Get In The Zone The basics of Reading Infrared Spectrometry Graphs What You Will Learn This is a tutorial to help you learn to recognize and read the peaks in Infrared Spectrometry Graphs. A Bit About Infrared Molecules are vibrating. When they absorb photons of the appropriate energy changes this vibration. What Do We Want to Know? We want to know how the structure is related to the energy that is absorbed. Why Should I care? This will let us make conclusions about the structure. What does an IR spectrum show us? FUNCTIONAL GROUPS!!! Sooo…what are we looking at? X AXIS: stretching frequency Y AXIS: number of photons absorbed More detail about the X and Y axis are explained in detail in Dr. Hardinger’s lectures. This tutorial is focused more on how to determine the peaks in the IR spectrum… What are we looking for? PEAKS! We are looking for downward spikes in the graph. Why are some peaks bigger than others? Polarity determines the peak’s strength. The more polar a molecule is, the stronger the peak is. Repetition can also determine strength; many of the same functional group leads to a larger peak. How does this help us? This helps us associate peaks with the bonded atoms. Prep Work!!! Calculate the DBE! You will need to look at the DBE value to determine the number of pi bonds. This will help you eliminate and differentiate among functional groups.
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Get In The Zone
The basics of Reading Infrared Spectrometry Graphs
What You Will Learn This is a tutorial tohelp you learn to recognize and read thepeaks in Infrared SpectrometryGraphs. A Bit About Infrared Molecules are vibrating.When they absorb photons of the appropriate energy changes thisvibration.What Do We Want to Know? Wewanttoknowhowthestructureisrelatedtotheenergythatisabsorbed.Why Should I care? Thiswillletusmakeconclusionsaboutthestructure.What does an IR spectrum show us? FUNCTIONALGROUPS!!!Sooo…what are we looking at? XAXIS:stretchingfrequencyYAXIS:numberofphotonsabsorbedMore detail about the X and Y axis are explained in detail in Dr. Hardinger’s lectures. ThistutorialisfocusedmoreonhowtodeterminethepeaksintheIRspectrum…What are we looking for? PEAKS!Wearelookingfordownwardspikesinthegraph.Why are some peaks bigger than others? Polaritydeterminesthepeak’sstrength.Themorepolaramoleculeis,thestrongerthepeakis.Repetition canalsodetermine strength;manyof the same functional group leads toa largerpeak.How does this help us? Thishelpsusassociatepeakswiththebondedatoms.Prep Work!!! CalculatetheDBE!YouwillneedtolookattheDBEvaluetodeterminethenumberofpibonds.Thiswillhelpyoueliminateanddifferentiateamongfunctionalgroups.
ZONE 1 Thiszonetakesplacebetween3700‐3200cm‐1(onthexaxis)
[citation(a)]
What are we looking for?
Alcohols,terminalalkynes,N‐Hstretches
Tips to Note: Peaksmayvaryinsize.Donotimmediatelyruleoutpeaksbecauseyouthinktheyaretoo
small!
T h e B r e a k d o w n
A l c o h o l s ( O – H ) Occuraround3650–3200cm‐1
Alcoholshaveaverydistinctstrongandbroadshape.Whenweseethissortofelongated“U”shapearoundthisregion,weknowthereisanalcoholgroup.Itwillgenerallylooklikethe
portionencircledinthepinkovalbelow.
[citation(a)]
T e r m i n a l A l k y n e s ( = C - H ) Occursaround3340‐3250cm‐1
TerminalAlkynesaregenerallystrongerandsharperinshape.Thepeakwithinthepinkovalbelowisaterminalalkyne.Beonthelookoutforsimilarshapesinthisregion.
[citation(b)]
N – H s t r e t c h e s Occursaround3500‐3200cm‐1
N–Hstretchescanmeanaminesoramides,butitdoesnotnecessarilyhavetoindicateeitherofthese.Thesearemediuminintensity,butarebroaderinappearance.
NOTE:Besuretolookatyourmolecularformulatomakesureanitrogenexistsinyourmolecule!AnitrogenisnecessaryfortheexistenceofanN‐Hstretch!
AllthreeofthefollowingareN‐Hstretches.Thefirstshowsaprimaryamine,andthesecondshowsatertiaryamine.ThelastimageshowsanN‐Hstretch,whichissignificantlysmallerthan
intheotherexamples,butstillcounts.
