752
J.
Phys. Chem. 1902.86, 752-757
This explanation is in essence similar to the isolated states proposed by McAdoo and McLafferty. The difference is that we assume them to be isomeric structures rather than electronic structures of the butanoic acid.
Acknowledgment. Tomas Baer acknowledges his sincere gratitude to Simon Bauer for providing an exciting and stimulating atmosphere during his 4 years at Cornell. Si's
enthusiasm for the new and the unexpected were then, and still are now, a constant source of inspiration. We are grateful to the National Science Foundation for the support of a portion of this work. The structure and energetic studies were supported by the Department of Energy. Maria L. Fraser-Monteiro and Luis Fraser-Monteiro thank their universities for a sabbatical leave and INVOTAN for a travel grant.
Thermochemistry and Dissociation Dynamics of State-Selected C,H,O,+ Acetate
Ions. 3. Ethyl
Luis Fraser-Monteiro,+ Maria L. Fraser-Yonteiro,' James J. Butler, and Tomas Baer' Department of Chemistry, University of North Carolina at Chapel Hill, Chapel H/II, North Carol/na 275 14 (Received: July 3 1, 198 1; I n Final Form: October 1, 1981)
The photoionization efficiency curves and appearance energies of C4H802+,C3HS02+, C4H60+,C2H502+, CzH50+, C2H30+,and C2H5+from ethyl acetate have been obtained. Structures and heats of formation of some of the ions are proposed. The ethyl acetate ion was found to be metastable in the vicinity of the first dissociation onset producing C4H60+( m / e 70) and HzO. The decay rates as a function of parent ion internal energy were measured between 10.4 and 10.7 eV. The comparison of the observed rates with those calculated by use of the RRKM/QET statistical theory indicates that the ethyl acetate ion isomerizes to a more stable ion prior to dissociation. The isomerized structure is neither that of dioxane nor butanoic acid, but may be the enol tautomer of ethyl acetate.
I. Introduction The third molecule in this series of C4H802 isomers is ethyl acetate. In common with dioxane and butanoic acid, no mass-analyzed photoionization (PI)data of ethyl acetate have been published. However, the ionic dissociationpaths have been investigated extensively by electron impact (EI) In a number of these studies it was mass noted that some of the fragment ions produced in the dissociation of the ethyl acetate ion me not necessarily the lowest energy ions.44411 Yet the hydrogen atoms have been shown to scramble to nearly the statistical limit in long-lived (metastable) Two dissociation paths have received particular attention. These are fragmentations to C4H60++ HzO and C2H5O2+ + CzH3. Godbole and Kebarle' using deuterium labeled ethyl acetate found that the methyl group attached to the carbonyl remained intact during the formation of CzH5O2+ ( m / e 61) while hydrogen atoms were randomly transferred from the ethyl group. More recently from the investigation of RC(OH)z+formation from a number of RCOOR' esters, Benoit et al.l0 concluded that the structure of the fragment ion is acid protonated at the carbonyl, and that the esters isomerize to the enol form prior to dissociation. The lowest energy enol form of ethyl acetate was subsequently shown to have a heat of formation ( A H f O ) 105 kJ/mol lower in energy than the keto form.12 The major fragment ion at low energies in the dissociation of ethyl acetate is C4H60+. Yeog carried out deuterium labeling studies and found that deuteriums on the ethyl group more readily end up in the neutral water fragment than do the deuteriums on the acetyl group. Faculty of Science and Technology, New University of Lisbon, Portugal. f Faculty of Science, University of Lisbon, Portugal. 0022-365418212086-0752$0 1.2510
