- Email: [email protected]
Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches
7 July 2000
Chemical Physics Letters 324 Ž2000. 265–272 www.elsevier.nlrlocatercplett
Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches R. Hermann a
a,)
, S. Naumov b, G.R. Mahalaxmi a , O. Brede
a
UniÕersity of Leipzig, Interdisciplinary Group Time-ResolÕed Spectroscopy, Permoserstrasse 15, D-04303 Leipzig, Germany b Institute of Surface Modification, Permoserstrasse 15, D-04303 Leipzig, Germany Received 10 March 2000
Abstract The deprotonation kinetics of phenol-type radical cations, formed via a very efficient electron transfer in the pulse radiolysis of non-polar solutions, for example n-chlorobutane, is governed mainly by electronic effects due to the nature of the phenol substituents, whereas steric effects are of minor importance; thiophenols, which are sulphur analogues of phenols, exhibit a similar behavior. Comparative quantum chemical calculations show that the calculated spin densities at the hetero atoms correlate well with the experimentally determined radical cation lifetimes. Not only the Density Functional Theory ŽDTF. B3LYP but also the semiempirical quantum chemical model PM3 can be applied for the open shell systems mentioned. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Using the pulse radiolysis technique, we recently investigated the electron transfer mechanism between primary formed n-butylchloride radical cations and very different phenolic molecules ŽArOH. w2–4x as described by reactions Ž1a. and Ž1b.
Unlike other electron transfer phenomena, the radiation-induced free electron transfer between solvent parent ions and solute molecules proceeds in a ) Corresponding author. Fax: q49-321-235-2317; e-mail: [email protected]
very rapid, unhindered manner. After a diffusioncontrolled encounter, electron transfer takes place within a few collisions. This has been monitored for solutes such as phenols and thiophenols by observing specific encounter-controlled product formations w2– 4x. At least two types of transients appear as products of the electron transfer: phenol-type radical cations ArOH Øq formed via electron donation from the aromatic ring Ž1a., and phenoxyl-type radicals ArO Ø formed via electron donation from the hetero group Ž1b. and immediate deprotonation of the solute precursor radical cation. Here we address the properties of the phenol radical cations, which can be well characterized spectroscopically and kinetically by pulse radiolysis as metastable species. The main decay channel of solute radical cations is described by reaction Ž2.
™ ArO q H
ArOH Øq
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 6 0 1 - 1
Ø
q
Ž n-C 4 H 9 Cl .
Ž 2.
266
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
Fig. 1. List of studied compounds; IRGANOX 1076 w s n-octadecyl-3-Ž3,5-di-tert.-butyl-4-hydroxyphenyl.-propionate.
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
It has thus been ascertained experimentally that the individual phenol radical cations disappear with characteristic lifetimes between about 100 ns and a few microseconds, their stability varying depending on their electronic structure, i.e. the nature and the position of the substituents. The phenols studied Žcf. Fig. 1. are characterized by an additional electronaccepting or -donating group, and in a few cases by an additional aromatic moiety, each of which causes typical spin density values at the hetero group via the ionization process. We suggest that the change in charge distribution and the corresponding spin density at the atom where deprotonation occurs determine the radical cation stability in the widest sense. Quantum chemistry ought to help quantitatively describe the kinetics of the radical cations obtained. The distributive feature of this Letter is the use of data for transient species of quite a large number of compounds for systematic correlation with quantum chemical results.
2. Methods Pulse radiolysis experiments were performed with high energy electron pulses Ž1 MeV, 15 ns duration. of a pulse transformer type electron accelerator ELIT ŽInstitute of Nuclear Physics, Novosibirsk, Russia. with an optical detection system. Details of the entire pulse radiolysis setup are contained elsewhere w1x. Using the semiempirical PM3 w5,6x and the Density Functional Theory hybrid B3LYP w7,8x with 6-31GŽd. parameter set methods, the geometries and quantum chemical parameters of a number of quite different phenols and thiophenols Žcf. Fig. 1. were calculated by means of the Gaussian 94W program. The solvent effect was taken into account by employing the Onsager ŽDipole and Sphere. Self-Consistent Reaction Field model ŽSCRFs Dipole. for the structure optimized in vacuum at the B3LYPr631GŽd. level. The restricted Hartree–Fock ŽRHF. approximation was used for the closed-shell systems and the unrestricted Hartree–Fock ŽUHF. option for the open-shell systems. We used Mulliken atomic charges, which are common in Molecular Orbital theory providing a reasonable representation of the 3D charge distribution within a molecule.
267
3. Results and discussion 3.1. Pulse radiolysis results As mentioned in the introduction, according to reactions Ž1. – Ž2., pulse radiolysis of solutions of phenols in n-butyl chloride yields solute radical cations ŽArOH Øq . and phenoxyl-type radicals ŽArO Ø.. In the case of thiophenols, instead of the phenoxyl radicals, thiol radical cations ŽArSH Øq . and thiyl radicals ŽArS Ø. are formed by an analogous mechanism w3,4x. Summaries of the spectroscopic data obtained for the generated solute radical cations, phenoxyl-type radicals and thiyl-type radicals are given in Tables 1 and 2, the transient absorptions observed appeared throughout the visible absorption range, showing superpositions of phenoxyl andror thiyl radical and phenol radical cation contributions. However, when using cation scavengers as ethanol or triethylamine ŽTEA., the origin and the nature of the absorbing transients were clearly identified.
