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Ground state reactants Ground state reactants hν Excited state reactants Ground state products Reaction Intermediates Ground state products 9.1 Criteria to establish reasonable mechanistic options In determining a plausible from a possible mechanistic set, we may first apply criteria relating to: 1. Energetics 2. Dynamics 3. Structure 4. Electronics 9.2 Additional criteria to select from among plausible mechanisms the most probable one • 1. Reactant and Product structure • 2. Structure of intermediates on the pathway from reactant to product • 3. Kinetic rate law • 4. Labeling experiments • 5. Structure-reactivity relationships 9.3 Calculated rate constants at 300 K for various preexponential factors and variable activation energies. 16 At 300 K 14 16 10 14 8 12 6 10 4 log (A/s–1) = 8 –1 log k (s ) 12 2 0 0 2 4 6 8 Ea (kcal/mol) 10 12 9.4 Norrish Type I reaction O O CH3 CH3 hν CH3 CH3 CH3 CH3 3n,π* ~ 100 ns O C• + The triplet lifetime is around 100 ns at room temperature. We assume that for this chemical reaction to contribute significantly to triplet decay, its rate constant will have to be at least 10 5 s -1 for the Norrish Type I cleavage. We calculate that the maximum activation energy is ~ 11 kcal/mol. Experimentally the measured values are A = 2 x 1012 s -1 and Ea = 7.3 kcal/mol and the (CH 3)3 C • cleavage occurs with a high quantum yield. 9.5 The Norrish Type II reaction O OH Ea (kcal/mol) hν R1 X H R2 Type II H-abstraction from the triplet state X R1 primary 7 R2 secondary 5 tertiary 4.5 Type II 1,4-biradical typical lifetimes are around 30-100 ns OH R1 + R2 (a) OH R1 (b) R1 X O X R2 X R2 X HO (c) O R1 X H R2 9.6 Norrish Type II reaction Pre-exponential factors H O A = 1013 s–1 H O A = 1012 s–1 Entropic factors are important in determining A factors 9.7 Quantum yield and state efficiency Quantum yield: Φ= moles of a given species formed or destroyed moles of photons absorbed by the system A mole of photons is also called an Einstein State efficiency : φ = moles of a given species formed or destroyed moles of a given state formed by absorption of an Einstein R hν φR* R* φI I Reactants φP P Products Φp = ΦR* x φI x φp 9.8 Some mechanistic tools and criteria 1. Does the overall reaction correspond to a standard reaction type or to a sequence of standard reaction types? 2. How does the product atomic composition and connectivity relate to the reactant atomic composition and connectivity? 3. How does the product stereochemistry relate to the reactant stereochemistry? Reactions can be classified in terms of mechanistic types, and for any given class of reactions only a small set of mechanistic types is likely to be required for a detailed analysis Hammond postulate If a transition state and an intermediate possess comparable energies and occur consecutively along a reaction coordinate, the chemical composition, the chemical constitution (structures) and chemical properties of the transition state will be similar to those of the intermediate 9.9 Excitation-decay of R* pulsed excitation 100 [R] Concentration (% of total) 80 60 40 20 hv → R Reactants [R * ] τR → Products R = Reactants [R*] 0 0 20 40 60 Time (arbitrary units) 80 100 Pulsed excitation at t = 10 excites 40% of the R molecules, Reading to an excited state (R*) that decays with τR = 20. Excited decay is accompanied by the concurrent regeneration of R. 9.10 Decay of triplet xanthone following pulsed laser excitation 0.025 O 0.020 Absorbance signal Decay of triplet xanthone monitored at 600 nm following 355 nm laser excitation in acetonitrile. The faster decay process is in the presence of 9 mM pyridine. Quenching is due to charge-transfer interactions O 0.015 no quencher 0.010 Lifetime of R*, no quencher 1 =k 1 τ1 with quencher 0.005 0.000 0 5 10 (nn.20) 15 20 25 Time, µs 30 35 40 Lifetime of R*, quencher present 1 = k + k [Q] = 1 + k [Q] 1 q q τ1 τ2 9.11 Stern-Volmer plot based on fluorescence 2.5 o Φ F/ Φ F 2 slope = k s τ 1.5 q F 1 0 0.05 0.1 0.15 0.2 0.25 0.3 [trans-1,2-dicyanoethylene], M Experimental plot of Φ0F/f ΦF vs. [trans-1,2-dicyanoethylene] for