J. Phys. Chem. 1984,88, 6162-6169
6162
predominantly through a stable triatomic intermediate. Our results indicate the reaction between I2and CIZproceeds at a rate comparable to the I2 Fz reactionZoand forms ICl. It seems reasonable to conjecture that the mechanisms may be similar for both reactions, and a stable IICl intermediate may account for the reaction between I2 and C12. Our experiments are, however, far from conclusive (for example, the possibility of a wall reaction on the surface of the flow tube cannot be eliminated). Further experiments are, therefore, needed to clarify the nature and rate of this reaction. When I(2Pl/2)is deactivated by IC1, the fast rate may also be due to the stable intermediate IICl. In addition, since there are twice as many ways to form I3from I(2Pl,z) Iz as there are to form IICl from I(2Pl,Z) IC1 (in the former case the I(2P1/2)can form a bond with either atom in I2 to form 13),the steric factor for I3 formation should be twice as large as the steric factor for IICl formation. Our results indicate the rate constant for I(2PljZ) deactivation by IC1 to be approximately half the previously determined rate constant for I(ZP1/2)deactivation by Iz, in agreement with this simple triatomic halogen model. After submittal of this manuscript for publication the authors have been informed that Houston and co-workersZ2have measured
+
+
+
1(2P1/2)deactivation by C1, in a photolysis experiment. They obtain a rate of 8 X cm3 molecule-’ s-’ in reasonable agreement with our value for k , . Note Added in Proof: Our analysis for the determination of the rate constant of the reactive channel of reaction 7, k7&,neglected the reaction C1+ IC1 C12 I. A previous determination of the rate constant for this reaction (ref 23) is inconsistent with our observed linear formation of C1. This indicates the measured rate constant for the reaction between C1 and IC1 may be too large or, alternatively, that the C1 formed in our experiment results from a secondary reaction (i.e. reaction 12). Further work to resolve this discrepancy is warranted.
-
+
Acknowledgment. We acknowledge Dr. J. D. Kelley and Dr. F. E. Hovis for helpful technical discussions during preparation of this manuscript. This research was conducted under Contract F29601-79-C-0087 sponsored by the Air Force Weapons Laboratory (AFWL), Air Force Systems Command (USAF), Kirtland Air Force Base, NM 871 17. Registry No. IC1, 7790-99-0; Clz, 7782-50-5; I, 14362-44-8.
-
(22) Houston, P. L., private communication. Houston suggests the possibility that the reactions C1 + CFJ IC1 + CF3 and CF, + C1, C1 C F X l form a chain reaction to Droduce IC1 and C1 which auench I(*P,!,\. Hetherefore interprets his meaiured value for kl as an uppk limit. (23) Clyne, M. A. A.; Cruse, H. W. J. Chem. SOC.,Faraday Trans. 2 1912, 68, 1377. +
(20) Valentini, J. J.; Coggiola, M. J.; Lee, Y. T. Faraday Discuss. Chem. SOC.1911, No. 62, 232. (21) Whitefield, P. D.; Davis, S.J. Chem. Phys. Lett. 1981, 83, 44.
\
+
A,”
Proton Transfer and Unlmolecular Decay In the Low-Energy Reaction Dynamics of H,O+ with Acetone W. R. Creasy and J. M. Farrar* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: July 20, 1983; In Final Form: August 13, 1984)
The title reaction was studied at collision energies of 0.83 and 2.41 eV with a crossed beam apparatus. At the lower collision energy both vibrationally excited and vibrationally relaxed reagent ions were used. Fcr all conditions, the proton transfer reaction has a direct mechanism, but there is poor agreement of measured distributions with the spectator stripping model. For the lower collision energy, regardless of reagent vibration, a similar fraction of total energy is transferred to product translation at low product translational energies. At higher collision energy, there is a significantly larger fraction of energy in product translation, which is consistent with an “induced repulsive energy release” effect. At the higher collision energy, three weaker reaction products were also detected: CH3CO+,C3HSt,and H2COH+. The last two products are produced from unimolecular decay of protonated acetone ions which are internally excited by vibrationally excited reagent ions. The CH3CO+ products are formed by a direct reaction.
Introduction The hydronium ion, H 3 0 + , plays an important role in many chemical systems. In the oxygen-rich region of flames, H30+is the most abundant ionic species.’q2 The ion, along with hydrated forms, H30+(H20),, is important in atmospheric chemistry as one of the predominant species in the tropo~phere.~The H30+ ion has been observed to be a very effective proton donor in the gas phase,es and it is also, of course, the predominant acidic species in most aqueous solutions. Despite the importance of the H30+ion, however, few detailed dynamical studies of its chemistry have been performed. Futrell et al.7 studied reactions 1 and 2 at collision energies of 0.65 to H30+ DzO H D 2 0 + H 2 0 (1)
+ H30++ DzO
--
+ H2DO+ + H D O
(2) 4.99 eV, using a crossed ion-neutral beam apparatus. The in-
* Alfred P. Sloan Foundation Fellow, 1981-1985. 0022-365418412088-6162$01.50/0
vestigators found that at high energies, reaction 1 was described well by the modified spectator stripping model,8 but at 0.65 eV, observation of multiple proton and deuteron transfers in reaction 2 suggested the formation of a persistent complex. Other gasphase studies of H30+have been conducted by Bohme et aL4s6 ~~~~
(1) D. K. Bohme in “Kinetics of Ion-Molecule Reactions”, P. Ausloos, Ed., Plenum, New York, 1979, p 323. ( 2 ) B. S . Fialkov and N. D. Shcherbakov, Russ. J. Phys. Chem., 54, 1513 (1980). ( 3 ) E. E. Ferguson in “Kinetics of Ion-Molecule Reactions”, P. Ausloos, Ed., Plenum, New York, 1979, p 377. (4) R. S.Hemsworth, J. D. Payzant, H. I. Schiff, and D. K. Bohme, Chem. Phys. Lett., 26, 417 (1974). ( 5 ) K. Hiraoka, Int. J. Mass. Spectrom. Ion Phys., 21, 139 (1979). (6) G. I. Mackay, S . D. Tanner, A. C. Hopkinson, and D. K. Bohme, Can. J . Chem., 57, 1518 (1980). (7) P. W. Ryan, C. R. Blakley, M. L. Vestal, and J. H. Futrell, J. Phys. Chem., 84, 561 (1980). (8) M. L. Vestal, A. L. Wahrhaftig, and J. H. Futrell, J. Phys. Chem., 80, 2892 (1976).
