3 Electric and magnetic field components of plane polarized light Note that refractive index reflect changes in speed of light in different mediaLight travels in z-directionElectric and magnetic fields travel at 90° to each other at speed of light in particular mediumc (= 3 × 1010 cm s-1) in a vacuum
4 Characterization of Radiation Note inverse relationship between energy and wavelength. Easy to get confused with the way scientists talk about changes (blue shift, red shift, etc)
5 Wavelength and Energy Units 1 cm = 108 Å = 107 nm = 104 =107 m (millimicrons)N.B. 1 nm = 1 m (old unit)Energy1 cm-1 = cal mol-1 of particles= 1016 erg molecule-1 = 1.24 10-4 eV molecule-1E (kcal mol-1) (Å) = 105E(kJ mol-1) = 1.19 105/(nm) 297 nm = 400 kJMuch of the data on organic molecules is recorded in millimicrons, but not to worry, same as nm.Radiation has lots of units. Must be careful because different types of spectroscopy use different units. IR uses energy units, while absorption spectroscopy uses wavelength.Usually due to instrumental considerations (hard to make monochromators linear in energy).Easy to work in energy now since computers can readily transform data, but is rarely done due to investigator comfort (why do we still use kcal?)
6 Absorption Spectroscopy Provide information about presence and absence of unsaturated functional groupsUseful adjunct to IRNeeded for chiroptic techniquesDetermination of concentration, especially in chromatographyFor structure proof, usually not critical data, but essential for further studiesNMR, MS not good for purity
7 Importance of UV data Particularly useful for Polyenes with or without heteroatomsBenzenoid and nonbenzenoid aromaticsMolecules with heteroatoms containing n electronsChiroptic tool to investigate optically pure molecules with chromophoresPractically, UV absorption is measured after NMR and MS analysis
9 UV and Visible Spectroscopy Vacuum UV or soft X-raysnmQuartz, O2 and CO2 absorb strongly in this regionN2 purge good down to 180 nmQuartz region200 – 350 nmSource is D2 lampVisible region350 – 800 nmSource is tungsten lampLots of talk now about deep uv, soft x-ray for microlithography. Not much useful chemical information here.Quartz (normal UV) and visible region of best use
10 All organic compounds absorb UV-light C-C and C-H bonds; isolated functional groups like C=C absorb in vacuum UV; therefore not readily accessibleImportant chromophores are R2C=O, -O(R)C=O, -NH(R)C=O and polyunsaturated compounds
11 Spectral measurementusually dissolve 1 mg in up to 100 mL of solvent for samples of D molecular weightdata usually presented as A vs (nm)for publication, y axis is usually transformed to or log10 to make spectrum independent of sample concentration
12 Preparation of samples Concentration must be such that the absorbance lies between 0.2 and 0.7 for maximum accuracyConjugated dienes have 8,000-20,000, so c 4 10-5 Mn* of a carbonyl have , so c 10-2 MSuccessive dilutions of more concentrated samples necessary to locate all possible transitions
14 Solvent choicesImportant features to consider are solubility of sample and UV cutoff of solventFiltration to remove particulates is useful to reduce scattered lightSolvent purity is very important
15 ChromophoresStructures within the molecule that contain the electrons being moved by the photon of lightOnly those absorbing above 200 nm are usefuln* in ketones at ca 300 nm is only isolated chromophore of interestall other chromophores are conjugated systems of some sort
16 Types of organic transitions (Chromophores) *Sat’d hydrocarbonsVacuum UVn*Sat’d hydrocarbons with heteroatomsPossibly quartz UV*OlefinsUVn*Olefins with heteroatomsOrganic molecules have a number of possible transitions, but practically, transitions involving the sigma symmetry orbitals are not observed
17 Modes of electronic excitation Remember, we don’t use the sigma transitions since they are in the vacuum UV
18 Simple lone pair system Presence of a lone pair containing heteroatom adds an additional transition
19 Simple olefinOlefins have two types of transitions, and the energies are now accessible to the quartz UV region
21 Examples of n* and * transitions Both 1,3-butadiene and 2-butenal have similar pi to pi* bands at 218 nm but 2-butenal has low intensity peak at 316 nm due to n to pi* transition
22 Molecular orbitals for common transitions Molecular orbital diagram for 2-butenalShows n * on rightShows * on leftBoth peaks are broad due to multiple vibrational sublevels in ground and excited states
23 Energy level diagram for a carbonyl Typical example of a system with both n and pi orbitals. This group allows a great deal of information to be inferred due to the number of potential transitions.
