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1、紫外可见光谱 I 有机波谱分析 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
17、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 extends below 100 nm(high energy),oxygen in the atmosphere is not transparent below 200 nmoxygen in the
18、 atmosphere is not transparent below 200 nm 8.8.Special equipment to study Special equipment to study vacuumvacuum or or far UVfar UV is requiredis required 9.9.Routine organic UV spectra are typically collected from 200Routine organic UV spectra are typically collected from 200-700 nm700 nm 10.10.T
19、his limits the transitions that can be observed:This limits the transitions that can be observed:p p n n p p p p p p alkanes carbonyls unsaturated cmpds.O,N,S,halogens carbonyls 150 nm 170 nm 180 nm -if conjugated!190 nm 280 nm 10 UV SpectroscopyUV Spectroscopy I.I.IntroductionIntroduction E.E.Band
20、StructureBand Structure 1.1.Unlike IR,or NMR,where there may be upwards of Unlike IR,or NMR,where there may be upwards of 5 or more resolvable peaks from which to 5 or more resolvable peaks from which to elucidate structural information,UV tends to give elucidate structural information,UV tends to g
21、ive wide,overlapping bandswide,overlapping bands 2.2.It would seem that since the electronic energy levels of a pure It would seem that since the electronic energy levels of a pure sample of molecules would be quantized,sample of molecules would be quantized,fine,discrete bands fine,discrete bands w
22、ould be observedwould be observed for atomic spectra,this is the casefor atomic spectra,this is the case 3.3.In molecules,when a bulk sample of molecules is observed,not all In molecules,when a bulk sample of molecules is observed,not all bonds(pairs of electrons)are in the same vibrational or rotat
23、ional bonds(pairs of electrons)are in the same vibrational or rotational energy states energy states 4.4.This effect will impact the wavelength at which a transition is This effect will impact the wavelength at which a transition is observed observed very similar to the effect of Hvery similar to th
24、e effect of H-bonding on the Obonding on the O-H H vibrational energy levels in neat samplesvibrational energy levels in neat samples 11 UV spectrum of 1,2,4,5UV spectrum of 1,2,4,5-TetrazoleTetrazole (a)r.t.,gas phase；(b)77K,in isopentene；(c)r.t.,in cyclohexane；(d)r.t.,in water.12 UV SpectroscopyUV
25、 Spectroscopy I.I.IntroductionIntroduction E.E.Band StructureBand Structure 5.5.When these energy levels are superimposed,the effect can be When these energy levels are superimposed,the effect can be readily explained readily explained any transition has the possibility of being any transition has t
26、he possibility of being observedobserved Energy VoV4V3V2V1DisassociationR1-RnR1-RnR1-RnR1-RnR1-RnE0 E1 VoV4V3V2V1DisassociationR1-RnR1-RnR1-RnR1-RnR1-Rn13 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra C.C.The SpectrumThe Spectrum 1.1.The xThe x-axis of t
27、he spectrum is in wavelength;200axis of the spectrum is in wavelength;200-350 nm for UV,350 nm for UV,200200-700 for UV700 for UV-VIS determinationsVIS determinations 2.2.Due to the lack of any fine structure,spectra are rarely shown in Due to the lack of any fine structure,spectra are rarely shown
28、in their raw form,rather,their raw form,rather,the peak maxima are simply reported as a the peak maxima are simply reported as a numerical list or numerical list or maxmax max=206 nm 252 317 376 ONH2O14 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra C.C.T
29、he SpectrumThe Spectrum 1.1.The yThe y-axis of the spectrum is in absorbance,axis of the spectrum is in absorbance,A A 2.2.From the spectrometers point of view,absorbance is the inverse of From the spectrometers point of view,absorbance is the inverse of transmittance:transmittance:A A =log=log1010
30、(I I0 0/I I)3.3.From an experimental point of view,three other considerations From an experimental point of view,three other considerations must be made:must be made:i.i.a longer a longer path length,l path length,l through the sample will cause through the sample will cause more UV light to be abso
31、rbed more UV light to be absorbed linear effectlinear effect ii.ii.the greater the the greater the concentration,cconcentration,c of the sample,the more of the sample,the more UV light will be absorbed UV light will be absorbed linear effectlinear effect iii.iii.some electronic transitions are more
32、effective at the some electronic transitions are more effective at the absorption of photon than others absorption of photon than others molar absorptivity,molar absorptivity,this may vary by orders of magnitudethis may vary by orders of magnitude 15 UV SpectroscopyUV Spectroscopy II.II.Instrumentat
33、ion and SpectraInstrumentation and Spectra C.C.The SpectrumThe Spectrum 4.4.These effects are combined into the BeerThese effects are combined into the Beer-Lambert Law:Lambert Law:A A =c lc l i.i.for most UV spectrometers,for most UV spectrometers,l l would remain constant would remain constant(sta
34、ndard cells are typically 1 cm in path length)(standard cells are typically 1 cm in path length)ii.ii.concentration is typically varied depending on the strength of concentration is typically varied depending on the strength of absorption observed or expected absorption observed or expected typicall
35、y dilute 0.001 Mtypically dilute 0.001 M iii.iii.molar molar absorptivitiesabsorptivities vary by orders of magnitude:vary by orders of magnitude:values of 10values of 104 4-10106 6 10104 4-10106 6 are termed are termed high intensity high intensity absorptionsabsorptions values of 10values of 103 3
36、-10104 4 are termed are termed low intensity absorptionslow intensity absorptions values of 0 to 10values of 0 to 103 3 are the absorptions of are the absorptions of forbidden forbidden transitionstransitions A is unitless,so the units for A is unitless,so the units for are cmare cm-1 1 MM-1 1 and a
37、re rarely and are rarely expressedexpressed 5.5.Since path length and concentration effects can be easily factored Since path length and concentration effects can be easily factored out,absorbance simply becomes proportional to out,absorbance simply becomes proportional to ,and the y,and the y-axis
38、is axis is sometimes expressed as sometimes expressed as directly or as the logarithm of directly or as the logarithm of 16 UV SpectroscopyUV Spectroscopy II.II.Instrumentation and SpectraInstrumentation and Spectra D.D.Practical application of UV spectroscopyPractical application of UV spectroscopy
39、 1.1.UV was the first organic spectral method,however,it is rarely used UV was the first organic spectral method,however,it is rarely used as a primary method for as a primary method for structure determinationstructure determination 2.2.It is most useful in combination with NMR and IR data to eluci
40、date It is most useful in combination with NMR and IR data to elucidate unique electronic features that may be ambiguous in those unique electronic features that may be ambiguous in those methodsmethods 3.3.It can be used to assay(via It can be used to assay(via maxmax and molar absorptivity)the pro
41、per and molar absorptivity)the proper irradiation wavelengths for photochemical experiments,or the irradiation wavelengths for photochemical experiments,or the design of UV resistant paints and coatingsdesign of UV resistant paints and coatings 4.4.The most ubiquitous use of UV is as a detection dev
42、ice for HPLC;The most ubiquitous use of UV is as a detection device for HPLC;since UV is utilized for solution phase samples vs.a reference since UV is utilized for solution phase samples vs.a reference solvent this is easily incorporated into LC designsolvent this is easily incorporated into LC des
43、ign UV is to HPLC what mass spectrometry(MS)is to GC;UV is to HPLC what mass spectrometry(MS)is to GC;but you would need to know what compounds could and what but you would need to know what compounds could and what compounds could not be detected by UV detector!(Structure!)compounds could not be detected by UV detector!(Structure!)