™ TEA q ArOH q C H OH ™ C H OH q ArO
ArOH Øqq TEA ArOH Øq
2
Øq
5
2
5
q 2
Ž 3. Ø
Ž 4.
In the case of the unsubstituted phenol, the absorption spectrum is mainly caused by the wellknown phenoxyl radical absorptions around 300 and 400 nm. Phenol radical cation absorption dominates around 440 nm. The addition of ethanol or TEA scavenges the phenol radical cations such that only the absorption bands of the phenoxyl radical remain. By way of illustration, corresponding spectra of 10y2 mol dmy3 4-chlorophenol Ž4-Cl–ArOH. in BuCl solution in the absence and presence of 10y1 mol dmy3 ethanol as observed by pulse radiolysis are shown in Fig. 2. Typical time profiles of the corresponding phenoxyl radical and phenol radical cation are given in the inset. Whereas the formation of the solute radical cations always proceeds with rate constants k 1a around 2 = 10 10 dm3 moly1 sy1 , the decay times of the radical cation of the various phenols differ considerably. The experimentally observed lifetimes Žtexp . of the radical cations derived for various phenols are listed in Tables 1–3. Depending on their structural features, these values vary between 100 ns and 2.5 ms
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R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
Table 1 Optical absorption maxima of the observed phenoxyl-type radicals and phenol radical cations as well as their experimental lifetimes in n-BuCl Compound 4-OH–ArSH 4-NO 2 –ArOH ArOH 4-CN–ArOH DtBP 4-Me–ArOH 4-Cl–ArOH DtBMeP IRGANOX 1076 w 4-MeO–ArOH DtBMeOP 4-NH 2 –ArOH 4-NMe 2 –ArOH 3-NMe 2 –ArOH 1-NpOH 2-NpOH 2-ByOH 3-ByOH 4-ByOH
Radical ŽArO Ø . lexp wnmx
Radical cation ŽArOH Øq . lexp wnmx
340, 440 300, 400 300, 400 Ž300., 440
550 510 440 Ž300., 480
Ž310., 400 Ž310., 420 310, 360, 400 310, 370, 400 Ž320., 410 310, 400 320, 340, 440 360, 420 340, 460 350, 390, 520 350, 380, 470 360, 400 340, 400, 530 340, 520
Ž320, 400., 430 Ž310., 460 Ž310., 440 Ž310., 450 Ž320., 450 450 320, 340, 450 Ž360., 470 Ž340., 500 380, 420, 580 360, 460, 570 360, 380, 600 370, 390, 650 390, 680
Lifetime t ŽArOH Øq . wmsx 0.06 0.23 0.27 0.11 0.30 0.42 0.32 0.48 0.37 0.34 0.7 0.68 1.16 1.27 1.9 1.5 2.4 2.2 2.1
The errors of the lifetimes are within "10%.
w2–4x. The data in Table 2 provide information about the decay characteristics of the corresponding thiophenol radical cations, which are in the range 60–300 ns. Experimental errors for the obtained radical cation lifetimes are within "10%. The lifetimes Žtexp . given in the tables provide a useful yardstick for the stability of the phenol radical cations. The competing decay reactions of ArOH Øq are dominated by deprotonation Ž2., whereas neutral-
ization Ž5. with the counter ions Cly only plays a role in the case of large t values.
™ products
ArOH Øqq Cly
Ž 5.
The deprotonation behavior is explained by electronic effects of the substituent on the aromatic ring and the effect of steric hindrance. Thus electrondonating groups Žmethoxy, amino and dimethylamino. stabilize the radical cation and cause less
Table 2 Optical absorption maxima of the observed thiyl-type radicals and phenol radical cations as well as their experimental lifetimes in n-BuCl Compound
Radical ŽArS Ø . lexp wnmx
4-OH–ArSH 1 ArSH 3-Me–ArSH 4-Me–ArSH 2-Me–ArSH 4-MeO–ArSH 2-NpSH
520 Ž300., 490 Ž300., 460 310, 490 310, 460 Ž330., 520 390, 500, 720
The errors of the lifetimes are within "10%. a Both types ArOH Øq and ArSH Øq are involved.
Radical cation ŽArSH Øq . lexp wnmx 550 Ž300., 550 Ž300., 560 Ž310., 550 Ž300., 550 Ž330., 570 Ž390., 650
Lifetime ŽArSH Øq . t wnsx 60 90 100 150 160 210 290
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
Fig. 2. Transient optical absorption spectra taken 100 ns after the electron pulse of N2 saturated solution of 0.01 M 4-Cl–ArOH n-BuCl in the absence Ž`. and in the presence Ž^. of 0.1 M ethanol. The inset shows typical time profiles of ArO Ø Ž420 nm. and ArOH Øq Ž460 nm.. At 310 nm ArO Ø and ArOH Øq are superimposed.
spin density at the hetero atom. Conversely, electron-withdrawing groups Žnitro, chloro and cyano. pull electrons away from the ring and destabilize the corresponding radical cation, leading to higher spin density at the hetero atom of the phenol. This is evident from the fact that unsubstituted phenol and phenols with withdrawing groups Ž4-Cl– ArOH, 4-NO 2 –ArOH, 4-CN–ArOH. exhibit shorter lifetimes in the range of a few hundred nanoseconds, while phenols with electron-donating groups Ž4NH 2 –ArOH, 4-NMe 2 –ArOH, 4-MeO–ArOH, 3NMe 2 –ArOH. have longer lifetimes of 0.5–2.5 ms. The influence of the steric hindrance is reflected in the case of phenols with tertiary butyl groups as substituents ŽDtBP, DtBMeP, DtBMeOP, IRGANOX 1076 w . 1 Comparing their data Žcf. tables. with those of the corresponding unhindered compounds, it can be seen that steric hindrance affects the radical cation lifetimes much less pronouncedly than electronic effects via individual substituents for e.g. Cl, OCH 3 , CH 3 , etc. However, the steric effect by bulky groups tends to slightly increase the radical cation lifetimes. Analysis of the radical cation decay behavior of naphthols and hydroxybiphenyls showed the extended aromatic moiety to play an important role in
1
IRGANOX 1076 w : n-octadecyl-3-Ž3,5-di-tert.-butyl-4-hydroxyphenyl.-propionate.
