UV Spectroscopy: Empirical Approach to Molecular Structures. Dr. Mishu Singh Department of Chemistry M. P.Govt P. G.

Transcription

1 UV Spectroscopy: Empirical Approach to Molecular Structures Dr. Mishu Singh Department of Chemistry M. P.Govt P. G.College, Hardoi

2 WHAT IS SPECTROSCOPY? Atoms and molecules interact with electromagnetic radiation (EMR) in a wide variety of ways. Atoms and molecules may absorb and/or emit EMR. Absorption of EMR stimulates different types of motion in atoms and/or molecules. The patterns of absorption (wavelengths absorbed and to what extent) and/or emission (wavelengths emitted and their respective intensities) are called spectra. spectroscopy is the interaction of EMR with matters to get specta,which gives information like, bond length, bond angle, geometry and molecular structure.

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4 The entire electromagnetic spectrum is used by chemists: g-rays nuclear excitation (PET) X-rays UV IR Microwave Radio core electron excitation (X-ray cryst.) electronic excitation (p to p*) molecular vibration molecular rotation Nuclear Magnetic Resonance NMR (MRI) Visible

5 Electromagnetic radiations, are a form of energy, displays the properties of both,particles and waves. The particle component is called a photon The term photon is implied to mean a small, massless particle that contains a small wave-packet of EM radiation/light. The important parameters associated with electromagnetic radiation are: Energy (E): Frequency (n) E = hn Wavelength (l) : Energy is directly proportional to frequency, and inversely proportion to wavelength, as indicated by the equation below l l = distance of one wave n = frequency: waves per unit time (sec -1, Hz) c = speed of light (3.0 x 10 8 m sec -1 ) h = Plank s constant (6.63 x J sec) 5

6 Principles of molecular spectroscopy organic molecule light hn organic molecule (excited state) relaxation organic molecule(gro und state) UV-Vis: valance electron transitions - gives information about p-bonds and conjugated systems Infrared: molecular vibrations (stretches, bends) - identify functional groups Radiowaves: nuclear spin in a magnetic field (NMR) - gives a map of the H and C framework

7 Ultra-Violet Spectroscopy It is a branch of spectroscopy in which transition occur due to the excitation of electrons from one energy level to higher one by the interaction of molecules with ultraviolet and visible lights. Absorption of photon results in electronic transition of a molecule, and electrons are promoted from ground state to higher electronic states. Since it involves an electron excitation phenomenon, so, also called as Electronic Spectroscopy. E hv LUMO HOMO

8 Range of UV-Visible Spectrum Visible Mid UV Far UV 800 nm 400 nm 200nm 100 nm Violet: nm Indigo: nm Blue: nm Green: nm Yellow: nm Orange: nm Red: nm

9 Absorption in in UV UV Spectroscopy Absorption of light in U V region is governed by Lambert s-ber s law. When a abeam of monochromatic light is passed through the homogenous medium of substance, a fraction of light is absorbed. The decreased in the intensity of light with the concentration of medium depends on the thickness and concentration of the medium and the phenomenon is given by the equation: A = - logt = ebc A = absorbance ( optical density ) ; and A = log I0 / I, T= Transmittance C= concentration of solutios b = length of the sample tube = molar extinction coefficient I0 = Intensity of incident light I = Intensity of transmitted light

10 Electronic transitions There are three types of electronic transition which can be considered; 1. Transitions involving charge-transfer electrons 2.. Transitions involving d and f electrons 3. Transitions involving p, s, and n electrons Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy.