Furthermore, the longer lived the parent ion (i.e., the lower the internal energy) the greater is the degree of scrambling. He concluded that the reaction is a complicated one and proposed that the C4H60+structure is 1. A number of
ch3-Y=fr b-bH,'
t
1
1
structures for this ion have been discussed by Terlouw et = 732 al.13 among which are CH2CHCHCHOH+ kJ/mol) and CH2COHCHCHz+( m H f o 2 9 8 = 761 kJ/mol). In a very recent study Holmes et al.ll investigated the role (1) J. D. Morrison and A. J. C. Nicholson, J . Chem. Phys., 20, 1021 (1952). (2) R. R. Bernecker and F. A. Long, J. Phys. Chem., 65,1565 (1961). (3) C. E. Brion and W. J. Dunning, Trans. Faraday SOC.,59, 647 (1963). (4) M. S. B. Munson and J. L. Franklin, J . Phys. Chem., 68, 3191 (1964). (5) A. G. Sharkey, J. L. Schultz, and R. A. Friedel, Anal. Chem., 31, 87 (1959). (6) J. H. Beynon, R. A. Saunders, and A. E. Williams, Anal. Chem., 33, 221 (1961). (7) E. W. Godbole and P. Kebarle, Trans. Faraday Soc., 58, 1897 (1962). (8) A. G. Harrison, A. Ivko, and D. Van Raalte, Can. J. Chem., 44,1625 (1966). (9) A. N. H. Yeo, Chem. Commun., 1154 (1970). (10) F. M. Benoit, A. G. Harrison, and F. P. Lossing, Org. Muss Spectrom., 12, 78 (1977). (11) J. L. Holmes, P. C. Burgers, and J. K. Terlouw, Can. J . Chem.,
in press. (12) J. L. Holmes and F. P. Lossing, J . Am. Chem. SOC.,102, 1591 (1980). (13) J. K. Terlouw, W. Heerma, J. L. Holmes, and P. C. Burgers, Org. Mass Spectrom., 15, 582 (1980).
0 1982 American Chemical Society
Thermochemistry of C,H,02+
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
Ions
of the oxygen atoms by the use of '80 labeling. They found that the oxygen scrambling was greatest at short times (Le., high C4H802+internal energy) which is just the opposite of the H atom scrambling results. These data would seem to indicate that there is a significant energy barrier to 0 atom migration, where as the H atoms have no significant barrier for scrambling but do so at a slow rate because of circuitous (high entropy) pathways. Holmes et al.ll also compared the collisional activation (CA) spectra of the C4H60+daughter ions with C4H60+ ions of known structures. Because the CA spectral4 are often very sensitive to the ion structure, it was pzmible to eliminate a number of C4H60+structures as acceptable products from ethyl acetate. However, Holmes et al.ll also concluded that the dissociation is very complicated and did not propose a mechanism for this reaction. Another relatively low energy dissociation channel is to C2H50+ + C2H30. Munson and Franklin4 reported AHfo(CH3CH20+)= 669 kJ/mol, a value supported by work of Harrison et a1.8 However, the existence of alternative structures and energies for this ion was suggested by the lack of agreement between the energetics of the CH3CH20+from ethyl acetate and those from other ions such as ethyl formate and diethyl ether.4 In this study of ethyl acetate we report the first accurate onsets for some of the fragment ions as well as dissociation rates as a function of parent ion internal energy for the H 2 0 loss channel. We conclude that the parent ion isomerizes to a mor? stable structure prior to dissociating. This is contrary to the behavior of the other two C4H802+isomers, dioxane15 and butanoic acid,16 which retain their ionic structure.
