596
J . Phys. Chem. 1993, 97, 596-603
Gas-Phase Metal Ion Ligation: Collision-Induced Dissociation of Fe( H20)f and Fe(CH4)xt ( x = 1-4) Richard H.Schultzt and P.B. Armentrout'*$ Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12 Received: August 24, 1992; In Final Form: November 12, I992
Guided-ion beam mass spectrometry is used to study the collision-induced dissociation (CID) of Fe(HzO),+ and Fe(CH4),+ ions, x = 1-4. Ions are produced in a flow tube source to ensure thermalization. By measuring the CID thresholds, we obtain the following 0 K bond dissociation energies (BDEs) for Fe(H20),+ clusters: D"o[Fe+-(HzO)] = 30.6 f 1.2 kcal/mol, D0o[(H20)Fe+-(H20)] = 39.3 f 1.0 kcal/mol, Doo[(HzO)zFe+(HzO)] = 18.2 f 0.9 kcal/mol, and Doo[(H20)jFe+-(H20)] = 19.6 f 1.2 kcal/mol. Our values for x = 1 and 2 are compared to and contrasted with prior literature measurements and calculations. We also report the first measurements of the BDEs for Fe(CH4),+ clusters, x = 1-4, obtaining the following 0 K values: Doo[Fe+-(CH4)] = 13.7 f 0.8 kcal/mol, Doo[(CH4)Fe+-(CH4)] = 23.3 f 1.0 kcal/mol, DOo[(CH&Fe+(CHd)] = 23.6 f 1.4 kcal/mol, and Doo[(CH4)jFe+-(CH4)] = 17.6 f 1.4 kcal/mol. The most surprising observation here is that D0o[(CH4)2Fe+-(CH4)] exceeds D0o[(HzO)2Fe+-(Hz0)]. This result and other trends in these BDEs are discussed by examining FeL,+ bonding in terms of the nature of the ligand and the electronic structure of the ion.
htroduction The nature of the solvation process has been the subject of considerable study for many years.' One approach to understanding the solvation process has been the study of ion-solvent clusters in the gas phase. The most extensively studied of these systems have been alkali-metal ion/water complexes.2 In these systems, it has been observed that the ligand binding energies decrease monotonically with the number of water molecules attached to the ion.3 Transition-metal ion/HzO complexes have not received nearly as much attention. In one early study, Foster and Beauchamp used ion cyclotron resonance (ICR) mass spectrometry to observe the exchange behavior of Fe(CO),+ ions with a variety of other ligand^.^ They discovered that H2O could, at thermal energy, replace the carbonyl of FeCO+ and all but the last carbonyl of Fe(CO),+ (x = 2-4), but did not replace any carbonyls of Fe(eo),+. The amount of thermodynamic information that can be derived from this study is limited, however. The reactant ions in that study were produced by electron impact on Fe(C0)s with no further cooling, a method known to produce internally hot ions.5 Due to this unknown amount of internal excitation of the FeCO+ ion, it is not clear whether Do(Fe+-H20) is actually greater than Do(Fe+-CO)as implied by theobservation of ligand substitution at thermal kinetic energies. Furthermore, ligand substitution, especially to a coordinatively saturated metal ion, may be kinetically rather than thermodynamically controlled under their experimental conditions. More recently, Castleman and co-workershave performed highpressure mass spectrometry equilibrium studies of the binding energies in M+-(H20),, with M = Cu and Ag and x = 3-5.6 These bond strengths were found to be relatively insensitive to the number of water molecules attached to the ion, about 16-17 kcal/mol for Cu+ and 15 kcal/mol for Ag+. It therefore came as something of a surprise when the Mich17and Squires" groups independentlyreported that the second H2O ligand is bound more strongly to most of the first-row gas-phasetransition-metal cations than the first is. Both groups obtained their values for the bond strengths from measuring collision-induced dissociation (CID) 7 Present address: Department of Chemistry, University of California, Berkeley, CA 94720. Camille and Henry Dreyfus TeacherScholar, 1987-1992.
