J. Phys. Chem. A 2010, 114, 5655–5665
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Reactions of Laser-Ablated Zirconium Atoms within a Supersonic Expansion: Insertion versus Radical Mechanism S. Soorkia,† C. Pothier, J. M. Mestdagh, and B. Soep* Laboratoire Francis Perrin (CNRS-URA-2453) CEA/IRAMIS/SerVice des Photons, Atomes et Mole´cules, CEN Saclay, F-91191 Gif-sur-YVette cedex, France
J. Lie´vin SerVice de Chimie Quantique et Photophysique, 50 AV. F.D. RooseVelt, CP 160/09 B-1050 Bruxelles, Belgium ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: March 19, 2010
In a laser ablation type microreactor followed by supersonic expansion, zirconium atoms have been reacted with methyl fluoride, CH3F (MeF), and mixtures of MeF and dimethylether, CH3-O-CH3 (DME) seeded in He. With both mixtures, only a number of simple fluorinated products are formed, and they have been identified by one-photon ionization. All products can be linked to radical reactions either with F atoms, CH3, or ZrF1, 2, 3 radicals. No insertion products of the Grignard reagent type, F -Zr-CH3 could be identified with or in the absence of DME. On the other hand, evidence has been found for the presence of organometallic compounds of the type ZrC2Hn)2, 4, 6, which could result from radical attack. Thus, even in conditions where intense solvation is at work, induced by clustering with polar DME molecules, which can act as stabilizing agents, a direct insertion mechanism into the C-F bond involving barrier suppression is not at work in our conditions. The reactivity due to radicals is very effective in this type of reactor, and the products that are efficiently formed can be quickly stabilized in the expansion. The radical attack supersedes, in the case of zirconium solvated by DME, the metastable mechanism with Zr(4d)3(5s)1, that is certainly energetically impossible in the absence of strong reaction barrier suppression by a solvent. High level ab initio calculations performed at the CASPT2 level of theory are used for characterizing the electronic and geometric structure of the inserted products. They also reveal striking features of the reaction mechanism that support the absence of observation of inserted products within solvated clusters of zirconium. 1. Introduction Organometallic species are key compounds in organic chemistry, as initially demonstrated with the preparation of organomagnesium compounds (Grignard reagents).1 Yet the observation and characterization of these molecules has essentially been performed in solutions, while the production in the gas phase of reactive organometallics is more recent and stems from the development of laser vaporization.2 Among these systems, compounds containing a metal-carbon bond, like Grignard reagents, are of high preparative value as they can functionalize C-H or C-F bonds. Whereas, in the gas phase, Grignard reagents have not been observed, except for a similar compound H-Zn-Cl,3,4 inserted transition metal ions have been prepared and characterized.5 Also, insertion mechanisms have often been shown to drive direct reactions of halogen-containing compounds with metals.6 For neutral compounds, a convenient method to characterize metal bonds is matrix isolation spectroscopy, where infrared analysis is used to assign the nature of the bonds of the inserted metal via the bond specific modes: metal stretch or bend infrared transitions.7 A thorough study of magnesium methyl halides has been performed using isotopic substitution and calculations, allowing the characterization of CH3-Mg-F.8 In this fashion, early on, much cryochemistry has been performed by codepos* To whom correspondence should be addressed. † Present address: Departments of Chemistry and Physics and Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720 USA.
iting transition metals with organic molecules9 producing organometallic systems. A variety of experiments on the reactivity of neutral transition metals with organic compounds or halogens10-12 has been performed in the gas phase by different techniques: crossed beams,11,13 beam-gas,12 and flowcells.14 Insertion mechanisms have been characterized for reactions involving neutral metals with hydrocarbons, where both C-C and C-H insertion have been found to be operative.13 In turn, reactions involving organohalogen molecules, for example, X-CH3 type compounds, which could lead to bond-breaking or bond-insertion reactions of the metal atom with the C-X bond, have been concerned to a much lesser extent in the gas phase. For this specific reason, it is of fundamental interest to study these latter gas phase reactions. Because inserted organometallic compounds are expected to be highly ionic in nature, a study of their reactivity in clusters could provide insight into how solvation influences the course of reactions. The reactions of laser ablated zirconium with MeF, have been studied in the present work under various conditions of concentration, expansion, and solvation. The various types of reactions taking place in these conditions have been searched for, and we have examined here the possibility to form and solvate insertion products of zirconium within a supersonic expansion. Here, laser ablation of Zr has been performed in the presence of MeF and DME, and mixtures of both diluted in helium. To investigate the possibility of forming inserted reaction products of the Grignard reagent type (F-Zr-CH3),
10.1021/jp911857m 2010 American Chemical Society Published on Web 04/20/2010
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Figure 1. Schematics of the laser ablation source. The gas pulse is actuated by a piezoelectric valve with a c.a. -500 V pulse, 200 µs long. The 532 nm Nd:YAG laser is focused (100 µm spot diameter) on the rotating and translating Zr rod, in the center of the channel (10 mm long, 1 mm in diameter), terminated with a broad angular aperture cone.
