited to avery simple model for the infrared spectrum.The readeris encouraged to refer to more complete texts.24/fordetailed discussion of the fundamentals.While it was stated that the fundamental infraredabsorption frequencies are not the only component to beevaluatedinaspectralinterpretation,theyaretheessenceand foundation of the art.For the most part,the basicmodel of the simple harmonic oscillator and its modifica-tion 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 Hookes law wecan express the fundamental vibrational frequency of amolecular ensemble according to Equation(3):n D12pcrk.3/where n D fundamental vibration frequency,k D forceconstant,and D reduced mass.The reduced mass,Dm1m2/.m1C m2/,where m1and m2are the componentmasses for the chemical bond under consideration.This simple equation provides a link between thestrength(or springiness)of the covalent bond betweentwoatoms(ormolecularfragments),themassoftheinter-acting atoms(molecular fragments)and the frequency ofvibration.Although simple in concept,there is a rea-sonably 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 energyspatial 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 essen-tially 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 3N?6(nonlinear).4/D 3N?5(linear).5/where N is the number of component atoms in themolecule.Inpractice,apartfromthesimplestofcompounds,mostmolecules have nonlinear structures,except where a spe-cific functional group or groups generate a predominantlinear arrangement to the component atoms.If we calcu-late the number of modes for a simple hydrocarbon,suchas methane(nonlinear,tetrahedral structure),a value ofnineisobtained.Thiswouldimplythatninesetsofabsorp-tion frequencies would be observed in the spectrum ofmethane gas.In reality,the number observed is far less,corresponding to the asymmetric and symmetric stretch-ing 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,thatis,thesameamountofenergyisrequiredfor 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(1cm?1).At relatively high resolu-tions,a fine structure is superimposed,originating fromrotational bands,which involve significantly lower energytransitions.Each of the sets of vibrationalrotationalabsorptions manifest this superimposed fine structurefor low-molecular-weight gaseous compounds,methanebeingagoodexample.Severalmedium-molecular-weight4INFR