2768
J. Phys. Chem. 1986, 90, 2768-2172
radical is formed in preference to the primary radical. Such a biradical mechanism has been proposed for the UV photolysis of epoxides and propylene sulfide in the liquid phase,21 for the mercury-photosensitized decomposition of butylene oxide,22and in the addition of oxygen atoms to olefins.23 We have examined the M N D O calculations for energy and structural features that may correlate with r e a ~ t i v i t y . ~Most ~ promising was the study of correlations between the reaction cross section and the extent to which the intermediate complex was structurally productlike. While certain such trends can be found, they were small and not convincingly significant. As mentioned earlier, the low reactivity of C3H3C130is unexpected. At issue is the low reactivity at the chlorine end of the molecule. We have performed rate measurements for the reaction of boron with a series of halomethaness and the chlorine atom has been found reactive. For example, the cross section for the reaction of boron with CC13H is 6 A2. Yet in the case of the epoxide the total reactivity of both the chlorine and oxygen sites is only 2 A2. The M N D O calculations do not indicate any significant difference between the chlorines in CC1,H and C,H,Cl,O. In both cases an unhindered reaction with chlorine is suggested. The observed reduction in reactivity may be explained if one assumes that a boron-chlorine collision complex of the coordinate-covalent type is formed along the reaction path. The essentially free rotation of the -CC1, moiety25would then bring the boron into the vicinity of the ring, specifically into the region of the vacant orbital between C, and 0. A further coordinative or (21) Gritter, R. J.; Sabatino, E. C. J . Org. Chem. 1964, 29, 1965. (22) Cventanovic, R. J.; Doyle, L. C. Can. J . Chem. 1957, 35, 605. (23) Cvetanovic, R. J. Adu. Photochem. 1963, 1, 127. (24) Tabacco, M. B. Ph.D. Thesis, Boston College, 1984. (25) Separate MNDO studies in our lab indicate a CI-CC1, rotational barrier of about 1.3 kcal.
“bridge” bond to anchor the boron in this region may then occur, such interactions having precedent in organoborane chemistry.2629 In this way reactions at the chlorine end would be aborted. The MNDO calculations do show local minima in the approach of boron to the tetrahedral cone formed by the -CCl, group. These minima arise when the boron atom is about 2.1 A from the chlorine atoms. Finally, we point out that, in the collision where the formation of BO is aborted, the system need not retrace the reaction path back to a boron atom plus epoxide. An alternate channel is energetically allowed. It is possible that from the ring-opened state rearrangement occurs and an aldehyde is formed by an intramolecular hydrogen This alternate path of a boron atom plus an aldehyde is exoergic by about 1 eV.
Acknowledgment. Our thanks to Drs. David McFadden and Dennis Sardella for their useful insights. This work was supported in part by the National Science Foundation Grant No. CPE8310767 and by the Air Force Office of Scientific Research. Registry No. Boron, 7440-42-8;ethylene oxide, 75-21-8; epifluorohydrin, 503-09-3;epibromohydrin, 3 132-64-7;cyclopentene oxide, 28567-6; styrene oxide, 96-09-3; cyclohexene oxide, 286-20-4; vinyloxabicycloheptane, 106-86-5. (26) Yarwood, J. Ed. Spectroscopy and Structure of Molecular Complexes; Plenum: London, 1973; p 468. (27) Andres, L.; Keefer, R. Molecular Complexes in Organic Chemistry; Holden-Day: San Francisco, 1964; pp 49-54. (28) Onak, T. Organoborane Chemistry; Academic Press: New York, 1975; p 25, 99. (29) Pasto, D. J.; Kang, S . 2.J . Am. Chem. SOC.1968, 90, 3797. (30) Stork, G.; Colvin, E. J . Am. Chem. SOC.1971, 93, 2080. (31) Weissberger, A., Ed. Heterocyclic Compounds with Three and Four Membered Rings, Part I; Interscience: New York, 1964; pp 230-288. (32) Levy, M. Prog. React. Kiner. 1981, 10, 154.
