J. Phys. Chem. 1995,99, 11387-11391
11387
Absorption Spectra of Ethynyl, Ethenyl, and Phenyl Peroxyl Radicals? M. Krauss* Center for Advanced Research in Biotechnology, National Institute of Standards and Technology, 9600 Gudelsky Drive, Rockville, Maryland 20850
R. Osman Department of Physiology and Biophysics, Mount Sinai School of Medicine, City University of New York, New York, New York 10029 Received: January 4, 1995; In Final Form: May 15, 1995@
Both the ethenyl and phenyl peroxyl radical absorb in the visible region in aqueous solution. Since the absorption spectra of alkyl peroxyl radicals is invariably in the ultraviolet, these observations were initially surprising. Ab initio determination of the electronic structure of the ground and excited states of the ethynyl, ethenyl, and phenyl peroxyl radicals provides a fundamental understanding of these electronic transitions. The electronic excited states of these radicals are low in energy because the n-type open-shell orbital localized on the oxygen atoms in the ground state couples to a relatively low-energy n or conjugated orbital system in the excited state. In the case of both the ethynyl and phenyl peroxyl radicals, the excitation of the C-C bond n orbital is relatively high in energy, and the in vacuo prediction for the absorption is far to the blue of the transition observed in solution. Large spectral red shifts are predicted, however, because all the radicals are polar in the ground state, and the dipole moment in the relevant excited state is substantially larger. In addition, for the ethenyl peroxyl radical there are two isomeric forms whose ground states are close in energy, and the observed spectrum can be assigned to the convoluted spectra of both isomers.
Introduction Peroxyl radicals are important intermediates in a wide variety of oxidative biochemical reactions.' Both nucleic acid and lipid peroxyl radicals are of interest, and recent experimental studies have shown that they can be identified from visible absorption Addition of oxygen to alkyl radicals yields peroxyl radicals that absorb below 250 nm.* However, the proximity of the localized open-shell orbital to an unsaturated bond, like that of the vinyl radical, suggests that a visible absorption is to be expected. Analogous electronic circumstances were studied by us in the hydrogen and hydroxyl adducts of ~ r a c i l . ~ Proximity of the open-shell and the possibility of coupling it to an unsaturated bond significantly lowered the excited states. In addition to the proximity of the bonds, the ease with which the 7~ electrons can be excited will also affect the extent of the coupling and the lowering of the energy of the excited state. In order to develop these ideas further, this study will examine three peroxyl radicals that are representativeof different degrees of unsaturation and coupling between the open shell and the n system. These are the ethynyl, ethenyl, and phenyl peroxyl radicals. The spectrum of the ethynyl peroxyl radical has not been presented either experimentally or theoretically. The ground state geometry has been obtained by ab initio gradient optimization at the MP2 level.I0 Boyd et al. also examined the ground state structural properties of the ethenyl peroxyl radical.I0 A comparison of the C-C distance in the correlated ethynyl peroxyl radical with the corresponding carbon radical shows that the multiple bond character is reduced when the C-0 bond is formed. However, for the ethynyl peroxyl radical the C-C bond distance is sufficiently small that significcpt multiple bond + Peroxyl calculations reported at the 8th Intemational Congress of Quantum Chemistry, Prague, June 20, 1994. Abstract published in Advance ACS Abstracts, June 15, 1995. @
0022-3654/95/2099-11387$09.00/0
character is still present. The differences in the electronic behavior between the ethynyl and ethenyl radicals should then provide insight into the mechanism for yielding low-energy excited states. The experimental spectrum of the ethenyl peroxyl radical is too broad for any structural information to be obtained.2 Theoretical calculationsI0suggest that cis and trans isomers are close in energy, but the experiment provides no means of identifying which isomer is present. A calculation of isomeric spectra for peroxynitrite anion' showed that substantial differences exist in the spectral peaks between the two isomers. Such differences could be exploited for the assignment of the peroxyl radical spectra as well as provide insight into formation mechanisms of these radicals. A broad visible absorption with a peak around 490 nm has been assigned to the phenyl peroxyl radical in ~ o l u t i o n It. ~ ~ ~ ~ ~ has also been suggested that the gas-phase radical may have an absorption peak in the same wavelength region.I2 A broad peak has been observed in the region between 495 and 530 nm and