*
0022-3654/58/2097-0596$04.00/0
thresholds in the middle quadrupole of a triple-quadrupole mass spectrometer. Marinelli and Squircs hypothesized that this unusual trend must be linked to the electronic configurations of the central metal ions. In order to try to explain these unusual results, Rosi and Bauschlichergperformed ab initio calculations on the bonding in all of the first row transition-metal M(H20),+ ions for x = 1 and 2. Their calculations agreed with the experimental findings that for M = V, Cr, Fe, Co, and Cu the second bond strength is greater than the first, while for M = Mn the first is greater. Their calculations predicted a slightly lower second bond strength for Ni, although theexperimental studies found it to beslightlygreater than the first. Experimental work in progress in our laboratory agrees with the theoretical findings for all of these metals.I0 The calculations predict that the bonding in these M+-H20 systems, much as in the M+-CO systems," is primarily electrostatic. Such a bond will balance the competing forces of the attractive ion-induceddipole interaction and the electron-electron repulsion of the ion and ligand. For one water bound to a transition-metalion, the ion can minimize this repulsion by altering the orientation of the holes in the 3d orbitals in two ways: 4s3d or 4s4p hybridization and 4s to 3d promotion. For Fe(H20)2+, 4s4p hybridization cannot reduce the repulsion, so 4s3d hybridization and 4s to 3d promotion become more important. The relative importance of these two considerations (hybridization vs 4s to 3d promotion) depends on the energy difference between the low-lying 3dn and 4s3d"I states of the metal ion. For the particular case of Fe+, the calculations predict that the ground state of Fe(H20)+ is 6 A arising ~ from the 6D(4s3d6)ground state of Fe+. For a single H2O ligand, promotion of the ion to the 4F(3d7) state does not increase the bonding sufficiently to compensate for the promotion energy. Rosi and Bauschlicher state that the calculated 6A1-4AIseparation of 4.4 kcal/mol is probably an overestimate, but that the energy ordering is unchanged by any of the errors in their calculation. For Fe(H20)2+, Rosi and Bauschlicher calculate a 4B1, ground state with the sextet states more than 20 kcal/mol higher in energy. We undertook the present study for several reasons. First, we wished to confirm the earlier experimental and theoretical reports of the relative bond strengths of Fe(H20)+ and Fe(H20)2+ and to extend the bond dissociation energy measurements to larger species. In addition, we performed CID on Fe(CH4),+ ions, for Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 597
Gas-Phase Metal Ion Ligation which there exist no reports in the literature, to compare and contrast the polar H2O and the nonpolar CH4 as ligands. Finally, we wished to extend the number and kinds of Fe+-ligand systems for which accurate thermochemistryis known in order to provide a data base that could provide information to deepen our understanding of the detailed nature of gas-phase ion-ligand bonding.
Experimental Section
General. The guided-ion beam instrument on which these experiments were performed has been described in detail previously.12J3 Ions are created in a flow tube source, described below, extracted from the source, accelerated,and passed through a magnetic sector for mass analysis. The mass-selected ions are then decelerated to the desired kinetic energy and focused into an octopole ion beam guide. This device uses radio-frequency electric fields to trap the ions in the radial direction and ensure complete collection of reactant and product ions. The octopole passes through a gas cell of effective length 8.26cm that contains Xe gas (Air Products, >99.995%, used as received except for multiple freeze-pumpthaw cycles to remove noncondensable impurities). The unreacted parent and product ions drift to the end of the octopole, from which they are extracted, passed through a quadrupole mass filter for mass analysis, and detected with a secondaryelectron scintillation ion detector using standard pulse counting techniques. Raw ion intensities are converted to cross sections as described previously.12 We estimate absolute cross sections to be accurate to *20%, while relative cross sections are accurate to *5%. Laboratory (lab) energiesareconvertedtocenter of mass (CM) energies by using the conversion ECM= E l r a / ( M m), where m and M are the ion and neutral masses, respectively. The absolute energy scale and corresponding full width at halfmaximum (fwhm) of the ion beam kinetic energy distribution are determined by using theoctopoleas a retardingenergy analyzer as describedpteviously.l2 The absoluteuncertainty in the energy scale is f0.05 eV (lab). The energy distributions are nearly Gaussian and have typical fwhms of 0.254.4 eV (lab). Ion Sources. Fe(HzO),+ ions (x = 1-3) were made by using an electron impact source in which the He flows between a heated tungsten filament and an extraction plate biased at 3G70 V. Fe(C0)5 is admitted to the flow tube several centimeters downstream from the filament, and various fragment ions are formed by charge