有机波谱分析双语课件 (39).pdf

紫外可见光谱 III 有机波谱分析 Organic Spectroscopic Analysis Ultraviolet and Visible(UV-vis)Spectroscopy III 2 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra A.A.Instrumentation Instrumentation 1.1.The construction of a traditional UVThe construction of a traditional UV-VIS spectrometer is very VIS spectrometer is very similar to an IR,as similar functions similar to an IR,as similar functions sample handling,irradiation,sample handling,irradiation,detection and output are requireddetection and output are required 2.2.Here is a simple schematic that covers most modern UV Here is a simple schematic that covers most modern UV spectrometers:spectrometers:sample reference detector I0 I0 I0 I log(I0/I)=A 200 700 l l,nm monochromator/beam splitter optics UV-VIS sources 3 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra A.A.Instrumentation Instrumentation 3.3.Two sources are required to scan the entire UVTwo sources are required to scan the entire UV-VIS band:VIS band:Deuterium lamp Deuterium lamp covers the UV covers the UV 200200-330330 Tungsten lamp Tungsten lamp covers 330covers 330-700700 4.4.The lamps illuminate the entire band of UV or visible light;the The lamps illuminate the entire band of UV or visible light;the monochromator(grating or prism)gradually changes the small monochromator(grating or prism)gradually changes the small bands of radiation sent to the beam splitter bands of radiation sent to the beam splitter 5.5.The beam splitter sends a separate band to a cell containing the The beam splitter sends a separate band to a cell containing the sample solution and a reference solutionsample solution and a reference solution 6.6.The detector measures the difference between the transmitted The detector measures the difference between the transmitted light through the sample(light through the sample(I I)vs.the incident light()vs.the incident light(I I0 0)and sends this)and sends this information to the recorder information to the recorder 4 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra A.A.Instrumentation Instrumentation 7.7.As each band is to be measured separately,As each band is to be measured separately,timetime is required to is required to cover the entire UVcover the entire UV-VIS band due to the mechanism of changing VIS band due to the mechanism of changing wavelengthswavelengths 8.8.A recent improvement is the A recent improvement is the diodediode-array spectrophotometerarray spectrophotometer -here here a prism(dispersion device)breaks apart the full spectrum a prism(dispersion device)breaks apart the full spectrum transmitted through the sampletransmitted through the sample 9.9.Each individual band of UV is detected by an individual diode on a Each individual band of UV is detected by an individual diode on a silicon wafer simultaneously silicon wafer simultaneously the obvious limitation is the size of the obvious limitation is the size of the diode,so some loss of resolution over traditional instruments is the diode,so some loss of resolution over traditional instruments is observedobserved sample Polychromator entrance slit and dispersion device UV-VIS sources Diode array 5 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra B.B.Instrumentation Instrumentation Sample HandlingSample Handling 1.1.Virtually all UV spectra are recorded in solutionVirtually all UV spectra are recorded in solution-phasephase 2.2.Cells can be made of plastic,glass or quartzCells can be made of plastic,glass or quartz 3.3.Only quartz is transparent in the full 200Only quartz is transparent in the full 200-700 nm range;plastic and 700 nm range;plastic and glass are only suitable for visible spectraglass are only suitable for visible spectra 4.4.Concentration(we will cover shortly)is empirically determinedConcentration(we will cover shortly)is empirically determined A typical sample cell(commonly called a A typical sample cell(commonly called a cuvettecuvette):):6 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra B.B.Instrumentation Instrumentation Sample HandlingSample Handling 5.5.Solvents must be transparent in the region to be observed;the Solvents must be transparent in the region to be observed;the wavelength where a solvent is no longer transparent is referred to wavelength where a solvent is no longer transparent is referred to as the as the cutoffcutoff 6.6.Since spectra are only obtained up to 200 nm,solvents typically Since spectra are only obtained up to 200 nm,solvents typically only need to lack conjugated only need to lack conjugated systems or carbonyls systems or carbonyls Common solvents and cutoffs:Common solvents and cutoffs:acetonitrile acetonitrile 190190 chloroformchloroform 240240 cyclohexanecyclohexane 195 195 1,41,4-dioxanedioxane 215215 95%ethanol95%ethanol 205205 n n-hexanehexane 201201 methanolmethanol 205205 isooctaneisooctane 