An Aspect of Substituents and Peripheral Structures in Chemical Reactivities of Molecules Hiroshi Fujimoto,* Yoshitaka Mizutani, and Koji Iwase Division of Molecular Engineering, Kyoto University, Kyoto 606, Japan (Received: November 4, 1985)
With a view to studying what orbitals are most suited for representing electron delocalization in chemical reactions, we have designed a kind of hybridized orbital that possesses the maximum amplitude on a certain atom or on a certain structural unit of a molecule. The shape of these reactive orbitals is determined not by the structure of molecules but by the mode of interaction. The potentials of these orbitals for electron donation and for electron acceptance are sensitive, however, to changes in molecular structure and substituents. Reactivities of a variety of molecules are shown to be interpreted in a unified manner by the use of these orbitals.
Introduction In order to interpret selectivities in chemical reactions, we often refer to the overlap of molecular orbitals (MO) between the reagent and the reactant.’-) For instance, stereochemistry in pericyclic processes was explained in terms of the in-phase and out-of-phase overlaps between the occupied MO’s of one part of the reacting system and the unoccupied MO’s of the other part.4 (1) (a) Fukui, K. Theory of Orientation and Stereoselection; Springer Verlag: West Berlin, 1974. (b) Fujimoto, H.; Fukui, K. In Chemical Reactiuiry and Reaction Paths; Klopman, G., Ed.; Wiley-Interscience: New York, 1974; pp 23-54. (2) Herndon, W. C. Chem. Rev. 1972, 72, 157. (3) Houk, K. N. Acc. Chem. Res. 1975, 8, 361. (4) Woodward, R. B.; Hoffmann, R. The Conseruation of Orbital Symmerry; Academic: New York, 1969.
0022-3654/86/2090-2768$0 1.50/0
The concept of orbital interactions has also been applied successfully to inorganic and organometallic systems to understand their structures and reactivities5 Thus, the development of theoretical chemistry is much indebted to the principle of maximum overlap6*’and/or the overlap and orientation principle.8 In experiments, on the other hand, mechanisms of chemical reactions are discussed in terms of the properties that are related to some particular regions of molecules, e.g., functional groups. In most chemical reactions, a few atoms participate directly in ( 5 ) See, for example: Hoffmann, R. Angew. Chem., Inr. Ed. Engl. 1982, 21, 711. (6) Pauling, L. C. The Nature of the Chemical Bond; Cornell University
Press: Ithaca, NY, 1960. ( 7 ) Weinhold, F.; Brunck, T. K. J . Am. Chem. SOC.1976, 98, 3745. (8) Mulliken, R. S . R e d . Trac. Chim. Acta 1956, 75, 845.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2769
Chemical Reactivities of Molecules interactions between molecules. In spite of apparent successes of chemical reactivity theories,I4 we often feel that the delocalized MO’s are not necessarily the best suited for representing the important concepts in chemistry. In this paper, we demonstrate that the orbitals, localized by a simple manipulation of canonical MO’s so as to give the maximum amplitude on specific structural units of molecules, lead to a concise interpretation of experimental results. Our view is much more intimate with useful concepts in chemistry than the existing chemical reactivity theories.
