J. Phys. Chem. 1992, 96,6663-6668
6663
Mechanism of Prooxidant Reaction of Vitamin E. Kinetic, Spectroscopic, and ab Initio Study of Proton-Transfer Reaction Shin-ichi Nagaoka,**+Kouhei Sawada: Youji Fukumoto,+ Umpei Nagashima,* Shunji Katsumata,g and Kazuo Mukai*it Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790, Japan; Institute for Molecular Science, Myodaiji, Okazaki 444, Japan; and Department of Fundamental Science, Iwaki Meisei University, Zwaki 970, Japan (Received: February 18, 1992; In Final Form: April 30, 1992)
To shed light on the mechanism of proton-transfer reactions, a kinetic, spectroscopic, and ab initio study of the prooxidant action of vitamin E derivatives has been carried out. The second-order rate constants (k,’s) for the reaction of six tocopheroxyl radicals (Toc”s) with five alkyl hydroperoxides (ROOHs) in benzene were determined spectrophotometrically. The first adiabatic ionization potentials ( I t s ) of ROOHs were obtained by means of photoelectron spectroscopy. The result indicates that k, increases as the electrondonathg capacity of the alkyl substituents of ROOH increases and I, decreases. The methylation at the aromatic ring of Toc’ reduces the k,for a given ROOH. k,for the reaction of deuterated alkyl hydroperoxides (ROODS) with a Toc’ in a mixed solution of benzene and ethanol-d, was also measured. A deuterium kinetic isotope effect on k, is observed. For a given Toc’, plots of log k, vs I, for various ROOHs and log k, vs Taft’s us constant of alkyl substituents of ROOH are found to be linear. The slope of the plot of log k, vs u* for ROOD is similar to that for ROOH. The geometries of ROOHs were optimized, and the Koopmans’ theorem first ionization potentials (IK’s) for those geometries were calculated with the ab initio method. A plot of log k, in the reactions of a Toc’ with various ROOHs vs I , of the ROOH is also found to be linear. From these results, it is considered that both charge transfer and proton tunneling play important roles in the prooxidant reaction of TocH. The transition state in the prooxidant reaction has properties of the chargetransfer species. The proton tunneling takes place below the transition state. Tunneling allows the proton to cut a corner on the potential energy surface. Our explanation will be widely applicable to many transfer reactions.
Introduction In recent years, proton transfer has been a topic of much interest because of its importance in many chemical and biological processes.14 As a representative of the proton-transfer reaction, we @-, 7-, and 6-tocohave chosen the reaction of vitamin E (a-, pherols) derivative^.^^^ The antioxidant property of tocopherol (TocH) has been ascribed to proton transfer (hydrogen transfer) from the OH group in TocH by a peroxyl radical (LOO’) as a whole. The proton transfer (hydrogen transfer) produces a tocopheroxyl radical (Toc’), which combines with another peroxyl radical (reactions 1 and 2).’ Here LOOH stands for a peroxide. LOO’ LOO’
-
+ TocH
+ Toc’
ki
k2
LOOH
+ Toc’
(1)
non-radical products
(2) In the previous papers,16 it is concluded that both charge transfer and proton tunneling play important roles in reaction 1. In contrast, several investigators showed that a-TocH in high concentration acts as a prooxidant during the autoxidation of polyunsaturated fatty acids (LH).*-1° This prooxidant effect of a-TocH leads to an increase of hydroperoxides with a conjugated diene structure. Loury et a1.8 and Terao et al? proposed that Toc”s participate in this prooxidant effect through the following reactions
+ LH
k3
+ L’ Toc’ + LOOH 2TocH + LOO’ Toc’
TocH
(3) (4)
where reaction 3 is proton transfer (hydrogen transfer) from the polyunsaturated fatty acids and is a chain-transfer reaction. Reaction 4 is a reversal of reaction 1. Previously, we determined the rate constant k3 for reactions of fatty acid esters and egg yolk lecithin with Toc’ spectrophotometrically.’I However, there remained an important question concerning the prooxidant reaction of TocH. Although the elucidation of reaction mechanisms is a major challenge in physical chemistry, the exact Ehime University. *Institute for Molecular Science. Present address: Department of Computational Science, Faculty of Science, Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo 11 2, Japan. lwaki Meisei University.
mechanism of reaction 4 has not been established well. In view of the wide occurrence of the proton transfer (hydrogen transfer) such as the prooxidant action of TocH in chemical and biological systems, the elucidation of its mechanism is particularly important. Accordingly, in the present paper, we have determined the second-order rate constant k, for reaction of five alkyl hydroperoxides (ROOH’s) with six Toc”s: Toc’
+ ROOH
k.
