UV Spectroscopy - Pennsylvania State University

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Spring 2014 .
UV Spectroscopy 2I IntroductionA UV radiation and Electronic Excitations1 The difference in energy between molecular bonding non bonding and anti bonding orbitals ranges from 125 650 kJ mole.
2 This energy corresponds to EM radiation in the ultraviolet UV region 100 350 nm and visible VIS regions 350 700 nm of the3 For comparison recall the EM spectrum rays X rays UV IR Microwave Radio4 Using IR we observed vibrational transitions with energies of 8 .
40 kJ mol at wavelengths of 2500 15 000 nm5 For purposes of our discussion we will refer to UV and VISspectroscopy as UV UV Spectroscopy 3I Introduction.
B The Spectroscopic Process1 In UV spectroscopy the sample is irradiated with the broadspectrum of the UV radiation2 If a particular electronic transition matches the energy of acertain band of UV it will be absorbed.
3 The remaining UV light passes through the sample and is4 From this residual radiation a spectrum is obtained with gaps at these discrete energies this is called an absorption UV Spectroscopy 4I Introduction.
C Observed electronic transitions1 The lowest energy transition and most often obs by UV istypically that of an electron in the Highest Occupied MolecularOrbital HOMO to the Lowest Unoccupied Molecular Orbital2 For any bond pair of electrons in a molecule the molecular.
orbitals are a mixture of the two contributing atomic orbitals forevery bonding orbital created from this mixing there isa corresponding anti bonding orbital of symmetrically higherenergy 3 The lowest energy occupied orbitals are typically the likewise .
the corresponding anti bonding orbital is of the highest4 orbitals are of somewhat higher energy and theircomplementary anti bonding orbital somewhat lower in energythan 5 Unshared pairs lie at the energy of the original atomic orbital .
UV Spectroscopy 5I IntroductionC Observed electronic transitions6 Here is a graphical representationUnoccupied levels.
Atomic orbital Atomic orbitalOccupied levelsMolecular orbitals UV Spectroscopy 6I Introduction.
C Observed electronic transitions7 From the molecular orbital diagram there are several possibleelectronic transitions that can occur each of a different relative alkanes carbonyls.
unsaturated cmpds n O N S halogensn carbonyls UV Spectroscopy 7I Introduction.
C Observed electronic transitions7 Although the UV spectrum extends below 100 nm high energy oxygen in the atmosphere is not transparent below 200 nm8 Special equipment to study vacuum or far UV is required9 Routine organic UV spectra are typically collected from 200 700.
10 This limits the transitions that can be observed alkanes 150 nm carbonyls 170 nm unsaturated cmpds 180 nm if conjugated n O N S halogens 190 nm.
n carbonyls 300 nm UV Spectroscopy 8I IntroductionD Selection Rules1 Not all transitions that are possible are observed.
2 For an electron to transition certain quantum mechanicalconstraints apply these are called selection rules 3 For example an electron cannot change its spin quantumnumber during a transition these are forbidden Other examples include .
the number of electrons that can be excited at one symmetry properties of the molecule symmetry of the electronic states4 To further complicate matters forbidden transitions aresometimes observed albeit at low intensity due to other factors.
UV Spectroscopy 9I IntroductionE Band Structure1 Unlike IR or later NMR where there may be upwards of 5 ormore resolvable peaks from which to elucidate structural.
information UV tends to give wide overlapping bands2 It would seem that since the electronic energy levels of a puresample of molecules would be quantized fine discrete bandswould be observed for atomic spectra this is the case3 In molecules when a bulk sample of molecules is observed not.
all bonds read pairs of electrons are in the same vibrationalor rotational energy states4 This effect will impact the wavelength at which a transition isobserved very similar to the effect of H bonding on the O Hvibrational energy levels in neat samples.
UV Spectroscopy 10I IntroductionE Band Structure5 When these energy levels are superimposed the effect can bereadily explained any transition has the possibility of being.
DisassociationV1 R1 RnE1 Vo R1 RnDisassociationEnergy R1 Rn.
V 1 R1 RnE0 Vo R1 Rn UV Spectroscopy 11II Instrumentation and SpectraA Instrumentation.
1 The construction of a traditional UV VIS spectrometer is verysimilar to an IR as similar functions sample handling irradiation detection and output are required2 Here is a simple schematic that covers most modern UVspectrometers .
log I0 I AUV VIS sources I0 Imonochromator beam splitter optics I0 I0 UV Spectroscopy 12.
II Instrumentation and SpectraA Instrumentation3 Two sources are required to scan the entire UV VIS band Deuterium lamp covers the UV 200 330 Tungsten lamp covers 330 700.
4 As with the dispersive IR the lamps illuminate the entire band ofUV or visible light the monochromator grating or prism gradually changes the small bands of radiation sent to the beam5 The beam splitter sends a separate band to a cell containing thesample solution and a reference solution.
6 The detector measures the difference between the transmittedlight through the sample I vs the incident light I0 and sendsthis information to the recorder UV Spectroscopy 13II Instrumentation and Spectra.
A Instrumentation7 As with dispersive IR time is required to cover the entire UV VISband due to the mechanism of changing wavelengths8 A recent improvement is the diode array spectrophotometer here a prism dispersion device breaks apart the full spectrum.
transmitted through the sample9 Each individual band of UV is detected by a individual diodes ona silicon wafer simultaneously the obvious limitation is the sizeof the diode so some loss of resolution over traditionalinstruments is observed.
Diode arrayUV VIS sourcesPolychromator entrance slit and dispersion device UV Spectroscopy 14.