0 1984 American Chemical Society
The Journal of Physical C h e m i s t r y , V o l . 88, No. 25, 1984 6163
Dynamics of H30+with Acetone using the flowing afterglow technique and by Hiraoka5 using miss spectrometry. These studies showed that proton transfer of H30+ to many neutrals is generally rapid, with rates approaching the Langevin limit in numerous cases. In previous work from this laboratory, we have studied the reactions of HCO+9-1' and H3O+I2with water, methanol, and ethanol in a crossed beam apparatus and have observed the unimolecular decomposition products of the protonated alcohols. In all cases, the proton transfer reactions of both ions were direct stripping processes, followed by partial decomposition of metastable parent ions which live many rotational periods. Reactions of H 3 0 + 6and H C 0 + I 3 with acetone have been observed by Bohme et al. In neither case was decomposition of the resulting protonated acetone observed at 300 K, even though the formation of CH3CO+ is allowed energetically at thermal energies. The unimolecular decay of protonated acetone has been studied by Harrison and c o - w o r k e r ~as ~ ~well , ~ ~as by Williams et a1.I6through investigations of the decay of alcohols fragmenting to protonated aldehydes and ketones as well as by chemical ionization (CI) mass spectrometry. Fragmentation of the molecular ions of 2-propanol and 2-butanol yields protonated acetone and protonated propionaldehyde, respectively. From measurements of the metastable decay of these latter ions, including abundance studies, kinetic energy measurements, and metastable appearance energies, Williams and co-workers'6 suggested that the formation of daughter ions C3H5+and CHzOH+through the reactions 3 and 4 proceeds through a rate-determining isomerization of protonated (CH3)2C=OH+
+
CH3CHZCHOH'
+ H20 CHZOH" + CzH4
-+
+
C3H5+
(3)
(4)
acetone to protonated propionaldehyde. More recent s t u d i e ~ ' ~ of the chemical ionization mass spectra of five isomers of C3H60, including acetone and propionaldehyde, have confirmed this rate-determining isomerization. Ir. these studies, Bowen and Harrison measured fragmentation ratios for C3H60H+isomers protonated by a wide variety of CI reagents gases including H2, Dz, N2/Hz,COz/Hz,and CO/H2. The protonated acetone cation was found to be particularly stable, and a consideration of the competition between unimolecular decay and collisional stabilization of C3H60H+led the investigators to conclude that a large fraction of the protonation reaction exothermicity is partitioned into internal excitation of the nascent parent ion. In this study, we have performed a dynamical study of the proton transfer reaction 5 using a crossed beam apparatus at
H30++ (CH,),C=O
-
(CH,),C=OH+
TABLE I: Heats of Formation AH, kcal mol
H30+ (CH,),C=O (CHJ2C=OHt HZO CH3COC
AH, kcal mol CH, C3H5+ CH20HC
143a -5 1.5' 119c -57.0' 152'
C2H4
-15.99d 226e 169' 14.6b
"M. A. Haney and J. L. Franklin, J. Chem. Phys., 50, 2028 (1969).
*H.M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, J. Phys. Chem. Ref. Data, Supp. 1, 6 (1977). cComputed from the proton affinity of acetone, 194.6 kcal mol-', reported by R. Yamdagni and P. Kebarle, J. A m . Chem. SOC.,98, 1320 (1976). dNatl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 27 (1971). 'F. P. Losing, Can. J. Chem., 50, 3973 (1972). f K . M. A. Refaey and W. A. Chupka, J. Chem. Phys., 48, 5205 (1969).
n
2 t
'r
C H ~ O H + +C
.]-\.+
~
H
~
C H ~ C O + + C H+ ~( H ~ o )
I-.-..-
v L'+ CH3CH2~H=dH
(CH3)2C=OH +
(H20)
Figure 1. Schematic reaction coordinate for reactants, protonated acetone ions, and the three less intense product channels; barrier heights taken from ref 16.
reaction dynamics or unimolecular decay of the protonated acetone parent ion. These reaction products were studied at a collision energy of 2.41 eV. The heats of formation used to calculate the energetics are summarized in Table I, and the energy relationships among the reactants and products are shown in Figure 1.