TocH
+ ROO’
(5) To examine whether or not charge transfer from LOOH or ROOH to Toc’ plays an important role in reactions 4 and 5, we have tried to evaluate the electron-donating capacity of ROOH. However, it is difficult to obtain the oxidation potential of ROOH by means of the cyclic voltammetry. Thus, we observed the first adiabatic ionization potential (I,) of ROOH by means of photoelectron spectroscopy. We have also optimized the geometries of ROOHs and have obtained the Koopmans’ theorem first ionization potential (ZK) with the ab initio method. The photoelectron experiments profit from a comparison with these molecular orbital calculations. The combination of the spectroscopy and ab initio method is a powerful means of investigating the prooxidant reaction of TocHs. Furthermore, to examine whether or not a deuterium kinetic isotope effect on k, is observed, k, for the reactions of deuterated alkyl hydroperoxides (ROODS) with a Toc’ was measured. From these experimental and calculated results, the mechanism of the prooxidant reaction of TocH is discussed in the present paper. In Figure 1, we give the structures of the molecules studied in this work. Experimental Section Sample Preparation. As reported previously,I2 vitamin E radicals are not stable. Thus,stable 5,7diethyl~pheroxylradial (l),5,7-diisopropyltocopheroxylradical (2), 5-methyl-7-tert-butyltocopheroxyl radical (3), 5-isopropyl-7-rert-butyltocopheroxyl radical (4), 5,7-diethyl-8-methyltocopheroxylradical (S), and 5,7-diisopropyl-8-methyltocopheroxyl radical (6) are used for the present work. Preparation of 5,7-diethyltocopherol, 5,7-diisopropyltocopherol, and 5-methyl-7-tert-butyltocopherol was reported in a previous paper.I3 5-Isopropyl-7-~ert-butyltocopherol was synthesized according to a procedure described previously.14 5,7-Diethyl-8-methyltocopheroland 5,7-diisopropyl-8-methylto-
0022-365419212096-6663$03.00/00 1992 American Chemical Society
6664 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Nagaoka et al.
*'
8g
-
.e
----
IO.----11
\.
5
6
Figure 1. Molecular structures of Two's,2,6-di-tert-butyl-4-(4'-meth-
oxyphenyl)phenoxyl, and ROOHs. copherol were prepared according to a method reported previ~ u s l y . ' The ~ tocopheroxyl radicals with substituent group at the 5-, 7-, and/or 8-positions (1-6) were prepared by the Pb02 oxidation of the corresponding tocopherol under nitrogen atmosphere, 2,6-Di-tert-butyl-4-(4'-methoxyphenyl)phenoxyl(7) was prepared according to a method described previ0us1y.l~ n-Butyl hydroperoxide (8), sec-butyl hydroperoxide (9), tertbutyl hydroperoxide (lo), n-propyl hydroperoxide (ll), and isopropyl hydroperoxide (12)were prepared according to the method of Williams et al.'6J7 Benzene and ethanol-do were purchased from Wako Pure Chemical Industries and Nihon Alcohol, were dried over sodium hydride and calcium oxide, respectively, and were purified by distillation. Ethanol-d, (C2HSOD)of 99.5% purity was purchased from Aldrich and was used without further purification. When ROOH is dissolved in ethanol-d,, replacement of the hydrogen atom of the OH group of ROOH by a deuteron is easily accomplished. It was verified by proton NMR. Benzene and ethanol-dl (benzene and ethanol-do) were mixed in a 20:l volume ratio to obtain a mixed solvent, B/EtOD (B/EtOH). Meas"& The setup and the experimental procedures for the measurements of the rate constants were described in detail elsewhere.I2 Briefly, the kinetic data were obtained with a JASCO spectrometer Model UVIDEC-660 or a Shimadzu UV-2100s spectrophotometerby mixing equal volumes of solutions of 1-7 and 8-12 under a nitrogen atmosphere. All the measurements were performed at 25.0 f 0.5 OC. The pseudo-first-order rate constant for reaction 5 (/cow) was determined by following a decrease in absorbance of 1-7 (for example, at 417 nm in 2'*). koW was obtained by eq 6, where -d[Toc'] /dt = k,W[Toc']
(6)
[Toc'] refers to the molar concentration of Toc'. kow is proportional to the initial concentration of the alkyl hydroperoxide ([ROOH]) are reported previously. As reported by Mahoney et aI.,l9 under the conditions such as k2[Toc'] >> k,[TocH] (see reactions 1 and 2), the rate of disappearance of Toc' is given by -d[Toc'] /dt = Zk,[ROOH] [Toc']
(7)
From q s 6 and 7, koM is given by kobd = 2k*[ROOH] (8) Accordingly, k, can be obtained by plotting kW against [ROOH]. The He1 photoelectron spectra of ROOHs were measured with a vacuum ultraviolet photoelectron spectrometer (JASCO Model PE-1A). The setup and the experimental procedures were described in detail elsewhere.*O Briefly, the photoelectron spectrometer contains a dc discharge lamp of helium, a hemispherical electrostatic analyzer ( l k m diameter), and an electron channel multiplier (Mullard B318/01). The energy resolution of the spectrometer was about 40 meV in full width at half-maximum, and the spectrum was measured with a scan voltage of 7 meV/one channel. The calibration of the ionization energy scale of the spectrum was carried out using the two lines of Ar and Xe (15.759
5000
8 g
I ,
*
-
+
3000
--_
IO---
-
m
z W
c
z
2000
.w c
V
I
B
I
Q. 5 lONlZATlON ENERGY
IO
10.5
laV1
Figure 3. Photoelectron spectra near the first ionization threshold of Arrows 8-10 indicate the position of Is's of 8-10, respectively.
