J. Phys. Chem. 1994, 98, 11714-11718
11714
Pyrene Radical Formation in Pulse Radiolysis of Liquid Methanol Guohong Zhang and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received: May 20, 1994; In Final Form: July 14, 1994@
Reaction of hydrogen atoms and pyrene in liquid methanol was studied via the hydrogen addition product, i.e., the pyrene radical, by nanosecond transient absorption spectroscopy. The rate constant for hydrogen addition is measured to be 7.2 x lo9 M-I s-l in methanol, and that of hydrogen abstraction from methanol is 2.0 x lo8 s - l . Oxygen is shown to react with the pyrene radical with a slower than diffusion-controlled rate 1.4 x lo8 M-' s-l. The G value of hydrogen atom formation from molecular dissociation in radiolysis of liquid methanol is measured as 1.5 per 100 eV.
-
Introduction Pyrene is a popular probe molecule for organized assemblies and s~rfaces,l-~ as its anion radical, cation radical, triplet state (TI), and singlet excited state (SI)are well characterized. The pyrene radical, produced via H atom addition, has not been reported in an unambiguous manner, although the Mataga group suggested that an absorption at 400 nm observed in the laser photolysis of pyrene with amine in hydrocarbon solvents is indeed the pyrene radical! It is now important to further characterize the pyrene radical, as pyrene is used extensively in many systems, under conditions where the radical might be found. For example, this radical has been tentatively identified with pyrene on silica-alumina ~urfaces.~ Rates of hydrogen abstraction to give H2 and H atom addition to unsaturated hydrocarbons in aqueous solution have been reported,6-8 and use is made of these early concepts in the present work. Here, H atoms are produced by radiolysis of liquid methanol, and their reaction is monitored by observation of the H-pyrene adduct. This species is characterized, and the rate of reaction of H atom with methanol in liquid methanol is also measured. The latter rate constant throws light on the nature of the non-diffusion-controlled reactions in condensed phases.
Experimental Section Spectroscopic grade methanol and chloroform were used as received from Aldrich. Pyrene was purified by passing its benzene solution through an activated alumina column and recrystallized three times from ethanol-benzene solution. Samples were deoxygenated by bubbling with nitrogen or saturated by bubbling with 1 atm oxygen for at least 5 min before they were used in different experiments. The nanosecond pulse radiolysis and transient absorption spectroscopy utilized in this work is based on the original design by Hunt and tho ma^.^ The excitation electron pulse delivered from a Febetron 706 (Field Emission) has a -2 ns width, 0.40.6 MeV output energy, and a dose of -200 kradpulse. Samples were irradiated in a 2 mm thick glass cell with a very thin window (-20 pm) made of cellophane film. Transient absorption from 350 to 650 nm was measured in the transmission mode with analyzing light passing through the sample cell at a cross configuration. The instrument response is about 10 ns for submicrosecond measurements where a Hamamatsu R928 PMT and a Tektronix 7A13 differential comparator (bandwidth @
Abstract published in Advance ACS Abstracts, October 15, 1994.
0022-365419412098- 117 14$04.50/0
100 MHz) were used. Data were acquired by a computer interfaced transient digitizer. Transient spectra were obtained by assembling measurements at different wavelength. For kinetic measurements on even faster time scale, the combination of an ITT's F-4014 planar photodiode (-90 ps rise time under 3 kV) and a Tektronix 71429 amplifier (bandwidth 1 GHz) captures signals with very high speed, with its response limited by the 2 ns electron pulse width. Transient signal $(t) was simulated by convoluting the model kinetics S(t) with the pulse response of the entire measuring system F(t).
-
S(t) = F(t) 8 S(t) = LtF(r)S(t-r) d r
(1)
A simple biexponential function is used for the model kinetics
S(t) = So{exp( -kbt) - exp( -kat)} where k, is the formation rate of the monitored transient and kb is its decay rate at the early stage. The time resolution of this fast measurement system has been tested by monitoring the hydrated electrons produced in water. Processes as fast as 500 ps can be measured by the reconvolution analysis.