as described by Andrews et al.,15 DME is mostly employed as a stabilizing agent by analogy to the preparation of R-Mg-X compounds in the condensed phase. The pressure and concentration have been systematically varied in order to form clusters offering determined solvation conditions. After the supersonic expansion, the reaction products have been analyzed by timeof-flight mass spectrometry and single-photon ionization. High level ab initio calculations at the CASPT2 level of theory have been used here to investigate the properties of inserted compounds: structure, energetics of the ground and lowest excited states, and the related spin orbit states. The static properties of the excited and ionic states have important consequences on the experimentally observed systems; the calculations not only support the observations, but they also inform on the ionic character of inserted compounds. 2. Experimental Techniques The experimental apparatus combines a pulsed supersonic beam generating the clusters and the products, nanosecond lasers and a time-of-flight mass spectrometer (TOF-MS). The components of the beam are ionized by the UV probe lasers and mass analyzed using the TOF-MS. 2.1. Beam Source and Detection. Cold clusters are generated using a conventional laser vaporization source, see Figure 1. It is composed of a pulsed piezoelectric valve, a nozzle of 1 mm diameter followed by a channel of 1 mm diameter and 8 mm overall length. There, a rotating/translating zirconium rod is ablated by the 532 nm doubled output of a Nd:YAG laser along the design of Smalley.16 A mixture of 0-5% DME, 0-10% MeF in pure helium is passed through the nozzle. The gases have been premixed in a cylinder and entrained in the channel where the Zr vapor is created by laser ablation. After the ablation zone, the length of the channel is reduced to 3 mm, followed by a small cone with a broad aperture, 120°. The beam is then free to expand. The combined geometry of the channel and the aperture cone ensures a cooling of the ablated zirconium shortly before expansion. It produces cold ZrF, ZrF2 radicals, clusters of Zr and DME, plus various types of clusters and mixed clusters containing MeF and DME. The operating conditions (see Section 2.4) are adjusted for the production of a variety of species by increasing the effective pressure within the nozzle to achieve a controlled number of collisions between zirconium atoms and the reagents in helium within the channel. This, of course, leads to cluster formation at the tip of the nozzle. The
Soorkia et al. effective expansion pressure is therefore adjusted: for convenience we set the backing pressure (P0) at 2 bar and modify the effective pressure by changing the high voltage applied to the piezoelectric valve. Increasing the voltage increases the valve opening and thus the pressure at the nozzle. This is monitored through the pressure in the source chamber (P1) which will be indicated in the graphs, proportional to the effective backing pressure. The molecular beam is formed by skimming a free jet expansion through a 2 mm diameter skimmer before entering the chamber where the detection is performed. After the skimmer, the molecular beam crosses the ionization laser(s) in the acceleration region of a 1.23 m Wiley-McLaren17 TOFMS collinear with the beam. The mass resolution is 1 at mass 500, and the zirconium isotopes are clearly identified in all compounds. The isotope intensity pattern serves as a template to identify zirconium compounds. 2.2. Lasers. A pulsed tunable dye laser (LambdaPhysik LPD 3000) is pumped by the 355 nm output of a nanosecond Nd: YAG Quantel laser. It is used to ionize the metallic compounds of the molecular beam with frequency-doubled tunable light between 207 and 240 nm (6.1-5.2 eV). An excimer laser, (GAM Laser EX5) operating with a F2 mixture at 157 nm, is also used to ionize at 7.9 eV. The energy of both these lasers is in the micro-Joule range per pulse, and their beam diameter is ∼5 mm, therefore biphotonic effects are not expected. 2.3. Laser Ablation. Ablation is achieved by a frequencydoubled Nd:YAG laser (532 nm) whose energy is attenuated by a Glan prism. The energy used for ablation is above 5 mJ (typically 6 mJ), and the laser is focused over a 100 µm spot diameter. This energy is necessary for a rather refractory metal such as zirconium and is 5 times larger than calcium18 or magnesium19 used in similar experiments. 2.4. Clocking. The time delays between the various components of the beam source and detection represent essential parameters to control. This is achieved via a digital delay generator controlled by a quartz resonator and computer controlled (designed and built at Laboratoire de PhotoPhysique Mole´culaire, Universite´ Paris-Sud). The time sequence is the following: the Nd:YAG lasers are first charged, then the ablation laser is fired, setting the time origin, t0. The pulsed valve is opened at t1. The delay t1 - t0 controls the injection of zirconium in the gas pulse close to its maximum, it is varied in 1 µs steps. The ionization laser is fired at t2 to allow the clusters to arrive in the observation zone, t2 - t0 is also controlled by 1 µs steps. 3. Computational Methods Large scale ab initio calculations have been performed in order to investigate the valence electronic structure of inserted compounds of Zr with CH3F. All calculations were performed with the MOLPRO quantum chemistry package20 running on the HP-XC 4000 cluster of the ULB/VUB computer center. The RS2 version21 of the CASPT2 method22,23 implemented in MOLPRO has been used for optimizing the geometries and determining the relative stabilities of singlet and triplet states. The Zr atom is described by the quasi-relativistic Wood-Boring pseudopotential with 28 core electrons24 and by the corresponding valence basis set from the Stuttgart library24 augmented by 3f and 2g Gaussian primitives playing the role of polarization functions.25 The C, F, and H atoms are described by the augcc-pVTZ correlation consistent basis sets.26,27 An active space of 10 electrons distributed into 7 molecular orbitals (MO) (4a′ and 3a′′ for planar Cs geometries) is used in the CASSCF28 calculations preparing the multireference of the CASPT2
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Figure 2. Clustering of Zr in a mixture of 2% of methyl fluoride in He at three different backing pressures (P0) increasing from bottom to top by a factor of 2. The pressure P0 is represented here via the source pressure P1 (see Section 2), i.e. (a), (b), and (c) at 0.4 × 10-4, 0.6 × 10-4, and 0.8 × 10-4 mbar, respectively. Also, higher P0 values are displayed in Figure 5. It appears that the signal of the complex undergoes an abrupt increase with the change in the backing pressure. The isotopic pattern of Zr is clearly identified on the Zr · · · F-CH3 complex, while Zr2 and Zr2C have more complex patterns due to isotopic combinations.
calculations, in which all 18 valence electrons are then correlated. The definition of this space is of particular importance for getting a balanced representation of the electron correlation between all investigated compounds. The full valence space (18 electrons in 17 active MOs) is unfortunately out of reach of present computer facilities. It generates 116 million configuration state functions (CSF) at CASSCF level and 4 billion of contracted CSF at the RS2 level. We thus performed preliminary test calculations, in which the size of the active space was progressively augmented and the convergence of the CASSCF and RS2 energy differences was investigated. We found that the 10-electron active space is a good compromise between accuracy and computational efficiency and that it should recover a large amount of the valence correlation energy through the RS2 treatment. This space generates 300 configuration state functions (CSF) at the CASSCF level and 5 million of contracted CSF in the RS2 calculation, which is a reasonable size for applying the CASSCF/RS2 treatment to all calculations. These include the geometry optimization calculations that were performed using a numerical quadratic steepest descent algorithm.29 4. Experimental Results We analyze here the reaction products formed in presence of MeF, DME, and MeF/DME mixtures, while changing the concentrations of reagents, the pressure conditions in the expansion and the ionization energies. The nature of the products and the complexes they form will provide insight into the gas phase reactivities of clustered and unclustered species. 4.1. Zirconium-Methyl Fluoride Clusters. At low MeF concentrations, that is, 0.6 × 10-4 mbar, then reaches a maximum. The (1:2) Zr · · · F-CH3 cluster is also observed with an intensity no larger than 2% of the
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Figure 3. The energetic scheme for the single photon ionization of the van der Waals complex Zr · · · F-CH3 is shown on the right-hand side. The reactivity in the ground neutral metal Zr(4d)2(5s)2, a3F2, and Zr+ ion (4d)2(5s)1, a4F(3/2), leading to the ZrF and ZrF+ products, respectively, is shown on the left-hand side. Double-headed arrows indicate differences in vertical energies with each corresponding value written on the right-hand side. Note that all values are given in eV. The experimentally determined value for the ionization energy (5.15 eV) of the van der Waals complex is encircled, and the ionization energies obtained by calculation are indicated in rectangles (see Section 4.5 and Table 2). The formation of ZrF can be found 1.82 eV lower than the reagents (see Section 5 for the calculation) and is underlined. The reaction pathway from the separated reagents Zr + F-CH3 on the right-hand side leads to ZrF + CH3, passing over a barrier and flying over the insertion well of F-Zr-CH3. The barrier (hypothetical value) corresponds to the repulsive penetration of the valence (5s)2 orbitals of zirconium within the C-F bond. In general, the barrier is reached at the surface crossing with the metastable configurations36 (4d)3(5s)1 for the neutral and (4d)3(5s)0 for the ion, which have favorable overlap with the F-CH3 molecule. Owing to a lesser repulsion, the barrier for the ion insertion will consequently be lower as compared to the neutral (as is sketched here). The dashed curve represents the ground state of the inserted compound F-Zr-CH3 in its ground 1A′ state, and the upper curve corresponds to the 3A′′ state. Both correlate to the same ZrF + CH3 limit. The arrows at 5.15 eV (240 nm) and 7.9 eV (157 nm) represent the highest ionization energies used in this work.
