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manganese(III) to manganese(II) ions by lactic acid as a substrate of this kind is accompanied by chemiex citation of Mn2+ with the chemiexcitation yield of 0.1,.
ISSN 00181439, High Energy Chemistry, 2015, Vol. 49, No. 5, pp. 316–321. © Pleiades Publishing, Ltd., 2015. Original Russian Text © Yu.B. Tsaplev, R.F. Vasil’ev, A.V. Trofimov, 2015, published in Khimiya Vysokikh Energii, 2015, Vol. 49, No. 5, pp. 356–361.

PHOTOCHEMISTRY

Chemiluminescence in Reactions of Manganese(III) Reduction by Lactic Acid: TwoElectron Mechanism of Chemiexcitation Yu. B. Tsaplev, R. F. Vasil’ev, and A. V. Trofimov Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119334 Russia email: [email protected] Received February 11, 2015

Abstract—Chemiluminescence is generated during reduction of manganese(III) ions with lactic acid to have a quantum yield as high as 0.1, a recordbreaking chemiexcitation value for inorganic emitters. Kinetic features of the chemiluminescence and a nonradical mechanism of lactic acid oxidation lead to the conclusion that che miexcitation results from twoelectron reduction of manganese(IV) ions in the complex with lactic acid. DOI: 10.1134/S0018143915050173

Chemiluminescence (CL) during the reduction of manganese(III) ions was first investigated in the reac tions with malonic acid by Karavaev et al. [1], who proved that the emitter is the chemiexcited manga nese(II) ion. We have shown that the CL quantum yield in these reactions increases with an increase in Mn(II) concentration [2]. This is due to competitive Mn(III) consumption via the light and dark reaction pathways. The dark pathway wins the competition in the reaction with malonic acid and the chemiexcita tion yield does not exceed 0.0013. To investigate the Mn2+ chemiexcitation mecha nism and the light pathway reactions, it is advisable to find substrates that make the light pathway of the Mn(III) consumption the main route and bring the chemiexcitation yield close to unity. It is likely that α hydroxycarboxylic acids are exactly this type of com pounds because hydroxymalonic acid, for example, is the first intermediate in the light pathway of malonic acid conversion. We have found that the reduction of manganese(III) to manganese(II) ions by lactic acid as a substrate of this kind is accompanied by chemiex citation of Mn2+ with the chemiexcitation yield of 0.1, which is recordbreaking for inorganic emitters. This work is devoted to the study of CL in reactions of man ganese(III) specifically with lactic acid. EXPERIMENTAL The following chemicals were used: MnSO4 ⋅ 5H2O, FeSO4 ⋅ H2O of the analytical grade, concen trated sulfuric acid of the special purity grade, KMnO4, rivanol, αhydroxyacids (citric, tartaric, malic, and lactic), hydroquinone of the reagent grade, acetaldehyde of extra pure grade (+99.5%). All solutions were prepared with deionized water. Manganese(III) solutions were prepared via oxida tion of an excess amount of MnSO4 with potassium

permanganate in 3.1 M sulfuric acid solution. The Mn(III) concentration was determined by titration with a FeSO4 solution in the presence of rivanol as a fluorescent indicator [3]. The prepared Mn(III) solutions had a concentration of 4.8 mmol/L and contained H2SO4 and MnSO4 with concentration of 3.1 mol/L and 2.6 mmol/L, respectively. The Mn(III) solutions did not change properties (titer and absorption spectra) during storage in a fridge for 2 to 3 weeks. To dilute the Mn(III) solution, a solu tion containing H2SO4 and MnSO4 with the above mentioned concentration was used, which main tained stability of the diluted Mn(III) solutions. Chemiluminescence was recorded with a chemilu minometer assembled on the basis of an H73603 pho ton counting head (Hamamatsu, Japan), a CNT202 photoncounting unit (Spetspribor, Belarus), and a personal computer. When the count rates were more than 1 MHz, the luminous flux was reduced 45fold using a neutral light filter. Microtubes of 2 mL capacity were used as reaction vessels. Prior to the experiment, an aliquot of a sub strate solution (50–100 μL) was added to the tube. During the experiment the reaction was initiated by injection of a Mn(III) solution into a tube located in the cuvette holder in front of the photodetector using a lightproof dispenser. The total volume of the reaction mixture did not exceed 100 μL. The CL quantum yield (ΦCL) determination procedure is described in [4]. The chemiexcitation yield (Φ*) was calculated according to equation: Φ* = ΦCL/Φ, where the lumi nescence quantum yield of excited Mn2+ (Φ) is 0.00055 [5].