11. Experimental Section The techniques applied to the study of ethyl acetate are the same as those described in paper 1of this series.15 Ions were produced by photoionization and state selected by photoion-photoelectron coincidence (PIPECO). Lifetimes of metastable ions were determined from asymmetric fragment ion time-of-flight (TOF) distributions. The heats of formation of fragment ions and/or neutrals were calculated from the onsets of mass-analyzed photoionization yields. 111. Results A . Photoionization Efficiency ( P I E ) Curves. The PIE curves for the parent and six fragment ions from 10 to 13.5 eV are shown in Figure 1. The data were collected with a quadrupole mass filter, but normalized to a constant ion collection efficiency with TOF mass analysis at selected photon energies. The adiabatic ionization energy of ethyl acetate was found to be 10.01 f 0.05 eV which agrees well with those of other workers who obtained 9.97,' 10.01: 10.13: and 10.2 eV4by electron impact (EI), and 10.09l' and 10.11 eV18 by photoionization without mass analysis. Two PES studies gave rather different values of 10.2419and 9.90 eVl0for the IE of ethyl acetate. The more recent value of 9.90 eV of (14)F.W. McLafferty, Phil. Trans. R. SOC.London, Ser. A., 293,93 (1979). (15)M. Fraser-Monteiro,L. Fraser-Monteiro,J. J. Butler, T. Baer, and J. R. Hass, J. Phys. Chem., paper 1 of this series, in this issue. (16)J. J. Butler, M. Fraser-Monteiro,L. Fraser-Monteiro,T. Baer, and J. R. Hass, J. Phys. Chem., paper 2 of this series, in this issue. (17)K. Watanabe, J. Chem. Phys., 26, 542 (1957). (18)K.Watanabe, T.Nakayama, and J. Mottle, J. Quant. Spectrosc. Radiat. Transfer,2,369 (1962). (19)D. A. Sweigart and D. W. Turner, J. Am. Chem. SOC.,94,5592 (1972).
753
PHOTON ENERGY (eV)
Figure 1. Photoionization efficiency (PIE) curves for ethyl acetate(+) (m/ e 881, C&02+ (m/e 73),C,H,O+ (m/ e 70), C2H502+(m / e 6 l), C2H50+( m l e 45), C2H30+( m / e 43), and CH ,+ , ( m / e 29). TABLE I: Fragment Appearance Energies of Ethyl Acetatea
a
m/e
fragments
expt AE,,,, eV
73 70 61 45 43 29
C,H,O,+ + CH, C,H,O + H,O C,H,O,+ + C,H, C,H,O+ + C,H,O C,H,O + C,H,O C,H*+ + C,H,O,
10.60 t 0.1 10.31 i 0.1 10.67 t 0.08 10.70 A 0.1
IE = 10.01 i 0.05
11.0 i 0.15 11.29 i 0.1
Qv.
Benoit et al. was obtained with a higher resolution and clearly indicated that the adiabatic IE is considerably below the 10.24 eV which is in fact the first vertical IE. The most recent photoionization measurements are therefore considerably lower than the monoenergetic E1 result of 10.16 eV obtained by Holmes and Lossing.12 The onsets for the various fragment ions are listed in Table I. They were determined by expanding the vertical scale of Figure 1. As with dioxane, no H loss peak was observed. The fine structures on each of the PIE curves is probably due to the incomplete normalization of the ion signal by the photon intensity. Because the photon spectrum from the H2 "many-line" source is highly structured, it is difficult to differentiate between this light source structure and weak but sharp autoionization. However, in a molecule as large as ethyl acetate, we do not expect to observe sharp autoionization structure. B. Dissociation Rates. The ethyl acetate ion is metastable over the energy range 10.4-10.7 eV in which the C4H60+ion is the only product. The parent ion lifetimes as a function of their internal energy were measured by collecting fragment ion TOF distributions in coincidence with zero kinetic energy electrons. Representative ion TOF distributions are shown in Figure 2 as points. The lines are calculated TOF distributions. The inputs for these calculations are the parent and fragment ion masses, the acceleration voltages and distances, and the one adjustable parameter, the mean ion lifetime or decay rate. The distribution of thermal kinetic energies broadens the TOF peaks. This was taken into account by convoluting the calculated TOF distribution with a Gaussian function of appropriate width.
754
The Journal of Physlcal Chemistry, Vol. 86, No.
5, 1982
Fraser-Monteiro et al.