attributed to the phenyl peroxyl radical.6 This gas-phase observation does not determine the peak absorption. Since the width at half-maximum of the solution spectrum exceeds 180 nm,5,7the position of the gas-phase peak is still not known. Such a low-energy excited state would suggest strong coupling between the n orbitals of the phenyl ring and the open-shell peroxyl orbital. Since the phenyl radical maintains the carbon skeletal geometry and the resonance stabilization of benzene,I3 such strong coupling is surprising. The transition is essentially the transfer of a ring 7~ orbital electron to the less strongly bound open-shell orbital on the oxygen. As we shall see, understanding the phenyl peroxyl absorption requires an analysis of the solvent shift for this polar molecule. In vacuo excitation energies will not provide correct assignments for these systems. Calculation of the excited states of radicals requires a multiconfiguration description of the electronic state. Multi0 1995 American Chemical Society
Krauss and Osman
11388 J. Phys. Chem., Vol. 99, No. 29, 1995 reference first-order configuration interaction (MR-FOCI) or self-consistent-field (MCSCF) descriptions of the excited states of benzene,14 the benzyl radical,l5,l6and the phenoxy1 provide reasonably accurate excitation energies. MCSCF calculations will be chosen here for the peroxyl radicals since the relevant valence excited molecular orbitals can be incorporated more readily in this method. Although reaction field interactions could be included in the Hamiltonian, comparable results were obtained between classical and quantum reaction field calculations of the spectral shifts for indole radicals.I8 A simpler and qualitatively correct estimate of the solvent shift can be obtained with the classical reaction field from the knowledge of the ground and excited state dipole moment^.'^,*^ This paper will examine the absorption spectra of the ethynyl, ethenyl, and phenyl peroxyl radicals. Such an analysis depends on the electronic properties at only the geometry of the ground state of the system. The energy surfaces of these systems are, in fact, quite complicated, and the peroxyl ground states represent quite high-energy local minima. We have begun to explore other interesting facets of these systems such as the properties of oxy-epoxide radicals and various dissociative energy surfaces in both the ground and excited states but will restrict ourselves here primarily to the electronic properties at the peroxyl ground state geometry. Method Optimized structures are generated with analytic gradients at the restricted open-shell Hartree-Fock ( R O W and complete active space multiconfiguration self-consistent-field (CASMCSCF) levels using the GAMESSZ1suite of codes. The ground states of all radicals are determined to be planar at the ROHF level. Optimization at the MCSCF level assumed a planar ground and excited state. The excited states may not be planar. The excited state energies were obtained in MCSCF calculations where the two-particle density matrix was constructed by averaging the ground and excited state. The core orbitals are replaced by compact effective potentials (CEP) used with the concomitant CEP-31G basis (DZ basis).22 Polarization functions were added only to the heavy atoms for the ethynyl and ethenyl radicals (DZd basis) but not to the phenyl due to the expense for the CAS-MCSCF calculation. However, the phenyl peroxyl radical was optimized at the ROHF level with the DZd basis and the CAS excitation energies obtained at this geometry with the DZ basis augmented with d polarization functions on just the oxygen atoms. The active space should include the valence orbitals required to correlate the C-0 and 0-0 bonding orbitals in the ethynyl and ethenyl peroxyl radicals as well as the antibonding n* of the C-C bond. Three valence orbitals can satisfy this condition. For the phenyl peroxyl radical, however, two valence JC orbitals are required to study the valence excited states of the phenyl moiety as was shown for benzeneI4 and the phenyl radical.23 Unfortunately, only three valence orbitals can be accommodated at this time in the active space for the MCSCF calculation. Two MCSCF calculations were performed starting either with two JC* and one a* or with one JC* and two a* valence orbitals. The total energies in both calculations are comparable, and the excitation energy differs between the two by only 600 cm-l. The In* natural orbitals with the largest occupation numbers are very similar in both cases. A similar behavior is observed with the l a * natural orbitals. The 1x* natural orbital serves to mainly correlate the C-0 bond and is delocalized to some extent on the ring while l a * is localized to correlate the 0-0 bond. The occupation number of the remaining natural orbitals, 2x* and 2a*, in the two cases is 0.06 and 0.04, respectively.