transfer from He+ or Penning ionization from He*. H20 is admitted at the first side port, - 5 cm further downstream, and Fe(HzO),+ ions formed by ligand exchange reactions with Fe(CO),+ fragments or three-bodycollisions with Fe+ and neutral molecules of the surrounding bath gas. For Fe(H20)4+, this method could not be used because Fe(H20)4+ has the same mass, 128 amu, as FeC203+, which is one of the ions made directly in the E1 source when Fe(C0)S is admitted to the flow tube.I4 Therefore, for this ion, a dc discharge source was used.5 This source comprises a water-cooledcarbonsteelcathode held a high negative voltage (1-3 kV). Argon is added to the He flow (typically about 5% of the total), where it is ionized and accelerated to the cathode, sputtering off Fe+ ions. H20 is admitted to the flow tube at the first side port, and Fe(HZO),+ ions are produced by three-body collisionswith bath gas molecules. To test for differences in ions produced by the two sources, Fe(H20)+ ions were also made by using the dc discharge source. No significant differences were found in the CID behavior of Fe(H20)+ ions made in the two ways. Fe(CH4),+ ions were made by using the dc discharge source and adding CH4 to the flow at the first side port. For the smaller ions, the amount of CH4 added was less than 1% of the total flow, and for the larger ones, the amount added was as much as 3% of the total flow. No obvious interference from FeO,+ ions was observed. Loss of one oxygen atom from such a species is not
+
expected to be a problem in any case, because the Fe+-O bond strength of 3.53 f 0.06 eVI5 is considerably higher than any of the Fe+-CH4 bond strengthswe measure here. For the Fe(CH&+ ions with x > 2,there might be B problem with interference from Fe(02),1(CH4),2+ ions. Because preliminary CID mass spectra16 always showed mass peaks every 16 amu below the parent, corresponding to loss of each CH4, and the peak from loss of one CH4 was always more intense than the others, we assume that there is no significant interference from ions containing 0 2 . The Fe(H20)4+ and Fe(CH&+ beams were less intense than those of the smaller ions by about an order of magnitude, as can be inferred from the relative signal to noise for the different experimental results. These larger ions were more difficult to make because they need more collisions in the flow tube to be formed. Another difficulty in the case of the Fe(H20),+ ions, x = 3 and 4,was the tendency of the source to preferentially make FeOH(H20),+ instead." In order to avoid passing these ions through the magnetic sector, which only has a resolution of AMJM 100, it was necessary to focus the magnet away from the peak, thereby reducing the ion intensity. In the case of Fe(CH4)4+, addition of two much CH4 to the flow tube caused formation of unknown impurity ions, presumably hydrocarbons, both 1 mass unit above and 1 mass unit below that of the desired parent. The Fe(H20)4+and Fe(CH&+ beams were still intense enough that it is possible to derive thresholds from their cross sections, however. We were unable to make any larger ions in sufficient quantities with which to perform meaningful experiments. We note that we have had similar difficulties in making other hydrated metal ions with more than four water molecules10 and that Graul and Squired8 reported that they could not make protonated water clusters with more than four water molecules in quantity either. Presumably, more elaborate experimental setups, such as a cooled flow tube or a supersonicexpansion, will be needed to make more highly ligated metal ions. Data Analysis. As previously reported for the CID cross sections of Fe(CO),+, x = 3-5,5 and cluster ions,Ig the cross sections for CID of the more highly ligated ions examined here show a marked dependenceon the Xe pressure in the gas cell due to the increasing probability of secondary collisionswith increasing pressure. In order to derive accurate CID thresholds, we therefore extrapolated the observed FeL,+ cross sections (x > 2) to zero pressure as described previo~sly.~J~ All cross sections shown below and all threshold analyses reported here for these species are for data that has been thus extrapolated. For x = 1 and 2, this procedure is unnecessary because no pressure dependence is observed. CID cross section thresholds are modeled by using the equationS
-
u = uo&(E
+ Ei + E,,, - Eo)"/E
(1) where E is the relative translational energy, EO is the CID threshold, uo is an energy-independent scaling parameter, and the exponent n is treated as a variable parameter. We have founds that in order to accurately and precisely model the experimentally observed cross sections of CID of these ion-ligand complexes, it is necessary to explicitly take into account their thermal internal energies. We thus model the cross section threshold by including the average 300 K rotational energy (Erot= 0.026 eV for linear molecules and 0.039 eV for nonlinear molecules) and the vibrational energy as a summation over vibrational levels i with populations gi and relative energies Et. We use the BeyerSwinehart algorithm20 to calculate a Maxwell-Boltzmann distribution of vibrational energies at 300 K. The resulting distribution of energies is then divided into up to 32bins (typically 100400cm-l wide), and the populations of the bins used as the factors gi in eq 1. We have described this threshold modeling procedure in detail elsewhere.5 Thevibrational frequencies used for each ion are given in Table I. There are no calculations in the literature of the vibrational
