High Energy Chemistry, Vol. 38, No. 3, 2004, pp. 174–179. Translated from Khimiya Vysokikh Energii, Vol. 38, No. 3, 2004, pp. 205–210. Original Russian Text Copyright © 2004 by Tsaplev.
PHOTOCHEMISTRY
Alkaline Hydrolysis of Nitroglycerin and Activation of Luminol Chemiluminescence Yu. B. Tsaplev1 Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia Received June 4, 2003
Abstract—Alkaline hydrolysis of nitroglycerin (G) was studied using the chemiluminescent reaction of the hydrolysis products with 4-dimethylaminophthalhydrazide (L). The chemiluminescence kinetics follows the pseudo-unimolecular law, with the rate constant k1 proportional to [OH–]. The apparent bimolecular constant k2 = k1/[OH–] is equal to 0.021 l mol–1 s–1. The chemiluminescence quantum yield per one nitroglycerin molecule ηG = (1.3 ± 0.3) × 10–3 photons per one molecule ([G0] [L]), and the reactant chemiluminescence quantum yield upon excitation by species X formed from G in the course of hydrolysis is (2.6 × 0.5) × 10–2 photons per molecule ([G] [L0]]). Hence, it follows that the hydrolysis of 100 nitroglycerin molecules results in about five X molecules exciting chemiluminescence. The effects of temperature, ionic strength, and composition of the solution on k2 were studied. Quantum-chemical calculation on the interaction of éç– ion with G molecule shows a possibility for peroxynitrite formation upon approach of the ion to the nitro group within the éNO2 plane. If the approach occurs not in the éNO2 plane, the conventional hydrolysis mechanism with substitution at nitrogen is realized. 1
The use of luminol and its analogs as chemiluminescent reagents for detection of various compounds has long been known. Interaction of luminol with nitrogen dioxide under heterophase conditions is one of unusual reactions among those described in the literature [1]. Chemiluminescence was found to appear upon blowing – – off air NO2 (but not NO 2 , NO 3 , or NO) on an alkaline luminol solution containing [1, 2]. The reaction was used for analytical purposes [1–5]; however, the chemiluminescence activation mechanism remained unclear. Peroxynitrite (ONOO–) was later shown to be an efficient oxidant inducing luminol chemiluminescence [6–9]. This finding was used for detection of NO in the luminol–ç2é2 system, in which peroxynitrite was formed as the result of NO interaction with ç2é2 [6–8]. The half-lifetime of ONOO– in solution is ~1 s [9]. We observed that nitroglycerin also activated luminol chemiluminescence in the course of alkaline hydrolysis. This reaction is of interest for both pharmaceutical and forensic chemistry. The present work is devoted to investigation of this reaction. EXPERIMENTAL Chemiluminescence was measured with a photodetector based on a photomultiplier operating in the single-photon counting mode. The kinetics of the emission yield (the total light) was recorded in ASCII codes with a computer, which permitted reading and processing the kinetic data with the Mathcad program (MathSoft). 1 E-mail:
[email protected]
Quantum-metric calibration was performed using the known procedure [10]. Polypropylene test microtubes (1.5 ml) were used as reactors for the reaction, in which reactants were mixed. The configuration of the setup was described earlier [11]. Temperature of solutions was measured with the temperature gage of a Hanna Instruments (model 8314) digital pH-meter. Trinitroglycerol (G) solutions were prepared from the pharmaceutical (1% G solution in oil) by dissolving it in acetone and subsequent dilution to necessary concentration. 4-Dimethylaminophthalhydrazide (ArmIREA) was used as a chemiluminescent reagent (L), which is an analog of luminol with the protected amino group and retains a high chemiluminescence quantum yield in a highly alkaline medium [12]. The program Statistica (StatSoft) was used for statistical data processing and regression analysis, and the program MOPAC 6.0 was employed for quantumchemical calculations of ionization potentials and formation heats [13]. RESULTS AND DISCUSSION Chemiluminescence Kinetics In Fig. 1, curve a shows the experimental dependence of the light emission yield (Σ(t)) on the time elapsed after mixing a highly alkaline L solution with a G solution, and curve b represents its transformation in Σ∞ the ln ---------------------versus time coordinates. The transΣ∞ – Σ ( t ) form of the kinetic curve is a straight line, which sug-