(1:1) complex at higher MeF concentrations, ∼4%. When changing the ionization wavelength to 240 nm, the (1:1) complex is barely observed, and at 250 nm it is no longer seen. This sets the ionization threshold at ∼5.15 eV via a one-photon excitation (the ionization laser has an intensity lower than 2 µJ over a 4 mm2 spot). The same expansion conditions have been explored with a fluorine laser at 7.9 eV, and no (1:1) cluster nor any larger cluster (1:2) were observed in these conditions. This sets the binding energy, of the ground state (1:1) complex or any larger complex (1:n), well below 1.27 eV (the binding energy is equal to the difference between the energy of the laser at 157 nm and the ionization energy of zirconium 6.63 eV), see Figure 3, which summarizes the energetics of these systems, experimental observations, theoretical calculations, and hypothetical values for barrier heights. There, the small binding energy of the complex is easily compensated by the energy of the 157 nm laser to produce the separated Zr+ + MeF fragments. 4.2. Zirconium-DME Clusters. Clusters with DME (possible solvent) are formed much more easily than with MeF. Indeed they were already observed in a flow tube with a binding energy of 0.83 eV.30 Therefore, the association complex likely exists even before the expansion. As displayed in Figure 4, at 212 nm a series of clusters are observed. As the pressure is increased, the size of clusters steadily increases, whereas the signal at mass 89.9 amu, the most abundant isotope for zirconium, decreases.
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Figure 4. Clustering of Zr in a 5% DME in He mixture at various backing pressures increasing from bottom to top, i.e. (a), (b), and (c) with P1 ) 1.1 × 10-4, 1.2 × 10-4 and 1.3 × 10-4 mbar, respectively. The wavelength of the ionization laser is set at 212 nm.
Figure 5. Solvated reaction products ionized at 212 nm with a mixture of 4% methyl fluoride in He. The backing pressure P0 increases from bottom to top, as follows with P1 ) (a) 0.4 × 10-4 mbar, (b) 0.8 × 10-4 mbar, (c) 1.1 × 10-4 mbar and (d) 1.4 × 10-4 mbar. It appears that the Zr · · · F-CH3 cluster passes over a maximum whereas the solvated products steadily increase with pressure.
4.3. Reaction Products with Pure MeF or Pure DME. In He/DME expansions, as the size of clusters increases, a new series grows in, with broadened peaks. It is readily identified to zirconium adducts owing to the isotopic patterns and it relates to the main peaks by a 15 amu mass decrease. The series is assigned to Zr-O-CH3(DME)n. Another series of solvated reaction products, ZrO, solvated by DME is also observed but with sharper peaks. In He/MeF expansions, as the backing pressure increases, the concentration of (1:1) Zr · · · F-CH3 clusters decreases while monitored with the 212 nm ionization laser. A new series of clusters appears that starts with 90ZrF3(MeF)nG2 at mass 214.9 amu with the number n of attached MeF increasing as shown in Figure 5. When monitoring the same expansion conditions at 157 nm, there appears the nonclustered ZrF3 molecule and its first (1:1) ZrF3 · · · F-CH3 complex (see Figure 6). The smaller radical 90ZrF2 is also ionized and observed at 127.9 amu with the 157 nm laser. The intensity of ZrF3+ ions is small but increases with pressure while that of ZrF2+ decreases. Likely, the ionization of ZrF3 is at threshold since the corresponding
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Figure 6. Clusterized reaction products ionized at 157 nm. The backing pressure P0 increases from bottom to top, as follows (with P1 ) (a) 0.4 × 10-4 mbar (b) 1.1 × 10-4 mbar, (c) 1.6 × 10-4 mbar and (d) 1.4 × 10-4 mbar.)
Figure 7. Details of the reaction products showing the ZrC2H2, 4, 6 products between ZrF and ZrF2, identified in the insert by isotopic modeling.
clusters ion signals are much more intense. At the same time, the intensity of ZrF2 is greater, indicating that its ionization threshold is well below 7.9 eV. When the concentration of MeF is increased to 8%, most of the zirconium has been transformed into products and their clusters at the highest backing pressures. The 157 nm laser also allows probing of other radicals formed in the beam as an intense group of masses in Figure 6 between 115 and 126 amu. These species correspond to the radicals ZrC2Hn)2, 4, 6, which are identified by their zirconium isotope abundance as shown in Figure 7. The same products are observed in pure He/DME mixtures using 157 nm photons and seem characteristic of hydrocarbon-containing species since we could also detect them in He/CH4 mixtures with the same isotopic pattern. 4.4. Reaction Products Formed in Mixed MeF/DME Expansions. When the expansion contains a mixture of DME, at concentrations >1%, and MeF at concentrations >2%, the (1:1) Zr · · · F-CH3 clusters completely disappear at 212 nm, as well as the majority of Zr · · · (DME)n clusters. It turns out that the increase in concentration of MeF in these mixed expansions favors the formation of products and their complexes, instead of clusters. In these conditions, the increase in the backing pressure results also in an increase of the intensity of the
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Figure 8. Reaction products ZrF3 · · · (MeF)n(DME)m formed in mixed expansion conditions: (a) the ionization laser is set at 212 nm while (b) it is at 157 nm. The (n, m) pair indexes respectively the number of MeF and DME molecules associated with ZrF3. The weak peak at 212 nm ionization at mass 258 amu is likely Zr-O-CH3 · · · (MeF)2(DME)1.