11 Vacuum UV or Far UV (λ<190 nm )

12 Selection Rules 1. Not all transitions that are possible are observed. Only those transitios which follow certain rule,are called as allowed transitions. 2. For an electron to transition, certain quantum mechanical constraints apply these are called selection rules 3. 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

13 Transition Probability ( Allowed and Forbidden Transitions ) The molar extinction coefficient, ξ depends upon: ξ max = 0.87 x x P x a Where, P = transition Probability varies from 0-1 a = target area of absorbing system(chromophores) Depending upon the value of ξ, and symmetry of orbital, the transition may be allowed or forbidden If, ξ > 10 4 ξ ~ ξ < 10 3 ; Strong / high intensity peak ( allowed transition) Medium / Moderate intensity peak( allowed Trans.) weak/low intensity peak ( forbidden transition)

14 Transitions s->s* UV photon required, high energy, shorter wavelength Methane at 125 nm ( All saturated comp ) Ethane at 135 nm n-> s* Saturated compounds with unshared e - N, O, S and Halogens, Absorption between 150 nm to 250 nm e between 100 and 3000 L cm -1 mol -1 Shifts to shorter wavelengths with polar solvents R-X, R-OH R-NH2

15 Transitions n->p* and p->p* Unsaturated Organic compounds, containing non-bonded electrons (O, N, S) and X wavelengths 200 to 700 nm n->p* low e (10 to 100), Shorter wavelengths Shorter wavelengths p->p* higher e (1000 to 10000), Longer wavelength as compared to n->p*

16 Presentation of U V Spectra

17 TERMINOLOGIES Chromophores and Auxochromes 1. A functional group capable of having characteristic electronic transitions is called a chromophore (color loving) C=C ; C=N, C=S, C=C; N=N etc 2. Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions 3. The attachment of substituent groups (other than H) can shift the energy of the transition 4. Substituent's,that could not absorbed light in UV region, but,increase the intensity and often wavelength of an absorption are called auxochromes 5. Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens

18 Hyperchromic Substituents may have any of four effects on a chromophore I. Bathochromic shift (red shift) a shift to longer l; lower energy II. Hypsochromic shift (blue shift) shift to shorter l; higher energy III. Hyperchromic effect an increase in intensity IV. Hypochromic effect a decrease in intensity Hypsochromic Hypochromic Bathochromic 200 nm 700 nm

19 Ethylene Chromophores CH2=CH2 174 CH3-(CH2)5-CH=CH CH2=CH-Cl 186 CH2=CH-OCH3 190 CH2=CH-S-CH3 210

20 Conjugation And Ethylene Chromophores Ethylene absorbs at 175 nm. Presence of either chromophores or auxochromes gives bathochromic shift. l max nm , , , ,000 L max = nm

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22 EXPLANATION OF HIGHER WAVE LENHTH FOR CONJUGATION When we consider butadiene, we are now mixing 4 p orbitals giving 4 MOs of an as compared to ethylene which has only 2 M Os Y 2 * Y 4 * LUMO Y 3 * LUMO Y 1 p HOMO Y 2 Y 1 HOMO 217 nm DE for the HOMO LUMO transition is reduced So, l max is increased.

23 Extending this effect to longer conjugated systems the energy gap becomes progressively smaller: Energy Lower energy = Longer wavelengths ethylene butadiene hexatriene octatetraene

24 Empirical Approach To Structure Determination WOODWARD-FEISER RULE *Woodward ( 1914 ) : gave certain rules for correlate with molecular structures with l max. *Scott-Feiser(1959):modified rule with more experimental data, the modified rule is known as Woodward-Feiser rule. *A more modern interpretation was compiled by Rao in 1975 (C.N.R. Rao, Ultraviolet and Visible Spectroscopy, 3 rd Ed., Butterworths, London, 1975) It is used to correlate the l max of for a given structure by relating position and degree of substitution of chromophores.

25 Woodward-Fieser Rules for Conjugated acyclic Dienes Woodward had predicted an empirical rule for calculating l max of conjugated acyclic and cyclic dienes based on base value and contribution of different substituent. The equation is: l max = Base value + Substituent's contribution + Other contribution Base value : Take a base value of 214/217 for any conjugated. diene or triene Incrementals: Add the following to the base value Group Increment Extended conjugation +30 Each exo-cyclic C=C Phenyl group Auxochromes: Alkyl +5 -OCOCH OR +6 -SR +30 -Cl, -Br +5 -NR 2 +60

26 Examples: Isoprene - acyclic butadiene = one alkyl subs. + 5 nm Experimental value 217 nm 222 nm 220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C + 5 nm 2 alkyl subs. +10 nm 232 nm Experimental value 237 nm