8-10.
and 12.130 eV in ionization energies,20respectively). The experiments were made under bulk-gas conditions. The measurement and analysis of the photoelectron spectrum were performed through the use of a microcomputer (NEC PC9801) system equipped with a home-built multichannel scaler.2'
Relationship between Rate Constant and Electron-Donating Capacity of ROOH Rate Constant for Reaction between Toc' and ROOH (k-,). Figure 2 shows decreases in absorbance at 417 nm with respect to time when benzene solution of 2 is mixed with benzene solution of 8-12 (1:l in volume). k,'s obtained for the reactions of 1-7 with 8-12 in benzene are listed in Table I. For a given Toc', the order of reactivity is 10 > 9 = 12 > 8 11. For a given ROOH, the change in k, for 1, 2, and 4 is due largely to steric factors (viz. 1 > 2 > 4). The methylation at the 8-position of Toc' reduces the k, for a given ROOH (1 > 5 and 2 > 6). Electnm-Doartjng Capncity of ROOH. Experimental Results. Figure 3 shows the photoelectron spectra near the first ionization threshold of 8-10. The photoelectron spectra in the ionization energy range 8-21 eV of 8-12 are available as supplementary material (see paragraph at end of paper). In the first and second ionization bands of hydroperoxide, electrons are ejected from n orbitals of the oxygen atoms, and these bands are not split.20Since the first and second ionization bands of 8-12 are not split as in the case of hydroperoxide, the vertical ionization potentials cannot be determined. Accordingly, in the present paper, we use the first adiabatic ionization potential (I,) obtained from the onset of the
-
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6665
Prooxidant Reaction of Vitamin E
TABLE I: I , for tk R ~ r ~ t ofi ~ROOH a (8-12) WMIT d (1-6) ud 7, I,,
Ip, d U* Of
8
9
3.94 X 1.34 X 1.22x 7.84 x 2.17 X 2.88 X 3.47 X
10-1 6.38 X 10-1 2.42 X lo-' 2.14 x 10-3 1.26 x lo-' 3.54 X 1W2 4.98 X 6.58 X
IJeV 9.35 IKfeV 9.26 -0.130
9.23 9.13 -0.210
1
2 3 4 5
6 7
e. 5
8
8. I
8.2
9.3
I.
8.4
ROOH (8-12)
~,/M-'s-~ 10 11 12 10-1 1.06 2.74 X lo-' 6.78 X 10-1 10-1 3.65 X 10-1 1.28 X 10-1 2.05 X 10-1 10-I 3.45 x 10-1 9.34x 1.84 x 10-1 10-2 1.91 x 10-2 7.98 x 10-3 1.21 x 10-2 10-1 6.47 X lo-' 1.95 X 10-I 3.65 X 10-1 6.95 X 2.54 X 4.72 X 9.31 X 3.20 X 5.16 X 9.11 8.77 -0.300
9.38 9.19 -0.115
9.28 9.09 -0.190
8.5
(eV1
Figure 4. Plot of I , vs I, in ROOH (8-12). This plot gives a linear fit with a slope of 1.62,an intercept of -5.93,and a correlation coefficient of 0.917.
t
1: f
€ .001
I1
12 9 11
'P
I
I
8.5
8
I*.
t
.001
i 7 . i'i,
, 8.1
8 I
I
8.5
IOVI
Figure 6. Plot of log &, vs IK of ROOH. Arrows 8-12 indicate the positions of IK'sof 8-12, respectively. The plots for 1-7 give good linear fits with s l o p of -1.00,-0.905, -1.01,-0.792,-1.02,-0.818,and -0.895, intercepts of 8.87,7.52,8.45,5.25, 8.81,6.05,and 6.84,and correlation coefficients of 0.836,0.904,0.870,0.932,0.929,0.857,and 0.875,respectively.
