M. Hoshino, S. Arai, and M. lmamura
2724
Anion Radicals of Aromatic Ketones in Amine Solutions as Studied by Radiolysis Mlkio Hoshlno,* The National lnstitute lor Environmental Studies, Yatabe-machi, lbaraki 300-2 1, Japan
Shigeyoshi Arai, and Masashi lmamura The lnstitute of Physical and Chemical Research, Wako-shi, Saitama 351, Japan (Received April 26, 1976) Publication costs assisted by The lnstitute of Physical and Chemical Research
Amine solutions of benzophenone and acetophenone have been studied by using matrix isolation and pulse radiolysis techniques a t various temperatures. In the case of benzophenone, the anion radical produced in sec- butylamine solution a t 77 K has an absorption peak at 780 nm, whereas the peak locates a t 680 nm at 153 K. The former peak is ascribed to a presolvated benzophenone anion radical and the latter to a solvated one. This solvated benzophenone anion radical was found to be in equilibrium with the ketyl radical in primary and secondary amine solutions. The enthalpy change ( A H ) was obtained for these systems to be ca. 6 kcal/ mol. In a triethylamine solution a t 183 K, the benzophenone anion radical has an absorption peak at 760 nm. The equilibrium between the anion and ketyl radicals was not observed in this solution. The acetophenone anion radical in the sec- butylamine solution showed the spectral shift due to solvation. An equilibrium between the anion and ketyl radicals was also studied.
Introduction The benzophenone anion radical has been studied extensively in photochemistry and radiation chemistry since Porter and Wilkinson’s observation in alkaline alcohol solution by flash photolysis.1 In previous papers, we have reported the time-dependent spectral shift of the benzophenone anion radical in alcohol solution observed by pulse radiolysis,2 and the spectral shift of the anion radical in alcohol solution by raising the temperature from 4 to ca. 80 K.3These facts have been interpreted based on the solvent reorientation around the anion radical. The other aromatic ketones showed similar behavior in alcohol solutions. Recently, flash and laser photolysis studies of benzophenone in amine solutions have been carried out at room temp e r a t ~ r e . 4Transients ,~ were the benzophenone anion radical and/or ketyl radical. However, the dynamic behavior of these transients is still unknown in amine solutions. In the present work, we have performed pulse radiolysis and y radiolysis studies on the solvation and successive reactions of benzophenone and acetophenone anion radicals in primary, secondary, and tertiary amine solutions. The solvated benzophenone anion radical was found to be in equilibrium with the ketyl radical in primary and secondary amine solutions. The ketyl radical produced probably forms hydrogen bonding with an imino anion in primary and secondary amine solutions. Detailed reaction mechanisms are discussed on the basis of solvation and its related equilibrium of the anion radical with the ketyl radical.
Experimental Section Benzophenone from Wako Pure Chemical Industries was recrystallized from an ethanol solution. Acetophenone was used without further purification. sec-Butylamine, n-propylamine, isopropylamine, diisopropylamine, and triethylamine, guaranteed reagents from Wako Pure Chemical Industries, were refluxed on sodium metal and fractionally distilled. These amines were stored on a metallic alloy of soThe Journal of Physical Chemistry, Vol. 80, No. 25, 1976
dium and potassium in vacuo. All the samples were prepared in vacuo to avoid moisture and oxygen. The irradiation cell for matrix study had an optical path length of 2 mm. y-Ray irradiation and measurements of absorption spectra were carried out a t 77 K on a Cary 14 R spectrometer. Samples were warmed after irradiation in a simple way where they were drawn out for a definite time from liquid nitrogen in a dewar vessel. The cells and apparatus for pulse radiolysis were essentially the same as described in earlier papers.6~~ The energy and duration of pulses were 2.7 MeV and 1.0 ys, respectively. Samples were cooled by blowing cold nitrogen gas which was supplied from a liquid nitrogen container equipped with an electric heater a t the bottom.