When the value of an occupation number in a specific state falls below 0.04, the effect on the relative energetics is assumed to be small. This relaxes slightly the criteria proposed by Boffil and PulayZ4for constructing an active space. The three valence orbitals are then chosen with two o* orbitals and one x* orbital to correlate the C-0 and 0-0 bonds as determined by solution of the MCSCF equations. The geometry optimizations of the ground and first excited states were performed separately with the active space containing two valence orbitals, 1n* and la*. This choice was required by the available computer resources since the number of configurations in the CAS increases rapidly with the number of valence orbitals. However, the dominant valence orbitals are evidently included since the smallest occupation number of the natural orbitals will not be large irrespective of the choice of valence orbital symmetries. Excitation energies for the ethynyl radical are reported with two active space cases to show the result when the number of valence orbitals is reduced from three to two. In case I 15 electrons are distributed over 11 active orbitals while in case I1 the active space consists of 21 electrons and 13 orbitals. This corresponds to choosing seven doubly occupied orbitals, one open shell orbital, and three additional valence orbitals in case I (7D,lA,3V) and a lOD,lA,2V active space for case 11. For the ethenyl radical 15 electrons are also distributed among 11 active orbitals in case I (7D,lA,3V). For case I1 each of the four states is optimized separately with an 11D,lA,2V active space. Here the adiabatic energy differences are reported rather than the vertical excitations given in case I. Three cases are reported in Table 3 for the phenyl peroxyl radical which all use the average density matrix for the ground and first excited state. All cases distribute 15 electrons in 11 orbitals (7D, 1A,3V). Three different geometries are compared. Cases I and I1 use the optimized geometry at the ROHF and CAS-MCSCF level, respectively, while the geometry with the DZd basis is chosen for case III. The DZd basis is prohibitively large for the MCSCF optimization at present, but the singlepoint energy calculation is tractable with the oxygen atoms represented with a polarized basis set. Results and Discussion The optimized geometries for the ground states of all peroxyl radicals at the MCSCF level are given in Table 1: Table 1 also includes ROHF optimized geometries for the phenyl peroxyl radical with both a DZ and a polarized DZd basis. As expected, the biggest differences between the ROHF and MCSCF geometries are in the C - 0 and 0-0 bonds that are correlated. A polarized basis set has a substantial effect on the 0-0 bond distance for the phenyl peroxyl radical. Excited state geometries are provided for the ethenyl peroxyl radical in Table 2. The ground state geometry of the ethynyl and ethenyl peroxyl radicals can be compared with earlier calculations.I0 The MC optimized geometry of the ethynyl radical is in reasonable agreement with the earlier geometry determined at the MP2 level. The 0-0 bond distance agrees to within 0.01 8, while the C-C distance is larger here by about 0.03 8,with a smaller value by 0.01 8, for the C - 0 distance. The MP2 structure is reported as linear from H to 01 in the MP2 structure but is found to be slightly bent in this study. As noted in the figure accompanying Table 1, the 0-0 bond is trans to the C-C bond. The ethenyl peroxyl structure was determined earlier at the ROHF level.8 Comparatively small changes observed between optimized geometries at the ROHF and MP2 levels for several test cases, including the ethynyl structure, were used to argue
Ethynyl, Ethenyl, and Phenyl Peroxyl Radicals