-
.
e
I
I I 1 1
8.3
8.4
8.2
II
8.5
IO
IlVl
Figure 5. Plot of log k, vs I, of ROOH. Arrows 8-12 indicate the positions of I,'s of 8-12, respectively. The plots for 1-7 give good linear fits with slopes of -2.01, -1.75, -2.03, -1.47, -1.91, -1.63, and -1.79, interccptsof 18.4,15.5,18.1, 11.7,17.2,13.7,and 15.3, andcorrelation coeffcients of 0.948,0.991,0.987,0.981,0.979,0.966,and 0.989,respcctively.
fmt photoelectron band. 1:s obtained for 8-12 are given in Table I. Calculated Results. Ab initio self-consistent field (SCF) calculations were carried out with the GAUSSIAN 86 programZ2 The basis set used in the present calculations is STO-3G, which has thoroughly been tested on organic molecules and reproduces the experimental results fairly ~ e l l . 2Fu ~ll geometry optimization was performed by the energy gradient method. The Koopmans' theorem first ionization potentials (IK%)for the optimized geometries were calculated with the ab initio method. ZK is simply equal in magnitude to the orbital energy (-e) of the highest occupied molecular orbital (HOMO) as shown in eq 9. Numerical
= (9) calculations were carried out at the Computer Center of Institute for Molecular Science (IMS). The optimized geometries of 8-12 are available as supplementary material. The structures of the COOH moieties are similar to those of methyl hydroper~xide.~~ The optimized geometries of 11 and 12 are similar to those of n-propyl hydroperoxide radical and isopropyl hydroperoxide radical, respect i ~ e l y ?except ~ for the COOH moieties. IK's obtained are listed in Table I.Z6 Figure 4 shows a plot of IK vs Za in ROOH. This plot is found to be linear. In fact, it is known in many fundamental organic IK
1
.01
*
001
-. 4
10
9 12
-.3
-.2
8 11
-. I
0
U*
Figure 7. Plot of log &, vs cr*. Arrows 8-12 indicate the positions of u*'s of 8-12, respectively. The plots for 1-7 give good linear fits with s l o p of -2.95,-2.55,-2.95,-2.16,-2.80,-2.38,and -2.59,intercepts of -0.818,-1.18,-1.32,-2.36,-1.02,-1.83,and -1.78,and correlation coefficients of 0.957,0.993,0.986,0.990,0.991,0.973,and 0.986,respectively.
molecules studied previouslyZothat there is a good correlation between IK's and the first ionization energies obtained by means of the photoelectron spectroscopy. But, the individual values of ZK are not in quantitative agreement with the experimental value. Relatiomhip between k, and Electron-Donathg Capacity of ROOK k,increases as the Taft's u* constant of alkyl substituents of 8-1227928(Table I) and I, (IK)of ROOH decreases. Figures 5-7 show plots of log k, vs I,, log k, vs fK, and log k, vs u*,
6666 The Journal of Physical Chemistry, Vol. 96, No. 16, 19'92
Nagaoka et al. L
TABLE 11: k,'~ for tbe R-cti~t~of ROOH psd ROOD (8-12) with Tm' (2) (kqH a d k,D, Respectively) and k,H/k*D k,H/M-l s-l k,D/M-l s-I k,Hlk,D 8 9 10 11 12
6.36 X 1.02 x 10-1 1.73 X 10-I 4.61 X 6.34 X
1.89 X 2.35 X 3.52 X 1.34 X 1.59 X
Deuterium Kinetic Isotope Effect k,'s for the reactions of ROODS with 2 (k,D) were obtained by mixing equal volumes of a benzene solution of 2 and a B/EtOD solution of 8-12 under a nitrogen atmosphere. These k-'s are listed in Table 11, together with k,'s obtained with B/EtOH solutions of ROOHs and a benzene solution of 2 (k,H) and the ratios k,H/k,D's. A deuterium kinetic isotope effect on k, is observed. Figure 8 shows plots of log k,H (log k,D) vs u* of the ROOH (ROOD). As in the case of ROOH, the plot for ROOD indicates a linear relationship. The slope of the plot for ROOD is similar to that for ROOHs. The intercept for ROOD is smaller than that for ROOH.