195195 waterwater 190190 7 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination A.A.DienesDienes 1.1.General FeaturesGeneral Features For acyclic butadiene,two conformers are possible For acyclic butadiene,two conformers are possible s s-cis cis and and s s-transtrans The The s s-ciscis conformer is at an overall higher potential energy than the conformer is at an overall higher potential energy than the s s-transtrans;therefore the HOMO electrons of the conjugated system;therefore the HOMO electrons of the conjugated system have less of a jump to the LUMO have less of a jump to the LUMO lower energy,longer wavelengthlower energy,longer wavelength s-trans s-cis 8 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination A.A.DienesDienes 1.1.General FeaturesGeneral Features Two possible Two possible *transitions can occur for butadiene*transitions can occur for butadiene 2 2 3 3*and and 2 2 4 4*The The 2 2 4 4*transition is not typically observed:transition is not typically observed:The energy of this transition places it outside the region The energy of this transition places it outside the region typically observed typically observed 175 nm175 nm For the more favorable For the more favorable s s-transtrans conformation,this conformation,this transition is forbiddentransition is forbidden The The 2 2 3 3*transition is observed as an intense absorption*transition is observed as an intense absorption s-trans s-cis 175 nm forb.217 nm 253 nm 175 nm Y Y4 4*Y Y2 2 Y Y1 1 Y Y3 3*9 UV Spectroscopy IV.Structure Determination A.Dienes 1.General Features The Y Y2 Y Y3*transition is observed as an intense absorption(e e=20,000+)based at 217 nm within the observed region of the UV While this band is insensitive to solvent(as would be expected),it is subject to the bathochromic and hyperchromic effects of alkyl substituents as well as further conjugation Consider:l lmax=217 253 220 227 227 256 263 nm 10 UV Spectroscopy IV.Structure Determination A.Dienes 2.Woodward-Fieser Rules Woodward and the Fiesers performed extensive studies of terpene(萜烯萜烯)and steroidal(类固醇类固醇)alkenes and noted similar substituents and structural features would predictably lead to an empirical prediction of the wavelength for the lowest energy *electronic transition This work was distilled by Scott in 1964 into an extensive treatise on the Woodward-Fieser rules in combination with comprehensive tables and examples (A.I.Scott,Interpretation of the Ultraviolet Spectra of Natural Products,Pergamon,NY,1964)A more modern interpretation was compiled by Rao in 1975 (C.N.R.Rao,Ultraviolet and Visible Spectroscopy,3rd Ed.,Butterworths,London,1975)11 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination A.A.DienesDienes 2.2.WoodwardWoodward-FieserFieser Rules Rules-DienesDienes The rules begin with a base value for The rules begin with a base value for l lmax max of the chromophore being of the chromophore being observed:observed:acyclic butadiene=214 nmacyclic butadiene=214 nm The incremental contribution of substituents is added to this base The incremental contribution of substituents is added to this base value from the group tables:value from the group tables:Group Increment Extended conjugation+30 Each exo-cyclic C=C+5 Alkyl+5 -OCOCH3+0-OR+6-SR+30-Cl,-Br+5-NR2+60 12 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination A.A.DienesDienes 2.2.WoodwardWoodward-FieserFieser Rules Rules-DienesDienes For example:For example:Isoprene Isoprene-acyclic butadiene=acyclic butadiene=214 nm214 nm one alkyl subs.one alkyl subs.+5 nm+5 nm 219 nm219 nm Experimental valueExperimental value 220 nm220 nm AllylidenecyclohexaneAllylidenecyclohexane -acyclic butadiene=acyclic butadiene=214 nm214 nm one exocyclic C=C one exocyclic C=C +5 nm+5 nm 2 alkyl subs.2 alkyl subs.+10 nm+10 nm 229 nm229 nm Experimental valueExperimental value 237 nm237 nm 13 UV SpectroscopyUV Spectroscopy IV.IV.Structure Determination Structure Determination A.A.DienesDienes 3.3.WoodwardWoodward-FieserFieser Rules Rules Cyclic DienesCyclic Dienes There are two major types of cyclic dienes,with two different base There are two major types of cyclic dienes,with two different base valuesvalues HeteroannularHeteroannular (transoidtransoid):):HomoannularHomoannular (cisoidcisoid):):e=5,000 e=5,000 15,00015,000 e=12,000e=12,000-28,00028,000 base base l lmax max=214=214 base base l lmax max=253=253 The increment table is the same as for acyclic The increment table is the same as for acyclic butadienesbutadienes with a with a couple additions:couple additions:Group Increment Additional homoannular+39 Where both types of diene are present,the one with the longer l l becomes the base 14 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination A.A.DienesDienes 3.3.WoodwardWoodward-FieserFieser Rules Rules Cyclic DienesCyclic Dienes In the preIn the pre-NMR era of organic spectral determination,the power of NMR era of organic spectral determination,the power of the method for discerning isomers is readily apparent the method for discerning isomers is readily apparent Consider abietic vs.Consider abietic vs.levopimariclevopimaric acid:acid:COOHCOOHlevopimaric acid abietic acid 15 UV Spectroscopy IV.Structure Determination A.Dienes 3.Woodward-Fieser Rules Cyclic Dienes For example:1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene heteroannular diene=214 nm 3 alkyl subs.