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Reactivity of vinyl chloride ionic clusters
Chemical Physics 239 Ž1998. 331–343
Reactivity of vinyl chloride ionic clusters b S. Martrenchard a , C. Dedonder-Lardeux a , I. Dimicoli b,) , G. Gregoire , C. Jouvet a , ´ M. Mons b, D. Solgadi a a
Laboratoire de Photophysique Moleculaire du CNRS, Bat. ´ ˆ 210, UniÕersite´ de Paris-Sud, F-91405 Orsay Cedex, France b CEA - Saclay, DSM-DRECAM, SerÕice des Photons, Atomes et Molecules, F-91911 Gif-sur-YÕette, France ´ Received 16 April 1998
Abstract The reactivity of vinyl chloride ionic clusters has been investigated by the Threshold PhotoElectron PhotoIon COincidences technique. In the case of the dimer, the competition between the three reactive channels ŽHCl, Cl Ø and CH 2 Cl elimination. has been studied. The main reactive channel is HCl elimination which proceeds through a 0.2 eV barrier. This elimination reaction is still observed in the trimer but not in larger clusters. For these clusters, cooling by evaporation of neutral vinyl chloride monomers seems to be the favored channel that hinders the HCl elimination step. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The reactivity in the gas phase is often quite different from the reactions in the condensed phase mostly because of solvent effects. The influence of the solvent molecules on a chemical reaction mechanism can be investigated in the gas phase by using molecular clusters as powerful model systems in which the reacting pair is gradually embedded in a solvent. Indeed, the energetics of the reaction is changed by adding more and more solvent molecules in the cluster and this effect has been clearly evidenced in several chemical reactions such as proton transfer w1–4x, ionic nucleophilic substitution w5–9x or cationic polymerization of unsaturated molecules w10,11x. The reactivity of vinyl halides, especially vinyl chloride C 2 H 3 Cl which will be denoted VCl, has )
Corresponding author. Fax: q33-1-69-08-87-07.
received significant attention since the ionization of VCl induces cationic polymerization. The first steps of this polymerization reaction has been previously studied by different methods and the important results of these works will be given in the following paragraphs. The ion molecule reaction of VClq with VCl has been studied in Ionic Cyclotronic Resonance ŽICR. experiments by different groups w12–14x. The first step of the reaction has been carefully studied by Ausloos and co-workers w14x: in this work, the ions are produced by one-photon ionization at two energies, the first one just above the vinyl chloride ionization threshold Ž10.03 eV. and the second at 11.7 eV, where the ions are produced with some internal energy. In the two cases, three reactive channels have been evidenced: – HCl elimination leading to C 4 H 5 Clq; – Cl Ø elimination leading to C 4 H 6 Clq; – CH 2 Cl elimination leading to C 3 H 4 Clq.
0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 5 5 - 9
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The branching ratio between the three channels has been found to change with the ions residence time in the ion trap and with the ionization energy. The observation of the three channels at 10.03 eV gives a lower limit of the activation barriers for these reaction channels. It is noteworthy that in this kind of experiments, the initial energy deposited in the collision pair cannot be less than the ionization energy of the molecule. For ‘non-thermalized’ ions Ži.e., at t s 0. the major channel at 10.03 eV is HCl elimination and CH 2 Cl elimination when a 11.7 eV photon is used. This implies that CH 2 Cl elimination channel presents a higher apparition threshold than HCl elimination but the corresponding rate constant is probably higher at 11.7 eV. The percentage of the HCl elimination product increases with time. The evolution of the branching ratio versus the residence time in the trap can be explained by the internal energy decay induced by collisions. At longer time, the internal energy of the ion decreases, therefore the most important channel will be the one with the lowest barrier, i.e. the HCl elimination. The successive polymerization steps have been followed in ‘high-pressure’ sources w13,15x, leading q and other to the formation of C 6 Hq 7 , C 6 H 8 Cl fragments which must come from ions which have much more internal energy that in the study presented here. Ionic reaction of VCl clusters has been studied by El-Shall and Schriver w11x in a molecular beam experiment by ionization through electron impact Ž70 eV.. In their mass spectrum, besides the parent mass peaks ŽVCl. n , several other peaks corresponding to addition–elimination reaction products have been observed. A mechanism, involving stepwise additions of VCl molecule followed by HCl or Cl Ø elimination has been proposed to account for the small clusters cationic polymerization. However, in electron impact experiments, the internal energy distribution of the ionized clusters is not known and is probably fairly large. This energy will be dissipated by an intensive evaporation process and therefore the information on the initial size of the reactive cluster is lost. Thus, this technique does not allow energetic measurements or size selective studies. Recently Sheng et al. w16x reported on the reactivity of the vinyl chloride clusters ionized by syn-
chrotron radiation. In this study the energy has been varied in a relatively large domain Ž9.4–10.5 eV. but no control on the internal energy distribution of the ions has been achieved. Only one reaction channel, the HCl elimination, has been observed and the reaction product appearance threshold coincides with the ionization threshold of the dimer. From this result it was concluded that this reaction proceeds without barrier. In the present paper, small ionized vinyl chloride clusters reactivity has been studied using synchrotron radiation to initiate the reaction. Owing to the temporal structure and high duty cycle of this source, a threshold photoelectron–photoion coincidences technique has been implemented to control the energy imparted to the cluster by the photon absorption. The photon energy has been continuously varied Ž9.3– 10.5 eV. in order to investigate precisely the energetic of the first steps of cationic polymerization as a function of cluster size.
2. Experimental setup The SAPHIRS experimental set-up, built on the Orsay synchrotron radiation facility has been described in detail previously w9x. It consists of a continuous supersonic beam coupled to a double electron – ion time-of-flight coincidence spectrometer situated at right angle to the plane defined by the molecular and photon beams. In the present experiments, the synchrotron radiation is used to ionize the clusters in the 9.3–10.5 eV range. The clusters are produced by expanding vinyl chloride ŽVCl. diluted in helium, through a 50 mm nozzle under typical backing pressures P0 between 1 and 3 bar. The cluster size distribution has been modified by varying the VCl concentration in the He buffer gas. At low concentration Ž5% VCl. and low pressure Ž P0 s 1.2 bar., the formation of only small ŽVCl. n clusters Ž n F 2 or 3. is obtained. At higher concentration Ž20% VCl. and higher pressure Ž P0 s 3 bar., clusters with sizes up to n s 7 are produced. The mass spectra of the ŽVCl. n clusters have been recorded using the Threshold PhotoElectron–PhotoIon COincidence ŽTPEPICO. method developed in Orsay by Guyon et al. w17x.
S. Martrenchard et al.r Chemical Physics 239 (1998) 331–343
The linear time-of-flight mass spectrometer combined with the detection of threshold electron is the one developed and tested by M. Richard-Viard w18x. Photoelectrons, accelerated by a small electric field Ž2 Vrcm., are selected by angular discrimination using a small diaphragm ŽB 2 mm. on the extraction electrode and by their time-of-flight ŽTOF., measured with respect to the photon pulse arrival time. The threshold electrons reach the detector in a time less than the photon pulse period Ži.e., 120 ns on the Super ACO ring when two positrons bunches are stored.. Energetic electrons arrive sooner or later depending whether they are emitted forward or backward with respect to the detector direction and a delayed time gate can be applied to select threshold photoelectrons for coincidence measurements. To record TPEPICO mass spectra, an ion extraction pulse Ž80 Vrcm. is triggered by the threshold electron signal. The coincident ion TOF is thus measured with respect to that of the threshold electron. Metastable ions which dissociate in the extraction and acceleration regions of the mass spectrometer will arrive at longer times. Analysis of the fragment peak shape enables to estimate reaction rates. Another contribution to the mass peak shape, even for fast reactions is the kinetic energy content in the fragment ion. The mass calibration was obtained from the proto.Ž . nated ammonia clusters ŽNHq 4 NH 3 n with n ranging from 2 to 20, the uncertainty being less than 0.2 mass unit at 100 amu. The mass resolution can be directly measured in the range of interest Ž80–100 amu. using the peaks of VCl–H 2 O. For this complex, the two peaks 80 and 82 amu are separated by 110 ns, i.e. more than 3.5 times the full width at half height ŽFWHHs 25 ns.. For cold molecules, at 100 amu, masses M and M q 1 can be separated. The energy resolution depends on the slit width of the monochromator. The resolution achieved in this study is ; 20 meV using 0.6 = 0.6 mm slits. The experimental interrogation on the reactivity of energy selected ionic clusters is based on the unique property of time and energy correlation between the ejected photoelectron and the corresponding ion. In particular, if the electron has no kinetic energy Žthreshold electron., the internal energy imparted to the corresponding ion is equal to the photon energy minus the adiabatic ionization threshold.
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This unique feature has been used to measure the ion yield for each mass peak as a function of the initial internal energy. The use of coincidence techniques for the study of clusters, i.e. in the presence of a great number of molecular species, has revealed some problems. Indeed, this detection technique is well adapted as long as only one molecular species can emit zero kinetic energy electrons at a given photon energy. Below the ionization threshold of the bare molecule, there is no difficulty to record the signal of the clusters. Above this threshold however, the cluster population represents at most 5% of the monomer population and the photon fluence must be lowered to avoid too many false coincidences due to the bare molecular ions. It then becomes difficult to detect the clusters. The existence of structured bands in the TPES of the free molecule allows to minimize this problem. The TPEPICO mass spectra of the clusters have been recorded for wavelengths where the threshold ionization efficiency of the bare molecule is minimum Ž10.1, 10.25, 10.4 eV..
3. Results The threshold photoelectron spectrum ŽTPES. of vinyl chloride is in good agreement with the previous PES spectrum of Mines and Thompson w19x, giving for the vertical and the adiabatic ionization potentials the same value: 10.01 " 0.02 eV. This spectrum presents several resolved vibrational bands at 10.06, 10.12 and 10.17 eV. The evolution of the reactivity as the cluster size increases has been studied using two different ŽVCl. n cluster distributions, small Ž n F 3. and large Ž n F 7., which will be presented separately. 3.1. Small clusters Mass spectra have been obtained for ionization energies ranging from 9.5 up to 10.5 eV. A typical TPEPICO mass spectrum using ‘small clusters’ expansion conditions is presented in Fig. 1 for a 9.8 eV ionizing photon. Two ionic species are observed: – the ionic dimer ŽVCl.q 2 : the peaks are narrow enough to discriminate the isotopomers with mass
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Fig. 1. TPEPICO time-of-flight mass spectrum obtained at 9.8 eV in the ‘mall clusters’ expansion conditions. Backing pressure P0 s 1.2 bar and VCl concentration 5% in He. In the insert a mass spectrum recorded in the same conditions but with an extraction field delayed by 1 ms is presented. In this case, the time origin has been shifted by 1 ms in order to have the same TOF scale. One clearly sees that the ‘product 1’ metastable tail decreases strongly.