Experimental Section The crossed beam apparatus used :n this study has been described in detail in the 1iterat~re.I~The H30+ions were prepared in two different ways: the first method employed electron impact in an ion source using a few percent H20in H2 gas according to the following chemical ionization reactions:
+ HzO AH = -1.3 eV ( 5 )
relative kinetic energies of 0.83 and 2.41 eV. At the lower collision energy, we have studied the role of reagent vibrational excitation by preparing H30+in two different ways, one of which yields vibrationally relaxed ions, the other producing ions with high vibrational excitation. We have also observed three ionic channels of lower intensity, corresponding to the ions CH3CO+,C3H5+,and CH20H+,with endothermicities, relative to protonated acetone, of 0.74, 2.1 7, and 2.71 eV, respectively. The experimental data provide insight into the reaction mechanisms for their production, either by direct (9) J. E. Moryl and J. M. Farrar, J . Phys. Chem., 86, 2016 (1982). (10) J. E. Moryl and J. M. Farrar, J. Phys. Chem., 86, 2020 (1982). (11) J. E. Moryl, W. R. Creasy, and J. M. Farrar, J . Phys. Chem., 87, 1954 (1983). (12) J. E. Moryl, W. R. Creasy, and J. M. Farrar, J . Chem. Phys., sub-
mitted for publication. (13) S . D. Tanner, G.1. Mackay, A. C. Hopkinson, and D. K. Bohme, Int. J. Mass Spectrom. Ion Phys., 29, 153 (1979). (14) C. W. Tsang and A. G. Harrison, Org. Mass Spectrom., 5, 877 (1971). (15) K. D. Bowen and A. G. Harrison, Org. Mass Spectrom., 16, 159 (1981). (16) G. Hvistendahl, R. D. Bowen, and D. H. Williams, J . Chem. SOC., Chem. Commun., 294 (1976).
We estimate that the total pressure in the ionization region is between 0.01 and 0.1 torr. As we have discussed in other work from our laboratory,l2 parent ions produced in this manner have substantial vibrational excitation, with a measurable fraction of the ions having excitation in excess of 1.3 eV. The second method of preparing H30+employs direct electron impact on pure water vapor, a procedure which yields vibrationally quenched hydronium ionsI8 via the following reaction:
+
H20+ H 2 0
-
H30+
+ OH
(8)
However, Koski and c o - w ~ r k e r s 'have ~ demonstrated that hydronium ions prepared in this manner may have a measurable component of an electronically excited metastable state. The proton transfer reaction 9, which is endothermic by 2.7 eV for H3O+ + H2 ---t H3+ + H2O
(9)
ground state ions, but becomes exothermic for the excited electronic state, as evidenced by the energy dependence of the reaction (17) R. M. Bilotta, F. N. Preuninger, and J. M. Farrar, J . Chem. Phys., 73, 1637 (1980). (18) J. L. Franklin and M. A. Haney, J. Phys. Chem., 73, 2857 (1969). (19) R. J. Cotter and W. S. Koski, J . Chem. Phys., 59, 784 (1973).
6164
The Journal of Physical Chemistry, Vol. 88, No. 25, 1984
Creasy and Farrar the final results; the equality of fluxes into a given solid angle in the two coordinate systems yields the following equality for a single Newton diagram:
"Ac Figure 2. Newton diagram depicting the relationship between laboratory
coordinates (~$3)and center-of-masscoordinates (u,O). total cross section, can be used as a qualitative indication of the excited state contribution to the beam. In the present experiments, we detected a very small H3+signal of about 0.1% of the protonated acetone signal intensity with beams produced by either method. We conclude that a very small component of electronis in the ion beam but too little to influence ically excited H30+ our results. The ions produced by these two techniques are fodsed, mass selected, and decelerated to the desired energy, and have fwhm energy and angular spreads of 0.3 eV and 2 O , respectively. Beam to 1 x lo4 4beams produced from currents range from 2 x pure water vapor were generally two to four times less intense than those produced by H2 C I and were significantly less stable, as well. Therefore, the only system studied under these conditions was the proton transfer at the lowest collision energy. The neutral beam is formed by bubbling Hz through 0 OC acetone and supersonically expanding the mixture through a 0.1-mm nozzle into a differentially pumped chamber, where the beam is collimated and modulated at 150 H z with a tuning fork chopper. Following this chamber, the beam enters the main torr by a trapped oil collision region, maintained at 5 X diffusion pump. The scattered products are energy analyzed, mass filtered, and counted by a multichannel scaler synchronized with the beam modulation under control of a minicomputer. A typical experiment consists of measuring laboratory fluxes at 6 to 10 angles, a t 50 different energies per laboratory scattering angle, each energy bin having a fixed energy width ranging from 0.04 to 0.06 eV. Typical count rates for detection of (CH&C=OH+ were approximately 50 to 100 cps. This large signal rate compares favorably with previous proton transfer experiments using the present a p p a r a t u ~ . ~ -The l ~ measured count rates for the daughter ion channels were 1 to 10 cps after subtraction of background counts. The low end of this range is near the limit of detection for this apparatus. The measured energy distributions for daughter ion production were noisy and were smoothed graphically prior to deconvolution.
Data Analysis The crossed molecular beam instrument acquires data in the form of laboratory flux at a particular velocity u and scattering angle 8; we denote such a flux by zlab(u,e), We seek to extract from these laboratory fluxes a set of corresponding polar fluxes ZcM(u,O) in center-of-mass coordinates, with speed denoted by u and scattering angle by 0. The relationship between laboratory coordinates (qe)and center-of-mass coordinates (u,0) is shown in the velocity vector kinematic Newton diagram of Figure 2. The transformation between laboratory and barycentric intensities has been discussed in many references,20 and we shall only mention (20) E. A. Entemann, Ph.D. Thesis, Harvard University, 1967; T. T. Warnock and R.B. Bernstein, J. Chem. Phys., 49, 1878 (1969); P. E. Siska, Ph.D. Thesis, Harvard University, 1969; K. T. Gillen, A. M. Rulis, and R. B. Bemstein, J . Chem. Phys., 54, 2831 (1971).