J. Phys. Chem., Vol. 99, No. 29, 1995 11389
TABLE 1: Geometry of Ground State Peroxyl Radicals
s-cis ethynyl Cl-C2 C2-03 03-04 C1-H5 Cl-H6 C2-H7 1-2-3 2-3-4 5-1-2 6-1-2 1-2-7
trans-ethenyl
’
cis-ethenyl
Bond Distances (A) 1.242 1.363 1.334 1.388 1.331 1.369 1.061 1.08 1 1.081 1.077
1.363 1.389 1.368 1.081 1.078 1.078
Angles (deg) 118.8 110.7 119.0 121.7 127.1
126.3 113.2 117.8 122.7 125.2
164.5 111.6 173.4
TABLE 2: Geometry of Excited States of the Ethenyl Peroxyl Radical“
phenyl DZ MC
DZ ROHF
DZd ROHF
Cl-C2 C2-C3 c3-c4 c4-c5 C5-C6 C6-01 01-02 C1-H C2-H C3-H C4-H C5-H
Bond Distances (A) 1.409 1.407 1.411 1.412 1.411 1.408 1.412 1.412 1.400 1.402 1.482 1.421 1.423 1.361 1.089 1.089 1.090 1.090 1.090 1.090 1.090 1.090 1.087 1.087
1.407 1.412 1.409 1.412 1.404 1.404 1.313 1.089 1.090 1.090 1.090 1.086
1-2-3 2-3-4 3-4-5 4-5-6 5-6-7 6-7-8
120.1 120.1 120.6 117.7 123.5 112.9
Angles (deg) 120.3 119.7 120.8 118.1 123.4 115.2
120.3 119.6 120.9 118.4 123.7 113.8
that the correlation effects would be small for the larger systems8 We find that electron correlation has a significant effect on the structure of the cis- and trans-ethenyl peroxyl isomers. At the ROHF level the C - 0 bond distance is calculated to be 1.375 8, for the trans isomer with the C-C and 0-0 distances considerably shorter at 1.316 and 1.298 A, respectively.8 Thus, the C-C bond distance in the peroxyl radical is even shorter than it is in the vinyl radicals8 As seen in Table 1, correlating the ethenyl peroxyl radical substantially increases the C-C and 0-0 bond distances to 1.363 and 1.369 A, respectively, with a small increase in C - 0 to 1.388 A. The double-bond character in the C-C bond has been reduced by mixing with the antibonding C-C orbital. There is little difference in the bond distances between the ethenyl peroxyl isomers in the ground state. Correlation has a substantial effect also on the C-0 and 0-0 distances in the phenyl peroxyl radical, but polarization functions are also important as seen in the ROHF optimization with DZd functions on the oxygen atoms. The ROHF(DZd) structure is similar to that reported at the UHF 1 e ~ e l . l MCSCF ~ energy calculations are reported using the truncated DZd basis, but optimization of the geometry with this basis at the MCSCF level was not not feasible at this time. It is evident that the structure of the phenyl peroxyl radical is not calculated accurately at this time. For an MCSCF optimized structure with a polarized basis
a
Cl-C2 C2-03 03-04 C1-H5 Cl-H6 C2-H7
A”(2) A’( 1) trans-Etheny1 1.432 1.363 1.273 1.385 1.739 1.448 1.079 1.081 1.080 1.081 1.081 1.077
1.538 1.379 1.449 1.077 1.077 1.072
1-2-3 2-3-4 5-1-2 6-1-2 1-2-7
120.5 104.7 119.2 120.6 121.7
119.4 107.6 118.8 121.8 125.0
111.2 108.0 119.7 119.0 129.0
Cl-C2 C2-03 03-04 Cl-H5 Cl-H6 C2-H7
cis-Ethenyl 1.434 1.273 1.625 1.080 1.073 1.085
1.363 1.375 1.440 1.081 1.079 1.078
1.545 1.384 1.448 1.078 1.076 1.073
1-2-3 2-3-4 5-1-2 6-1-2 1-2-7
126.8 107.5 117.4 121.5 120.5
127.2 110.0 117.9 123.2 124.7
122.7 111.1 117.8 121.5 126.3
A’@)
Bond distances are in angstroms and angles in degrees.
TABLE 3: Peroxsl Radical Excitation Enereies ~~
total energy excitation energy A”(1) (au) A’(1) (cm-I) A”(2) (nm) A’(2) (nm) ethynyl (15,11)‘ (21,131 ethenyl (s-trans) (15,111 (23,l4)b ethenyl (s-cis) (15,111 (23, 14)b phenyl (15,111 (DZ-ROHF)’ (15,11) (DZ-MCSCF)’ (1.511) (DZd-ROHF)’
-42.819 81 -42.752 89
11036 5400
296.1 300.4
296.8 291.7
-44.072 27 -44.038 87
8107 6378
382.8 488.2
248.7 299.5
-44.070 64 -44.037 23
6703 5325
430.0 531.6
242.9 292.2
-67.049 36
5260
351.7
201.1
-67.050 88
4181
361.0
-67.086 86
5501
336.6
(15,ll) represents 15 electrons in 11 orbitals active space. Adiabatic excitation energy. Level of optimization calculation.
