有机波谱分析双语课程 (37).pdf

紫外可见光谱 II 有机波谱分析 Organic Spectroscopic Analysis Ultraviolet and Visible(UV-vis)Spectroscopy II UV Spectroscopy(II)UV Spectroscopy(II)2 Chromophore and substituent effectChromophore and substituent effect 3 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores A.A.DefinitionDefinition 1.1.Remember the Remember the electronselectrons present in organic molecules present in organic molecules areare involved involved inin covalent bondscovalent bonds or or lone pairs of electronslone pairs of electrons on atoms such as O or on atoms such as O or N N 2.2.Since similar functional groups will have electrons capable of Since similar functional groups will have electrons capable of discrete classes of transitions,discrete classes of transitions,the characteristic energy of these the characteristic energy of these energies is more representative of the functional group than the energies is more representative of the functional group than the electrons themselveselectrons themselves 3.3.A functional group capable of having characteristic electronic A functional group capable of having characteristic electronic transitions is called a transitions is called a chromophorechromophore (color lovingcolor loving)4.4.Structural or electronic changes in the chromophore can be Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic quantified and used to predict shifts in the observed electronic transitionstransitions 4 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores B.B.Organic ChromophoresOrganic Chromophores 1.1.Alkanes Alkanes only posses only posses -bonds and no lone pairs of electrons,so bonds and no lone pairs of electrons,so only the high energy only the high energy *transition is observed in the*transition is observed in the far UVfar UV This transition is destructive to the molecule,causing cleavage of This transition is destructive to the molecule,causing cleavage of the the -bondbond *CCCC5 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores B.B.Organic ChromophoresOrganic Chromophores 2.2.Alcohols,ethers,amines and sulfur compoundsAlcohols,ethers,amines and sulfur compounds in the cases of in the cases of simple,aliphatic examples of these compounds the simple,aliphatic examples of these compounds the n n *is the*is the most often observed transition;like the alkane most often observed transition;like the alkane *,it is most*,it is most often at shorter often at shorter than 200 nmthan 200 nm Note how this transition occurs from the HOMO to the LUMONote how this transition occurs from the HOMO to the LUMO *CN CN nN sp3 CNCNCNCNanitbonding orbital6 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores B.B.Organic ChromophoresOrganic Chromophores 3.3.Alkenes and AlkynesAlkenes and Alkynes in the case of isolated examples of these in the case of isolated examples of these compounds the compounds the *is observed at 175 and 170 nm,respectively*is observed at 175 and 170 nm,respectively Even though this transition is of lower energy than Even though this transition is of lower energy than *,it is still*,it is still in the far UV in the far UV however,this transition energy is sensitive to however,this transition energy is sensitive to substitutionsubstitution *7 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores B.B.Organic ChromophoresOrganic Chromophores 4.4.CarbonylsCarbonyls unsaturated systems incorporating N or O can undergo unsaturated systems incorporating N or O can undergo n n *transitions(285 nm)in addition to*transitions(285 nm)in addition to *Despite the fact this transition is forbidden by the selection rules(Despite the fact this transition is forbidden by the selection rules(=15),it is=15),it is the most often observed and studied transitionthe most often observed and studied transition for for carbonylscarbonyls This transition is also sensitive to substituents on the carbonylThis transition is also sensitive to substituents on the carbonyl Similar to alkenes and alkynes,nonSimilar to alkenes and alkynes,non-substituted carbonyls undergo substituted carbonyls undergo the the *transition in the vacuum UV(188 nm,*transition in the vacuum UV(188 nm,=900);sensitive=900);sensitive to substitution effects.to substitution effects.8 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores B.B.Organic ChromophoresOrganic Chromophores 4.4.Carbonyls Carbonyls n n *transitions(285 nm);*transitions(285 nm);*(188 nm)*(188 nm)*n CO transitions omitted for clarity OOCO9 