24 Beer’s Law Io = Intensity of incident light I = Intensity of transmitted light = molar extinction coefficientl = path length of cellc = concentration of sample
25 Transition EnergiesElectronic transitions are quantized, so sharp bands are expectedIn reality, absorption lines are broadened into bands due to other types of transitions occurring in the same moleculesFor electronic transitions, this means vibrational transitions and coupling to solventQuantum mechanics says that to a first approximation, transitions are narrow. This works for atoms (consider AA and related techniques), but polyatomic systems have vibrations. These get coupled to the electronic transitions, yielding broadened transitions.
26 Actual transition with vibrational levels Note that presence of vibrational levels in the excited state gives rise to broadened absorption bands. Vibrational levels in the ground state are important for fluorescence and phosphorescence
27 Spectrum for energy level diagram shown on previous slide Spectrum resulting from energy level diagram on previous slide. Note that the nomenclature indicates the starting vibrational energy level going to the final vibrational level (nomenclature is a little casual here, but is effective).
28 Vibrational fine structure Rigid molecules such as benzene and fused benzene ring structures often display vibrational fine structureExample is benzene in heptaneUsually only observed in gas phase, but rigid molecules do display thisReal spectra showing multiple transitions of benzene. We’ll go into the details later, but note the fine structure on the low energy band. Heptane is pretty nonpolar, facilitating the fine structure
29 Benzene (note use of m in this older data) Solvent effect. Note the drastic change in the spectrum shifting from a nonpolar to a polar solvent.
30 PyridineSubstitution of the heteroatom deresolves the fine structure relative to benzene. Extra vibrations and some degree of structural flexibility is at play here.
32 Intensities of transitions Strictly speaking, one should work with integrated band intensitiesHowever, overlap of bands prevents clean isolation of transitions (hence the popularity of fluorescence in photophysical studies)Therefore, intensities are used
33 Selection RulesAfter resonance condition is met, the electromagnetic radiation must be able to electrical work on the moleculeFor this to happen, transition in the molecule must be accom-panied by a change in the electrical center of the moleculeSelection rules address the requirements for transitions between states in moleculesSelection rules are derived from the evaluation of the properties of the transition moment integral (beyond scope of this course
34 Selection Rule Terminology Transitions that are possible according to the rules are termed “allowed”Such transitions are correspond-ingly intenseTransitions that are not possible are termed “forbidden” and are weakTransitions may be “allowed” by some rules and “forbidden” by others
35 Common Selection Rules Spin-forbidden transitionsTransitions involving a change in the spin state of the molecule are forbiddenStrongly obeyedRelaxed by effects that make spin a poor quantum number (heavy atoms)Symmetry-forbidden transitionsTransitions between states of the same parity are forbiddenParticularly important for centro-symmetric molecules (ethene)Relaxed by coupling of electronic transitions to vibrational transitions (vibronic coupling)
36 Intensities P is the transition probability; ranges from 0 to 1 a is the target area of the absorbing system (the chromophore)chromophores are typically 10 Å long, so a transition of P = 1 will have an of 105
37 Intensities, con’t.this intensity is actually observed, and has been exceeded by very long chromophoric systemsGenerally, fully allowed systems have > 10,000 and those with low transition probabilities will have < 1000Generally, the longer the chromophore, the longer wavelength is the absorption maximum and the more intense the absorption
38 Intensities - Important forbidden transitions near 300 nm in ketones caIn benzene and aromaticsband around 260 nm and equivalent in more complex systems > 100Prediction of intensities is a very deep subject, covered in Physical Methods next year
