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
+
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2773
CH302+ Product of the 02+ CH4 Reaction of reaction 1. The occurrence of reaction 1 implies AHr(CH302') < 208 kcal/mol. Two endothermic channels are observed at elevated kinetic en erg^^.^ or with vibrationally excited 02+7*9
02+ + CH4
-
-
+ H 0 2 - 5.5 kcal/mol CH4+ + O2- 13.8 kcal/mol
CH3+
-
(7)
(8)
and reaction 7 is barely detectable, k cm3 s-I, a t 300 K. Above -0.5-eV center-of-mass kinetic energy, (7) and (8) become the dominant reaction channels. The total rate coefficient increases to a value of 1 X 1O-Io cm3 s-I at 1-eV center-of-mass kinetic energy.7 Villinger et a1.I0J1 reported that the CH3O2+ product ion of (1) was protonated formic acid, HC(OH)2+,which has a heat of formation'* of 96 kcal/mol. This was deduced from isotope exchange reactions with D 2 0 and from collisional dissociation behavior both of which were reported to be identical for the product of (1) and for protonated formic acid produced by the reaction of formic acid ion with formic acid. Holmes et al." also recently reported that the product ion of the 02+ CHI reaction is protonated formic acid. In addition, they noted that HC(OH)2+ and the isomeric ion, C H 2 0 0 H + ,give the same dominant collision fragments, HCO+ and H30+, in their collisional activation mass spectral studies. They placed an upper limit of 188 kcal/mol on the heat of formation of the C H 2 0 0 H + ion. Recently Ha and Nguyen14concluded from theoretical structure calculations that the product of reaction 1 is HC(0)OH2+, with an energy 28 kcal/mol greater than that of protonated formic acid. Either of the product ions, HC(OH)2+or HC(0)OH2+, poses a very serious problem in terms of the reaction mechanism, since the breaking and remaking of four bonds is required, a quite remarkable situation. W e conclude in this paper that neither of these two CH3O2+ isomeric forms is produced in the 02+ CH4 reaction; rather, the novel chemistry of the product ion conclusively determines its structure to be CH200H+. This ion transfers HO' to alkenes and compounds containing carbonyl and related groups, and H+ to ethylene and several other molecules. The C H 2 0 0 H +ion also abstracts hydride from alkanes and other neutral compounds. This represents the first gas-phase study of the chemistry of a hydroperoxy cation.
+
+
Experimental Section The experiments were performed with a selected ion flow tube (SIFT) which has been described previ0us1y.l~ The reactions of CH3O2' ions were studied in the following manner. 02+ ions were created by electron impact on O2 in a remote high-pressure (-60 mtorr) ion source. These ions were mass selected with a quadrupole mass filter and injected into the flow tube through a venturi inlet. Approximately 15 mtorr of ultrahigh-purity methane (99.999+%) was introduced 13 cm downstream of the injector. The neutral reactant was introduced through one of a series of stationary inlets located between 63 and 112 cm from the injector. ions react with methane before the Greater than 99% of the 02+ first of these inlets. The isotopically labeled ion CH31*02+ was formed by electron impact on I8O2in the high-pressure source, mass selection, and injection of l S 0 2 + into the flow tube and subsequent reaction with CH,. In the investigation of the reactions of protonated formic acid, this ion was created by electron impact ionization of formic acid
-
(9) Tanaka, K.; Kato, T.; Koyano, I. J. Chem. Phys. 1986, 84, 750. (10) Villinger, H.; Richter, R.; Lindinger, W. Inr. J. Mass Specrrom. Ion Phys. 1983, 51, 25. (1 1) Villinger, H.; Saxer, A.; Richter, R.; Lindinger, W. Chem. Phys. Left. 1983, 96,5 13. (12) Lias, S . G.; Liebman, J. F.;Levin, R. D. J . Phys. Chem. Ref. Data 1984, 13, 695. (13) Holmes, J. L.; Mommers, A. A.; DeKoster, C.; Heerma W.; Terlouw, J. L. Chem. Phys. Left. 1985, 115, 437. (14) Ha, T.K.; Nguyen, M. T.J. Phys. Chem. 1984, 88, 4295. (15) Grabowski, J. J.; DeF'uy, C. H.; Bierbaum, V. M. J . Am. Chem. Soc. 1983, 105, 2565.