598
Schultz and Armentrout
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
TABLE I: Vibrational Freuueacies (cur')'
5,00jo
species frequencies used free H206 1594.7,3651.1,3755.9 Fe(HZO)+ A: free H20 + 60 (2), 200 B: free H20 + 60 (2), 300 C: free H20 + 100 (2),300 Fe(H20)2+ A: free HzO (2)+ 75 (4), 200 (3), 250 (2) B: free H20 (2)+ 100 (4), 200 (3),300 (2) Fe(Hz0)3+ A: free HzO (3) + 100 (6), 300 (6),400 (3) B: free H20 (3) + 100 (6),200 (3),300 (3), 400 (3) Fe(H20)4+ A: free H20 (4)+ 100 (9),400 (8),500 (4) B: 75 (9),300 (8), 600 (4), 1750 (4),4000 (4),4130 (4) free CHdb 1306 (3), 1534 (2), 2916.5 (l), 3018.7 (3) Fe(Ch)+ A: free CH4 + 60 (2), 300 B: free CH4 + 40 (2), 240 Fe(CH,)z+ A: free CH4 (2)+ 60 (4),240 (2),400 (3) B: free CHI (2)+ 40 (4),240 (2),400 (3) F ~. ( C H + 100 (61,300(3). 500 (6), 1430 (9),1690 (a), .- I ~A:
'
ENERGY k V . Lab) 2;5 , 5;o
,
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3210 (3), 3320 (9)
B: 60 (6),240 (3),400 (3), 500 (3), 1430 (9),1690 (a), 3210 (3). 3320 (9)
C: 300 (6),-500(3),-700(6), 1430 (9), 1690 (6), 3210 (3), 3320 (9) Fe(CHA+ . , A 100 (9). 300 (4). 500 (8),1430 (12), 1690 (8), 3210 (4), 3320 (12) B: 60 (9),240 (4),400 (4),500 (4),1430 (12),1690 (8), 3210 (4), 3320 (12) a Degeneracies in parentheses. Frequencies for free HzO and CH4 taken from ref 24.
frequencies for M(H20),+ or M(CH4),+, nor is there a neutral species on which to base choices of vibrational frequencies. We therefore use the vibrational frequencies of the free ligands for the small ions and increase them by 10% for the larger ones where the steric crowding is expected to be greater. We estimate the Fe+-L bending and stretching frequencies based on those used previously for the Fe(CO),+ specie^.^ In all cases, we used at least two sets of frequencies for each ion. Thresholds derived from eq 1 are not very sensitive to the particular choice of frequencies made. Before comparison with the data, the model cross section of eq 1 is convoluted with the ion and neutral translational energy distributions as described previously.I2 The model cross section is then optimized by using a nonlinear least-squares fit, and the resulting value of EOtaken to be the threshold for a given data set. The reported uncertainties in EO include averages over different data sets, different sets of vibrational frequencies used for eq 1, different values of n,and the absolute uncertainty in the energy scale. Because all sources of reactant energy are explicitly accounted for, the thresholds are believed to correspond to 0 K. These thresholds correspond directly to the dissociation energies of the bonds broken if there are no activation barriers in excess of the reaction endothermicities. Such activation barriers are unlikely in these systems because they are simple bond fission reactionsthat dissociate along attractive ion-dipole or ion-induced dipole surfaces. A possible exception is if the dissociation asymptotecorresponds toan excited state of theseparated species. This possibility is discussed in more detail below. The cross sections for loss of one ligand generally peak at about the threshold for loss of the second. Because the bonds here are often fairly weak, all cross sections were modeled both in the threshold region and also over longer range by including highenergy falloff behavior in the model fit as described in detail elsewhere.21 Both methods yield threshold energies that are the same to within the stated experimental error, showing that the falloff is sufficiently above the threshold that it does not affect the threshold fit. Some of the models shown here include this high-energy falloff. Results Fe(H20),+. Cross sections for the reaction of Fe(H20)+ with Xe are shown in Figure 1. The cross section for CID rises from
010
110
410
210 310 ENERGY lev. CM)
5:O
Figure 1. Cross sections for reaction of Fe(H20)+with Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). Open circles are cross sections for CID to form Fe+, and closed circles are ligand exchange to form FeXe+. The dashed line is a fit to the CID data, and the solid line shows the same fit convoluted over the ion and neutral translational energy distributions. The vertical arrow shows the CID threshold of 1.33 eV. ENERGY lev. Lab) 0 .o
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Figure 2. Cross sections for CID of Fe(H20)2+by Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). The circles show the cross sections for loss of one HzO, and the diamondsshow those for loss of two H20 molecules multiplied by 10.The dashed line is a fit to these data, and the solid line is the same fit convoluted over the beam and neutral translational energy distributions. The vertical arrow shows the threshold for loss of one H20 of 1.70 eV.