0018-1439/04/3803-0174 © 2004 MAIK “Nauka /Interperiodica”
gests the pseudo-unimolecular kinetics of the process. The analytical expression of the kinetic curve is Σ‡(t) = Σ∞(1 – e
–k 1 t
). The mean relative error of the analytical
description given by
2 [ Σi – Σa ] 999 -------------------------i=0 1000Σ i
∑
(where Σi is the
experimental value at the time ti) does not exceed 0.3%. The time dependence of the chemiluminescence intensity (I(t)) was obtained from the emission yield kinetics I(ti) = (Σi + 1 – Σi)/(ti + 1 – ti). The chemiluminescence intensity reaches its maximum 1–2 s after mixing the reactants; then it decreases exponentially. The steepness of the front profile does not depend on luminol concentration in the range 10–100 µmol l–1 ([L] [G0]). Since the chemiluminescence rise time is much shorter than its decay time, the maximum intensity can be considered in the kinetic analysis as the intensity at the time of mixing the reactants (I0), and, therefore, chemiluminescence is measured under quasi-steadystate conditions. The emission yield in the reaction and I0 vary proportionally to the initial G concentration in the reactor (the squared correlation coefficient r2 is 0.99 and 0.985, respectively, at 1 nmol [G]0 [L] ~ 0.1 mmol l–1). The pseudo-unimolecular rate constant k1 depends on [OH–]. Since the reaction conditions require a high alkali concentration and the effect of variation in the ionic strength of the medium is noticeable, solutions with a constant ionic strength were taken to obtain the dependence of k1 on [OH–], using a KCl solution as a supporting electrolyte. At an ionic strength of 0.75 mol l–1, the linear regression equation is k1 = 0.021[OH–] (r2 = 0.999). The apparent bimolecular constant k2 = k1/[OH–] at the constant ionic strength does not depend on [OH–] and is equal to 0.021 l mol–1 s–1. Dependence of Emission Yield on [L] The emission yield increases with an increase in the luminol concentration, thus indicating the participation of an unstable transient (denoted as X) in chemiluminescence excitation: X
at a rate constant kc (light pathway). The rate constant ratio kt/kc can be obtained from the dependence of the emission yield on luminol concenVol. 38
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8 6
b 4 2 0
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200
300 Time, s
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500
No. 3
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Fig. 1. (a) The kinetics of the emission yield and (b) its linΣ∞ - versus time. ear transform in the coordinates ln ---------------------Σ∞ – Σ ( t ) The conditions were as follows: at a time t = 0, aqueous nitroglycerin solution (50 µl) was added to the reactor containing 450 µl of aqueous solution of L, KOH, and disodium ethylenediaminetetraacetate (Trilon B) (used to suppress the background reagent luminescence). The concentrations of L, KOH, Trilon B, and G were 86.6 µmol l–1, 0.9 mol l–1, 1.2 mmol l–1, and 0.37 µmol l–1, respectively. The temperature was 20.5°C.
Σ0/Σ 4 3 2 1
0
2
4
6 [L0]/[L]
8
10
12
Fig. 2. Plot of Σ0/Σ versus [L0]/[L] at [L0] = 86.6 µmol l–1, [KOH] = 0.8 mol l–1, and [G] = 0.22 (filled circles) and 2.2 (open circles) µmol l–1. The reactor volume was 0.5 ml. The linear transform equation is Σ0/Σ = 0.256[L0]/[L] + 0.744 (r2 = 0.991).
