J. Phys. Chem. 1995,99, 14207-14216
14207
ARTICLES Study of Chemiionization Reactions in the 0 -t C2H2 Reaction Mixture: Evidence for Involvement of the CH(X211) and CH(a4X-) States John M. Dyke,* Andrew M. Shaw, and Timothy G. Wright Department of Chemistry, The University, Highjield, Southampton, SO1 7 IBJ, U.K. Received: June 12, 1995@
Chemielectron and chemiion spectra have been recorded for the reaction of oxygen atoms with ethyne, C2H2, Four chemielectron bands have been observed with maxima at 0.06 f 0.04, 0.10 & 0.05, 0.23 f 0.04, and 0.50 f 0.10 eV, whose relative intensities depend upon the relative partial pressure of the reactants used. The chemiion spectra show HCOC to be a primary ion under all experimental conditions used. This evidence, as well as the behavior of the chemielectron bands with added quenching gases (N20, C h ,and NO) and comparison of the high kinetic energy onsets of the chemielectron bands with calculated reaction enthalpies, allows the 0.06 and 0.23 eV bands to be assigned to the 0 CH(X211) HCO’ e- and 0 CH(a4Z-) HCO+ e- reactions, respectively. The associative ionization reactions responsible for the bands at 0.10 and 0.50 eV cannot definitively be assigned, but suggestions are made on the basis of available evidence.
-
+
+
I. Introduction It has been known for some time that the collision of two neutral species can, under favorable conditions, lead to the formation of ionic products.’s2 Such reactions, termed “chemiionization reactions”, have been defined by Beny3s4as “processes that lead to the formation of free charges, electrons and ions under the conditions of chemical reactions”. This definition, which includes Penning ionization, is more general than a definition proposed by Fontijn’ which states that such processes should only include reactions “in which the number of elementary charge caniers is increased as a direct result of the formation of new chemical bonds”. This latter definition, however, is still rather broad for use in the present investigation. A more specific definition will, therefore, be adopted which is more appropriate to the present work where new bonds are formed in associative ionization reactions in the 0 C2H2 reaction mixture. A chemiionization reaction will be defined as a gas-phase reaction between two neutral reagents which results in the formation of new chemical bonds and the production of positive ions and electrons. Reactions of this type have been investigated in some detail in Southampton in recent years by electron spectroscopy and mass spectrometry, with most work being performed on metal oxidation reaction^.^-'^ Recently, however, chemielectron and chemiion spectra have been recorded for the reaction mixture of oxygen atoms with 2-butyne.” Two chemielectron bands were observed with band maxima at 0.06 f 0.04 and 0.27 f 0.06 eV, whose relative intensity depended upon the relative partial pressure of the reagents used. The experimental evidence suggested that these two bands could be assigned to the associative ionization reactions 0 CH(X211) HCO+ eand 0 CH3C CH3CO+ e-, respectively. A. Chemiionization Processes in Hydrocarbon Flames. Flames have attracted the interest of scientists for centuries. The presence of charged species in flames has been known since 1802 when ErmanI2 measured a potential between two elec-
+
+
-
+
+
-
+
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstmcrs, September 1, 1995.
0022-3654/95/2099-14207$09.00/0
-
+
+
trodes in a flame. Much later, it was realizedI3 that the levels of ionization in flames are far above the equilibrium values expected from the Saha eq~ati0n.I~ The first suggestion that this excess of charge may be a result of chemical reaction appears to have been made by Tufts in 1906.15 There are two major problems in identifying the ions arising from associative ionization reactions in a flame (the primary ions): the rapidity of ion-molecule reactions and the range of reactive neutral species present. These two facts can lead to the rapid scrambling of the nascent ion distribution, making the identification of primary ions very difficult. There are, however, two main suggestions in the literature for the primary ion in hydrocarbodoxygen flames: HCO+ and C3H3+. The suggestion that HCO+ is a primary ion was first made by Calcote;” however, the evidence presented in ref 17, and in many subsequent studies, was indirect. C3H3+ was first suggested as a primary ion in flames by Kistiakowsky et a1.I6 on the basis of shock tube experiments, where C3H3+ was the first ion to be observed. A direct method for the detection of primary ions has been presented by Bayes et a1.18,19This involved the use of a voltage to accelerate ions out of the 0 hydrocarbon reaction region into a mass spectrometer. Since the extraction voltage effectively reduced the residence time of ions in the reaction region, it was argued that the time in which ion-molecule reactions could occur was reduced. The total ion current was expected to increase with extraction voltage until a plateau is reached when the ion current is constant with applied voltage. At higher voltages the current is expected to increase again, since the energy imparted to the ions at these higher voltages allows collisional ionization to occur, which generates two charge caniers for each ion-molecule For identification of primary ions, it is necessary to perform a voltage dependence study on the total current, identify the plateau region, and then record the mass spectrum in this region. In the work of Bayes et ~ 1 . on ~ the ~ 30 ~ C2H2 ~ reaction mixture, a plateau was observed in the total ion current; the HCO+ signal was also observed to plateau in this region and was deduced to be a primary ion. Mass spectrometric evidence has been
+
+
0 1995 American Chemical Society
Dyke et al.
