CHEM0019Vibrational Spectroscopy

CHEM0019 Computational Chemistry:Vibrational SpectroscopyTheworkinthisexperimentcombinestheprinciplesofvibrationalspectroscopyofdiatomicmolecules,thatyoulearntinCHEM0019,withgrouptheory,thatyoumaybelearninginelsewhereorcanreadaboutinVincent,“MolecularSymmetry andGroup Theory”(whichisavailableasane-book).In thisexperiment,we willusetheGaussian programviatheWebMO interface to calculate normal modevibrationsandusethesetointerpretIR spectraandcharacterisepotentialenergysurfaces.LogontoWebMo by going towebmo.chem.ucl.ac.ukinabrowser.1The IR spectrumof water1.Startanew job.2.Draw awatermoleculeinthebuildwindow.3.Cleanupthestructureusing“Comprehensive -Mechanics”4.Symmetrisethemolecule.Select“Calculate→Symmetry→SymmetriseMolecule”.A windowshouldopenwiththedetectedsymmetry-itshouldrecognisethatwaterisC2v.Clickon“Symmetrise”.Then “OK”.Move on tosubmitacalculation,bypressingthearrowatthebottom right.CallitH2O631andselectan“Optimise+ VibFreq”Hartree-Fockcalculationwitha6-31G(d)basisset.Submitthisjob.RepeatthisprocesstocalculatethefrequenciesattheMP2 (MollerPlesset2)level.CallthecalculationH2OMP2 and selectatheorylevelof MP2.Intheresultswindowscrolldowntofind“VibrationalModes”isthelistoffrequenciesforthewatermolecule.Vieweachinturn.Themagnifyingglassdrawsarrowstorepresentthevibrationalmotion.The filmstripanimatesthe vibration-seehow themotion corresponds tothearrows.The vibrationsareclassifiedas“bend”,“symmetric-stretch”and“anti-symmetric-stretch”.Identifywhichiswhichandfilloutthefollowingtablewiththefrequencies(harmonicwavenumbers)from bothHartree-Fock(HF)andMoller-Plesset(MP2) calculations.The experimental spectrum hasbands at1595,3657,and 3756cm−1.Addthesevaluestothetable,assignedtotherelevantvibration,andcommentonhowtheycomparewiththecalculatedvalues.

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