(1)
Activation of L by X
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tration (provided that [L] [X] and variation in [L] is negligible). For scheme (1),
Deactivation of X at a rate constant kt, X+L
Σ(t) × 10–10, photons; ln[Σ/(Σ–Σ(t))]
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k [ L 0 ] ----c [ L 0 ] kt Σ0 1 - , - + ------------------------- ------------- = -----------------------k c [ L0 ] [ L ] k c [ L0 ] Σ 1 + --------------- 1 + --------------kt kt
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3.34 –3.1
3.36
1/T × 103, deg–1 3.38 3.40 3.42 3.44
3.46
3.48
–3.3 –3.5 log k2
–3.7 –3.9 –4.1 –4.3 –4.5 –4.7 –4.9 Fig. 3. Temperature dependence of the bimolecular rate constant k2 (l mol–1 s–1) at an ionic strength of 0.9 mol l–1, [L] = 86.6 µmol l–1, and [G]0 = 0.37 µmol l–1.
0
Ionic strength, mol l–1 0.5 1.0
–1.6
1.5 a
log k2
b –1.8
–2.0
–2.2 Fig. 4. Dependence of k2 on the ionic strength of the solution. The conditions were as follows: the reactor volume was 600 µl; the concentrations of L and Trilon B were 72.5 µmol l–1 and 0.9 mmol l–1, respectively; [G]0 = 0.3 µmol l–1; the electrolyte was KOH–KCl; the temperature was 21°C. Curves a and b show the approximating functions log ( k 2 ) = –2.243 + 1.012i0.5 – 0.405i – 0.018i1.5 and log ( k 2 ) = –2.037 + 1.051i – 0.656i1.5, respectively.
where Σ0 and Σ are the emission yields at the reagent concentrations [L0] and [L], respectively. Figure 2 presents the data on competition according to scheme (1) (plots in the coordinates Σ0/Σ versus [L0]/[L]). A statistical analysis shows that the data obtained at [G] = 0.22 and 2.2 µmol l–1 belong to the same set and Σ0/Σ = 0.256[L0]/[L] + 0.744 with high confidence, which confirms the validity of scheme (1). The estimation of the ratio kt/kÒ gives 29(±9%) µmol l–1. Hence, the correction coefficient for calculating the emission yield
at [L] ∞ is Σ([L] → ∞) = Σ[L]([L] + 29 µM/[L]). Taking into account the correction coefficient, the limiting chemiluminescence quantum yield per nitroglycerin molecule (ηG) was determined as ηG = (1.3 ± 0.3) × 10−3 photon/molecule. The value of ηG is equal to the product ηïηL, where ηL is the quantum yield of L chemiluminescence by excitation with the species X and ηï = m/n is the ratio of the stoichiometric coefficients m and n equal to the numbers of X molecules produced from one nitroglycerin molecule and reacting with one L molecule, respectively. The ηL value was measured at [L] [X], when L was completely consumed, which was checked by addition of control portions of nitroglycerin to the reactor. It was found that ηL = (2.6 ± 0.5) × 10–2 photon/molecule, and, hence, ηï ~ 0.05. If the species X were formed in the hydrolysis of each ester bond in the nitroglycerin molecule and each X excited L chemiluminescence, ηï would be equal to 3. In actuality, ηï 1; therefore, m 1 since it is unlikely that n exceeds 2. Consequently, the formation of the X species is a side process that insignificantly contributes to the main pathway of alkaline hydrolysis of G. In this case, the kinetics of the emission yield of L chemiluminescence reflects the consumption of G in this main (dark under the given experimental conditions) pathway. Alkaline Hydrolysis of Nitroglycerin It is reasonable to relate the apparent bimolecular products. rate constant k2 to the reaction G + éç– To confirm this