set, it is possible that the C - 0 and 0-0 bonds may be considerably shortened. ‘We can study the sensitivity of the absorption spectra to the geometry and particularly the 0-0 distance by using the various optimized geometries obtained here which span a range of 0-0 distances. The phenyl C-C distances are close to those calculated for the phenyl radical and benzene with the same baskz3 suggesting little reduction in the resonance behavior of the n orbitals in benzene. The in vacuo excitation energies are given in Table 3 at the geometries of the MCSCF optimized ground state structures of the ethynyl and ethenyl peroxyl radicals with the exception of case I1 for the ethenyl radical where the adiabatic energy differences are also provided. For the phenyl peroxyl radical, comparable excitation energies are found for the three geometries listed in Table 1 even though the geometries vary substantially. The lowest A’ excited state represents the rotation of the orbital hole localized on the oxygens from a n to a planar orientation. The orbital hole is not delocalized in the A‘ excited states so the excitation energy to the second A’ state, A’(2),
11390 J. Phys. Chem., Vol. 99, No. 29, 1995
Krauss and Osman
TABLE 4: Solvent Shift of Ethenyl and Phenyl Excitation Energies excitation energy (nm) dipole moment (D) A"( 1) A"(2) in vacuo reaction field" ethynyl
ethenyl cis trans
phenyl
2.26
6.09
300.4
332.2
2.48 2.41 3.21
3.85 4.43 8.37
430.0 382.8 361.0
462.2 403.6 410.8, 458.3
Reaction field radius a = 5.84, 6.09, 6.27, 7.23, and 6.0 au for ethynyl, cis- and trans-ethenyl, and the two phenyl values, respectively. provides an example of what is obtained when n orbital delocalization does not occur. The A'(2) and A"(2) excitation energies for the ethynyl radical are very similar. This is expected since excitation of the bonding orbitals is most difficult for the multiple C-C bond in the ethynyl peroxyl radical. Although the A'(2) excitation energy is predicted in the farW for the ethenyl and phenyl peroxyl radicals, the A"(2) transition moves further to the red for both, presumably as it becomes easier to excite the n bonding orbital. In the ground state of all peroxyl radicals the open shell hole is localized on the terminal oxygen atom. This is in agreement with an earlier study.I0 Correlation weakly couples this hole to the other bonds in the radicals in the ground state. In the excited states two types of excited configurations are found to be significant. First, the n-type orbitals are excited into their antibonding counterparts. Second, the hole is permuted among the n-type occupied orbitals located on the C-C and 0-0 bonds. This leads to substantial charge transfer in all peroxyl radicals and accounts for the large increase in the dipole moment of the excited state. Delocalization of the hole in the excited state in the ethenyl and phenyl peroxyl radicals leads to lower excitation energies for the A" states only. The first excited A' state, A'(2), is formed entirely by the n-to-n*-type excitations without delocalizing the hole on the terminal oxygen. Thus, the excitation energy is not affected by the presence of the hole on the oxygen. Only the phenyl peroxyl radical has been observed in the gas phase6 as a broad continuum in the 500 nm region. We believe our calculations are compatible with the experiment since the absorption peak has not been determined. Peroxy radicals have a very broad dissociative transition. The width at half-maximum for the absorption peak of phenyl peroxyl is over 180 nm.5,7 In addition, the phenyl peroxyl radical may not be the only species that is observed in the gas phase since there are lower energy isomeric species.I3 There is about 40 kcal/mol of excess vibrational energy in the peroxyl radical after the initial recombination event. Activation barriers to the more stable epoxide and oxy1 isomers are certainly lower than 40 kcavmol. In the gas-phase reaction of oxygen and the vinyl radical, the reaction proceeds to formaldehyde and formyl radical without evidence of the peroxyl radical.25 At the higher pressure recombination of phenyl and oxygen, apparently, deactivation is rapid enough to prevent dissociation of the 0-0 bond but may not prevent some isomerization to more stable isomers. This argument is of less weight than the first but also demonstrates the ambiguity of the attribution of the broad continuum to the phenyl peroxyl isomer without recording the entire spectral region. We are primarily concerned about the absorption spectra in aqueous solution. The solvent shift in Table 4 is estimated from classical theories for the effect of the solvent reaction field.'9.20 The excitation is assumed to be sufficiently rapid that the permanent dipoles of the solvent do not reorient, but polarization