Mecbanism of Prooxidant Reaction of TocH The elucidation of reaction mechanisms is a major challenge in the present study. On the basis of the experimental and calculated results mentioned above, we will try to explain the mechanism of the prooxidant action of TocH in this section. k, increases as u* of ROOH and Z, (IK)decrease and the electron-donating capacity of ROOH increases. On the other hand, the substitution of an electron-donating group (methyl group) for the hydrogen atom at the 8-position of Toc' reduces k, for a given ROOH. From these results, it is considered that the electron transfer from LOOH to Toc' (mechanism 10) plays an important role in reaction 4 and is rate controlling. This Tw' LOOH -.* [Tw:----LOOH+] TwH + LOO' (10) electron transfer will correspond to the 'normal region" in Marcus theoryz9 and/or Rehm-Weller equations.30 The fast proton transfer from LOOH' to Tot:- follows the charge transfer. Since the electron is much ligher than the hydrogen atom, the electron transfer takes place before the proton transfer (hydrogen transfer). The proton transfer occurs to compensate the charge separation produced by the electron transfer. Furthermore, straight-line relationships can be seen in Figures 5-7. The straight-line relationship shown in Figures 5 and 6 can be expressed mathematically by log k, = -CIZa + C, (11) log k-, = -C,,ZK + Cz, (12) +
respectively, where C,, C,, C,,,and C,,are constants. The relation between k, and the activation energy of reaction 5 (Eact)can be given by log k, = -C,E,,t
+ C4
(13) where C, and C4are constants (the Arrhenius equation). From eqs 11-13, it is suggested that EaQincludes the terms of the energy required to eject an electron from ROOH. Accordingly, this result also supports our view that the transition state in reaction 4 has the property of the charge-transfer species such as Tot:---LOOH+. On the other hand, as shown in Figure 8, a deuterium kinetic isotope effect on k, is observed (k,H/k,D = 3.37in 8). Within the framework of direct hydrogen atom transfer (mechanism 14), Toc' + LOOH TocH LOO' (direct path) (14)
-
+
0 ROOH 0 ROOD
3.37 4.34 4.91 3.44 3.99
respectively. In Figures 5-7, the plots for a given Toc' are found to be linear and the slopes in each figure are similar to one another. It is clearly more than coincidental that the above-mentioned values cluster near the straight line throughout the various systems studied.
+
_I
.01
.001
t
10
9 12
8 11
-. 4
1
-. 3
1>1
11,
I
I I
-. 2
I I
-. I
0
I
6.
Figure 8. Plots of log k , for the reaction of ROOH (ROOD)with 2 vs u* of the ROOH (ROOD).Arrows 8-12 indicate the positions of u*'s of 8-12, respectively. These plots for ROOH and ROOD give good linear fits with slopes of -2.90 and -1.98, intercepts of -1.64 and -2.07, and correlation coefficients of 0.952 and 0.895, respectively.
h h
n L
m
E
w
1
>
OH(0D) d i s t a n c e Figure 9. Schematic sketch of potential energy curve in general proton transfer (hydrogen transfer).
the deuterium kinetic isotope effect could be expressed as the difference of the isotopic zero-point energy in the following way.' Figure 9 shows a schematic sketch of a potential energy curve in a general proton transfer (hydrogen transfer). The reaction coordinate of the direct hydrogen atom transfer nearly coincides with the normal coordinate of the OH stretching vibration. As the mass of the H (D) atom increases, the zero-point energy decreases,)' the activation energy of the direct hydrogen atom transfer increases (Figure 9),and the rate constant may decrease. Such deuterium kinetic isotope effects may fall in the range kH/kD = 2-10.' As mentioned above, one can suggest two different mechanisms that play major roles in the prooxidant reaction of TocH; the electron transfer from LOOH to Toc' (mechanism 10) and the direct hydrogen atom transfer from LOOH to Toc' (mechanism 14). Mechanism 10 is in conflict with the experimental results of the deuterium kinetic isotope effect. Mechanism 14 is also in conflict with the results that k, increases with an increase and a decrease in electron-donating capacity of ROOH and Tw', respectively. Therefore, we offer a new probable mechanism which satisfactorily accounts for all of the above-mentioned experimental and calculated results. Our new explanation for the mechanism of the prooxidant reaction of TocH is as follows. Figure 10 shows a schematic contour map of the potential energy surface in the prooxidant reaction of LOOH and deuterated LOOH (abbreviated as LOOD)with Toc'. The distance between Toc' and LOOH, that is, the distance between the oxygen atom in Toc' and that in the OH group of LOOH (rm), is plotted as ordinate and the 0-H bond length of LOOH (raH) as abscissa. I, I*, 11, III*, and I11 denote the following: I, Toc' + LOOH; I*, a partial charge-transfer species (Toc'"---LOOH6+); 11, the chargetransfer species Toc:----LOOH+; III*, TocH---Loo'; and 111, TocH + LOO'. Since there are two factors playing major roles
