Dissociation Dynamics and Thermochemistry of Tin Species,(CH3

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J. Phys. Chem. A 2011, 115, 402–409

Dissociation Dynamics and Thermochemistry of Tin Species, (CH3)4Sn and (CH3)6Sn2, by Threshold Photoelectron-Photoion Coincidence Spectroscopy Juan Z. Da´valos,*,† Rebeca Herrero,† Nicholas S. Shuman,‡,§ and Tomas Baer*,‡ CSIC, Instituto de Quı´mica Fı´sica Rocasolano, c/Serrano 119, Madrid, 28006 Spain, and Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599, United States ReceiVed: NoVember 25, 2010

The dissociative photoionization of tetramethyltin (Me4Sn) and hexamethylditin (Me6Sn2) has been investigated by threshold photoelectron-photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me4Sn+ dissociates rapidly by methyl loss, with a 0 K onset of E0 ) 9.382 ( 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 µs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me3Sn• loss to their 0 K dissociation onsets, which were found to be 8.986 ( 0.050 and 9.153 ( 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me4Sn and Me6Sn2 are used to derive the following o 298.15 K gas-phase standard heats of formation, in kJ · mol-1: ∆fHm (Me3Sn+,g) ) 746.3 ( 2.9; o + o • ∆fHm(Me5Sn2 ,g) ) 705.1 ( 7.5; ∆fHm(Me3Sn ,g) ) 116.6 ( 9.7; ∆fHom(Me2Sn,g) ) 123.0 ( 16.5; ∆fHom(MeSn+,g) ) 877.8 ( 16.4. These energetic values also lead to the following 298.15 K bond dissociation enthalpies, in kJ · mol-1: BDE(Me3Sn-Me) ) 284.1 ( 9.9; BDE(Me3Sn-SnMe3) ) 252.6 ( 14.8. 1. Introduction The ability of multiple carbon atoms to bond together and form complex organic molecular structures is unique. It is thus of interest to compare the C-C bond energy with other group IV atoms, namely, the Si-Si, Ge-Ge, Sn-Sn, and Pb-Pb bonds in similar molecules. We recently determined the Si-Si bond strength in Me3Si-SiMe3 by measuring the dissociative photoionization onsets in a variety of Si-containing molecules.1 The approach is shown in the following reactions, where we measure the onset energies, E1 and E2

Me4X + hν f Me3X+ + Me• + e-

E1

Me3X-XMe3 + hν f Me3X+ + Me3X• + e-

(1)

E2 (2)

With a knowledge of the heats of the formation of the neutral precursors, it is possible to determine the heat of formation of the Me3X+ ion from a measurement of E1. This value can then be used in the second reaction to determine the heat of formation of the Me3X• radical, from which the X-X bond energy can be calculated. In this paper, we report the thermochemistry of the tetramethyltin and hexamethylditin compounds. Because the tetramethyltin and hexamethylditin compounds have rather well established heats of formation (discussed in detail below), we * To whom correspondence should be addressed. E-mail: jdavalos@ iqfr.csic.es (J.Z.D.); [email protected] (T.B.). † Instituto de Quı´mica Fı´sica Rocasolano. ‡ University of North Carolina. § Current address: Air Force Research Laboratory, Space Vehicles Directorate, Hanscom Air Force Base, Massachusetts 01730, United States.