assumption, we studied the effects of temperature, ionic strength, and composition of the solution on the value of k2. Figure 3 shows the temperature dependence of k2 in the Arrhenius coordinates k2 = Aexp(–E‡/RT) at an ionic strength of 0.9 mol l–1, where A = 2 × 1017 l mol–1 s–1 and E = 106.9 kJ mol–1 (the correlation coefficient is 0.99). At [OH–] ~ 1 mol l–1, on the average two to three hydroxide ions occur in the immediate vicinity of G; therefore, it does not make sense to interpret the quantity A in terms of the collision theory, this value only suggests a high activation entropy. The value of k2 increases with the ionic strength (Fig. 4). Therefore, in the reaction sequence characterized by k2, there is a limiting step of interaction between either like ions or an ion and a polar molecule. The effect of the ionic strength of a solution on the activity coefficients of ions (γi) and polar molecules (γm) in a 1 : 1 electrolyte is known to be described with a high accuracy by the semiempirical equations 2 0.5
log ( γ i ) = – ωz i i
1.5
+ αi i + βi i ,
log ( γ m ) = χ m i, HIGH ENERGY CHEMISTRY
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k 0.5 1.5 log ----ii- = 2ωz 1 z 2 i + αi + βi , k °ii k im 1.5 - = αi + βi . log ----- k °im
(2)
In Fig. 4, curves a and b map approximating functions defined by Eq. (2). The correlation coefficients for a and b (0.99 and 0.98, respectively) are virtually the same, and it does not seem possible to choose between the two variants of the mechanism. The composition of the solution has a strong effect on the value of k2. Figure 5 shows the dependence of the ratio k2/k20 (where k20 is the value of k2 in H2O) on the mole fraction of dimethylsulfoxide (DMSO) in the solution. At a DMSO mole fraction of 0.2, the rate constant increases 75 times! Such amounts of DMSO have no effect on luminol chemiluminescence [17]. It is known that DMSO solvates cations but is extremely ineffective in the solvation of anions [15, 16]. Therefore, it can be concluded from the data on the DMSO effect on k2 that the reaction sequence characterized by k2 has a limiting step involving anions. The alkali hydrolysis of organic nitrates was intensively studied in the 1950–1960s [18]. In summary, the following reactions can occur simultaneously or separately depending on the nitrate structure and conditions: substitution at the carbon atom according to the SN1 or SN2 mechanisms R 1 R 2 CH 2 –O–NO 2 + OH
–
(a)
–
R 1 R 2 CH 2 –OH + NO 3 , substitution at the nitrogen atom R 1 R 2 CH 2 –O–NO 2 + OH
–
(b)
–
R 1 R 2 CH 2 O + HO–NO 2 , hydrogen abstraction from the β-carbon atom RCH 2 CH 2 –O–NO 2 + OH
–
(c)
–
RCH=CH 2 + NO 3 + H 2 O, HIGH ENERGY CHEMISTRY
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k2/k20
where i is the ionic strength, ω is the limiting coefficient determined by the Debye–Hückel theory (ω = 0.506 at 20°ë); zi is the ion charge; and αi, βi, and χm are empirical coefficients [14–16]. Hence, the dependence of the rate constant for the reaction between two ions (kii) and between an ion and a molecule (kim) on the ionic strength of a solution, according to the Brönsted equation, can be expressed as follows:
177
10
1 0
0.05
0.10 0.15 DMSO mole fraction
0.20
0.25
Fig. 5. The dependence of k2 on the mole fraction of DMSO in the solution. k20 is the rate constant of the reaction in water.