269
the stability of the solute radical cations. The second aromatic ring yields a decrease in the spin density at the OH-group. Here steric effects are rather small. Therefore, a less pronounced change of the observable radical cation decay is expected within this class of phenols. Their cations disappear within about 2 ms, as can be seen in Table 1. Thiophenols exhibit decays of less than 300 ns – considerably shorter than the corresponding phenols. This is due to the destabilizing effect of the –SH group on the radical cation. In contrast to the phenols, the cation radical kinetics of the thiophenols are similar Žcf. the lifetimes in the tables., indicating the less pronounced effects of the electron-donating and electron-withdrawing groups and the extended aromatic rings compared to the influence of the sulphur of the hetero group. 3.2. Quantum chemical calculations and correlation of data Since deprotonation of the studied radical cations appears via the OH- or SH-group, it makes sense to consider the atomic spin densities and charge distributions at the hetero atom. By analyzing the results of quantum chemical calculations, we found that the values obtained for the spin density at the hetero atom and the alteration in the charge of the hetero group ŽSH or OH. could be correlated with the radical cation lifetimes measured. The spin density is an exact quantum chemical value, while the quantity of the charge depends upon the approximation applied. Using Mulliken charges, although the absolute values for charge distribution obtained by the two methods are very different, the differences in charges between ground state and radical cation are well comparable. The results of calculations obtained using both methods are presented in Tables 3 and 4. It can be seen that the larger the spin density at the oxygen atom and the change of the Mulliken charge at the –OH group, the shorter the lifetimes of the phenol radical cations. Fig. 3 correlates the atomic spin densities calculated at the oxygen atom with the lifetimes Žtexp . of the phenol radical cations investigated. Quantum chemical parameters obtained with the semiempirical PM3 method tally well with those calculated using DFT B3LYPr6-31GŽd..
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
270
Table 3 DFT B3LYPr6-31GŽd. and semiempirical PM3 calculated in vacuum and in BuCl Ž ´ s 8. Cation radical
texp wmsx
Log k
D wDebyex DFT
SŽO. DFT Vacuum
SŽO. DFT BuCl
SŽO. PM3 Vacuum
DQŽOH. DFT Vacuum
DQŽOH. DFT BuCl
DQŽOH. PM3 Vacuum
4-NO 2 –ArOH ArOH 4-CN–ArOH DtBP 4-Cl–ArOH 4-Me–ArOH DtBMeP IRGANOX w 4-MeO–ArOH 4-NH 2 –ArOH DtBMeOP 4-NMe 2 –ArOH 3-NMe 2 –ArOH 1-NpOH 2-NpOH 2-ByOH 3-ByOH 4-ByOH
0.23 0.27 0.11 0.30 0.32 0.42 0.48 0.37 0.34 0.68 0.70 1.16 1.27 1.9 1.5 2.4 2.2 2.1
6.64 6.57 6.96 6.52 6.50 6.38 6.32 6.43 6.47 6.17 6.15 5.94 5.90 5.72 5.82 5.66 5.68 5.68
8.76 1.46 5.61 2.28 3.68 1.60 2.54
0.204 0.197 0.170 0.176 0.168 0.177 0.167 0.157 a 0.151 0.133 0.146 0.110 0.005 0.099 0.098 0.088 0.069 0.111
0.242 0.201 0.204 0.175 0.189 0.181 0.166
0.198 0.186 0.175 0.184 0.124 0.165 0.170 0.171 0.139 0.086 0.150 0.072 -0.039 0.099 0.095 0.078 0.038 0.102
0.203 0.213 0.187 0.177 0.187 0.195 0.161
0.241 0.213 0.204 0.174 0.203 0.199 0.159
0.172 0.161 0.140 0.140 0.089 0.132 0.142 0.103 0.113 0.139
0.176 0.136 0.132 0.133 0.084 0.139 0.143 0.109 0.113 0.145
0.227 0.223 0.205 0.208 0.162 0.201 0.211 0.189 0.167 0.115 0.163 0.098 0.070 0.144 0.152 0.086 0.108 0.138
0.45 3.37 2.50 2.42 2.04 1.90 1.51 1.76 0.36 1.84
0.153 0.118 0.142 0.103 0.003 0.106 0.102 0.093 0.076 0.119
Atomic spin density at the oxygen SŽO., difference of Mulliken charges at the OH-group between cation radical and singlet ground state DQŽOH., and DFT calculated dipole D of ArOH Øq in comparison with log k s logŽ1rtexp .; texp is the experimentally obtained lifetime of the cation radicals in BuCl. Errors for the lifetimes are within "10%. a B3LYPr6-31GŽd. atomic spin density at PM3 obtained geometry.