316

Experiments were conducted at 20–23°C.

CHEMILUMINESCENCE IN REACTIONS OF MANGANESE(III) REDUCTION

317

Chemiluminescence quantum yield and yield of chemiexcitation in reactions with αhydroxycarboxylic acids and malonic acid Reactant concentration, mol/L Substrate (acid)

ΦCL

Formula

Φ* MnIII

substrate

H2SO4

Malonic

CH2(COOH)2

7.0 × 10–7

0.0013

0.0013

0.014

1.8

Citric

(HO2CCH2)2C(OH)CO2H

1.8 × 10–6

0.0033

0.0007

0.057

1.3

Tartaric

HO2C(CHOH)2CO2H

5.1 × 10–6

0.0092

0.0016

0.067

1.0

Malic

HO2CCH2CH(OH)CO2H

5.5 × 10–6

0.010

0.0016

0.067

1.0

Lactic

CH3CH(OH)CO2H

5.2 × 10–5

0.095

0.0008

0.13

1.0

RESULTS AND DISCUSSION CL Yield in Reactions with αHydroxycarboxylic Acids

the Mn(III) reduction reaction with lactic acid (see [7]): 3+

The CL accompanying the reduction of Mn(II) in sulfuric acid solutions of αhydroxycarboxylic acids has many common features. The CL yield for these substrates under conditions when the substrate con centration is much higher than the initial Mn(III) concentration changes only slightly upon varying the initial Mn(III) concentration. The results of measure ments are presented in the table. Despite the general similarity, the recordhigh chemiexcitation yield for inorganic emitters, Φ* ≈ 0.10, was obtained only in the reaction with lactic acid (LA). This value equals the ratio of the number of chemiexcited Mn2+ ions to the number of Mn2+ ions formed during the Mn(III) reduction. Note that the number of Mn2+ ions formed during the reduction of Mn(III) under condition when [Mn(III)] Ⰶ [LA] equals to the number of Mn(III) ions entering the reaction. The quantity Φ*LA , defined as the ratio of the num ber of chemiexcited Mn2+ ions to the number of oxi dized LA molecules under conditions when [Mn(III)] Ⰷ [LA], is also of interest. It was found that Φ*LA Φ* = 2.1 ± 0.3; hence, Φ*LA ≈ 2Φ *Mn . It is known that lactic acid is oxidized first to acetalde hyde and further to acetic acid in the presence of excess of Mn(III) [6]. The measurements showed that the CL quantum yield in the reactions of Mn(III) with acetaldehyde (at [AcH] >> [Mn(III)] did not exceed 3.5 × 10–7, which is more than two orders of magnitude below that in the reactions with LA. This indicates that practically the entire light in the reactions with LA was emitted in the step of LA oxidation to acetaldehyde. Based on the equation of HIGH ENERGY CHEMISTRY

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LA + 2 ( Mn ) aq → CO 2 + CH 3 CHO +

(1)

2+

+ 2H + 2 ( Mn ) aq , we obtain for the chemiluminescent reaction: 3+

LA + 2 ( Mn ) aq → CO 2 + CH 3 CHO + 2H + 2 ( 1 – Φ* )Mn

2+

+

(2)

2+

+ 2Φ* ( Mn )*.