14208 J. Phys. Chem., Vol. 99, No. 39, 1995
o,/o
presented by Hayhurst and Jones22.23which shows that two chemiionization processes occur in hydrocarbon flames. One of these processes is shown to dominate in the reaction region of the flame, leading to C3H3+, and the other is shown to dominate in the “feather” of a flame leading to HCO’. (It should be noted that pyrolysis of hydrocarbons in the absence of oxygen24and reactions of photoexcited acetylenez5have also been shown to give rise to ions.) The reaction that is now almost unanimously agreed to give rise to HCO’ in low-pressure O/hydrocarbon reaction mixtures is
+
o(~P) CH
-
HCO+
+ e-
(1)
If this reaction is accepted as the main source of ions in hydrocarbon flames, the next question that arises is: what electronic states of CH are involved? Is only the ground state, CH(X211), involved or are excited states also involved? Although some workers have emphasized the importance of CH excited states,26others have concluded that CH excited states are not major contributors to chemiionization processes in hydrocarbon ~ x i d a t i o n . ~Obviously ~-~~ the role of CH excited states in hydrocarbon flames needs to be clarified, but it is noteworthy that enhancement of the CH(A2A) and CH(B2Z-) populations, by laser excitation from the CH(X211) state, in a low-pressure hydrocarbon flame increases the rate of the chemiionization reaction leading to HCO+ by 3 orders of m a g n i t ~ d e . ~ ~Of, ~ particular ’ relevance to this issue is the suggestion that the CH(a4Z-) metastable state is responsible for a large fraction of the chemiionization in hydrocarbon flames. This proposal was first made by B a y e based ~ ~ ~ on the observation that measured rate constants for reactions of CH(XZn)with a range of molecules were incompatible with the rate expression derived for chemiionization in the O/CzHz reaction system. Subsequently, a molecule in a 4E state was detected by Nelis et ~ 2 1 in . ~ an ~ O/C2H2 reaction mixture using laser magnetic resonance spectroscopy and it was deduced that this was the CH(a4Z-), state. Further work allowed a rotational spectrum to be recorded, which confirmed the a~signment.~~ Later, experiments by Phippen and B a y e on ~ ~the ~ effects of quenching gases on the chemiion current showed that there are at least two chemiionization routes leading to HCO+ in low-pressure O/C2H2 reaction mixtures at room temperature:
+ - HCO+ + eO(3P)+ CH(a4Z-) - HCO+ + e-
o(~P) CH(X’II)
(2)
(3)
Further work by Hou and B a y e ~ ~has ~ . ~led ’ to the measurement of the rate constants of CH(a4E-) with a variety of simple molecules, and these authors have critically examined the implications of these rate constants on the possible role of CH(a4X-) in hydrocarbon combustion chemistry. Also, HCO’ is a well-established species in interstellar clouds and one mechanism for its productions is thought to be chemiionizati~n.~~-~I B. Chemielectron Spectroscopy. The term “chemielectron spectroscopy” was first introduced over 20 years ago by Jonathan and c o - w o r k e r ~to~describe ~ their study of the O/C2Hz gas phase reaction mixture with electron spectroscopy. As only one chemielectron band was observed with a band maximum at 0.23 f 0.01 eV, it was concluded that there was only one chemiionization reaction occurring, in their experiments. Also, as the maximum of the chemielectron band coincided reasonably well with the expected exothermicity of reaction 2, it was concluded that the dominant associative ionization reaction,
C,H* Electrons
\’
Photon Beam
Figure 1. Schematic diagram of the reaction cell used in this work.
-
+
under the conditions used, was the reaction O(3P) CH(X211) HCO+ e-. It should be noted, however, that there were tails in the experimental electron kinetic energy distribution to higher and lower kinetic energy of the band maximum, which were attributed to experimental effects in the original work. Since then, little work has been carried out using this technique until a series of experiments were performed on associative ionization reactions of ~ r a n i u m , some ~ . ~ l a n t h a n i d e ~ ,and ~.~~~~ the alkaline earth metals9 with a number of oxidants. During the course of these investigations it was realized that the exothermicity of a reaction should be compared with the high kinetic energy onset (HKEO) of a chemielectron band rather than the band maximum.8 The position of the band maximum (the most probable kinetic energy (MPKE)) has no thermochemical significance; it is, however, the most obvious feature to measure in a chemielectron band and serves as a way of labeling a particular band in the presence of other chemielectron bands. It corresponds to the transition with the largest FranckCondon factor between the classical turning point of the reactant channel and the product ion potential curve.s In the present work, the chemiionization reactions occurring in the gas-phase O/C2H2 reaction mixture are reinvestigated. This work extends a recent study of the chemiionization reactions occurring in the 0/2-butyne reaction mixture’ I where a chemielectron band with an MPKE of 0.06 f 0.04 eV was assigned to the O(3P) 3. CH(X211) chemiionization reaction, reaction 2. If this assignment is correct, then the band at 0.23 eV observed in ref 42 from the O/C2H2 reaction mixture must be reassigned. Hence, one of the main aims of this work was to reinvestigate the associative ionization reactions occurring in the O/CZHZreaction system using a recently developed chemielectron spectrometer to provide a consistent assignment of the observed chemielectron bands.
+
11. Experimental Section The two spectrometers used in this work have been described previously” and are therefore described only briefly here. In both instruments, effusive beams of the reactants were introduced into a reaction cell (see Figure 1) from two orthogonal directions, in front of the entrance slits of a hemispherical electrostatic electron energy analyzer. Ethyne was introduced through the top of the ionization chamber of the spectrometer and oxygen atoms, produced by a 2.45 GHz microwave discharge of flowing oxygen in a boric acid coated glass tube, was passed into the reaction cell from the side. The microwave discharge cavity was positioned approximately 30 cm away from the reaction cell on the glass inlet tube, which was bent into a right angle close to where the inlet tube enters the ionization chamber of the spectrometer. The microwave discharge was positioned such that there was no direct line of sight between the reaction cell and the discharge. The use of a reaction cell
Chemiionization Reactions in the 0
+ C2H2 Reaction