with subsequent proton transfer to the neutral parent. This ion was mass selected and injected into the flow tube and allowed to react with neutral compounds added through the stationary inlets described above. All neutral reagents were obtained from commercial suppliers and most were used without further purification. The aldehydes were treated with N a H C 0 3 and methyl formate was dried with CaH2. Neutral reactant flow rates were monitored by measuring the pressure increase with time in a calibrated volume. Rate coefficients were determined by monitoring the ion signal as a function of reaction distance. All reactions were studied at 0.4401~ helium pressure at room temperature.
Results In order to elucidate the structure of CH3O2+ produced in reaction 1 we studied the reactions of this ion with a number of neutral compounds. These reactions and their rate coefficients are summarized in Table I. This section presents the results, grouping sets of reactions by the neutral reactant. We will also report some pertinent reactions of protonated formic acid. The use of high flows of methane introduced some impurities into the flow tube, which might have produced trace ions. Therefore, we report only the major observed reaction products. It is unlikely that a mixture of isomers of CH3O2+ or a significant population of energetically excited CH3O2+ ions is formed. N o curvature was observed in the kinetic plots of CH3O2' reaction with any of the neutral compounds investigated. Alkanes. CH3O2' reacts with linear alkanes containing three or more carbon atoms by hydride abstraction to form a single reaction product, the (M - 1)+ ion of the corresponding alkane (see Table I). For instance, CH3O2' reacts with n-pentane (C5HI2)to form C5H11+.The atom content of this product was confirmed by allowing CH3I8O2+to react with n-pentane; m / z 7 1 was the only product ion observed. CH3O2+ does not react with ethane (C2H,). CH3O2' reacts with neopentane, C(CH3)4,in a fashion similar to its reaction with linear alkanes. The hydride abstraction product, (M - l)+, is observed, as well as an (M - 15)' product ion. This latter ion corresponds to methide abstraction from neopentane by CH3O2+. Carbonyl Compounds. In the reactions of CH3O2' with acetaldehyde, CH3C(0)H, and propionaldehyde, C2H5C(0)H,three major products are formed. As was observed for alkanes, the (M - 1)' product ion corresponding to the hydride abstraction product is observed. However, the major products are the proton-transfer product (M + 1)' and an ion of m / z (M 17)' which corresponds to addition of HO+ to the aldehyde. CH3O2' reacts with acetone, CH3C(0)CH3,and with methyl formate, C H 3 0 C ( 0 ) H , to form ions by proton transfer and by HO' addition. The (M - 1)' product ion is not observed in either of these reactions. Alkenes. The reaction of CH3O2' with ethylene leads to the formation of two product ions, (M + 1)' and (M + 17)+, corresponding to proton transfer16from CH3O2+ to ethylene and HO+ addition to ethylene, respectively. Propylene also reacts with CH3O2' by H+ and HO+ addition and also by hydride ion abstraction." In addition, an ion of mass 45 is produced, which was determined to contain one oxygen atom from CH3I8O2+ studies and is presumably C2H50+. Alkyl Chlorides. CH3O2' reacts with methyl chloride, CH3Cl, to form an (M + 17)' product ion. The observation and intensity of the isotope peak two mass units higher indicates that the product ion contains a single chlorine atom. In order to distinguish between the two possible (M + 17)+ ions, CClH40+and C102+,CH3I8O2+ was allowed to react with methyl chloride. The product ion observed was m / z (M + 19)'. Therefore, the product of CH3O2' reacting with methyl chloride is CC1H40+, the HO+ addition
+
(16) Because of an initial impurity at m / z 29 we are not able to distinguish C2HS+from HCO* unambiguously. (17) The C3Hs+ ion is formed upon addition of propylene; however, CH302+can not be conclusively identified as the source.
