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1、紫外可見光譜 I 有機(jī)波譜分析 Organic Spectroscopic Analysis Ultraviolet and Visible(UV-vis)Spectroscopy I 2 UltravioletUltraviolet and and VisibleVisible (UV(UV-vis)Spectroscopyvis)Spectroscopy AbsorptionAbsorption of UltravioletUltraviolet and and Visible LightVisible Light due to the Transition to due to the T

2、ransition to ElectronsElectrons to Higher Energy Levelsto Higher Energy Levels UV-vis Spectroscopy 3 Typical UV/Vis Spectrum p-p*Shorter wavelength,stronger absorption n-p*Longer wavelength,weaker absorption CCH3OCH3non-bondingelectronsp electronsp*are excited pelectronsUV Spectrum of Acetone 4 UV S

3、pectroscopyUV Spectroscopy I.Introduction A.UV radiation and Electronic Excitations 1.The difference in energy between molecular bonding,non-bonding and anti-bonding orbitals ranges from 125-650 kJ/mole 2.This energy corresponds to electromagnetic(EM)radiation in the ultraviolet(UV)region,100-350 nm

4、,and visible(VIS)regions 350-700 nm of the spectrum 3.For comparison,recall the EM spectrum:4.Using IR we observed vibrational transitions with energies of 8-40 kJ/mol at wavelengths of 2500-15,000 nm 5.For purposes of our discussion,we will refer to UV and VIS spectroscopy as UV UV X-rays IR g g-ra

5、ys Radio Microwave Visible 5 UV SpectroscopyUV Spectroscopy I.I.IntroductionIntroduction B.B.The Spectroscopic ProcessThe Spectroscopic Process 1.1.In UV spectroscopy,the sample is irradiated with the broad In UV spectroscopy,the sample is irradiated with the broad spectrum of the UV radiationspectr

6、um of the UV radiation 2.2.If a particular electronic transition matches the energy of a certain If a particular electronic transition matches the energy of a certain band of UV,it will be absorbedband of UV,it will be absorbed 3.3.The remaining UV light passes through the sample and is observedThe

7、remaining UV light passes through the sample and is observed 4.4.From this residual radiation a spectrum is obtained with“gaps”at From this residual radiation a spectrum is obtained with“gaps”at these discrete energies these discrete energies this is called an this is called an absorption spectrumab

8、sorption spectrum p pp p p pp p 6 UV SpectroscopyUV Spectroscopy I.I.IntroductionIntroduction C.C.Observed electronic transitionsObserved electronic transitions 1.1.The lowest energy transition(and most often obs.by UV)is typically The lowest energy transition(and most often obs.by UV)is typically t

9、hat of an electron in the Highest Occupied Molecular Orbital that of an electron in the Highest Occupied Molecular Orbital(HOMO)to the Lowest Unoccupied Molecular Orbital(LUMO)(HOMO)to the Lowest Unoccupied Molecular Orbital(LUMO)2.2.For any bond(pair of electrons)in a molecule,the molecular For any

10、 bond(pair of electrons)in a molecule,the molecular orbitals are a mixture of the two contributing atomic orbitals;for orbitals are a mixture of the two contributing atomic orbitals;for every bonding orbital“created”from this mixing(every bonding orbital“created”from this mixing(,p p),there is a),th

11、ere is a corresponding anticorresponding anti-bonding orbital of symmetrically higher energy bonding orbital of symmetrically higher energy(*,p p*)3.3.The lowest energy occupied orbitals are typically the The lowest energy occupied orbitals are typically the ;likewise,;likewise,the corresponding ant

12、ithe corresponding anti-bonding bonding *orbital is of the highest energyorbital is of the highest energy 4.4.p p-orbitals are of somewhat higher energy,and their complementary orbitals are of somewhat higher energy,and their complementary antianti-bonding orbital somewhat lower in energy than bondi

13、ng orbital somewhat lower in energy than *.*.5.5.Unshared pairs lie at the energy of the original atomic orbital,most Unshared pairs lie at the energy of the original atomic orbital,most often this energy is higher than often this energy is higher than p p or or (since no bond is formed,(since no bo

14、nd is formed,there is no benefit in energy)there is no benefit in energy)7 UV SpectroscopyUV Spectroscopy I.I.IntroductionIntroduction C.C.Observed electronic transitionsObserved electronic transitions 6.6.Here is a graphical representationHere is a graphical representation Energy p p pp n Atomic or

15、bital Atomic orbital Molecular orbitals Occupied levels Unoccupied levels 8 UV SpectroscopyUV Spectroscopy I.I.IntroductionIntroduction C.C.Observed electronic transitionsObserved electronic transitions 7.7.From the molecular orbital diagram,there are several possible From the molecular orbital diag

16、ram,there are several possible electronic transitions that can occur,each of a different relative electronic transitions that can occur,each of a different relative energy:energy:Energy p p pp n p p n n p p p p p p alkanes carbonyls unsaturated compounds O,N,S,halogens carbonyls 9 UV SpectroscopyUV Spectroscopy I.I.IntroductionIntroduction C.C.Observed electronic transitionsObserved electronic transitions 8.8.Although the UV spectrum extends below 100 nm(high energy),Although the UV spectrum ext