in the spectrometer ionization chamber allowed higher local pressures to be used than the pumping system normally permitted. The first apparatus, which was used only for chemielectron studies, is a high-temperature photoelectron spectrometer'"3modified slightly for the proposed experiments. The second apparatus allowed both a chemielectron and a chemiion mass spectrum to be recorded under the same reaction conditions. On this instrument, which will be described elsewhere,# electron and ion acceleration voltages were applied to the reaction cell on a 20 ms duty cycle. These voltages were -2 V for electrons and typically f 1 5 to +45 V for chemiions. A gating unit allowed the consecutive detection of both positive ions and electrons from an O/C2H2 reaction mixture in the reaction cell.' I Alternatively, ions or electrons could be detected separately by choosing the extraction voltage appropriately. As described previously,' the mass-resolved and total ion currents were recorded using a quadrupole mass spectrometer (VG Quadrupoles, SXP600) at a series of different positive voltages applied to the reaction cell. As in the previous study of the 0 f 2-butyne reaction mixture," calibration of the energy scale of each chemielectron spectrum was achieved by using the He I (21.22 eV) photoelectron spectrum of 02(X3Z,)45 recorded at the same time as a chemielectron spectrum. This was achieved by allowing He I radiation from a helium discharge lamp, positioned on the side of the ionization chamber of the spectrometer, to enter the reaction cell. A beam blocker attached to the reaction cell was used to allow chemielectron spectra to be recorded in the absence of photoelectrons, but it could be moved backwards (see Figure 1) to allow He I radiation to enter the reaction cell and produce photoelectrons from the species ( 0 2 , 0, C2H2) in the cell. In most cases, calibration was achieved using the 0,' (B2Zi) 02(X3Z,) He I photoelectron band. Good agreement with the relative vibrational spacings and relative intensities of the vibrational components with those reported in ref 45 for the fifth band of molecular oxygen was always obtained, showing that the transmission of the spectrometer at low electron kinetic energies was good. As described previously,'' the first apparatus had better electron energy resolution than the second and so chemielectron spectra recorded on the first instrument are presented in this work; the chemielectron spectra obtained from the second apparatus, on which the chemiion spectra were recorded, are very similar. Also, the first apparatus, owing to the presence of smaller slits in the reaction cell and the absence of the mass spectrometer, was capable of working with higher local reagent pressures than the second apparatus. By recording photoelectron spectra of the oxygen discharge in the 11.O-14.0 eV ionization energy region, the yields of 0 2 (a'Ag) and O(3P) relative to 0,(X3Z,) could be monitored. From the known photoionization cross sections at the He I the relative partial pressures of 0,(X3CH), 02(a'A,), and O(3P) in the reaction cell could be calculated from relative photoelectron band intensities.'-I0 In a typical experiment, the relative 0(3P):02(alA,):02(X3Zi)partial pressures were estimated as 30:10:60, and it was found that over the oxygen pressure range used the [O] partial pressure was proportional to the total oxygen pressure. The pressures quoted in this work are estimated pressures in the reaction cell, based on pressures measured by an ionization gauge positioned on the wall of the ionization chamber of the spectrometer. (Independent experiments have estimated the difference between the pressure in the reaction cell and the pressure at the ionization chamber walls as approximately a factor of 20, over the reagent pressure ranges
-
J. Phys. Chem., Vol. 99, No. 39, 1995 14209
Figure 2. Chemielectron spectrum recorded for the reaction of discharged oxygen with ethyne at partial pressures of 2 x mbar of ethyne and 2 x mbar of discharged oxygen. Ordinate, counts-s-I; abscissa, electron kinetic energy/eV.
used.) The residence time of the neutral reagents in the reaction cell is estimated as approximately 0.5 ms. Photoelectron spectra recorded for discharged oxygen show bands associated with O(3P), 02(a'Ag), and O2(X3Z;).' When ethyne is added to the reaction cell, photoelectron and chemielectron spectra recorded for the reaction mixture with the oxygen discharge on, the discharge on but with O(3P) deacti~ a t e d and , ~ the discharge off confirm that the observed chemielectron bands arise from the 0(3P)/C2H2 reaction mixture. Also, experiments in which 0 atoms were used to initiate the reaction and a mixture of 02(a'Ag) and 02(X3Z,) was added independently produced nb change in the observed chemielectron bands apart from a dilution effect.
111. Results and Discussion
A. Chemielectron Studies. Figures 2-5 show chemielectron spectra recorded under different relative concentrations of 0 atoms and ethyne. Figure 2 shows the spectrum obtained under approximately equal partial pressures of the two reagents (typically 2 x mbar of ethyne and 2 x mbar discharged oxygen, one-third of which consists of 0 atoms). It consists of two chemielectron bands, labeled B and C, with maxima at 0.10 f 0.05 eV and 0.23 =k 0.04 eV. The latter band appears to be the same chemielectron band as observed by Jonathan et al.42 in the first investigation of this reaction with chemielectron spectroscopy; band B was not observed in the earlier work, probably because of the lower resolution compared to that achieved in the present work. Under conditions of excess 0 atoms, the chemielectron spectrum changed to that shown in Figure 3. Bands B and C in Figure 2 have been replaced by one band, labeled A, with a maximum at 0.06 f 0.04 eV. In contrast, under conditions of excess ethyne, typically 8 x mbar of C2Hz and 2 x mbar of discharged oxygen, the chemielectron spectrum changed to that shown in Figure 4. Band B is seen more strongly than in Figure 2, band C is not resolved, and a new band with a maximum at 0.50 f0.10 eV, labeled D, was observed. At even higher partial pressures of ethyne, band D becomes more intense than band B and dominates the spectrum (see Figure 5). As described elsewhere,* the high kinetic energy onset (HKEO) of a chemielectron band should, in principle, be equal to the exothermicity of the associative ionization reaction giving rise to that
Dyke et al.
14210 J. Phys. Chem., Vol. 99, No. 39, 1995 70
300
7
HCO+
60
50
A
.-8 50 C 3
2 40
H,CCO+ C,H,+
v
+
30 2
0
-E 20 0
I
I
10
H,O+
0
2
0
Figure 3. Chemielectron spectrum recorded for the reaction of discharged oxygen with ethyne at partial pressures of 2 x mbar of ethyne and 8 x mbar of discharged oxygen. Ordinate, countss-I; abscissa, electron kinetic energy/eV.
300
D
-?
v1
Y
C 3
s
0 0.5 K.E.leV Figure 4. Chemielectron spectrum recorded for the reaction of discharged oxygen with ethyne at partial pressures of 8 x mbar of ethyne and 2 x mbar of discharged oxygen. Ordinate, counts-s-I; abscissa, electron kinetic energy/eV.
103
D
-
?