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6667
Prooxidant Reaction of Vitamin E
-
separate from each other ( I I P 111). Therefore, the total reaction scheme is given by mechanism 15. TN'
'0-H
(
'0-D
)
Figure 10. Schematic contour map of potential energy surface in prooxidant reaction of LOOH (LOOD). The distance between Toc' and LOOH (LOOD), that is, the distance between the oxygen atom in Toc' and that in the OH (OD) group of LOOH or LOOD (rW), is plotted as ordinate and the 0-H (0-D) bond length of LOOH (reH) or LOOD (reD) as abscissa. I, I*, 11, III*, and I11 denote the following: I, Toc' + LOOH; I*, a partial charge-transfer species Toc" ---LOOH6+; 11, the chargetransfer species Toc:----LOOH+; HI*, TocH---Loo'; and 111, TocH Loo'. The reaction trajectory corresponding to the electron transfer from LOOH to Toc' followed by fast proton transfer from LOOH+to Tot:-, the direct hydrogen atom transfer, and the real one are represented by I I1 111, I 111 (direct path), and a bold-face line (I I* III* 111), respectively. The solid and dotted parts of the bold-face line denote the classical and tunneling motions in the real reaction trajectory, respectively. The dotted parts H and D correspond to the tunneling paths for the reactions of LOOH and LOOD, respectively.
+
- - -- - -
in the reaction (electron transfer and hydrogen atom transfer), the potential energy should be a function of two coordinates. The reaction trajectory of the electron transfer from LOOH to Toc' followed by the fast proton transfer from LOOH' to Tot:is represented by I I1 I11 (mechanism 10) and the reaction trajectory of the direct hydrogen atom transfer corresponds to the direct one from I to I11 (mechanism 14). However, we consider that the real typical reaction trajectory can be represented by a bold-face line in Figure 10 and that the deuterium kinetic isotope effect observed here is due to proton tunneling. The solid and dotted parts of the bold-face line denote the classical and tunneling motions in the reaction trajectory, respectively. The dotted parts H and D correspond to the tunneling paths for the reactions of LOOH and LOOD, respectively. The tunneling path of LOOD is longer than that of LOOH. As the mass of the H (D) atom increases, the zero-point energy decreases and the thickness of the potential barrier increases (Figure 9). The probability of proton tunneling shows a decrease accompanying an increase in the mass of the H (D) atom and the thickness of the potential barrier." As a result, the deuterium kinetic isotope effect on k, is considered to be observed. Next let us look more closely at the reaction pathway on the potential energy surface shown in Figure 10. Each of the possible paths connecting I and I11 will have an energy maximum. Since there is a high and thick potential barrier between I and 111, the direct hydrogen atom transfer (direct path from I to 111) is not preferred as a path of the prooxidant reaction of TocH in either classical jump or tunneling. The reaction roughly follows, instead, the path along which the energy gradient is minimum. In the initial stage of the reaction, Toc' and LOOH approach each other and their electron clouds begin to overlap (I I*). Since Toc' and LOOH are relatively susceptible to accepting and donating an electron, respectively, the decrease in rm will induce the electron transfer from LOOH to Toc'. The final goal of this process is the transition state (saddle point, the lowest energy maximum in Figure 10) which has the property of the chargetransfer species (11). When Toc' and LOOH approach each other to some extent, the proton tunneling takes place below the transition state (the dotted part of the bold-face line in Figure 10, I* III*). Here, I* has the property of a partial charge-transfer species (Tocob---LOOH6+). Tunneling allows the proton to cut a comer on the potential energy surface. Finally, TocH and LOO'
--
-
-
-
--
+ LOOH -* ([T~c*~---LOOH~+] proton tunneling [TocH---LOO'])
TocH
+ LOO' (15)
It is, thus, considered that both of the charge transfer and the proton tunneling play important roles in the prooxidant reaction of TocH. The abovementioned mechanism satisfactorilyaccounts for the prooxidant action of TocH. Our explanation for the prooxidant reaction of vitamin E allows us to recognize its important features very well. We have shown that the antioxidant reaction also proceeds by a mechanism similar to that of the prooxidant r e a c t i ~ n . ~The . ~ implication of this paper will be important in various proton transfer reactions.
