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Proton transfer mechanism in the ionic methanol dimer |
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PhysChemComm,
Volume 2,
Issue 4,
1999,
Page 15-19
S. Martrenchard,
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摘要:
Proton transfer mechanism in the ionic methanol dimer S. Martrenchard,a G. Grégoire,b C. Dedonder-Lardeux,a C. Jouveta and D. Solgadia a Laboratoire de Photophysique Moléculaire du CNRS Bât 210 and Laboratoire LURE, Bât 201, Université Paris sud, 91405 Orsay Cedex, France b DRECAM/SPAM CEA, Saclay 91190, Gif/Yvette, France Received 26th April 1999, Accepted 11th May 1999, Published 17th May 1999 The intracluster proton transfer reaction in the methanol ionic dimer has been re-investigated using VUV ionization with synchrotron radiation. The energetic thresholds of the proton transfer issued from the methyl group or the hydroxy group have been obtained. These results are discussed in view of previous calculations and experiments. The non observation of unprotonated clusters in most of the experiments leads to the conclusion that the proton transfer reaction is very efficient and that the vertical ionization threshold of clusters is higher than the proton transfer reaction threshold. However, unprotonated clusters have been observed in some experiments,2 which means either that the threshold of reactions leading to (CH3OH)n-1H+ is higher than the adiabatic ionization threshold of (CH3OH)n, or that there is a barrier to the proton transfer reaction.Thus only soft ionization technique should be able to evidence the barrier (if there is one) or the threshold of the reaction. A recent experiment of Baer and coworkers9 using synchrotron radiation and TPEPICO technique shows that (CH3OH)2+ can be observed at 10.2 eV.The comparison of this mass peak intensity with the one of the protonated dimer (CH3OH)2H+ leads the authors to the conclusion that dissociative ionization (in this case proton transfer followed by dissociation) is the major channel for the ionic dimer, at this energy. Photon induced ionization of methanol clusters has also recently been investigated, particularly for the proton transfer reaction in the dimer by Tsai et al. using VUV laser light.10 As seen below, the energy used in their experiment was higher than that used by the present authors. The unprotonated ionic dimer has not been observed. However, experiments with deuterated CD3OH have been performed to characterize the competition between proton transfer from the methyl group and proton transfer from the hydroxy group, as well as detailed ab initio calculations of the potential energy surface to understand the reaction mechanisms.In the present work, we have performed new experiments mostly on the methanol dimer using tunable one-photon ionization with synchrotron radiation. This method presents the advantage of a good accordance of the ionization energy and the possibility to select the internal energy of reactive ions by threshold photoelectron–photoion coincidence experiments. Introduction Cluster ions formed from molecules with protic hydrogens and lone pair acceptors undergo proton transfer reactions that can be studied under stepwise solvation. Ionization of neutral clusters usually leads to the observation of protonated cluster ions through a complex mechanism of intracluster proton transfer, rearrangement and fragmentation or evaporation. The study of the proton transfer reaction within clusters has already received a great deal of attention.In spite of its apparent simplicity, the mechanism and energetics of the proton transfer and its evolution with cluster size are not yet fully understood. Among the simplest systems like H2O and NH3, the methanol case is interesting since it is one of the smallest molecules in which the ionic proton transfer can come either from an OH or a CH bond. It has already been shown that ionization of methanol clusters either with synchrotron radiation,1 electron impact2–4 or multiphoton ionization5,6 leads to protonated clusters through two possible reactions: (CH3OH)n® [(CH3OH)n]+®(CH3OH)n–1H+ + CH3O• (1) Hydroxy group proton transfer (CH3OH)n®[(CH3OH)n]+®(CH3OH)n–1H+ + CH2OH• (2) Methyl proton transfer The two mechanisms, proton transfer from the hydroxy group and the methyl group, can be differentiated by using deuterated methanol CD3OH or CH3OD.These two channels have also been characterized in ion–molecule reactions.7,8 A cluster size dependence of the ratio between methyl and hydroxy group proton transfer has been observed in electron impact studies3,4 but this ionization technique usually leads to intense