unoccupied
occupied
Method We consider here an orbital function 6, for a molecule that is assumed to represent most appropriately the interaction with a reagent. This referenct function 6, is not confined to an atomic orbital (AO) but can be a linear combination of several AOs. It may be chosen so as to give the maximum overlap with the relevant orbital of the reagent that is not involved explicitly. Then, we try to find the orbital which maximizes the contribution of 6, either in the occupied space or in the unoccupied space of the reactant molecule under the constraint that the remaining orbitals are orthogonal to the orbital of interest. This occupied orbital &(fir) is defined simply by projecting 6, onto the occupied space m
’
‘12
‘13
m
4,(6r) = Cdi,,4i/(Cdi,12)1/2
Figure 1. Occupied and unoccupied orbitals of benzene localized so as to possess the maximum amplitude on C(1). The *-type canonical MOs
i= 1
i= 1
are shown for reference. where M and m signify the number of AO’s and the number of the occupied MO’s, respectively, and where d,,, denotes the coefficient of the canonical M O c$~ in the reference orbital 6,:
canonical MO’s. Those orbitals were calculated at the STO-3G level.I0 The occupied orbital is constructed by a linear combination of x l and a3MO’s. The x2 M O is excluded by symmetry, yielding 4,(2pl)
The coefficients, di,, for the occupied M O c$~ and d,+,,, for the unoccupied M O &,,+j, are calculated easily by using the inverse matrix of the LCAO M O coefficients denoted here by cf,[: (3)
The potential of this molecular hybrid orbital 4, for electron donation in interaction may roughly be estimated by m
Xoc(6,) = Cdi,rZti/ Cdi,? i= I
i= I
(4)
where t i is the energy of the canonical M O & The unoccupied that has the maximum contribution of the reforbital &,(6,) erence orbital function 6, can be defined similarly: M-m
Anm(6r)
=
hi-m
C d m + j , r + m + j / ( jC dm+j,r2)’” j= I - 1
(5)
The electron affinity of this hypothetical orbital may be determined primarily by M-m
Xunm(6r)
M-m
= C dm+j,,2cm+j/ C dm+j,rl j= 1
+ O.7824(~3)
This orbital exhibits the maximum amplitude on C( 1) but has a significant contribution of the adjacent carbons C(2) and C(6). The unoccupied orbital that has the maximum amplitude on C( 1) can be determined similarly as A ” 2 P I ) = 0.8464(~5)+ 0.5334(~6)
M
4f = fCC,,fXf =1
m
= 0.6234(?f1)
j= 1
(6)
Results and Discussion Let us take the aromatic substitution reaction of benzene as the first example. It is reasonable to assume that electrophiles or nucleophiles attack one of the carbon atoms in this reaction. Then, we may take the 2px A 0 of C ( 1) as the reference orbital function 6,. We disregard the contribution of u MOs a t resent.^ The orbitals 4,(2pl) and &,,(2p1) that have been determined by using eq 1 and 5 are shown in Figure 1 together with the x-type (9) The s and p functions are mixed automatically in the course of orbital transformations in nonplanar molecules, even when we take a p orbital as the starting reference.
This orbital is shown also to be delocalized significantly on the adjacent atoms. By introduction of substituent groups, each canonical MO changes. However, calculations have revealed that the orbitals which have been localized so as to give the maximum amplitude on a carbon px A 0 are much alike. The combinations of several MO’s give rise to reactive orbitals almost the same in shape for different molecules and for different positions. Interestingly, however, the X value changes significantly. We will discuss this important point later in more detail, but we first try to connect this trend with some experimental results. In Figure 2, the A, and X,u, of the orbitals of substituted benzenes are plotted against the Hammett u constants.I1 The benzene orbital shown in Figure 1 was chosen as the standard (denoted by [HI). One can see an excellent correlation between the difference in X values and the c constants. As X of the occupied , gets higher, the molecule will be a better reactive orbital 6 electron donor in the reaction.12 Alternatively, as the X value of the unoccupied reactive orbital &, gets lower, the molecule will be a better electron acceptor. The orbitals give different X values for m- and p-substituted benzene molecules. It is now possible to compare the reactivities of different molecules and of different positions in a unified manner. (10) Binkley, J. S.;Whitesides, R. A.; Krishnan, R.; Seeger, R.; DeFrees, J.; Schlegel, H. B.; Topiol, S.;Kahn, L. R.; Pople, J. A. QCPE 1981, 13, No. 406. (1 1) (a) Hammett, L. P. Physical Orgunic Chemistry; McGraw-Hill: New York, 1940; pp 184-199. (b) McGary, C. W.; Okamoto, Y.; Brown, H. C. J . Am. Chem. SOC.1955, 77, 3037. (12) An interpretation of the Hammett equation by means of the frontier orbitals was reported. See: Henri-Rousseau, 0.;Texier, F.J . Chem. Educ. 1978, 55, 437.
2770 The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Fujimoto et al.
a I
t
\\ l.ooo
0.05
pNHa
i
1
0.894
lSU1
h
J
cf
0
o.o~
t
I
1\
1.323 1.273
0.91 1
:fi
1.626
1.212
1.337
68 1.317
\ /
" 0 2
8
0.721
0
P-W?