the cycloaddition of acetone singlets to the ethylene. The intercept is 1.0 and the slope is 7 M-1 = kqτ. CN O* + kq CN S 1 (n,π*) CN CN 1 9.12 Stern-Volmer plots in laser photolysis CH3 O* CH3 Butyrophenone triplet + 1-Methylnaphthalene ground state [ MeN *] ∞ = 1 ∆OD∞ CH3 Butyrophenone ground state k q [ MeN ] τ -1 + k [MeN ] º = a + CH3 O Energy transfer q + * 1-Methylnaphthalene triplet (~420 nm absorption) [ BTP * ] º a kq τ0 [ MeN ] 9.13 Detecting the excited quencher S1 91 T1 80 ener gy trans fer triplet state with a characteriatic signature T1 60 transient absorption 87 S1 solvent: methanol hν 320 S0 S0 360 400 440 Wavelength, nm 480 CH3 O H 3C R 9.14 Kinetics of Reactions Involving More than One Excited State O hν CH3 H3C S1 T1 CH3 COCH3 + CH2 =CHCH3 Relative efficiencies Relative rates S φq φqT = kSq τ S Norrish Type II products formed from both S 1 and T 1 kq τT T rate of S quenching 1 rate of T quenching 1 = kqS [diene] k qT [diene] = k Sq k qT 9.15 Possible forms of Stern-Volmer plots Plot type A) Linear Cause 1) Only one excited state reacts 2) Two rapidly equilibrating excited states with either or both quenched 3) Two non-interconverting states coincidentally quenched with the same value of kq Z 4) The first of two consecutively formed states is quenched, with either one reacting B) Downward curvature 1) Reaches a zero slope. Two reactive excited states, but only one quenched 2) Reaches a positive asymptotic slope. (a) Two non-interconverting excited states quenched with different kq values, or (b) Two interconverting excited states with the shorter lived or both reactin the quencher disrupts the interconversion equilibrium. Final slop corresponds to initial shorter state C) Upward curvature 1) Two consecutive excited states are quenched, but only the second one is reactive 2) Two non-interconverting excited states are both reactive and quenched with different kq2 values 3) Two interconverting excited states, with only the lower one reacti and with the quencher disrupting the interconversion equilibrium 4) Same as B2 5) A sensitized reaction with both sensitizers and substrate states quenched 6) Static or time dependent quenching. 9.16 Two different approaches to the study of short lived reaction intermediates • By making the technique sufficiently fast that it can access the time scale in which the intermediate naturally lives. • By isolating the intermediate under conditions where its lifetime is long enough for the technique of choice, such as in matrix isolation. 9.17 Sensitization: why and how The idea of triplet sensitization is to test if a given triplet reaction can be induced while completely bypassing the singlet manifold of that compound. Thus, for efficient and selective sensitization, the sensitizer should have the following characteristics: (1) Triplet energy higher than that of the molecule to be sensitized. (2) An absorption in a region where the molecule under study is transparent, so that selective excitation can be readily achieved. (3) Either a very short singlet lifetime, or a sufficiently large S-T energy gap to place the sensitizer S1 level significantly above the S1 level of the sensitized molecule. 9.18 The Norrish Type II reaction An example of intramolecular hydrogen abstraction Ph OH Me Ph O* • OH 2 ns H Me Me Ph Ph Me 100 ns OH • Me Me Ph n,π* triplet O H Me γMeVLP Me hν Acetophenone (ACP) 9.19 Experimental tests for the involvement of biradicals in the Norrish Type II reaction Hydrogen abstraction CH3 Ph + CH3 Ph RS–H OH OH CH3 CH3 H + RS • Electron transfer CH3 + H 3C N N CH3 Ph + CH3 MV+ • O MV2+ Initiation of polymerization H 3C + CO2CH 3 polymerization CH2 Addition to reactive double bonds + H 3C But C Se Ph But OH But Se C But CH3 9.20 The Norrish Type I reaction of cyclic ketones O Ph O* Ph Ph O Ph hν fast ↑ Ph ↑ – CO Ph Ph ↑ ↑ Ph Ph ↑ ↓ k > 10 8 s–1 Ph Ph + Ph Ph Ph The Norrish Type I reaction of 2,6-diphenylcyclohexane involves the fragmentation of a triplet biradical to another triplet biradical following rapid decarbonylation. 