2774 The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
TABLE I: Reactions of CH3O2' Ions Produced from the 02' + CH, Reaction
neutral reactant C2H6
N R ~
C3H8
Cd7'
n-C4H10
C4H9'
n-C5H12
CSHII'
C(CH314
C5Hll'
CHIC(0)H
C(CH,),+ C2H30+ C2HSO'
C2H5C(0)H
C,H50+ C,H70+
1)
8.5 (f0.9)
1)
9.2 (f2.3) X lo-"
1)
5.5 (f0.3)
X
X
IO-''
15) 1.2 (fo.3) x 10-9
1.5 (f0.2)
C3H702'
(M - 1) ( M + 1) (M + 17)
C3H70'
(M
+ 1) + 17) (M + 1) (M + 17) (M + I)* (M + 17)
2.6 (fo.8) x 10-9
(M - 1)' ( M + 1) (M + 17)
1.1 (fO.l) X IO-''
C3H702'
CHIOC(0)H
(M (M (M (M (M -
(M - 1) (M + 1) (M + 17)
C2H502'
CH$(O)CH,
1)
rate coefficient" cm3 SKI I 1 x 10-12 4.6 (f0.5) X lo-''
ion products
C2H5021 C2H503 C2H5'
C2H5O' C3H5' C3H7' C3H70+ C2H50t
X
lo9
(M
1.2 (fo.1) x 10-9 6.2 (f0.1)X
CH3CI
CH3C10H' CH30CHz' CI'CH20
(M
+ 17)
1.5 (f0.1)X
C2H5CI
C2HSCIOH' C2HSOCH2'
(M
+ 17)
5.9 (f0.3)
CS2
CS2OH'
cos
COSOH' HISO'
(M (M
+ 17) + 17)
X
4.4 (f0.4) X 8.6 ( f 1 . 2 ) X
7.4 (f0.2) x 10-10
CH,O' H,S+ H2SOH' HSO'
(M + 1) (M + 17)
CH3SCH2' (CH3)zSOH'
(M - 1) (M + 17)
1.8 ( ~ 0 . 1 )x 10-9
NH4' NH,OH'
(M
+ 1) (M + 17) (M + 1) (M + 17) (M + 1)
1.2 ( ~ 0 . 3 )x 10-9
H,O' D2HO' H,l80+
+
Discussion Structure of CH3O2' Produced from 02' + CH,. The differences in reactivity of protonated formic acid and the CH3O2' ion produced from the reaction of 02'with CH, clearly indicate that this latter ion is not protonated formic acid, as previously proposed. Moreover, the CH302' ion is not a cluster of the form CH3'.02, HCO'.H20, or H30+.C0since solvent-switching reactions are not observed with H 2 0 , DzO, or H2180. The key to the structure of CH302' is supplied by its chemistry. CH3O2' is a strong oxidizing agent. Its characteristic reactions are hydride ion abstraction (formation of (M - 1)') and HO' transfer (formation of (M 17)'). It also transfers a proton to some molecules (formation of (M 1)'). In solution, peroxy acids oxidize alkenes, aldehydes, and ketones by oxygen atom addition, similar to the oxidation reactions observed here for CH302'. We believe that the only structure consistent with all of our data is the methylene hydroperoxy structure shown below.'* This ion has two resonance forms.
