UV Spectroscopy

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Slide 1:

1 CHMBD 449 – Organic Spectral Analysis Fall 2005 Chapter 7: UV Spectroscopy UV & electronic transitions Usable ranges & observations Selection rules Band Structure Instrumentation & Spectra Beer-Lambert Law Application of UV-spec

Slide 2:

2 UV Spectroscopy Introduction UV radiation and Electronic Excitations The difference in energy between molecular bonding, non-bonding and anti-bonding orbitals ranges from 125-650 kJ/mole This energy corresponds to EM radiation in the ultraviolet (UV) region, 100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum For comparison, recall the EM spectrum: Using IR we observed vibrational transitions with energies of 8-40 kJ/mol at wavelengths of 2500-15,000 nm For purposes of our discussion, we will refer to UV and VIS spectroscopy as UV UV X-rays IR g -rays Radio Microwave Visible

Slide 3:

3 UV Spectroscopy Introduction The Spectroscopic Process In UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiation If a particular electronic transition matches the energy of a certain band of UV, it will be absorbed The remaining UV light passes through the sample and is observed From this residual radiation a spectrum is obtained with “gaps” at these discrete energies – this is called an absorption spectrum

Slide 4:

4 UV Spectroscopy Introduction Observed electronic transitions 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) For any bond (pair of electrons) in a molecule, the molecular orbitals are a mixture of the two contributing atomic orbitals; for every bonding orbital “created” from this mixing ( s , p ), there is a corresponding anti-bonding orbital of symmetrically higher energy ( s * , p * ) The lowest energy occupied orbitals are typically the s; likewise, the corresponding anti-bonding s * orbital is of the highest energy p -orbitals are of somewhat higher energy, and their complementary anti-bonding orbital somewhat lower in energy than s *. Unshared pairs lie at the energy of the original atomic orbital, most often this energy is higher than p or s (since no bond is formed, there is no benefit in energy)

Slide 5:

5 UV Spectroscopy Introduction Observed electronic transitions Here is a graphical representation Energy s* p s p* n Atomic orbital Atomic orbital Molecular orbitals Occupied levels Unoccupied levels

Slide 6:

6 UV Spectroscopy Introduction Observed electronic transitions From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy: Energy s* p s p* n s s p n n s * p * p * s * p * alkanes carbonyls unsaturated cmpds. O, N, S, halogens carbonyls

Slide 7:

7 UV Spectroscopy Introduction Observed electronic transitions Although the UV spectrum extends below 100 nm (high energy), oxygen in the atmosphere is not transparent below 200 nm Special equipment to study vacuum or far UV is required Routine organic UV spectra are typically collected from 200-700 nm This limits the transitions that can be observed: s s p n n s * p * p * s * p * alkanes carbonyls unsaturated cmpds. O, N, S, halogens carbonyls 150 nm 170 nm 180 nm √ - if conjugated! 190 nm 300 nm √

Slide 8:

8 UV Spectroscopy Introduction Selection Rules Not all transitions that are possible are observed For an electron to transition, certain quantum mechanical constraints apply – these are called “ selection rules ” For example, an electron cannot change its spin quantum number during a transition – these are “ forbidden ” Other examples include: the number of electrons that can be excited at one time symmetry properties of the molecule symmetry of the electronic states To further complicate matters, “forbidden” transitions are sometimes observed (albeit at low intensity) due to other factors

Slide 9:

9 UV Spectroscopy Introduction Band Structure Unlike IR (or later NMR), where there may be upwards of 5 or more resolvable peaks from which to elucidate structural information, UV tends to give wide, overlapping bands 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 In molecules, when a bulk sample of molecules is observed, not all bonds (read – pairs of electrons) are in the same vibrational or rotational energy states 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

Slide 10:

10 UV Spectroscopy Introduction Band Structure When these energy levels are superimposed, the effect can be readily explained – any transition has the possibility of being observed Energy E 0 E 1

Slide 11:

11 UV Spectroscopy Instrumentation and Spectra Instrumentation The construction of a traditional UV-VIS spectrometer is very similar to an IR, as similar functions – sample handling, irradiation, detection and output are required Here is a simple schematic that covers most modern UV spectrometers: sample reference detector I 0 I 0 I 0 I log( I 0 / I ) = A 200 700 l , nm monochromator/ beam splitter optics UV-VIS sources

Slide 12:

12 UV Spectroscopy Instrumentation and Spectra Instrumentation Two sources are required to scan the entire UV-VIS band: Deuterium lamp – covers the UV – 200-330 Tungsten lamp – covers 330-700 As with the dispersive IR, the lamps illuminate the entire band of UV or visible light; the monochromator (grating or prism) gradually changes the small bands of radiation sent to the beam splitter The beam splitter sends a separate band to a cell containing the sample solution and a reference solution The detector measures the difference between the transmitted light through the sample ( I ) vs. the incident light ( I 0 ) and sends this information to the recorder

Slide 13:

13 UV Spectroscopy Instrumentation and Spectra Instrumentation As with dispersive IR, time is required to cover the entire UV-VIS band due to the mechanism of changing wavelengths A recent improvement is the diode-array spectrophotometer - here a prism (dispersion device) breaks apart the full spectrum transmitted through the sample Each individual band of UV is detected by a individual diodes on a silicon wafer simultaneously – the obvious limitation is the size of the diode, so some loss of resolution over traditional instruments is observed sample Polychromator – entrance slit and dispersion device UV-VIS sources Diode array

Slide 14:

14 UV Spectroscopy Instrumentation and Spectra Instrumentation – Sample Handling Virtually all UV spectra are recorded solution-phase Cells can be made of plastic, glass or quartz Only quartz is transparent in the full 200-700 nm range; plastic and glass are only suitable for visible spectra Concentration (we will cover shortly) is empirically determined A typical sample cell (commonly called a cuvet ):

Slide 15:

15 UV Spectroscopy Instrumentation and Spectra Instrumentation – Sample Handling Solvents must be transparent in the region to be observed; the wavelength where a solvent is no longer transparent is referred to as the cutoff Since spectra are only obtained up to 200 nm, solvents typically only need to lack conjugated p systems or carbonyls Common solvents and cutoffs: acetonitrile 190 chloroform 240 cyclohexane 195 1,4-dioxane 215 95% ethanol 205 n -hexane 201 methanol 205 isooctane 195 water 190

Slide 16:

16 UV Spectroscopy Instrumentation and Spectra Instrumentation – Sample Handling Additionally solvents must preserve the fine structure (where it is actually observed in UV!) where possible H-bonding further complicates the effect of vibrational and rotational energy levels on electronic transitions, dipole-dipole interacts less so The more non-polar the solvent, the better (this is not always possible)

Slide 17:

17 UV Spectroscopy Instrumentation and Spectra The Spectrum The x-axis of the spectrum is in wavelength; 200-350 nm for UV, 200-700 for UV-VIS determinations Due to the lack of any fine structure, spectra are rarely shown in their raw form, rather, the peak maxima are simply reported as a numerical list of “lamba max” values or l max l max = 206 nm 252 317 376

Slide 18:

18 UV Spectroscopy Instrumentation and Spectra The Spectrum The y-axis of the spectrum is in absorbance, A From the spectrometers point of view, absorbance is the inverse of transmittance: A = log 10 ( I 0 / I ) From an experimental point of view, three other considerations must be made: a longer path length, l through the sample will cause more UV light to be absorbed – linear effect the greater the concentration, c of the sample, the more UV light will be absorbed – linear effect some electronic transitions are more effective at the absorption of photon than others – molar absorptivity, e this may vary by orders of magnitude…

Slide 19:

19 UV Spectroscopy Instrumentation and Spectra The Spectrum These effects are combined into the Beer-Lambert Law: A = e c l for most UV spectrometers, l would remain constant (standard cells are typically 1 cm in path length) concentration is typically varied depending on the strength of absorption observed or expected – typically dilute – sub .001 M molar absorptivities vary by orders of magnitude: values of 10 4 -10 6 10 4 -10 6 are termed high intensity absorptions values of 10 3 -10 4 are termed low intensity absorptions values of 0 to 10 3 are the absorptions of forbidden transitions A is unitless, so the units for e are cm -1 · M -1 and are rarely expressed Since path length and concentration effects can be easily factored out, absorbance simply becomes proportional to e , and the y-axis is expressed as e directly or as the logarithm of e

Slide 20:

20 UV Spectroscopy Instrumentation and Spectra Practical application of UV spectroscopy UV was the first organic spectral method, however, it is rarely used as a primary method for structure determination It is most useful in combination with NMR and IR data to elucidate unique electronic features that may be ambiguous in those methods It can be used to assay (via l max and molar absorptivity) the proper irradiation wavelengths for photochemical experiments, or the design of UV resistant paints and coatings The most ubiquitous use of UV 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) will be to GC

Slide 21:

21 UV Spectroscopy Chromophores Definition Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N Since similar functional groups will have electrons capable of discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves A functional group capable of having characteristic electronic transitions is called a chromophore ( color loving ) Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions

Slide 22:

22 UV Spectroscopy Chromophores Organic Chromophores Alkanes – only posses s -bonds and no lone pairs of electrons, so only the high energy s  s * transition is observed in the far UV This transition is destructive to the molecule, causing cleavage of the s -bond s* s

Slide 23:

23 UV Spectroscopy Chromophores Organic Chromophores Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic examples of these compounds the n  s * is the most often observed transition; like the alkane s  s * it is most often at shorter l than 200 nm Note how this transition occurs from the HOMO to the LUMO s* CN s CN n N sp 3

Slide 24:

24 UV Spectroscopy Chromophores Organic Chromophores Alkenes and Alkynes – in the case of isolated examples of these compounds the p  p * is observed at 175 and 170 nm, respectively Even though this transition is of lower energy than s  s *, it is still in the far UV – however, the transition energy is sensitive to substitution p* p

Slide 25:

25 UV Spectroscopy Chromophores Organic Chromophores Carbonyls – unsaturated systems incorporating N or O can undergo n  p * transitions (~285 nm) in addition to p  p * Despite the fact this transition is forbidden by the selection rules ( e = 15), it is the most often observed and studied transition for carbonyls This transition is also sensitive to substituents on the carbonyl Similar to alkenes and alkynes, non-substituted carbonyls undergo the p  p * transition in the vacuum UV (188 nm, e = 900); sensitive to substitution effects

Slide 26:

26 UV Spectroscopy Chromophores Organic Chromophores Carbonyls – n  p * transitions (~285 nm); p  p * (188 nm) p p* n s CO transitions omitted for clarity It has been determined from spectral studies, that carbonyl oxygen more approximates sp rather than sp 2 !

Slide 27:

27 UV Spectroscopy Chromophores Substituent Effects General – from our brief study of these general chromophores, only the weak n  p * transition occurs in the routinely observed UV The attachment of substituent groups (other than H) can shift the energy of the transition Substituents that increase the intensity and often wavelength of an absorption are called auxochromes Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens

Slide 28:

28 UV Spectroscopy Chromophores Substituent Effects General – Substituents may have any of four effects on a chromophore Bathochromic shift (red shift) – a shift to longer l ; lower energy Hypsochromic shift (blue shift) – shift to shorter l ; higher energy Hyperchromic effect – an increase in intensity Hypochromic effect – a decrease in intensity 200 nm 700 nm e Hypochromic Hypsochromic Hyperchromic Bathochromic

Slide 29:

29 UV Spectroscopy Chromophores Substituent Effects Conjugation – most efficient means of bringing about a bathochromic and hyperchromic shift of an unsaturated chromophore: l max nm e 175 15,000 217 21,000 258 35,000 n  p * 280 27 p  p * 213 7,100 465 125,000 n  p * 280 12 p  p * 189 900

Slide 30:

30 UV Spectroscopy Chromophores Substituent Effects Conjugation – Alkenes The observed shifts from conjugation imply that an increase in conjugation decreases the energy required for electronic excitation From molecular orbital (MO) theory two atomic p orbitals, f 1 and f 2 from two sp 2 hybrid carbons combine to form two MOs Y 1 and Y 2 * in ethylene Y 2 * p Y 1 f 1 f 2

Slide 31:

31 UV Spectroscopy Chromophores Substituent Effects Conjugation – Alkenes When we consider butadiene, we are now mixing 4 p orbitals giving 4 MOs of an energetically symmetrical distribution compared to ethylene Y 2 * p Y 1 Y 1 Y 2 Y 3 * Y 4 * D E for the HOMO  LUMO transition is reduced

Slide 32:

32 UV Spectroscopy Chromophores Substituent Effects Conjugation – Alkenes Extending this effect out to longer conjugated systems the energy gap becomes progressively smaller: Energy ethylene butadiene hexatriene octatetraene Lower energy = Longer wavelengths

Slide 33:

33 UV Spectroscopy Chromophores Substituent Effects Conjugation – Alkenes Similarly, the lone pairs of electrons on N, O, S, X can extend conjugated systems – auxochromes Here we create 3 MOs – this interaction is not as strong as that of a conjugated p -system Y 2 p Y 1 p * n A Y 3 * Energy

Slide 34:

34 UV Spectroscopy Chromophores Substituent Effects Conjugation – Alkenes Methyl groups also cause a bathochromic shift, even though they are devoid of p - or n -electrons This effect is thought to be through what is termed “hyperconjugation” or sigma bond resonance

Slide 35:

35 UV Spectroscopy Next time – We will find that the effect of substituent groups can be reliably quantified from empirical observation of known conjugated structures and applied to new systems This quantification is referred to as the Woodward-Fieser Rules which we will apply to three specific chromophores: Conjugated dienes Conjugated dienones Aromatic systems

Slide 36:

36 UV Spectroscopy Structure Determination Dienes General Features For acyclic butadiene, two conformers are possible – s-cis and s-trans The s-cis conformer is at an overall higher potential energy than the s-trans ; therefore the HOMO electrons of the conjugated system have less of a jump to the LUMO – lower energy, longer wavelength s - trans s - cis

Slide 37:

37 UV Spectroscopy Structure Determination Dienes General Features Two possible p  p * transitions can occur for butadiene Y 2  Y 3 * and Y 2  Y 4 * The Y 2  Y 4 * transition is not typically observed: The energy of this transition places it outside the region typically observed – 175 nm For the more favorable s-trans conformation, this transition is forbidden The Y 2  Y 3 * transition is observed as an intense absorption s - trans s - cis 175 nm –forb. 217 nm 253 nm 175 nm Y 4 * Y 2 Y 1 Y 3 *

Slide 38:

38 UV Spectroscopy Structure Determination Dienes General Features The Y 2  Y 3 * transition is observed as an intense absorption ( e = 20,000+) based at 217 nm within the observed region of the UV While this band is insensitive to solvent (as would be expected) it is subject to the bathochromic and hyperchromic effects of alkyl substituents as well as further conjugation Consider: l max = 217 253 220 227 227 256 263 nm

Slide 39:

39 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules Woodward and the Fiesers performed extensive studies of terpene and steroidal alkenes and noted similar substituents and structural features would predictably lead to an empirical prediction of the wavelength for the lowest energy p  p * electronic transition This work was distilled by Scott in 1964 into an extensive treatise on the Woodward-Fieser rules in combination with comprehensive tables and examples – (A.I. Scott, Interpretation of the Ultraviolet Spectra of Natural Products , Pergamon, NY, 1964) A more modern interpretation was compiled by Rao in 1975 – (C.N.R. Rao, Ultraviolet and Visible Spectroscopy , 3 rd Ed., Butterworths, London, 1975)

Slide 40:

40 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules - Dienes The rules begin with a base value for l max of the chromophore being observed: acyclic butadiene = 217 nm The incremental contribution of substituents is added to this base value from the group tables: Group Increment Extended conjugation +30 Each exo-cyclic C=C +5 Alkyl +5 -OCOCH 3 +0 -OR +6 -SR +30 -Cl, -Br +5 -NR 2 +60

Slide 41:

41 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules - Dienes For example: Isoprene - acyclic butadiene = 217 nm one alkyl subs. + 5 nm 222 nm Experimental value 220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C + 5 nm 2 alkyl subs. +10 nm 232 nm Experimental value 237 nm

Slide 42:

42 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules – Cyclic Dienes There are two major types of cyclic dienes, with two different base values Heteroannular (transoid): Homoannular (cisoid): e = 5,000 – 15,000 e = 12,000-28,000 base l max = 214 base l max = 253 The increment table is the same as for acyclic butadienes with a couple additions: Group Increment Additional homoannular +39 Where both types of diene are present, the one with the longer l becomes the base

Slide 43:

43 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules – Cyclic Dienes In the pre-NMR era of organic spectral determination, the power of the method for discerning isomers is readily apparent Consider abietic vs. levopimaric acid: levopimaric acid abietic acid

Slide 44:

44 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules – Cyclic Dienes For example: 1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene heteroannular diene = 214 nm 3 alkyl subs. (3 x 5) +15 nm 1 exo C=C + 5 nm 234 nm Experimental value 235 nm