Testing for the formation of free radicals in the oxi dation reactions of an organic substrate with transition metals is very important in investigation of these reac tions [8, 9]. The test is based on the initiation of poly merization of olefin monomers by an oxygenfree reaction mixture. The positive result of the test indi cates the formation of free radicals of the oxidized sub strate in the reaction mixture. The formation of radi cals during oxidation of malonic [10, 11], citric [12], tartaric, and malic [13] acids with manganese(III) was confirmed using this test. The effective formation of radicals in the reactions with malonic and citric acids is used in polymer chemistry practice for polymeriza tion and copolymerization [14]. The radical formation test confirms the special position of lactic acid among the substrates listed in the table, since it shows the negative result [15]. Hence, the high chemiexcitation yield of manga nese(II) ions corresponds to a nonradical mechanism of the substrate oxidation. Next it was important to measure the light yield in the reactions of Mn(III) with the substrate oxidized exclusively via the radical mechanism. We have selected hydroquinone as such a substrate because (1) the radical mechanism of oxidation is proven for this substrate [16, 17] and (2) the nonradical oxidation

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TSAPLEV et al.

of hydroquinone is impossible as it suggests the forma tion of at least two coordination bonds between the metal ion and the oxidized substrate. Our measure ments showed that Mn(III) reduction with hydro quinone is not accompanied by chemiluminescence. Thus, there are two types of competition in the reactions of Mn(III) with substrates, namely, between the light and dark pathways of Mn(III) con sumption in its reduction reaction and between the radical and nonradical mechanisms of the substrate oxidation with manganese(III). They are related in such a manner that the CL yield is the highest if the oxidation follows the nonradical mechanism and CL is absent when the oxidation proceeds solely via the radical mechanism. The nonradical oxidation mechanism for reac tion (1) is identical to the twoelectron oxidation mechanism, which implies the preliminary forma tion of a complex of the substrate with tetravalent manganese [18]:

Mn

III

+ S ←⎯⎯ → [Mn S], K e1

III

K e2 Mn III + [Mn IIIS] ←⎯⎯ → [Mn IV S] + Mn II,

(3) (4)

[MnIVS] → MnII + products. (5) In any chemiluminescent reaction, there is one elementary step for which the sum of heat and activa tion energy is no less than the energy of electronically excited state (energy of the emitted CL quantum): (–ΔHr + Ea) ≥ hν, where ΔHr is the reaction enthalpy, Ea is the activa tion energy, and ν is the wave number of emitted light. Approximately half of the quanta emitted in chemiluminescent reaction (2) has an energy of 41– 48 kcal/mol (spectral range 600–700 nm). The enthalpy of reaction (1) can be calculated from known standard enthalpies of the reactants and prod ucts using the Hess law. These values for reaction (1) are known [19–21], and their values are presented below in the sequence determined by the equation of reaction (1): (–164.0) + 2 × (–23.8) → (–94.1) + + (–39.6) + 2 × 0 + 2 × (–52.4). Hence, it follows that Δ H r0 = –26.9 kcal/mol, a value that is significantly lower than the energy of emitted photons. Thus, the reaction scheme should include endothermic reactions that store energy in the prod ucts, whose further transformation will be sufficiently exothermic in the chemiexcitation step. From this point of view, the only step in the scheme (3)–(5) that can be involved in chemiexcitation in the case of S = LA is the following reaction: k6 [MnIV LA] ⎯⎯⎯ → (Mn2+)* + CO2 + CH3CHO. (6)

The CL kinetics in the reaction of Mn(III) with LA was investigated with the goal to verify the scheme

including steps 3,4, 6, and the step of light emission from the chemiexcited particle: k7 (Mn2+)* ⎯⎯ ⎯ → Mn2+ + hν,

(7)

Chemiluminescence Kinetics in Reactions with Lactic Acid The CL intensity is at maximum at the time of mix ing the reactants with the initial Mn(III) concentra tion of 8 × 10–5 to 1.6 × 10–3 mol/L and LA concentra tion of 1 × 10–6 to 0.1 mol/L under all the investigated conditions. The CL kinetics was investigated in detail under conditions when the Mn(III) concentration was much higher than the LA concentration. In this case, it is easy to derive the time dependence of the CL intensity using the quasi steadystate approximation for [MnIVLA] and (Mn2+)*: iCL ~ k7 [(Mn2+)*] = k6[MnIVLA] = Lαexp(–αt), (I) where L is the total concentration of LA at the begin ning of the reaction (L = [LA] + [MnIIILA] + [MnIVLA] at t = 0) and k6 K e1K e2[Mn III ]2 . [Mn II ] + K e1 ⋅ [Mn II ] ⋅ [Mn III ] + K e1K e2[Mn III ]2 In the experiment, the change in CL intensity with time obeys the firstorder law, and the semilogarithmic transform of the rate curve I(t) is close to the straight line (Fig. 1). This allows describing the CL kinetics (I(t)) with two parameters, the initial intensity I0 and the effective constant characterizing the intensity decay rate, keff: α=