124, 126 and 128 amu due to the 35 Cl and 37Cl isotopes. – a broad asymmetric peak Žlabeled ‘product 1’. which mass lies between 88 and 91 amu Žflight time between 9.5 and 10.5 ms in Fig. 1., with a long metastable tail. The assignment of this peak is not obvious. It can result from the elimination of HCl, Cl Ø or both from the dimer, following:
appearance threshold is at 9.75 " 0.05 eV. The product ion yield increases more rapidly with energy than the dimer one.
C 2 H 3 Clqq C 2 H 3 Cl ™ C 4 H 5 Clq
Ž m s 88r90 amu. q HCl , or C 2 H 3 Clqq C 2 H 3 Cl ™ C 4 H 6 Clq
Ž m s 89r91 amu. q Cl Ø . No mass peak corresponding to ŽVCl.q 3 is observed. TPEPICO ion yield spectra for ŽVCl.q and 2 ‘product 1’ are presented in Fig. 2. The ionic dimer appearance threshold is at 9.55 " 0.05 eV. For ‘product 1’, two curves are displayed: one is the integral of the whole mass peak Žfrom 9.5 to 10.5 ms. and the other corresponds to the metastable tail Ž9.8 to 10.5 ms.. In both cases, the ‘product 1’
Fig. 2. TPEPICO photoionization yield spectra of ŽVCl. 2 and ‘product 1’ recorded in ‘small clusters’ expansion conditions. The curve labeled ‘product 1’ corresponds to the integral of the whole mass peak Žin Fig. 1. between 9.5 and 10.5 ms and the curve labeled ‘metastable’ corresponds to the integral of long time range signal Ž9.8–10.5 ms..
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This experiment indicates that the reaction threshold lies 0.2 eV above the dimer ionization threshold, in contradiction with the results of Sheng et al. w16x. The assignment of the ‘product 1’ peak necessitates further investigations. The peak broadening can have two origins: Ž1. temporal broadening: when a reaction is ‘slow’ at the time scale of the experiment Ži.e., a few hundreds of ns., some clusters react in the extraction–acceleration region and this leads to an asymmetric broadening of the product peak towards long times Žmetastable tail.; Ž2. kinetic broadening: if the exothermicity of the reaction is large, a part of the available energy is released as kinetic energy of the fragments. This leads to a symmetrical broadening of the mass peak. These two effects seem to be present in the ‘product 1’ peak shape of Fig. 1: a symmetrical broadening as well as a metastable tail. The temporal effect is clearly seen in mass spectra recorded with a delayed ion extraction field. In the insert of Fig. 1, where this delay is set at 1 ms, the product peak metastable tail disappears but the symmetrical enlargement remains the same. The evolution of the kinetic energy release ŽKER., linked to the symmetrical shape of the product peak, has been carefully examined as a function of energy: there is no significant change in the width of the peak between 9.75 Žproduct appearance threshold. and 10.0 eV. As previously explained, only a few mass spectra have been recorded above the ionization threshold of the bare molecule Žat 10.1, 10.25 and 10.4 eV. and Fig. 3 presents that obtained at 10.25 eV: – the broad peak called previously ‘product 1’ remains the major peak, but its shape is modified above 10.1 eV Žsee Section 4.. – additional peaks at 75 and 77 amu are observed: they correspond to the reaction already seen in ICR experiments w14x: C 2 H 3 Clqq C 2 H 3 Cl ™ C 3 H 4 Clqq CH 2 Cl. This reaction occurs only above 10.1 eV but the mass peak remains narrow as compared to the one called ‘product 1’. 3.2. Larger clusters Mass spectra have been obtained for ionization energies ranging from 9.3 to 10.0 eV. A typical mass
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Fig. 3. TPEPICO time-of-flight mass spectrum at 10.25 eV obtained in the ‘small clusters’ expansion conditions. Backing pressure P0 s1.2 bar and VCl concentration 5% in He. The free molecule mass peak has been cut.
spectrum recorded with ‘large clusters’ expansion conditions for a 9.7 eV photon energy is presented in Fig. 4. The most intense peaks have the same masses as the ŽVCl.q n homogeneous clusters. These peaks present for n ) 2 a metastable tail induced by a slow dynamical process. This can be indicative of the evaporation of at least one VCl molecule in the ions. Ž The ŽVCl.q 2 mass peaks are narrow the three isotopomers can be distinguished. and correspond to the excitation of the dimer which does not react at this energy. The absence of strong kinetic or metastable enlargement of the peaks suggests that larger clusters do not significantly evaporate to give the dimer. The smallest observed reaction product ŽTOF: 9.75 ms. is not as broad as the one observed at higher energy for small clusters and can be assigned to C 4 H 535 Clq and C 4 H 537Clq isotopomers, corresponding to HCl elimination ŽFigs. 4 and 5b.. The peak shapes recorded with the two different expansion conditions show significant differences as presented in Fig. 5. In this figure, the delay between the electron detection and the extraction pulse is 500 ns in order to minimize the temporal broadening and to enlighten the kinetic broadening. The vinyl chloride–water complex is presented on Fig. 5a as a reference: the two isotopomers Ž80 and 82 amu. are well separated and the FWHH of the peak is 25 ns showing the resolution of the mass spectrometer.
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S. Martrenchard et al.r Chemical Physics 239 (1998) 331–343
Fig. 4. TPEPICO time-of-flight mass spectrum at 9.7 eV obtained in the ‘large clusters’ expansion conditions. Backing pressure P0 s 3 bar and VCl concentration 20% in He. The peak labeled with an asterisk ) corresponds to the elimination of HCl from ŽVCl.q 3 . The peak q labeled with two asterisks )) results from the elimination of 2 HCl from ŽVCl.q peak is shown in an expanded 3 . In the insert the C 4 H 5 Cl scale: upper spectrum obtained in the ‘small clusters’ expansion conditions at 9.8 eV; lower spectrum obtained in the ‘large clusters’ expansion conditions at 9.6 eV.
In the large clusters case, at 9.6 eV Ženergy at which no reaction products are observed for small clusters., the two isotopomers of C 4 H 5 Clq are clearly separated but the width of these peaks, assuming a Gaussian shape becomes 55 ns ŽFWHH. ŽFig. 5b.. The broadening to 55 ns is due to the reactive process. The peaks are well reproduced without adding any contribution from the Cl Ø elimination channel, thus at this low energy Ž9.6 eV. no significant Cl Ø elimination is observed. When the energy is high enough to allow the reaction of the small clusters Ži.e., above 9.75 eV., the two C 4 H 5 Clq isotopomers are not separated and the peaks broaden up to 120 ns ŽFig. 5c., assuming a Gaussian shape, which however does not reproduce correctly the overall peak shape. The peak shape is better simulated with rectangular profiles with a width of 150 ns as shown on Fig. 5d Žsee Section 4.. Concerning the reactivity of larger clusters, elimination of HCl Žor Cl Ø. from the trimer is clearly observed Žpeak labeled with an asterisk ) in Fig. 4.,
for ŽVCl. ns 4 the elimination channel is barely detected and is not seen anymore for larger clusters. Successive HCl eliminations is not a dominant channel; only a weak peak corresponding to 2 consecutive eliminations is observed Žpeak labeled with two asterisks )).. This small ratio productrŽVCl. n has also been observed by El-Shall and Schriver w11x in their electron impact experiment. Appearance thresholds have been derived from the TPEPICO ion yield curves Žsimilar to Fig. 2 but not shown here.: ; 9.3 eV for ŽVCl.q 3 , 9.55 eV for ŽVCl.q and 9.55 eV for C 4 H 5 Clq and can be 2 compared with the results obtained in ‘small clusters’ expansion conditions: Ž1. the dimer appearance threshold remains the same; Ž2. the appearance threshold of C 4 H 5 Clq is lowered by 0.2 eV as compared to the ‘product 1’ one; Ž3. the TPEPICO ion yield curve of ŽVCl.q 2 is increasing constantly as the energy increases whereas previously a plateau was obtained above 9.8 eV.
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tion mechanism will be discussed using the following approach: Ž1. comparison between experimental data obtained in the two different expansion conditions; Ž2. comparison of experimental mass peak shapes and the shapes deduced from simple kinetic energy distribution corresponding to different reaction mechanisms; and Ž3. comparison with the results of previous experiments. 4.1. Precursors of the reaction products Since the evaporation of one or several neutral VCl molecules after ionization can occur, the reactive precursor size of a given product is not obvious to assign and may depend on the cluster size distribution. ‘Small clusters’ and ‘large clusters’ expansion conditions will be discussed separately.
Fig. 5. Enlargement of some mass peaks recorded with a delay between the detected electron and the extraction pulse of D t s 500 ns. Ža. Non-reactive VCl–H 2 O van der Waals complex obtained by adding water in the expansion: this gives the resolution of the mass spectrometer in the range of interest. Žb. ‘Product 1’ peak recorded at 9.6 eV in large cluster conditions: the peaks are enlarged as compared to the VCl–H 2 O signal: the data can be fitted assuming only HCl elimination. Žc. ‘Product 1’ peak recorded at 9.95 eV in small cluster conditions: The peaks are broadened and not very well fitted with a Gaussian peak shape Žsee text.. Žd. ‘Product 1’ peak recorded at 9.95 eV in small cluster conditions and fit with rectangular profiles according to the impulsive model.
This indicates that larger clusters evaporate at high photon energy.