an apparent threshold near 1 eV to a peak cross section of about 4 A2 above 3 eV. This cross section does not decline appreciably after reaching its peak. Ligand exchange to form FeXe+ rises from a somewhatlower threshold to a peak near the CID threshold, above which it declines rapidly. Because these experiments were done under low-resolution conditions in our detector mass spectrometer, the absolute magnitudesof the FeXe+cross sections have not been corrected for the Xe isotopes. Cross sections for CID of Fe(H20)2+ are shown in Figure 2. Primary CID to lose one H20 molecule rises from a threshold between 1 and 2 eV to a peak cross section of about 11 A2above 5 eV. Loss of two water molecules is a much less probable process, rising slowly from an apparent threshold of 3 eV to a peak cross
Gas-Phase Metal Ion Ligation
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 599
ENERGY lev. Lab) 5 .o 10 .o
0 .o
+ Xe
Fe (H,O)
ENERGY l e v , Lab) 15 .O
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1
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3.0
4.0
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lev, CMI Figure 5. Cross scctions for reaction of Fe(CH4)+ with Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). CID is shown as open circles and ligand exchange as solid circles. The dashed line shows a fit to these data, and the solid line shows the same fit convoluted over the ion and neutral translational energy distributions. The vertical arrow shows the CID threshold of 0.59 eV.
TABLE II: 0 K Bond Strengths Measured in This Study and Fitting Parameters Used in Eq I bond Fe+-(H2O) (H20)Fe+-(H20) (H20)zFe+-(H20) (H20)3Fe+-(H20) Fe+-(CHd) (CHI) Fe+-(CH4) (CWZF~+-(CHI) (CH4)3Fet(CH4)
20.0
0 w
2 0
1 .o
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Figure 3, Cross sections for CID of Fe(H20)3+ by Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). Cross sections for loss of one H20 are shown by triangles, loss of two by circles, and loss of three (multiplied by 10) by diamonds. The dashed line shows a fit to these data, and the solid line shows the same fit convoluted over the ion and neutral translational energy distributions. The vertical arrow shows the threshold for loss of one H20 at 0.79 eV.
30.0
0 .o
10.0 ENERGY lev.
10 .o CM)
F i i 4. Cross sections for CID of Fe(H20)4+ by Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). Cross sections for loss of one H20 are shown as squares, loss of two by triangles, and loss of three by circles. The dashed line shows a fit to these data, and the solid line shows the same fit convoluted over the ion and neutral translational energy distributions. The vertical arrow shows the threshold for loss of one H20 at 0.85 eV.
section of less than 0.5 Az at 10 eV. Ligand exchange could not be observed because of the mass limit of our detector quadrupole mass spectrometer. Cross sectionsfor CIDof Fe(H20)3+ and Fe(H20)4+ are shown in Figures 3 and 4. They both show similar behavior, with primary CID rising from an apparent threshold below 1 eV to peak at around 2 eV. In both cases, secondary CID processes are much less probable, with loss of two water molecules rising slowly to peak cross sections (above 5 eV in both cases) about a factor of 6 smaller than loss of one. Loss of three water molecules is even less probable, rising slowly to a peak cross section below 1 A2 at 10 eV or above. A very small amount of Fe+ (maximum cross section ca. 0.1 A2) was detected from Fe(H20)4+, although it is not shown in Figure 4. In all cases, the secondary thresholds
OEO
BDE,’ kcal/mol 30.6 k 1.2 39.3 f 1.0 18.2 f 0.9 19.6 f 1.2 13.7 f 0.8 23.3 f 1.0 23.6 17.6 f 1.4 1.5
00
n
5.9 f 1.0 15.8 f 1.1 27.7 f 2.0 37.8 f 2.7
1.4 f 0.1 1.7 f 0.1 1.8 f 0.1 1.6 f 0.1
13.0 f 0.5 38.8 f 2.3 26.7 f 0.8 59.1 f 0.4
1.6 f 0.1 1.3 f 0.2 1.4 f 0.1 1.5 f 0.1
(eq 1).