[citation(b)]
[citation(b)]
[citation(c)]
ZONE 2
Thiszonetakesplacebetween3200‐2700cm‐1(onthexaxis)
[citation(a)]
What are we looking for? Sp2carbons,sp3carbons,aldehydes,carboxylicacid
Tips to Note:
LookforC‐Hbondinginthiszone!Weareinorganicchemistry;nearlyeverymoleculethatwedealwithwillinvolveC‐Hbonding.Useyourwritingutensiltomapoutareasandfocusinon
thesespecificareastomakesureyouarereadingthegraphaccurately!
T h e B r e a k d o w n
S p 2 C a r b o n - H y d r o g e n ( C - H ) Occursaround3100‐3000cm‐1
Sp2carbonsmayvaryinsizeandshapeItisimportanttonoteexactlywhereyouseethepeak!Thispeakwilloccurinaverysmallrange,highlightitandnoteitdown!
[ProfessorStevenHardinger’sThinkbook]
S p 3 C a r b o n - H y d r o g e n / A l k y l ( C - H ) Occursaround2960‐2850cm‐1
Sp3carbonsmayalsovaryinsizeandshapeItisimportanttonoteexactlywhereyouseethepeak!Thispeakwilloccurinaverysmallrange,highlightitandnoteitdown!
[ProfessorStevenHardinger’sThinkbook]
A l d e h y d e ( C - H ) Onepeakaround2900cm‐1andanotheraround2700cm‐1
Aldehydesareofmediumlengthandhavetwopeaks.Becareful!!Thefirstpeakisoftendistortedbysp3carbonsandmaynotbeeseen!Lookforthesecondpeakaround2700cm‐1.
[ProfessorStevenHardinger’sLectureSupplement]Thesecondpeakiscircledinpinkintheimageabove
C a r b o x y l I c A c I d ( O - H ) Occursaround3000‐2500cm‐1
Wecandetermineacarboxylicacidbylookingforastrong,broadpeak.BECAREFUL!!!Acarboxylicacidhastwocomponents,theremustalsobeaC=Oinzone4tohaveacarboxylic
acid!!!
NoticethatthereisanOHstretchinzone2andaC=Ostretchinzone4!
[citation(d)]Here’sanotherexample
[citation(a)]
ZONE 3
Thiszonetakesplacebetween2300‐2000cm‐1(onthexaxis)
[citation(a)]
T h e B r e a k d o w n
A l k y n e ( C ≡ C ) Around2260‐2000cm‐1
Alkyneshavevariableandsharppeaks,suchasthepeakcircledinpinkbelow.Note:LookatyourDBEvalue!Youneed2pibondstoformatriplebond
[citation(e)]
N i t r i l e ( C ≡ N )
Onearound2260‐2220cm‐1Alkyneshavevariableandsharppeaks,suchasthepeakinthepinkcirclebelow
NOTE:REMEMBERTOUSEYOURMOLECULARFORMULA!!!Iftherearenonitrogenspresent,youwillnothaveanitrilegroup!
ALSONOTE:Also,mindthenitrogensinzone1!DOUBLYALSONOTE:LOOKATYOURDBE!Youneed2pibondstoformanitrilegroup.
[citation(b)]
ZONE 4
Thiszonetakesplacebetween1850‐1650cm‐1(onthexaxis)Thiszonecontainscarbonyls.
REMEMBER:CarbonylsarecarbonsdoublebondedtoOxygens.PIBONDSARENECESSARY!LOOKATYOURDBEVALUE!
ALSOREMEMBER:Manyofthesecarbonylsrelatetootherpeaksinvariousotherzonesofthegraph.Forsomeofthesefunctionalgroupstoexist,theremustbeothercorrespondingpeaks.
LOOKFORTHEM!DOUBLYALSOREMEMBER:Allofthepeaksinthiszoneappearstrong.Payattentiontothe
numberofphotonsabsorbed(theyaxisvalue)!!!
[citation(a)]
T h e B r e a k d o w n
K e t o n e ( C = O ) Around1750‐1705cm‐1
Ketoneswillhaveastrongpeak.GENERALLYLookfornumberofphotonsinthelow1700s.Thismaynotalwaysbethecase,however.Remembertolookforsignsofconjugation!
[citation(d)]
NOTE:1715isacommonvalueforaketone,butcanalsoapplytoaldehydes.Theyfallwithinsimilarranges.HowcanItellthedifference?Ifthereisanaldehyde,therewillalsobethedoublepeakedformationinzone2!Beonthelookoutandkeeprelatingwhatyoualreadyknowtowhatyouarefindinginthefollowingzones!