(3 x 5)+15 nm 1 exo C=C +5 nm 234 nm Experimental value 235 nm 16 UV SpectroscopyUV Spectroscopy IV.IV.Structure Determination Structure Determination A.A.DienesDienes 3.3.WoodwardWoodward-Fieser Rules Fieser Rules Cyclic DienesCyclic Dienes COOHheteroannular diene=214 nm 4 alkyl subs.(4 x 5)+20 nm 1 exo C=C +5 nm 239 nm homoannular diene=253 nm 4 alkyl subs.(4 x 5)+20 nm 1 exo C=C +5 nm 278 nm COOHlevopimaric acid abietic acid 17 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination B.B.EnonesEnones 1.1.General FeaturesGeneral Features For For auxochromicauxochromic substitution on the carbonyl,pronounced substitution on the carbonyl,pronounced hypsochromichypsochromic shifts are observed for the shifts are observed for the n n *transition(*transition(l lmaxmax):This is explained by the inductive withdrawal of electrons by O,N or halogen from the carbonyl carbon this causes the n-electrons on the carbonyl oxygen to be held more firmly It is important to note this is different from the auxochromic effect on *which extends conjugation and causes a bathochromic shift（red shift）In most cases,this bathochromic shift is not enough to bring the *transition into the observed range HOCH3OClONH2OOOOHO293 nm 279 235 214 204 204 Blue shiftBlue shift C C CC C CCCOO18 UV SpectroscopyUV Spectroscopy IV.IV.Structure Determination Structure Determination B.B.EnonesEnones 1.1.General FeaturesGeneral Features Conversely,if the C=O system is conjugated both the Conversely,if the C=O system is conjugated both the n n *and*and *bands are*bands are bathochromicallybathochromically shifted shifted(red shift)(red shift)Here,several effects must be noted:Here,several effects must be noted:i.i.the effect is more pronounced for the effect is more pronounced for *ii.ii.if the conjugated chain is long enough,the much higher if the conjugated chain is long enough,the much higher intensity intensity *band will overlap and band will overlap and drown outdrown out the the n n *bandband iii.iii.the shift of the the shift of the n n *transition is not as predictabletransition is not as predictable For these reasons,empirical WoodwardFor these reasons,empirical Woodward-FieserFieser rules for conjugated rules for conjugated enonesenones are for the higher intensity,allowed are for the higher intensity,allowed *transitiontransition C C CC C CCCOO19 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination B.B.EnonesEnones 1.1.General FeaturesGeneral Features These effects are apparent from the MO diagram for a conjugated These effects are apparent from the MO diagram for a conjugated enoneenone:Y Y1 1 Y Y2 2 Y Y3 3*Y Y4 4*n *n OO20 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination B.B.EnonesEnones 2.2.WoodwardWoodward-FieserFieser Rules Rules-EnonesEnones Aldehydes,esters and carboxylic acids have different base values Aldehydes,esters and carboxylic acids have different base values than ketonesthan ketones Unsaturated system Base Value Aldehyde 208 With or alkyl groups 220 With,or,alkyl groups 230 With,alkyl groups 242 Acid or ester With or alkyl groups 208 With,or,alkyl groups 217 Group value exocyclic,double bond+5 Group value endocyclic,bond in 5 or 7 membered ring+5 21 UV SpectroscopyUV Spectroscopy IV.IV.Structure Determination Structure Determination B.B.Enones Enones 2.2.WoodwardWoodward-Fieser Rules Fieser Rules-EnonesEnones Group Increment 6-membered ring or acyclic enone Base 215 nm 5-membered ring parent enone Base 202 nm Acyclic dienone Base 245 nm Double bond extending conjugation 30 Alkyl group or ring residue,and higher 10,12,18-OH,and higher 35,30,18-OR,35,30,17,31-O(C=O)R,6-Cl,15,12-Br,25,30-NR2 95 Exocyclic double bond 5 Homocyclic diene component 39 C C CC C CCCOO22 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination B.B.Enones Enones 2.2.WoodwardWoodward-Fieser Rules Fieser Rules-EnonesEnones Unlike conjugated alkenes,Unlike conjugated alkenes,solvent does have an effect on solvent does have an effect on l lmaxmax These effects are also described by the WoodwardThese effects are also described by the Woodward-Fieser rulesFieser rules Solvent correction Increment Water+8 Ethanol,methanol 0 Chloroform-1 Dioxane-5 Ether-7 Hydrocarbon-11 23 UV SpectroscopyUV Spectroscopy IV.IV.Structure DeterminationStructure Determination B.B.EnonesEnones 2.2.WoodwardWoodward-FieserFieser R