J. Phys. Chem. 1980, 84, 827-830
827
Quenching of the Triplet State of Acetylene by Foreign Gases Masao Tsukada Laboratory of Physical Chemistry, School of Medicine, Juntendo University, Narashino, Chiba 275, Japan (Received October 1, 1979) Publication costs assisted by Juntendo University
The relative rate constants for quenching of the excited triplet state of acetylene by foreign gases have been studied by measuring the phosphorescence of biacetyl induced by energy transfer to biacetyl from acetylene in the mercury photosensitization of acetylenebiacetyl-quencher-nitrogen mixtures. The rate of energy transfer to biacetyl occurs at 5.2 times self-quenching of triplet acetylene. The rate for quenching of triplet acetylene by hydrogen sulfide is 3.4 times the quenching by propylene which is 1.6 times self-quenching of acetylene. Ethylene scarcely quenches triplet acetylene, in contrast to propylene.
Introduction Unsaturated hydrocarbons in their lowest triplet states are important chemical intermediates in photochemistry. In the mercury-photosensitized reaction of unsaturated hydrocarbons, which is one of the most extensively investigated photochemical reactions, their excited triplet states are formed by energy transfer from excited mercury at0ms.l However, excited triplet states of alkenes have generally not been observed by spectroscopic methods, in contrast to aromatic triplet species. Thus many interesting questions about their properties remain unanswered. For the simplest representative of aliphatic unsaturated hydrocarbons, acetylene, Burton and Hunziker2 showed that a metastable triplet formed in the mercury-photosensitized reaction enhances the biacetyl phosphorescence in the mixture of acetylene, biacetyl, and nitrogen under 253.7nm radiation. Although considerable information is available concerning the quenching efficiency of foreign gases on the excited states of molecules, relatively little is known for the small molecules which do not give off fluorescence or phosphorescence. In this paper, relative rate constants for quenching of the triplet state of acetylene by foreign gases have been investigated. The method is based on the measurement of biacetyl phosphorescence sensitized by acetylene in the presence or absence of a quencher.
Experimental Section The reactants were circulated through a closed-loop system having a mercury vapor saturator and a glass pump circulator. The temperature of the mercury vapor saturator was kept constant at 20 "C. The reaction cell was a quartz tube, 3 cm in diameter and 55 cm long, which was kept a t a constant temperature of 25 "C. The exciting lamp was 21 70-W low-pressure mercury lamp (Ushio Electric, UL1-7UQ) which operated by direct current. Between the lamp and the reaction cell, an interference filter was placed. Another low-pressure mercury lamp (Hamamatsm-TV, N128A) was used as a light source to measure thie 3P0mercury atom concentration. A 0.2-m monochromator (Ritzu Applied Optics, MC-BON) and a lock-in amplifier (Brookdeal, 9501) were used to monitor the emission intensity. The biacetyl phosphorescence emission signal was measured at a wavelength of 510 nm. The 3P0mercury atom concentration was determined from the absorption of the 404.7-nm line. Acetylene was obtained from Matheson Gas Products. Acetone (the main impurity) was removed by distilling the acetylene under reduced pressure at -120 "C and passing it through acti0022-3654/80/2084-O827$0 1.OO/O
TABLE I : Relative Rate Constants for Quenching of Hg(3P,) Atoms quencher k
--
C A (CH[,CO),
CZ%
c H[, 3
H2S
1.00 0.94 'r 0.98 t 1.08 f 1.15 f 0.89 'r
0.02 0.03 0.02 0.02 0.05
vated charcoal. Acetylene-d2was prepared by passing D20 through high-purity carbide and purified by bulb-to-bulb distillation. Nitrogen (99.999%) was passed through a U-shaped glass tube packed with small tips of copper a t 350 "C to eliminate a trace of oxygen. The nominal purity of other gases was 99.8% for C2H4,99.9% for C3H6,99.0% for H2S,and 99.99% for COz. These gases were purified by bulb-to-bulb distillation in vacuum.