and hydrogen abstraction from the α-carbon atom according to the concerted E2 mechanism RCH 2 CH 2 –O–NO 2 + OH –
–
(d)
RCH 2 CH=O + NO 2 + H 2 O. Reaction (d) is not observed under the solvolytic conditions, and its carbanionic mechanism was rejected as inconsistent with experimental data [19]. None of these reactions suggests the formation of species capable of activating L chemiluminescence. This means that the scheme of formation of stable final products, being correct in general, overlooks important intermediate steps. A specific feature of molecules, such as G, having many electrophilic centers is their ability to delocalize an accepted electron. Electron transfer from éç– to G is energetically favored, as can be seen from comparison of the ionization potentials of these molecules (14.6 and 8.3 eV for G and éç–, respectively) and the energies of molecular orbitals (the nitroglycerin LUMO G is higher than the HOMO of éç– by 5.6 eV). Electron loss by the hydroxyl ion converts the ion into the chemically reactive radical that occurs together with G in the same solution cage, thus predetermining the attachment of the radical to the nitroglycerin molecule. Figure 6 presents the results of PM3 quantumchemical calculations on energy changes (heats of for0 mation ∆ H f ) for the approaching of éç– to a nitroglycerin nitro group. The energy decreases upon the approach to a distance of 0.17–0.18 nm in the direction perpendicular to the nitro group plane; the closer approach leads to the formation of an N–OH bond and the simultaneous weakening of the CO–N bond; i.e., reaction (b) is actually realized. If the approach occurs in the nitro group plane, the dependence of the potential energy on the distance has two minimums separated by
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∆Hf0, kJ mol–1
1.0 –400
1.5
x, A 2.5
2.0
3.0
3.5
4.0
–500
–600
–700 Fig. 6. Change in the heat of formation ∆Hf upon OH– approach to a nitroglycerin nitro group (circles) in the nitro group plane (where x is the distance between the oxygen atoms of the approaching groups) and (squares) perpendicular to the nitro group plane (where x is the N···OH– distance).
sidered above. Figure 7 presents the configurations of the molecules at the energy minimum along the reaction coordinate. It may be assumed that the low yield of species X compared to other products is due to the steric factor. In accordance with the concept developed earlier [20], the activation energy of the reaction between the hydroxyl and a polar molecule in solution is determined by energy consumption for reorganization of solvent molecules surrounding the reactants. As the result, reactions that could occur with zero activation energy in the gas phase have an activation energy of 20–30 kcal mol–1 in a liquid (for example, reactions of alkyl halides with the hydroxyl ion). On the basis of the information available, the following concise final kinetic scheme describing the processes of nitroglycerin hydrolysis and chemiluminescence activation can be proposed: k2
G + OH–
kx
(GOH)– (GOH)– (‡) H
H
H
O O
N O
O–
H
X+L
O
H O O N O
N O
O H
X
H
O
O O N
O–
(3)
activation of L,
deactivation of X,
kc[L ] – – I ( t ) = η G -----------------------k 2 [ OH ] [ G ] 0 exp ( – k 2 [ OH ] t ). kt + kc[L ]
O
H H O
P,
is the product of hydroxyl addition to G where with an electron transferred to G, X = NO(OOH), and P are hydrolysis products other than X. Since ηï 1, kx/(kx + ky) ≈ kx/ky 1. In quasi-steady-state approximation providing that (kt + kc[L]) kx and ky k2[OH–], the emitted light intensity is expressed as follows:
H
H
ky
X,
(Géç)–
(b) H
kÚ
kc
(GOH)–,
N
O O
O N O Fig. 7. Configurations of molecules at the energy minimum along the reaction coordinate upon hydroxyl ion approach to the nitro group in the direction (a) perpendicular to the ONO2 plane and (b) in the nitro group plane.