Discussion
k,H/k,D does not change appreciably with the Wl substitution of ROOH as shown in Table 11. Accordingly, it is considered that the alkyl substitution does not have a large influence on the probability of proton tunneling. The thickness of the potential barrier and the shape of the potential energy surface around the transition state (saddle point in Figure 10) will not depend largely on the substitution. The substitution is likely to affect E,, alone. In each of Figures 5-7, the slopes of the linear relationships are close to one another. These facts indicate that the substitution effect in ROOH and Toc' contributes to k , independently and additively. If this is equally true of reaction 4, we can predict kl from the substituents of LOOH and Toc'. The results shown here will be useful for future studies concerning protection of polyunsaturated lipids or fatty acids from peroxidation. We have combined two approaches-spectrophotometric technique and ab initio calculation. Comparing calculated ZK's of ROOHs with the experimental values of k*'s and Z,'s of ROOHs, we have found some systematic correlations, which are given in Figures 4 and 6. Such correlations are expected to be useful in future use of the molecular orbital calculations for predicting those experimental values without experiments. Naturally, we have examined other possibilities for the reason why k , depends on the alkyl substituent of ROOH. First, the calculated OH bond energy and the observed frequency of the OH stretching vibration of ROOH do not change appreciably with the alkyl substitution. Accordingly, it is considered that the alkyl substitution does not have a large influence on the probability of OH dissociation of ROOH in itself. Second, since the order of reactivity is prim-ROOH < seoROOH < tert-ROOH for a given Toc', the change in k,is unlikely to be determined by steric factors of ROOH. Thirdly, we examine whether the proton tunneling mechanism is required in the reaction. If the proton tunneling does not take place, the reaction roughly follows the path along which the energy gradient is minimum and the classical trajectory of the reaction passes near the saddle point in the potential energy surface (I I1 111 in Figure 10). In the present system, the saddle point has the property of the charge-transfer species as described in a previous section. If the reaction trajectory passes through such a saddle point, the charge transfer is rate controlling and the deuterium kinetic isotope effect cannot be observed. Since this explanation is not consistent with the experimental results, the proton tunneling mechanism is considered to be required. Thus, in summary, it seems most likely that the real reaction mechanism can be represented by mechanism 15.
--
Conclusion
A kinetic and ab initio study of the prooxidant action of vitamin E derivatives has been carried out. It is concluded that both charge transfer and proton tunneling play important roles in the prooxidant reaction of TocH. The transition state in the prooxidant reaction has the property of the charge-transfer species. The proton tunneling takes place below the transition state. Tunneling allows the proton to cut a comer on the potential energy surface. The mechanism of the prooxidant action of TocH can be explained in terms of reaction 15. Figure 10 shows a schematic contour map
J. Phys. Chem. 1992, 96, 6668-6674
6668
of the potential energy surface in the prooxidant reaction of TocH. Acknowledgment. We thank the Computer Center of the Institute for Molecular Science (IMS) for the use of the HITAC M-680H and S-820/80 computer and the Library Program G A U S S I A N ~ ~We . ~ ~express our sincere thanks to Dr. Shiro Urano of Tokyo Metropolitan Institute of Gerontology for his help concerning the synthesis of 5,7-diethyl-8-methyltocopheroland 5,7-diisopropyl-8-methyltocopheroland to Mr. Yasuhiro Kohno of Ehime University for his help in the early stage of this work. RWtry NO, 1, 129666-54-2; 1-OH,142129-71-3; 2, 142129-70-2; 2-OH, 142129-72-4; 3, 129666-55-3; SOH, 142129-73-5; 4, 12966656-4; &OH, 124041-50-5; 5, 129666-52-0; SOH, 142129-74-6; 6, 129666-53-1; &OH, 142129-75-7; 7, 6257-34-7; 8, 4813-50-7; 9, 13020-06-9; 10, 75-91-2; 11, 6068-96-8; 12, 3031-75-2; 02, 7782-39-0.
Supplementary Material Available: Photoelectron spectra in the ionization energy range 8-21 eV of 8-12 (Figures 11-15) and optimized geometries of 8-12 with the ab initio method (Figures 16-20) (1 1 pages). Ordering information is given on any current masthead page.