C
3
0
0.5
K.E./eV Figure 5. Chemielectron spectrum recorded for the reaction of discharged oxygen with ethyne at partial pressures of 12 x mbar of ethyne and 2 x mbar of discharged oxygen. Ordinate, c o u n t s - I ; abscissa, electron kinetic energylev.
band; however, in practice, the experimental HKEO value may be lower than the actual reaction enthalpy because the true onset may not be observed because of poor Franck-Condon factors
19
27
29
39
42
51
52
mlz a.m.u. Figure 6. Chemiion spectrum obtained under the same reaction conditions as used to obtain the chemielectron spectrum in Figure 2, with f 2 0 V on the reaction cell. This positive ion mass spectrum shows relative ion intensities. The measured total ion current was 0.6 x lo-' A. It should be noted that, as shown, the ion signals of H30+, CzH3+, and HCO+ have been divided by 50 in this plot.
in the onset region. In this work, HKEOs could be measured for bands A, C, and D, as these bands could be recorded with no overlapping features on their high electron kinetic energy sides. However, because experimental spectra showing band B also contained contributions from bands C and D, the HKEO for band B could only be obtained from an approximate extrapolation. The HKEO values obtained for all four bands, averaged over all chemielectron spectra recorded under different conditions, are given in Table 1 together with their band maxima (the most probable kinetic energy, MPKE, values). The errors quoted in Table 1 for the MPKE and HKEO values were derived from a combination of the experimental error in the measurements as well as twice the standard deviations of the measured values taken from more than a hundred spectra. B. Chemiion Studies. The chemiion spectrum obtained from the reaction cell using the reagent partial pressures used to obtain Figure 2 is shown in Figure 6. It was recorded with +20 V on the reaction cell. As can be seen from this figure, the most intense ion signals were observed for HCO+ and C2H3+, consistent with the ion spectra obtained previously by Bayes and co-w~rkers'~ under similar conditions. Plotting the total ion current against the voltage applied to the reaction cell, for fixed reagent partial pressures, gave the graph shown in Figure 7a. The total ion current shows a plateau (the saturation current) at approximately +17 V, which continued to approximately +25 V before increasing again. A voltage in this plateau region, +20 V, was therefore chosen to record subsequent mass spectra at different reagent partial pressures. It was found that the HCO+ ion signal increases with applied reaction cell voltage whereas C*H3+, the only other prominent ion, decreases (see Figure 7, a and b). This result, which has been reported previously,'* is consistent with HCO+ being a primary chemiion in the O/C2H2 reaction system. The accepted production route of HCO+ is reaction 1 with C2H3' being produced from reaction 4. HCO'
+ C,H,
-
+
C2H3+ CO, AH = -0.35 eV (4)
As the oxygen atom [O] partial pressure was increased at fixed [C2H2], the HCO+:C2H3+ratio increased and signals from the hydrocarbon ions C3H3+, C4H3+, and C4H4+ decreased (see
Chemiionization Reactions in the 0
+ C2H2 Reaction
60
50
J. Phys. Chem., Vol. 99, No. 39, I995 14211 350
i
HCO'
60 . . . . . . . . . . . . . . ./ 50 . . . . . . . . . . . . . . . . ........
40
-
30
[CzH,]. The calculated reaction enthalpy of reaction 2 of -0.22 f 0.03 eV agrees well with the experimental HKEO value of band A of 0.25 f 0.10 eV. Hence, consistent with the assignment of this band presented in the earlier study of the OR-butyne reaction mixture with chemielectron spectroscopy," band A is assigned to the chemiionization reaction between O(3P) and CH(X211) (reaction 2). Attention was then turned to the possibility of assigning band C or D to the O(3P) CH(a4Z-) reaction, reaction 3. As shown in Figures 4 and 5 , bands B and D were observed together when [C2H2] > [O] and band D dominates when [C2H2] >> [O]; also, band C was observed when [C2H2] % [O] (see Figure 2 ) . Although no meaningful quenching experiments could be performed on band D, band C appeared to be unaffected by CHq addition but was efficiently quenched by NO. This result would imply, based on the work of Phippen and B a y e ~that ,~~ band C arises from a reaction involving CH(a4Z-). Band C is therefore assigned to the O(3P) CH(a4Z-) chemiionization reaction, reaction 3. If this assignment is accepted, then the measured HKEO of this band of 0.63 f 0.06 eV is lower than the calculated reaction enthalpy of 0.96 f 0.03 eV. This could be because of poor Franck-Condon factors in the onset region: a commonly observed feature of chemielectron It is felt, therefore, that this work, combined with the previous studies of Bayes et a1.,'8,32,35 provides strong evidence of 0 CH associative ionization reactions involving CH(X211) and CH(a42-) in the O/C2H2 reaction mixture. A very recent discharge flow, molecular beam mass spectrometric study of the 0 C2H2 reactions2 has led to the suggestion that CH(a4Z-) and CH(X211) are produced from the following reaction sequence
+
+
+
+
+
C2H2 0 H -t HCCO
HCCO
-
HCCO
+H
(9)
CH2 -t CO
(10)
+ o - CH(X~II) + CO, - CH(a4Z-) + CO,
(1 la)
(11b)
with a sizeable fraction of CH arising from reaction 11 being produced in the a4C- state. CH(X211) is also produced later in the O/C2H2 reaction scheme via H CH2 CH H2.'* This reaction is, however, not sufficiently exothermic to produce CH(a4Z-). In Figure 2 band C at 0.23 f 0.04 eV dominates and in the early chemielectron study of Jonathan et a1.$2 the 0.23 eV band was the only resolved band. Assignment of band C to the O(3P) CH(a4C-) chemiionization reaction is therefore consistent with a sizeable fraction of CH being produced from reaction 11 in the a4Z- state. An interesting feature of the quenching experiments performed in the present work was that when N20 was added under conditions when band C was observed, small partial pressures of N20 had little effect on this band (as expected for a reaction involving CH(a4Z-)), but larger additions caused an increase in the intensity of band C. No explanation can be given for this effect, although it appears that since only band C is affected, either N2O increases the production of CH(a4X-) or it blocks a reaction that removes it. Additions of N20 also caused the total ion current to increase. This effect has been observed previo u ~ l ybut , ~ no ~ explanation of this observation could be given. As can be seen on comparing Tables 1 and 2, the HKEO of band D is in very good agreement with the calculated enthalpy for the reaction 0 CH(a4C-) HCO+ e-; however, band
+
-
+
+
-
+
+
TABLE 1: Most Probable Kinetic Energies (MPKEs) and High Kinetic Energy Onsets (HKEOs) of the Chemielectron Bands Observed from the O/C*Hz Reaction Mixture band MPWeV HKEO/eV A 0.06 f 0.04 0.25 f 0.10 B 0.10 f 0.05 0.36 f 0.10 C 0.23 f 0.04 0.63 f 0.06 D 0.50 f 0.10 1.10 f 0.10 TABLE 2: Calculated Reaction Enthalpies for Possible Reactions Producing HCO+ and C3H3+ as Primary Ion9 AHfOleV reaction no. CH(X2n)+ 0 HCO+ e-0.22 f 0.03 2 CH(a4Z-)+ 0 HCO+ + e-0.96 f 0.03 3 -3.09 f 0.03 CH(A2A)+ 0 HCO+ + eCH(X2n)+ C2H2 C3H3+ + e2.63 f 0.02 7a 1.89 f 0.02 CH(a4Z-) + C2H2 C3H3+ + e7b CH(A2A) + C2H2 C3H3+ + e7c -0.24 f 0.02 0.95 & 0.05 8a C2(X'Zi)+ CH3 C3H3+ + eC2(a3n,) CH3 C3H3+ + e0.86 f 0.05 8b -0.70 f 0.05 8c C2(c3c)+ CH3 C3H3+ + e- 1.52 f 0.05 C2(d3n,)+ CH2 C3H3+ + e8d " Heats of formation at 298 K taken from refs 47- 4 9 , 63, and 64.