In conclusion, there are some remarks on the possibility of identification of X. Peroxynitrite has an absorption maximum at 302 nm with an absorption coefficient of 1670 l mol–1 cm–1 [6]; however, taking into account the low solubility of G in water (about 1 mmol l–1), the nitroglycerin own absorption with ε ~ 10 l mol–1 cm–1 [18], kt ~ 1 s–1 [9] and the fact that the maximum concentration of X is equal to –
a potential barrier of 57 kJ mol–1. The principal and the second minimums are at distances of ~0.19 and ~0.15 nm, respectively, between oxygen atoms of the hydroxyl and the nitro group; they correspond to the formation of the peroxy derivative [RCH2–O– NO(OOH)]–. This derivative has the O–NO(OOH) bond order decreased to 0.37 and probably decomposes into the RCH2O– ion and the NO(OOH) molecule. The latter molecule plays the role of the intermediate X con-
k k 2 [ OH ] 1 0.02 [ X ] max ≤ ----x -------------------[ G 0 ] ∼ ------ ---------- [ G ] 0 50 1 ky kt –4
= 4 × 10 [ G ] 0 , according to scheme (3), it is unlikely that X can be detected spectrophotometrically. An attempt was made to find a reagent that could be specific to X. It is easy to check the specificity of the reaction of this reagent with X by the presence or absence of competitive quenching of chemiluminesHIGH ENERGY CHEMISTRY
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cence appeared as the result of X interaction with L. The attempt failed. Phenols (thymol, hydroquinone) quench chemiluminescence in the system; however, this quenching is due to their interaction with reactive species produced from L after activation, not with X. ACKNOWLEDGMENTS The author is grateful to R.F. Vasil’ev and A.F. Vanin for the helpful remarks and recommendations. REFERENCES 1. Maeda, Y., Aoki, K., and Munemori, M., Anal. Chem., 1980, vol. 52, no. 2, p. 307. 2. Wendel, G.J., Stedman, D.H., Cantrell, C.A., and Damrauer, L., Anal. Chem., 1983, vol. 55, no. 6, p. 937. 3. Zhang, D., Maeda, Y., and Munemori, M., Anal. Chem., 1985, vol. 57, no. 13, p. 2552. 4. Mikuska, P. and Vecera, Z., Anal. Chem., 1992, vol. 64, no. 18, p. 2187. 5. Mikuska, P. and Vecera, Z., Anal. Chim. Acta, 2000, vol. 410, no. 1/2, p. 159. 6. Kikuchi, K., Nagano, T., Hayakawa, H., Hirata, Y., and Hirobe, M., Anal. Chem., 1993, vol. 65, no. 13, p. 1794. 7. Kikuchi, K., Nagano, T., Hayakawa, H., Hirata, Y., and Hirobe, M., J. Biol. Chem., 1993, vol. 268, no. 31, p. 23106. 8. Evmiridis, N. and Yau, D., Anal. Chim. Acta, 2000, vol. 410, no. 1/2, p. 167.
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9. Dyke, K. van, McConnell, P., and Marquardt, L., Luminescence, 2000, vol. 15, no. 1, p. 37. 10. Vladimirov, Yu.V., Sherstnev, M.P., Piryazev, A.P., and Belyakov, V.A., Zh. Prikl. Spektrosk., 1989, vol. 50, no. 2, p. 341. 11. Tsaplev, Yu.B., Zh. Fiz. Khim., 1992, vol. 66, no. 12, p. 3348. 12. Lind, J., Merenyi, G., and Eriksen, T.E., J. Am. Chem. Soc., 1983, vol. 105, no. 26, p. 7655. 13. Stewart, J.J.P. MOPAC Manual, Colorado Springs: US Air Force Academy, 1990. 14. Hammet, L., Physical Organic Chemistry: Reaction Rates, Equilibria and Mechanisms, New York: McGrawHill, 1970. 15. Emanuel, N.M. and Knorre, D.G., Kurs khimicheskoi kinetiki (Course of Chemical Kinetics), Moscow: Vysshaya Shkola, 1984. 16. Denisov, E.T., Sarkisov, O.M., and Likhtenshtein, G.I., Khimicheskaya kinetika (Chemical Kinetics), Moscow: Khimiya, 2000. 17. Lee, J. and Seliger, H.H., Photochem. Photobiol., 1972, vol. 15, no. 2, p. 227. 18. Comprehensive Organic Chemistry: The Synthesis and Reactions of Organic Compounds, vol. 2: Nitrogen Compounds, Sutherland, I.O., Ed., Oxford: Pergamon, 1979. 19. Buncel, E. and Bourns, A.N., Can. J. Chem., 1960, vol. 38, p. 2457. 20. Moelwyn-Hughes, E.A., Physical Chemistry, London: Pergamon, 1961.