products at the expense of Zr atoms. At the highest pressures, Zr and ZrF almost disappear. A new series of compounds is best seen at concentrations of 8% MeF and 2% DME in helium. The general formula of these species is ZrF3(MeF)n(DME)m and it starts with n ) 0, m ) 0 when ionized at 157 nm and at n ) 1, m ) 1 at 212 nm, as shown in Figure 8. On the one hand, at 212 nm a simple cluster progression of ZrF3 · · · (DME)n>2 is prominent and ZrF3 · · · (MeF)1(DME)1 is also distinctly observed as seen in Figure 8. Two products are seen at 157 nm: ZrF2 and ZrF3 with their MeF clusters starting at n ) 1. On the other hand, at the same ionization wavelength no compound of general formula Zr(MeF)(DME)n can be observed that would match the mass of a solvated inserted compound. Also, as stated above, no broadened Zr(MeF)n peaks can be seen, as resulting from evaporation within ions. 4.5. Theoretical Calculations. The lowest singlet and triplet states of the compounds formed by addition of methyl fluoride with zirconium have been characterized at the CASPT2 level of theory (see Section 3). The results include the van der Waals complex Zr · · · F-CH3 and two possible inserted compounds, the Grignard reagent type F-Zr-CH3 and the methylidene CH2dZrHF. Geometry optimizations have been performed without imposing any symmetry constraints, and it is found that all systems adopt quasi-planar Cs geometries for the following low-lying states: 1A′, 1A′′, 3A′, and 3A′′. The equilibrium geometries are listed in Tables 3 and 4 using the labeling of atoms defined in Figure 9. The relative stabilities of all states with respect to the ground state of the F-Zr-CH3 inserted compound are given in Table 1. These tables also report for comparison purpose the B3LYP DFT results obtained by Cho and Andrews15 for some of the states of the inserted compounds. Mulliken atomic populations are inserted in Tables 3-5, together with dipole moment values since they give insight in the charge distribution of all compounds. Atomic populations on the 5s and 4d atomic orbitals of zirconium are also reported in order to characterize the electronic structure of the transition metal within the molecules. The ionization energies (IE) have also been calculated for the vertical ionization of all species from the ground states (see Table 2). The 2A′ and 2A′′ electronic states of the corresponding cations were considered. These
Figure 9. Structures of the (1:1) Zr · · · F-CH3 complex and of the inserted compounds F-Zr-CH3 and CH2dZrHF as discussed in the text in Section 4.5.
TABLE 1: Calculated Relative Stability (eV) of the (1:1) Zr · · · F-CH3 van der Waals Complex, the Grignard Reagent Type F-Zr-CH3, and the Methylidene Type CH2dZrHF Inserted Products with Respect to the 1A′ Ground State of F-Zr-CH3a (1:1) Zr · · · F-CH3
F-Zr-CH3
CH2dZrHF
A′′
5.59
A′ A′′ 1 A′
5.62 6.43 6.94
1.18 -0.407 3.14 2.18 0 0
2.36 0.68 1.02 0.32 -0.11 -0.056
3
3 1
a
The values in italic refer to Cho and Andrews.15
TABLE 2: Calculated Vertical Ionization Energies (eV) of: the (1:1) Zr · · · F-CH3 Complex Relative to Its Ground 3A′′ State, the Grignard Reagent Type F-Zr-CH3, and the Methylidene Type CH2dZrHF Inserted Products Relative to Their Ground 1A′ State 2 2
(1:1) Zr · · · F-CH3
F-Zr-CH3
CH2dZrHF
6.26 6.28
5.67 5.57
4.80 7.03
A′′ A′
results shed light on the experimental observations (see Table 2). The van der Waals complex is reasonably stabilized in its ionic form by charge dipole interactions, and the IE is calculated as 6.27 eV. The inserted compounds exhibit a lower IE, that is, 5.57 eV for the Grignard reagent type F-Zr-CH3 and 4.8 eV for the methylidene type CH2dZrHF. 5. Discussion The discussion will review the formation conditions of the different compounds which have been observed, mainly fluorinated zirconium ZrF1, 2, 3 and clusters with MeF and DME. It will assign the formation of these compounds to the reaction of radicals, fluorine, and subsequently zirconium-containing
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TABLE 3: Geometrical Parameters of the Inserted Methylidene Compound CH2dZrHF 1
parameters
a
this work
r(C-H1), r(C - H2) r(C-Zr) r(Zr-H3) r(Zr-F) ∠H1CH2 ∠CZrF ∠CZrH3 ∠H3ZrF ∠H1CZr, H2CZr ∠H1CZrH3, H2CZrH3 ∠H1CZrF, H2CZrF q(C) q(H1, H2, H3) q(Zr) [q(5s), q(4d)] d q(F) µ(D)
b
1
A′ Cho and Andrews
1.102, 1.090 1.966 1.917 1.966 111.2 112.9 120.5 126.6 106.0,142.7 180.0, 0.0 0.0, 180.0 -1.34 0.41, 0.41, -0.52 1.77 [0.14, 1.85] -0.73 0.79
c
this work
1.085, 1.113 1.965 1.873 1.945 112.9 112.7 105.8 118.6 96.5, 149.9 26.8, -139.3 158.1, -7.9 -0.77 -0.25, 0.13, 0.09 1.30 -0.49 3.58
3
A′′ b
this work
1.090, 1.092 2.241 1.894 1.998 109.1 121.3 118.0 120.6 128.5, 122.3 180.0, 0.0 0.0, 180.0 -1.48 0.41, 0.40, -0.62 2.01 [0.32, 1.54] -0.72 0.40
3
A′ b
this work
1.087, 1.097 2.057 1.880 2.057 109.2 121.6 112.7 125.7 135.0, 115.7 180.0, 0.0 0.0, 180.0 -1.23 0.44, 0.37, -0.52 1.70 [0.44, 1.68] -0.77 2.81
A′′
b
Cho and Andrewsc
1.079, 1.077 2.206 1.865 1.934 108.6 115.7 120.7 123.6 126.9, 124.5 180.0, 0.0 0.0, 180.0 -1.45 0.43, 0.42, -0.61 1.91 [0.36, 1.58] -0.69 1.45
1.092, 1.095 2.199 1.867 1.940 109.9 121.0 117.9 121.1 123.5, 126.6 180.0, 0.0 0.0, 180.0 -0.80 -0.30, 0.12, 0.12 1.38 -0.53 2.05
a ˚ and degrees, respectively. b This work, CASPT2 calculations. See Figure 9 for the labeling of the atoms. Bond lengths and angles are in A Cho and Andrews,15 B3LYP/6-311+G(2d,p)/LANL2DZ calculations. d Mulliken atomic populations. q(5s) and q(4d) are the Mulliken populations on the 4s and 5d orbitals of Zr, respectively. Small amounts of the population, not reported here, are also found on the 5p orbital and on the f and g polarization functions. c
TABLE 4: Geometrical Parameters of the Inserted Compound F-Zr-CH3 1
1
A′
3
A′′
3
A′
A′′
parametersa
this workb
this workb
this workb
this workb
Cho and Andrewsc
r(C-H1), r(C-H2), r(C-H3) r(C-Zr) r(Zr-F) ∠H1CH3, H1CH3, H2CH3 ∠CZrF ∠H1CZr, H2CZr, H3CZr ∠H1CZrF, H2CZrF, H1CZrF q(C) q(H) q(Zr) d [q(5s), q(4d)] q(F) µ(D)
1.086, 1.101, 1.101 2.159 1.891 108.9, 108.9, 107.5 115.6 123.2, 123.2, 103.6 0.0, 113.6, -113.6 -1.34 0.34, 0.36, 0.36 0.91 [1.18, 1.50] -0.64 1.72
1.092, 1.089, 1.089 2.182 1.883 105.7, 105.7, 106.8 109.2 106.4, 115.7, 115.7 0.0, 98.0, 98.0 -1.51 0.34, 0.33, 0.3 1.16 [0.70, 1.79] -0.64 0.32
1.088, 1.089, 1.089 2.594 1.968 119.8, 119.8, 119.8 166.8 77.5, 100.3, 100.3 0.0, 117.4, -117.4 -1.28 0.42, 0.44, 0.44 0.70 [0.75, 2.41] -0.72 6.32
1.085, 1.093, 1.093 2.176 1.896 115.5, 115.5, 110.8 120.6 106.5, 106.5, 106.1 0.0, 108.5, -108.5 -1.52 0.32, 0.31, 0.31 1.23 [0.69, 1.82] -0.66 1.5
1.093, 1.101, 1.101 2.223 1.942 107.7, 108.0, 108.0 121.4 114.4, 109.3, 109.3 0.0, 121.2, -121.2 -0.95 0.13, 0.13, 0.13 1.12 -0.56 2.56
a ˚ and degrees, respectively. b This work, CASPT2 calculations. See Figure 9 for the labeling of the atoms. Bond lengths and angles are in A Cho and Andrews,15 B3LYP/6-311+G(2d,p)/LANL2DZ calculations. d Mulliken atomic populations. q(5s) and q(4d) are the Mulliken populations on the 4s and 5d orbitals of Zr, respectively. Small amounts of the population, not reported here, are also found on the 5p orbital and on the f and g polarization functions. c