II Instrumentation and SpectraB Instrumentation Sample Handling1 Virtually all UV spectra are recorded solution phase2 Cells can be made of plastic glass or quartz3 Only quartz is transparent in the full 200 700 nm range plastic.
and glass are only suitable for visible spectra4 Concentration we will cover shortly is empirically determinedA typical sample cell commonly called a cuvet UV Spectroscopy 15II Instrumentation and Spectra.
B Instrumentation Sample Handling5 Solvents must be transparent in the region to be observed thewavelength where a solvent is no longer transparent is referredto as the cutoff6 Since spectra are only obtained up to 200 nm solvents typically.
only need to lack conjugated systems or carbonylsCommon solvents and cutoffs acetonitrile 190chloroform 240cyclohexane 195.
1 4 dioxane 21595 ethanol 205n hexane 201methanol 205isooctane 195.
UV Spectroscopy 16II Instrumentation and SpectraB Instrumentation Sample Handling7 Additionally solvents must preserve the fine structure where itis actually observed in UV where possible.
8 H bonding further complicates the effect of vibrational androtational energy levels on electronic transitions dipole dipoleinteracts less so9 The more non polar the solvent the better this is not always UV Spectroscopy 17.
II Instrumentation and SpectraC The Spectrum1 The x axis of the spectrum is in wavelength 200 350 nm for UV 200 700 for UV VIS determinations2 Due to the lack of any fine structure spectra are rarely shown in.
their raw form rather the peak maxima are simply reported asa numerical list of lamba max values or max max 206 nm UV Spectroscopy 18II Instrumentation and Spectra.
C The Spectrum1 The y axis of the spectrum is in absorbance A2 From the spectrometers point of view absorbance is the inverseof transmittance A log10 I0 I 3 From an experimental point of view three other considerations.
must be made 1 a longer path length l through the sample will causemore UV light to be absorbed linear effect2 the greater the concentration c of the sample themore UV light will be absorbed linear effect.
3 some electronic transitions are more effective at theabsorption of photon than others molar absorptivity this may vary by orders of magnitude UV Spectroscopy 19II Instrumentation and Spectra.
C The Spectrum4 These effects are combined into the Beer Lambert Law A i for most UV spectrometers l would remain constant standard cells are typically 1 cm in path length ii concentration is typically varied depending on the strength.
of absorption observed or expected typically dilute sub 001 Miii molar absorptivities vary by orders of magnitude i values of 104 106 are termed high intensityabsorptions.
ii values of 103 104 are termed low intensity absorptionsiii values of 0 to 103 are the absorptions of forbiddentransitionsA is unitless so the units for are cm 1 M 1 and are rarely UV Spectroscopy 20.
II Instrumentation and SpectraD Practical application of UV spectroscopy1 UV was the first organic spectral method however it is rarelyused as a primary method for structure determination2 It is most useful in combination with NMR and IR data to.
elucidate unique electronic features that may be ambiguous inthose methods3 It can be used to assay via max and molar absorptivity theproper irradiation wavelengths for photochemical experiments or the design of UV resistant paints and coatings.
4 The most ubiquitous use of UV is as a detection device for HPLC since UV is utilized for solution phase samples vs a referencesolvent this is easily incorporated into LC designUV is to HPLC what mass spectrometry MS will be to GC UV Spectroscopy 21.
III ChromophoresA Definition1 Remember the electrons present in organic molecules areinvolved in covalent bonds or lone pairs of electrons on atomssuch as O or N.
2 Since similar functional groups will have electrons capable ofdiscrete classes of transitions the characteristic energy of theseenergies is more representative of the functional group than theelectrons themselves3 A functional group capable of having characteristic electronic.
transitions is called a chromophore color loving 4 Structural or electronic changes in the chromophore can bequantified and used to predict shifts in the observed electronictransitions UV Spectroscopy 22.
III ChromophoresB Organic Chromophores1 Alkanes only posses bonds and no lone pairs of electrons soonly the high energy transition is observed in the far UVThis transition is destructive to the molecule causing cleavage.
of the bond UV Spectroscopy 23III ChromophoresB Organic Chromophores Alcohols ethers amines and sulfur compounds in the cases of.
simple aliphatic examples of these compounds the n is themost often observed transition like the alkane it is mostoften at shorter than 200 nmNote how this transition occurs from the HOMO to the LUMOanitbonding.
UV Spectroscopy 24III ChromophoresB Organic Chromophores3 Alkenes and Alkynes in the case of isolated examples of thesecompounds the is observed at 175 and 170 nm .
respectivelyEven though this transition is of lower energy than it isstill in the far UV however the transition energy is sensitive tosubstitution UV Spectroscopy 25.
III ChromophoresB Organic Chromophores4 Carbonyls unsaturated systems incorporating N or O canundergo n transitions 285 nm in addition to Despite the fact this transition is forbidden by the selection rules.
15 it is the most often observed and studied transition forThis transition is also sensitive to substituents on the carbonylSimilar to alkenes and alkynes non substituted carbonylsundergo the transition in the vacuum UV 188 nm 900 sensitive to substitution effects.
UV Spectroscopy 26III ChromophoresB Organic Chromophores4 Carbonyls n transitions 285 nm 188 nm It has been.
determined fromn spectral studies thatcarbonyl oxygenmore approximatessp rather than sp2 .
* UV Spectroscopy Visible Spectroscopy Color General One of the most common class of colored organic molecules are the azo dyes: From our discussion of di-subsituted aromatic chromophores, the effect of opposite groups is greater than the sum of the individual effects – more so on this heavily conjugated system Coincidentally, it is necessary ...

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