fragmentation. In most of the experiments, the unprotonated ionized clusters have not been observed clearly except in one case, in the work of Garvey and coworkers with methanol–argon clusters.2 In this case, the evaporative cooling of the ionized cluster after "argon mediated ionization" enables the observation of different ions than in the case of direct ionization of pure methanol clusters, and in particular the unprotonated clusters are observed.PhysChemComm, 1999, 4 Experimental The SAPHIRS experimental set-up built on the SuperACO storage ring at LURE (Orsay) consists of a continuous supersonic beam coupled to a double time of flight (ionsand electrons) spectrometer and has been described in a previous publication.11 The clusters are produced by expanding helium seeded with methanol through a 50 mm nozzle under a typical backing pressure Po of 1 bar leading to a pressure of about 10–4 Torr in the expansion chamber.The beam is skimmed before entering the ionization chamber which is maintained at a pressure of 10–6 Torr. Clusters are ionized with the synchrotron radiation in the 9.6 to 10.9 eV energy range using a three meter normal incidence monochromator to select the wavelength. The ions are detected in coincidence with the electrons in a tandem time of flight mass spectrometer (PEPICO photoelectron–photoion coincidence technique) providing time of flight spectra, directly related to clusters' masses. In the simplest experiments, the ions and electrons are extracted by a 80 V cm–1 dc field, in which case no selection on the electron energy is achieved.The selectivity of the experiment can be increased by detecting the ions in coincidence with zero kinetic energy electrons (TPEPICO threshold photoelectron–photoion coincidence). In this case, the energy imparted to the corresponding ion is well defined and is equal to the photon energy minus the ionization threshold of the species. Discrimination of electron kinetic energies is obtained in two ways. There is first a geometrical discrimination: the electrons are detected through a small hole (2 mm in diameter) and a weak electric field (1 V cm–1) is applied before the ion extraction. Under these conditions, all the fast electrons are eliminated (and only the near zero kinetic energy electrons are detected) except those which are ejected in the direction of the detector.The selectivity can then be increased by discriminating the electrons by their time of flight which ensures that only coincidences with threshold photoelectrons are detected. The typical resolution obtained in these experiments is 0.05 eV since when working on clusters the slits of the monochromator are wide open (600 mm wide). Results Fig. 1 presents a selection of the mass spectra recorded by ionizing neutral (CH3OH)n clusters with photon energies varying from 9.7 to 10.5 eV. These spectra have been recorded without selecting the zero kinetic energy electrons (80 V cm–1 extraction field). The main results are the following: (i) At high energy, peaks corresponding to masses 33 and 65 m/z, i.e.to (CH3OH) H+ and (CH3OH)2H+, are observed. These peaks result from the proton transfer reaction as expected when ionizing methanol clusters following the mechanism: (CH3OH)n ® (CH3OH)n+ ® (CH3OH)n–m–1H+ + CH3O• or CH2OH• + mCH3OH (ii) beside the peak at 65 m/z, we clearly see a peak corresponding to m/z 64, i.e. to (CH3OH)2+, the unprotonated dimer. These experiments enable the vertical appearance threshold of the methanol dimer at 9.7 ± 0.05 eV to be measured. (iii) the appearance threshold of protonated methanol ion (CH3OH)H+ is 10.15 ± 0.05 eV in good agreement with the Fig. 1 Mass spectra obtained for different photon energies. At low energies (9.8 and 10 eV) only the dimer and protonated dimer are observed. The protonated methanol ion is observed above 10.15 eV.previous experiments of ref. 1. It can be seen on Fig. 1, that the shape of the peak changes from 10.2 to 10.4 eV. At 10.2 eV, mass peak 33 m/z presents a long tail extending towards large times of flight. This peak shape is characteristic of a slow reactive process (a few 100 ns time scale) occurring in the extraction region of the mass spectrometer: the ion is first accelerated as a parent ion, then undergoes the reaction and is further accelerated as a daughter ion. When the methanol concentration is decreased in the expansion, only (CH3OH)H+ and (CH3OH)2+ mass peaks remain. In Fig. 2 a mass spectrum is presented recorded under the same expansion conditions, for two photon energies of 10 and 10.2 eV, in which the internal energy of the ion is selected by performing coincidences with threshold electrons.The comparison with previous