9.21 The paradigm for biradical reactions RULES Rule # 1 Rule # 2 Biradical paradigm concepts 1 3 BR will only yield singlet products BR will only yield triplet products. These reactions are uncommon, unless another biradical is produced (see the example of Scheme nn.11) Rule # 3 Biradicals undergo monoradical-like reactions with essentially the same rate constants a typical monoradicals. Whether or not these processes dominate will largely depend on the dynamics of biradical-specific reactions (see rule # 4) Rule # 4 3 BR will undergo biradical-specific reactions when they interact with paramagnetic species, such as oxygen, nitroxides and certain transition metal ions Rule # 5 3 1 Intersystem crossing ( BR ↔ BR) plays an important role in biradical reactions. Equilibration is rarely achieved. For triplet biradicals intersystem crossing is frequently irreversible and determines their lifetime Rule # 6 1 The decay of BR to molecular products can be fast enough to compete with bond rotations. As a result the partition anong different biradical products may depend on th conformation at which intersystem crossing occurs Rule # 7 3 The lifetime of BR will depend on the factors that control spin-spin interactions, such as distance, S-T energy gap, and spin-orbit coupleing interactions (these effects will be covered in detail in Chapter zz) Rule # 8 Structures in which odd electrons are of the same spin are more stable the fusther apart are the location of the unpaired electrons 9.22 Oxygen in Organic Photochemistry a common statement: The reaction is quenched by oxygen, thus, it must be a triplet reaction true or false? 10.1 Electronic structure The basic electronic structure of the oxygen molecule in the ground state can be written as: O2 (1σg)2 (1σµ)2 (2σg)2 (2σµ)2 (3σg)2 (1π µ)4 (1π g)2 or O2 (1σ)2 (1σ∗)2 (2σ)2 (2σ∗)2 (3σ)2 (1π) 4 (1π*) 2 10.2 Energy levels for molecular oxygen 1 1 ∆ g, ∆ g 44.8 kcal/mol, 15764 cm 1 Energy Σg 37.5 kcal/mol, 13121 cm 1∆ g -1 -1 22.4 kcal/mol, 7882 cm-1 3 Σg (ground state) Energy levels for molecular oxygen. Excited triplet states have not been included because they are much higher in energy. The 1∆g state is the one normally refereed to as singlet oxygen. 10.3 Dimol emissions Oxygen shows several "dimeric" emissions, the best known of which is the dimol emission at ~635 nm. No dimer is formed under normal laboratory conditions. Kasha has pointed out that the simultaneous transition for a pair of emitting species does not require an actual complex to exist, although the two molecules must be within contact distance; i.e. close enough for electron exchange to be possible. The process has also been described as energy pooling and is reminiscent of triplet-triplet annihilation processes ( ) ( ) ( ) ( ) O 2 1∆ g + O2 1∆ g → O 2 3 Σg + O2 3 Σg + hν 10.4 Jablonski diagram for the excited states of molecular oxygen 1Σ g; t = 130 ns (sum of knr from 1 Σg = 7.6 x 106 s-1) krad = 3.4 x 103 s-1 knr = Φ em = 4.5 x 10-4 krad = 0.40 s -1 knr = 1 ∆g ; t = 87 ms Φ em = 5.2 x 10-8 CCl4 krad = 1.1 s-1 knr = 10.4 s-1 Φ em = 0.087 3 Σg 10.5 Singlet oxygen spectroscopy Solvent Φ em τo∆ (µs) C6H6 4.7 x 10 -5 31 CH3CN 7.1 x 10 -5 75 CHCl3 3.6 x 10 -4 207 CS2 0.040 34000 CCl4 0.087 87000 Freon-113 0.15 99000 H 2O CH3OH Sources: (Schmidt, 1989), krad (s-1) ~5 10.4 Singlet oxygen (1∆g) lifetimes, emission quantum yields and radiative decay rate constants (krad) in various solvents at room temperature 10.6 Effect of deuteration on singlet oxygen lifetimes O2 (1∆g) lifetimes in Pr = CH3OH 10.4 µs CH3OD 37 µs CD3OD 227 µs k M[ M] −1 τ + kM [ M] Probability of reaction 10.7 Effect of deuterium on reaction yields Pr = k M[ M] −1 τ + kM [ M] Probability of reaction There are two extreme situations in which we would NOT expect an effect: (i) if the reaction does not involve singlet oxygen; and (ii) if the reaction with singlet oxygen is extremely fast 10.8 Simple rules let us anticipate changes in O2 ( 1∆ g) lifetime as a function of the solvent • The longest lifetimes are observed in perhalogenated solvents. • τo∆ decreases on increasing the number of H atoms in the solvent molecule. • The shortest τo∆ values are observed with solvents having O–H groups, notably water. • The presence of heavy atoms reduces τo∆ . • Solvent deuteration invariably increases τo∆ . 