Since real experiments are performed with beams characterized by dispersion in velocity and intersection angle, and detection systems with finite energy and angular resolution, the observed lab flux at a given u and 8 is a convolution of barycentric fluxes over a range of scattering angles and velocities, the distribution for which depends upon the ion and neutral beam velocity distributions. Extracting the true barycentric differential cross section from experimentally determined fluxes requires numerical deconvolution procedures. Our standard method is the iterative method of Siska;21the specific manner in which we perform the calculation has been described in our previous p~b1ications.l~The result of the deconvolution procedure is a set of energy-independent barycentric fluxes, which, when transformed to the laboratory coordinate system with averaging over initial beam conditions, reproduce the experimental data with a standard deviation of 2-4%. The barycentric fluxes IcM(u,O) may be presented as contours of constant polar flux in the polar coordinates u and 0. Because the barycentric polar fluxes are actually doubly differential cross sections in velocity and angle, they may be integrated over appropriate coordinates, with the proper Jacobian, to yield singly differential cross sections. Two such singly differential cross sections of particular utility in understanding the experiental data are the product translational distribution P(ET') and the barycentric angular distribution g(0), obtained by integrating ZcM(u,O) over angle and velocity, respectively. In actuality, the energy and angular distributions are obtained from the data by summation over a finite number of points:
Relative cross section ratios are computed by integrating over the laboratory angular distributions of the appropriate channels. The mean velocity of the neutral beam is required for kinematic analysis and was determined empirically by adjusting the acetone beam velocity until the barycentric flux distribution for proton transfer was symmetric about the relative velocity vector. This procedure indicated that the neutral beam corresponded to a gas mixture composed of 7.5% acetone in H2gas. This concentration implies that the carrier gas was not saturated with acetone vapor at 0 "C, although the beam conditions were readily reproduced from day to day, with small variations.
Results The proton transfer reaction from hydronium to acetone was studied at a collision energy near 0.8 eV with vibrationally relaxed H30+as well as with the excited reagent produced by H2chemical ionization. At a relative collision energy of 2.41 eV, we studied the proton transfer reaction with unquenched reagents only. The barycentric polar flux contour maps for the lower energy experiments are shown in Figures 3 and 4, while the contour map for the higher energy experiment is shown in Figure 5. In both cases, the product peaks very sharply in the backward direction, relative to the incoming H 3 0 +ion, corresponding to abstraction of the proton by the incoming acetone molecule. This behavior is similar to that observed in our previous studies of proton transfer from HCO+ and H 3 0 + to alcohol^.^-^^ The reaction dynamics for the proton transfer reaction appear to be direct at both collision energies, with the incident acetone molecule removing a proton from H30+and forming a product moving in the same direction as the incident neutral. The spectator stripping (SS) provides the classic description of this (21) P. E. Siska, J . Chem. Phys., 59, 6052 (1973). (22) A. Henglein, K. Lacmann, and G. Jacobs, Ber. Bunsenges. Phys. Chem., 69, 279 (1965).
The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6165
Dynamics of H30+ with Acetone H30+
E,,I
+
=
(CH3)zCO
-
0 . 83 eV.
E,ib
(CH3)2COHf =
0
6 3
+
H20
eV
eo
8 Figure 6. Barycentric angular distribution g(0) for all (CH3)zCOH' experiments, as indicated.
v
5 a I
o4
I .o
crn/ssc
WE);
"(CH3)2CO
Figure 3. Barycentric polar flux contour map for (CH3),COHt production at 0.83 eV with reagent vibrational energy of 0.63 eV. The Newton diagram and the spectator stripping (SS) velocity are indicated. The barycentric origin is denoted by CM.
5
IO
0
20 E ? (eV)
30
Ere1 = 0 8 3 0 V
fz 0'
180.
"
(CH312C0 5 x
I O ~ C ~ / P . C
0
Figure 4. Barycentric polar flux contour map for (CH3),COHt production at 0.83 eV with reagent vibrational energy of 3.1 eV. H30+ E,,
+ (CH312CO=
(CH3)2COH+ + H 2 0
2 . 4 1 eV
CM
i -900
/ "iCHJI2CO
Figure 5. Barycentric polar flux contour map for (CH3),COH+ production at 2.41 eV. The Newton diagram and the spectator stripping ( S S ) velocity are indicated. The barcentric origin is denoted by C M .
process and invokes the assumption that the speed of the water molecule left in the collision has the same speed as the incoming hydronium before the collision. The contour maps indicate only
f
l'
Figure 7. Barycentric translational energy distributions for 0.8-eV collision energy (CH3),COH+ reaction with two reagent vibrational energies, as indicated. Top panel: Product translational energy distribution P(&'). Spectator stripping ( S S ) energy is indicated. Lower panel: Distribution of the fraction of total energy in product translation Pcfr').