4. Discussion The present experimental data, TPEPICO mass spectra and ion yield curves, confirm that ionization of ŽVCl. n clusters induces chemical reactions. The identification of products, the energetic and the reac-
4.1.1. Small clusters In the ‘small clusters’ experiments, no ŽVCl.q n with n ) 2 have been observed. The only peaks observed in the mass spectra are the dimer ŽVCl.q 2 and the ‘product 1’. The TPEPICO ion yield curves ŽFig. 2. shows a correlation between the ‘product 1’ and ŽVCl.q 2 intensities as the ionization energy increases: the ŽVCl.q 2 intensity levels off as the product intensity increases. Such a behavior is not expected if ŽVCl.q 2 is also produced by evaporation Ž .q from larger clusters ŽVCl.q 3 ™ VCl 2 . It is thus reasonable to assume that very little trimer is present in the expansion. Therefore, only ŽVCl.q 2 can be the precursor for ‘product 1’, and the measured appearance threshold of ŽVCl. 2 Ž9.55 eV. corresponds to the vertical ionization threshold of the dimer, i.e. to ŽVCl. 2 ™ ŽVCl.q 2 . The appearance threshold of ‘product 1’ is higher at 9.75 eV. The energy difference between these two thresholds Ž0.2 eV. then corresponds to a reaction barrier. At this stage of the discussion, the barrier value derived from the data can be questioned. Indeed, to be accurate, the estimation should use the adiabatic ionization potential value and not the measured vertical ionization threshold Žwhich is equal or higher than the adiabatic one.. Furthermore, the measured ‘product 1’ appearance threshold is an upper value of the energy necessary to cross the barrier. Indeed the product are observed at their mass only if the
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reaction occurs in a time much shorter than the time spent in the extraction region of the mass spectrometer and the reaction rate increases with the excess energy. The appearance thresholds measured either on the metastable part or on the prompt part of the reactive peak are very similar, which indicates that the ‘kinetic shift’ Ždifference between the observed barrier and the real one. is small Žwithin the experimental energy resolution 0.05 eV.. 4.1.2. Large clusters In ‘large clusters’ expansion conditions, additional peaks appear corresponding to reaction products issued from larger ŽVCl.q n G 3 precursors. The smallest reaction product mass peak is narrow Žsee insert of Fig. 4 at 9.6 eV and Fig. 5b. and can be assigned to C 4 H 535 Clq and C 4 H 537Clq ŽHCl elimination.. These masses Ž88 and 90 amu. were included in the mass range of the broad ‘product 1’ peak issued from the ionic dimer reaction. The peak width is larger than for a cold product ŽFig. 5a. but still relatively narrow, indicating that the kinetic energy released in the product is small. The C 4 H 5 Clq appearance threshold is at 9.55 eV, i.e. 0.2 eV lower than the threshold for ‘product 1’ observed in ‘small clusters’ expansion conditions. Clearly between 9.55 q and 9.75 eV, ŽVCl.q precursor 2 is not the C 4 H 5 Cl and this product must come from the trimer, the reaction being followed by evaporation of one VCl molecule: q
Ž VCl. 3 ™ Ž VCl. –C 4 H 5 Clqq HCl ™ C 4 H 5 Clqq VCl . In this scheme the first step is the ejection of the neutral HCl molecule. The kinetic energy is shared between this neutral fragment and the ionic one according to their mass ratio. Therefore as the cluster size increases the kinetic energy taken by the heavy ionic cluster decreases. The subsequent evaporation will not change notably this kinetic energy leading to narrow mass product peak. The reactions observed in the ‘large clusters’ expansion conditions can thus be summarized as follows: below 9.75 eV, HCl elimination leading to C 4 H 5 Clq occurs from ŽVCl.q 3 whereas above 9.75 eV, the broad ‘product 1’ is coming from ŽVCl.q 2.
4.2. ‘Product 1’ identification A careful analysis of the mass peak shape is necessary to establish the nature of ‘product 1’ issued from the dimer. In ICR experiments w14x, the HCl elimination channel presents the lowest barrier, thus it can reasonably be assumed that the ‘product 1’ corresponds at least in part to C 4 H 5 Clq. The broadening of ‘product 1’ peak can have three causes: Ž1. slow reaction, Ž2. kinetic energy released in the fragment C 4 H 5 Clq after reaction of ŽVCl.q 2 , or Ž3. presence of an additional reaction channel: Cl Ø elimination which will induce two additional peaks at 89 and 91 amu, superposed to the HCl elimination products at 88 and 90 amu. A slow reaction rate for the HCl elimination is responsible for the metastable tail of the mass peak but cannot explain completely the broadening. Indeed the peak remains broad when the ion extraction is delayed Žup to 2 ms. with respect to ionization, even though the metastable tail is considerably reduced Žsee insert in Fig. 1.. However, the metastable part of the product is minor and the ratio productrdimer does not substantially increase with the extraction delay which shows that the reaction takes essentially place within 100 ns after the ionization. Kinetic energy released in the fragment can account for the symmetrical peak broadening. Indeed, HCl elimination from ŽVCl.q 2 is largely exothermic by 1.5 eV 1 in agreement with an estimated value of 1.8 eV w14x. Assuming that the binding energy of the neutral van der Waals dimer ŽVCl. 2 is of the order of 0.1 eV, the ionization threshold of bare VCl being 10.0 eV, the absorption of a 9.8 eV photon by ŽVCl. 2 produces a ŽVCl.q 2 ion with an energy of 0.3 eV below the dissociation limit in VClqq VCl. For an estimated exothermicity with respect to this limit of 1.5 eV, the available energy for the reaction is 1.2 eV. Thus near the reaction threshold Ž9.8 eV. around 1.2 eV have to be shared between the product and the fragment HCl as kinetic and internal energy. This
1
Calculated value in the present work using the MOPAC program at AM1 level.
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hypothesis enables to explain the lack of variation of the peak width when the ionization energy is varied from 9.75 to 10 eV: a 0.25 eV energy increase is small with respect to the total available energy. In order to test the assumption that kinetic energy released during the reaction process is responsible for ‘product 1’ mass peak broadening and to extract information about the dissociation mechanism, the mass peak shape has been simulated. Two limiting distributions for the kinetic energy released in the HCl elimination reaction have been used: a statistical and an impulsive model. In the statistical model, the available energy is redistributed between the different degrees of freedom before the dissociation. In the impulsive model, on the contrary, the fragment is ejected monokinetically. This implies that there is no energy redistribution: as soon as the dimer overcomes the reaction barrier, the ejection of HCl takes place to give the C 4 H 5 Clq product. Starting from a given kinetic energy distribution, the mass peak width has been calculated in the case of an isotropic velocity distribution, using the known voltages and geometry of the mass spectrometer. This width has been convoluted by the ‘apparatus function’ which can be derived from the cold bare molecule peak.
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with a ; 50 ns half width before convolution with the apparatus function. Fig. 6 shows the ‘product 1’ peak recorded at 9.8 eV together with the simulation obtained with the statistical model Ždashed line.. There is a clear disagreement of this simulation with the experimental peak shape. 4.2.2. ImpulsiÕe model In the case of an impulsive model, all the fragments being ejected with the same velocity, the product mass peak has a rectangular profile. The simulated mass peak for a kinetic energy of 1000 cmy1 in the C 4 H 5 Clq fragment is displayed in Fig. 5d and in Fig. 6 Ždotted line.. The impulsive model enables to reproduce quite well the experiment. It should be noticed that in the frame of this model, it is not necessary to involve the Cl Ø elimination channel to explain the mass peak shape. However, in the frame of the impulsive model, all the available energy should be shared between C 4 H 5 Clq and HCl fragments and a value of ; 3000 cmy1 is expected for the kinetic energy of the C 4 H 5 Clq fragment. Our best-fit value Ž; 1000 " 200 cmy1 . is clearly lower than the expected one. Similar low kinetic energy content has also been observed in the HCl elimination from ionized ethylchloride w22x.
4.2.1. Statistical model Using Klots theory w20x, the average kinetic energy released in a dissociation ² E kin : can be calculated for a given available energy and the vibrational frequencies of the system 2 . The kinetic energy distribution takes the approximate form of the exponential function expŽyE kinr² E kin :.. This approach has been previously used by T. Baer and co-workers w22x in the ethylchloride dissociative ionization study. In the case of a 9.8 eV ionizing photon, one obtains an average translational energy Žtotal kinetic energy. of ; 800 cmy1 . The ‘product 1’ average kinetic energy ² E kin : is around 230 cmy1 according to the mass ratio between C 4 H 5 Clq and HCl. The resulting calculated mass peak shape is approximately Gaussian
2
The Klots model requires the knowledge of the vibrational frequencies. They have been obtained from Ref. w21x. A variation of 10% of these frequencies do not change the KER more than 4%.
Fig. 6. Comparison between the calculated mass peak shapes of C 4 H 5 Clq and the experimental ‘product 1’ peak Žfull line. recorded at 9.8 eV in the ‘small clusters’ expansion conditions with no delay between the detected electron and the extraction pulse. The zero time-of-flight corresponds to the C 4 H 5 35 Clq TOF in our calibrated mass spectrometer. Dashed line: statistical model Ž- - - - -.; dotted line: impulsive model ŽPPPPPP..
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This behavior can be explained by assuming that the reaction proceeds through a late barrier Žlate transition state.. Before the late barrier crossing, the energy is shared between all the degrees of freedom Žstatistical behavior.. Then, at the late transition state there is an abrupt bond rupture, triggering the expulsion of the HCl fragment without interaction with the other product. This leads to a final impulsive dissociation but not with the total initial available energy. This reactive scheme has already been proposed in
the case of the reaction of O q C 2 H 4 w23x or in the photodissociation of HFCO w24x. The reaction path for HCl elimination from ŽVCl.q 2 is sketched in Fig. 7: – The photon hn promotes the dimer on the VCl–VClq ionic surface Ž1.. – In the ionic dimer, the radical cation VClq ŽCH 2 CHClqØ. known to be highly reactive towards electron donor molecules Žsee, e.g., Ref. w25x. reacts with the neutral CH 2 CHCl through a barrier Ž2. of
Ž .q Fig. 7. Scheme of the reaction paths of ŽVCl.q 2 and VCl 3 . Dotted line: reaction in the dimer. Above the entrance barrier the reaction leads to HCl elimination Žchannel a .. Full line: reaction in the trimer: Stabilization of a stable addition complex through VCl evaporation Žchannel b .: this channel is not seen for the trimer but seems to be dominant for larger clusters. Reaction leading to HCl elimination followed by VCl evaporation Žchannel g .. Successive elimination of two HCl fragments Žchannel d ..