can determine the Me3Sn+ and, perhaps, Me3Sn• heats of formation with good precision. This is, of course, not the first study of these themochemical values. Recently, Jover et al.2 reviewed the enthalpies of formation of over 130 organometallic compounds in an attempt to develop a quantitative structure property relationship (QSPR) among them. Although certainly useful, such reviews highlight the continuing problems in finding reliable thermochemical values. Most of the reviews3 and compilations of organometallic compounds4 rely on old experimental values which have not been confirmed by newer methods. As pointed out in the Jover study, high-level calculations for organometallic compounds are difficult to perform, which results in very large discrepancies between calculated and experimental heats of formation, discrepancies that often exceed 30 kJ · mol-1. Interestingly, the Jover compilation failed to note a recent high-level coupled cluster calculation of the Ge(CH3)4 heat of formation by Koizumi et al.,5 which reported a value of -123 kJ · mol-1 with an estimated uncertainty in the 4 kJ · mol-1 range. In comparison, the Jover compilation lists an experimental value of -107.5 and a calculated value of -83.5 kJ · mol-1. Clearly, more reliable thermochemical data with realistic error limits are badly needed for organometallic compounds. 2. Experimental Methods A. Materials. Tetramethyltin (Me4Sn, purity 98%) and hexamethylditin (Me6Sn2, purity 99%) were purchased from Aldrich and used without further purification. In order to obtain a good signal rate for the hexamethylditin, the vapor pressure of the liquid sample was increased by using a heated inlet system. The tetramethyltin compound had sufficient vapor pressure to use at room temperature. B. Threshold Photoelectron-Photoion Coincidence (TPEPICO) Spectrometer. The TPEPICO experiment has been thoroughly described in previous publications.1,6,7 Ions are

10.1021/jp111229d  2011 American Chemical Society Published on Web 12/23/2010

Dissociation Dynamics and Thermochemistry of Tin Species energy-selected by measuring the time-of-flight distribution of ions collected in coincidence with initially zero energy (threshold) electrons. The resolution of the photon and threshold electron detection optics is about 10 meV. The ion internal energy is thus given by Eion ) hν - IE + Eth, where IE is the molecule’s ionization energy and Eth is the molecule’s initial internal energy. Because the neutral room-temperature thermal energy distribution is much larger than the 10 meV electron energy width, the ion thermal energy distribution is explicitly taken into account when modeling the data. Because of the many tin isotopes, some of the experiments were carried out with a reflectron TOF (ReTOF) mass spectrometer and others with a linear TOF (LinTOF). The ions in both spectrometers were accelerated to 100 eV over a 5 cm long acceleration region. In the LinTOF instrument, they were accelerated a second time to about 260 eV and drifted for 30 cm prior to detection by a multichannel plate detector. In the case of the ReTOF, the ions drifted at 100 eV without a second acceleration until they reached the reflectron and then drifted another 30 cm to the detector. An important difference in the two spectrometers is the mass resolution, which is much higher in the ReTOF than that in the LinTOF. In addition, the total flight time of the ions is a factor of 5 longer in the ReTOF, allowing ions in the ReTOF a longer time to dissociate before being detected. In the past, we have noted that metastable ions which dissociate in the acceleration and drift regions have a lower collection efficiency in the ReTOF than they do in the LinTOF. As demonstrated by SIMION calculations, this is a result of the kinetic energy lost in the dissociation event, which causes fragment ions in the ReTOF from slowly dissociating parent ions to have trajectories that deviate significantly from the parent ions and thus miss the ion detector. The loss in detection efficiency is tolerable for methyl loss from Me6Sn2+ but becomes significant for the Sn-Sn bond-breaking step. Two types of information are utilized in this experiment. The fractional abundance of the parent and various daughter ions can be plotted as a function of the photon energy to obtain a breakdown diagram. In addition, if the ion dissociation time is on the same order of magnitude as the time required to extract the ion from the source to the detector, the resulting fragment ion TOF will exhibit quasi-exponential tailing toward the long TOF side. These peak shapes can be analyzed to obtain absolute rate constants for ion dissociation. Thus, the breakdown diagram provides relative rate information, while the ion TOF distribution provides absolute rate information. The rate data can be modeled with the statistical theory of unimolecular reactions8 in order to extrapolate the rate constant to the dissociation limit. 3. Quantum Calculations Quantum chemical calculations were carried out using the Gaussian 03 package.9 The ground-state geometries of the neutral and ionic species were optimized by using density functional theory (DFT), with the Becke 3 parameter and the Lee, Yang, Parr (B3LYP) functional method.10 The 6-311+G(d,p) basis set was used for C and H, whereas the combination of the SDB-aug-cc-pVTZ basis set and the LANL2DZdp effective core potential (ECP) were used to describe tin. This combination was recommended by Whittleton et al.11 for the prediction of the geometries of organotin compounds. The vibrational frequencies obtained in these calculations, and shown in Table S1b of the Supporting Information, were used without scaling. In the RRKM analysis described below, approximate transitionstate (TS) vibrational frequencies for barrierless dissociations