10.9 Bond dissociation energies for selected oxygen containing species Molecule O2 Bond type O=O BDE (kcal/mol) 119.0 H2O2 O–O 51 H–O H2O H–O H–O 102.2 119.3 H2O2 H–O 88.1 HO2 H–O ROH ROOH H–O H–O 105 R2O2 ketone O–O C=O 37 methanol H–O 104.4 10.10 Redox properties • Oxygen is a good electron acceptor, but a very poor donor. • Reduction of oxygen can lead to O2• , H O2• , H O2–, H 2O2 and HO• • It is usually the first electron transfer to O2 that is the rate limiting step. • The O2/ O2• couple has an immense importance in nature. •It has an E˚ of -0.15 V in water and -0.60 in dimethylformamide. • Under many conditions, O 2• is itself a good reductant. • Superoxide is a poor oxidant, since E˚ (O2• / O22–) < -1.7 V 10.11 Redox properties of singlet oxygen Singlet oxygen is a better oxidant than ground state oxygen. When the excitation energy of singlet oxygen is taken into consideration the values of E˚ (1O2/ O2• ) are 0.34 V in dimethylformamide and 0.79 V in water. Singlet oxygen oxidizes molecules such as N,N,N',N'-tetramethyl-pphenylenediamine to its radical cation. Me 2 N NMe 2 10.12 Energy transfer Singlet oxygen as a donor There are not many examples of energy transfer from oxygen, largely because few molecules have such a low excitation energy 1O 2 1∆ g + β-C 3O 2 3Σ g + 3β-C* CH3 CH3 CH3 CH3 CH3 CH3 H 3C CH3 CH3 CH3 β-carotene (β-C) 10.13 Singlet oxygen: chemical quenching H 3C H 3C CH3 H 3C 1O CH2 2 H 3C CH3 CH3 OOH (A) 1 O2 O (B) O CH3O CH3O 1O CH3O 2 CH3O O O (C) 10.14 The Schenck or ene reaction H3C H3C H3C CH3 1 O2 H3C CH3 (A) CH3 OOH H O H CH2 O O O 10.15 Singlet oxygen: reversible addition Addition to a conjugated system can be reversed thermally with regeneration of singlet oxygen O 1O 2 O ∆ + 1O2 endoperoxide Provides a chemical mechanism for ‘storing’ singlet oxygen 10.16 Dioxetane formation is not reversible H3C CH3 CH3 + H3C 1 O2 CH3 H3C O H3C O minor CH3 dioxetane CH3 H3C O H3C O CH3 H3C ∆ Ea ~ 27 kcal/mol H3C O* + H3C ΦS ~ 0.25 ΦT ~ 0.35 CH3 H3C H3C O O• H3C O• CH3 ? 10.17 Interaction of oxygen with excited singlet states S1 T2 T1 Possible but unlikely ( ) 1 * X + 3O 2 → 3X* + O 2 1∆ g requires large S-T gap So Common Assisted intersystem crossing 1 * X + 3O 2 → 3 X* + 3 O 2 10.18 Energy transfer processes Quenching by oxygen of excited triplet states 2 ]* X + 1O 2 singlet path [X····O2 ]* X + 3 triplet path 1 [X····O 1/9 3 X* + 3 1/3 O2 3 O2 5/9 5 [X····O 2 ]* quintet path Spin statistics plays a key role indetermining the probabilities of the vatious reaction paths between an excited triplet state and molecular oxygen 10.19 Detecting singlet oxygen The emission at 1270 nm provides a convenient tool to study the chemistry of singlet oxygen in the 1∆g state. The next higher electronic state is the 1Σg, 37.5 kcal/mol or 13121 cm -1 above the ground state. It emits weakly by decay to both the 1∆g state and the ground state. Its lifetime of 135 ns in carbon tetrachloride is surprisingly long for an upper electronic state (Kasha's Rule) laser 1Σ g; t = 130 ns (sum of knr from 1 Σg = 7.6 x 106 s-1) krad = 3.4 x 103 s-1 knr = Φ em = 4.5 x 10-4 krad = 0.40 s -1 1∆ g ; t = 87 ms sample power monitoring silicon filter germanium diode knr = -8 Φ em = 5.2 x 10 krad = 1.1 s-1 knr = 10.4 s-1 Φ em = 0.087 3Σ g signal monitoring 10.20 Paradigm for electronic energy transfer from a triplet sensitizer to molecular oxygen • It cannot occur if the sensitizer energy is significantly below 22 kcal/mol. • It can only populate the 1∆g level of molecular oxygen if the sensitizer energy is between 22 and 37 kcal/mol, since population of the 1Σg level would be energetically unfavorable. • If the sensitizer energy exceeds 38 kcal/mol, excitation of oxygen to either the 1∆g or 1Σg levels is possible. • If the energy of the sensitizer is between ~21 and ~25 kcal/mol, it is possible for the process to be reversible, with 1O2 (1∆g) also transferring energy back to repopulate the triplet state of the sensitizer. • If the energy of the sensitizer is in the neighborhood of 37-40 kcal/mol – i.e. matching reasonably well the energy of 1O2 (1Σg) – the process is not expected to show reversibility. This is due to the fact that the 1Σg state is too short-lived for reversible transfer to occur with any significant probability at the concentrations of organic solutes frequently used in photochemistry. The process is possible but not probable. 