appear to be shifted above their thermodynamic thresholds (Table 11)22when the cross sections are extrapolated to zero pressure. Fe(C!H&+. Cross sections for reaction of Fe(CH4)+ with Xe are shown in Figure 5 . CID rises from an apparent threshold below 1 eV to a peak cross section of about 10 A2. The larger cross section for FeCH4+ CID compared to that of Fe(H20)+ and FeCO+ (the latter is about 2 A2, ref 5 ) is presumably due to its smaller bond dissociation energy. Although ligand exchange occurs even at the lowest kinetic energy we could measure, a close examination reveals that this process is actually slightly endothermic, as the cross section levels out at the very lowest energies, rather than continuing to rise as is typical of exothermicreactions. As above, the magnitude of the ligand exchange cross section is not corrected for the Xe isotopes. CID of Fe(CH&+ is shown in Figure 6. Loss of one methane molecule rises from a threshold between 0.5 and 1 eV to a peak cross section of about 20 A2 above 2 eV. The fluctuations in the cross section at higher energy are an experimental artifact that we occasionally see in CID of ligated metals and are due to rf coupling between the octopole electric field and the ions. As with Fe(H20)2+, loss of both ligands occurs inefficiently,peaking at about 1 A2 above 5 eV, and rises slowly from its threshold below 2 eV. Cross sectionsfor CID of Fe(CH&+ and Fe(CH4)4+ are shown in Figures 7 and 8. For both ions, primary CID m u r s from an apparent threshold below 1eV, and secondary CID processes rise slowly and inefficiently from thresholds that appear to be shifted upward in energy relative to the thermodynamic thresholds that would be calculated from the primary thresholds given in Table
Schultz and Armentrout
600 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 ENERGY
0 .o
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lev. Lab)
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lev, Lab)
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relative kinetic energy (lower x axis) and laboratory energy (upper x axis). The circles show cross sections for loss of one methane molecule and the diamonds for loss of two (multiplied by 5 ) . The dashed line shows a fit to these data, and the solid line the same fit convoluted over the ion and neutral translational energy distributions. The vertical arrow shows the threshold for loss of one CHI of 1.01 eV. ENERGY
(eV,
0 .o
2 .o
6 .O 8 .o lev. CM) Figure 8. Cross sections for CID of Fe(CH4)4+ by Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). Squares show cross sections for loss of one methane molecule and triangles for loss of two. The dashed line shows a fit to these data, and the solid line shows the same fit convoluted over the ion and neutral translational energy distributions. Thevertical arrow shows the threshold for loss of one CH4 molecule of 0.76 eV. 4
.o
ENERGY
Figure 6. Cross sections for CID of Fe(CH&+ by Xe as a function of
0 .o 20.0 !
~
TABLE IIk (HzO),IFe+-(H20) BDEs (kcal/mol)
Lab)
5 .o
10 .o I
I
x
this studp Magnera et a!lPe
1 3 0 . 6 i 1.2 2 3 9 . 3 i 1.0 3 1 8 . 2 i 0.9 4 1 9 . 6 i 1.2
Marinelli and Squiresd t h c o r p 32.8 i 4 40.8 i 4
28.8 i 3 38 i 3
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Figure 7. Cross sections for CID of Fe(CH4)3+ by Xe as a function of relative kinetic energy (lower x axis) and laboratory energy (upper x axis). Triangles show cross sections for loss of one methane molecule, circles for loss of two, and diamonds for loss of three (multiplied by 5). The dashed line shows a fit to these data, and the solid line shows the same fit convoluted over the ion and neutral translational energy distributions. The vertical arrow shows the threshold for loss of one CH4 molecule at 1.02 eV.
11. A small amount of Fe(CHd)+ (having a peak cross section of