39 Fundamentals of spectral interpretation Examining orbital diagrams for simple conjugated systems is helpful (lots of good programs available to do these calculations)Wavelength and intensity of bands are both useful for assignments
40 Solvent effects Franck-Condon Principle nuclei are stationary during electronic transitionsElectrons of solvent can move in concert with electrons involved in transitionSince most transitions result in an excited state that is more polar than the ground state, there is a red shift ( nm) upon increasing solvent polarity (hexane to ethanol)
41 Solvent effects Hydrocarbons water * n* Weak bathochromic or red shiftn*Hypsochromic or blue shift (strongly affected by hydrogen bonding solvents)Solvent effects due to stabilization or destabilization of ground or excited states, changing the energy gap
42 Solvent effects, con’t n* in ketones is the exception there is a blue shiftthis is due to diminished ability of solvent to hydrogen bond to lone pairs on oxygenexample - acetonein hexane, max = 279 nm ( = 15)in water, max = nm
43 Band assignments: n* < 2000Strong blue shift observed in high dielectric or hydrogen-bonding solventsn* often disappear in acidic media due to protonation of n electronsBlue shifts occur upon attachment of an electron-donating groupAbsorption band corresponding to the n* is missing in the hydrocarbon analog (consider H2C=O vs H2C=CH2Usually, but not always, n* is the lowest energy singlet transition* transitions are considerably more intense
44 Searching for chromophores No easy way to identify a chromophoretoo many factors affect spectrumrange of structures is too greatUse other techniques to helpIR - good for functional groupsNMR - best for C-H
45 Identifying chromophores complexity of spectrumcompounds with only one (or a few) bands below 300 nm probably contains only two or three conjugated unitsextent to which it encroaches on visible regionabsorption stretching into the visible region shows presence of a long or polycyclic aromatic chromophore
46 Identifying chromophores Intensity of bands - particularly the principle maximum and longest wavelength maximumSimple conjugated chromophores such as dienes and unsaturated ketones have values from 10,000 to 20,000Longer conjugated systems have principle maxima with correspondingly longer max and larger
47 Identifying chromophores Low intensity bands in the nm (with ca ) are result of ketonesAbsorption bands with ,000 almost always show the presence of aromatic systemsSubstituted aromatics also show strong bands with > 10,000, but bands with < 10,000 are also present
48 Next steps in spectral interpretation Look for model systemsMany have been investigated and tabulated, so hit the literatureMajor referencesOrganic Electronic Spectral Data, Wiley, New York, Vol 1-21 ( )Sadtler Handbook of Ultraviolet Spectra, Heyden, London
50 Substituted acyclic dienes max shiftsPresence of substituentsLength of conjugation
51 Conjugated dienes Strong UV absorber max affected by geometry and substitution patternS-trans 217 nmS-cis 253 nmReplacement of hydrogen with alkyl or polar groups red shift these base valuesExtending conjugation also red shifts max
59 Molecular orbitals for common transitions Molecular orbital diagram for 2-butenalShows n * on rightShows * on leftBoth peaks are broad due to multiple vibrational sublevels in ground and excited states
60 Orbital Diagram for Carbonyl Group n* bands are weak due to unfavorable orientation of n electrons relative to the * orbitals
61 Rules for calculation of * max for conjugated carbonyls
63 Selected ReferencesHarris, D. C., Bertolucci, M. D., Symmetry and Spectroscopy, Dover, 1978.Pasto, D. J., Johnson, C. R., Organic Structure Determination, Prentice-Hall, 1969.Drago, R. S., Physical Methods for Chemists, Surfside Publishing, 1992.Nakanishi, K., Berova, N., Woody, R. W., Circular Dichroism, VCH Publishers, 1994Williams, D. H., Fleming, I., Spectroscopic methods in organic chemistry, McGraw-Hill, 1987.
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