qualitative agreement with the predictions of SS, however, a point which we will discuss later. Angular distributions for parent ion formation, obtained by integration of the barycentric fluxes, as described previously, are shown in Figure 6 . Consistent with the contour maps, the angular distributions confirm the backward scattered nature of parent ion formation. At the lower collision enegy, the barycentric angular distribution shows a prominent backward peak of half-width 20" and a very weak plateau extending to smaller angles. At 2.41 eV, this backward peak decreases in width to loo, essentially the instrumentally limited resolution, and the weak plateau to smaller angles disappears. Of particular interest is the fact that the barycentric angular distributions for the lower energy experiments are invariant to the degree of reagent vibrational excitation. The product translational energy distributions for the 0.8-eV collision energy experiments are shown in Figure 7; the top panel shows the distributions plotted as a function of the product translational energy ET'. Although the effect of increased reagent vibrational energy is clearly apparent in this graphs as a shift to higher kinetic energy release, the total energy available to the collision system is also higher and a more proper portrayal of this energy partitioning is given in terms of the reduced variable fT', defined as follows: fT'
=
ET'/Etotal
The variable fT' represents the fraction of the available energy appearing in product translation. The total energy of the system is the sum of kinetic energy, reaction exothermicity, and reagent internal excitation. The two methods of reagent preparation yield
6166 The Journal of Physical Chemistry, Vol. 88, No. 25, 1984
Creasy and Farrar
Erel = 2 41 eV
IO
I
I
#-
E;
(eV)
Figure 9. Barycentric polar flux contour map for production of CH3COt, at a collision energy of 2.41 eV. The Newton diagram, the center-ofmass origin (CM), and the velocity of the peak of the (CH3)$OHt
distribution (P) are indicated. f
T’
Figure 8. Barycentric translational energy distributions for 2.41-eV
collision energy (CH&COH+ reaction. Top panel: Product translational energy distribution P(ET’). Spectator stripping (SS)energy is indicated, as well as the threshold (T) to dissociation to C3H5+,assuming no barrier exists and both reagent H30+and product H 2 0 are vibrationally cold. Lower panel: Distribution of the fraction of total energy in product translation Pur’) using Etotal= 6.8 eV, Evlb= 3.1 eV. ions with very different vibrational state distributions; in the case of hydronium ion production by reaction 8, the reaction exothermicity is 1.O eV and the work of Franklin and HaneyIs indicates that the average product internal excitation is 0.63 eV, divided among H30+and 0I-jdegrees of freedom. We use this value as an upper limit for the internal energy of the vibrationally “cold” reagent ions; In contrast, the exothermicity of the C I reaction yielding hydronium is 3.1 eV, and there are no extant data to allow us to estimate the product vibrational distribution or its mean. Our experimental data on the reaction of hydronium ions with methanol and the subsequent unimolecular decay of the protonated methanol indicate that a measurable fraction of the ions have internal excitation in excess of 1.3 eV, and that the large excess of hydrogen in the ion source partially quenches the nascent distribution.’* Because of this poorly characterized internal energy distribution, we have chosen to assign the value of 3.1 eV, the exothermicity of the protonation reaction, to the H 3 0 +internal excitation, representing an upper limit. Our previous work on proton transfer from Hz+ to H2OZ3provides some justification of this conclusion in that this reaction, in which a proton is transferred from a light group to a heavier group, appears to create reaction products which are maximally excited internally, subject to constraints of product stability. The SS model also predicts high internal excitation in the reaction prducts for this particular mass combination. The unknown dispersion in the reagent iriternal energy and, therefore, in the total available energy in the present proton transfer reactions of H30+creates considerable uncertainty in assessing energy partitioning in this reactive system. When these criteria are used for assigning the total available energy, the vibrationally relaxed ions colliding at 0.83 eV bring a total energy of 2.76 eV to the collision system, while the vibrationally “hot” ions correspond to a total energy of 5.24 eV. The transbtional energy distributions for these two experiments are plotted as a function of fT’ in the lower panel of Figure 7. Although the shapes of these curves depend in large measure on the choice of total energy, the similarities between the PUT’) distributions for “hot” and “cold” H 3 0 +reagents are apparent. The average values OffT’ are 0.190 and 0.314 for hot and cold reagents, respectively. The only significant difference between the two plots is the depletion of high recoil energy products for
+
(23) R. M. Bilotta and J. M. Farrar, J . Phys. Chem., 85, 1515 (1981).
> 0.5 with vibrationally “hot” reagents. The fact that the vibrational excitation of H30+reagents created by Hz chemical ionization is partially quenched, reducing the number of ions with the maximum possible internal excitation, may be responsible in part for this absence of products with largefT’ values. The top panel of Figure 8 shows the translational energy distriution for production formation a t a translational energy 2.41 eV plotted as a function of ET’, while the bottom panel shows the PUT’)distribution using the internal energy assigned to Uhot” reagent ions. The average value of fT’ for this experiment is 0.35 1. Also indicated on the plot is the minimum threshold energy for dissociation by reaction 3, from reagents in their ground internal energy states. The presence of reagent internal excitation moves this threshold to higher translational energies. A significant fraction of the reaction products appear at translational energies below the threshold; some of these products correspond to metastable ions which may survive transit through the detector, while others correspond to the formation of reaction product pairs whose combined internal energy exceeds the bond dissociation energy for protonated acetone, but for which this internal excitation is shared between the protonated acetone and water products. This point will be addressed further in the discussion which follows. The predictions of the S S model are also shown on these translational energy distributions. With vibrationally cold reagents at the lower collision energy, the model overestimates the most probable kinetic energy. With vibrationally excited ions at 0.83 eV, the most probable kinetic energy is in reasonable accord with SS, but the experimental distribution is quite broad, in contrast to the sharp, kinematically constrained energy release predicted by SS. At high collision energy, SS seriously underestimates the most probable kinetic energy release. These observations are consistent with a variety of proton transfer reactions examined in our laboratory recently, which indicate the marginal utility of this simplistic model in complex collision systems. At the higher collision energy of 2.41 eV, the depletion of intensity in the recoil energy distribution at low values of ET’ suggests that a fraction of the protonated acetone parent cations undergo fragmentation prior to the detector. Accordingly, we have measured the laboratory flux distributions for allyl cation, C3H5+, and protonated formaldehyde, CHzOH+,hypothesized by Bowen and Harrison15 to occur through the sequential isomerization and fragmentation reactions 3 and 4. We have also measured the flux distributions for acetyl cation, CH3COf, whose production is endothermic by 0.74 eV relative to protonated acetone. Contour maps for these daughter ions channels are shown in Figures 9-1 1, indicating that these reaction products appear in the same region of velocity space as the parent ion. Also shown on Figures 9-1 1 is the point of parent ion maximum intensity. The C3H5+and CHzOH+ products appear at barycentric speeds less than the most probable speed of the parent, in a region of
fT’
7’he Journal ofphysical Chemistry, Vol. 88, No. 25, 1984 6167
Dynamics of H30+with Acetone H30+
+
(CH3)2C0 -C3H5+
+H20 +H20 ~
Erel =2.41 eV
Figure 10. Barycentricpolar flux contour map for production of C3H5+.