spectra is interesting: the dimer mass peak (CH3OH)2+ disappears at 10.2 eV when the (CH3OH)H+ product begins to be observed. This is a clear indication of a parent/daughter ion relationship: at 10.2 eV, the protonated methanol is issued from the methanol ionic dimer. In order to discriminate between the hydroxy group proton transfer and the methyl proton transfer, experiments with deuterated methanol CD3OH have been done. Mass spectra recorded in the range 10–10.5 eV are presented in Fig. 3. The reaction product mass peak splits into two components with m/z 36 and 37, corresponding to proton transfer from the hydroxy group (36) or the methyl group (37): (CD3OH)2+ ® (CD3OH) H+ + CD3O• (M=36) (CD3OH)2+ ® (CD3OH) D+ + CD2OH• (M=37)The appearance thresholds of these two peaks, 36 and 37, are very close to each other.Within the experimental resolution, it seems that peak 37 appears at an energy slightly lower (ca. 10.1 ± 0.05 eV) than peak 36 (10.2 ± 0.05 eV). Fig. 3 shows that in the spectrum recorded at 10.2 eV, the M=36 mass peak is absent and the M=37 mass peak is very broad, indicating a slow reaction for this channel at this energy. At higher energies, between 10.4 and 10.8 eV, no significant modification of the ratio of the intensities for peaks 36/37 is observed. Fig. 2 TPEPICO mass spectra: when increasing the energy, the dimer peak disappears while the protonated methanol product peak increases.The comparison of the reaction products peak shape at 36 and 37 m/z is interesting: mass peak 36 [(CD3OH)H+] is narrow, even near the threshold while mass peak 37 [(CD3OH) D+] presents the same broad asymmetric profile as (CH3OH)H+ at 10.2 eV. From the shape of this peak, it is possible to deduce a reaction time. From a mass spectrum obtained at 10.2 eV using coincidences with threshold electrons, the reaction time deduced is around 300 ns at this energy. The difference in the reaction rate between the two channels is clearly evidenced in Fig. 4 recorded at 10.2 eV. There, the delay between the detected electron and the extraction pulse is varied from 80 ns to 1 ms.If the two channels have the same reaction rate the intensity at the maximum of the two peaks should be independent of this delay. If one reaction is slow, at a small delay time some daughter ions are accelerated as parent ions, leading to a broad peak (metastable peak), whereas when the delay is large, all the parent ions have reacted before extraction and the ions are all observed at the mass of the product. Here, the mass 37 peak intensity is greater after a long delay which shows that the rate for this reactive channel is slower than the one leading to mass 36 (hydroxy group proton transfer). Fig. 3 PEPICO mass spectra recorded at different energies using deuterated CD3OH methanol. At 10 eV no reaction product.At 10.2 eV there is a broad peak assigned to a slow reaction giving CD3OHD+ (M=37 ) is observed. At 10.3 and 10.5 eV the other channel (M=36) is observed. Fig. 4 Variation of the peak shape as a function of the delay between ionization event and 80 V cm–1 extraction pulse application. The CD3OHD+ peak becomes narrower as the delay increases. The ionization energy is 10.2 eV.Discussion The unexpected point in these experiments is the observation of the unprotonated ionized methanol dimer at M=64. The narrow shape of this mass peak, as well as the observation of this peak even when the methanol concentration in the expansion is small seem to indicate that the (CH3OH)2+ or (CD3OH)2+ dimer ion is obtained by direct vertical ionization of the neutral dimer. Indeed, if it was coming from evaporation of larger clusters, the peak would be broadened by the kinetic energy released in the dissociation as can be seen for comparison on the M=65 peak (CH3OH)2H+ that comes from a larger cluster involving at least three molecules (Fig.2). Until now, it was presumed that vertical ionization would be above the proton transfer reaction threshold and thus that unreactive ionized methanol dimer could not be observed. The vertical ionization threshold measured in this experiment, 9.7 ± 0.05 eV, can give a lower value of the ionic dimer binding energy since the adiabatic ionization potential is equal to or lower than this value. Assuming a value of 0.13 eV (ref. 4) for the CH3OH–CH3OH binding energy in the neutral dimer, and since the free methanol ionization potential is 10.84 eV, the binding energy of the ionic dimer is at least 10.84 + 0.13 – 9.7=1.27 eV.This binding energy can be larger if the adiabatic ionization threshold is significantly different from the vertical threshold measured here. The second point concerns the reactivity of the dimer ion. Below 