Results and Discussion Evaluation of the experiments reported herein requires the knowledge of the relative quenching rate constants for Hg(3Po)atoms of acetylene, biacetyl, ethylene, propylene, hydrogen sulfide, and carbon dioxide. In this study, the concentration of Hg(3Po)atoms was determined from the absorption of the 404.7-nm line in the presence of excess nitrogen. The relation of the reciprocal of Hg(3Po)~ atom concentration to the concentration of quenching molecules was explained well by the Stern-Volmer equation. The ratio of the slopes of the linear Stern-Volmer plots represents the ratio of the quenching rate constants, which are given in Table I. There is no report for hydrogen sulfide and only one report for biacety12 where it was measured at a higher temperature than we employed. The values for other gases which we obtained are about the same as other values previously reported in the liter~sture.~ Details of the determination of these quantities will be published separately. With acetylene (0.1 torr), biacetyl(O.1 torr), and nitrogen (657 torr) passing through the reactor, biacetyl phosphorescence was generated; the spectrum is in good agreement with the spectrum observed by Burton and Hunziker.2 These conditions of partial pressures were usually employed and will be denoted as standard conditions. The intensity of the biacetyl emission was observed to be affected by changes in the biacetyl partial pressure. When the partial pressures of acetylene and nitrogen were kept constant and that of the biacetyl was varied, the results shown in Figure 1 were obtained. Observations
0 1980 American Clhemical Society
828
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980
0
0 05
010
0.15
((CH3C0)2)
Tsukada
0 20
torr
0.001 0
2
4
8
I
10
(C2H2) I ((CH3C0)2)
Figure 1. Dependence of sensitized emission at 510 nm on biacetyl pressure.
6
Figure 3. Variation of the reciprocal of emission intensity vs. the reciprocal of biacetyl pressure.
for decomposition was calculated to be about 0.02; cistrans isomerization of 2-butene was employed as an actinometer. Therefore reaction 6 is probably favored over reaction 5. By the steady-state treatment, we get the following relation: -1 =-
alp
I 0
0.25
0.50
0.75
(C2bI
similar to those obtained for the change of biacetyl partial pressure were made when the partial pressure of acetylene was varied and the other components were kept constant (Figure 2). According to the usual mechanism of mercury photosensitization, the following reaction scheme may be reasonable. Hg hv Hg* (1)
-
+
Hg* + Nz
+ (CH,C0)2 Hgo* + CZH2
Hgo*
C2H2*' C2H2*
fragmentation
+ + --
C2H2*'
+ Nz
-+ Hgo*
Hg
CZH2*'
fragmentation
Nz
+ (CH3CO)Z
+
C2H2* C2Hz (CH3C0)2*
(CH3CO)Z"
(2)
(3) (4)
(5)
+ N2
(6)
CzH2 + (CH3C0)2*
(7)
C2H2*
a=
o
g + hie) + M kh7k9I
[CZH~I [(CH3C0)2l
h3[(CH3C0)21 + k4[CzHzl kJC2HZI
1.00
torr
Figure 2. Dependence of sensitized emission at 510 nm on acetylene pressure.