1.260
0
1.o
0.0
-1.0
1.328 1.279 1.627
8 \
1.333
unoccupied o occupied
0.862 1.357 1.253 1.287
Hammett U
Figure 2. Relation between the electron donating and accepting abilities of reactive orbitals of substituted benzenes and Hammett u constants. The benzene orbitals [HI were taken as the reference.
9
1.270
e:'
1 A64
1.674
1.185
1.120
TABLE I: The Unoccupied Reactive Orbitals of Formic Acid and Its Derivatives Localized on Carbonyl Carbons molecule
H-COCI CC1,CH3Ph-
H-COOCOH H-COOH CC1,CH3PhH-COOCH, CCI3CH3Ph-
au 0.240 0.214 0.267 0.275 0.219 0.308 0.216 0.334 0.339 0.311 0.280 0.335 0.340
localn on C. % 82.9 83.9 85.0 85.0 83.5 84.3 85.2 86.2 86.2 84.3 85.2 86.2 86.2
Table I shows the X value of the orbitals of formic acid and its derivatives. These orbitals were localized in the unoccupied space in order to have the maximum amplitude on the carbonyl carbon p?' A 0 in each of the molecules examined here. They are delocalized over the carbonyl group and look very similar to each other, as indicated by almost the same extent of localization of the orbitals on the reaction center. This result can be related to the reactivities of carbonyl carbons against the attack of nuc l e o p h i l e ~ . ~Acyl ~ chloride is indicated to be more reactive than its acid anhydride which is shown to be more reactive than its acid and its ester in accordance with experiments. Methyl and phenyl groups are suggested to reduce the reactivity, while the trichloromethyl group is predicted to enhance the reactivity of carbonyl groups. The idea of localizing the orbitals on the specific reactive region of molecules can also be applied to the simple Huckel MO calculations. Since the overlap integrals are neglected there, the coefficients di,rin eq 1 and 4 and dm+j,rin eq 5 and 6 are replaced , ~c,,,+~,respectively. One by the LCAO M O coefficients c ~and leads a t once to A, shown in Figure 3 for some conjugated hydrocarbons. In the open-chain conjugated systems, the terminal carbons exhibit the highest activities against electrophiles in agreement with experiments.14 The naphthalene 1-position is shown to be more reactive than the 2-position in conformity with (13) See, For example: March, J. Advanced Organic Chemistry, 2nd ed.; Academic: New York, 1977; pp 347-357. (14) (a) Mislow, K.; Hellman, H. M. J . Am. Chem. SOC.1951, 73, 244. (b) Mislow, K. Ibid. 1953, 75, 2512. (c) Farmer, E.H.; Laroria, B. D.; Switz, T. M.; Thorpe, J. F. J . Chem. SOC.1927, 2937.
1.093 1.345
A
1.61 1
2.182 1.811
Figure 3. The X values of reactive orbitals localized so as to have the maximum amplitude on a carbon atom in the occupied space of conjugated hydrocarbons (in @ units).