10.1 eV, no protonated methanol is observed. As soon as the protonated (or deuterated) methanol ion appears, the dimer mass peak disappears as can be seen clearly in the TPEPICO mass spectrum of Fig. 2. This shows that the precursor of (CH3OH)H+ [or (CD3OH)H+ and (CD3OH) D+] is the ionized methanol dimer. In the case of methanol–4H, it is not possible to discriminate between proton transfer from the hydroxy group or the methyl group but in the case of (CD3OH)2+, the two product mass peaks (CD3OH)H+ and (CD3OH)D+ show a different behaviour. Proton transfer from the hydroxy group (mass 36 product) For proton transfer occurring from the hydroxy group, the threshold is higher (ca.10.2 eV) than for the other channel (methyl proton transfer threshold at 10.1 eV) and no metastable peak is observed indicating a fast reaction rate near threshold. The energetic threshold of this reaction has been calculated to be at 10.37 eV (ref. 10) or 10.15 eV (ref. 4) very close to the present measure: it then seems that the measured threshold corresponds to energetic threshold and that the reaction proceeds without barrier.Proton transfer from the methyl group (mass 37 product) In the proton transfer from the methyl group reaction, the product peak observed at ca. 0.1 eV above the threshold is metastable (broad and asymmetric see Fig. 2) which leads to a long reaction time of ca. 300 ns. The calculated reaction enthalpy leads to an energetic threshold of 9.75 eV (ref. 4) or 10.08 eV (ref. 10) for this reaction. The binding energy difference between CH3O• and CH2OH• is 0.4 eV (ref. 3) and therefore if the 10.2 eV value is the energetic threshold for the hydroxy group proton transfer, the methyl proton transfer threshold should appear at 9.8 eV. This value is smaller than the measured threshold and one can deduce that the reaction proceeds through a barrier.A value of this barrier can be derived from the difference between the appearance threshold of the protonated methanol and the dimer ionization threshold: 10.1 – 9.7=0.4 ± 0.1 eV. This barrier is not very high and it explains why at 0.3 eV above the reaction threshold, the mass peak becomes narrow indicating that the process occurs on the nanosecond time scale or faster. These results are summarized in Fig. 5. Comparison with the recent experimental work of Tsai et al.10 can be made. Mass spectra of ionized methanol clusters have been recorded using VUV laser radiation for several photon energies. The mass spectrum recorded at a medium energy of 10.49 eV is very similar to these presented here. At higher energy (10.9 eV, above methanol ionization threshold), the authors present a mass spectrum in which the ratio (CD3OH)H+/(CD2OH)D+ is larger than 3.It leads them to the conclusion that proton transfer from the hydroxy group is slow at the threshold and becomes faster than the other reaction at higher energy. We have no evidence of any change on the mass peak 36 profile that would indicate a change in the reaction rate. Fig. 5 Proton transfer reaction diagram for (CD3OH)2+, the thick lines are the values measured in the present work. The values of reaction enthalpies are issued from thermodynamical data3 and represented with narrow lines. Comparison with calculations The reaction mechanism proposed by Lee et al.4 can account for our results.Two minima have been found in the potential energy surface of the ionic dimer: both correspond to structures with an hydrogen bond. The dissociation of these dimers leads either to CH3OH2+ + CH3O• (hydroxy group proton transfer) or CH3OH2+ + CH2OH• (methyl group proton transfer). Due to the hydrogen bond already present in the neutral dimer, the proton transfer from the hydroxy group is geometrically favoured in the vertical ionization. Thus proton transfer from the hydroxy group is calculated to occur without barrier, corroborated by our measurement where the narrow shape of mass peak 36, even near the threshold, indicates a fast reaction.On the contrary, proton transfer from the methyl group requires a large geometry change. Lee et al.4 found a barrier corresponding to this rearrangement in the reaction path from the vertical ionized dimer to the second isomer.This barrier may be the one which has been measured in this experiment. The calculated energy difference between the vertical ionization potential of the dimer and the top of the barrier is 0.37 eV, which is very close to our measured value: 0.4 eV. However, some discrepancies between the experiment and the calculations can be found. First, the calculated vertical ionization potential of methanol dimer (10.2 eV) is much too high since it is higher than the proton transfer