10.21 Reaction paths available in the interaction of sigma singlet oxygen with the substrate RX krxn 1O 1Σ 2 g RX kbypass Products 3O 3Σ 2 g (The upper state of singlet oxygen) kΣ∆ O2 1∆ g 1 10.22 Quenching by oxygen of excited triplet states. Chemical trapping In the majority of cases, interaction of oxygen with triplet states involves energy transfer. In a few examples, notably diketones, a chemical reaction occurs between the triplet state and O 2: addition to a carbonyl carbon (Schenck mechanism) and subsequent C–C bond cleavage to an acylperoxy and an acyl radical, which itself is scavenged by molecular oxygen to yield a second acylperoxy radical Schenck mechanism 3* O R 3 O O2 3 Σg R • O• R R O R O O O + R 3 O O• O• O2 3 Σg O 2 R O O• 10.23 Efficiency of singlet oxygen, O2(1∆ g), generation: selecting a good singlet oxygen sensitizer Φ∆ = ΦISC • S∆ • kq [O2] τ–1 + kq [O2] S∆ is a true indicator of the 'quality' of a triplet sensitizer in the generation of singlet oxygen 10.24 Values of S∆ for some singlet oxygen sensitizers • The π,π* triplet states of polynuclear aromatics are generally highly efficient, frequently with S∆ ≥ 0.8. Many other π,π* triplet states are also very efficient. • The n,π* triplet states of ketones have low values of S∆, for example for benzophenone in the 0.3-0.4 range. There is a modest increase in the value of S∆ with decreasing triplet energy. Sensitizer Solvent S∆ naphthalene cyclohexane 1.0 anthracene benzophenone benzene benzene 0.8 0.3 fluorenone benzene 0.8 tetraphenylporphyrin Ru(bipy) 3Cl2 benzene 0.58 methanol 0.92 α-Terthienyl benzene 0.8 phenazine acridine benzene acetonitrile 0.83 0.82 10.25 Parameters to take into consideration in selecting the singlet oxygen sensitizer and conditions • High value of S∆. • Long triplet lifetime in order to maximize triplet quenching. • High rate constant for triplet quenching by oxygen (true in almost all cases), and low rate constant for triplet quenching by substrate. • High sensitizer stability toward singlet oxygen. Some good (i.e. high S∆) sensitizers may also trap singlet oxygen efficiently, thus reducing their own usefulness. • Good spectral properties making possible the selective excitation of the sensitizer (as opposed to the substrate) with a readily available light source. • A sensitizer with efficient intersystem crossing under the experimental conditions (that could include oxygen-assisted intersystem crossing). • A solvent with good solubility for oxygen (e.g. halogenated) and where singlet oxygen has a long lifetime. • Easy sensitizer removal. For synthetic applications it may be desirable to eliminate the sensitizer at the end of the reaction. Some heterogeneous sensitizers have been developed (e.g. on polymer particles) that can be readily filtered at the end of the oxidation. 10.26 Reaction of oxygen with reaction intermediates: Mechanisms and kinetics Free radical scavenging by oxygen Carbon-centered free radicals frequently react with oxygen with rate constants exceeding 109 M -1 s-1 in fluid solution, to yield a peroxyl radical R• + O 2 → R –OO• ROO• + RH → R• + ROOH ANTIOXIDANTS CH 3 OH Bu t Bu t HO CH 3 H 3C CH 3 BHT O CH 3 CH 3 CH 3 CH 3 CH 3 Vitamin E 10.27 Biradical scavenging by oxygen A) Assisted intersystem crossing 3 1 3 O2 Products H H Ph Ph OH 3O 2 OH E.g. + + PhCOCH 3 B) Hydroperoxide formation 3 3 OO• O2 H OOH H C) Peroxide formation 3 3 O O2 O H H 10.28 Reactions of carbenes with oxygen [( C6 H5) 2 C:] 3 3 + O2 → ( C 6H5 ) 2COO Carbonyl oxide or Criegee intermediate Making carbenes: Diazo precursor of carbene with triplet ground state N N 1 3 hν fast –N2 Diazirine precursor of carbene with singlet ground state N N Cl 1 hν –N2 Cl 10.29