+
-----
+
D was only observed at high [C2H2]:[0] partial pressure ratios. No quenching experiments were possible on this band, and the results of the quenching experiments that were performed favor assignment of band C to this reaction (Le., reaction 3). The bands which remain to be assigned, bands B and D, are favored by low oxidant and high C2H2 partial pressures. Assignment of these bands is rather difficult as HCO+ is observed as the dominant chemiion under all reaction conditions used and bands associated with reactions 2 and 3 have already been identified. As noted earlier, C3H3+ has been suggested as a primary chemiion in hydrocarbon flames, although it was only observed weakly in the present work. It did, however, become more intense as the C2H2 partial pressure was increased and it became comparable in intensity to HCO+ when N20 was added to a O/C2H2 reaction mixture. Therefore, chemiionization reactions which produce C3H3+ were considered as possible candidates for assignment of bands B and D. Associative ionization reactions of CH, in either the X or the a states, with C2H2 to form C3H3+, via reaction 7, are endothermic ( A H 7 = 2.6 eV for CH(X211) and A H 7 = 1.9 eV for CH(a4Z-)) (see Table 2). Excitation of CH(X2H) to CH(A2A) is required to make the reaction exothermic. There is, however, no evidence for significant population of this state in the O/C2H2 reaction mixture5' and no chemielectron bands associated with 0 CH(A2A) HCO+ e- (AH = -3.1 eV, see Table 2) could be identified in this work. Reaction 8 should also be considered, as under fuel-rich conditions significant quantities of unoxidized carbon-containing species will be present. In particular, CH3 and excited states of C2 are known to be present in low-pressure O/C2H2 reaction mixtures and flames, although in low relative concentration^.^^.^^.^^ From Table 2, it can be seen that two chemiionization reactions involving excited states of C2 are exothermic and produce C3H3+ as the primary ion, reactions 8c and 8d. The calculated reaction enthalpy of reaction 8c is -0.70 eV (see Table 2) compared with the high kinetic energy onset of band B of 0.36 f 0.10 eV. This difference is not unexpected as the minimum energy geometry of C3H3' is a cyclic ~ t r u c t u r ewhereas ~ ~ . ~ ~ the C3H3+ produced from the C2 CH3 reaction is expected to have an open, noncyclic structure. The most stable noncyclic C3H3+ structure is the propargyl cation whereas the 1-propynyl cation (H$--C=C+) is higher in energy.5s Hence the associative ionization envelope is expected to be broad with low Franck-Condon factors in the
-
+
+
+
Dyke et al.
14214 J. Phys. Chem., Vol. 99, No. 39, 1995 onset region. Therefore, band B could possibly be assigned to reaction 8 involving C2(c3&+) (reaction 8c in Table 2) and band D could possibly be assigned to reaction 8 involving C2(d311,) (reaction 8d in Table 2). The calculated reaction enthalpy for this latter reaction is - 1.52 eV compared with the measured onset of 1.1 k 0.1 eV. It must, however, be emphasized that this assignment of bands B and D to reaction 8 involving different excited states of C2 is only a proposal at this stage because of the lack of evidence which establishes C3H3+ as a primary ion. However, if C3H3+ is a primary ion arising from reaction 8 its concentration must be reduced relative to that of HCO+ under the conditions used as the observed signals of C3H3+ were very low relative to those of HCO+. Possible ion-molecule reactions which could achieve this are
+
C3H3+ H,CCO
-
+
C3H3 H,CCO’
+ C3H3-t HCO’ C3H3++ HCCO - C3H2+ H2CCO+ C3H3+ HCO
(12)
(13) (14)
Available heats of formation show that reactions 12, 13, and 14 have enthalpies of -1.6, +1.5, and f 1 . 3 eV, respectively. As noted previously?2 HCCO is the dominant primary product of the 0 C2H2 reaction and H2CCO is a secondary product. Reactions 12 and 14 will convert C3H3+ into H2CCO’. This would be consistent with the observed ion spectra, recorded with +20 V on the reaction cell, where signals associated with C2H20+ and C3H3+ increase as the initial C2H2:O ratio is increased. Also, it should be noted that the enthalpies of reactions 12 to 14 have been calculated using the heat of formation of cyclic C3H3+, the most stable form of this ion. If, however, a substantialfraction of C3H3+ is produced in the linear form, the propargyl ~ a t i o n , ~ then ~ -the ~ ~enthalpies of reactions 12-14 are reduced by 1.0 eV, to -2.6, 0.5, and 0.3 eV. As has already been noted, bands A and C have been assigned to the O(3P) CH(X211) and O(3P) CH(a4C-) chemiionization reactions to give HCO+ e-, where HCO+ is the main chemiion observed under all reaction conditions. In previous studies of associative ionization reactions with electron spectroscopy, chemielectron spectra have been interpreted in terms of a classical turning point me~hanism.~.*,~’ In this mechanism, the reactants 0 and CH approach each other until the left-hand turning point of an HCO* state is reached. Autoionization then occurs onto an HCO+ curve and the most intense vibrational component in the chemielectron band corresponds to the component with the greatest overlap of the initial and final vibrational wavefunctions, in accordance with the FranckCondon principle. For the ground state reactants, O(3P) and CH(X211),the following states of HCO are symmetry allowed for a linear 0-CH approach: 4,2C-, 4,2C+,432A, and 4 3 2 1 1 ; for the CH(a42-) + O(3P) reaction, the allowed states in C,, symmetry are or 6 3 4 . 2 1 1 . If it is assumed that the ground electronic state of HCO+ is formed in the observed chemiionization processes (bands A and C), as seems likely as the next highest HCO+ state is ~5 eV higher in and is hence inaccessible, then the HCO* state formed must be in a doublet state for the autoionization process to HCO+ X’Z+ to be spinallowed. Also, production of the isoformyl cation (COH+) can be discounted as the ground state of COH+ lies x 1.7 eV higher in energy than the ground state of HC0+.60 Thus, for a collinear approach of the reactants, the CH(X211) 0 reaction can proceed via a ’E+,’X-,*II,or 2A HCO* intermediate and the CH(a4Z-) 0 route can proceed through a 2X+ or a 211HCO* intermediate.