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0.2 eV Žmeasured in the present experiment. to form an addition distonic complex ØCH 2 –CHCl–CH 2 – CHClqŽ 3. or one of its isomers: ØCHCl–CH 2 – CH 2 –CHClq or ØCH 2 –CHCl–CHCl–CHq 2. – This addition complex will rearrange to the Ž4. ŽCH 2 5CCl– stable polymeric species C 4 H 6 Clq 2 CH 2 –CH 2 Clq or one of its isomers. which has been calculated to be 1 eV below the VCl q VClq limit 3. – The HCl elimination from C 4 H 6 Clq 2 proceeds impulsively through a late barrier Ž5. and leads to the formation of C 4 H 5 Clqq HCl Ž6. and the kinetic energy release measured for the C 4 H 5 Clq product is around 1000 cmy1 so that the total kinetic energy release in this dissociation should be around 3000 cmy1 . It can be noticed here that the formation of the addition complex can be seen as the elementary process for the polymerization reaction, which should be repeated many times to get the polyvinylchloride ions and that HCl elimination can be considered as the ending process. 4.3. Other reactiÕe channels Above 10 eV, additional reaction channels have been observed. The CH 2 Cl elimination channel appears between 10.1 and 10.25 eV Žsee Fig. 3.: q ŽVCl.q 2 ™ C 3 H 4 Cl q CH 2 Cl. Since the two 35 q C 3 H 4 Cl and C 3 H 437Clq mass peaks are resolved, the kinetic energy released in the reaction is smaller than in the case of HCl elimination. Another result is the modification of the ‘product 1’ peak shape. Fig. 8 displays a comparison between mass peaks recorded at 10.25 and 9.95 eV. One possible explanation is the appearance of the Cl Ø elimination channel. Indeed, an attempt to reproduce the new shape of ‘product 1’ mass peak in the frame of the previous impulsive model, with only C 4 H 535 Clq and C 4 H 537Clq peaks ŽHCl elimination. broadened by kinetic energy, has been unsuccessful. If we assume that the peak shape results from the superposition of the latter peaks and C 4 H 635 Clq and C4 H637Clq Žresulting from Cl Ø elimination without kinetic energy., a better agreement with the experi-
3
Calculated value in the present work using the MOPAC program at AM1 level.
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Fig. 8. Comparison of TPEPICO mass spectra of ‘product 1’ recorded at 9.95 Žthin dotted line. and 10.25 eV Žfull line. in ‘small clusters’ expansion conditions. The mass peak shape at 10.25 eV has been simulated Žthick dotted line. by adding to the peak shape at 9.95 eV Ždue to C 4 H 5 Clq coming from the ŽVCl. 2 dimer. two narrow mass peaks corresponding to the Cl Ø elimination channel leading to C 4 H 6 35 Cl and C 4 H 637 Cl Ždashed line.. The shape of these peaks is that of VCl–H 2 O shifted by 0.54 ms to match the position of a cold C 4 H 6 Clq product.
mental results is obtained Žsee Fig. 8.. It is difficult to estimate precisely the branching ratio between the two channels without using a reflectron time-of-flight mass spectrometer which would suppress the KER broadening. However, HCl seems to remain the major product. The present results can be compared with the observations realized in ICR experiments w14x. In both cases, HCl elimination is the dominant channel. An estimation of the three reactions enthalpies is given in Ref. w14x: HCl elimination is the most exothermic reaction Žy1.8 eV. which can explain the large amount of kinetic energy in the fragment measured in the present work and the two other channels are also exothermic Žy0.2 eV for CH 2 Cl elimination and y0.9 eV for Cl Ø elimination.. However, we do not observe them at the dimer ionization threshold. This implies that, as in the case of HCl elimination, a barrier to these reactions is present. Unfortunately, a precise measurement of these barriers was not possible because of the low signal to noise ratio for the experiments performed above the bare molecule ionization threshold. In the free VClq ion, the dissociation of the C–Cl bond requires 2.4 eV of excess energy Žthreshold at 12.4 eV.w13x. From the observation of the Cl Ø elimi-
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nation at 10.25 eV and knowing the dimer ionization threshold it is clear that the Cl Ø elimination requires less energy in the complex Žless than 10.25–9.55 s 0.7 eV. than in the free ions. Similar results have been obtained in the nucleophilic substitution of Cl Ø by NH 3 in the fluorochlorobenzene–NH 3 system where the formation of an intermediate sigma complex weakens the C–Cl bond to 0.5 eV w8x. 4.4. Reaction in larger clusters The last point concerns the apparent absence of reaction products issued from large clusters ŽVCl.q n ) 3. In the case of HCl elimination, one expects a mechanism similar in the dimer and in larger clusters with a similar barrier to the addition Ž0.2 eV.. Since the observed VClq mass peaks present a small n metastable tail, this implies that these ions are obtained from evaporation of larger clusters in the acceleration region of the mass spectrometer. The absence of elimination products from large clusters implies that the reaction is stopped during the reactive pathway. This evaporation process can dissipate enough energy to stop the reactive process behind a barrier along the reactive path. Furthermore, the evaporation is probably the most important when the available energy is maximum, i.e. above the minima of the potential energy surface. 4.4.1. The trimer case A reactive scheme is proposed for the case of the Ža., a first trimer in Fig. 7. Starting from VClq 3 addition reaction between VClq and one neutral VCl occurs, through a 0.2 eV barrier Žb. like in the dimer, Ž . leading to C 4 H 6 Clq 2 PPP VCl d . The potential energy is just shifted by the stabilization energy induced by the third molecule. This addition product Žd. can: Ž1. eliminate HCl through a late barrier Že. to give C 4 H 5 Clq . . . VCl Žf.. The subsequent evaporation of VCl will lead to the C 4 H 5 Clq product Ž6. Žchannel g . observed in the experiment Žsee Figs. 4 and 5b.. Ž2. continue the polymerization process by adding the third VCl molecule and lead, after rearrangeŽ . ments, to the stable addition complex C 6 H 9 Clq 3 g . Here the final process can be elimination of one Žh. or two Ži. HCl molecules from this stable addition polymer Žchannel d ., as observed in Fig. 4 Žpeaks labelled ) and )), respectively..
Ž3. be stabilized by the evaporation of one VCl Ž . Žchannel b, dashed molecule to give C 4 H 6 Clq 2 4 arrow.. It should be noticed that these addition polyvinylchloride ions have the same masses as the van der Waals complexes Žchannel g .; however, they may be distinguished from the direct ionization of the cold complexes by an enlargement of the mass peaks. The absence of kinetic broadening and metastable tail on the dimer peak indicates that the trimer does not follow this channel but peaks corresponding to larger clusters are enlarged, which may reflect this process. 4.4.2. Larger clusters This reaction scheme can be extended to the case of larger clusters. Successive addition processes should lead to the formation of polyvinylchloride ions but the HCl elimination channel has not been observed: the reactive process may be stopped in the addition intermediate state by the evaporative cooling of VCl molecules. The observed mass peaks may thus correspond to stable addition complexes Žand not van der Waals clusters.. The exact chemical nature of the peaks observed at the masses of VCl n cannot be resolved at the moment and more experimental andror theoretical work is necessary. In particular, one cannot exclude that the reactive process is rather stopped in the exit channel: the eliminated HCl may be bound to the remaining cluster by strong ion–dipole interaction and cannot escape from the cluster, the evaporation of a VCl molecule being energetically more favorable than evaporation of HCl. In any case, the absence of HCl evaporation observed in large clusters can be seen as an evidence for a cage effect: the reaction stops since it is easier to evaporate a surrounding molecule than to cross over a barrier along the reaction coordinate. 5. Conclusions The reactivity of ionized vinyl chloride clusters has been investigated by the TPEPICO technique which enables a control of the internal energy of the reactive ions. In the case of the dimer ŽVCl.q 2 , a first reactive channel, the HCl elimination, has been evidenced with an appearance threshold 0.2 eV above the ion-
S. Martrenchard et al.r Chemical Physics 239 (1998) 331–343
ization threshold of the dimer. The kinetic energy released in the C 4 H 5 Clq reaction product has been estimated through the simulation of the mass peak shape assuming that the C 4 H 5 Clq and HCl fragments are ejected monokinetically Žimpulsive model. but only a part of the total available energy is released as fragment kinetic energy. The following reactive path has been proposed to account for this results: the initial step is the formation of a stable addition complex C 4 H 6 Clq 2 after crossing the 0.2 eV barrier. This can be considered as the first step of the polymerisation process. The final step is the HCl elimination from this addition complex. This elimination proceeds also through a barrier and the dissociation is prompt Žimpulsive model.. Two other reactive channels have been evidenced for higher energies: CH 2 Cl and Cl Ø elimination. The evolution of the polymerisation reaction in the larger clusters has been investigated. The polyvinylchloride ions resulting from successive additions of the VCl molecules in the cluster can be very stable. As the size increases, the HCl elimination is less observed. This seems to be due to competition with evaporation processes which cool the system in the intermediate addition polymer: the reaction is stopped and the final process ŽHCl elimination. is not observed anymore. The cluster acts both as a thermal bath and as a cage.
References w1x R. Knochenmuss, S. Leutwyler, J. Chem. Phys. 91 Ž1989. 1268. w2x J. Syage, Faraday Discuss. 97, 97r24 Ž1994..