TABLE 11: Thermochemical Data Relevant t o the Dissociation of Ethyl Acetate
-.
ion
m/e 88
C,H,O,+
CH ,COOC,H,
73
C,H,02+
CH 3
70
C'lH',O'
61
C,H,O,+
C,H,
45
CH ,CHOH+
C2H,O
43
C,H,O+
29
CP,'
2,3y,
AH
(21) T. Schimanouchi,Natl. Stand. Ref. Data Ser., Natl. Bur. Stand.,
No. 39 (1972).
755
structure CH3C0
= -5.4 kcal/mol) for the neutral,
(26) D. H. Aue and M. T. Bowers in "GasPhase Ion Chemistry",Vol. 2, M. T. Bowers, Ed., Academic Press, New York, 1979, p 28. (27) N. E. Middlemiss and A. G. Harrison, Can. J . Chem., 57,2827 (1979). (28) J. L. Holmes and J. K. Terlouw, Org. Mass Spectrom., 15, 383 (1980).
756
The Journal of Physical Chemistty, Vol. 86, No. 5, 1982
we derive a thermochemical AE of 10.30 eV. The difference of 0.40 eV between it and the experimental value is far too large to be a result of a kinetic shift which in this case would be less than 0.1 eV. However, higher energy structures for either the neutral or the ion products or a reverse activation barrier could account for the 0.4-eV discrepancy. Because of the m / e 43 peak, we could not measure the kinetic energy release by TOF. In converting the AHfom(CH3CO)to AHfoo(CH3CO) we used the following vibrational frequencies taken in part from those of acetaldehyde:213005,2965,2915,1745,1440, 1420, 1350, 1115, 920, 865, 510, and 150 cm-'. 5. C2H30+( m l e 43) + C2H50. The most recent calcu1ationP of the possible structures for this ion indicate that the CH3C=O+ form is most stable, with a AHf0298= 630 k J / m 0 1 . ~Very ~ little is known about the neutral C2H50-, although a A H f O 2 9 8 has been determined to be -23 k J / Using these values we calculate a thermochemical AE of 10.76 eV which is close to the experimental value of 11.0 eV. The difference can be attributed to kinetic shift or reverse activation barrier. The conversion of the A H:, to AHm' for C2H50*was accomplished by taking vibrational frequencies from acetaldehyde and methanol.21 They were 3680,3005,2965,2915,2820,1745, 1440,1420, 1400, 1345, 1305, 1115, 920, 865, 765, 510, 350, and 150 cm-l. It is interesting that the m / e 43 signal in Figure 1 changes abruptly at about 11.9 eV. It suggests that a new channel for this ion opens up. This could be a direct cleavage of the C-0 bond in the enol form of ethyl acetate giving structure 10. However, without further information or calculations on C2H,0 structures and energies, it is not possible to be certain about this change in the PIE curve.
1'
h
Ob
10
6. C2H6+( m l e 29) + C2H302.The AH:o(C,H5+) is now well-known having been determined recently by two independent methods to be 913 kJ/m01.~?~l Using this value and the measured AP gives a AHfo0(C2H302)= -227 kJ/mol. The AHfoB8(C2H302) = -238 kJ/mol was calculated by using the following vibrational frequencies which were obtained from those of dichloroethylene, methanol, and acetic acid?' 3680, 3035, 1625, 1400, 1345, 1180, 1180, 1095, 875, 685, 655, 580, 580, 535, and 535 cm-'. The derived heat of formation of C2H3O2 from ethyl acetate agrees to within 4 kJ/mol with that obtained from the butanoic acid dissociation. The onsets from both precursors are high and therefore they may be subject to considerably kinetic shift. Nevertheless, the agreement between the two values suggests that the C2H302structures are the same, and that they are probably the lowest energy structures. B. Dissociation Rates. RRKMIQET Calculations. The decay rates derived from Figure 2 are plotted as a function of the ion energy in Figure 4. The solid line is an RRKM/QET calculation of the dissociation rate. Three parameters are needed to calculate the decay rate of an ion by the statistical theory. These are the dissociation energy and the set of vibrational frequencies for both the precursor ion and the transition state. A first attempt was (29) D. R. Yarkony and H. F. Schaefer, 111, J. Chem. Phys., 63,4317 (1975). (30) T. Baer, J . Am. Chem. Soc., 102, 2482 (1980). (31) D. K. Bohme and G. I. Mackay, J . Am. Chem. Soc., 103, 2173 (1981).