27 Woodward-Fieser Rules Cyclic Dienes There are two major types of cyclic dienes, with two different base values Heteroannular (transoid): Homoannular (cisoid): base l max = 214 base l max = 253 base l max = 253 Groups Extended conjugation Pheny group Increment Each exo-cyclic C=C +5 Alkyl / ring residue +5 -OCOCH OR +6 -SR +30 -Cl, -Br +5 -NR Additional homoannular +39

28 Eexamples: 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

29 C O OH heteroannular diene = 214 nm 1 alkyl subs. (1 x 5) RR exo C=C +0 5 nm 239 nm C O OH homoannular diene = 253 nm 1alkyl subs. (1x 5) +05nm 3 RR +15nm 1 exo C=C + 5 nm 278 nm

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31 Be careful with your assignments three common errors: R 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

32 Base - Transoid/Heteroannular Substituents- 5 Ring Residues 1 Double bond extending conjugation Other Effects- 3 Exocyclic Double Bond nm 5 x 5 = + 25 nm + 30 nm + 15 nm Calculated λmax Observed λmax 285 nm 283 nm

33 Component Core- Homoannular/Cisoid diene Substituents- 5 alkyl substituents Double bond extending conjugation Other Effects- 3 Exocyclic double bonds Calculated λ max Contribution 253 nm 5 x 5 = + 25 nm + 30 nm 3 x 5 = + 15 nm 323 nm Observed λ max 321

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35 Limitations of Woodwards rule for dienes The Woodward rule is applicable to conjugated acyclic dienes with a maximum no. of 04 double bonds, it fails beyond that. Conjugated dienes more than 04 double bonds ; Fischer and Khun rule is applicable. λ max = M + n ( n) 16.5 R endo 10 R exo ε max = (1.74 x 10 4 ) n M = the number of alkyl substituents and ring residues n = the number of conjugated double bonds Rendo = the number of rings with endocyclic double bonds Rexo = the number of rings with exocyclic double bonds

36 Name of Compound Base Value all-trans-lycophene 114 nm M (number of alkyl & RR substituents) 8 n (number of conjugated double bonds) 11 R endo (number of endocyclic double bonds) 0 R exo (number of exocyclic double bonds) 0 Substituting in equation λ max = M + n ( n) 16.5 R endo 10 R exo λ max observed practically Calculate ε max using equation: ε max = (1.74 x 10 4 ) n = (8) + 11 ( (11)) 16.5 (0) 10 (0) = (29.3) 0 0 = Calc. λ max = nm 474 nm = (1.74 x 10 4 ) 11 Calc. ε max = x 10 4 Practically observed ε max 18.6 x 10 4

37 Name of Compound Base Value β-carotene 114 nm M (number of alkyl & RR substituents) 10 n (number of conjugated double bonds) 11 R endo (number of endocyclic double bonds) 2 R exo (number of exocyclic double bonds) 0 Substituting in equation λ max = M + n ( n) 16.5 R endo 10 R exo λ max observed practically Calculate ε max using equation: ε max = (1.74 x 10 4 ) n = (10) + 11 ( (11)) 16.5 (2) 10 (0) = (29.3) 33 0 = Calc. λ max = nm 452nm = (1.74 x 10 4 ) 11 Calc. ε max = x 10 4 Practically observed ε max 15.2 x 10 4

38 Enones Enone, called as alpha beta unsaturated carbonyl compound shows two types of electronic transitions. p* The, p p* and n p* ; Both show longer λ max than isolated enone n p Conjugation of a double bond with a carbonyl group leads to π π*, at 200 ~ 250 nm absorption (ε = 8,000 to 20,000), Predictable And n π*, at 210 ~ 330 nm, much less intense (ε= 50 to 100), not predictable

39 The effects of conjugation are apparent from the MO diagram for an enone: Y 4 * E p* Y 3 * p* n n Y 2 p Y 1 p O O

40 Group Woodward-Fieser Rules - Enones b a d g b a b C C C d C C C C C O O Increment 6-membered ring or acyclic enone Base 215 nm 5-membered ring parent enone Base 202 nm Acyclic dienone Base 245 nm Aldehyde,Ester & Acid Substituents: Double bond extending conjugation Phenyl group b Base 208 nm 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