+
hg + h hgI
quenching
(8)
(CH3CO)B + h v ,
(9)
(CH3CO)z
(10)
In this scheme, Hg* is the Hg(3P1)atom and Hgo* is the Hg(3Po)atom. Reactions involving Hg(3P1)atoms with acetylene and biacetyl cannot effectively compete with reaction 2 and therefore are not considered further. The excited state of acetylene, produced via reaction 4, is thought to be vibronically excited. It is indicated by an asterisk-prime combination. The quantum yield of decomposition of acetylene was estimated under standard conditions but with biacetyl omitted. The quantum yield
where I is the light intensity of the mercury lamp and Ip is the intensity of the biacetyl emission. A t pressures normally employed, biacetyl can compete effectively for metastable mercury atoms. This causes the biacetyl phosphorescence to decrease with increasing biacetyl pressure. The decrease of emission intensity due to the quenching of Hg(3Po)atoms by biacetyl is compensated by the term a. The reciprocal of aZ as a function of the reciprocal concentration of biacetyffalls into a good linear plot. From the ratio of the intercept and the slope in Figure 3, we get the ratio of k , to k7 (k7 = 5.24k8). In the reaction scheme, the triplet-triplet annihilation reaction of the excited biacetyl molecules is ignored. The ratio of the rate constant (ha)for self-quenching of triplet acetylene to the rate constant (k,) for energy transfer to biacetyl can be determined from the experimental relation between the emission intensity and the acetylene partial pressure, as well as the biacetyl partial pressure. We obtain about the same values from those two different conditions. So, it seems that the triplet-triplet annihilation reaction makes a relatively small contribution to the overall reactions. Sensitized phosphorescence of biacetyl in the mercury photosensitization of a mixture of acetylene, biacetyl, and nitrogen has been reported previously by Burton and Hunzikera2 They showed that the energy transfer to biacetyl occurs at 12 times the rate of self-quenching of triplet acetylene. This is about two times larger than the value obtained by us. However, it is difficult to refer directly to the discrepancy because of the difference between reaction temperatures. The intensity of the biacetyl emission was observed to be affected by adding foreign gases as quenchers of excited acetylene. Figure 4 shows the effects of quenchers on the biacetyl emission. When a quencher was added t o the mixture, the partial pressures of acetylene, biacetyl, and
The Journal of Physical Chemistry, Vol. 84,
Quenching of the Triplet State of Acetylene
No. 8, 1980 829
2.0 I
-
co2
b--
U
t
I -
0 0
0 10
0 05
0.15
torr
[Ql
1.5
0
Figure 4. Effects of COP,C,H,, C,H,, and H2Son the biacetyl emission. Figure 5. Variation of p I p o I p I , vs.
TABLE 11: Relative Rate Constants for Quenching of Triplet Acetylene -quencher k quencher k GH, (CH,CO), c 2
H,
c 3
H,
HZS CO Z
1.00 5.24 i0.02 i1.57 i5.37 * 0.00
0.34 0.02 0.17 0.35
CZDZ (CH,CO),
1.00 5.34 i 0.33
C,H, H,S
1.17 6.43
i!
0.15 0.37
nitrogen were kept constant. It can be seen from Figure 4 that the excited state of acetylene is little quenched by carbon dioxide. The additives used in this study do not quench the biacetyl emitting state. This was established by noting that the biacetyl phosphorescence was not quenched by the additives in the pressure range of the experiment when biacetyl was! excited directly in the 365-nm region by a Xe lamp. Wheiti the mixtures of biacetyl and those quenchers did not contain acetylene, the biacetyl emission was absent. It is thus apparent that the energy transfer from an excited state of a quencher to biacetyl can be ignored. In the presence of a quencher, reactions 11and 12 should be added to the reaction scheme presented earlier. Then Hgo* + Q quenching (11) CzHz*+ Q
--
quenching
(12)
we obtain the following relation by the steady-state treatment: Pl‘po hdQ1 -_ =1+ P6 k,[(CH3CO)21 + h8[CzH2j
p
+ h4[CzH21 hdC2H21
=: h3[(CH&O)21 -
hii[&1 -
where Ipor Tpo is the emission intensity in the presence or the absence of a quencher. The decrease of the emission intensity by quenching of Hg(3E’,J a t o m by biacetyl and a quencher is compensated by the term1 P. Since the ratio of k7 to k 8 was determined as previously stated and both the pressure of biacetyl and that of acetylene were equal, we come to the final expression,
PI,, = 1 + hi2 [Ql PIP
6.24h8 [C,H2]
The values of PIpo/pIpas a function of the concentration of a quencher are plotted in Figure 5 . From the slope of
[Q]/[C,H,].