Figure 11. CH20H+.
Barycentric polar flux contour map for production of
velocity space where a fraction of the parents created by proton transfer from vibrationally excited reagents would have sufficient internal energy to dissociate. These observations support the claim the C3HS+and C H 2 0 H +result from unimolecular decay of the parent. In contrast, the CH,CO+ flux distributions is incomplete, with the apparent maximum occurring at the angular detection limit of our instrument. The maximum intensity in the CH3CO+ flux distributions corresponds to a larger barycentric speed than the most probable parent ion speed. Such parent ions would be less highly internally excited than those produced at the maximum and therefore less likely to decay. Therefore, it seems unlikely that the CH3CO+fragment ions result from unimolecular decay of excited parent ions, but from a direct reaction instead. We have further examined the unimolecular decay channels yielding C3H5+and C H 2 0 H +and the direct reaction producing CH3CO+ by measuring the branching ratios for these products at a collision energy of 2.41 eV, where the total energy is 4 eV above the highest dissociation limit of the protonated acetone cation. At this collision energy the reaction products are in the following ratios:
(CH3)2C=OH+:CH20H+:C3Hs+:CH3CO+ = 1.0:0.07:0.04:0.04 The fractional abundances of the C3HS+and CH20H+fragments, measured here, are in reasonable agreement with the CI mass spectra of acetone reported by Bowen and H a r r i ~ 0 n . l ~
Discussion The angular distributions for proton transfer at kinetic energies of 0.83 eV for both sets of H30+reagents and at 2.41 eV for vibrationally excited reagents indicate that the proton transfer reaction is a direct one in which acetone removes a proton from
H30+and forms a product scattered in the backward direction relative to the incoming H30+. Despite this apparent “stripping” behavior, the contour maps of Figures 3-5 and the recoil distributions of Figures 7 and 8 indicate that the most probable recoil speeds are generally not in agreement with the predictions of the simplistic spectator stripping (SS) modeLZ2 Vibrationally “quenched” reagents at 0.83 eV yield a broad distribution of translational energies, with the most probable recoil energy significantly less than the predictions of SS, while the peak for the vibrationally hot reagents at 0.83 eV appears to be in better accord with this impulsive model. At the higher collision energy of 2.41 eV, the most probable recoil energy is far in excess of the model prediction, an observation we have made in essentially every proton transfer reaction system we have studied.+12 Despite the fact that the reaction involves transfer of a light particle, the failure of the simplistic SS model implies that there are large interactions between departing products which lead to considerable momentum transfer. At the lower translational energy, the recoil energy distributions of Figure 7 indicate that a small fraction of the excess reagent translational energy appears as product translation, but that when the data are interpreted in terms of the partitioning of the total of available energy, the PUT’) distributions are essentially superimposable a t 1OwfT’. For the production of the most highly internally excited reaction products, the amount of energy available, but not its origin, determines the form of the translational distribution. The “prior” translational energy distribution, which assumes that products states are populated in accord with a microcanonical ensemble, depends only on fT’ and not on how the total energy is partitioned between translational and internal e ~ c i t a t i o n . ~Despite ~ the fact that the reaction appears to have direct dynamics, this independence of product energy disposal on the form of reagent energy hints at an underlying statistical behavior in the proton transfer dynamics. The PUT’) distributions differ significantly in their forms at highf,’. Because of considerable uncertainty in assigning the total energy of vibrationally excited ions, a lack of H 3 0 + with internal energy near the 3.1-eV exothermicity of reaction 7 would deplete the highf,’ tail of the curve. In addition, proton transfer reactions which leave the initial H30+vibrational excitation in the H 2 0 product would cause less energy to appear in translation than one would expect statistically. At the higher collision energy, the reaction products are more translationally excited than predicted by impulsive models. The polarization stripping model of Wolfgang and collaborators,2s modified by inclusion of electrostatic interactions in the spirit of the average dipole orientation (ADO) model of Bowers and coworkers,26 can account qualitatively for a small fraction of the enhanced translational energy release in this reaction. As H30+ and acetone approach, they are accelerated by the attractive interaction between the ionic charge and the neutral dipole and polarizability. In the exit channel, the smaller attraction of the charged product with the dipole moment and polarizability of H 2 0 decelerates the separating products to a lesser extent, resulting in a net transfer of energy to product translation. At higher collision energies, the phenomenon of “induced repulsive energy release”,27in which trajectories with high translational energy sample the “corner” of the potential energy surface where both the newly formed bond and the breaking bond are compressed, may lead to ejection of products into the exit valley of the potential energy surface with high translation and negligible internal excitation. The transfer of a proton between two heavy groups takes place on a potential energy surface with a small skew (24) R.D.Levine and J. L. Kinsey, in “Atom-Molecule Collision Theory”, R. B. Bernstein, Plenum, Ed., New York, 1979, p 693. ( 2 5 ) 2.Herman, J. Kerstetter, T. Rose, and R. Wolfgang, . . Discuss. Faraday SOC.,44, 123 (1967). (26) L. Bass, T. Su,W. M. Chesnavich, and M. T. Bowers, Chem. Phys. Lett., 34. 119 (1975); T. Su and M. T. Bowers, J . Chem. Phys., 58, 3027 (1973). (27) A. M. G. Ding,L. J. Kirsch, D.S. Perry, J. C. Polanyi, and J. L. Schreiber, Faraday Discuss., Chem. SOC.,55, 252 (1973).