Results Benzophenone Anion Radical. Figure 1 shows the absorption spectra obtained by y irradiation of a sec-butylamine solution containing 10 mol % benzophenone a t 77 K. The spectrum for the solution without warming has a peak at 780 nm and a small peak at 560 nm. The main peak gradually shifted to shorter wavelengths upon warming of the solution, and its final location was a t 680 nm. When the solution was further warmed, the 680-nm band decreased without shift and then disappeared completely. There was no band appearing concurrently with the decay of the 680-nm band. During the shift from 780 to 680 nm the intensity of the band was found to increase by a factor of almost 2. Such a phenomenon was not observed when ethanol was used as a m a t r i ~ . ~ Figure 2 shows the spectra obtained by pulse radiolysis of a liquid sec-butylamine solution of 3 mol % benzophenone at various temperatures. The spectrum at room temperature had an intense band at 560 nm, which agrees with the well-known band of a benzophenone ketyl radical. As the temperature was decreased, the intensity of the 560-nm band decreased and the other band which is due to the benzophenone anion radical appeared at 680 nm. When the solution was irradiated at 193 K, only the 680-nm band was observable. A clear isosbestic point is seen at about 580 nm in Figure 2, indicating an equilibrium between the ketyl and anion radi-
Anion Radicals of Aromatic Ketones in Amine Solutions
2725
0.5
> Icn
0.4
2 W
0
0.3
-I
f
O 2
c
n 0
0.1
0
WAVELENGTH
(nrn)
Figure 1. Absorption spectra for a y-irradiated sec-butylamine solution containing 10 mol % benzophenone at 77 K: (A) before warming; (B) warmed for 65 s; (C) for 95 s;(D)for 135 s; (E) for 185 s; (F) for 285 s. Irradiation and optical measurement carried out at 77 K. 5-FOLD EXPANSION
0.5-
0.3
L
E
; - 0.2
0.3;
frl
1
2+ 0.2-
cn
2
Y
f3 0.1-
0
a
2- 0.1 I-
n
WAVELENGTH (nm)
0
Figure 4. Absorption spectra for a pulse-irradiated triethylamine solution of 4 mol % benzophenone at 183 K. A and B determined at 2 and 47 ps after the pulse, respectively. The ordinate is fivefold expanded in the region above 600 nm. WAVELENGTH
Inm)
Flgure 2. Absorption spectra for a pulse-irradiated sec-butylamine solution of 3 mol % benzophenone. Every spectrum obtained at 25 ps after the pulse: (A) 291 K; (B) 258 K; (C) 222 K; (D) 193 K.
cals. From a comparison of the peak intensities for the solution at 291 and 193 K, the value of &0G291~/&0G193 K is obtained to be 1.7, where ( and G express the molar extinction coefficient and G value of the tra+nsients, respectively. Figure 3 represents the spectra determined at various times after the pulse for the solution at 230 K. Each spectrum shows the same shape, irrespective of the time. This fact means that the ratio of the absorbing species at 560 and 680 nm is constant throughout their decays. In all primary and secondary amine solutions and at all temperatures studied, the decay behavior of the 560- and 680-nm bands was very similar to each other and fits the second-order rate law. On the basis that the molar extinction coefficient of a benzophenone ketyl radical is 3.7 X lo3 M-l cm-l, the rate constant for the decay of the radical is evaluated as 1.5 X lo7 M-l s-l in the sec-butylamine solution at room temperature.8 n-Propylamine, isopropylamine, and diisopropylamine solutions gave essentially the same results as that observed for the sec-butylamine solution. In a triethylamine solution, however, the spectrum obtained immediately after the pulse had an intense band a t 560 nm and a weak band at 760 nm even at 183 K, as shown in Figure 4.Furthermore, the 760-nm band disappeared almost completely within 100 ps after the
pulse, although the lifetime of the 560-nm band was as long as 0.3 s. The decay of the 760-nm band was accompanied by the formation of an appreciable portion of the 560-nm band. Hence, no equilibrium exists between both absorbing species in this solution. Acetophenone Anion Radical. Figure 5 shows the transient absorption spectra obtained by the pulse radiolysis of the sec -butylamine solution containing 3 mol % acetophenone at 173 K. The spectrum observed immediately after the pulse decayed over the wavelengths studied. The spectral shape did not change during the decay of the band. This species is ascribed to the acetophenone anion radical. y radiolysis of the sec-butylamine solution containing 3 mol % acetophenone was carried out a t 77 K. As shown in Figure 6, the absorption band obtained before warming was at longer wavelengths than that obtained after warming the solution. The shape of the latter band was quite similar to that observed for the pulse radiolysis a t 173 K.
Discussion Absorption Spectra of the Benzophenone Anion Radical in Amine Solutions. A possible reaction mechanism may be represented for amine solutions from the analogy of alcohols as: RzCH-NH2
+
RzCH-N+H2, emobiie,H, R2C-NH2, R2CH-NH, and others (1) The Journal of Physical Chemistry, Vol. 80, No. 25, 1976
M. Hoshino, S.Arai, and M.
2726
lmamura
TABLE I: Absorption Peak Wavelengths of the BenzoDhenone Anion Radical in Amine Solutions (in nm) s-BA" 77 K 193 K
780 680
n-PAb iPAc 680
660
(i-P)7Ad
TEAe
680
760
Dia sec -Butylamine. * n-Propylamine. a Isopropylamine. isopropylamine. e Triethylamine.