A similar trend was found for the thiophenols Žcf. Table 4 and Fig. 4.: the larger the spin density at the sulphur atom and the change of the Mulliken charge at the –SH group, the shorter the lifetimes of the
thiophenol-type radical cation. Although the semiempirical PM3 method predicts geometrical parameters with high accuracy, the calculated spin densities on the sulphur atom differ systematically from those
Table 4 DFT B3LYPr6-31GŽd. and semiempirical PM3 calculated in vacuum and in BuCl Ž ´ s 8.O Cation radical
t wnsx
log k
D wDebyex DFT
SŽS. DFT Vacuum
SŽS. DFT BuCl
SŽS. PM3 Vacuum
DQŽSH. DFT Vacuum
DQŽSH. PM3 Vacuum
4-OH–ArSH
60
7.22
0.62
90 100 150 160 210 290
7.05 7.00 6.82 6.80 6.72 6.53
0.99 1.67 1.10 1.40 0.36 1.23
0.329 0.133 a 0.425 0.401 0.376 0.390 0.305 0.237
0.323 0.135a 0.424 0.410 0.386 0.389 0.302 0.227
0.638 0.077 a 0.797 0.783 0.735 0.755 0.670 0.657
0.351 0.161a 0.412 0.403 0.377 0.382 0.330 0.286
0.571 0.095a 0.661 0.652 0.626 0.602 0.549 0.569
ArSH 3-Me–ArSH 4-Me–ArSH 2-Me–ArSH 4-MeO–ArSH 2-NpSH
Atomic spin density at the oxygen SŽS., difference of Mulliken charges at the SH-group between cation radical and singlet ground state DQŽSH., and DFT calculated dipole D of ArSHqP in comparison with logk s logŽ1rtexp .; texp is the experimentally obtained lifetime of the cation radicals in BuCl. Errors of the lifetimes are within "10%. a Atomic spin density at oxygen SŽO.; difference of Mulliken charges at the OH-group DQŽOH. between cation radical and singlet ground state.
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
271
dipole of the cation radical: the greater the inherent dipole, the greater the effect of the reaction field. The solvent effect thus obtained is always relatively small and does not result in any significant changes to the spin densities or the charge distributions of the cations on the cation radical, as can be seen from the data of Tables 3 and 4 as shown in Figs. 3 and 4. Both methods show reasonable correlations between the spin densities calculated at the oxygen or sulphur, changes of the Mulliken charge Žcharge difference between singlet and cation radical. at the OH- or SH-group, and the lifetimes of the cation
Fig. 3. Spin density at oxygen w SŽO.x and change in charge distribution on OH w D q ŽOH.x vs. experimentally observed radical cation lifetimes wlogŽ1rtexp .x for the phenols studied Žincluding NpOHqByOH..
obtained by the DFT method. This is mainly due to the importance of the sulphur 3d orbitals w11x, which are neglected in PM3. In comparison to phenols, the thiophenols show much faster decaying radical cations. This can be explained by the higher values of spin densities and change of Mulliken charges at the hetero group in thiophenols compared to phenols, leading to more localization of charge in the former. The influence via the solvent ŽBuCl, ´ s 8. estimated by the Onsager model depends on the inherent
Fig. 4. Spin density at sulphur w SŽS.x and change in charge distribution on SH w D q ŽSH.x vs. experimentally observed radical cation lifetimes wlogŽ1rtexp .x for the thiophenols studied.
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R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
radicals. However, DFT data correlate slightly better with experimental data. The phenol cation radicals with the largest spin density and the largest change in the positive charge at the hetero atom display the shortest lifetimes and consequently the highest reactivity. This is reflected in Figs. 3 and 4, where linear regression analysis reveals good correlations between the atomic spin at the oxygen ŽFig. 3. or the sulphur ŽFig. 4.. The logŽ k ., where k s Žtexp .y1 , is the reciprocal of the decay rate measured for the corresponding radical cation.
4. Conclusions Pulse radiolysis experiments of various phenols and thiophenols yielded unstable radical cations exhibiting different first-order decay times. It has been shown that the solute cation stability is mainly determined by electronic effects due to the substituents; steric effects were found to be of minor importance. Electron-accepting substituents cause shorter radical cation lifetimes as opposed to electron-donating groups, which yield longer lifetimes for these radical cations. Calculating the spin densities and the changes in the charge distribution of the different hetero groups via the ionization, a good correlation with the experimentally determined radical cation lifetimes of the molecules studied was obtained. High spin densities at oxygen or sulphur were found in the case of phenols with electron-withdrawing substituents. As radical cations of naphthols and hydroxybiphenyls are stabilized via the delocalization of the positive charge, they have smaller spin densities at the hetero group due to the influence of the second
aromatic moiety. Therefore the corresponding lifetimes are the longest among the compounds studied here. Although thiophenols follow the same trend, the influence of the SH-group by the electron-accepting effect dominates over all other influences. Comparative calculations revealed that the semiempirical PM3 method can also be used as an alternative to the Density Functional Theory to calculate the discussed open-shell data of phenolic compounds in solution.
5. Uncited references w9,10x
References w1x O. Brede, H. Orthner, V. Zubarev, R. Hermann, J. Phys. Chem. 100 Ž1996. 7097. w2x H. Mohan, R. Hermann, S. Naumov, J.P. Mittal, O. Brede, J. Phys. Chem. A 102 Ž1998. 319. w3x G.R. Dey, R. Hermann, S. Naumov, O. Brede, Chem. Phys. Lett. 310 Ž1999. 137. w4x R. Hermann, G.R. Dey, S. Naumov, O. Brede, Phys. Chem. Chem. Phys. 2 Ž2000. . w5x J.P.P. Stewart, J. Comp. Chem. 10 Ž1989. 209, 221. w6x J.P.P. Stewart, J. Comp.-Aided Mol. Design 4 Ž1990. 1. w7x A.D. Becke, J. Chem. Phys. 98 Ž7. Ž1993. 5648. w8x C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 Ž1988. 785. w9x R. Hermann, S. Naumov, O. Brede, J. Mol. Structure ŽTheochem., submitted. w10x G.R. Mahalaxmi, R. Hermann, O. Brede, unpublished results. w11x J.M. Morley, Int. J. Quant. Chem. 66 Ž1998. 141.