the selectivity in aromatic substitution^.'^ The anthracene 9- and 10-positions exhibit strong reactivities for electrophiles. This is also in agreement with experiment^.'^ Quinodimethane and its homologues are shown to possess extraordinarily high-lying occupied orbitals at the methylene carbons. They are suggested to be very reactive and substitutions of hydrogens by bulky groups will be necessary in order to isolate some of these molecules.16 On the other hand, cyclic bicaeicene is suggested to be not so reactive. Bromination is known to occur at the carbons in the five-membered rings." Here we discuss briefly the connection with the frontier orbital theory. When an atom has a large population in the highest occupied MO, the occupied reactive orbital that has been localized so as to give the maximum amplitude around that atom consists largely of the highest occupied MO. The X value of the resulting hybrid orbital is small (in negative) and the atom will be able to denote electronic charge easily. When an atom has a large population in the lowest unoccupied MO, we obtain a low-lying (1 5) (a) Shieman, G.; Gueffroy, W.; Wikelmuller, W. Ann. Chem. 1931, 487, 270. (b) Glaser, C. Ibid. 1965, 135, 41. (c) Meisenheimer, J.; Con-
nerade, E. Ibid. 1903,330, 164. In the case of phenanthrene, our calculation has suggested the order of reactivity for electrophilic substitutions to be 9 > 1 > 4 > 3 > 2, whereas experimental studies of nitration have shown the order of reactivity to be 9 > 1 > 3 > 2 > 4. The C4 position is assumed to be deactivated by steric effects. See: Dewar, M. J. S.; Warford, E. W. T. J. Chem. SOC.1956, 3570. (16) Some o-quinodimethane derivatives are stable in solution at room temperature. No derivatives of p-quinodimethane have been reported to be isolable. See: (a) Miller, R. D.; Kolc, J.; Michl, J. J. Am. Chem. SOC.1976, 98, 8510. (b) Dolbier, Jr., W. R.; Matsui, K.; Michl, J.; Horak, D. V. Ibid. 1976, 99,3876. (c) Palensky, F. J.; Morrison, H. A. Ibid. 1977, 99,3507. (d) Steiner, R. P.; Miller, R. D.; Dewey, H. J.; Michl, J. Ibid. 1979, 101,1820. (e) Ackermann, J.; Angliker, H.; Hasler, E.; Wirtz, J. Angew. Chem., Int. Ed. Engl. 1982, 21, 618. (f) Inagaki, S.; Iwase, K. Nouu. J . Chim. 1984, 8, 7 3 . (17) (a) Yoshida, Z. Pure Appl. Chem. 1982, 54, 1059. (b) Yoneda, S.; Shibata, M.; Kida, S.; Yoshida, Z.; Kai, Y.; Miki, K.; Kasai, N. Angew. Chem. 1984, 96, 75.
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2771
Chemical Reactivities of Molecules
67
68
70
1.000
0.930
0.863
60
1.315
Figure 4. Sketch of the reactive orbitals of naphthalene and a large aromatic system localized so as to have the maximum amplitude on C(l) and C(4) in the occupied space. Extent of localization (upper in C ) and the X value (lower in 0 units) are compared with benzene.
unoccupied hybrid orbital that is localized around that atom. Then, the atom is suggested to have a strong reactivity for nucleophiles. Our concept of localized reactive orbitals does not contradict the frontier orbital theory but contains it in a more generalized form. We may refer next to multicentered reactions. The Diels-Alder type cycloaddition is taken as an example. The reference function in the dienophile part should be chosen as either (2pr - 2p,) or (2pr 2pJ in which r and s are located at positions 1 and 4. By looking at the canonical MO's of butadiene or benzene, one can choose at once the antisymmetric combination for the occupied space and the symmetric combination for the unoccupied space. Figure 4 illustrates the occupied reactive orbitals of anthracene and a large condensed aromatic hydrocarbon for cycloaddition at the para positions.18 They bear close resemblance to the r3 M O of benzene molecule. The anthracene 9-, 10-positions yield a high-lying 1,4-conjugated occupied orbital which agrees with its high reactivity for the Diels-Alder type cycloaddition at the central ring.Ig On the other hand, the large condensed aromatic system is predicted to be unreactive.20 The same conclusion is derived also for the unoccupied orbitals.
+
Figure 5. STO-3Ginteracting orbitals of benzene obtained for the attack of a proton (upper) and for the attack of a hydride ion (lower). The interacting orbitals calculated for a planar benzene with the attacking ion 1.5 A above the C(l) are shown on the right-hand side for reference. The orbitals of ions are not presented. ,./ ........... .. .............. .........
;
w , ............... .. . . . . ......................