Slide 45:

45 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules – Cyclic Dienes heteroannular diene = 214 nm 4 alkyl subs. (4 x 5) +20 nm 1 exo C=C + 5 nm 239 nm homoannular diene = 253 nm 4 alkyl subs. (4 x 5) +20 nm 1 exo C=C + 5 nm 278 nm

Slide 46:

46 UV Spectroscopy Structure Determination Dienes Woodward-Fieser Rules – Cyclic Dienes Be careful with your assignments – three common errors: This compound has three exocyclic double bonds; the indicated bond is exocyclic to two rings This is not a heteroannular diene; you would use the base value for an acyclic diene Likewise, this is not a homooannular diene; you would use the base value for an acyclic diene

Slide 47:

47 UV Spectroscopy Structure Determination Enones General Features Carbonyls, as we have discussed have two primary electronic transitions: p p* n Remember, the p  p * transition is allowed and gives a high e , but lies outside the routine range of UV observation The n  p * transition is forbidden and gives a very low e, but can routinely be observed

Slide 48:

48 UV Spectroscopy Structure Determination Enones General Features For auxochromic substitution on the carbonyl, pronounced hypsochromic shifts are observed for the n  p * transition ( l max ) : This is explained by the inductive withdrawal of electrons by O, N or halogen from the carbonyl carbon – this causes the n -electrons on the carbonyl oxygen to be held more firmly It is important to note this is different from the auxochromic effect on p  p * which extends conjugation and causes a bathochromic shift In most cases, this bathochromic shift is not enough to bring the p  p * transition into the observed range 293 nm 279 235 214 204 204

Slide 49:

49 UV Spectroscopy Structure Determination Enones General Features Conversely, if the C=O system is conjugated both the n  p * and p  p * bands are bathochromically shifted Here, several effects must be noted: the effect is more pronounced for p  p * if the conjugated chain is long enough, the much higher intensity p  p * band will overlap and drown out the n  p * band the shift of the n  p * transition is not as predictable For these reasons, empirical Woodward-Fieser rules for conjugated enones are for the higher intensity, allowed p  p * transition

Slide 50:

50 UV Spectroscopy Structure Determination Enones General Features These effects are apparent from the MO diagram for a conjugated enone: p Y 1 Y 2 Y 3 * Y 4 * p* n p p* n

Slide 51:

51 UV Spectroscopy Structure Determination Enones Woodward-Fieser Rules - Enones Group Increment 6-membered ring or acyclic enone Base 215 nm 5-membered ring parent enone Base 202 nm Acyclic dienone Base 245 nm Double bond extending conjugation 30 Alkyl group or ring residue a, b, g and higher 10, 12, 18 -OH a, b, g and higher 35, 30, 18 -OR a, b, g, d 35, 30, 17, 31 -O(C=O)R a, b, d 6 -Cl a, b 15, 12 -Br a, b 25, 30 -NR 2 b 95 Exocyclic double bond 5 Homocyclic diene component 39

Slide 52:

52 UV Spectroscopy Structure Determination Enones Woodward-Fieser Rules - Enones Aldehydes, esters and carboxylic acids have different base values than ketones Unsaturated system Base Value Aldehyde 208 With a or b alkyl groups 220 With a,b or b,b alkyl groups 230 With a,b,b alkyl groups 242 Acid or ester With a or b alkyl groups 208 With a,b or b,b alkyl groups 217 Group value – exocyclic a,b double bond +5 Group value – endocyclic a,b bond in 5 or 7 membered ring +5

Slide 53:

53 UV Spectroscopy Structure Determination Enones Woodward-Fieser Rules - Enones Unlike conjugated alkenes, solvent does have an effect on l max These effects are also described by the Woodward-Fieser rules Solvent correction Increment Water +8 Ethanol, methanol 0 Chloroform -1 Dioxane -5 Ether -7 Hydrocarbon -11

Slide 54:

54 UV Spectroscopy Structure Determination Enones Woodward-Fieser Rules - Enones Some examples – keep in mind these are more complex than dienes cyclic enone = 215 nm 2 x b - alkyl subs. (2 x 12) +24 nm 239 nm Experimental value 238 nm cyclic enone = 215 nm extended conj. +30 nm b -ring residue +12 nm d -ring residue +18 nm exocyclic double bond + 5 nm 280 nm Experimental 280 nm