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w3x C. Jouvet, C. Dedonder-Lardeux, M. Richard-Viard, D. Solgadi, A. Tramer, J. Phys. Chem. 94 Ž1990. 5041. w4x J.J. Breen, L.W. Peng, D.M. Wilberg, A. Heikal, P. Cong, A. Zewail, J. Chem. Phys. 92a Ž1990. 805. w5x B. Brutschy, Chem. Rev. 92 Ž1992. 1567. w6x C. Riehn, C. Lahmann, B. Brutschy, J. Phys. Chem. 96 Ž1992. 3626, and references therein. w7x T. Maeyama, N. Mikami, J. Phys. Chem. 94 Ž1990. 6973. w8x S. Martrenchard, C. Dedonder-Lardeux, C. Jouvet, U. Rockland, D. Solgadi, J. Phys. Chem. 99 Ž1995. 13716. w9x C. Dedonder-Lardeux, I. Dimicoli, C. Jouvet, S. Martrenchard-Barra, M. Richard-Viard, D. Solgadi, M. Vervloet, ¨ Chem. Phys. Lett. 240 Ž1995. 97. w10x M.T. Colbaugh, G. Vaidyanathan, J.F. Garvey, Int. Rev. Phys. Chem. 13 Ž1994. 1. w11x M.S. El-Shall, K.E. Schriver, J. Chem. Phys. 95 Ž1991. 3001. w12x V.G. Anicich, M.T. Bowers, Int. J. Mass Spectrom. Ion Phys. 12 Ž1973. 231. w13x J.A. Herman, K. Herman, T.B. McMahon, Can. J. Chem. 69 Ž1991. 2038, and references therein. w14x L.W. Sieck, R. Gorden Jr., S.G. Lias, P. Ausloos, Int. J. Mass Spectrom. 15 Ž1974. 181. w15x B.M. Hughes, T.O. Tiernan, J.H. Futrell, J. Phys. Chem. 73 Ž1969. 829. w16x L.S. Sheng, F. Qi, H. Gao, Z.Y. Luo, Y.W. Zhang, J. Electron Spectrosc. 80 Ž1996. 33. w17x P.M. Guyon, T. Baer, L.F.A. Ferreira, I. Nenner, Aabche´ Fouhaile, ´ R. Botter, T.R. Govers, J. Phys. B 11 Ž1978. L141. w18x M. Richard-Viard, These, ` Universite´ de Paris-Sud, Orsay Ž1998.. w19x G.W. Mines, H.W. Thompson, Electrochim. Acta 29 Ž1973. 1377. w20x C.E. Klots, J. Chem. Phys. 58 Ž1973. 5364. w21x C.W. Gullikson, J.R. Nielsen, J. Mol. Spectrosc. 1 Ž1957. 158. w22x J.A. Booze, K.M. Weitzel, T. Baer, J. Chem. Phys. 94 Ž1991. 3649. w23x J.C. Loison, C. Dedonder-Lardeux, C. Jouvet, D. Solgadi, Faraday Discuss. 97 Ž1994. 379. w24x S.R. Langford, A.D. Batten, M. Kono, M.N.R. Ashfold, J. Chem. Soc., Faraday Trans. 93 Ž21. Ž1997. 3757. w25x A. Nixdorf, H.F. Grutzmacher, J. Am. Chem. Soc. 119 ¨ Ž1997. 6544.
Reactivity of vinyl chloride ionic clusters b S. Martrenchard a , C. Dedonder-Lardeux a , I. Dimicoli b,) , G. Gregoire , C. Jouvet a , ´ M. Mons b, D. Solgadi a a
Laboratoire de Photophysique Moleculaire du CNRS, Bat. ´ ˆ 210, UniÕersite´ de Paris-Sud, F-91405 Orsay Cedex, France b CEA - Saclay, DSM-DRECAM, SerÕice des Photons, Atomes et Molecules, F-91911 Gif-sur-YÕette, France ´ Received 16 April 1998
Abstract The reactivity of vinyl chloride ionic clusters has been investigated by the Threshold PhotoElectron PhotoIon COincidences technique. In the case of the dimer, the competition between the three reactive channels ŽHCl, Cl Ø and CH 2 Cl elimination. has been studied. The main reactive channel is HCl elimination which proceeds through a 0.2 eV barrier. This elimination reaction is still observed in the trimer but not in larger clusters. For these clusters, cooling by evaporation of neutral vinyl chloride monomers seems to be the favored channel that hinders the HCl elimination step. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The reactivity in the gas phase is often quite different from the reactions in the condensed phase mostly because of solvent effects. The influence of the solvent molecules on a chemical reaction mechanism can be investigated in the gas phase by using molecular clusters as powerful model systems in which the reacting pair is gradually embedded in a solvent. Indeed, the energetics of the reaction is changed by adding more and more solvent molecules in the cluster and this effect has been clearly evidenced in several chemical reactions such as proton transfer w1–4x, ionic nucleophilic substitution w5–9x or cationic polymerization of unsaturated molecules w10,11x. The reactivity of vinyl halides, especially vinyl chloride C 2 H 3 Cl which will be denoted VCl, has )
Corresponding author. Fax: q33-1-69-08-87-07.
received significant attention since the ionization of VCl induces cationic polymerization. The first steps of this polymerization reaction has been previously studied by different methods and the important results of these works will be given in the following paragraphs. The ion molecule reaction of VClq with VCl has been studied in Ionic Cyclotronic Resonance ŽICR. experiments by different groups w12–14x. The first step of the reaction has been carefully studied by Ausloos and co-workers w14x: in this work, the ions are produced by one-photon ionization at two energies, the first one just above the vinyl chloride ionization threshold Ž10.03 eV. and the second at 11.7 eV, where the ions are produced with some internal energy. In the two cases, three reactive channels have been evidenced: – HCl elimination leading to C 4 H 5 Clq; – Cl Ø elimination leading to C 4 H 6 Clq; – CH 2 Cl elimination leading to C 3 H 4 Clq.
0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 5 5 - 9
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The branching ratio between the three channels has been found to change with the ions residence time in the ion trap and with the ionization energy. The observation of the three channels at 10.03 eV gives a lower limit of the activation barriers for these reaction channels. It is noteworthy that in this kind of experiments, the initial energy deposited in the collision pair cannot be less than the ionization energy of the molecule. For ‘non-thermalized’ ions Ži.e., at t s 0. the major channel at 10.03 eV is HCl elimination and CH 2 Cl elimination when a 11.7 eV photon is used. This implies that CH 2 Cl elimination channel presents a higher apparition threshold than HCl elimination but the corresponding rate constant is probably higher at 11.7 eV. The percentage of the HCl elimination product increases with time. The evolution of the branching ratio versus the residence time in the trap can be explained by the internal energy decay induced by collisions. At longer time, the internal energy of the ion decreases, therefore the most important channel will be the one with the lowest barrier, i.e. the HCl elimination. The successive polymerization steps have been followed in ‘high-pressure’ sources w13,15x, leading q and other to the formation of C 6 Hq 7 , C 6 H 8 Cl fragments which must come from ions which have much more internal energy that in the study presented here. Ionic reaction of VCl clusters has been studied by El-Shall and Schriver w11x in a molecular beam experiment by ionization through electron impact Ž70 eV.. In their mass spectrum, besides the parent mass peaks ŽVCl. n , several other peaks corresponding to addition–elimination reaction products have been observed. A mechanism, involving stepwise additions of VCl molecule followed by HCl or Cl Ø elimination has been proposed to account for the small clusters cationic polymerization. However, in electron impact experiments, the internal energy distribution of the ionized clusters is not known and is probably fairly large. This energy will be dissipated by an intensive evaporation process and therefore the information on the initial size of the reactive cluster is lost. Thus, this technique does not allow energetic measurements or size selective studies. Recently Sheng et al. w16x reported on the reactivity of the vinyl chloride clusters ionized by syn-
chrotron radiation. In this study the energy has been varied in a relatively large domain Ž9.4–10.5 eV. but no control on the internal energy distribution of the ions has been achieved. Only one reaction channel, the HCl elimination, has been observed and the reaction product appearance threshold coincides with the ionization threshold of the dimer. From this result it was concluded that this reaction proceeds without barrier. In the present paper, small ionized vinyl chloride clusters reactivity has been studied using synchrotron radiation to initiate the reaction. Owing to the temporal structure and high duty cycle of this source, a threshold photoelectron–photoion coincidences technique has been implemented to control the energy imparted to the cluster by the photon absorption. The photon energy has been continuously varied Ž9.3– 10.5 eV. in order to investigate precisely the energetic of the first steps of cationic polymerization as a function of cluster size.
2. Experimental setup The SAPHIRS experimental set-up, built on the Orsay synchrotron radiation facility has been described in detail previously w9x. It consists of a continuous supersonic beam coupled to a double electron – ion time-of-flight coincidence spectrometer situated at right angle to the plane defined by the molecular and photon beams. In the present experiments, the synchrotron radiation is used to ionize the clusters in the 9.3–10.5 eV range. The clusters are produced by expanding vinyl chloride ŽVCl. diluted in helium, through a 50 mm nozzle under typical backing pressures P0 between 1 and 3 bar. The cluster size distribution has been modified by varying the VCl concentration in the He buffer gas. At low concentration Ž5% VCl. and low pressure Ž P0 s 1.2 bar., the formation of only small ŽVCl. n clusters Ž n F 2 or 3. is obtained. At higher concentration Ž20% VCl. and higher pressure Ž P0 s 3 bar., clusters with sizes up to n s 7 are produced. The mass spectra of the ŽVCl. n clusters have been recorded using the Threshold PhotoElectron–PhotoIon COincidence ŽTPEPICO. method developed in Orsay by Guyon et al. w17x.
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The linear time-of-flight mass spectrometer combined with the detection of threshold electron is the one developed and tested by M. Richard-Viard w18x. Photoelectrons, accelerated by a small electric field Ž2 Vrcm., are selected by angular discrimination using a small diaphragm ŽB 2 mm. on the extraction electrode and by their time-of-flight ŽTOF., measured with respect to the photon pulse arrival time. The threshold electrons reach the detector in a time less than the photon pulse period Ži.e., 120 ns on the Super ACO ring when two positrons bunches are stored.. Energetic electrons arrive sooner or later depending whether they are emitted forward or backward with respect to the detector direction and a delayed time gate can be applied to select threshold photoelectrons for coincidence measurements. To record TPEPICO mass spectra, an ion extraction pulse Ž80 Vrcm. is triggered by the threshold electron signal. The coincident ion TOF is thus measured with respect to that of the threshold electron. Metastable ions which dissociate in the extraction and acceleration regions of the mass spectrometer will arrive at longer times. Analysis of the fragment peak shape enables to estimate reaction rates. Another contribution to the mass peak shape, even for fast reactions is the kinetic energy content in the fragment ion. The mass calibration was obtained from the proto.Ž . nated ammonia clusters ŽNHq 4 NH 3 n with n ranging from 2 to 20, the uncertainty being less than 0.2 mass unit at 100 amu. The mass resolution can be directly measured in the range of interest Ž80–100 amu. using the peaks of VCl–H 2 O. For this complex, the two peaks 80 and 82 amu are separated by 110 ns, i.e. more than 3.5 times the full width at half height ŽFWHHs 25 ns.. For cold molecules, at 100 amu, masses M and M q 1 can be separated. The energy resolution depends on the slit width of the monochromator. The resolution achieved in this study is ; 20 meV using 0.6 = 0.6 mm slits. The experimental interrogation on the reactivity of energy selected ionic clusters is based on the unique property of time and energy correlation between the ejected photoelectron and the corresponding ion. In particular, if the electron has no kinetic energy Žthreshold electron., the internal energy imparted to the corresponding ion is equal to the photon energy minus the adiabatic ionization threshold.