of the solvent occurs. The contribution of the solvent polarization is a substantial part of the spectral shift. Quantum and classical reaction field methods yield comparable spectral shifts so we used the classical approach here. The necessary dipole moments of the ground and excited states are obtained from in vacuo calculation of each state at the MCSCF level. As seen in Table 4, the dipole moments of the ground and excited states are very different, and a large solvent shift is expected. Very recently, a solvent shift has been reported for the absorption maximum of the trichloroethenyl peroxyl radical from 580 nm in water to 540 nm in neat tetrachloroethene,26 which is comparable energetically to that predicted here for the ethenyl peroxyl radical. Polarizabilities of the ground and excited states are not expected to be substantially different so the classical analysis was restricted to the shift generated by the change in the solute dipole moment upon electronic excitation. For now, no experimental absorption spectra for the ethynyl radical have been reported. Our calculations predict an absorption around 300 nm in vacuo, but in water it is red-shifted to 330 nm. The experimental absorption assigned to the phenyl peroxyl radical has a peak about 490 nm in water.2 In methanol the peak is reported at 470 nm.5 Considering the relative dielectric properties of methanol and water, 20 nm is a substantial shift. The gas-phase continuum spectra attributed to phenyl peroxyl is not observed over a sufficient wavelength region to determine the peak absorpti~n.'~It may also be contaminated with the absorption due to isomeric species. The in vacuo excitation energy is calculated at 361 nm, but the excited state dipole is very much larger than the value in the ground state. Reaction field spectral shifts are strongly dependent upon the choice of the cavity radius which should be considered as a parameter in a multipolar expansion truncated to the dipole term. An estimate of the radius, 7.23 bohrs, is obtained from the volume of the charge den~ity.~'However, planar molecules of the size of the phenyl peroxyl radical are often parametrized with smaller radii.20 Using a value of 6 bohrs will substantially shift the peak to the red as seen in Table 4. The shift between water and methanol calculated with this radius is only 6 nm, but actually a larger difference of 20 nm is observed. This would suggest that a smaller radius or a higherorder multipole expansion is warranted. Nevertheless, the simplified model suggests that a large red shift is expected. The in vacuo excitation excitation energy of phenyl peroxyl is less accurate than the results for ethynyl and ethenyl peroxyl because of the limitation in the basis set and the restricted correlation that could be done with a relatively large molecule. When the geometry obtained with a DZd basis is used, the excitation energy shifts to higher energy which can be correlated to the reduction in the 0-0 bond distance. Nonetheless, the present results do not vary much, considering the substantial differences in the three geometries and especially the 0-0 distances. At all of the geometries used here, the excitation energy is predicted to be substantially to the blue of the absorption assigned to phenyl peroxyl in water. Analysis of the gas-phase kinetics experiments" has to keep this result in mind. Unfortunately, no gas-phase absorption spectra have been reported. The ethenyl radical is correlated with a DZd basis used for all heavy atoms. The trans isomer is about 4 kJ more stable than the cis. Reaction field calculations do not change the relative energies in solution because the radius parameters and the in vacuo ground state dipole moments for the isomers are quite similar. Assuming that the isomers are produced randomly and can equilibrate, both should be present under the experimental conditions if the calculated relative energies are accepted.
J. Phys. Chem., Vol. 99, No. 29, 1995 11391