Chemical Physics Letters 324 Ž2000. 265–272 www.elsevier.nlrlocatercplett
Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches R. Hermann a
a,)
, S. Naumov b, G.R. Mahalaxmi a , O. Brede
a
UniÕersity of Leipzig, Interdisciplinary Group Time-ResolÕed Spectroscopy, Permoserstrasse 15, D-04303 Leipzig, Germany b Institute of Surface Modification, Permoserstrasse 15, D-04303 Leipzig, Germany Received 10 March 2000
Abstract The deprotonation kinetics of phenol-type radical cations, formed via a very efficient electron transfer in the pulse radiolysis of non-polar solutions, for example n-chlorobutane, is governed mainly by electronic effects due to the nature of the phenol substituents, whereas steric effects are of minor importance; thiophenols, which are sulphur analogues of phenols, exhibit a similar behavior. Comparative quantum chemical calculations show that the calculated spin densities at the hetero atoms correlate well with the experimentally determined radical cation lifetimes. Not only the Density Functional Theory ŽDTF. B3LYP but also the semiempirical quantum chemical model PM3 can be applied for the open shell systems mentioned. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Using the pulse radiolysis technique, we recently investigated the electron transfer mechanism between primary formed n-butylchloride radical cations and very different phenolic molecules ŽArOH. w2–4x as described by reactions Ž1a. and Ž1b.
Unlike other electron transfer phenomena, the radiation-induced free electron transfer between solvent parent ions and solute molecules proceeds in a ) Corresponding author. Fax: q49-321-235-2317; e-mail: [email protected]
very rapid, unhindered manner. After a diffusioncontrolled encounter, electron transfer takes place within a few collisions. This has been monitored for solutes such as phenols and thiophenols by observing specific encounter-controlled product formations w2– 4x. At least two types of transients appear as products of the electron transfer: phenol-type radical cations ArOH Øq formed via electron donation from the aromatic ring Ž1a., and phenoxyl-type radicals ArO Ø formed via electron donation from the hetero group Ž1b. and immediate deprotonation of the solute precursor radical cation. Here we address the properties of the phenol radical cations, which can be well characterized spectroscopically and kinetically by pulse radiolysis as metastable species. The main decay channel of solute radical cations is described by reaction Ž2.
™ ArO q H
ArOH Øq
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 6 0 1 - 1
Ø
q
Ž n-C 4 H 9 Cl .
Ž 2.
266
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
Fig. 1. List of studied compounds; IRGANOX 1076 w s n-octadecyl-3-Ž3,5-di-tert.-butyl-4-hydroxyphenyl.-propionate.
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
It has thus been ascertained experimentally that the individual phenol radical cations disappear with characteristic lifetimes between about 100 ns and a few microseconds, their stability varying depending on their electronic structure, i.e. the nature and the position of the substituents. The phenols studied Žcf. Fig. 1. are characterized by an additional electronaccepting or -donating group, and in a few cases by an additional aromatic moiety, each of which causes typical spin density values at the hetero group via the ionization process. We suggest that the change in charge distribution and the corresponding spin density at the atom where deprotonation occurs determine the radical cation stability in the widest sense. Quantum chemistry ought to help quantitatively describe the kinetics of the radical cations obtained. The distributive feature of this Letter is the use of data for transient species of quite a large number of compounds for systematic correlation with quantum chemical results.
2. Methods Pulse radiolysis experiments were performed with high energy electron pulses Ž1 MeV, 15 ns duration. of a pulse transformer type electron accelerator ELIT ŽInstitute of Nuclear Physics, Novosibirsk, Russia. with an optical detection system. Details of the entire pulse radiolysis setup are contained elsewhere w1x. Using the semiempirical PM3 w5,6x and the Density Functional Theory hybrid B3LYP w7,8x with 6-31GŽd. parameter set methods, the geometries and quantum chemical parameters of a number of quite different phenols and thiophenols Žcf. Fig. 1. were calculated by means of the Gaussian 94W program. The solvent effect was taken into account by employing the Onsager ŽDipole and Sphere. Self-Consistent Reaction Field model ŽSCRFs Dipole. for the structure optimized in vacuum at the B3LYPr631GŽd. level. The restricted Hartree–Fock ŽRHF. approximation was used for the closed-shell systems and the unrestricted Hartree–Fock ŽUHF. option for the open-shell systems. We used Mulliken atomic charges, which are common in Molecular Orbital theory providing a reasonable representation of the 3D charge distribution within a molecule.