Fraser-Monteiro et al.
L
ti
I03
l 1
1
104
I
I
105 106 107 108 PHOTON ENERGY ( e V )
I
I
IO9
110
Flgwe 4. Decay rate, k ( E ) , as a function of the photon energy. The solid line is an RRKMlQET calculation with the frequencies of Table 111.
TABLE 111: Molecular Ion and Transition State Frequencies (cm-l) Used in the R R K M Calculations molecular ion 2983, 2983, 2983, 2983, 2941, 2910, 2910, 2 8 8 4 , 1 7 1 6 , 1 4 6 0 , 1 4 6 0 , 1 4 4 2 , 1413,1413,1373,1346,1263,1263, 1182,1108,1089,1077,997,952, 937, 7 6 8 , 7 6 0 , 590, 545, 460, 413, 378, 226, 226, 2 1 0 , 1 0 6
transition state
2983, 2983, 2983, 2983, 2983, 2983, 2941, 2941, 2910, 2 9 1 0 , 2 8 8 4 , 1 7 1 6 , 1460,1460,1422,1413,1413,1373, 1346,1263,1182,1108,1089,1077, 997, 952, 937, 768, 760, 590, 590, 545, 545, 4 6 0 , 4 1 3
made by using the precursor ion and the transition state. A first attempt was made by using the difference in the IE of ethyl acetate (10.01 eV) and the AE(10.31 eV) of C4H60+as the dissociation energy. This calculation gave rates which were about 100 times larger than the measured rates. In addition, the slope of k(E)vs. E was much too steep. We therefore conclude that the parent ethyl acetate ion rearranges to a more stable structure prior to dissociation. By carrying out a series of RRKM/QET calculations in which the parent ion energy was varied we obtained an energy which brought the calculated rates in agreement with the observed rates. This energy is 9.39 eV, Le., 0.62 eV lower than the IE of ethyl acetate. In doing these calculations, we fixed the 298-K dissociation limit a t 10.31 eV (AEo= 10.43) because there is no evidence for a kinetic shift. That is, the dissociation rate at the observed PI onset of 10.31 eV is considerably higher than the lower limit of our sensitivity. In order to reproduce the observed rates over the whole range of the data, it was necessary to assume a loose parent ion and a tight transition state (Table 111). That is, the three lowest ethyl acetate frequencies were replaced by high frequencies. This set of frequencies is characteristic for a ring structure. It is difficult to conclude much about the dissociation of the metastable ethyl acetate ion. The structures of the reacting molecular ion, and the transition state are unknown; and therefore the mechanism cannot be determined. We have considered some structures for the C4H802+molecular ion. These are structures 11-14 which are graphically compared in Figure 3. The heat of for(32) These heats of formation have been kindly given to us by J. L. Holmes.
J. Phys. Chem. 1982, 86. 757-759
/"\
acetate does not isomerize to its enol form. Structure 13 has a heat of formation which places it just 0.17 eV below the isomerized ethyl acetate ion. There may be enough uncertainty in the AH? for it to be considered. However, without additional evidence, it is not really justified.
CH, CH,CH#H
CH3 0 , CH,CH3
12
11
AH^^^^^ = 490"
V. Conclusions In summary, we know the energy of the isomerized molecular ion, but not its structure; we suspect that the transition state is cyclic; and we believe the product ion is structure 6 (CH3COCHCH2+).With all this information it should be possible to derive a mechanism and determine the structure of the molecular ion. However, as was already pointed outgJ1the loss of H20 is a very complicated reaction.