41 SOLVENT CORRECTION Unlike conjugated alkenes, solvent does have an effect on l max These effects are also described by the Woodward-Fieser rules Common solvent cuttoff: acetonitrile 190 chloroform 240 cyclohexane 195 1,4-dioxane % ethanol 205 n-hexane 201 methanol 205 isooctane 195 water 190 Solvent correction Increment Water +8 Ethanol, methanol 0 Chloroform -1 Dioxane -5 Ether -7 Hydrocarbon -11

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44 Examples keep in mind these are more complex than dienes O cyclic enone = 215 nm 2 x b- alkyl subs. (2 x 12) +24 nm 239 nm Experimental value 238 nm O R cyclic enone = extended conj. b-ring residue d-ring residue exocyclic double bond 215 nm +30 nm +12 nm +18 nm + 5 nm 280 nm Experimental 280 nm

45 Base - cyclopentenone nm Substituents at α-position 0 Substituents at β-position- 2 Ring Residue Other Effects- 1 Exocyclic Double Bond Calculated λ max Observed λ max 2 x 12= + 24 nm + 5 nm 231 nm 229 nm

46 Base - cyclohexenone Substituents at α-position: 0 Substituents at β-position: 1 Ring Residue Substituents at γ-position Substituents at δ-position: 0 Substituents at ε-position: 1 Ring Residue Substituents at ζ-position: 2 Ring Residue Other Effects: 2 Double bonds extending conjugation Homoannular Diene system in ring B 1 Exocyclic double bond Calculated λ max Observed λ max nm + 12 nm nm 2 x 18 = + 36 nm 2 x 30 = + 60 nm + 35 nm nm 388 nm

47 Problem can these two isomers be discerned by UV-spectroscopy? O O Eremophilone allo-eremophilone

48 Aromatic Compounds Benzene gives three bands in UV absorption.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 6 * p 4 * p 5 * 200 nm (forbidden) p 2 p nm (forbidden) 180 nm (allowed) p 1

49 Substituent Effects Aromatic Compound No matter what electronic influence a group exerts, EWG or EDG, their presence shift l towards longer side. Substituent l max H CH Cl 210 -Br 210 -OH 211 -OCH NH CN 224 C(O)OH 230 -C(O)H 250 -C(O)CH NO 2 269

50 Polynuclear aromatics When the number of fused aromatic rings increases, the l max for the primary and secondary bands also increased. For heteroaromatic systems spectra,become complex 50

51 Woodward Rule for Aromatic Ketones Parent Chromophore R = alkyl or ring residue l max 246 O R G R = H 250 R = OH or O-Alkyl 230 Substituent increment G o m p Alkyl or ring residue O-Alkyl, -OH, -O-Ring O Cl Br NH NHC(O)CH NHCH N(CH 3 )

52 Base value = Ring residue in o- position = 1 x 3 = Polar group -OCH 3 in p- position = 246 nm 3 nm 25 nm λ max = 274 nm Observed = 274 nm

53 PROBLEMS 1. The enol acetate (ester) of the following compounds has λ max at 238nm, suggest the structure of the ester. 2. The following triene on practical hydrogenation gives three products, separated by GLC. How UV spectra anwoodwards rule will help to identify the products.

54 3.. Pentene-1 absorbs at λ max 178nm. Three isomeric dienes A,B and C of molecular composition C 5 H 8 absorbs at: λ max, for A=179nm for B=217nm for C=219nm Find out A, B and C 4. Four isomeric ketones A, B, C and D having molecular formula C6H8O The UV spectra gives absorption at 228 nm. Which is probable structure of the ketone.

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56 More Complex Electronic Processes Fluorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of the same multiplicity Phosphorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of different multiplicity Singlet state: spins are paired, no net angular momentum (and no net magnetic field) Triplet state: spins are unpaired, net angular momentum (and net magnetic field) Fluorescence is a process that involves a singlet to singlet transition. Phosphorescence is a process that involves a triplet to singlet transition