this figure, the relative rate constants for quenching the excited acetylene are obtained. The same experiment was done with acetylene-d2. The experimental results are summarized in Table 11. The data provide no firm basis for lengthy speculation on the mechanistic details of the quenching process. However, the quenching process can be suggested as follows: (1)The quenching of the triplet state of acetylene by propylene results from the energy transfer from excited acetylene. (2) Carbon dioxide and ethylene scarcely quench the excited acetylene because their lowest triplet states should be situated at higher levels than acetylene. (3) The quenching process by hydrogen sulfide would be the hydrogen-atom abstraction reaction from hydrogen sulfide by excited acetylene. We infer the quenching process for hydrogen mlfide from the dependence of benzene formation on hydrogen sulfide partial pre~sure.~ In the mercury photosensitization of acetylene in the absence of biacetyl, the yield of benzene decreased whereas thiophene increased with increasing amounts of hydrogen sulfide. It has been confirmed that thiophene is formed in the reaction of HS radical with acetylene whereas the reaction of hydrogen atom with hydrogen sulfide gives a HS radical. However, the efficiency of ethylene for inhibition of benzene formation was one-tenth that of the hydrogen sulfide, although the efficiency of ethylene for hydrogen-atom scavenging is at most two times larger than that of hydrogen ~ u l f i d e . ~ Therefore, most of the HS radicals are not formed from the reaction of hydrogen sulfide with hydrogen atoms produced by decomposition of excited acetylene but formed from the reaction of hydrogen sulfide with excited acetylene. As shown in Table 11, the quenching efficiency by propylene for acetylene-h2is larger than that for acetylene-dz, whereas the quenching efficiency by hydrogen sulfide for acetylene-h2 is smaller than that for acetylene-dz. This reversal in tendencies at the quenching efficiencies for acetylene-hz and acetylene-d2 comparisons would inot be understandable if both the quenching processes wore the energy-transfer mechanism. But it would be explained on the basis of the quenching processes mentioned above; that is, the energy transfer to propylene from acetylene-h, could more effectively occur than from acetylene-d2,whereas the hydrogen-atom abstraction reaction from hydrogen sulfide by acetylene-h2 would be much slower than by acetylene-& The latter process would be reasonable from the results6 that the reaction rate of hydrogen atom with
830
J. Phys. Chem. 1980, 8 4 , 830-833
acetylene-h2is smaller than the corresponding reaction rate with acetylene-dz. References a n d Notes
(2) C. S. Burton and H. E. Hunziker, J. Chem. Phys., 57,339 (1972). (3) H. Horguchiand S. Tsuchiya, Bull. Chem. Soc.Jpn., 47,2768 (1974). (4) M. Tsukada, T. Oka, and S. Shida, Chem. Lett., 437 (1972). (5) G.R. Woolley aml R. J. Cvetanovic, J. Chem. phys., 50,4697 (1969). (6) E. B. Gordon, 8. I. Ivano9, A. P. Perminov, and V. E. Balalaev, Chem. Phys., 35, 79 (1978).
(1) See,for example, J. G. Calved and J. N. Pitt, "Photochemistry", Wiley, New York, 1966.
Reduction of Nitro Blue Tetrazolium by COP- and 02-Radicals Benon H. J. Blelski," Grace G. Shiue, and Stanley Bajuk Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973 (Received November 1, 1979) Publication costs assisted by Brookhaven National Labcratory
The reduction of nitro blue tetrazolium (NBT2+)by COf and Of has been studied by the stopped-flow method and the pulse-radiolysis technique. Both reductants form the tetrazolinyl radical, and the rate constants are hNBTZ++ coz- = (6.4 f 0.2) x 109 ~ - s-1 1 and h N B p + + 02-= (5.88 f 0.12) X lo4 M-I s-'. In the absence of other reactants, the tetrazolinyl radicals (NBT',) disappear by a pH-dependent second-order disproportionation reaction to produce monoformazan (MF') and nitro blue tetrazolium (NBT2+)ions. The tetrazolinyl radical (NBT+.) ~ 000 ~ f has an absorption maximum at 405 nm with a pH-independent molar extinction coefficient ~ 4 0 =5 15 350 M-' cm-l. Monoformazan (MF+)has an absorption maximum at 530 nm; its molar extinction coefficient varies from E~~~~~ = 25 400 f 1200 M-' cm-l at pH 9.5-11.0 to €53onm = 12 800 f 640 M-l cm-' at pH 5.7-6.7. Mechanisms for the overall reaction are discussed.