I(t) = I0exp(–kefft). The dependence of the I0 and keff values on the initial LA concentration are presented in Fig. 2. The I0 value is directly proportional to [LA]0, and keff remains almost unchanged, in agreement with theoretical equation (I). The dependences of I0 and keff on the MnIII con centration are plotted in Fig. 3 in the 1/I0–1/[MnIII], and 1/ keff–1/[MnIII] coordinates, respectively. It can be seen that the 1/I0 vs 1/[MnIII] curve is very close to the 1/keff vs 1/[MnIII] curve if the ordinate axes are appropriately scaled. These two curves can be approx imated (with R 2 > 0.998) by the sum of two terms, lin ear and quadratic for 1/[MnIII]. The ratio of coeffi cients at the linear and quadratic terms is 150 L mol–1 in one case and 210 L mol–1 in the other; according to Eq. (I), this ratio is Ke1. The dependences of 1/I0 and 1/keff on the MnII concentration are presented in Fig. 4. Both depen dences are linear. When an appropriate scale is selected for the ordinate axis, both straight lines repre senting these dependences merge into one, which is in agreement with the theoretical dependence of α on the MnII concentration in (I): 1/α ~ с0 + с1 ⋅[MnII]. The HIGH ENERGY CHEMISTRY

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1000000

3

2 100000 I(t), count/s

1

10000

Instrument background

1000 0

200

600

400

800

1000

1200

t, s Fig. 1. Chemiluminescence rate curves in semilogarithmic coordinates at a Mn(III) concentration 1.6 mmol/L and LA concen trations of (1) 0.017, (2) 0.033, and (3) 0.067 mmol/L. The reactants are mixed at a time t = 10 s. The concentrations of H2SO4 and MnSO4 are 1.0 mol/L and 0.87 mmol/L, respectively.

Thus, the dependences of I0 and keff on LA, Mn , and MnII concentrations support the applicability of the scheme including steps 3, 4, 6, and 7 to description of the CL kinetics. The twoelectron reduction mechanism is com monly considered as a sequential transfer of two elec trons. In the case when the energy released after the transfer of the first electron has a chance to dissipate before the second electron transfer, there is no advan tage of the twoelectron mechanism for the chemiex citation of the product. The advantage appears only if the pair of electrons is transferred. In this connection it is important to understand the structure of the [MnIV LA] species. It can be presented by three forms (A, B, C) that differ in the character of bonds between the central atom and the ligand. HIGH ENERGY CHEMISTRY

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0.040 0.035 0.030

100000

0.025 0.020 0.015

10000

0.010 0.005 0

1000 0.000001 0.0001 0.0000001 0.00001

0.001

[LA], mol/L Fig. 2. Dependence of I0 (䉫, solid line) and keff (䉭, dashed line) on lactic acid concentration. For conditions, see Fig. 1.

keff, s–1

III

1000000

I0, count/s

III 2 ratio of coefficients с0/с = K e1K e2[Mn III] , and from 1 + K e1[Mn ] the data (Fig. 4) the value of this parameter is approx imately 0.0004 mol/L, which leads to the Ke2 ≈ 1 at Ke1 ≈ 200 L/mol, [MnIII] = 1.6 × 10–3 mol/L.

0.00006

3000

0.00005

2500

0.00004

2000

0.00003

1500

0.00002

1000

0.00001

500

0 0

500

1000

1500

2000

2500

1/keff, s

TSAPLEV et al.

1/I0, s/count

320

0 3000

1/[Mn(III)], L/mol Fig. 3. Dependence of I0 (䉫, solid line) and keff (䉭, dashed line) on manganese(III) concentration. The concentrations of H2SO4 and MnSO4 are 1.0 mol/L and 0.87 mmol/L, respectively, and the initial LA concentration is 33 µmol/L.