threshold whereas we measured it at 9.7 eV. Then the position of the barrier in the methyl transfer pathway is not compatible with what we observed.Indeed, the top of this barrier is higher than both exit channels, and in particular higher than the hydroxy group proton transfer channel which proceeds without barrier. If it were the case, methyl proton transfer should be very unfavoured and probably not observed. Concerning these points, the agreement with the recent calculation of Tsai et al.10 is better. They found a value of 9.74 eV for the dimer vertical ionization which is very close to our measurement. This value is also below the calculated proton transfer reaction thresholds, indicating that non reactive ionized methanol dimer could be observed. In these calculations, two new isomers of the ionized methanol dimer have been found leading to a revision of the reaction mechanism. For the hydroxy group proton transfer, it has been calculated that, in addition to the direct channel without barrier leading to the observation of CD3OH2+ with the elimination of CD3O•, there is a more exothermic reaction leading of CD2OD• through a barrier.Our measurements are consistent with the observation of the direct mechanism without a barrier, and the calculated threshold for the direct channel (10.37 eV) is close to the experimental value (10.2 eV). For the methyl group proton transfer, the authors found another reaction path than Lee et al.4 which enables CD3OHD+ to be produced by isomerization of the ionized dimer with a very low barrier, below the vertical ionization threshold. If this were the case, we should have measured an appearance threshold of CD3OHD+ corresponding to the energetic threshold at 9.8 eV (i.e.0.4 eV lower than the other channel) and not 10.1 eV. This reaction is observed to be slow at 10.2 eV which seems to be consistent with the presence of a barrier along the reactive path. However one cannot exclude that this barrier is smaller than the gap between the expected threshold and the observed one if this reaction is too slow below 10.2 eV to be detected in the experiment (large kinetic shift). Our experiment shows only that there is a barrier in the reactive pathway leading to methyl proton transferred product, but it is still not clear if this barrier corresponds to the rearrangement of the vertical ionized isomer4 or if there is another reactive pathway with a lower activation barrier.10 Conclusion The reactivity of the ionized methanol dimer has been studied.The vertical ionization threshold of the unreactive dimer has been measured at 9.7 eV. At higher energy, two proton transfer reactions occur: methyl group proton transfer, which is the most exothermic but which presents a barrier and hydroxy group proton transfer which occurs at threshold without barrier. We are now investigating by soft ionization and internal energy discrimination techniques the reactivity of the larger clusters and especially the trimer to study the evolution of the hydroxy vs. methyl proton transfer ratio, in particular to understand the "concentration" effect seen in refs. 3 and 4 (relative increase of the hydroxy group proton transfer reaction as the cluster size increases). Acknowledgements We want to thank Dr C-K. Ni for his very helpful discussions and for providing us with his results before publication. Prof. I. Dimicoli is also thanked for her help in setting up the experiment, as well as the staff of LURE for operating the Super-ACO storage ring. Paper 9/03329G References 1 K. D. Cook, G. G. Jones and J. W. Taylor, Int. J. Mass. Spectrom. Ion Phys., 1980, 35, 273. 2 G. Vaidyanathan, M. T. Coolbaugh, W. R. Peifer and J. F. Garvey, J. Chem. Phys., 1991, 94, 1850. 3 M. S. El-Shall, C. Marks, L. W. Sieck and M. Meot- Ner, J. Phys. Chem., 1992, 96, 2045. 4 S. Y. Lee, D. N. Shin, S. G. Cho, K-H. Jung and K. W. Jung, J. Mass Spec., 1995, 30, 969. 5 S. Morgan, R. G. Keesee and A. W. Castleman, J. Am. Chem. Soc., 1989, 111, 3841. 6 S. Morgan and A. W. Castleman, J. Phys. Chem., 1989, 93, 4544. 7 K. R. Ryan, L. W. Sieck and J. H. Futrell, J. Chem. Phys., 1964, 41, 111. 8 L. W. Sieck, F. P. Abramson and J. H. Futrell, J. Chem. Phys., 1966, 45, 2859. 9 S-T. Tsai, J-C. Jiang, Y. T. Lee, A. H. Kung, S. H. Lin and C-K. Ni, submitted to J. Chem. Phys. 10 J. A. Booze and T. Baer, J. Chem. Phys., 1992, 96(7), 5541. 11 C. Dedonder-Lardeux, I. Dimicoli, C. Jouvet, S. Martrenchard-Barra, M. Richard-Viard, D. Solgadi and M. Vervloet, Chem. Phys. Lett., 1995, 240, 97. PhysChemComm © The Royal Society of Chemistry 1999
ISSN:1460-2733
DOI:10.1039/a903329g
出版商:RSC
年代:1999
数据来源: RSC
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