Slide 55:

55 UV Spectroscopy Structure Determination Enones Woodward-Fieser Rules - Enones Take home problem – can these two isomers be discerned by UV-spec Eremophilone allo- Eremophilone Problem Set 1: (text) – 1,2,3a,b,c,d,e,f,j, 4, 5, 6 (1 st , 2 nd and 5 th pairs), 8a, b, c Problem Set 2: outside problems/key -Tuesday

Slide 56:

56 UV Spectroscopy Structure Determination Aromatic Compounds General Features Although aromatic rings are among the most widely studied and observed chromophores, the absorptions that arise from the various electronic transitions are complex On first inspection, benzene has six p -MOs, 3 filled p , 3 unfilled p * p 4 * p 5 * p 6 * p 2 p 1 p 3

Slide 57:

57 UV Spectroscopy Structure Determination Aromatic Compounds General Features One would expect there to be four possible HOMO-LUMO p  p * transitions at observable wavelengths (conjugation) Due to symmetry concerns and selection rules, the actual transition energy states of benzene are illustrated at the right: p 4 * p 5 * p 6 * p 2 p 1 p 3 A 1 g B 2 u B 1 u E 1 u 260 nm (forbidden) 200 nm (forbidden) 180 nm (allowed)

Slide 58:

58 UV Spectroscopy Structure Determination Aromatic Compounds General Features The allowed transition ( e = 47,000) is not in the routine range of UV obs. at 180 nm, and is referred to as the primary band The forbidden transition ( e = 7400) is observed if substituent effects shift it into the obs. region; this is referred to as the second primary band At 260 nm is another forbidden transition ( e = 230), referred to as the secondary band. This transition is fleetingly allowed due to the disruption of symmetry by the vibrational energy states, the overlap of which is observed in what is called fine structure

Slide 59:

59 UV Spectroscopy Structure Determination Aromatic Compounds General Features Substitution, auxochromic, conjugation and solvent effects can cause shifts in wavelength and intensity of aromatic systems similar to dienes and enones However, these shifts are difficult to predict – the formulation of empirical rules is for the most part is not efficient (there are more exceptions than rules) There are some general qualitative observations that can be made by classifying substituent groups --

Slide 60:

60 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Substituents with Unshared Electrons If the group attached to the ring bears n electrons, they can induce a shift in the primary and secondary absorption bands Non-bonding electrons extend the p -system through resonance – lowering the energy of transition p  p * More available n -pairs of electrons give greater shifts

Slide 61:

61 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Substituents with Unshared Electrons The presence of n -electrons gives the possibility of n  p * transitions If this occurs, the electron now removed from G, becomes an extra electron in the anti-bonding p * orbital of the ring This state is referred to as a charge-transfer excited state

Slide 62:

62 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Substituents with Unshared Electrons pH can change the nature of the substituent group deprotonation of oxygen gives more available n -pairs, lowering transition energy protonation of nitrogen eliminates the n -pair, raising transition energy Primary Secondary Substituent l max e l max e -H 203.5 7,400 254 204 -OH 211 6,200 270 1,450 -O - 235 9,400 287 2,600 -NH 2 230 8,600 280 1,430 -NH 3 + 203 7,500 254 169 -C(O)OH 230 11,600 273 970 -C(O)O - 224 8,700 268 560

Slide 63:

63 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Substituents Capable of p -conjugation When the substituent is a p -chromophore, it can interact with the benzene p -system With benzoic acids, this causes an appreciable shift in the primary and secondary bands For the benzoate ion, the effect of extra n -electrons from the anion reduces the effect slightly Primary Secondary Substituent l max e l max e -C(O)OH 230 11,600 273 970 -C(O)O - 224 8,700 268 560

Slide 64:

64 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Electron-donating and electron-withdrawing effects No matter what electronic influence a group exerts, the presence shifts the primary absorption band to longer l Electron-withdrawing groups exert no influence on the position of the secondary absorption band Electron-donating groups increase the l and e of the secondary absorption band

Slide 65:

65 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Electron-donating and electron-withdrawing effects Primary Secondary Substituent l max e l max e -H 203.5 7,400 254 204 -CH 3 207 7,000 261 225 -Cl 210 7,400 264 190 -Br 210 7,900 261 192 -OH 211 6,200 270 1,450 -OCH 3 217 6,400 269 1,480 -NH 2 230 8,600 280 1,430 -CN 224 13,000 271 1,000 C(O)OH 230 11,600 273 970 -C(O)H 250 11,400 -C(O)CH 3 224 9,800 -NO 2 269 7,800 Electron donating Electron withdrawing