0.000025 1000 0.000022 800

0.000015 600 0.000010

y = 0.0013x + 4 × 10–7 R2 = 0.996

1/keff, s

1/I0, s/count

y = 58700x + 24 R2 = 0.996

400

0.000005

0

200

0 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 [MnSO4], L/mol

Fig. 4. Dependence of I0 (䉫) and keff (䉭) on manganese(II) concentration. The concentrations of H2SO4 and MnSO4 are 1.0 mol/L and 1.6 mmol/L, respectively, and the initial LA concentration is 67 µmol/L. HIGH ENERGY CHEMISTRY

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CHEMILUMINESCENCE IN REACTIONS OF MANGANESE(III) REDUCTION

H3C

H

q

O O

H

H3C

O

H

Mn

H A

H3C

H O

Mn

H O

O Mn C

(8)

H (MnII)* + CO2 + CH3CHO.

MnIV

Form “A” is an entity in which the central atom is linked to the ligand by bonds formed using lone elec trons of the oxygen atom. The shift of electron density to manganese facilitates deprotonation and transfor mation to form ‘B’. Form ‘C’ of [MnIVLA] can result from the reaction between the hydroxyl group of LA and the hydroxyl of the cation hydration shell:

The bonds linking the manganese cation with the oxygen atoms in form “C” are the strongest. The elec tron density in the fivemembered ring formed by two carbon atoms, two oxygen atoms, and manganese atom is displaced toward manganese, thereby destabi lizing the C–C bond. Its homolytic cleavage inevitably results in decomposition of the ligand into CO2 and acetaldehyde and simultaneous twoelectron reduc tion of the central ion (8). These changes are accom panied by the release of a significant amount of energy that is required for chemiexcitation.

9. Littler, J.S. and Waters, W.A., J. Chem. Soc., 1959, p. 1299. 10. Kemp, T.J. and Waters, W.A., J. Chem. Soc., 1964, p. 1489.

12. Nayak, P.L., Samal, R.K., and Nayak, M.C., Eur. Polym. J., 1978, vol. 14, no. 4, p. 287. 13. Levesley, P. and Waters, W.A., J. Chem. Soc., 1955, p. 217. 14. Yagci, Y., Reetz, I., and Mishra, M.K., Handbook of Vinyl Polymers: Radical Polymerization, Process, and Technology, Mishra, M.K. and Yagci, Y., Eds., New York: CRC Press Taylor & Francis Group, 2009, ch. 5. 15. Poloju, S., Cholkar, K., Kouassi, G.K., Van Horn, J.D., Rangappa, K.S., and Gowda, N.M.M., Am. J. Org. Chem., 2012, vol. 2, no. (3), p. 58. 16. Davies, G. and Kustin, K., Trans. Faraday Soc., 1969, vol. 65, p. 1630.

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8. Drummond, A.Y. and Waters, W.A., J. Chem. Soc., 1953, p. 3119.

11. Singh, H., Thampy, R.T., and Chipalkatti, V.B., J. Polym. Sci., 1965, vol. 3, no. 12, p. 4289.

[Mn(OH)(H2O)4CH3CHOHCO2]2+ → [Mn(H2O)4CH3CHOCO2]2+ + H2O.

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H

O

O O

O

H3C

O

B

H

q–2

q–1

O

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17. Wells, C.F. and Kuritsyn, L.V., J. Chem. Soc. A, 1970, p. 676. 18. Kemp, T.J. and Waters, W.A., J. Chem. Soc., 1964, p. 1192. 19. Saville, G. and Gundry, H.A., Trans. Faraday Soc., 1959, vol. 55, p. 2036. 20. Vatani, A., Mehrpooya, M., and Gharagheizi, F., Int. J. Mol. Sci., 2007, vol. 8, no. 5, p. 407. 21. Lidin, R.A., Andreeva, L.L., and Molochko, V.A., Konstanty neorganicheskikh veshchestv (Constants of Inorganic Substances), Moscow: Drofa, 2008.

Translated by L. Brovko