(M + 1) ( M + 1)
1.4 (f0.2) x 10-9
2.1 (fO.l)
X
2.6 (fO.l) X
IO-" IO-"
product. This reaction also leads to the formation of ions of m / z These ions are tentatively identified as CH30CH2+and Cl'.CH20. The reaction of CH3O2' with ethyl chloride similarly forms the HO' addition product. In addition, m / z 59 is formed in this reaction. When CH3'802' is allowed to react with ethyl chloride, m/z 61 is observed, indicating that this ion contains a single oxygen atom and thus corresponds to CH3CH20CH2+. CS2,COS,and C 0 2 . The reactions of CH3O2' with carbon disulfide (CS,) and carbonyl sulfide (COS) form (M + 17)' product ions, corresponding to addition of HO' to the neutral reactants. COS also reacts with CH3O2+ to form an ion of m / z 51. The observation of the sulfur isotope peak two mass units
+
H
H
'c=ot
'Values represent the total rate coefficient. For most reactions, the rate coefficient represents the average of three measurements using different flows of neutral reactants. The standard deviations of these measurements are in parentheses. Systematic errors are estimated to be f25%. *See ref 16. 'See ref 17. dNo reaction. 45 and m / z 6 5 .
higher indicates the presence of sulfur in the product ion, presumably H3SOf. No reaction occurs with C 0 2 . Hydrogen Sulfide and Dimethyl Sulfide. CH302' reacts with H2Sto form CH30+,(M + l)', (M + 17)+, and m / z 49 product ions. This latter ion is tentatively identified as HSO' although the sulfur isotope peak, if present, is masked by the (M 17)' product ion. The reaction between CH3O2' and dimethyl sulfide forms the (M - 1)' and (M + 17)' product ions. NH3 and ND3. Ammonia reacts with CH302' to form principally (M + l)', the proton-transfer product, NH,'. An (M + 17)' product ion is also observed. ND3 reacts analogously with CH3O2'. No hydrogen-deuterium exchange was observed. H 2 0 , DzO, and H 2 l 8 0 . The only product observed in the reactions of these three isotopes of water with CH3O2' is the (M + 1)' ion. No hydrogen-deuterium exchange was observed. Reactions of Protonated Formic Acid. Protonated formic acid was generated in the remote high-pressure ion source as described above. The structure of this ion was verified by bracketting its proton affinity. The proton affinity was found to be between that of H2S (170 kcal/mol)I2 and methyl formate (189 kcal/mol),12 which is consistent with the value reported in the literature for formic acid (179 kcal/mol).I2 Protonated formic acid does not react with propane, n-butane, ethylene, water, or hydrogen sulfide; however, this ion exhibits two hydrogen-deuterium exchanges upon reaction with D 2 0 .
+
N R ~
ND,H' ND,OH'
Van Doren et al.
H
/
H
\0/
\ t o
H
7-7
/H
0
The collision-induced dissociation spectra of the methylene hydroperoxide ion, generated from C H 3 0 0 H , and of the ion produced in reaction 1 support the present structure a~signment.'~ The structure proposed is in disagreement with the structure suggested by Villinger et al.lo*l'and Holmes et al.13 as well as the structure proposed by Ha and Nguyen.14 Private communications with Professors Lindinger and Holmes indicate that their experiments apparently suffered from formic acid contamination so that the ion studied was not the product of reaction 1 but rather protonated formic acid arising from contamination. Heat of Formation of CH302+.CH3O2' abstracts a hydride ion (H-) from normal alkanes with three, four, and five carbons, but not from ethane (Table I). A lower limit on AHf(CH3O2') may be calculated from the reaction with propane, which is evidently exothermic, or at most only slightly endothermic (not more than -0.5 kcal/mol endothermic when k Ik, exp(-AHIRT) is used). It is almost certain that the failure of CH302' to react with ethane implies that the reaction is endothermic, which gives (18) Protonated dioxirane, although consistent with the characteristic reaction of HOC transfer, is not accessible energetically (ref 14; higher level calculation, ref 25). (19) Kirchner, N. J.; Van Doren, J. M.; Bowers, M. T., work in progress.