267
3. Results and discussion 3.1. Pulse radiolysis results As mentioned in the introduction, according to reactions Ž1. – Ž2., pulse radiolysis of solutions of phenols in n-butyl chloride yields solute radical cations ŽArOH Øq . and phenoxyl-type radicals ŽArO Ø.. In the case of thiophenols, instead of the phenoxyl radicals, thiol radical cations ŽArSH Øq . and thiyl radicals ŽArS Ø. are formed by an analogous mechanism w3,4x. Summaries of the spectroscopic data obtained for the generated solute radical cations, phenoxyl-type radicals and thiyl-type radicals are given in Tables 1 and 2, the transient absorptions observed appeared throughout the visible absorption range, showing superpositions of phenoxyl andror thiyl radical and phenol radical cation contributions. However, when using cation scavengers as ethanol or triethylamine ŽTEA., the origin and the nature of the absorbing transients were clearly identified.
™ TEA q ArOH q C H OH ™ C H OH q ArO
ArOH Øqq TEA ArOH Øq
2
Øq
5
2
5
q 2
Ž 3. Ø
Ž 4.
In the case of the unsubstituted phenol, the absorption spectrum is mainly caused by the wellknown phenoxyl radical absorptions around 300 and 400 nm. Phenol radical cation absorption dominates around 440 nm. The addition of ethanol or TEA scavenges the phenol radical cations such that only the absorption bands of the phenoxyl radical remain. By way of illustration, corresponding spectra of 10y2 mol dmy3 4-chlorophenol Ž4-Cl–ArOH. in BuCl solution in the absence and presence of 10y1 mol dmy3 ethanol as observed by pulse radiolysis are shown in Fig. 2. Typical time profiles of the corresponding phenoxyl radical and phenol radical cation are given in the inset. Whereas the formation of the solute radical cations always proceeds with rate constants k 1a around 2 = 10 10 dm3 moly1 sy1 , the decay times of the radical cation of the various phenols differ considerably. The experimentally observed lifetimes Žtexp . of the radical cations derived for various phenols are listed in Tables 1–3. Depending on their structural features, these values vary between 100 ns and 2.5 ms
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R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
Table 1 Optical absorption maxima of the observed phenoxyl-type radicals and phenol radical cations as well as their experimental lifetimes in n-BuCl Compound 4-OH–ArSH 4-NO 2 –ArOH ArOH 4-CN–ArOH DtBP 4-Me–ArOH 4-Cl–ArOH DtBMeP IRGANOX 1076 w 4-MeO–ArOH DtBMeOP 4-NH 2 –ArOH 4-NMe 2 –ArOH 3-NMe 2 –ArOH 1-NpOH 2-NpOH 2-ByOH 3-ByOH 4-ByOH
Radical ŽArO Ø . lexp wnmx
Radical cation ŽArOH Øq . lexp wnmx
340, 440 300, 400 300, 400 Ž300., 440
550 510 440 Ž300., 480
Ž310., 400 Ž310., 420 310, 360, 400 310, 370, 400 Ž320., 410 310, 400 320, 340, 440 360, 420 340, 460 350, 390, 520 350, 380, 470 360, 400 340, 400, 530 340, 520
Ž320, 400., 430 Ž310., 460 Ž310., 440 Ž310., 450 Ž320., 450 450 320, 340, 450 Ž360., 470 Ž340., 500 380, 420, 580 360, 460, 570 360, 380, 600 370, 390, 650 390, 680
Lifetime t ŽArOH Øq . wmsx 0.06 0.23 0.27 0.11 0.30 0.42 0.32 0.48 0.37 0.34 0.7 0.68 1.16 1.27 1.9 1.5 2.4 2.2 2.1
The errors of the lifetimes are within "10%.
w2–4x. The data in Table 2 provide information about the decay characteristics of the corresponding thiophenol radical cations, which are in the range 60–300 ns. Experimental errors for the obtained radical cation lifetimes are within "10%. The lifetimes Žtexp . given in the tables provide a useful yardstick for the stability of the phenol radical cations. The competing decay reactions of ArOH Øq are dominated by deprotonation Ž2., whereas neutral-
ization Ž5. with the counter ions Cly only plays a role in the case of large t values.
™ products
ArOH Øqq Cly
Ž 5.
The deprotonation behavior is explained by electronic effects of the substituent on the aromatic ring and the effect of steric hindrance. Thus electrondonating groups Žmethoxy, amino and dimethylamino. stabilize the radical cation and cause less
Table 2 Optical absorption maxima of the observed thiyl-type radicals and phenol radical cations as well as their experimental lifetimes in n-BuCl Compound
Radical ŽArS Ø . lexp wnmx
4-OH–ArSH 1 ArSH 3-Me–ArSH 4-Me–ArSH 2-Me–ArSH 4-MeO–ArSH 2-NpSH
520 Ž300., 490 Ž300., 460 310, 490 310, 460 Ž330., 520 390, 500, 720
The errors of the lifetimes are within "10%. a Both types ArOH Øq and ArSH Øq are involved.
Radical cation ŽArSH Øq . lexp wnmx 550 Ž300., 550 Ž300., 560 Ž310., 550 Ž300., 550 Ž330., 570 Ž390., 650
Lifetime ŽArSH Øq . t wnsx 60 90 100 150 160 210 290
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
Fig. 2. Transient optical absorption spectra taken 100 ns after the electron pulse of N2 saturated solution of 0.01 M 4-Cl–ArOH n-BuCl in the absence Ž`. and in the presence Ž^. of 0.1 M ethanol. The inset shows typical time profiles of ArO Ø Ž420 nm. and ArOH Øq Ž460 nm.. At 310 nm ArO Ø and ArOH Øq are superimposed.