Introduction Nitro blue tetrazolium (NBT2+)is a widely used reagent for the detection of superoxide radicals.'-12 Although it has merit as a qualitative indicator, its use as a quantitative reagent has been less successful partly because of its complex chemistry13-19and partly because its reduction products (mono- and diformazans) are only slightly soluble in aqueous solutions. Formally, NBT is known as 3,3'-(3,3'-dimethoxy[l,l'biphenyl]-4,4'-diyl) bis [ 2- (4-nitrophenyl)-5-phenyl]-2Htetrazolium dichloride. It is a ditetrazolium salt which can be reversibly reduced to diformazan (DF)15-17by the stepwise addition of four electrons with the formation of and one stable the transient free radicals (NBT'., MF,)20-22 intermediate consisting of one tetrazolium center and one formazan center (MF+); the simplified notation given under the formulas in Scheme I will be used in this paper. In aqueous solutions the chemistry of formazans is further complicated by their acid-base properties. An earlier has shown that formazans exist in three forms (cationic, anionic, and neutral) and have in general pKs in the range from 4.0 to 5.0 and from 8.4 to 10.0. These acid-base properties are apparently dependent upon the molecular configuration of the given formazan. The purpose of this investigation is the elucidation of the reduction mechanism of NBT2+by superoxide radicals. Since preliminary experiments had revealed that the perhydroxyl radical (HOz,the conjugated acid of 0,) does not react with NBT2+,the study was limited to the pH range between 5.5 and 11.0. Also, the complexity of the overall system was greatly simplified when it was found that under certain experimental conditions only one product, the monoformazan, was formed. Part of this project is the pulse-radiolytic study of the short-lived NBT+- free radical. As will become apparent, the understanding of the chemical properties of the NBT+. species is essential for the elucidation of the mechanism by which Oz- reduces NBT2+to MF'. The NBT'. radical was conveniently generated by reduction of NBT2+with C02- in the well-known formate system.24 0022-3654/80/2084-0830$0 1 .OO/O
Experimental Section Materials. All solutions were prepared with highly purified water which was obtained by passing distilled water through a Millipore ultrapurification system. Sodium phosphate and sodium formate were purified by recrystallization. A filtered source of UHP nitrogen (99.99%, Matheson Co.) or nitrous oxide was used for sparging of solutions. Nitro blue diformazan (grade 111) and nitro blue diformazan (grade I), both Sigma products, were used without further purification. Apparatus and Techniques. Pulse-radiolysis experiments were carried out with a %MeV Van de Graaff accelerator or a Febetron. The sample cell had a 6.1-cm optical light path. All data were automatically evaluated on a PDP-11 computer. The energy input into the sample per pulse was evaluated from ferrous dosimetry calibrations by using G(Fe3+)= 15.6, where the G value represents the number of molecules formed or changed per 100 eV of energy d i ~ s i p a t e d .All ~ ~experiments were carried out a t ambient temperature (23-24 "C). Results and Discussion The radiation-induced reduction of NBT2+to MF+ in nitrous oxide saturated formate solutions can be described by eq I, 1-5). H20
--
H, OH, ea;,
+ HCOOeaq- + NzO + HzO OH + HCOOCOO- + NBT2+ NBT'. + NBT+* H
+
+
H30+,H20z,H2
(I)
+ COZNz + OH- + OH H2O + COZCOZ + NBT'.
(1)
H2
NBT2+
+ MF+
(2) (3) (4) ( 5)
It was established that only one stable endproduct, MF', is formed when the ratio (NBT2+)/(COz-)> 100. Hence pulse-radiolysis experiments were designed so that a l-ps pulse of 2-MeV electrons generated 1pM COz- in a 0.1 M 0 1980 American Chemical Society