Results and Discussion Pulse radiolysis of a deoxygenated 10 mM methanol solution of pyrene produces a pyrene triplet-like absorption band with its maximum around 400 nm, the pyrene anion band around 490 nm, and the broad absorption above -450 nm due to the solvated electron. The 400 nm band is assigned to a pyrene radical on the basis of oxygen effect, competition kinetics, and acid effect as described below. The same absorption band was observed by Mataga in laser photolysis of pyrene with amines in hydrocarbon solutions and was assigned to 1-hydropyrenyl r a d i ~ a l .Similar ~ absorption bands, red-shifted from the ground state absorption of the substrate molecules, have been observed in previous work on hydrogen addition to benzene and naphthalene. They were assigned to the corresponding H atom adducts: cyclohexadienyl radical for benzene with the absorption maximum Am= = 315 nmloS1land 1-hydronaphthylradical for naphthalene with Amax = 340 and 390 nm.11.12The formation of 1-hydronaphthyl radical as the major addition product was explained by a larger H atom affinity for the 1-position than the 2-position and was further confirmed by quantum mechanical calculation." For pyrene radical, however, no rigorous calculation of the electronic structures and transitions is available at this time. Considering the high order of 1-2 x-bond (0.669), the maximum free valence index at the 1-position (0.468). and 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 45, 1994 11715
Pyrene Radical Formation
9
-c
0 C C
c
7
Figure 1. 1-Hydropyrenyl radical as the major H-pyrene adduct: numbering of pyrene rings and the position of H atom addition.
the high chemical reactivity of the 1-position of pyrene,13 the H atom is considered to add to the 1-position of the pyrene molecule and form a 1-hydropyrenyl radical (Figure 1). The decay of the pyrene radical is nonexponential with a halflifetime measured as til, 1.2 ps. Since the kinetics cannot be fitted to a simple second-order reaction of A A type, the pyrene radical might react with other radicals such as 'CHzOH, which is the major intermediate species in radiolysis of methanol. The decay trace monitored at 490 nm consists of two components: a fast decay with -20 ns lifetime due to the solvated electron reacting with pyrene and a much slower decay due to the pyrene anion. The ln(0D) t plot of the pyrene anion shows a faster decay at earlier time but approaches a straight line, Le., exponential decay, after the first 200 ns with a decay rate of 1.9 x lo6 s-l. The exponential decay of anions of some aromatic molecules within microseconds was also observed in pulse radiolysis of alcohols by Dorfman and coworkers and was attributed to the proton transfer reaction between aromatic anions and MeOH.14 The faster decay at early time due to the ion recombination reaction was not observed in their microsecond experiments. The major processes involved in formation and further reaction of these transient species are outlined as follows
-
+
-
-
+H MeOH - MeOH' + e,MeOH' - MeO' + H+ e,- + H+-H MeOH
MeO'
(3) (4) (5)
k6
k6 = 5.2 x 10" M-' s-' e,-
(6)
(ref 15)
-
+ Py k7 Py-
(7)
k, = (5-7) x lo9M-' s-' (ref 16) H
+ MeOH -'CH,OH + H, k8
-
+ Py H + 0, H
k,
k10
(8)
300
400
(9)
HOO'
(10)
Figure 2. Transient absorption spectrum of irradiated methanol solution with 10 mM pyrene. The spectrum was taken at the end of the electron pulse. An air-equilibrated sample was used to suppress the solvated electron absorption at longer wavelength.
( k = 7.0 x lo9 M-' s-l from ref 17) is not significant in the first tens of nanoseconds and is neglected in the discussion. Other slow reactions involving H atoms15 are also not considered in our fast kinetic study. The addition reaction between 'CH2OH (a methyl-like radical) and pyrene is not included in the above scheme simply because the reaction rate is about 3 orders of magnitude slower than that of the hydrogen addition reaction.18 The slow rate of reaction between 'CH20H radical and aromatics has also been discussed in the early work by Sauerlo and D01-fman.l~ Oxygen Effect. In an air-equilibrated 10 mM pyrene/MeOH sample, the solvated electron is removed by oxygen.
+ 0, -0,kl I
e,-
k , , = 2 x 10" M-'
s-l
(ref 16)
-
The transient absorption spectrum taken at the end of the pulse ( t 10 ns) shows only two transient species: the pyrene radical and the pyrene anion (Figure 2). The lifetime of pyrene anion is reduced to 44 ns due to the diffusion-controlled electron transfer reaction between pyrene anion and 0 2 . By using the known oxygen solubility in methanol,2othe rate constant of this reaction is estimated as -1.1 x 1OloM-' s-l from the oxygen quenching data. However, the half-lifetime of the pyrene radical is only reduced to t112 700 ns, indicating a non-diffusioncontrolled reaction. When the same sample is saturated with 1 atm of oxygen, the lifetime of pyrene anion is reduced to -10 ns while the decay rate of the pyrene radical only increases to 1.4 x lo6 s-l. Oxygen might react with pyrene radical via an addition reaction to produce a peroxide radical. The bimolecular reaction rate is measured as 1.4 x lo8 M-' s-l, which is 2 orders of magnitude slower than the diffusion-controlled rate in methanol.