500
400
600
7 00
WAVELENGTH ( r m j
Figure 5. Absorption spectra for a pulse-irradiated sec-butylamine solution of 3 mol % acetophenone at 173 K. A and B determined at 5 and 130 hs after t h e pulse, respectively.
WAVELENGTH ( n m )
Figure 6. Absorption spectra for a y-irradiated sec-butylaminesolution containing 3 mol % acetophenone at 77 K: (A) before warming; (B) warmed for 60 s; (C) for 140 s.
R~CH-N'HZ
+ RzCH-NHz
-
+
R & - N H 2 R&H-NfH3 or R2CH-NH R2CH-N+H3 ( 2 )
+
The amine cation radical may produce a relatively stable protonated cation and a neutral amine radical in which an odd electron localizes on a carbon or nitrogen atom as shown in reaction 2. When benzophenone molecules are present, they capture mobile electrons to form benzophenone anion radicals. The anion radical may additionally be produced by the following reaction of benzophenone and the neutral amine radical:
+
R ~ C - N H Z (CsH&CO +
R2C=N+H2
+ (C6Hj)&O-
(C6H&CO-- - -sec-CdHgNHn A
+ (C,&)&OH-
(3)
In Table I are listed the peak wavelengths of the benzophenone anion radical produced in several kinds of amine solutions. Our previous studies have shown that the absorption peak of a presolvated benzophenone anion radical locates a t 780 nm and that of a solvated one a t 640 nm in an ethanol s o l u t i ~ n .In ~ ,the ~ present study, it is reasonable to assign the 780- and 680-nm bands observed in sec- butylamine to the presolvated and solvated anion radicals, respectively. The pulse radiolysis results of a triethylamine solution are evidently different from those of primary and secondary amine solutions. The spectrum had the peaks a t 760 and 560 nm. The latter band is obviously due to the ketyl radical, while the former can be assigned to the anion radical. The Journal of Physical Chemistry, Vol. 80, No. 25, 1976
A large shift of the absorption band due to solvation was observed for alcohol and primary and secondary amine solutions. This fact suggests that the H in OH, "2, and NH groups approaches an oxygen atom of the benzophenone anion radical in the solvated state. I t is likely that the anion radical is a hydrogen bonded complex with a solvent molecule in these solvents. A significant increase in band intensity on going from 780 to 680 nm does not result from the difference in extinction coefficients between solvated and presolvated anion radicals, because such an increase was not seen in an ethanol solution. Reactive species trapped in the sec -butylamine solution probably produces additional benzophenone anion radicals a t temperatures higher than 77 K according to reaction 3. Absorption Spectra of the Acetophenone Anion Radical in sec-Butylamine Solution. The absorption band of the anion radical produced in the sec-butylamine solution shows a spectral shift to shorter wavelengths by warming the solution. An increase in the band intensity of the anion radical was also observed similarly to the case of the benzophenone anion radical. Because the absorption band of the acetophenone ketyl radical locates a t shorter wavelengths, we were unable to observe it in the wavelength region studied. The transient absorption spectrum observed for the pulse radiolysis of acetophenone in sec-butylamine solution a t 173 K is in good agreement with the spectrum obtained after warming the solution at 77 K. Therefore, the 460-nm band of the acetophenone anion radical is due to the solvated species. Equilibrium between the Anion Radical and the Ketyl Radical. The existence of the isosbestic point and the similar decay behavior observed for the sec -butylamine solution of benzophenone leads to a conclusion that the anion radical is in equilibrium with the ketyl radical. Since the equilibrium is not established in a triethylamine solution, the reaction is considered to be proton transfer from a solvent molecule to an anion radical
- -sec-CdHgN-H
(4) B where A and B are the solvated benzophenone anion radical and the ketyl radical hydrogen bonded with the sec- butylimino anion. If the ketyl radical and sec-butylimino anion formed by reaction 4 are free in the solution, the ratio of [A] to [(C6H&COH] depends on the concentration of. secC4HgN-H, as expressed in the equation, [A]/[(C~HS)~COH] = [sec-C4HgN-H]/K, where K is the equilibrium constant of reaction 4. This relation is contrary to the experimental finding that the ratio is constant throughout their decays. From K = [B]/[A] and [A] [B] = [C,], one obtains
+
K = ([Co] - [Al)/[Al = (EJ[CoI - Eal[Al)/Ea~[Al= (DO- Da)/Da
(5)
Anion Radicals of Aromatic Ketones in Amine Solutions
2727
TABLE 11: AH for the Equilibrium between the Benzophenone Anion and the Ketyl Radical a
AH, kcal/mol a
I
D C B
S-BA
n-PA
i-PA
(i-P)*A
5.5
6.6
6.3
5.9
Abbreviations are the same as in Table I.