TABLE 5: Geometrical Parameters of the van der Waals Complex (1:1) Zr · · · F-CH3 1
parameters
a
r(C-H1), r(C-H2), r(C-H3) r(C-F) r(Zr-F) H1CH3, H1CH3, H2CH3 ∠ZrFC ∠H1CF, H2CF, H3CF ∠H1CFZr, H2CFZr, H1CFZr q(C) q(H) q(Zr) [q(5s), q(4d)] c q(F) µ(D)
1
A′
this work
b
1.083, 1.073, 1.073 1.448 2.308 112.6, 112.6, 115.1 130.8 106.6, 104.4, 104.4 0.0, 119.4, -119.4 -0.51 0.36, 0.36, 0.36 -0.17 [1.83, 2.07] -0.39 5.85
3
A′′
this work
b
1.085, 1.072, 1.072 1.428 2.345 111.6, 111.6, 114.8 132.1 107.2, 105.4, 105.4 0.0, 119.1, -119.1 -0.53 0.32, 0.35, 0.35 -0.11 [1.88, 2.16] -0.38 5.29
3
A′
this work
b
1.084, 1.072, 1.072 1.432 2.338 111.8, 111.8, 114.9 130 107.0, 105.3, 105.3 0.0, 119.1, -119.1 -0.53 0.33, 0.35, 0.35 -0.11 [1.85, 2.11] -0.37 5.28
A′′
this workb 1.085, 1.073, 1.073 1.426 2.399 111.6, 111.6, 114.6 131.1 107.2, 105.6, 105.6 0.0, 119.1, -119.1 -0.52 0.33, 0.35, 0.35 -0.08 [1.78, 2.09] -0.41 5.06
a ˚ and degrees, respectively. b This work, CASPT2 calculations. See Figure 9 for the labeling of the atoms. Bond lengths and angles are in A Mulliken atomic populations. q(5s) and q(4d) are the Mulliken populations on the 4s and 5d orbitals of Zr, respectively. Small amounts of the population, not reported here, are also found on the 5p orbital and on the f and g polarization functions. c
radicals that are formed. We shall comment here upon the nondetection of insertion compounds, previously observed in rare gas matrices after laser ablation of zirconium15 and compare with quantum chemistry calculations.
5.1. Zr Clustering and the Reactivity of the Metal Atom. It appears that translationally cold ground-state zirconium does not react with either MeF or DME, since complexes can efficiently be formed with either molecule. One sees, for instance
Reactions of Laser-Ablated Zirconium Atoms in Figure 2, that at moderate expansion pressures and low MeF concentrations, Zr · · · F-CH3 is the only detectable species by ionization at 212 nm. The same is true when DME is passed at low concentrations in helium through the laser ablation, only Zr · · · (DME)1, 2 clusters are observed. Hence, there exists a barrier to the reaction in the ground state of Zr (4d)2(5s)2 3F2 with either compound, although both reactions with MeF and DME are highly exothermic. The formation of ZrF (the only compound documented) is indeed found 1.82 eV lower than the reagents (from ZrF ) 6.5 eV31 and F-CH3 ) 4.68 eV32). The same is true for the Zr+ · · · (MeF)1, 2 ions, which are observed intact, indicating an important barrier for the reaction of the Zr+ ion with MeF. A similar situation was found in the case of calcium ion complexes with MeF, where a barrier separated the ion complex from the lower lying products, allowing the observation of the (1:1) Ca+ · · · (MeF)1, 2 cluster ions.33 This justifies the barriers that have been drawn for the neutral reaction of Zr + MeF and Zr+ + MeF as indicated in Figure 3. It can be surmised that DME has a less pronounced excess energy for the formation of Zr-O-CH3 and a similar type of barrier exists for all the Zr · · · (DME)n complexes since they are all easily observed in the expansion. This results from the fact that, in a very broad sense, O-CH3 is isoelectronic to F. However, in contrast to fluorinated compounds, the energy barrier to reaction can be overcome in Zr+ · · · (DME)n cluster ions at 212 nm, since Zr-O-CH3+ · · · (DME)m products can be weakly observed in Figure 4. We infer that the reaction occurs after ionization rather than in the neutral clusters because these fragment mass peaks are broadened, as are those of larger clusters of Zr+ · · · (DME)n. The diffuseness of cluster peaks is typical of fragmentation of clusters in the ionization chamber. Due to internal excess energy deposited in the ion by the laser at 212 nm (5.9 eV) after ionization or reaction, clusters fragment during their flight in the accelerating region of the mass spectrometer. At 212 nm, the (1:1) DME cluster is easily ionized and so are larger clusters, but, owing to the intense solvation in the cluster ion and compared to the neutral, at this wavelength the energy becomes sufficient to fragment larger clusters. The peaks for the reaction products from cluster ions also appear as diffuse in the mass spectra because they have undergone fragmentation/evaporation processes. 5.2. Fluorinated Zr Compounds. Although barely apparent at 212 nm, ZrF+ can be observed intensely at 157 nm, in expansions of ∼4% MeF in He mixtures at low backing pressures, as shown in Figure 7. The laser energy (7.9 eV) at 157 nm is well above the ionization energy of ZrF, 6.5 eV.31 However, the intensity of ZrF is quickly superseded by that of ZrF2 and ZrF3 when either the backing pressure or the MeF concentration are increased. Not surprisingly, ZrF4 is not observed, even at 157 nm, since its ionization threshold is 14.5 eV.31 The high ionization energy of ZrF4 is due to its saturated structure, in which all four of the valence electrons of the zirconium atom are engaged in bonds with F atoms. This high ionization energy also precludes the observation of clusters of ZrF4 with MeF that would require more than 5.6 eV of stabilization energy of the ion with respect to the neutral to bring the ionization energy down to 7.9 eV. However, ZrF3 · · · MeFn>2 clusters appear at 212 nm. This is consistent with the ionization threshold of 7.8 eV, requiring a stabilization of only 2 eV by two MeF, to reach the ionization threshold of 5.9 eV provided by the 212 nm photons, as seen in Figure 5. It is clear that the increase in intensity of these ZrFn products with pressure and MeF concentration is due to reactions of
J. Phys. Chem. A, Vol. 114, No. 18, 2010 5661 ZrFn)0, 1, 2 in the short reaction channel, prior to supersonic expansion into vacuum. The peaks associated with these species in the mass spectra are narrow, indicating a neutral reaction followed by expansion cooling. It is specifically interesting to note that no clusters of ZrF or ZrF2 have been detected with MeF. Since these species have been formed in the channel, they have been cooled in the expansion and have undergone further collisions with MeF or MeF clusters. The latter binary collisions could form the ZrF1, 2 · · · MeFn, as the corresponding ZrF3 · · · MeFn are, in difference, observed. If existing, ZrF1, 2 · · · MeFn clusters should have been detected, since the ionization energy of ZrF · · · F-CH3 is in the range accessible with the 212 nm laser, given the 6.5 eV31 ionization threshold of ZrF and the expected stabilization of the ionic complex with MeF by ∼1.5 eV.34 We thus infer that almost no barrier to the reaction exists between ZrF1, 2 and MeF, within ZrF1, 2 · · · MeFn clusters. We shall now discuss the probable mechanisms of formation of the different fluorinated species in the ablation channel by first listing some plausible mechanisms. Considering eqs 4-7, the formation of ZrF radicals in the ablation channel can have two origins. The whole process can be divided into 2 steps as described below: (1) Precursors are formed in the laser ablation zone:
Zrsolid + hν f Zr*
(1)
Zr+ + e- f Zr**
(2)
CH3F f CH3 + F
(3)
(2) Reactions forming the ZrFn)0, 1, 2 compounds:
Zr* + CH3F f ZrF* + CH3
(4)
Zr + F + M f M* + ZrF
(5)
ZrF + F f ZrF2
(6)
ZrF + CH3F f ZrF2 + CH3
(7)
5.2.1. Formation of ZrF. Zirconium atoms can be formed as translationally hot neutral atoms (eq 1) in the laser ablation process, or as electronically hot metastable states that are either directly formed in the ablation process or formed by electron-ion recombination in the ablation plasma (eq 2). Regardless of how they are formed, spin statistics favor the production of highspin quintet metastable states. Equations 4 and 5 suggest alternative routes to the formation of ZrF and we do not believe that either the translationally or electronically excited Zr atoms can be responsible for the majority of the observed products by reaction with MeF, although MeF is the dominant species in the gas flow of the reactive channel, according to eq 4. As illustrated in Figure 3, translational energy will be effective in surmounting the reaction barrier, and translationally hot Zr atoms are expected to efficiently lead to ZrF product under low pressure conditions. This process would become less efficient at higher overall pressures, owing to the increased collisional cooling of the translationally hot Zr atoms, however in our experiments the concentration of ZrF grows approximately
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linearly with increasing pressure. This rules out this channel as a dominant one. Metastable Zr atoms, on the other hand, would be expected to retain their electronic energy in a high-pressure environment and would react as they undergo multiple collisions with the methyl fluoride molecules. Such metastables are expected to represent only a fraction of the total number of Zr atoms, and at high concentrations of MeF and at high pressures, the metal atoms have all reacted away (see Figure 6 at P1 ) 1.6 × 10-4 mbar, for example). Since only a fraction of Zr atoms are in metastable states, this observation shows that an effective reaction path for ground state atoms must exist, namely, the alternative route following eq 5. Then the dominant process is the dissociation of CH3F molecules in the plasma by electron impact or eventually by multiphoton absorption, into CH3 radicals and F atoms. These F atoms react at hard sphere collision rates with zirconium atoms (and ZrF1, 2) as shown in eqs 3-6. We favor this latter possibility, since the companion product ZrC2H6 is also found by ionization at 157 nm as seen in Figure 6. This product appears intensely and is most likely formed by the addition of two methyl radicals, produced by the dissociation of methyl fluoride, to a zirconium atom. The pressure increase also stabilizes the newly formed ZrF, eq 5. 5.2.2. Formation of ZrF2, 3, 4. The formation of ZrF radicals induces further reactions. Indeed, further fluorination of ZrF should occur with collisions with MeF (eq 7), or (MeF)m clusters, rather than with F atoms (eq 6), since MeF is more abundant by an order of magnitude than F atoms in the gas flow of the reactive channel. We have observed that ZrF1, 2 are reactive radicals, without reaction barrier with MeF, as noted in Section 5.2. Thus, these ZrFn radicals do not need the presence of F atoms to form ZrFn+1. Indeed, when the backing pressure is increased, then the number of collisions of ZrF1, 2 with MeF increases in the reactive channel. In these conditions, fluorine atoms are scavenged by collisions with abundant MeF35 at hard collision efficiency, to form HF and CH2F. From their quickly decreasing number along the channel, they become no longer available for reactions with the newly formed ZrF1, 2, while the number of collisions with MeF does not decrease at the same location. As the concentration of the ZrF2, 3 products increases with the backing pressure, we consider that these radicals are formed by radical reactions of ZrF1, 2 with MeF or subsequently with (MeF)n clusters. In this view, the concentration of F atoms should sharply decrease after the ablation by reactions with Zr atoms or MeF within the channel. Then, with a certainly lesser efficiency, the newly born ZrF1, 2 radicals will react with MeF in the channel. After the expansion, as noted, we observe only ZrF3 · · · (MeF)n>2 as clusters of products. Once ZrF1, 2 have been captured by the cluster there remains more than 100 µs before the detection, thus much time for reaction and cooling by evaporation of MeF molecules. Although collisions of ZrF1, 2 with MeF are more frequent than with the abundant MeF clusters (several % concentration of MeF in helium), the capture in the cluster should give an advantage to that reaction, forming ZrF4. In summary, the formation of the fluorinated compounds is stepwise and the reactivity of the radicals likely decreases as F > ZrF > ZrF2 . ZrF3. This stems from both the decreasing number of unbonded electrons on zirconium and the increasing steric effect on the radical considered. In this reactivity, the reactive channel and the expansion both play a role. 5.2.3. Other Zr Reaction Products. Three Zr-hydrocarbon compounds have been identified as ZrC2Hn)2, 4, 6, and there may be others in smaller quantities. The structure of n ) 6 can be CH3-Zr-CH3 and results from the recombination of Zr with
Soorkia et al. two methyl radicals. This is confirmed by identical results obtained with several hydrocarbon sources of radicals, all of which generated the same three products. The resulting hot product in the laser plasma can dehydrogenate to form these products with reasonable probability. The nonobservation of mixed bonds F-Zr-CH3 by radical recombination is understandable since F atoms are scavenged by Zr and MeF35 (see discussion in Section 5.2.2), whereas CH3 radicals have no other scavengers than Zr, aside from CH3 in lesser quantities. In other words, to form these mixed bond compounds with reasonable probability, an equal collision probability must exist between Zr and the F or CH3 radicals, which is unlikely from the lower F atom concentration as referred to CH3. The formation of the inserted hydrocarbon zirconium products via a radical pathway could be the preferred route in the gas phase but was not observed here. 5.3. Search for MeF Inserted Compounds. We scrutinized the various results for the formation of inserted compounds of the type F-Zr-CH3 · · · (DME)n>2. We knew that in the gas phase the outcome of insertion reactions of zirconium or yttrium with saturated or sometimes unsaturated hydrocarbons is generally the elimination of H2 molecules after flying over the insertion well,36,37,11 since a high barrier has to be overcome, as shown in Figure 3. In the reaction of Ru with MeF studied by quantum chemistry,38 the insertion does not proceed into the C-H bond but into the C-F bond with a large