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Figure 1. Molecular geometry for Me4Sn (left) and staggered Me6Sn2 (right), optimized at the B3LYP level with the 6-311+G(d,p) basis set for C and H and a combination of the SDB-aug-cc-pVTZ basis set and the LANL2DZdp effective core potential (ECP) for Sn.

were obtained by stretching the bond in question to approximately 4 Å at the B3LYP level of theory. The geometry of the Me4Sn neutral molecule (Figure 1) has Td symmetry. The Sn-C and C-H bond lengths (2.147 and 1.093 Å, respectively) and angles C-Sn-C and Sn-C-H (109.5°, 110.9°) observed are in good agreement with experimental values obtained from gas-phase electron diffraction data.12 Me6Sn2 neutral molecules have two conformers Is (staggered) and Ie (eclipsed) with D3d and D3h symmetries, respectively. Our calculations show that hexamethylditin (Me6Sn2), similar to analogues of X2H6 (X ) C, Si, Ge, Sn, Pb), exhibits staggered conformational preference rather than eclipsed, with an internal rotation barrier (enthalpy change of reaction Is T Ie) of ∆rH ) 1.4 kJ · mol-1. This value is practically the same for the Sn2H6 case reported in the literature.13,14 A sample of gaseous Me6Sn2 at ∼298.15 K is an equilibrating mixture (Is T Ie) of 83 and 17% staggered and eclipsed, respectively. Taking into account these results and also because the estimated rotation barrier is almost 1 kJ · mol-1 (it is lower than the Boltzmann energy at room temperature), we can conclude that there will be considerable floppiness in the Me3Sn groups around the Sn-Sn bond in Me6Sn2, although its conformational preference is still present. 4. Results and Discussion A. Me4Sn and Me6Sn2 Heats of Formation. The two starting compounds, tetramethyltin and hexamethylditin, have experimentally determined heats of formation. All values for SnMe4 listed in recent compilations, such as the NIST Website,15 Rabinovich et al.,4 Martinho Simoes et al.,3 Cox and Pilcher,16 and Pilcher,17 refer back to the1963 heat of combustion work of Davies et al.,18 who reported a liquid-phase heat of formation of -52.3 ( 1.9 kJ · mol-1. However, when this is corrected for a more recent SnO2(s) heat of formation19 (which was available but not used in previous reviews), the value increases to -49.2 ( 1.9 kJ · mol-1. Finally, with the latest suggested heat of vaporization of 32.0 ( 0.67 kJ · mol-1,20 we end up with a gasphase ∆fH°m(Me4Sn,g) ) -17.2 ( 2.1 kJ · mol-1, a value listed in Table 1. The heat of formation for Me6Sn2(l) has a more complicated history. Pedley et al.21 measured the heat of bromination of this compound (-293.9 ( 2.1 kJ · mol-1), a reaction that produced 2Me3SnBr(l). The heat of formation of the brominated product was determined by measuring the Me4Sn(l) + Br2(g) f Me3SnBr(l) + MeBr(g) heat of reaction (-202.1 ( 2.9 kJ · mol-1). Since the time of these measurements, the heats of formation of the three other molecules in this reaction have all changed. When we use the latest values [-49.2 (Me4Sn(l)), 30.9 (Br2(g)),19 and -35.4 (MeBr(g))22], we obtain a ∆fH°m(Me3SnBr(l)) of -185.0 ( 3.7 kJ · mol-1. With this updated

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TABLE 1: Auxiliary and Derived Gas-Phase Thermochemical Data (in kJ · mol-1) ∆fH°0K Me4Sn Me6Sn2 Me5Sn2+ Me3Sn+ Me3Sn• Me2Sn MeSn+ Me•