-
+ 0, -PyHOO' k12
k,, = 1.4 x lo8 M-'
Here H+ is the proton in an associated cluster form. Recombination of atomic hydrogen to give molecular hydrogen
600
Wavebnglh (nm)
PyH' PyH'
500
(12)
s-l
The effect of oxygen on the pyrene radical is shown in Figure 3, indicating both static and dynamic quenching. The static decrease of the initial yield of pyrene radical by 0 2 is due to
11716 J. Phys. Chem., Vol. 98, No. 45, 1994
Zhang and Thomas
0 36
03
03
0 24
02
J
IO.18
f
I-
0 12
0.1
0 06
0
0
0
1
2
3
4
5
7
6
8
9
10
flmo (mlcrosecondr)
Figure 3. Oxygen effect on pyrene radical yield and decay kinetics. The same sample of 10 mM pyrene/methanol solution was irradiated under different conditions: (1) deoxygenated by bubbling with N2, (2) air equilibrated, (3) oxygen (1 atm) saturated. The decay profdes of pyrene radical were monitored at 400 nm. 0.6 I
0.4
z
F
0
10
20 30 Tlmo (nmorroondr)
0.02
0 03
[Wenel (M)
Figure 4. Growth of pyrene radical yield with increasing pyrene concentration showing the competition between pyrene and methanol for hydrogen atom. The transient absorbance of the pyrene radical was measured right after the pulse. The line through the data points is back-calculated from the linear least-squares fit of 1/OD l/[Py] plot by using k& = 36.5 M-' and setting the relative yield of the pyrene radical to one (a= 1) at infiiite pyrene concentration.
-
-
the reaction of H atom with oxygen, and the rate constant is estimated to be klo 3 x 1Olo M-' s-l in methanol. Competition Kinetics. Competition between reactions 8 and 9 was studied in deoxygenated acidic methanol solutions with different concentration of pyrene. HCl (0.1 M) was used to convert all the solvated electrons into H atoms and completely suppress reaction 7. The yield of pyrene radical measured at the end of the pulse (t 10 ns) obeys the Stem-Volmer kinetics (Figure 4).
-
where a is the relative yield of pyrene radical, Le., the fraction
50
Figure 5. Time-resolved profde of the pyrene radical formation and decay was fitted by reconvoluting the model kinetics S(t) = exp(-kbt) - exp(-kat) using the excitation electron pulse. The rate of the pyrene radical formation is measured as &, = 3.4 x los s-', and its decay rate at the early stage is kb = 6.5 x lo6 s-'. The electron pulse shape was obtained by monitoring the Cerenkov emission from a quartz plate. of H atoms that react with pyrene. The ratio between the two rate constants is obtained from fitting the plot, k$kg = 36.5 M-I. Measurements on even shorter time scales combined with convolution analysis show that the absolute rate of pyrene radical formation in a deaerated 20 mM pyrene/methanol solution is 3.4 x lo8 s-l (Figure 3,which then gives the rate constants kg = 2.0 x lo8 s-l and k9 = 7.2 x lo9 M-' s-'. Acid Effect. The extinction coefficient of the pyrene radical is determined from the effect of acid on the pyrene anion and the pyrene radical. The reduction of the pyrene anion and the concomitant growth of the pyrene radical with increasing concentration of HC1 is due to the fact that pyrene and H+ compete for the solvated electrons, i.e., reactions 6 and 7, which defines the following relationship between the observed reduction and growth of pyrene transients.
AA(PyH', 400 nm) 0.01
0.00
40
AA(Py-, 490 nm)
=a
~ ( P y s400 , nm) r(Py-, 490 nm)
(14)
From (13) and (14), and using the measured M's,we have the following equation which relates the extinction coefficients of the pyrene transients.
~ ( P y s400 , nm) = 0.7Oc(Py-, 490 nm)
(15)
The extinction coefficient of the pyrene radical in methanol is thus determined ~ ( 4 0 0nm) = 34 400 M-' cm-' by using the known value of pyrene anion extinction coefficient ~ ( 4 9 0nm) = 49 200 M-' cm-'.& Electron Scavenger Effect and Yield of H Atom. There are two channels for H atom formation, Le., from ion recombination, reaction 6 , and from molecular dissociation, reaction 3. The total yield of H atoms therefore consists of contributions from the above two processes.
The ion recombination is subject to electron scavenging by chloroform. Different amounts of chloroform were added to 5 mM pyrene/MeOH solution to scavenge the solvated electrons.
Pyrene Radical Formation
J. Phys. Chem., Vol. 98, No. 45, 1994 11717 where E = r(PyH', 400 nm)/c(Py-, 490 nm) = 0.70 and a = 0.15 for [Py]= 5 mM. Gm(H)is obtained by using the transient absorbance values (A) of the pyrene anion and the pyrene radical measured at 490 and 400 nm, respectively.