,
CeHe
CeHsNHn
Significance of the Localized Reactive Orbitals
We saw above an interesting aspect of chemical interactions. The active regions of molecules are almost fixed in each reaction for an analogous series of molecules. However, the same structural unit exhibits different potentials for electron donation and electron acceptance in different molecules. That is, the change in the environment due to the presence of substituent groups and the difference in periphery of molecules is reflected on the potential of the common reactive unit. This view appears to be much closer to our experiences in chemistry than the usual interpretation of reactivities and selectivities in terms of delocalized MOs. Here we have to examine whether or not the intuitively taken reference orbital functions, e.g., the p r A 0 of a carbon atom in a benzene ring for aromatic substitutions, are valid in modeling chemical interactions. With a view to investigate the orbitals that should participate actually in electron delocalization between molecules, we proposed a method of transforming the MO's simultaneously within each of the two fragment species.21 The interacting orbitals are calculated rigorously by combining unitary transformations of canonical MO's with the configuration analysis of the ab initio wave function for a composite interacting system.22*23Thus, these interacting orbitals can be utilized in order (18) Simple dienes usually play the electron donor part. See,for example: (a) Benghiat, I.; Becker, E. I. J . Org. Chem. 1958, 23, 885. (b) Sustman, R. Tetrahedron Lett. 1971, 2721. (c) Eisenstein, 0.; Lefour, J. M.; Anh, N. T. Tetrahedron 1977, 33, 531. (d) Houk, K. N. J . Am. Chem. SOC.1973,95, 4092. (e) Fujimoto, H.; Inagaki, S.; Fukui, K. Ibid. 1976, 98, 2670. (19) Diels, D.; Alder, K. Justus Liebigs Ann. Chem. 1931, 486, 191. (20) Graphite is known to undergo oxidation under extreme conditions. (21) Fukui, K.; Koga, N.; Fujimoto, H. J . Am. Chem. Soc. 1981,103,196.
CsHsF
Figure 6. 6-31G* interacting orbitals of benzene and substituted benzenes taking part in electron donation to a proton.
to verify the use of the reference functions. Figure 5 illustrates the pairs of interacting orbitals that take part in electron delocalization between a benzene molecule and a proton and between a benzene molecule and a hydride ion.24 The ions were located 1.5 A away from C(1) and the geometries were optimized at the STO-3G level for each system. Since an orbital function is allotted to the attacking reagents in this basis set, electron delocalization can be represented by a single interacting orbital of benzene that is paired with the reagent orbital in each case. One sees that the orbitals derived from this analysis on the composite reacting system bear close resemblance to the orbitals in Figure 1, obtained by simple manipulation for the (22) Fujimoto, H.; Koga, N.; Hataue, I. J . Phys. Chem. 1984, 88, 3539. (23) Fujimoto, H.; Kato, K.; Yamabe, S.; Fukui, K. J . Chem. Phys. 1974, 60, 572. (24) The fragment orbitals are paired by the orbital transformationsso that an occupied orbital of the electron donor part interacts exclusively with its paired counterpart in the unoccupied space of the electron acceptor part.
2772
J . Phys. Chem. 1986, 90, 2772-2777
isolated benzene molecule so as to have the maximum amplitude on C(1). Figure 6 compares the interacting orbitals of benzene with the orbitals of aniline and fluorobenzene for the attack of a proton. The structure of the six-membered ring was fixed at the one optimized above for the C6H6 + H+ system at the STO-3G level. The 6-31G* calculation adopted here gives rise to the second pair of interacting orbitals that are antibondingSz5 The major interacting orbital on the left-hand side, however, looks similar to that obtained for the minimal basis calculation. Anyhow, substituents are shown to have little influence on the orbital shape. This result is in agreement with the fact that the reactive orbitals obtained for isolated benzene molecules in Figure 2 are almost the same irrespective of the substituents. The orbitals of dienes in [2 41 cycloadditions calculated by using eq 1 and 5 look also very similar to the interacting orbitals reported previously.22 Our intuitively selected starting functions based on the principle of maximum overlap are concluded to be relevant so far. As shown above electron delocalization determines the active regions of the reagent and reactant molecules. The inclusion of the overlap repulsion and polarization effects in chemical interactions gives rise to the paired interacting orbitals that are localized not in a specific structural unit but on the reaction center.26 The
+
(25) The contribution of the second pair of orbitals to the stabilization is much less significant.
fact that organic chemistry has been systematized on the basis of the concept of functional groups is regarded as indicating most clearly the significance of electron delocalization in chemistry.