spin density at the hetero atom. Conversely, electron-withdrawing groups Žnitro, chloro and cyano. pull electrons away from the ring and destabilize the corresponding radical cation, leading to higher spin density at the hetero atom of the phenol. This is evident from the fact that unsubstituted phenol and phenols with withdrawing groups Ž4-Cl– ArOH, 4-NO 2 –ArOH, 4-CN–ArOH. exhibit shorter lifetimes in the range of a few hundred nanoseconds, while phenols with electron-donating groups Ž4NH 2 –ArOH, 4-NMe 2 –ArOH, 4-MeO–ArOH, 3NMe 2 –ArOH. have longer lifetimes of 0.5–2.5 ms. The influence of the steric hindrance is reflected in the case of phenols with tertiary butyl groups as substituents ŽDtBP, DtBMeP, DtBMeOP, IRGANOX 1076 w . 1 Comparing their data Žcf. tables. with those of the corresponding unhindered compounds, it can be seen that steric hindrance affects the radical cation lifetimes much less pronouncedly than electronic effects via individual substituents for e.g. Cl, OCH 3 , CH 3 , etc. However, the steric effect by bulky groups tends to slightly increase the radical cation lifetimes. Analysis of the radical cation decay behavior of naphthols and hydroxybiphenyls showed the extended aromatic moiety to play an important role in
1
IRGANOX 1076 w : n-octadecyl-3-Ž3,5-di-tert.-butyl-4-hydroxyphenyl.-propionate.
269
the stability of the solute radical cations. The second aromatic ring yields a decrease in the spin density at the OH-group. Here steric effects are rather small. Therefore, a less pronounced change of the observable radical cation decay is expected within this class of phenols. Their cations disappear within about 2 ms, as can be seen in Table 1. Thiophenols exhibit decays of less than 300 ns – considerably shorter than the corresponding phenols. This is due to the destabilizing effect of the –SH group on the radical cation. In contrast to the phenols, the cation radical kinetics of the thiophenols are similar Žcf. the lifetimes in the tables., indicating the less pronounced effects of the electron-donating and electron-withdrawing groups and the extended aromatic rings compared to the influence of the sulphur of the hetero group. 3.2. Quantum chemical calculations and correlation of data Since deprotonation of the studied radical cations appears via the OH- or SH-group, it makes sense to consider the atomic spin densities and charge distributions at the hetero atom. By analyzing the results of quantum chemical calculations, we found that the values obtained for the spin density at the hetero atom and the alteration in the charge of the hetero group ŽSH or OH. could be correlated with the radical cation lifetimes measured. The spin density is an exact quantum chemical value, while the quantity of the charge depends upon the approximation applied. Using Mulliken charges, although the absolute values for charge distribution obtained by the two methods are very different, the differences in charges between ground state and radical cation are well comparable. The results of calculations obtained using both methods are presented in Tables 3 and 4. It can be seen that the larger the spin density at the oxygen atom and the change of the Mulliken charge at the –OH group, the shorter the lifetimes of the phenol radical cations. Fig. 3 correlates the atomic spin densities calculated at the oxygen atom with the lifetimes Žtexp . of the phenol radical cations investigated. Quantum chemical parameters obtained with the semiempirical PM3 method tally well with those calculated using DFT B3LYPr6-31GŽd..
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
270
Table 3 DFT B3LYPr6-31GŽd. and semiempirical PM3 calculated in vacuum and in BuCl Ž ´ s 8. Cation radical
texp wmsx
Log k
D wDebyex DFT
SŽO. DFT Vacuum
SŽO. DFT BuCl
SŽO. PM3 Vacuum
DQŽOH. DFT Vacuum
DQŽOH. DFT BuCl
DQŽOH. PM3 Vacuum
4-NO 2 –ArOH ArOH 4-CN–ArOH DtBP 4-Cl–ArOH 4-Me–ArOH DtBMeP IRGANOX w 4-MeO–ArOH 4-NH 2 –ArOH DtBMeOP 4-NMe 2 –ArOH 3-NMe 2 –ArOH 1-NpOH 2-NpOH 2-ByOH 3-ByOH 4-ByOH
0.23 0.27 0.11 0.30 0.32 0.42 0.48 0.37 0.34 0.68 0.70 1.16 1.27 1.9 1.5 2.4 2.2 2.1
6.64 6.57 6.96 6.52 6.50 6.38 6.32 6.43 6.47 6.17 6.15 5.94 5.90 5.72 5.82 5.66 5.68 5.68
8.76 1.46 5.61 2.28 3.68 1.60 2.54
0.204 0.197 0.170 0.176 0.168 0.177 0.167 0.157 a 0.151 0.133 0.146 0.110 0.005 0.099 0.098 0.088 0.069 0.111
0.242 0.201 0.204 0.175 0.189 0.181 0.166
0.198 0.186 0.175 0.184 0.124 0.165 0.170 0.171 0.139 0.086 0.150 0.072 -0.039 0.099 0.095 0.078 0.038 0.102
0.203 0.213 0.187 0.177 0.187 0.195 0.161
0.241 0.213 0.204 0.174 0.203 0.199 0.159
0.172 0.161 0.140 0.140 0.089 0.132 0.142 0.103 0.113 0.139
0.176 0.136 0.132 0.133 0.084 0.139 0.143 0.109 0.113 0.145
0.227 0.223 0.205 0.208 0.162 0.201 0.211 0.189 0.167 0.115 0.163 0.098 0.070 0.144 0.152 0.086 0.108 0.138
0.45 3.37 2.50 2.42 2.04 1.90 1.51 1.76 0.36 1.84
0.153 0.118 0.142 0.103 0.003 0.106 0.102 0.093 0.076 0.119
Atomic spin density at the oxygen SŽO., difference of Mulliken charges at the OH-group between cation radical and singlet ground state DQŽOH., and DFT calculated dipole D of ArOH Øq in comparison with log k s logŽ1rtexp .; texp is the experimentally obtained lifetime of the cation radicals in BuCl. Errors for the lifetimes are within "10%. a B3LYPr6-31GŽd. atomic spin density at PM3 obtained geometry.