13.5 ( 2.1 24.7 ( 5.7b 741.4 ( 7.5f 768.5 ( 2.9f 139.3 ( 9.7f 138.0 ( 16.5f 886.3 ( 16.4f 150.3 ( 0.4o b

(H°298K - H°0K)a

∆fH°m

∆fH°m (lit. values)

30.62 51.07 45.04 25.39 24.85 18.80 11.52

-17.2 ( 2.1 -19.4 ( 5.7c 705.1 ( 7.5b 746.3 ( 2.9b 116.6 ( 9.7b 123.0 ( 16.5b 877.8 ( 16.4b c

-19.2 ( 2.5 ; -19.0 ( 2e -26.9 ( 8.4d; -29 ( 8e d

757.3g; 771h; 778i; 795j; 765.7k 130 ( 17l; 125.5g; 104.6m; 113.9h; 146i; 134 ( 25j 151.5 ( 7.9n 1058.6k

a Calculated using vibrational frequencies obtained from the B3LYP method with a combination of SDB-aug-cc-pVTZ and LANL2DZdp ECP bases. b Converted from (or to) the 298.15 K value. c Evaluated from literature values using updated ancillary heats of formation and vaporization (see text). d Pilcher (1992).17 e Rabinovich et al. (1999).4 f This work. g Pietro and Hehre (1982).39 h Lappert et al. (1971).25 i Yergey and Lampe (1965).40 j Yergey and Lampe (1968).41 k Hobrock and Kiser (1961).42 l Martinho Simoes et al. (1995).3 m Jackson (1979).43 n Allendorf and Melius (2005).44 o Derived from dissociative photoionization of CH4 by Weitzel et al. (1999)45 and the CH3 ionization energy of Blush and Chen (1992).46

modeled using the integral of the thermal energy distribution and the fact that the 0 K dissociation onset (E0) is approximately where the parent ion disappears.6,23,24 The tetramethyltin ion dissociates exclusively by loss of the methyl group (reaction 3) up to at least 10 eV.

Me4Sn + hν f Me3Sn+ + Me• + e-

Figure 2. Breakdown diagram for Me4Sn+. The points are the experimental abundances of the parent and fragment ions, while the solid line is the modeled fit from which the 0 K dissociation energy is extracted.

value for the trimethylbromotin heat of formation, we can obtain the hexamethylditin heat of formation from the Me6Sn2(l) + Br2(l) f 2Me3SnBr(l), which yields a ∆fH°m(Me6Sn2(l)) of -76.0 ( 5.6 kJ · mol-1. We now need the heat of vaporization to obtain the gas-phase heat of formation. According to Cox and Pilcher,16 the literature value of 50.2 ( 4 kJ · mol-1 is based on Trouten’s rule. It leads to a gas-phase heat of formation for hexamethylditin of -25.9 ( 6.9 kJ · mol-1. In order to verify this value and to reduce its uncertainty, we determined the Me6Sn2(l) heat of vaporization from the heats of sublimation (75.1 ( 1.1 kJ · mol-1) and fusion (18.5 ( 0.2 kJ · mol-1) measurements (full details are given as Supporting Information). The measured heat of vaporization determined was found to be 56.6 ( 1.1 kJ · mol-1. Thus, the standard gas-phase heat of formation obtained for Me6Sn2 is ∆fH°m(Me6Sn2(g)) ) -19.4 ( 5.7 kJ · mol-1. This is considerably higher than the literature values of -26.9 ( 8.417 and -29 ( 8,4 which are based on the same Pedley experiment. B. Dissociative Photoionization of Me4Sn: Thermochemistry of the Trimethyltin ion (Me3Sn+). Coincidence TOF distributions for the methyl loss reaction were obtained in the vicinity of the dissociation limit. The fractional abundances of parent and daughter ions are plotted as a breakdown diagram, shown in Figure 2. The daughter peaks in the TOF distribution are symmetric, indicating that the dissociation rate for methyl loss is rapid with a rate constant > 107 s-1. This means that if an ion has a total energy, given by hν + Eth, in excess of the dissociation limit, it will dissociate instantly (on the µs time scale of our experiment). Thus, the breakdown diagram can be