Conclusion The present study has demonstrated that the orbitals that participate actively in electron delocalization in reactions are alike in a series of molecules for the same type of reaction. The important conclusion is that the same structural unit has different potentials for electron donating and accepting interactions in different environments. Molecules have many active sites or reactive structural units and, hence, undergo various reactions. The orbitals that are designed specifically so as to characterize each type of chemical reaction lead to a much more realistic view of chemical interactions than the usual orbital interaction scheme. Incidentally, the present study provides a straightforward verification of the useful principle of maximum overlap in molecular interactions. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (No. 59104003) from the Ministry of Education, Japan. A part of calculations was carried out at the Computer Center, Institute for Molecular Science. Registry No. Benzene, 7 1-43-2; naphthalene, 9 1-20-3. (26) Fujimoto, H.; Yamasaki, T.; Mizutani, H.; Koga, N. J . Am. Chem. SOC.1985, 107,6157.
Chemistry and Structure of the CH302+Product of the 0,'
-I-CH, Reaction
J. M. Van Doren,* S. E. Barlow, C. H. DePuy,* V. M. Bierbaum, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-021 5
I. Dotan,' The Weizmann Institute, Rehovot, Israel
and E. E. Ferguson* Aeronomy Laboratory, NOAA. Boulder, Colorado 80303 (Received: November 8, 1985)
The CH302+product ion of the thermal energy reaction between 02+ and CH4 has been studied in reaction with a number of neutral molecules. The reactivity pattern demonstrates very convincingly that the structure of the ion is methylene hydroperoxy cation, CH200H+,in contrast to previous reports that the ion is protonated formic acid, HC(OH)2+or HC(0)OH2+. From these and other data the heat of formation of this ion is determined to be between 182 and 188 kcal/mol.
Introduction The reaction between 02+ and CHI at low energy is one of the most extensively studied of all ion-molecule The rate coefficient is relatively small at 300 K, 5 X lo-'* cm3 s-', and increases to 5 X cm3 s-l with decreasing temperature to 20 K.6 The rate coefficient also increases at temperatures higher than 300 K, at elevated relative kinetic en erg^,^,^ and with 02+ vibrational e~citation.',~ (1) National Research Council Senior Associate, NOAA. (2) Franklin, J. L.; Munson, M. S.B. Symp. (In?.)Combust., [Proc.],10th 1965, 561-568. ( 3 ) Dotan, I.; Fehsenfeld, F. C.; Albritton, D. L. J. Chem. Phys. 1978,68, 5665. (4) Smith, D.; Adams, N. G.; Miller, T. M. J . ,-hem. phys. 1978, 69,308, (5) Nestler, V.;Warneck, P. Chem. Phys. Lett. 1977, 45, 96. ( 6 ) Rowe, B. R.;Dupeyrat, G.; Marquette, J. B.; Smith, D.; Adams, N. G.; Ferguson, E. E. J. Chem. Phys. 1984, 80, 241. (7) DurupFerguson, M.;Bahringer, H.; Fahey, D. W.; Fehsenfeld, F. C.; Ferguson, E. E. J. Chem. Phys. 1984, 81, 2657. (8) Hollebone, B. R.; Bohme, D. K. J. Chem. Soc.,Faraday Tram. 2 1973, 69, 1569.
0022-3654/86/2090-2772$01 S O / O
There are several possible exothermic channels for this reaction at room temperature: 02'
+ CH,
CH302'
-
--*
+
+ H + AE
H30+ + HCO
+ 113 kcal/mol H20++ CHzO + 53 kcal/mol C H 2 0 + + H 2 0 + 95 kcal/mol HCO' + H + H 2 0 + 71 kcal/mol C H 3 0 + + OH + 78 kcal/mol
(1) (2) (3) (4) (5) (6)
Only reaction 1 is observed and its exothermicity depends on the structure of the CH3O2' ion produced. The most exothermic reaction would be the production of protonated formic acid, HC(OH)2+,for which AE = 111 kcal/mol. Production of HC(0)OH2+would be about 83 kcal/mol exothermic. These are not the Product ions, however, and one Purpose of this study is to delineate the structure of the product CH302+and the energetics 0 1986 American Chemical Society