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects General General from our brief study of these general chromophores,only the from our brief study of these general chromophores,only the weak weak n n *transition occurs in the routinely observed UV*transition occurs in the routinely observed UV The attachment of substituent groups(other than H)can shift the The attachment of substituent groups(other than H)can shift the energy of the transitionenergy of the transition Substituents that increase the intensity and often wavelength of an Substituents that increase the intensity and often wavelength of an absorption are called absorption are called auxochromesauxochromes Common Common auxochromesauxochromes include include alkyl,hydroxyl,alkoxy and amino alkyl,hydroxyl,alkoxy and amino groups and the halogensgroups and the halogens 10 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects GeneralGeneral Substituents may have any of four effects on a chromophoreSubstituents may have any of four effects on a chromophore i.i.Bathochromic shift(red shift)Bathochromic shift(red shift)a shift to longer a shift to longer ;lower energy;lower energy ii.ii.HypsochromicHypsochromic shift(blue shift)shift(blue shift)shift to shorter shift to shorter ;higher energy;higher energy iii.iii.Hyperchromic effect Hyperchromic effect an increase in intensityan increase in intensity iv.iv.Hypochromic effect Hypochromic effect a decrease in intensitya decrease in intensity 200 nm 700 nm Hypochromic Hypsochromic Hyperchromic Bathochromic 11 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects 1.1.Conjugation Conjugation most efficient means of bringing about a most efficient means of bringing about a bathochromic and hyperchromic shift of an unsaturated bathochromic and hyperchromic shift of an unsaturated chromophore:chromophore:H2CCH2-caroteneOO max nm 175 15,000 217 21,000 258 35,000 n *280 27 *213 7,100 465 125,000 n *280 12 *189 900 12 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects 1.1.Conjugation Conjugation AlkenesAlkenes The observed shifts from conjugation imply that The observed shifts from conjugation imply that an increase in an increase in conjugation decreases the energy required for electronic excitation conjugation decreases the energy required for electronic excitation From molecular orbital(MO)theory two atomic From molecular orbital(MO)theory two atomic p p orbitals,orbitals,1 1 and and 2 2 from two spfrom two sp2 2 hybrid carbons combine to form two MOs hybrid carbons combine to form two MOs 1 1 and and 2 2*in ethylenein ethylene Y Y2 2*Y Y1 1 1 1 2 2 13 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects 2.2.Conjugation Conjugation AlkenesAlkenes When we consider butadiene,we are now mixing 4 p orbitals giving When we consider butadiene,we are now mixing 4 p orbitals giving 4 MOs of an energetically symmetrical distribution compared to 4 MOs of an energetically symmetrical distribution compared to ethyleneethylene Y Y2 2*Y Y1 1 Y Y1 1 Y Y2 2 Y Y3 3*Y Y4 4*D DE for the HOMO LUMO transition is reduced 14 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects 2.2.Conjugation Conjugation AlkenesAlkenes Extending this effect out to longer conjugated systems the energy Extending this effect out to longer conjugated systems the energy gap becomes progressively smaller:gap becomes progressively smaller:Energy ethylene butadiene hexatriene octatetraene Lower energy=Longer wavelengths 15 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects 2.2.Conjugation Conjugation AlkenesAlkenes Similarly,the lone pairs of electrons on N,O,S,X can extend Similarly,the lone pairs of electrons on N,O,S,X can extend conjugated systems conjugated systems auxochromesauxochromes Here we create 3 MOs Here we create 3 MOs this interaction is not as strong as that of a this interaction is not as strong as that of a conjugated conjugated -system system Y Y2 2 Y Y1 1 A*nA Y Y3 3*Energy 16 UV SpectroscopyUV Spectroscopy III.III.ChromophoresChromophores C.C.Substituent EffectsSubstituent Effects 2.2.Conjugation Conjugation AlkenesAlkenes Methyl groups also cause a bathochromic shift(red shift),even Methyl groups also cause a bathochromic shift(red shift),even though they are devoid of though they are devoid of -or or n n-electronselectrons This effect is thought to be through what is termed This effect is thought to be through what is termed“hyperconjugation”“hyperconjugation”（超共轭作用）（超共轭作用）or sigma bond resonanceor sigma bond resonance CCCHHH