Slide 66:

66 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Di-substituted and multiple group effects With di-substituted aromatics, it is necessary to consider both groups If both groups are electron donating or withdrawing, the effect is similar to the effect of the stronger of the two groups as if it were a mono -substituted ring If one group is electron withdrawing and one group electron donating and they are para - to one another, the magnitude of the shift is greater than the sum of both the group effects Consider p -nitroaniline:

Slide 67:

67 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Di-substituted and multiple group effects If the two electonically dissimilar groups are ortho- or meta- to one another, the effect is usually the sum of the two individual effects ( meta - no resonance; ortho -steric hind.) For the case of substituted benzoyl derivatives, an empirical correlation of structure with observed l max has been developed This is slightly less accurate than the Woodward-Fieser rules, but can usually predict within an error of 5 nm

Slide 68:

68 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Di-substituted and multiple group effects Substituent increment G o m p Alkyl or ring residue 3 3 10 -O-Alkyl, -OH, -O-Ring 7 7 25 -O - 11 20 78 -Cl 0 0 10 -Br 2 2 15 -NH 2 13 13 58 -NHC(O)CH 3 20 20 45 -NHCH 3 73 -N(CH 3 ) 2 20 20 85 Parent Chromophore l max R = alkyl or ring residue 246 R = H 250 R = OH or O-Alkyl 230

Slide 69:

69 UV Spectroscopy Structure Determination Aromatic Compounds Substituent Effects Polynuclear aromatics When the number of fused aromatic rings increases, the l for the primary and secondary bands also increase For heteroaromatic systems spectra become complex with the addition of the n  p * transition and ring size effects and are unique to each case

Slide 70:

70 UV Spectroscopy Visible Spectroscopy Color General The portion of the EM spectrum from 400-800 is observable to humans- we (and some other mammals) have the adaptation of seeing color at the expense of greater detail 400 500 600 800 700 l , nm Violet 400-420 Indigo 420-440 Blue 440-490 Green 490-570 Yellow 570-585 Orange 585-620 Red 620-780

Slide 71:

71 UV Spectroscopy Visible Spectroscopy Color General When white (continuum of l ) light passes through, or is reflected by a surface, those ls that are absorbed are removed from the transmitted or reflected light respectively What is “seen” is the complimentary colors (those that are not absorbed) This is the origin of the “color wheel”

Slide 72:

72 UV Spectroscopy Visible Spectroscopy Color General Organic compounds that are “colored” are typically those with extensively conjugated systems (typically more than five) Consider b -carotene l max is at 455 – in the far blue region of the spectrum – this is absorbed The remaining light has the complementary color of orange

Slide 73:

73 UV Spectroscopy Visible Spectroscopy Color General Likewise: l max for lycopene is at 474 – in the near blue region of the spectrum – this is absorbed, the compliment is now red l max for indigo is at 602 – in the orange region of the spectrum – this is absorbed, the compliment is now indigo!

Slide 74:

74 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 for these to be opposite for the original synthetic preparation!

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75 UV Spectroscopy Visible Spectroscopy Color General These materials are some of the more familiar colors of our “environment”

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76 The colors of M&M’s Bright Blue Common Food Uses Beverages, dairy products, powders, jellies, confections, condiments, icing. Royal Blue Common Food Uses Baked goods, cereals, snack foods, ice-cream, confections, cherries. Orange-red Common Food Uses Gelatins, puddings, dairy products, confections, beverages, condiments. Lemon-yellow Common Food Uses Custards, beverages, ice-cream, confections, preserves, cereals. Orange Common Food Uses Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections

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77 UV Spectroscopy Visible Spectroscopy Color General In the biological sciences these compounds are used as dyes to selectively stain different tissues or cell structures Biebrich Scarlet - Used with picric acid/aniline blue for staining collagen, recticulum, muscle, and plasma. Luna's method for erythrocytes & eosinophil granules. Guard's method for sex chromatin and nuclear chromatin.

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78 UV Spectroscopy Visible Spectroscopy Color General In the chemical sciences these are the acid-base indicators used for the various pH ranges: Remember the effects of pH on aromatic substituents

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