Ethynyl, Ethenyl, and Phenyl Peroxyl Radicals The absorption observed in water can be interpreted as the convolution of two broad absorptions (half-width in excess of 50 nm) from both the cis and trans isomers with a dominant contribution from the trans. The absorption is broad because the excited state equilibrium geometry is substantially shifted from that of the ground state. As a result, the difference between the vertical and adiabatic energies for the transition to the A”(2) state is large. It is probable that the vertical transition is essentially dissociative. The analysis of the ground and excited energy surfaces for C2H302 will be presented later, but the ease with which this excited state may dissociate an oxygen atom is reflected in the increase of the 0-0 distance to about 1.7 A in the A”(2) excited state. Conclusion Three prototypical peroxyl radicals, the ethynyl, ethenyl, and phenyl, are calculated to be planar in the ground state, and each has an A” ground state. The A”( 1) to A”(2) electronic transition transfers substantial charge in all the radicals, leading to large increases in the dipole moment of the excited state. Relatively large red shifts are predicted for all the radicals. Delocalization of the n orbital electron hole in the excited A“ state of the phenyl and ethenyl peroxyl radicals is suggested as the basis for the substantial lowering of the energy of this excited state. The in vacuo excitation energy for the A”( 1) to A”(2) transition in the phenyl radical is predicted to be about 360 nm, which is far from the experimental peak observed at 490 nm in water.* However, the observation of this peak,at 470 nm in methanol5 is suggestive of the large red shift to be expected in a highdielectric media such as water or methanol. Reaction field calculations suggest a shift to the red in excess of 100 nm from the in vacuo prediction. Two isomers are predicted to be close in energy for the ethenyl peroxyl radical, but they have substantially different excitation energies. The reactive behavior on either the ground or excited state will depend on the isomer or initial conformation. Differences in the absorption spectrum between isomers could assist the assignment. However, the dominant electronic transition has a broad vibrational envelope due to a substantial shift in the geometry upon electronic excitation. The 0-0 bond is very weak in the excited state, and photodissociation is also
expected. The convolution of two broad peaks may prevent the observation of the individual isomers. References and Notes (1) von Sonntag, C. Chemical Basis of Radiation Biology; Taylor and Francis: London, 1987. (2) Mertens, R.; von Sonntag, C. Angew. Chem., lnt. Ed. Engl. 1994, 33, 1262. (3) Verbeek, J.-M.; Stapper, M.; Krijnen, E. S.; van Loon,J.-D.; Lodder, G.; Steenken, S. J. Phys. Chem. 1994, 98, 9526. (4) Alfassi, Z. B.; Marguet, S . ; Neta, P. J. Phys. Chem. 1994,98, 8019. (5) Alfassi, 2.B.; Khaikin, G.; Neta, P. J. Phys. Chem. 1995, 99, 265. (6) Yu, T.; Lin, M. C. J. Am. Chem. SOC. 1994, 116, 9571. (7) Fang, X.; Mertens, R.; von Sonntag, C. To be published in J. Chem. Soc., Perkins Trans. 2. (8) Wallington, T. J.; Dagaut, P.; Kurylo, M. J. Chem. Rev. 1992, 92, 667. (9) Krauss, M.; Osman, R. J. Phys. Chem. 1994, 98, 13515. (10) Boyd, S . L.; Boyd, R. J.; Barclay, L. R. C. J. Am. Chem. Soc. 1990, 112, 5724. (11) Krauss, M. Chem. Phys. Lett. 1994, 222, 513. (12) Sommeling, P. M.; Mulder, P.; Louw, R.; Avila, D. V.; Lusztyk, J.; Ingold, K. U. J. Phys. Chem. 1993, 97, 8361. (13) Mebel, A. M.; Lin, M. C. J. Am. Chem. SOC. 1994, 116, 9577. (14) Mat0s.J. M. 0.;Roos, B. 0. J. Am. Chem. SOC. 1988, 110, 7664. (15) Rice, J. E.; Handy, N. C.; Knowles, P. J. J. Chem. Soc., Faraday Trans. 2 1987, 83, 1458. (16) Krauss, M. J. Mol. Struct. (THEOCHEM) 1994, 307, 47. (17) Chipman, D. M.; Liu, R.; Zhou, X.; Pulay, P. J. Chem. Phys. 1994, 100, 5023. (18) Krauss, M.; Garmer, D. R. J. Phys. Chem. 1993, 97, 831. (19) Amos, A. T.; Burrows, B. L. Adv. Quantum Chem. 1973, 7, 289. (20) Lami, H.; Glasser, N. J. Chem. Phys. 1986, 84, 597. (21) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A,; Elbert, S . T.; Gordon. M. S.: Jensen, J. H.: Kokeski, S . ; Matsunapa, N.; Npuven, K. A.; Su, S ; Windus, T. L.; Dupuis, M.; Montgomery, J-A. J. CGrnjut. Chem. 1993, 14, 1347. (22) Stevens, W. J.; Basch, H.; Krauss, M. J. Chem. Phys. 1984, 81, 6026. (23) Krauss, M.; Roszak, S . J. Mol. Struct. (THEOCHEM) 1994, 310, 155. (24) Boffil, J. M.; Pulay, P. J. Chem. Phys. 1989, 90, 3637. (25) Slagle, I. R.; Park, J.-Y.;Heaves, M. C.; Gutman, D. J. Am. Chem. Soc. 1984, 106,4356. (26) Mertens, R.; von Sonntag, C. J. Photochem. Photobiol. 1995, 85, 1. (27) GAUSSIAN 1992: Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A,; Replogle, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S . ; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.: Pople, J. A. Gaussian Inc., Pittsburgh, PA, 1992. JP950025Y