AH^^^^^ = 523
I+
I
OH
A
CH, CHCH,OH
757
1
13
AH^',^^ = 44432
mation of our isomerized C,H8O2+ion derived from the RRKM/QET fit of Figure 4 is 460 f 6 kJ/mol. From our in modeling the k(E) vs. E curves with the statistical theory we conclude that the theory is accurate for establishing energies to within 0.1 eV. The enol form of ethyl acetate (structure 14) has an energy 0.26 eV below our isomer. The rate data, therefore, indicate that ethyl (33) A. S. Werner and T. Baer, J. Chem. Phys., 62, 2900 (1975). (34) T. Baer, J.Electron Spectrosc. Relat. Phenom., 15, 225 (1979). (35) G. D. Willett and T. Baer, J.Am. Chem. Soc., 102,6774 (1980).
Acknowledgment. Tomas Baer acknowledges his sincere gratitude to Simon Bauer for providing an exciting and stimulating atmosphere during his 4 years at Cornell. Si's enthusiasm for the new and the unexpected were then, and still are now, a constant source of inspiration. We are grateful to the National Science Foundation for the support of a portion of this work. The structure and energetic studies were supported by the Department of Energy. We thank Professor John Holmes for sending us a preprint on the CA spectra of the C4H60+isomers. Finally, Maria L. Fraser-Monteiro and Luis Fraser-Monteiro thank their universities for a sabbatical leave and INVOTAN for a travel grant.
Formation of Vibrationally Excited CO in the O('D2)
+ C2H2Reaction
W. M. Shaub,+ T. L. Burks,t and M. C. Lln' Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375 (Receive& August 3, 198 1; In Final Form: September 8, 198 1)
The reaction of O(lD,) atoms with C2Hzhas been studied at room temperature by using the CO laser resonance absorption technique. Observed vibrational populations of the CO product were found to be consistent with the overall mechanism O('D2) + CzH2 CHZCO*+ Cot + CHZ(@A1).
-
Introduction We have recently studied the reaction of O(3PJ)atoms with 1-alkynes,including C2H2,using a CO laser to directly monitor the production of CO in various vibrational s t a t e ~ . l - ~The observed vibrational energy distributions of the nascent CO product could be reasonably accounted for by statistically expected distributions based on the well-accepted mechanism3g4 O(3PJ) RC2H RCHCOf* RCH COf
+
-
-
+
where R = H, CH3, C2H5,n-C3H7,and n-C4H9,and AHo = -47 to -50 kcal/mol. In this work we have investigated the reaction of the electronically excited oxygen atom, O(lD2),with CzH2in an attempt to detect CO as well as to measure its vibrational energy content if it is promptly formed. To our knowledge, no information is currently available in the 'National Bureau of Standards, Washington, D. C. 20234 Medical School, Georgetown University, Washington, D. C. 20007
*
-
literature concerning the kinetics and mechanism of the O(lD2)+ C2H2reaction.
Experimental Section We have previously reported the experimentalprocedure for the flash photolysis-CO laaser resonance absorption experiments.'~~ Briefly, the output from a frequencystabilized, line-tuned, CW CO laser was directed along the axis of a quartz flash photolysis tube that was temperature controlled to 293 f 1K. 0(lDz)atoms were produced by photodissociation of NOz at X L 200 nm. Typically, a mixture of C2H2,NOz, and SFB(which was used as a diluent in these experiments to assure rotational-transla(1) M. C. Lin, R. G. Shortridge, and M. E. Umstead, Chem. Phys. Lett., 37, 279 (1976). (2) M. E. Umstead, R. G. Shortridge, and M. C. Lin, Chem. Phys., 20, 271 (1977). (3) W. M. Shaub. T. L. Burks. and M. C. Lin. Chem. Phvs.. - . 45.455 . (1980). (4) J. T. Herron and R. E. Huie, Prog. React. Kinet., 8, 1 (1975). (5) M. C. Lin and R. G. Shortridge, Chem. Phys. Lett., 24,42 (1974).
This article not subject to U.S. Copyright. Published 1982 by the American Chemical Society