barrier, 30 kcal mol-1 (1.30 eV), from the ground atomic state a5F. Thus, the purpose of a third body in a cluster was 2-fold: (1) to collisionally stabilize the system into the insertion well during its course over it and, most importantly, (2) the second effect of the cluster should be to decrease the barrier to insertion, making a facile passage into the insertion well. The barrier suppression effect adds to the third-body effect and decreases the total energy to dispose in the recoil of this third body. The Zr-F bond is strongly ionic, whereas the other bond Zr-CH3 is less ionic as seen in Table 4 and Section 5.4. Preliminary calculations have also shown that all insertion intermediates, en route to the insertion well, have an almost positive unit of charge on Zr. Although the neutral reagents are weakly stabilized, the intermediate charge transfer and the final products should be stabilized by polar solvent molecules relative to the neutral compounds, thus decreasing the barrier. A typical case of stabilization of ionic products by reactions with clusters has been found in the reactions of Mg+ with (H2O)n clusters.39 This is explained by a more efficient stabilization for MgOH+ than for Mg+. Thus, the threshold for production of MgOH+ is observed for MgOH+ · · · (H2O)n)5. Hence, if the insertion mechanism were active, we would expect the detection of systems of general formula F-Zr-CH3(MeF)n(DME)m, in the highest expansion conditions, owing to barrier suppression for the ionic intermediate. The bottom of the inserted well has been calculated as 5.59 eV below the ground state 3A′′ of the Zr · · · F-CH3 van der Waals complex, as represented in Figure 3. The vertical ionization energy for the inserted product F-Zr-CH3 is found as 5.57 eV by calculations (see Table 1), thus it should be observed when solvated, at 212 nm. In all the spectra, there are hardly any unassigned peaks in mixed helium MeF, or helium, MeF, and DME expansions that we have investigated; as can be seen in Figures 5 or 8, most masses relate to fluorinated Zr reaction products as described in the preceding section but none to clusters of F-Zr-CH3. The mass peaks at ∼124 amu in Figures 2 and 5 correspond to the (1:1) Zr · · · F-CH3 complex as described in Section 5.1, which has the same mass as the free inserted species, F-Zr-CH3. The (1:1) complex appears
Reactions of Laser-Ablated Zirconium Atoms already at increasing P0 pressures in dilute mixtures of MeF in helium (∼2%), at higher concentrations it increases with pressure, then decreases as seen in Figure 5. The peaks cannot be assigned to the inserted free product since the increase of P0 should favor their growth and the appearance of solvated inserted products F-Zr-CH3(MeFn). This we do not observe and exclude the observation of nonsolvated F-Zr-CH3 inserted products. 5.3.1. Comparison with Results in Rare-Gas Matrices. The solvated inserted products have not been observed in any of our conditions. In more diluted mixtures of MeF and DME, the solvation effects may be insufficient to suppress the barrier and solvate the inserted products; at higher solvent concentrations, other reaction products are formed instead. However, in cryogenic matrices, Cho and Andrews15 have found evidence of two inserted compounds that interconvert by irradiation at 240 nm, F-Zr-CH3 and the methylidene CH2dZrHF. These two compounds have equivalent energies within 0.21 eV (see Table 1) and are converted by R hydrogen migration after the passage of a barrier. Both compounds are formed during the codeposition of ablated zirconium atoms and methyl fluoride. There is a major difference in the reaction conditions with our cluster work: in the matrix deposition, the ablated Zr atoms, containing a proportion of metastables and ions are swept into the matrix where MeF is independently deposited with argon. No F atoms are formed there, and metastable and ions have time to react in the definitive presence of methyl fluoride, forming exclusively the inserted product (or simply quenching the electronic excitation). This can account for the formation of F-Zr-CH3 but does not explain the production of the methylidene CH2dZrHF, since to reach it there is a further barrier due to the R migration of a H atom from the methyl. Also, the insertion reaction is immediately arrested at the surface of argon in matrix deposition. Insertion reactions are difficult to prepare away from condensed phases for exothermic reactions after barriers, since the potential surface gradient drives away the system directly to the noninserted products, flying above the deep inserted potential well, see Figure 3.6,37 There needs to be caging within a matrix to prevent ZrF and CH3 from flying away from each other. From the results of Cho and Andrews,15 we expected that at sufficient concentrations of MeF, solvation and caging should be effective in producing the inserted systems. From the preceding we are led to infer that the reaction leading to the inserted compounds originates from metastable zirconium atoms in the matrix codeposition experiment. The reaction within metastable should have a barrier, probably as for the Zr+ ion (not reactive with MeF), although both barriers should be lower than for ground neutral zirconium. In this respect, we can refer to the calculations on Ru + MeF where a barrier exits,38 although lower, for the 3F Ru metastable state. However on this excited state surface, nonadiabatic relaxation can occur to the ground state surface, providing the necessary excess energy to overcome the barrier in the ground state. The 1P Ca + MeF excited state reaction was explained in this way via a non adiabatic relaxation to a triplet surface.18 We expect that unless this excitation is achieved within the cluster, the reaction cannot reach the inserted compound. The result of the electronic excitation of zirconium will be addressed for the (1:1) Zr · · · F-CH3 complex in a forthcoming paper.40 The same applies to ground state ions that, as represented in Figure 3, have a barrier to reaction, which allows for their observation. 5.4. Theoretical Results and Inserted Compounds. DFT and CASPT2 calculations agree for predicting that the ground