E0

(3)

The derived 0 K onset for methyl loss is E0 ) 9.382 ( 0.020 eV. This can be compared to the only other available value for this onset, obtained by electron impact, of 9.58 ( 0.19 eV.25 These values are usually too high because of the poor electron energy resolution and the unfavorable threshold law. Very roughly, our breakdown diagram would be the second derivative of the electron impact onset, which means that the former is much sharper. Using the thermochemical cycle associated with reaction 3 and the heats of formation shown in Table 1, we can obtain the gas-phase heat of formation at 0 K of the trimethyltin ion, ∆fH°0K(Me3Sn+(g)) ) 768.5 ( 2.9 kJ · mol-1, which is converted to ∆fH°m(Me3Sn+(g)) ) 746.3 ( 2.9 kJ · mol-1 at 298.15 K. The Lappert et al.25 value for the 298.15 K enthalpy is 771 kJ · mol-1. C. Dissociation of Me6Sn2. The dissociation of hexamethylditin is significantly more complicated than that of tetramethyltin. The parent ion (Me6Sn2+) dissociates to a number of products, as shown in the TOF distributions obtained with a linear time-of-flight apparatus at several photon energies in Figure 3. The dominant channel is methyl loss to produce Me5Sn2+, with Sn-Sn bond cleavage to form Me3Sn+ as a minor channel, and trace amounts of Me2Sn+ formed presumably through a rearrangement producing tetramethyltin neutral. (In solution, there is a similar process, which is a cobalt-catalyzed disproportion reaction of Me6Sn2 into Me4Sn and Me2Sn.26) Because of the many tin isotopes (there are seven major isotopes from 116 to 124 amu) and the slow dissociation, which causes asymmetric tailing of the methyl loss peak toward higher time-of-flight, extracting the methyl loss peak signal from the parent ion was not entirely problem-free. Thus, to check the results, we repeated the experiment using a reflectron mass spectrometer, ReTOF. Figure 4 shows two Me6Sn2+ ReTOF distributions at low and high photon energies. The peaks are fully resolved in the higher-energy spectrum. At 9.180 eV, the dissociation rate was sufficiently slow that the ions dissociated while accelerating in the first region as well as in the 40 cm long drift distance prior to the reflectron. This means that the peaks were asymmetrically broadened toward higher masses. In addition, those ions that dissociated in the

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Figure 5. The breakdown diagram for Me6Sn2+ in the 8.5 and 11.5 eV range. Solid lines are simulations for experimental linear TOF points, and dotted lines are for ReTOF. Figure 3. TPEPICO ion time-of-flight (linear TOF) distributions for Me6Sn2 at several selected photon energies in the range of 8.5-11.5 eV.

Figure 6. The dissociation scheme of Me6Sn2+ ions. The energies are given in eV. Note that the Me3Sn+ ions are produced via two different channels. Figure 4. TPEPICO high-resolution ion time-of-flight distributions (ReTOF) for Me6Sn2 at the two indicated photon energies. Black lines are experimental data, while the red lines are modeled, taking into account all of the major Sn isotopes. At a photon energy of 9.18 eV, the dissociation rate constant is low, and the peaks are asymmetric, while at 9.670 eV, the peaks are narrow and symmetric. Arrows in the low-energy spectrum indicate peaks due to metastable ions that dissociate in the drift region, resulting in mass peaks that do not appear at integral mass peaks.