A
Flgure 7. Logarithmic plots of Kvalues against 1ITobtained with the systems of benzophenone-amines : (A) seobutylamine; (E) diisopropylamine; (C) isopropylamine;(D) n-propylamine.
-0.21 \.
-3.4 1
where Ea, I , and D , are a molar extinction coefficient of the anion radical, an optical path length, and an optical density at 680 nm, respectively. DOis equal to tal[CO]and corresponds to the optical density a t 680 nm for the solution a t 193 K, where the equilibrium shifts far to the left. Equation 5 is valid only when [CO]is constant, irrespective of temperature studied. This condition may be satisfied in the present experiment, since all the spectra have an isosbestic point over the entire temperature range studied. In Figure 7 are shown the logarithmic plots of K values against 1/T in four amine solutions. The good straight lines gave the values of enthalpy change ( A H ) of 5.5 kcal/mol for see-butylamine, 6.6 kcal/mol for n propylamine, 6.3 kcal/mol for isopropylamine, and 5.9 kcal/ mol for diisopropylamine solutions (Table 11). In the pulse radiolysis of the sec-butylamine solution of acetophenone, it is difficult to examine the equilibrium between the anion radical and the ketyl radical, since the ketyl radical has the absorption band below 400 nm. A C-T absorption due to acetophenone and amine prevented observation at short wavelengths. However, by assuming this equilibrium, the apparent K values were obtained a t various temperatures by using eq 5 as in the case of benzophenone. Logarithmic plots of the K values against 1/T give a convex curvature as shown in Figure 8, indicating complex features of this equilibrium. The apparent AH values were estimated to be 4.1 kcal/mol from the slope of the plots in the lower temperature region and 2.0 kcal/mol from that in the higher temperature region. A number of studies have been published on the photochemical reduction of benzophenone in amine solutions and/or solutions containing a m i n e ~ . ~ ,The ~ , ~well J ~ accepted mechanism involves the formation of a charge transfer triplet state followed by its direct dissociation into a ketyl radical and a neutral amine radical
Logarithmic plots of apparent K values against 1/ T obtained with a system of acetophenone-sec-butylamine. Figure 8.
However, the other pathway for the formation of the ketyl radical is more conceivable from the results obtained here. A benzophenone anion radical produced from a charge transfer triplet state may abstract a proton from a neighboring amine molecule according to reaction 4 and form the ketyl radical. When the sec-butylamine solution of 3 mol % benzophenone was irradiated by uv light a t 173 K, the benzophenone anion radical was observed together with an unspecified yellow species that is more stable than the anion radical.ll The benzophenone ketyl radical was not detected a t this temperature. These results should support the reaction mechanism that the benzophenone anion radical is initially produced in the sec -butylamine solution upon uv irradiation. The reaction mechanism is
-
(CsH5)2CO*+ R2CH-NH2 [C-TI* [(CsH5)2CO-], e (C&Is)aCOH- - -RzCH-N-H ---+
Acknowledgment. The authors express their thanks to Dr. Akira Kira for his kind assistance in the matrix isolation experiment and for stimulating discussions throughout this study. References a n d Notes (1)G. Porter and F. Wilkinson, Trans. Faraday Soc., 57, 1686 (1961). (2)M. Hoshino, S.Arai, and M. Imamura, J. Phys. Chem., 78, 1473 (1974). (3)M. Hoshino, S. Arai, M. Imamura, and A. Namiki, Chem. Phys. Lett., 26, 582 (1974). (4) R . S. Davidson, P. F. Lanbeth, and M. Santanam, J. Chem. SOC.,Perkin Trans. 2, 2351 (1972). (5) S.Arimitsu and H.Masuhara, Chem. Phys. Lett., 22,543 (1973). (6)A. Kira, S. Arai, and M. Imamura, Rep. inst. Phys. Chem. Res. (Jpn.), 47, 139 (1971). (7)S. Arai, M. Hoshino, and M. Imamura, J. Phys. Chem., 79,702 (1975). (8) 6.W. Hodgson, J. P. Keene, E. J. Land, and A. J. Swallow, J. Chem. Phys , 63,3761 (1975). (9)S.G.Cohen and R. J. Baurngarten, J. Am. Chem. SOC.,87,2996 (1965). (IO)A. H. Parola. A. W.Rose, and S.G. Cohen, J. Am. Chem. Soc., 97,6202 (1975). (11) M. Hoshino and S. Arai, unpublishedresults.
The Journal of Physical Chemistry, Voi. 80, NO. 25, 1976