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angle p;28on such a surface, enhanced translation preferentially samples the “cornern of the surface, leading to enhanced repulsive energy release. The H30+-acetone system is consistent with other “heavy-light-heavy” systems in exhibiting such energy partitioning at high collision e n e r g i e ~ . ~ - ’ ~ At the 2.41-eV collision energy, the combined incident kinetic energy plus reaction exothermicity yields a total energy for vibrationless reagents of 3.7 eV. This total energy exceeds the dissociation energies for channels 3 and 4, yielding allyl and protonated formaldehyde ions, respectively. The metastable mass spectral data of Williams and co-workersI6 suggest that these daughter ions are formed over an isomerization barrier in the range 2.35 to 2.78 eV above protonated acetone. When the maximum value of 2.78 eV was used as the threshold for decay, the parent ion translational energy, below which protonated acetone is unstable when formed in concert with a vibrationally cold H 2 0 molecule, is 3.70-2.78 = 0.92 eV. The translational energy distribution, shown in the top panel of Figure 8, indicates significant product intensity down to 0.1 eV. The appearance of reaction products in this “forbidden” region can be analyzed in terms of metastable products surviving transit through the detector and via the production of protonated acetone ions in concert with vibrationally and rotationally excited H 2 0 molecules which stabilize the parent against dissociation. Kinetic shift calculations described in Appendix A indicate that metastable ions with internal excitation as high as 0.4 eV in excess of a 2.35-eV barrier or 0.6 eV above a 2.78-eV barrier can survive energy analysis before decaying to daughter ions. This “kinetic shift” indicates that much of the parent ion intensity below 0.92 eV may be accounted for by metastable ions. Nevertheless, a statistically significant fraction of the protonated acetone parents may be formed in concert with internally excited H 2 0 products. An analysis of the translational energy distribution for protonated acetone cation formation at 2.41 eV reveals some interesting insight into energy disposal at this collision energy. All reaction products formed with translational energies higher than 3.7 eV must be formed from vibrationally excited H30+reagents, and approximately 30% of the products are so formed. Sixty percent of the reaction products have translational energies in excess of 2.41 eV and must, therefore, receive energy from H30+ reagent vibration and reaction exothermicity. Only 10% of the reaction products dissociate to allyl or protonated formaldehyde cation, however. The reaction dynamics and the high barriers to dissociation partition the available energy of the reaction preferentially into product kinetic energy rather than the vibrational energy of protonated acetone. This result contrasts with the inferences of Harrison and co-workersI5 that a large fraction of the reaction exothermicity for protonation appears in parent ion internal excitation. The reaction producing the acetyl cation, CH3CO+, appears to be direct, in contrast to the production of C3H5+and CH20H+. The appearance of reactively scattered flux at velocities forward of the most probable parent ion velocity argues against unimolecular decay as the mechanism for acetyl formation. Bohme et aL6 have suggested that the reaction proceeds by direct protonation of the methyl carbon in acetone, followed by C-C bond cleavage:
H30++ (CH3)2C=0
-
CH3CO+ + CH,
+ H20 AH = -0.6 eV
This reaction was not observed in thermal energy studies, suggesting the presence of a large activation barrier arising from the geometric constraint of protonation on the carbon, rather than the nucleophilic carbonyl oxygen. The mass spectrometric studies of C3H60 compound^'^ also do not indicate the occurence of a mass 58 43 metastable, indicating that unimolecular decay on the microsecond timescale is not the mechanism for acetyl formation. Because of the indeterminate kinematics of collision induced dissociation, it is difficult to be quantitative about the dynamics
-
(28) J. 0. Hirschfelder, Int. J. Quantum Chem., 3S,17 (1969). @isabout 15” for the H30c-acetone system.
Creasy and Farrar TABLE II: Molecular Frequencies for Protonated Acetone Isomerization u/
mode
assignment
activated ion
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
CH, d stretch CH, s stretch CO stretch CH, d bend CH, s bend CH, rock C C stretch CCC bend CH, d stretch CH3 d deform CH3 rock torsion CH3 d stretch CH3 s stretch C H d bend CH, s bend CC stretch CH3 rock C O ip bend CH3 d stretch CH, d bend CH, rock C O op bend torsion 0 - H stretch 0 - H bend 0 - H bend
3019 2937 1700 1435 1364 1066 777 385 2963 1426 877 105 3019 2937 1410 1364 1216 891 530 2972 1454 1091 484 109 3700 1345 1345
cm-‘ transition state
RC6 2937
500 1435 1364 1066 1000 600 2963 1426 877 105 3019 2937 1410 1364 1500 89 100 2972 1454 1091 100 109 3700 1345 1345
Vibrational frequencies adapted from T. Shimanouchi, Natl. Stand. Ref.Data Ser., Natl. Bur. Stand., No. 39 (1972). bReaction coordinates.
of CH20H’ and C3H5+ formation. The appearance of the fragments of barycentric speeds smaller than the most probable parent speed is energetically consistent with the interpretation of these channels as arising from unimolecular decay of protonated parent. Since the most probable speeds of the C H 2 0 H + and C3H5+in the center of mass of the H,O+-acetone collision system correspond to the decay of parents which were formed with translational energies of 2.0 to 2.4 eV, while the total energy of a collision with vibrationally cold H30+is 3.7 eV, unimolecular decay to products over a barrier of height 2.4 to 2.8 eV requires participation of vibrationally excited H30+reagents. The initial reaction dynamics which form protonated acetone must be direct, resembling a stripping reaction in analogy to the “stripping plus dissociation” mechanism proposed by Mahan and S c h ~ b a rfor t~~ the formation of DC02+ from the reaction of C 0 2 +with D2, and its subsequent decay to DCO+. The branching ratio for CH20H+ to C3H5+favors the production of the more endothermic CH,OH+ channel by a factor of 1.8, consistent with the results of Williams et a1.,16 and supportive of the interpretation that the transition state for allyl cation formation is tight, with an appreciable exit channel barrier. The schematic reaction coordinate of Figure 1 indicates that C3H5+and C H 2 0 H + may be produced by unimolecular decay of protonated acetone. The direct production of CH3CO+ is also indicated, along with an entrance channel activation barrier.