drift tube appeared at nonintegral masses, whose precise location depended on the voltage settings on the ReTOF. In this case, they appeared 1.8 mass units after the nominal mass peak. Two of these drift tube dissociation peaks are indicated by arrows in Figure 4. These peaks appear more intense in the simulated spectrum (red) because the experimental peaks (black) are broadened by kinetic energy release in the dissociation event, a broadening that is not taken into account in our simulation. We can construct breakdown diagrams (Figure 5) by plotting the fractional abundances of the various mass peaks as a function of the photon energy for both the LinTOF and ReTOF data. The diagrams differ only in the region of the methyl loss peak because the ions in the ReTOF are given a longer time to dissociate (61 vs 12 µs) so that at a given energy, a larger fraction of them end up as fragment ions. The breakdown diagram shows the onsets of the various channels up to 11.5 eV. It is evident that the Me3Sn+ ion is produced by two pathways. Between 9 and 10.5 eV, the Me3Sn+ + Me3Sn• reaction remains a minor channel, below 10%. However, above 10.5 eV, the Me3Sn+ signal increases rapidly at the expense of the Me5Sn2+ ion, from which it is clearly produced by a sequential dissociation to generate Me3Sn+ + Me2Sn + Me•.

The other main sequential channel is the formation of MeSn+ + Me4Sn + Me•. On the basis of the fitting of the breakdown diagram, we can construct a dissociation scheme, as shown in Figure 6. Determining the dissociation onset for each of these channels requires simultaneous modeling of each of the competitive processes. Making matters significantly more complicated, the asymmetric peak shapes in the TOF spectra in Figure 3 show that the initial dissociation of hexamethylditin ions is slow on the time scale of our experiment. That is, a significant fraction of the rate curve lies below about 107 s-1, resulting in dissociations at various positions along the flight path in the spectrometer. Some fraction of ions with sufficient energy to dissociate will not do so by the time that they are detected. The resulting “kinetic shift” of the breakdown diagram to higher photon energies requires accurate dissociation rates in order to extract the onsets. By modeling the asymmetric peaks, we can directly measure dissociation rates for the first channel (Me• loss) between about 103 and 107 s-1. The portion of the rate curve below 103 s-1 must be extrapolated from the higher-energy rates, and this low-energy portion of the rate curve is critical in determining E0. Previous attempts at the k(E) extrapolation have employed Rice-Ramsperger-Kassel-Marcus (RRKM) theory,27-30 or the equivalent quasi-equilibrium theory of Rosenstock et al.31 RRKM assumes that a dissociation proceeds through an energyindependent TS. However, barrierless ionic dissociations (such as those that we assume for all channels from which thermochemical data are extracted) lack such a TS. We and others have recently shown that in cases where the kinetic shift is greater than about 200 meV, RRKM is unreliable in determining

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E0.32,33 Variational transition-state theory (VTST)34,35 may be employed; however, doing so requires significant knowledge of the potential along the reaction coordinate, requiring arduous calculation in order to extract a relatively small amount of kinetic information. Instead, a simplified version of the statistical adiabatic channel model (SSACM), which requires knowledge only of the initial and final states of the reaction, is used. SSACM successfully determines E0 for a variety of ionic dissociations, including those of several benzene derivatives33,36 and di-t-butyl peroxide.37 The details of the method can be found elsewhere.32,33 Briefly, SSACM assumes that orbiting transition-state phase space theory (OTS-PST) correctly describes the rate of ionic dissociations with the exception of not accounting for anisotropy in the potential along the reaction coordinate. Any anisotropy restricts the orbiting motion of the separating products, reducing the reaction rate below the PST limit. SSACM accounts for an increasingly anisotropic potential at small interfragment distances by introducing a “rigidity factor”38 into the calculation of the rotational number of states of the products such that †,PST †,SSACM frigidNrot (E - E0) ) Nrot (E - E0)

(4)

† where Nrot is the number of states of the orbital motion of the products, E is the ion internal energy, and E0 is the threshold to dissociation. The rotational number of states is subsequently convoluted with the PST vibrational number of states. The energy dependence of frigid is modeled by a simple exponential such that

frigid ) e-(E-E0)/c

(5)

where c is treated as an adjustable parameter but should roughly increase with the polarizability of the neutral fragment. Finally, the dissociation rate constant k is determined in the spirit of statistical theory

k)

σN†(E - E0) hF(E)