References and Notes (1) Proton-Transfer Reactions; Caldin, E., Gold, V., Eds.; Chapman and Hall: London, 1975. (2) Spectroscopy and Dynamics of Elementary Proton Transfers in Polyatomic Systems; (special issue of Chem. Phys.); Barbara, P. F., Trommsdorff, H. P., Us.North-Holland: ; Amsterdam, 1989; Vol. 136, pp 153-360. (3) Nagaoka, S.;Nagashima, U. Chem. Phys. 1989, 136, 153. (4) Nagaoka, S.Kagaku To Kogyo 1991,44, 182. (5) Nagaoka, S.;Kuranaka, A.; Tsuboi, E.; Nagashima, U.; Mukai, K. J. Phys. Chem. 1992, 96, 2754. (6) Kuranaka, A.; Sawada, K.; Nagashima, U.; Nagaoka, S.; Mukai, K. Vitamins 1991, 65, 453. (7) Reference cited in ref 5. (8) Loury, M.; Bloch, C.; Francois, R. Rev. Fr. Corps Gras 1966,13,747. (9) Terao, J.; Matsushita, S. Lipids 1986, 21, 255. (10) References cited in ref 9. (1 1) Nagaoka, S.; Okauchi, Y.; Urano, S.;Nagashima, U.; Mukai, K. J. Am. Chem. Soc. 1990,112,8921. (12) Mukai, K.;Kohno, Y.; Ishizu, K. Biochem. Biophys. Res. Commun. 1988, 155, 1046.
(13) Mukai, K.; Kageyama. Y.; Ishida, T.; Fukuda, K. J. Org. Chem. 1989, 54, 552. (14) Nilsson, J. L. G.; Sievertsson, H.; Selander, H. Acta Chem. Scand. 1968, 22, 3160. (15) Mukai, K.; Kikuchi, S.; Urano, S.Biochim. Biophys. Acra 1990, 1035, 77. (16) Williams, H. R.; Mosher, H. S.J. Am. Chem. Soc. 1954, 76,2984. (17) Williams, H. R.; Mosher, H. S. J. Am. Chem. SOC.1954, 76,2987. (18) Mukai, K.; Watanabe, Y.; Ishizu, K. Bull. Chem. Soc. Jpn. 1986,59, 2899. (19) Mahoney, L. R.; Darooge, M. A. J. Am. Chem. Soc. 1970,92,4063. (20) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press/Halsted Press: Tokyo/New York, 1981. (21) Shindo, Y. Research Institute of Applied Electricity Technical Report (in Japanese) 1984, No. 3. (22) Frisch, M. J.; Binkley, J. S.;Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; DeFrees, D. J.; Seeger, R.; Whiteside, R. A.; Fox, D. J.; Fluder, E. M.; Topiol, S.; Pople, J. A. GAUSSIAN86; Camegie-Mellon Quantum Chemistry Publishing Unit, Camegie-Melon University: Pittsburgh, registered at IMS Program Library by N. Koga, S. Yabushita, K. Sawabe, and K. Morokuma (IMS). (23) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (24) Koller, J.; Hod&k, M.; Plesnihr, B. J. Am. Chem. Soc. 1990,112, 2124. (25) Boyd, S. L.; Boyd, R. J.; Barclay, L. R. C. J. Am. Chem. Soc. 1990, 112, 5724. (26) Although ROOH geometries may show tendencies toward intramolecular hydrogen bonding between OH and C (0)atoms (bridging by hydrogen), it does not have a significant influence on IK ( methylacetylene > ethylacetylene > phenylacetylene > benzene. A triplet-state mechanism involving long-lived nonradiative excited NO2was invoked to explain the production mechanism of this chemiluminescence.
Introduction The visible photochemistry of NO2has been extensively studied due to the absorption in the entire visible spectral region, the relative ease of sample preparation, and the important role it plays in atmospheric chemistry. Recently much attention has focused on the photochemistry of NO2 following multiphoton excitation. Matsumi et a1.l determined that highly vibrationally excited 02(u’up to 25) was produced following the multiphoton excitation of NO2 with visible light. This study in addition to similar experiments by Nagai et al.2 and Jusinski et a1.j led to the determination of reactions involving O(IS) as follows: 0022-3654/92/2096-6668$03.00/0
-
-
+ nhu O(%) + N O N O + OZ(v?, AH = -597 kJ/mol
NO2
+ NO2 (1) A reaction scheme involving O( ‘D) production following multiphoton excitation was reported by Fujimura et al.:4 NO2 + h~ N O Z ( ~ B ~ ) NO2 + nhu O(ID) + N O
O(’S)
--
O(’D) + N02(2Bz) NO(A22+)+ 02 AH = -106 kJ/mol (A = 475 nm) 0 1992 American Chemical Society
(2)