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This unique feature has been used to measure the ion yield for each mass peak as a function of the initial internal energy. The use of coincidence techniques for the study of clusters, i.e. in the presence of a great number of molecular species, has revealed some problems. Indeed, this detection technique is well adapted as long as only one molecular species can emit zero kinetic energy electrons at a given photon energy. Below the ionization threshold of the bare molecule, there is no difficulty to record the signal of the clusters. Above this threshold however, the cluster population represents at most 5% of the monomer population and the photon fluence must be lowered to avoid too many false coincidences due to the bare molecular ions. It then becomes difficult to detect the clusters. The existence of structured bands in the TPES of the free molecule allows to minimize this problem. The TPEPICO mass spectra of the clusters have been recorded for wavelengths where the threshold ionization efficiency of the bare molecule is minimum Ž10.1, 10.25, 10.4 eV..
3. Results The threshold photoelectron spectrum ŽTPES. of vinyl chloride is in good agreement with the previous PES spectrum of Mines and Thompson w19x, giving for the vertical and the adiabatic ionization potentials the same value: 10.01 " 0.02 eV. This spectrum presents several resolved vibrational bands at 10.06, 10.12 and 10.17 eV. The evolution of the reactivity as the cluster size increases has been studied using two different ŽVCl. n cluster distributions, small Ž n F 3. and large Ž n F 7., which will be presented separately. 3.1. Small clusters Mass spectra have been obtained for ionization energies ranging from 9.5 up to 10.5 eV. A typical TPEPICO mass spectrum using ‘small clusters’ expansion conditions is presented in Fig. 1 for a 9.8 eV ionizing photon. Two ionic species are observed: – the ionic dimer ŽVCl.q 2 : the peaks are narrow enough to discriminate the isotopomers with mass
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Fig. 1. TPEPICO time-of-flight mass spectrum obtained at 9.8 eV in the ‘mall clusters’ expansion conditions. Backing pressure P0 s 1.2 bar and VCl concentration 5% in He. In the insert a mass spectrum recorded in the same conditions but with an extraction field delayed by 1 ms is presented. In this case, the time origin has been shifted by 1 ms in order to have the same TOF scale. One clearly sees that the ‘product 1’ metastable tail decreases strongly.
124, 126 and 128 amu due to the 35 Cl and 37Cl isotopes. – a broad asymmetric peak Žlabeled ‘product 1’. which mass lies between 88 and 91 amu Žflight time between 9.5 and 10.5 ms in Fig. 1., with a long metastable tail. The assignment of this peak is not obvious. It can result from the elimination of HCl, Cl Ø or both from the dimer, following:
appearance threshold is at 9.75 " 0.05 eV. The product ion yield increases more rapidly with energy than the dimer one.
C 2 H 3 Clqq C 2 H 3 Cl ™ C 4 H 5 Clq
Ž m s 88r90 amu. q HCl , or C 2 H 3 Clqq C 2 H 3 Cl ™ C 4 H 6 Clq
Ž m s 89r91 amu. q Cl Ø . No mass peak corresponding to ŽVCl.q 3 is observed. TPEPICO ion yield spectra for ŽVCl.q and 2 ‘product 1’ are presented in Fig. 2. The ionic dimer appearance threshold is at 9.55 " 0.05 eV. For ‘product 1’, two curves are displayed: one is the integral of the whole mass peak Žfrom 9.5 to 10.5 ms. and the other corresponds to the metastable tail Ž9.8 to 10.5 ms.. In both cases, the ‘product 1’
Fig. 2. TPEPICO photoionization yield spectra of ŽVCl. 2 and ‘product 1’ recorded in ‘small clusters’ expansion conditions. The curve labeled ‘product 1’ corresponds to the integral of the whole mass peak Žin Fig. 1. between 9.5 and 10.5 ms and the curve labeled ‘metastable’ corresponds to the integral of long time range signal Ž9.8–10.5 ms..
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This experiment indicates that the reaction threshold lies 0.2 eV above the dimer ionization threshold, in contradiction with the results of Sheng et al. w16x. The assignment of the ‘product 1’ peak necessitates further investigations. The peak broadening can have two origins: Ž1. temporal broadening: when a reaction is ‘slow’ at the time scale of the experiment Ži.e., a few hundreds of ns., some clusters react in the extraction–acceleration region and this leads to an asymmetric broadening of the product peak towards long times Žmetastable tail.; Ž2. kinetic broadening: if the exothermicity of the reaction is large, a part of the available energy is released as kinetic energy of the fragments. This leads to a symmetrical broadening of the mass peak. These two effects seem to be present in the ‘product 1’ peak shape of Fig. 1: a symmetrical broadening as well as a metastable tail. The temporal effect is clearly seen in mass spectra recorded with a delayed ion extraction field. In the insert of Fig. 1, where this delay is set at 1 ms, the product peak metastable tail disappears but the symmetrical enlargement remains the same. The evolution of the kinetic energy release ŽKER., linked to the symmetrical shape of the product peak, has been carefully examined as a function of energy: there is no significant change in the width of the peak between 9.75 Žproduct appearance threshold. and 10.0 eV. As previously explained, only a few mass spectra have been recorded above the ionization threshold of the bare molecule Žat 10.1, 10.25 and 10.4 eV. and Fig. 3 presents that obtained at 10.25 eV: – the broad peak called previously ‘product 1’ remains the major peak, but its shape is modified above 10.1 eV Žsee Section 4.. – additional peaks at 75 and 77 amu are observed: they correspond to the reaction already seen in ICR experiments w14x: C 2 H 3 Clqq C 2 H 3 Cl ™ C 3 H 4 Clqq CH 2 Cl. This reaction occurs only above 10.1 eV but the mass peak remains narrow as compared to the one called ‘product 1’. 3.2. Larger clusters Mass spectra have been obtained for ionization energies ranging from 9.3 to 10.0 eV. A typical mass
335
Fig. 3. TPEPICO time-of-flight mass spectrum at 10.25 eV obtained in the ‘small clusters’ expansion conditions. Backing pressure P0 s1.2 bar and VCl concentration 5% in He. The free molecule mass peak has been cut.
spectrum recorded with ‘large clusters’ expansion conditions for a 9.7 eV photon energy is presented in Fig. 4. The most intense peaks have the same masses as the ŽVCl.q n homogeneous clusters. These peaks present for n ) 2 a metastable tail induced by a slow dynamical process. This can be indicative of the evaporation of at least one VCl molecule in the ions. Ž The ŽVCl.q 2 mass peaks are narrow the three isotopomers can be distinguished. and correspond to the excitation of the dimer which does not react at this energy. The absence of strong kinetic or metastable enlargement of the peaks suggests that larger clusters do not significantly evaporate to give the dimer. The smallest observed reaction product ŽTOF: 9.75 ms. is not as broad as the one observed at higher energy for small clusters and can be assigned to C 4 H 535 Clq and C 4 H 537Clq isotopomers, corresponding to HCl elimination ŽFigs. 4 and 5b.. The peak shapes recorded with the two different expansion conditions show significant differences as presented in Fig. 5. In this figure, the delay between the electron detection and the extraction pulse is 500 ns in order to minimize the temporal broadening and to enlighten the kinetic broadening. The vinyl chloride–water complex is presented on Fig. 5a as a reference: the two isotopomers Ž80 and 82 amu. are well separated and the FWHH of the peak is 25 ns showing the resolution of the mass spectrometer.
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Fig. 4. TPEPICO time-of-flight mass spectrum at 9.7 eV obtained in the ‘large clusters’ expansion conditions. Backing pressure P0 s 3 bar and VCl concentration 20% in He. The peak labeled with an asterisk ) corresponds to the elimination of HCl from ŽVCl.q 3 . The peak q labeled with two asterisks )) results from the elimination of 2 HCl from ŽVCl.q peak is shown in an expanded 3 . In the insert the C 4 H 5 Cl scale: upper spectrum obtained in the ‘small clusters’ expansion conditions at 9.8 eV; lower spectrum obtained in the ‘large clusters’ expansion conditions at 9.6 eV.
In the large clusters case, at 9.6 eV Ženergy at which no reaction products are observed for small clusters., the two isotopomers of C 4 H 5 Clq are clearly separated but the width of these peaks, assuming a Gaussian shape becomes 55 ns ŽFWHH. ŽFig. 5b.. The broadening to 55 ns is due to the reactive process. The peaks are well reproduced without adding any contribution from the Cl Ø elimination channel, thus at this low energy Ž9.6 eV. no significant Cl Ø elimination is observed. When the energy is high enough to allow the reaction of the small clusters Ži.e., above 9.75 eV., the two C 4 H 5 Clq isotopomers are not separated and the peaks broaden up to 120 ns ŽFig. 5c., assuming a Gaussian shape, which however does not reproduce correctly the overall peak shape. The peak shape is better simulated with rectangular profiles with a width of 150 ns as shown on Fig. 5d Žsee Section 4.. Concerning the reactivity of larger clusters, elimination of HCl Žor Cl Ø. from the trimer is clearly observed Žpeak labeled with an asterisk ) in Fig. 4.,
for ŽVCl. ns 4 the elimination channel is barely detected and is not seen anymore for larger clusters. Successive HCl eliminations is not a dominant channel; only a weak peak corresponding to 2 consecutive eliminations is observed Žpeak labeled with two asterisks )).. This small ratio productrŽVCl. n has also been observed by El-Shall and Schriver w11x in their electron impact experiment. Appearance thresholds have been derived from the TPEPICO ion yield curves Žsimilar to Fig. 2 but not shown here.: ; 9.3 eV for ŽVCl.q 3 , 9.55 eV for ŽVCl.q and 9.55 eV for C 4 H 5 Clq and can be 2 compared with the results obtained in ‘small clusters’ expansion conditions: Ž1. the dimer appearance threshold remains the same; Ž2. the appearance threshold of C 4 H 5 Clq is lowered by 0.2 eV as compared to the ‘product 1’ one; Ž3. the TPEPICO ion yield curve of ŽVCl.q 2 is increasing constantly as the energy increases whereas previously a plateau was obtained above 9.8 eV.
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tion mechanism will be discussed using the following approach: Ž1. comparison between experimental data obtained in the two different expansion conditions; Ž2. comparison of experimental mass peak shapes and the shapes deduced from simple kinetic energy distribution corresponding to different reaction mechanisms; and Ž3. comparison with the results of previous experiments. 4.1. Precursors of the reaction products Since the evaporation of one or several neutral VCl molecules after ionization can occur, the reactive precursor size of a given product is not obvious to assign and may depend on the cluster size distribution. ‘Small clusters’ and ‘large clusters’ expansion conditions will be discussed separately.