0.6
0 Pyrene Anion
A
Pyrene Radical
G,(H) = 0.45G(eS-)
0.4
(22)
0
0 L
.-a e
E
I-
0.2
0
-
A
A
I)
0 0.0
0.0
0.2
0.1
[CHC13]
0.3
(M)
Figure 6. Electron scavenger effect on the yields of pyrene transients
produced in irradiated methanol solution. Pyrene anion is completely quenched at high chloroform concentration while the yield of the pyrene radical is approaching a steady level. Since the reaction between H atom and CHC13 is very slow, the decrease of the pyrene radical yield when [CHCl3] 5 0.24 M is completely due to the electron scavenging (see text). The solid line shows the minimal effect of chloroform on the unscavengable pyrene radicals which are derived from those H atoms produced via molecular dissociation. All samples were deoxygenated. Chloroform competes very efficiently with H+ and pyrene for the solvated electrons by the electron detachment reaction. CHCI,
+ e,-
'CHC1,
+ C1-
(17)
k,, = 3.0 x lO'OM-' s-' (ref 16) Figure 6 shows that the yield of pyrene anion is quenched almost to zero at [CHC13] = 0.24 M, while the pyrene radical is reduced to a steady level due to the unscavengable H atoms from molecular dissociation. The slow abstraction reaction between H atoms and chloroform only leads to a very slight decrease of the pyrene radical yield at high chloroform concentrations. CHCI,
+ H -'CC1, + H, or 'CHCI, + HC1 kl8
(18)
k,, = 1.1 x io7M - ' s - ~ (ref 17) The yield of pyrene radical is related to that of H atoms by the Stern-Volmer kinetics.
Without added scavenger, the solvated electrons initially produced either react in geminate pairs with H+ via reaction 6 or react homogeneously with pyrene via reaction 7.
The yield of hydrogen atoms from molecular dissociation Gm(H) can be deduced from eqs 19 and 20
If we take the initial yield of the solvated electrons as G(e,-) = 3.3,21 the yield of H atoms from molecular dissociation is Gm(H) = 1.5, and the total yield of H atom and therefore HZ gas in acidic MeOH is -4.8. With pyrene added in neutral methanol, the yields of the pyrene anion and the pyrene radical depend on the concentration of pyrene. In case of [Py] = 5 mM, the transient yields are G(Py-) = 1.0 and G(PyH') = 0.59. Higher yields were measured in the steady state y-radiolysis by product analysis:15G,(H) = 2.0 in methanol with iodine as the electron scavenger, and G(H2) = 6.0 in acidified methanol. The present pulsed method of measuring the yields of shortlived intermediates is straightforward and avoids the complications from secondary reactions in the steady state method. Hydrogen Abstraction Reaction. Hydrogen abstraction reactions with alcohols as substrates have been found to be activation controlled in the gas phase.,, The activation energy for the gas phase reaction H MeOH is E, = 35.9 kJ/mol, and it drops to about 26 kJ/mol in aqueous solution.23 The reaction of H atom with 2-propanol in water was also shown to be activation controlled with a preexponential factor A = 1.0 x 10'' M-' s-l and an activation energy E, = 16 k.J/mol,* which is 10 kJ lower than Ea = 26.8 kJ/mol of the corresponding gas phase reaction.,, Thomas6 and Schuler7 studied the reaction between H atoms and MeOH in aqueous solution and measured the rate constant k = 1.6 x IO6 M-' s-l. The recommended value on the basis of a comprehensive evaluation of the measurements by different groups is k = 2.6 x lo6 M-' s-l.17 It is much slower than the diffusion-controlled rate kd 1.O x 1O'O M-' s-l for H atoms in water.17 The reaction rate of H atoms with MeOH in pure methanol has not been previously measured. This work shows that the second-order rate constant of the abstraction reaction in liquid methanol, k = 8.1 x IO6 M-' s-' with the concentration of MeOH taken as 24.7 M, is significantly faster than k = 2.6 x lo6 M-' s-l measured in water. Extrapolation of the rate constant obtained in aqueous solution to liquid methanol is not valid. In aqueous solution, the reaction proceeds via encounters of reactants that occur by diffusion.
+
-
H
+ MeOH
kd
k-d
H:MeOH
4
'CH,OH
+ H,
(23)
kd is the diffusion-controlled rate for formation of caged encounter pairs H:MeOH, k-d is the rate for the reactants to escape the encounter, and kr is the reaction rate of the encounter pairs. For the slow activated reaction k,
