1、 I Organic Spectroscopic Analysis Ultraviolet and Visible(UV-vis)Spectroscopy I 2 Ultraviolet and Visible(UV-vis)Spectroscopy Absorption of Ultraviolet and Visible Light due to the Transition to Electrons to Higher Energy Levels UV-vis Spectroscopy 3 Typical UV/Vis Spectrum p-p*Shorter wavelength,st
2、ronger absorption n-p*Longer wavelength,weaker absorption CCH3OCH3non-bondingelectronsp electronsp*are excited pelectronsUV Spectrum of Acetone 4 UV Spectroscopy I.Introduction A.UV radiation and Electronic Excitations 1.The difference in energy between molecular bonding,non-bonding and anti-bonding
3、 orbitals ranges from 125-650 kJ/mole 2.This energy corresponds to electromagnetic(EM)radiation in the ultraviolet(UV)region,100-350 nm,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/m
4、ol 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-rays Radio Microwave Visible 5 UV Spectroscopy I.Introduction B.The Spectroscopic Process 1.In UV spectroscopy,the sample is irradiated with the broad spectrum of the UV
5、 radiation 2.If a particular electronic transition matches the energy of a certain band of UV,it will be absorbed 3.The remaining UV light passes through the sample and is observed 4.From this residual radiation a spectrum is obtained with“gaps”at these discrete energies this is called an absorption
6、 spectrum pppp6 UV Spectroscopy I.Introduction C.Observed electronic transitions 1.The lowest energy transition(and most often obs.by UV)is typically that of an electron in the Highest Occupied Molecular Orbital(HOMO)to the Lowest Unoccupied Molecular Orbital(LUMO)2.For any bond(pair of electrons)in
7、 a molecule,the molecular orbitals are a mixture of the two contributing atomic orbitals;for every bonding orbital“created”from this mixing(,p),there is a corresponding anti-bonding orbital of symmetrically higher energy(*,p*)3.The lowest energy occupied orbitals are typically the;likewise,the corre
8、sponding anti-bonding*orbital is of the highest energy 4.p-orbitals are of somewhat higher energy,and their complementary anti-bonding orbital somewhat lower in energy than*.5.Unshared pairs lie at the energy of the original atomic orbital,most often this energy is higher than p or(since no bond is
9、formed,there is no benefit in energy)7 UV Spectroscopy I.Introduction C.Observed electronic transitions 6.Here is a graphical representation Energy p p n Atomic orbital Atomic orbital Molecular orbitals Occupied levels Unoccupied levels 8 UV Spectroscopy I.Introduction C.Observed electronic transiti
10、ons 7.From the molecular orbital diagram,there are several possible electronic transitions that can occur,each of a different relative energy:Energy p p n p n n p p p alkanes carbonyls unsaturated compounds O,N,S,halogens carbonyls 9 UV Spectroscopy I.Introduction C.Observed electronic transitions 8
11、.Although the UV spectrum extends below 100 nm(high energy),oxygen in the atmosphere is not transparent below 200 nm 8.Special equipment to study vacuum or far UV is required 9.Routine organic UV spectra are typically collected from 200-700 nm 10.This limits the transitions that can be observed:p n
12、n 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 Spectroscopy I.Introduction E.Band Structure 1.Unlike IR,or NMR,where there may be upwards of 5 or more resolvable peaks from which to elucidate structural information,UV tends
13、 to give wide,overlapping bands 2.It would seem that since the electronic energy levels of a pure sample of molecules would be quantized,fine,discrete bands would be observed for atomic spectra,this is the case 3.In molecules,when a bulk sample of molecules is observed,not all bonds(pairs of electro
14、ns)are in the same vibrational or rotational energy states 4.This effect will impact the wavelength at which a transition is observed very similar to the effect of H-bonding on the O-H vibrational energy levels in neat samples 11 UV spectrum of 1,2,4,5-Tetrazole(a)r.t.,gas phase;(b)77K,in isopentene
15、;(c)r.t.,in cyclohexane;(d)r.t.,in water.12 UV Spectroscopy I.Introduction E.Band Structure 5.When these energy levels are superimposed,the effect can be readily explained any transition has the possibility of being observed Energy VoV4V3V2V1DisassociationR1-RnR1-RnR1-RnR1-RnR1-RnE0 E1 VoV4V3V2V1Dis
16、associationR1-RnR1-RnR1-RnR1-RnR1-Rn13 UV Spectroscopy II.Instrumentation and Spectra C.The Spectrum 1.The x-axis of the spectrum is in wavelength;200-350 nm for UV,200-700 for UV-VIS determinations 2.Due to the lack of any fine structure,spectra are rarely shown in their raw form,rather,the peak ma
17、xima are simply reported as a numerical list or max max=206 nm 252 317 376 ONH2O14 UV Spectroscopy II.Instrumentation and Spectra C.The Spectrum 1.The y-axis of the spectrum is in absorbance,A 2.From the spectrometers point of view,absorbance is the inverse of transmittance:A=log10(I0/I)3.From an ex
18、perimental point of view,three other considerations must be made:i.a longer path length,l through the sample will cause more UV light to be absorbed linear effect ii.the greater the concentration,c of the sample,the more UV light will be absorbed linear effect iii.some electronic transitions are mor
19、e effective at the absorption of photon than others molar absorptivity,this may vary by orders of magnitude 15 UV Spectroscopy II.Instrumentation and Spectra C.The Spectrum 4.These effects are combined into the Beer-Lambert Law:A=c l i.for most UV spectrometers,l would remain constant(standard cells
20、 are typically 1 cm in path length)ii.concentration is typically varied depending on the strength of absorption observed or expected typically dilute 0.001 M iii.molar absorptivities vary by orders of magnitude:values of 104-106 104-106 are termed high intensity absorptions values of 103-104 are ter
21、med low intensity absorptions values of 0 to 103 are the absorptions of forbidden transitions A is unitless,so the units for are cm-1 M-1 and are rarely expressed 5.Since path length and concentration effects can be easily factored out,absorbance simply becomes proportional to,and the y-axis is some
22、times expressed as directly or as the logarithm of 16 UV Spectroscopy II.Instrumentation and Spectra D.Practical application of UV spectroscopy 1.UV was the first organic spectral method,however,it is rarely used as a primary method for structure determination 2.It is most useful in combination with
23、 NMR and IR data to elucidate unique electronic features that may be ambiguous in those methods 3.It can be used to assay(via max and molar absorptivity)the proper irradiation wavelengths for photochemical experiments,or the design of UV resistant paints and coatings 4.The most ubiquitous use of UV
24、is as a detection device for HPLC;since UV is utilized for solution phase samples vs.a reference solvent this is easily incorporated into LC design UV is to HPLC what mass spectrometry(MS)is to GC;but you would need to know what compounds could and what compounds could not be detected by UV detector!(Structure!)