A similar trend was found for the thiophenols Žcf. Table 4 and Fig. 4.: the larger the spin density at the sulphur atom and the change of the Mulliken charge at the –SH group, the shorter the lifetimes of the
thiophenol-type radical cation. Although the semiempirical PM3 method predicts geometrical parameters with high accuracy, the calculated spin densities on the sulphur atom differ systematically from those
Table 4 DFT B3LYPr6-31GŽd. and semiempirical PM3 calculated in vacuum and in BuCl Ž ´ s 8.O Cation radical
t wnsx
log k
D wDebyex DFT
SŽS. DFT Vacuum
SŽS. DFT BuCl
SŽS. PM3 Vacuum
DQŽSH. DFT Vacuum
DQŽSH. PM3 Vacuum
4-OH–ArSH
60
7.22
0.62
90 100 150 160 210 290
7.05 7.00 6.82 6.80 6.72 6.53
0.99 1.67 1.10 1.40 0.36 1.23
0.329 0.133 a 0.425 0.401 0.376 0.390 0.305 0.237
0.323 0.135a 0.424 0.410 0.386 0.389 0.302 0.227
0.638 0.077 a 0.797 0.783 0.735 0.755 0.670 0.657
0.351 0.161a 0.412 0.403 0.377 0.382 0.330 0.286
0.571 0.095a 0.661 0.652 0.626 0.602 0.549 0.569
ArSH 3-Me–ArSH 4-Me–ArSH 2-Me–ArSH 4-MeO–ArSH 2-NpSH
Atomic spin density at the oxygen SŽS., difference of Mulliken charges at the SH-group between cation radical and singlet ground state DQŽSH., and DFT calculated dipole D of ArSHqP in comparison with logk s logŽ1rtexp .; texp is the experimentally obtained lifetime of the cation radicals in BuCl. Errors of the lifetimes are within "10%. a Atomic spin density at oxygen SŽO.; difference of Mulliken charges at the OH-group DQŽOH. between cation radical and singlet ground state.
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
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dipole of the cation radical: the greater the inherent dipole, the greater the effect of the reaction field. The solvent effect thus obtained is always relatively small and does not result in any significant changes to the spin densities or the charge distributions of the cations on the cation radical, as can be seen from the data of Tables 3 and 4 as shown in Figs. 3 and 4. Both methods show reasonable correlations between the spin densities calculated at the oxygen or sulphur, changes of the Mulliken charge Žcharge difference between singlet and cation radical. at the OH- or SH-group, and the lifetimes of the cation
Fig. 3. Spin density at oxygen w SŽO.x and change in charge distribution on OH w D q ŽOH.x vs. experimentally observed radical cation lifetimes wlogŽ1rtexp .x for the phenols studied Žincluding NpOHqByOH..
obtained by the DFT method. This is mainly due to the importance of the sulphur 3d orbitals w11x, which are neglected in PM3. In comparison to phenols, the thiophenols show much faster decaying radical cations. This can be explained by the higher values of spin densities and change of Mulliken charges at the hetero group in thiophenols compared to phenols, leading to more localization of charge in the former. The influence via the solvent ŽBuCl, ´ s 8. estimated by the Onsager model depends on the inherent
Fig. 4. Spin density at sulphur w SŽS.x and change in charge distribution on SH w D q ŽSH.x vs. experimentally observed radical cation lifetimes wlogŽ1rtexp .x for the thiophenols studied.
272
R. Hermann et al.r Chemical Physics Letters 324 (2000) 265–272
radicals. However, DFT data correlate slightly better with experimental data. The phenol cation radicals with the largest spin density and the largest change in the positive charge at the hetero atom display the shortest lifetimes and consequently the highest reactivity. This is reflected in Figs. 3 and 4, where linear regression analysis reveals good correlations between the atomic spin at the oxygen ŽFig. 3. or the sulphur ŽFig. 4.. The logŽ k ., where k s Žtexp .y1 , is the reciprocal of the decay rate measured for the corresponding radical cation.
4. Conclusions Pulse radiolysis experiments of various phenols and thiophenols yielded unstable radical cations exhibiting different first-order decay times. It has been shown that the solute cation stability is mainly determined by electronic effects due to the substituents; steric effects were found to be of minor importance. Electron-accepting substituents cause shorter radical cation lifetimes as opposed to electron-donating groups, which yield longer lifetimes for these radical cations. Calculating the spin densities and the changes in the charge distribution of the different hetero groups via the ionization, a good correlation with the experimentally determined radical cation lifetimes of the molecules studied was obtained. High spin densities at oxygen or sulphur were found in the case of phenols with electron-withdrawing substituents. As radical cations of naphthols and hydroxybiphenyls are stabilized via the delocalization of the positive charge, they have smaller spin densities at the hetero group due to the influence of the second
aromatic moiety. Therefore the corresponding lifetimes are the longest among the compounds studied here. Although thiophenols follow the same trend, the influence of the SH-group by the electron-accepting effect dominates over all other influences. Comparative calculations revealed that the semiempirical PM3 method can also be used as an alternative to the Density Functional Theory to calculate the discussed open-shell data of phenolic compounds in solution.
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