+
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2775
CH302+ Product of the 02+ CHI Reaction
exothermicity is lower than those calculated for the five other possible reactions, (2)-(6), or for the production of HC(OH)2+ or HC(0)OH2+. Chemistry of CH302+.CH302+reacts with normal alkanes only by hydride abstraction to form the (M - 1)' ion of the corresponding alkane. The rate coefficients for hydride abstraction from normal alkanes increase as the number of carbon atoms (n) increase in the molecule (see Table I). This trend is consistent H2C=bOH + C3HB C3H7+ C H 3 0 0 H (9) with a decrease in hydride affinity as n increases, as well as the increased number of exothermic channels (number of exothermic from ethane, we can deduce'2~2"-22 205 (f2) > AHf > 184 (f2) sites for hydride abstraction) documented in the l i t e r a t ~ r e . The ~~ kcal/mol for CH302'. An upper limit of 208 kcal/mol is also lack of reaction with ethane allows us to bracket the hydride deduced from AH (eq 1) I0. A second possibility is that hydride affinity of CH3O2' between that of C2H5+, 270 (f2) k~al/mol,~O ion is abstracted by HO', forming water and formaldehyde (eq and that of C3H7+,250 (f2) k~al/mol,~O assuming that the re10). If this pathway were followed we can d e d ~ ~ e ' that ~ * ~ ~ , ~ ~ , ~ ~ action with ethane is endothermic. CH302+exhibits both hydride abstraction and methide abH2C=bOH C3Hs C3H7+ CH20 H 2 0 (10) straction reactions with "pentane. In analogy with the hydride abstraction reactions, we assume that the neutral product in the the heat of formation of CH302+must be greater than 132 (f2) latter reaction is ethyl hydroperoxide. kcal/mol (because it reacts with propane) and less than 152 (f2) All of the carbonyl-containing compounds react with CH3O2+ kcal/mol (because it does not react with ethane). by HO+ transfer to form an (M 17)' product ion, with formAt this point we invoke theory for guidance. Ha and Nguyen14 aldehyde generally as the neutral product. have calculated AHf(CH200H+)to be 180 kcal/mol using an + + S C F calculation with a 3-21G basis set. F r i s ~ h ?using ~ a higher H,C=OOH R2C-O RZC=OOH + CH2O (1 1) level of theory including correlation and polarization corrections We were unable to identify the structure of the (M 17)' product (MP2/6-3l+G*/HF/3-21G), calculates AHf = 191 kcal/mol. ions chemically. However, there are two reasonable structures These calculations indicate that AHf cannot be as low as 152 for these ions. The first structure is a hydroperoxide ion (the ( M kcal/mol. In addition, DeFrees has calculated the energy of - 1)+ of the corresponding hydroperoxide), analogous to the reaction for eq 9 and 10 as well as for the corresponding reactions reactant ion. If the reaction of acetaldehyde is used as an example, with ethane as the neutral reactant.26 His results (using (a) the product ion arising from bond formation to the carbonyl MP2/6-3l+G*/HF/3-2lG, scaled 3-21G ZPE and (b) MP4/ oxygen would be ethylidene hydroperoxide ion, CH3CHOOH+. 6-31 l++G**/HF/3-21G, scaled 3-21G ZPE) indicate that This ion must be 14 kcal/mol more stable than CH3O2' for formation of CHzO H 2 0with either ethane or propane as the the reaction to be exothermic. There are few thermochemical data neutral reactant in the hydride abstraction reaction is highly on hydroperoxides and no information on their hydride affinities. exothermic. Formation of C H 3 0 0 H , however, is endothermic We can, however, compare the heats of formation of the correwhen ethane is the neutral reactant and approximately thermosponding neutral compounds. BensonZoestimates the heat of neutral (-1.6 kcal/mol with (a) and +3.0 kcal/mol with (b)) when formation of C H 3 0 0 H to be --31.3 kcal/mol. The heat of propane is the neutral reactant. All these calculations suggest formation of C 2 H 5 0 0 Hhas been calculated from solution results very strongly that the neutral product is CH300H. This gives to be -48 f 14 kcal/mol.21 Since alkyl cations are stabilized by limits on AHF(CH302+) of >184 (f2) and