+
+
+
+
634,2X+
+
+
Recently, Metropoulos et aL6’ have performed ab initio molecular orbital calculations at the MRDCI level on the chemiionization reactions 2 and 3, but only collinear 0-CH approaches were considered. Potential energy curves were computed for several states of HCO and the ground state of HCO+, the XIZ+ state, as a function of the 0-CH separation. It was concluded6’‘that the O(3P) CH(X211) reaction could occur via the HC0(12C+) state, and the O(3P) CH(4C-) reaction could occur via a classical tuming point on the HCO(X211)surface. If nonlinear geometries are considered then the X211 state will split into 2A’ and 2A” states and autoionization from both of these states could contribute to the observed chemielectron bands. Metropoulos et ~ 1 . point ~ ‘ out that the 0 CH(a4Z-) chemiionization reaction, via the HCO(X211) potential surface, can only occur provided nonadiabatic coupling between the 2211and X211states of HCO occurs in the incoming channel at relatively long 0 CH distances. It is of note that the 0 CH(X211) HCO(X211) HCO+ e- chemiionization pathway has a barrier of ~ 0 . 4eV in the incoming channel. It seems possible that this barrier will be lowered to near zero if nonlinear geometries are considered. In contrast, the 0 CH(a4X-) HC0(22C+) HCO+ e- pathway has a large barrier ( ~ 1 . eV) 6 in the entrance channel and it seems unlikely that this will be reduced to near zero if nonlinear orientations are considered and hence allow reaction to occur. Prior to the work of Metropoulos et u ~ . , ~ Ithe only other investigation that has been carried out to calculate possible states of HCO involved in the 0 CH associative ionization reaction was that of MacGregor and Berry.62 In that work, potential curves of the ground states of HCO and HCO+ were computed using the INDO semiempirical method. Linear and nonlinear geometries were considered, and potential surfaces of excited states of HCO were obtained from the ground state orbital eigenvalues using an approximate method devised by Huzinaga. This method is clearly very approximate, but it did identify an excited state of HCO of 2C- symmetry as being an important HCO* intermediate state for chemiionization. In fact, in ref 62 it is concluded that 0 CH(X211) chemiionization via this HC0(2C:-) state is the dominant channel. Angular momentum considerations demonstrate that a state of 2C- symmetry can only be produced from the 0 CH(X211)reaction and not from the 0 CH(a4X-) reaction. HCO ?Z- states were not considered in the work of Metropoulos et a1.,6I who only calculated HCO states of 211,?Z+, and 2A symmetry. Therefore, on the basis of the results of refs 61 and 62, the following reactions can be identified as possible chemiionization reactions leading to HCO+ e-.
+
+
+
+
-
+
-
+
--+
-
+
+
+
+
+
+
+
+ CH(X211) - HCO( 122+)(A’) 0 + CH(a42-) - HCO(X211) (A’”’’) 0 + CH(X211) - HCO(X211) (A’”’’) 0 + CH(X211) - HC0(2Z-) (A”) 0
(i) (ii) (iii) (iv)
(term symbols are given first in C,, and then in C, symmetry)
+
Hence, if 0 CH reactions are considered with linear approach of the reagents, three chemielectron bands would be expected, as reaction iii has too high a barrier ( ~ 0 . eV) 4 in its entrance channel. Alternatively, if nonlinear approaches are considered a maximum of six bands is expected. It is possible that the four chemielectron bands observed in this work arise from these routes.
Chemiionization Reactions in the 0
+ C2H2 Reaction
J. Phys. Chem., Vol. 99, No. 39, 1995 14215
It is clear, therefore, that although the initial molecular orbital calculations6'*62are useful, further calculations are required on HCO* states in C, as well as C,, symmetry. In particular, as well as the 2Z+, 211,and 2A states considered in ref 61, 2Zstates of HCO should also be considered. Related calculations are also required on the reaction C2* CH3 C3H3+ e- to investigate the possibility that bands B and D can be assigned to reactions 8a and 8b. E. Summary of Available Evidence for Assignment of Bands B and D. As is evident from the previous discussion, there are two possible chemiionization reactions which need to be considered for assignment of bands B and D:
+
-
+
+ CH HCO' + eC2* + CH3 - C3H3' + e0
.-..
(1)
(8)
Considering reaction 8 first, this would seem unlikely as HCO' was always observed in this work as the major chemiion under most experimental conditions. It was only when N20 was added to the reaction mixture that the [C3H3+] signal became comparable in intensity to the [HCO+] signal. Also, although reactions can be proposed which might reduce the [C3H3+]partial pressure at high [C2H2]:[0] ratios (e.g., reaction 12), other reactions can be proposed, which are exothermic, which would be expected to convert HCO+ to C3H3+ at high ethyne partial pressures, e.g.