J. Phys. Chem. A, Vol. 114, No. 18, 2010 5663 state of CH2dZrHF is a closed shell singlet state, which is close in energy (within 0.1 eV) to the corresponding singlet state in F-Zr-CH3. However the equilibrium geometry of the ground state of CH2dZrHF obtained by both methods is different: whereas CASPT2 converges to a planar structure, DFT predicts a significantly nonplanar one. Our result is yet compatible with an ethylene-like CH2dZrHF structure corresponding to a 1A′ assignment for the ground state of this compound. Another disagreement concerns the energy position of the 3A′′ state with respect to the ground singlet state that is found to be significantly more stable by the DFT calculations. The discrepancy amounts to 1.6 and 1.7 eV for F-Zr-CH3 and CH2dZrHF, respectively, and leads to the prediction by DFT of a 3A′′ ground state for F-Zr-CH3. This result is rather surprising because it would suggest that the open-shell structure would be more stable than the closed one. Some mismatch is also observed in the charge distribution (see Mulliken charges and dipole moment values. These discrepancies are probably due to the fact that B3LYP calculations are not well suited for describing the electronic structures of such compounds involving noncovalent interactions and exhibiting a multiconfigurational character induced by the zirconium atom. This justifies our use of a multireference approach like CASPT2 for a proper description of the properties of such systems. Also the increased stabilization of the singlet surfaces in the inserted compounds can be presumably linked with the strong ionic character of zirconium in the product that destabilizes the triplet product. As expected, the van der Waals complex Zr · · · F-CH3 is found to be higher in energy than the inserted compounds. It exhibits a triplet ground state (3A′′) and a close lying 3A′ excited state, compatible with the a3F2 ground state of the zirconium atom. This shows that the insertion of MeF does not fundamentally shuffle the spin orbit coupling in zirconium, as the free metal is in a 3F2 ground state. Taking the 1A′ state of F-Zr-CH3 as the reference, we thus predict an energy difference of ca. 5.6 eV with respect to the 3A′′ state of Zr · · · F-CH3. The difference between both inserted compounds CH3-ZrF (open shell) and CH2dZrHF (closed shell) amounts to 0.11 eV only, which can be considered as insignificant. Zirconium is, as expected, found to be almost neutral in Zr · · · F-CH3 with an electron population close to the (5s)2(4d)2 ground state atomic configuration. On the other hand, zirconium bears between one and two positive charges in the inserted compounds, with the lost fractions of electron charge coming from the 5s and 4d orbitals. It is interesting to examine from Tables 3-5 how some of the geometrical parameters change from one state to another of the same species. As expected, no spectacular geometry changes occur within the van der Waals complex, but more important variations are observed in the inserted compounds. In particular, the F-Zr-C bending angle is found to be sensitive to the nature of the electronic state and coupled to the angular positions of the methyl hydrogens. Let us mention the geometry of the 3A′ state of F-Zr-CH3, which exhibits a F-Zr-C angle not so far from linearity (166.8° to be compared to the values around 110° of the other states) and a large C-Zr distance. The calculations, which have been conducted at a high level, are supportive of the reaction mechanism, forming here mainly ZrFn and not the inserted compounds, as we have previously emphasized. The reaction path after the barrier passes quickly over the insertion well of F-Zr-CH3 3A′ correlating at infinite distance with ZrF + CH3, as illustrated in Figure 3, with an excess energy of 1.82 eV. F-Zr-CH3 is a strongly ionic species with a 1.7 positive charge on the zirconium atom, where almost
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two electrons have been transferred to the adjacent fluorine and carbon atoms, see Table 4. Therefore, even in the presence of solvent to lower the reaction barrier, a two electron process has to be achieved to form this compound. This can be achieved probably only too late, along the reaction pathway. Ultimately, if formed, in a 3A′ state, this compound could interconvert into the ground singlet state 1A′. There is another very interesting result of the calculations: the ionic compound F-Zr-CH3+ in the ground state 2A′, is almost at the same energy as the complex Zr · · · F-CH3 3A′. This situation is unusual and can result in chemionization where the reaction flux encompasses the ion well and can be quenched into that well by autoionization. Unfortunately, in our experiment these ions cannot be detected as they are blocked by a grid in the time-of-flight mass spectrometer. Conclusions In a laser ablation microreactor, zirconium atoms have been reacted with MeF alone and in presence of DME. With MeF, simple products have been identified by one photon ionization, of general formula ZrFi(MeF)n or ZrC2Hn)2, 4, 6. Our studies suggest that the majority of the products are formed via radical reactions either with F, CH3 or ZrF1, 2, 3. Organometallic products have been found containing hydrocarbon groups ZrC2Hn)2, 4, 6, some of them should have an inserted structure (probably ZrC2H6). They result from radical attack of Zr by CH3 radicals. No products of the type F-Zr-CH3 could be observed. Alternatively, in conditions where intense solvation is at work, the direct insertion mechanism involving barrier suppression is also not observed. These conditions are in marked contrast with those of matrix deposition for similar compounds,15 where there is a strong caging effect by argon that can allow the formation of these inserted zirconium-organohalogen compounds likely via a metastable configuration of the metal. In the present experiment, the radical attack supersedes the mechanism with metastable atoms Zr(4d)3(5s)1, which is certainly inactive for insertion in absence of solvation. This situation could be met in reactions proceeding at the surface of argon clusters for immediate stabilization and in presence of solvent to suppress the barrier to insertion. The mechanism involving other metal atom configurations10,5 relies on the hybridization of the (n 1)dm orbitals of the metal with a half vacant exterior ns1 orbital. In the cation, this latter orbital stands as ns1 while in the neutral only low lying metastable excited electronic states bear the appropriate (n - 1)dm+1ns1 instead of (n - 1)dmns2, which is filled and repulsive to the approach of reactants. This might be at the origin of the role played by solvent molecules in neutral atom reactions in preferentially lowering the metastable configuration with respect to the closed shell ...ns2 configuration. Also the solvation mechanism can be electrostatic in nature with a strong stabilization of the ionic products and transition states. This situation could be sought in transition metals for which the insertion within the ions is facile and documented, since in a broad sense the neutral potential energy surfaces parallel the ionic ones in transition metal reactions. High level ab initio calculations at the CASPT2 level of theory reveal important features of the inserted product: its very low ionization energy, placing the inserted ion lower than the neutral complex. Also the Mulliken charge on zirconium is high in the inserted complex and close to +1.23. These two properties have important consequences on any possible formation of inserted products in the gas phase via clusters and support its description as a Grignard-type intermediate and the fact that we did not observe them. The calculations show also the existence of a low lying methylidene product in a 1A′ ground state.
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