Conclusions Reactive scattering studies of proton transfer from hydronium ion to acetone at collision energies of 0.83 and 2.41 eV indicate that the reaction is a direct process; however, simplistic models, such as spectator stripping, fail to account quantitatively for the collision dynamics because of significant momentum transfer between reactants and products. The preferential partitioning of excess translational energy into product translation is consistent with “induced repulsive energy on the highly skewed potential energy surfaces characteristic of proton transfer between (29) B. H. Mahan and P.J. Schubart, J . Chem. Phys., 66, 3155 (1977).
The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6169
Dynamics of H30+with Acetone
the instrument transit time, and the detector sensitivity. A parent ion with energy E* will decay to thejth daughter ion with a rate constant kJ(E*). For parents or daughters detected at time T , we may calculate the parent abundance Ap(E*,7)or daughter abundance A ~ ( E * , Tfrom ) the following relations:30
6 5
Ap(E*,7) = exp[-Ck,(E*)~] J
AJ(E*,T) = (k,/CkJ)(l - exp[-Ck,(E*)~I) J
\
;; .25
a
\I
0
2.4
2.6
2.8 3.0 E*, eV
3.2
3.4
Figure 12. Top panel: RRKM rate constant calculations for rate-limiting isomerization of protonated acetone to protonated propionaldehyde, with frequencies listed in Table 11. Isomerization barrier heights of 2.35 and 2.78 eV are indicated. Bottom panel: Parent ion abundance vs. internal excitation for isomerization barrier heights of 2.35 and 2.78 eV.
two heavy groups. The role of excess vibrational energy in the H30+reagents at a collision energy of 0.83 eV suggests that the fraction of the total energy appearing in the most high internally excited products does not depend on whether this energy is translational or vibrational in origin. Production of daughter ions CH3CO+,C3HS+,and C H 2 0 H + appears to involve direct reaction dynamics for acetyl formation and unimolecular decay of protonated acetone for allyl and protonated formaldehyde generation. The latter appear to be formed by decay of protonated acetone parents formed in stripping reaction of vibrationally excited H30+which partition energy into vibration rather than translation. The relatively small fraction of protonated ions with internal excitation sufficient to surmount dissociation barriers decay with a C3H5:CH,0H+ branching ratio of 1.8, in qualitative agreement with the results of Harrison and co-workers.
Acknowledgment. We thank the U S . Department of Energy for its support of this research. W.R.C. acknowledges the University of Rochester for the award of a Sherman Clarke Fellowship and the Hooker Foundation for support as an Elon Huntington Hooker Graduate Fellow. Appendix A. Kinetic Shift Calculation The large number of degrees of freedom accessible to the protonated acetone parent ion and the finite flight time of ions through the crossed beam instrument lead to the possibility that parent ions excited above their dissociation limit may survive transit through the instrument and be detected as parents rather than daughters. This survival of superexcited molecules results in an apparent displacement of the dissociation threshold to larger values than the true value; this displacement is referred to as the "kinetic shift" and depends on the lifetime of the decaying complex,
I
The summation extends over all decay channels accessible to the parent at energy E*. The detection time corresponds to the transit time through the instrument prior to final mass selection; for these experiments, the transit time is approximately 60 ps. Although the protonated acetone cation decays to the allyl cation as well as protonated formaldehyde, the work of Williams and collaborators,16 as well as that of Bowen and H a r r i s ~ n , ' ~ indicates that the isomerization of protonated acetone to protonated propionaldehyde is rate determining. Accordingly, we have computed the parent ion abundance using rate constants for this isomerization step. Bowen and H a r r i ~ o n 'concluded ~ that the CH2CH(CH3)0H+ion was a likely intermediate in the isomerization; because this ion's heat of formation is -2.43 eV higher than that of protonated acetone, comparable to the observed activation energy for fragmentation, these workers noted that the transition state for the isomerization must be similar in structure, and their work indicated that the activation barrier lies between 2.35 and 2.78 eV in height. Our RRKM rate constant calculat i o n ~assume ~ ~ a protonated acetone structure for the energized molecule, and a transition state resembling the CH2CH (CH3)OH+ structure. The frequencies chosen for these calculations are listed in Table 11. The computed rate constants are relatively insensitive to small variations in the frequencies but depend very sensitively on the barrier height. Because the work of Bowen and Harrison indicates a 0.2-eV uncertainty in the isomerization barrier, we have performed rate constant and parent ion abundance calculations with barrier heights of 2.35 and 2.78 eV to yield a range of energy-dependent abundances. The results of these calculations are shown in Figure 12 and indicate that the threshold rates are quite low, generally less than 100 s-'. The parent ion abundances drop to 10% between 0.4 and 0.6 eV above threshold. At a relative collision energy of 2.41 eV, the standard deviation of the maximum observed counting rate for parent ions is 10% for a 100-s count. Therefore, we estimate that our instrument sensitivity limitation corresponds to a parent abundance of 10%. With this detection sensitivity, we conclude that the kinetic shift for this particular ion under the present experimental conditions is 0.4 to 0.6 eV. The RRKM calculations that we employed do not include angular momentum conservation. The geometries suggested for the energized ion and the critical configuration differ significantly only in the placement of hydrogen atoms. We therefore do not expect that the explicit inclusion of angular momentum conservation in the calculations will change these estimates of the kinetic shift perceptibly. Registry No. H,O+, 13968-08-6; (CH&C=O, 67-64-1. (30) M. Vestal and J. H. Futrell, J . Chem. Phys., 52, 978 (1970). (31) W. L. Hase and D. L. Bunker, Quantum Chemistry Program Exchange, Catalog No. QCPE-234.