(6)

where σ is the symmetry number of the dissociation, h is Planck’s constant, and F is the density of states of the dissociating ion. Each dissociation channel then has two adjustable parameters, the dissociation onset E0 and the rigidity factor constant c. These parameters were optimized for the parallel dissociation of Me6Sn2+ between 8.5 and 10.5 eV, as described in detail elsewhere.6 The minor channel producing Me2Sn+ was omitted from this analysis. Typically, the necessary dissociation rate information is determined by the asymmetry of the daughter ion peaks in the TOF spectral; however, in this case, the large number of stable isotopes significantly complicates such a fitting. Instead, rate information was obtained by exploiting the significantly different draw out times of the linear TOF and the ReTOF setups. In the former, only ions dissociating within the first 12 µs of flight are detected as daughter ions, whereas in the latter, ions dissociating within the first 61 µs are detected as daughter ions, with the ratio of the measured abundances at each draw-out time determining the dissociation rate of the parent ion. Fits to the LinTOF and ReTOF breakdown diagrams are optimized simultaneously with the best-fit onsets for the

Me loss and Me3Sn• loss channels and determined uncertainties appearing in Figures 5 and 6. The resulting best-fit rate curve using a c value of 21 cm-1 for the Me loss reaction provides simulated TOF spectra in good agreement with experiment (Figure 4) across the energy range producing metastable Me6Sn2+. Because the metastable ions dissociate to Me5Sn2+ at a range of locations along the flight path, the corresponding TOF peaks are asymmetric; at lower photon energies corresponding to slower rates, the broad tails to longer TOF for each isotope peak cause the peaks to be unresolved, while at higher photon energies and faster dissociation rates, the isotope peaks become symmetric, sharp, and resolved. As stated above, dissociations in the drift region of the ReTOF yield Me3Sn+ fragments with kinetic energy sufficiently different than that of the parent ion to cause ion loss to a degree that we cannot directly quantify (this is not a complication for the methyl loss channel because the neutral fragment is a much smaller percentage of the parent mass). Fortunately, for this particular system, evidence shows that Me3Sn• loss in the drift region has a minimal effect on the measured abundances. First, the parent ions with the lowest internal energies will have the longest lifetimes and be disproportionately represented among those ions which do not dissociate until reaching the drift region. The product branching for these low-energy ions is heavily weighted toward the lower energy methyl loss channel. Additionally, the dissociation rate of the Sn-Sn cleavage channel may be directly derived from the shape of the Me3Sn+ daughter peak in the LinTOF spectra (unlike the methyl loss product, this peak is resolved from the parent ion) and is in excellent agreement, using a c value of 45 cm-1, with the rate curve derived from comparing the LinTOF and ReTOF data. At photon energies above 10.5 eV, a sequential dissociation of the Me5Sn2+ daughter ion occurs. The onsets in this region were determined in a similar manner to that described above, with the parameters of the Me6Sn2+ dissociation channels held constant. Owing to a lack of ReTOF data in this region, dissociation rate information was determined as best as possible from the asymmetry of the daughter ion peaks in TOF spectra. Of the two dissociation channels in the region, (i) RRKM was used to model the rate of the (MeSn+ + Me4Sn) channel. The dissociating ion undergoes a rearrangement and likely passes over a real energetic barrier along the reaction coordinate, and SSACM is inappropriate to describe the dissociation rate. (ii) SSACM was used to model the rate of the Sn-Sn bond cleavage. Taking into account energetic considerations, the production of Me3Sn+ + Me2Sn is more favorable than that of Me3Sn• + Me2Sn+ (see section S2 of Supporting Information). The best-fit onsets are shown in the breakdown diagram (Figure 5) and the energy level diagram (Figure 6). It is important to mention that at energies above 11.5 eV, the abundance of Me5Sn2+ ion levels off at roughly 20%. Because the Me5Sn2+ dissociates at lower energies, this behavior is only possible if the ion isomerizes to a more stable geometry or dissociates by H loss, the product ion from which would not be differentiable from the Me5Sn2+ peak in the LinTOF spectra, and no ReTOF data were collected at these energies. Although we cannot determine from our data which of these scenarios occurs, we believe that the latter is more likely as we cannot determine a lower-energy structure of Me5Sn2+, whereas H loss could lead to Me2Sn(CH2)SnMe2+, which DFT calculations suggest is a stable ion accessible at these total energies. The measured dissociative photoionization onsets permit us to determine the heat of formation at 0 K, ∆fH°0K, of Me5Sn2+, Me3Sn+, Me3Sn•, Me2Sn, and MeSn+ (Table 1) using the well-