Fig. 5. Enlargement of some mass peaks recorded with a delay between the detected electron and the extraction pulse of D t s 500 ns. Ža. Non-reactive VCl–H 2 O van der Waals complex obtained by adding water in the expansion: this gives the resolution of the mass spectrometer in the range of interest. Žb. ‘Product 1’ peak recorded at 9.6 eV in large cluster conditions: the peaks are enlarged as compared to the VCl–H 2 O signal: the data can be fitted assuming only HCl elimination. Žc. ‘Product 1’ peak recorded at 9.95 eV in small cluster conditions: The peaks are broadened and not very well fitted with a Gaussian peak shape Žsee text.. Žd. ‘Product 1’ peak recorded at 9.95 eV in small cluster conditions and fit with rectangular profiles according to the impulsive model.
This indicates that larger clusters evaporate at high photon energy.
4. Discussion The present experimental data, TPEPICO mass spectra and ion yield curves, confirm that ionization of ŽVCl. n clusters induces chemical reactions. The identification of products, the energetic and the reac-
4.1.1. Small clusters In the ‘small clusters’ experiments, no ŽVCl.q n with n ) 2 have been observed. The only peaks observed in the mass spectra are the dimer ŽVCl.q 2 and the ‘product 1’. The TPEPICO ion yield curves ŽFig. 2. shows a correlation between the ‘product 1’ and ŽVCl.q 2 intensities as the ionization energy increases: the ŽVCl.q 2 intensity levels off as the product intensity increases. Such a behavior is not expected if ŽVCl.q 2 is also produced by evaporation Ž .q from larger clusters ŽVCl.q 3 ™ VCl 2 . It is thus reasonable to assume that very little trimer is present in the expansion. Therefore, only ŽVCl.q 2 can be the precursor for ‘product 1’, and the measured appearance threshold of ŽVCl. 2 Ž9.55 eV. corresponds to the vertical ionization threshold of the dimer, i.e. to ŽVCl. 2 ™ ŽVCl.q 2 . The appearance threshold of ‘product 1’ is higher at 9.75 eV. The energy difference between these two thresholds Ž0.2 eV. then corresponds to a reaction barrier. At this stage of the discussion, the barrier value derived from the data can be questioned. Indeed, to be accurate, the estimation should use the adiabatic ionization potential value and not the measured vertical ionization threshold Žwhich is equal or higher than the adiabatic one.. Furthermore, the measured ‘product 1’ appearance threshold is an upper value of the energy necessary to cross the barrier. Indeed the product are observed at their mass only if the
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reaction occurs in a time much shorter than the time spent in the extraction region of the mass spectrometer and the reaction rate increases with the excess energy. The appearance thresholds measured either on the metastable part or on the prompt part of the reactive peak are very similar, which indicates that the ‘kinetic shift’ Ždifference between the observed barrier and the real one. is small Žwithin the experimental energy resolution 0.05 eV.. 4.1.2. Large clusters In ‘large clusters’ expansion conditions, additional peaks appear corresponding to reaction products issued from larger ŽVCl.q n G 3 precursors. The smallest reaction product mass peak is narrow Žsee insert of Fig. 4 at 9.6 eV and Fig. 5b. and can be assigned to C 4 H 535 Clq and C 4 H 537Clq ŽHCl elimination.. These masses Ž88 and 90 amu. were included in the mass range of the broad ‘product 1’ peak issued from the ionic dimer reaction. The peak width is larger than for a cold product ŽFig. 5a. but still relatively narrow, indicating that the kinetic energy released in the product is small. The C 4 H 5 Clq appearance threshold is at 9.55 eV, i.e. 0.2 eV lower than the threshold for ‘product 1’ observed in ‘small clusters’ expansion conditions. Clearly between 9.55 q and 9.75 eV, ŽVCl.q precursor 2 is not the C 4 H 5 Cl and this product must come from the trimer, the reaction being followed by evaporation of one VCl molecule: q
Ž VCl. 3 ™ Ž VCl. –C 4 H 5 Clqq HCl ™ C 4 H 5 Clqq VCl . In this scheme the first step is the ejection of the neutral HCl molecule. The kinetic energy is shared between this neutral fragment and the ionic one according to their mass ratio. Therefore as the cluster size increases the kinetic energy taken by the heavy ionic cluster decreases. The subsequent evaporation will not change notably this kinetic energy leading to narrow mass product peak. The reactions observed in the ‘large clusters’ expansion conditions can thus be summarized as follows: below 9.75 eV, HCl elimination leading to C 4 H 5 Clq occurs from ŽVCl.q 3 whereas above 9.75 eV, the broad ‘product 1’ is coming from ŽVCl.q 2.
4.2. ‘Product 1’ identification A careful analysis of the mass peak shape is necessary to establish the nature of ‘product 1’ issued from the dimer. In ICR experiments w14x, the HCl elimination channel presents the lowest barrier, thus it can reasonably be assumed that the ‘product 1’ corresponds at least in part to C 4 H 5 Clq. The broadening of ‘product 1’ peak can have three causes: Ž1. slow reaction, Ž2. kinetic energy released in the fragment C 4 H 5 Clq after reaction of ŽVCl.q 2 , or Ž3. presence of an additional reaction channel: Cl Ø elimination which will induce two additional peaks at 89 and 91 amu, superposed to the HCl elimination products at 88 and 90 amu. A slow reaction rate for the HCl elimination is responsible for the metastable tail of the mass peak but cannot explain completely the broadening. Indeed the peak remains broad when the ion extraction is delayed Žup to 2 ms. with respect to ionization, even though the metastable tail is considerably reduced Žsee insert in Fig. 1.. However, the metastable part of the product is minor and the ratio productrdimer does not substantially increase with the extraction delay which shows that the reaction takes essentially place within 100 ns after the ionization. Kinetic energy released in the fragment can account for the symmetrical peak broadening. Indeed, HCl elimination from ŽVCl.q 2 is largely exothermic by 1.5 eV 1 in agreement with an estimated value of 1.8 eV w14x. Assuming that the binding energy of the neutral van der Waals dimer ŽVCl. 2 is of the order of 0.1 eV, the ionization threshold of bare VCl being 10.0 eV, the absorption of a 9.8 eV photon by ŽVCl. 2 produces a ŽVCl.q 2 ion with an energy of 0.3 eV below the dissociation limit in VClqq VCl. For an estimated exothermicity with respect to this limit of 1.5 eV, the available energy for the reaction is 1.2 eV. Thus near the reaction threshold Ž9.8 eV. around 1.2 eV have to be shared between the product and the fragment HCl as kinetic and internal energy. This
1
Calculated value in the present work using the MOPAC program at AM1 level.
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hypothesis enables to explain the lack of variation of the peak width when the ionization energy is varied from 9.75 to 10 eV: a 0.25 eV energy increase is small with respect to the total available energy. In order to test the assumption that kinetic energy released during the reaction process is responsible for ‘product 1’ mass peak broadening and to extract information about the dissociation mechanism, the mass peak shape has been simulated. Two limiting distributions for the kinetic energy released in the HCl elimination reaction have been used: a statistical and an impulsive model. In the statistical model, the available energy is redistributed between the different degrees of freedom before the dissociation. In the impulsive model, on the contrary, the fragment is ejected monokinetically. This implies that there is no energy redistribution: as soon as the dimer overcomes the reaction barrier, the ejection of HCl takes place to give the C 4 H 5 Clq product. Starting from a given kinetic energy distribution, the mass peak width has been calculated in the case of an isotropic velocity distribution, using the known voltages and geometry of the mass spectrometer. This width has been convoluted by the ‘apparatus function’ which can be derived from the cold bare molecule peak.
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with a ; 50 ns half width before convolution with the apparatus function. Fig. 6 shows the ‘product 1’ peak recorded at 9.8 eV together with the simulation obtained with the statistical model Ždashed line.. There is a clear disagreement of this simulation with the experimental peak shape. 4.2.2. ImpulsiÕe model In the case of an impulsive model, all the fragments being ejected with the same velocity, the product mass peak has a rectangular profile. The simulated mass peak for a kinetic energy of 1000 cmy1 in the C 4 H 5 Clq fragment is displayed in Fig. 5d and in Fig. 6 Ždotted line.. The impulsive model enables to reproduce quite well the experiment. It should be noticed that in the frame of this model, it is not necessary to involve the Cl Ø elimination channel to explain the mass peak shape. However, in the frame of the impulsive model, all the available energy should be shared between C 4 H 5 Clq and HCl fragments and a value of ; 3000 cmy1 is expected for the kinetic energy of the C 4 H 5 Clq fragment. Our best-fit value Ž; 1000 " 200 cmy1 . is clearly lower than the expected one. Similar low kinetic energy content has also been observed in the HCl elimination from ionized ethylchloride w22x.
4.2.1. Statistical model Using Klots theory w20x, the average kinetic energy released in a dissociation ² E kin : can be calculated for a given available energy and the vibrational frequencies of the system 2 . The kinetic energy distribution takes the approximate form of the exponential function expŽyE kinr² E kin :.. This approach has been previously used by T. Baer and co-workers w22x in the ethylchloride dissociative ionization study. In the case of a 9.8 eV ionizing photon, one obtains an average translational energy Žtotal kinetic energy. of ; 800 cmy1 . The ‘product 1’ average kinetic energy ² E kin : is around 230 cmy1 according to the mass ratio between C 4 H 5 Clq and HCl. The resulting calculated mass peak shape is approximately Gaussian
2
The Klots model requires the knowledge of the vibrational frequencies. They have been obtained from Ref. w21x. A variation of 10% of these frequencies do not change the KER more than 4%.
Fig. 6. Comparison between the calculated mass peak shapes of C 4 H 5 Clq and the experimental ‘product 1’ peak Žfull line. recorded at 9.8 eV in the ‘small clusters’ expansion conditions with no delay between the detected electron and the extraction pulse. The zero time-of-flight corresponds to the C 4 H 5 35 Clq TOF in our calibrated mass spectrometer. Dashed line: statistical model Ž- - - - -.; dotted line: impulsive model ŽPPPPPP..