+ C,H3 + HCO' C3H2 HCO'
-
+ CO, AH = -3.5 eV C3H3++ HCO, AH = -1.5 eV
C3H3+
(15) (16)
Also, C2* and CH3, the reactants in reaction 8, are expected to have very low steady state concentrations under the experimental conditions used; the C2* states proposed for assignment of bands B and D having allowed transitions to lower state^.^^,^^,^ This evidence means that assignment of bands B and D to reaction 8 is unlikely. The proposal that bands B and D can be associated with reaction 1, as are bands A and C, was also considered. This would be consistent with HCO+ being the dominant chemiion under all experimental conditions in the absence of added quenching gas. However, bands A, B, C, and D behaved independently as the partial pressures of [O] and [C2H2] were changed, and bands A, B, and C also behaved differently with added quenching gas (N20 and CH& It is possible that the reaction 0 CH(A2A) HCO+ e- is occurring but that the experimental HKEO value (e.g., 1.10 f.0.10 eV measured for band D) is a lot less than the reaction exothermicity (-3.09 eV, see Table 2) because of the Franck-Condon factors associated with the chemiionization process. It is also possible that as the partial pressures of the reagents are changed, CH(X2n)and CH(a4Z-) are produced with different degrees of excitation which allow different regions of the autoionizing HCO* surfaces to be accessed, thus giving rise to different chemielectron band shapes. These suggestions must await further experimental and theoretical investigations.
+
-
dominant chemiion. Two of the chemielectron bands were firmly assigned to the associative ionization reactions
+
IV. Conclusions
+
In this work, chemiionization reactions occurring in the 0 ethyne reaction mixture have been studied in the gas phase under effusive flow conditions, by monitoring the chemielectron energy distributions and the chemiions produced at different reagent partial pressures. Over the range of conditions used, four chemielectron bands were observed and HCO+ was the
o(~P)
+ CH(X~II) -HCO+ + e-
O(3P)
+ CH(a4Z)
-
HCO'
+ e-
The other two bands cannot be firmly assigned on the basis of the available evidence, although the possibility of assigning both of these bands to either C2* CH3 C3H3+ e- or 0 CH HCO+ e- has been considered.
+
-
+
+
-
+
Acknowledgment. The authors are grateful for financial support from the EPSRC and the European Community. The authors are also very grateful to Professor K. D. Bayes (UCLA, Los Angeles) for many useful discussions on this topic. References and Notes (1) Fontijn, A. Pure Appl. Chem. 1974, 39, 287. (2) Fontijn, A. frog. React. Kinet. 1972, 6 , 75. (3) Berry, R. S. In Molecular Beams and Reaction Kinetics; Schlier, C., Ed.; International School of Physics "Enrico Fermi"; Academic Press: New York, 1970. (4) (a) Nielsen, S. B.; Berry, R. S. In Recent Developments in Mass Spectrometry; Ogata, K., Hayakawa, T., Eds.; University Press: Baltimore, 1970. (b) Berry, R. S. Adv. Mass Spectrom. 1974, 6, 1. ( 5 ) Dyke, J. M.; Ellis, A. M.; Fehtr, M.; Morris, A. Chem. Phys. Lett. 1988, 145, 159. (6) Baker, J.; Barnes, M.; Cockett, M. C. R.; Dyke, J.; Ellis, A. M.; Fehtr, M.; Lee, E. P. F.; Morris, A,; Zamanpour, H. J . Electron Spectrosc. Relat. Phenom. 1990, 51, 487. (7) Cockett, M. C. R.; Dyke, J. M.; Ellis, A. M.; Fehtr, M.; Wright, T. G. J . Electron Spectrosc. Relat. Phenom. 1990, 51, 529. (8) Cockett, M. C. R.; Nyuliszi, L.; Veszprtmi, T.; Wright, T. G.; Dyke, J. M. J . Electron Spectrosc. Relat. Phenom. 1991, 57, 373. (9) Shaw, A. M.; Dyke, J. M.; Zengin, V.; Suzer, S. Chem. Phys. 1994, 179, 455. (10) Cockett, M. C. R.; Dyke, J. M.; Ellis, A. M.; Wright, T. G. J . Chem. Soc., Faraday Trans. 1991, 87, 19. (1 1) Dyke, J. M.; Shaw, A. M.; Wright, T. G. J . Phys. Chem. 1994, 98, 6327. (12) Erman, G. Ann. Phys. Leipzig 1802, 11, 150 (in German). (13) See for example: Sugden, T. M. Annu. Rev. Phys. Chem. 1962, 13, 369. (14) Saha, M. Philos. Mag. 1920, 40, 472. (15) Tufts, F. L. Phys. Rev. 1906, 22, 193. (16) (a) Kistiakowsky, G. B.; Michael, J. V. J . Chem. Phys. 1964, 40, 1447. (b) Glass, G. P.; Kistiakowsky, G. B.; Michael, J. V.; Niki, H. Tenth Symposium (International) on Combustion, Proceedings; Combustion Institute: Pittsburgh, 1965; p 513. (17) Calcote, H. F. Eighth Symposium (International) on Combustion, Proceedings; Combustion Institute: Pittsburgh, 1962; p 182. (18) Gardner, M. P.; Vinckier, C.; Bayes, K. D. Chem. Phys. Lett. 1975, 31, 318. (19) Vinckier, C.; Gardner, M. P.; Bayes, K. D. Sixteenth Symposium (International) on Combustion, Proceedings; Combustion Institute: Pittsburgh, 1977; p 881. (20) Cool, T. A. Appl. Opt. 1984, 23, 1558. (21) Lawton, J.; Weinberg, F. J. Proc. R. Soc. (London) 1964, A277, 468. (22) (a) Hayhurst, A. N.; Jones, H. R. N. Nature 1982, 296, 61. (b) McAllister, T. Nature 1982, 300, 199. (c) Hayhurst, A. N.; Jones, H. R. N. Nature 1982, 300, 200. (23) Hayhurst, A. N.; Jones, H. R. N. J . Chem. Soc., Faraday Soc. 2 1987, 83, 1. (24) (a) Bowser, R. J.; Weinberg, F. J. Combust. Flame. 1976, 27, 21. (b) Abrahamson, J.; Kenney, E. R. Twelfh Bienn. Carbon Conf., Pittsburgh, 1975, 169. (c) Tse, R. S.; Michaud, P.; Delfau, J. L. Nature 1978, 272, 153.