A l d e h y d e ( C = O )
Around1740‐1720cm‐1Aldehydeswillhaveastrongpeak.Lookinthelow1700s,butGENERALLYclosertothe1720
range.Thismaynotalwaysbethecase,however.Remembertolookforsignsofconjugation!
Noticethealdehydepeakinzone2aswell!
[ProfessorStevenHardingerLectureSupplement]
E s t e r s ( C = O ) Around1750‐1735cm‐1
Esterswillhaveastrongpeak.Numbersaroundthe1740srangearetypical.Thismaynotalwaysbethecase,however.Remembertolookforsignsofconjugation!
[citation(d)]
C a r b o x y l I c A c I d ( C = O )
Around1250‐1700cm‐1Thecarbonylwillhaveastrongpeak.Lookaround1710sto1720s.Again,thesenumbersmay
vary,thisisjustageneralguidelineforwhatwouldbeconsideredtypical.
NoticetheO‐Hstretchinzone2!!!Thisisnecessaryforacarboxylicacid!
[citation(d)]
A m I d e ( C = O ) Around1690‐1650cm‐1
Thecarbonylstretchwillbestrong,again.LookforthenumberofNitrogensinyourmolecule.Also,lookfortheamidgroupinzone1!
[citation(b)]
ZONE 5
Thiszonetakesplacebetween1680‐1450cm‐1(onthexaxis)NOTE:Thiszoneincludesdoublebonds!LookatyourDBEvalues!!!
T h e B r e a k d o w n
A l k e n e ( C = C ) Around1680to1620cm‐1
Thispeakmayvary.Itmayhelptohighlightthisregiontobetterseethepeak.CheckyourDBEvalue!
[citation(d)]
B e n z e n e ( C = C ) Onepeakaround1600cm‐1andanotheraround1500‐1450cm‐1
Thispeakmayvary.Thepeakat1600mayhave2peaks.LookforbothpeaksandaDBEvalueofatleast4!
[ProfessorStevenHardinger’sLectureSupplement]
Additional Notes
BEWAREOFCONJUGATION!!!We remember that conjugation lowers the energy of a molecule. This implies in reading IRgraphsaswell. If functionalgroupsareconjugated, thepibondcanwill lower the stretchingfrequency. More conjugation leads to A lower stretching frequency. You can suspectconjugationwhenpeaksareatthelowerendoftheirrange.HIGHLIGHTTHEZONES!!!When getting used toworkingwith the different zones, itmay be helpful to color code thezonessoyoucanzeroinonspecificareasanddifferentiatepeaksthatway.DON’TWORRYABOUTMEMORIZATION!!!ThetableofzonesandstretchingfrequencieswillbegiventoyouonatestREMEMBERYOURDBEVALUES!!!Pibondingisessentialindeterminingandrulingoutcertainfunctionalgroups.ALWAYSREFERBACKTOTHEFORMULA!!!Iftheformulaisgiventoyou,remembertoreferbacktoit.ThisformulagivesyouESSENTIALinformationontheatomsinvolvedinthemolecule.
Work Cited
(a) Clark,Jim."InterpretingInfra‐redSpectra."InterpretingInfraredSpectra.2000.Web.11May2012.<http://www.chemguide.co.uk/analysis/ir/interpret.html>.
(b) Glagovich,Neil."InfraredSpectroscopy."CHEM316.2012.Web.11May2012.<http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html>.
(c) Matt."InfraredAbsorbanceSpectroscopy(IR)."MendelSet.Mendelset,19July2011.Web.11May2012.<http://www.mendelset.com/articles/686/infrared‐absorbance‐spectroscopy‐ir>.
(d) "IRSpectroscopyTutorial."IR:CarboxylicAcids.UniversityofColorado,Boulder,ChemistryandBiochemistryDepartment,2011.Web.11May2012.<http://orgchem.colorado.edu/Spectroscopy/irtutor/carbacidsir.html>.
(e) "ChemWiki:TheDynamicChemistryTextbook."ChemWikiDavis.UniversityofCalifornia,Davis,7Oct.2011.Web.11May2012.<http://chemwiki.ucdavis.edu/>.
(f) ProfessorStevenHardinger’sLectureSupplement(g) ProfessorStevenHardinger’sThinkbook
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