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This behavior can be explained by assuming that the reaction proceeds through a late barrier Žlate transition state.. Before the late barrier crossing, the energy is shared between all the degrees of freedom Žstatistical behavior.. Then, at the late transition state there is an abrupt bond rupture, triggering the expulsion of the HCl fragment without interaction with the other product. This leads to a final impulsive dissociation but not with the total initial available energy. This reactive scheme has already been proposed in
the case of the reaction of O q C 2 H 4 w23x or in the photodissociation of HFCO w24x. The reaction path for HCl elimination from ŽVCl.q 2 is sketched in Fig. 7: – The photon hn promotes the dimer on the VCl–VClq ionic surface Ž1.. – In the ionic dimer, the radical cation VClq ŽCH 2 CHClqØ. known to be highly reactive towards electron donor molecules Žsee, e.g., Ref. w25x. reacts with the neutral CH 2 CHCl through a barrier Ž2. of
Ž .q Fig. 7. Scheme of the reaction paths of ŽVCl.q 2 and VCl 3 . Dotted line: reaction in the dimer. Above the entrance barrier the reaction leads to HCl elimination Žchannel a .. Full line: reaction in the trimer: Stabilization of a stable addition complex through VCl evaporation Žchannel b .: this channel is not seen for the trimer but seems to be dominant for larger clusters. Reaction leading to HCl elimination followed by VCl evaporation Žchannel g .. Successive elimination of two HCl fragments Žchannel d ..
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0.2 eV Žmeasured in the present experiment. to form an addition distonic complex ØCH 2 –CHCl–CH 2 – CHClqŽ 3. or one of its isomers: ØCHCl–CH 2 – CH 2 –CHClq or ØCH 2 –CHCl–CHCl–CHq 2. – This addition complex will rearrange to the Ž4. ŽCH 2 5CCl– stable polymeric species C 4 H 6 Clq 2 CH 2 –CH 2 Clq or one of its isomers. which has been calculated to be 1 eV below the VCl q VClq limit 3. – The HCl elimination from C 4 H 6 Clq 2 proceeds impulsively through a late barrier Ž5. and leads to the formation of C 4 H 5 Clqq HCl Ž6. and the kinetic energy release measured for the C 4 H 5 Clq product is around 1000 cmy1 so that the total kinetic energy release in this dissociation should be around 3000 cmy1 . It can be noticed here that the formation of the addition complex can be seen as the elementary process for the polymerization reaction, which should be repeated many times to get the polyvinylchloride ions and that HCl elimination can be considered as the ending process. 4.3. Other reactiÕe channels Above 10 eV, additional reaction channels have been observed. The CH 2 Cl elimination channel appears between 10.1 and 10.25 eV Žsee Fig. 3.: q ŽVCl.q 2 ™ C 3 H 4 Cl q CH 2 Cl. Since the two 35 q C 3 H 4 Cl and C 3 H 437Clq mass peaks are resolved, the kinetic energy released in the reaction is smaller than in the case of HCl elimination. Another result is the modification of the ‘product 1’ peak shape. Fig. 8 displays a comparison between mass peaks recorded at 10.25 and 9.95 eV. One possible explanation is the appearance of the Cl Ø elimination channel. Indeed, an attempt to reproduce the new shape of ‘product 1’ mass peak in the frame of the previous impulsive model, with only C 4 H 535 Clq and C 4 H 537Clq peaks ŽHCl elimination. broadened by kinetic energy, has been unsuccessful. If we assume that the peak shape results from the superposition of the latter peaks and C 4 H 635 Clq and C4 H637Clq Žresulting from Cl Ø elimination without kinetic energy., a better agreement with the experi-
3
Calculated value in the present work using the MOPAC program at AM1 level.
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Fig. 8. Comparison of TPEPICO mass spectra of ‘product 1’ recorded at 9.95 Žthin dotted line. and 10.25 eV Žfull line. in ‘small clusters’ expansion conditions. The mass peak shape at 10.25 eV has been simulated Žthick dotted line. by adding to the peak shape at 9.95 eV Ždue to C 4 H 5 Clq coming from the ŽVCl. 2 dimer. two narrow mass peaks corresponding to the Cl Ø elimination channel leading to C 4 H 6 35 Cl and C 4 H 637 Cl Ždashed line.. The shape of these peaks is that of VCl–H 2 O shifted by 0.54 ms to match the position of a cold C 4 H 6 Clq product.
mental results is obtained Žsee Fig. 8.. It is difficult to estimate precisely the branching ratio between the two channels without using a reflectron time-of-flight mass spectrometer which would suppress the KER broadening. However, HCl seems to remain the major product. The present results can be compared with the observations realized in ICR experiments w14x. In both cases, HCl elimination is the dominant channel. An estimation of the three reactions enthalpies is given in Ref. w14x: HCl elimination is the most exothermic reaction Žy1.8 eV. which can explain the large amount of kinetic energy in the fragment measured in the present work and the two other channels are also exothermic Žy0.2 eV for CH 2 Cl elimination and y0.9 eV for Cl Ø elimination.. However, we do not observe them at the dimer ionization threshold. This implies that, as in the case of HCl elimination, a barrier to these reactions is present. Unfortunately, a precise measurement of these barriers was not possible because of the low signal to noise ratio for the experiments performed above the bare molecule ionization threshold. In the free VClq ion, the dissociation of the C–Cl bond requires 2.4 eV of excess energy Žthreshold at 12.4 eV.w13x. From the observation of the Cl Ø elimi-
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nation at 10.25 eV and knowing the dimer ionization threshold it is clear that the Cl Ø elimination requires less energy in the complex Žless than 10.25–9.55 s 0.7 eV. than in the free ions. Similar results have been obtained in the nucleophilic substitution of Cl Ø by NH 3 in the fluorochlorobenzene–NH 3 system where the formation of an intermediate sigma complex weakens the C–Cl bond to 0.5 eV w8x. 4.4. Reaction in larger clusters The last point concerns the apparent absence of reaction products issued from large clusters ŽVCl.q n ) 3. In the case of HCl elimination, one expects a mechanism similar in the dimer and in larger clusters with a similar barrier to the addition Ž0.2 eV.. Since the observed VClq mass peaks present a small n metastable tail, this implies that these ions are obtained from evaporation of larger clusters in the acceleration region of the mass spectrometer. The absence of elimination products from large clusters implies that the reaction is stopped during the reactive pathway. This evaporation process can dissipate enough energy to stop the reactive process behind a barrier along the reactive path. Furthermore, the evaporation is probably the most important when the available energy is maximum, i.e. above the minima of the potential energy surface. 4.4.1. The trimer case A reactive scheme is proposed for the case of the Ža., a first trimer in Fig. 7. Starting from VClq 3 addition reaction between VClq and one neutral VCl occurs, through a 0.2 eV barrier Žb. like in the dimer, Ž . leading to C 4 H 6 Clq 2 PPP VCl d . The potential energy is just shifted by the stabilization energy induced by the third molecule. This addition product Žd. can: Ž1. eliminate HCl through a late barrier Že. to give C 4 H 5 Clq . . . VCl Žf.. The subsequent evaporation of VCl will lead to the C 4 H 5 Clq product Ž6. Žchannel g . observed in the experiment Žsee Figs. 4 and 5b.. Ž2. continue the polymerization process by adding the third VCl molecule and lead, after rearrangeŽ . ments, to the stable addition complex C 6 H 9 Clq 3 g . Here the final process can be elimination of one Žh. or two Ži. HCl molecules from this stable addition polymer Žchannel d ., as observed in Fig. 4 Žpeaks labelled ) and )), respectively..
Ž3. be stabilized by the evaporation of one VCl Ž . Žchannel b, dashed molecule to give C 4 H 6 Clq 2 4 arrow.. It should be noticed that these addition polyvinylchloride ions have the same masses as the van der Waals complexes Žchannel g .; however, they may be distinguished from the direct ionization of the cold complexes by an enlargement of the mass peaks. The absence of kinetic broadening and metastable tail on the dimer peak indicates that the trimer does not follow this channel but peaks corresponding to larger clusters are enlarged, which may reflect this process. 4.4.2. Larger clusters This reaction scheme can be extended to the case of larger clusters. Successive addition processes should lead to the formation of polyvinylchloride ions but the HCl elimination channel has not been observed: the reactive process may be stopped in the addition intermediate state by the evaporative cooling of VCl molecules. The observed mass peaks may thus correspond to stable addition complexes Žand not van der Waals clusters.. The exact chemical nature of the peaks observed at the masses of VCl n cannot be resolved at the moment and more experimental andror theoretical work is necessary. In particular, one cannot exclude that the reactive process is rather stopped in the exit channel: the eliminated HCl may be bound to the remaining cluster by strong ion–dipole interaction and cannot escape from the cluster, the evaporation of a VCl molecule being energetically more favorable than evaporation of HCl. In any case, the absence of HCl evaporation observed in large clusters can be seen as an evidence for a cage effect: the reaction stops since it is easier to evaporate a surrounding molecule than to cross over a barrier along the reaction coordinate. 5. Conclusions The reactivity of ionized vinyl chloride clusters has been investigated by the TPEPICO technique which enables a control of the internal energy of the reactive ions. In the case of the dimer ŽVCl.q 2 , a first reactive channel, the HCl elimination, has been evidenced with an appearance threshold 0.2 eV above the ion-
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ization threshold of the dimer. The kinetic energy released in the C 4 H 5 Clq reaction product has been estimated through the simulation of the mass peak shape assuming that the C 4 H 5 Clq and HCl fragments are ejected monokinetically Žimpulsive model. but only a part of the total available energy is released as fragment kinetic energy. The following reactive path has been proposed to account for this results: the initial step is the formation of a stable addition complex C 4 H 6 Clq 2 after crossing the 0.2 eV barrier. This can be considered as the first step of the polymerisation process. The final step is the HCl elimination from this addition complex. This elimination proceeds also through a barrier and the dissociation is prompt Žimpulsive model.. Two other reactive channels have been evidenced for higher energies: CH 2 Cl and Cl Ø elimination. The evolution of the polymerisation reaction in the larger clusters has been investigated. The polyvinylchloride ions resulting from successive additions of the VCl molecules in the cluster can be very stable. As the size increases, the HCl elimination is less observed. This seems to be due to competition with evaporation processes which cool the system in the intermediate addition polymer: the reaction is stopped and the final process ŽHCl elimination. is not observed anymore. The cluster acts both as a thermal bath and as a cage.
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