(25) Koyano, I.; Tanaka, I.; Omura, I. J . Chem. Phys. 1964, 40, 2734. (26) Kinbara, T.; Noda, K. Thirteenth Symposium (International) on Combustion, Proceedings; The Combustion Institute: Pittsburgh, 1971; p 333. (27) Amngton, C. A,; Brennen, W.; Glass, G. P.; Michael, J. V.; Nib, H. J . Chem. Phys. 1965, 43, 525, 1489. (28) Glass, G.P.; Kistiakowsky, G. B.; Michael, J. V.; Niki, H. J . Chem. Phys. 1965, 42, 608. (29) Zallen, D. M. Arch. Termodyn. Spalania 1975, 6 , 5. (30) Cool, T. A.; Tjossen, P. J. H. In Gas Phase Chemiluminescence and Chemi-Ionization;Fontijn, A., Ed.; North-Holland: Amsterdam, 1975.
14216 J. Phys. Chem., Vol. 99, No. 39, 1995 (31) Cool, T. A,; Tjossen, P. J. H. Chem. Phys. Lett. 1984, 111, 82. (32) Bayes, K. D. Chem. Phys. Lett. 1988, 152, 424. (33) Nelis, T.; Brown, J. M.; Evenson, K. M. J . Chem. Phys. 1988, 88, 2087. (34) Nelis, T.; Brown, J. M.; Evenson, K. M. J . Chem. Phys. 1990, 92, 4067. (35) Phippen, D. E.; Bayes, K. D. Chem. Phys. Lett. 1989, 164, 625. (36) Hou, Z.; Bayes, K. D. J . Phys. Chem. 1992, 96, 5685. (37) Hou, Z.; Bayes, K. D. J . Phys. Chem. 1993, 97, 1896. (38) See for example: Duley, W. W.; Williams, D. A. Interstellar Chemistry; Academic Press: London, 1984. (39) Dalgarno, A.; Oppenheimer, M.; Beny, R. S. Astrophys. J . 1973, 183, L21. (40) Millar, T. J.; Williams, D. A. Mon. Not. R. Astron. SOC.1975, 170, 51P. (41) Petrie, S.; Bohme, D. K. Astrophys. J . 1994, 436, 411. (42) Jonathan, N.; Morris, A.; Okuda, M.; Smith, D. J. J . Chem. Phys. 1971, 55, 3046. (43) (a) Dyke, J. M.; Moms, A,; Josland, G. D.; Hasting, M. P.; Francis, P. D. High Temp. Sci. 1986, 22, 95. (b) Bulgin, D. K.; Dyke, J. M.; Goodfellow, F.; Lee, E.; Morris, A. J . Electron Spectrosc. Relat. Phenom. 1977, 12, 67. (44) Dyke, J. M.; Shaw, A. M.; VeszprCmi, T. Manuscript in preparation. (45) Edqvist, 0.;Lindolm, E.; Selin, L. E.; Asbrink, L. Phys. Scr. 1970, 1, 25. (46) van der Meer, W. J.; van Der Meulen, P.; Volmer, M.; de Lange, C. A. Chem. Phys. 1988, 126, 385. (47) (a) Brix, P.; Herzberg, G. Can. J . Phys. 1954, 32, 110. (b) Herzberg, G. Molecular Spectra and Molecular Structure Ill: Spectra of Diatomic Molecules; van Nostrand Co., Inc.: New York, 1950. (48) Traeger, J. C. Int. J . Mass Spectrom. Ion Phys. 1985, 66, 271. (49) JANAF Thermochemical Tables, 2nd ed.; Stull, D. R., hophet, H., Eds.; Nat. Stand. Ref. Data Ser.; Natl. Bur. Stand: Washington, DC, 1971; Vol. 37. (50) Kasdan, A.; Herbst, E.; Lineberger, W. C. Chem. Phys. Lett. 1975, 31, 78.
Dyke et al. (51) Dyke, J. M.; Shaw, A. M.; Wright, T. G. In Gas-Phase Metal Reactions; Fontijn, A,, Ed.; North-Holland Elsevier: Amsterdam, 1992. (52) (a) Peeters, J.; Langhams, I.; Boullart, W.; Nguygen, M. T.; Devriendt, K. J. Phys. Chem. 1994, 98, 11988. (b) Peeters, J.; Boullart, W.; Devriendt, K. J. Phys. Chem. 1995, 99, 3583. (53) Abe, H.; Hayashi, H. Chem. Phys. 1992, 162, 225. (54) (a) Jones, A.; Padley, P. J. Chem. Phys. Lett. 1973, 20, 104. (b) Meier, U.; Kohse-Hoinghaus, K. Chem. Phys. Lett. 1987, 142, 498. (55) Li, W. K.; Riggs, N. V. J . Mol. Struct. 1992, 257, 189. (56) (a) Xie, Y.; Boggs, J. E. J . Chem. Phys. 1989, 90, 4320. (b) Lee, T. J.; Willetts, A.; Gaw, J. F.; Handy, N. C. J . Chem. Phys. 1989, 90,4330. (c) Cameron, A.; Leszczynski, J.; Zemer, M. C.; Weiner, B. J. Phys. Chem. 1989, 93, 139. (57) Blush, J. A.; Clauberg, H.;Kohn, D. W.; Minsek, D. W.; Zhang, X.; Chen, P. A m . Chem. Res. 1992, 25, 385. (58) Dyke, J. M.; Jonathan, N. B. H.; Moms, A,; Winter, M. J. Mol. Phys. 1980, 39, 629. (59) Bruna, P. J.; Peyerimhoff, S. D.; Buenker, R. J. Chem. Phys. 1975, 10, 323. (60) Ma, N. L.; Smith, B. J.; Radom, L. Chem. Phys. Lett. 1992, 197, 573. (61) Metropoulos, A.; Engels, B.; Peyerimhoff, S. D. Chem. Phys. Lett. 1993, 204, 333. (62) Macgregor, M.; Beny, R. S. J. Phys. E 1973, 6, 181. (63) Rosenstock, H.M.; Draxl, K.; Steiner, B. W.: Herron, J. T. J. Phys. Chem. R e t Data 1977, 6, Suppl. 1. (64) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV: Constants of Diatomic Molecules; van Nostrand Reinhold Co.: New York, 1979. (65) (a) Jones, I. T. N.; Bayes, K. D. Fourteenth Symposium (Inremational)on Combustion, Proceedings; Combustion Institute: Pittsburgh, 1973; p 277. (b) Jones, I. T. N.; Bayes, K. D. Proc. R. SOC.London 1973, A335, 547. (66) Becker, K. H.; Bayes, K. D. J. Chem. Phys. 1968, 48, 653 JP95 1613E