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TABLE 2: Experimental Bond Dissociation Enthalpies (BDE) at 298.15 K, in kJ · mol-1 X

Me3X-H

Me3X-Me

Me3X-XMe3

C Si Ge Sn Pb

403.8 ( 1.7 396.2 ( 4.6b 364.0c; 340 ( 10d; 345.6 ( 2.1d 309.5 ( 10.7f; 322.7 ( 17d; 314 ( 15g

366.1 ( 1.7 395.1 ( 3.5b 347 ( 17d; 339 ( 13d 284.1 ( 9.9f; 297 ( 17d 238 ( 17d

328.9 ( 2.9 333.3 ( 5.8b 280.3e 252.6 ( 14.8f; 286.5 ( 25d; 276h; 258e

a

a

a

a Blanksby and Ellison (2003).51 b Davalos and Baer (2006).1 c Laarhoven et al. (1999).50 d Martinho Simoes et al. (1995)3 (see text for error discussion). e Lappert et al. (1971).25 f This work (to calculate the Me3Sn-H BDE, we assumed ∆fH°m(Me3SnH(g)) ) 25 ( 4 kJ · mol-1 from Rabinovich et al. (1999)4). g Brinkman et al. (1995).47 h Becerra et al. (2005).52

established literature enthalpy of formation values for the others species, which are also listed in Table 1. The conversion of the heat of formation from 0 to 298.15 K, and vice versa, was made by means of the usual thermochemical cycle, such as is described in ref 1 and 5, where the parameter (H°298K - H°0K) for the molecule or ion is determined using theoretical DFT vibrational frequencies. The literature values for Me4Sn and Me6Sn2 in Table 1 are based on the same experiments as our values but differ only in the assumed ancillary heats of formation for SnO2 and the heats of vaporization. For the case of hexamethylditin, we determined a new heat of vaporization, which is described in the Supporting Information. The heat of formation of Me3Sn+ is based on the methyl loss from Me4Sn. Because both the parent molecule and the methyl radical have well-established heats of formation, we know the heat of formation of the trimethyltin ion to a very high precision. It is much lower than previous measurements, which were based primarily on electron impact ionization studies. Our value for the trimethyltin radical heat of formation, from which we can determine various Sn-X bond energies, falls in the middle of the numerous values found in the literature, most of which have no reported error bars and differ by as much as 40 kJ · mol-1 from each other. D. Calculation of Bond Dissociation Enthalpies (BDE). The measurement of the Me3Sn• radical heat of formation permits us to determine the Me3Sn-X BDE via eq 7

Me3X-Y f Me3X•+Y•

∆rH298 ) BDE

(7)

Our values for the BDE of Me3Sn-H, Me3Sn-Me, and Me3Sn-SnMe3 are listed as 309 ( 10.6, 284.1 ( 9.7, and 252.6 ( 14.6 kJ mol-1, respectively. The BDE of Me3Sn-H requires a knowledge of the ∆fH°m(Me3SnH(g)), which we obtained from the Rabinovich et al.4 compilation as 25.1 ( 4.2 kJ · mol-1. Because this is based on an old 1964 value that appears to be published only in a Ph.D. dissertation, we checked its value by calculating it using the following isodesmic reaction: 3/4 Me4Sn + 1/4 SnH4 f Me3SnH. This calculation yielded ∆fH°m(Me3SnH(g)) ) 27.6 kJ · mol-1, which lends a measure of confidence to the experimental value. Details are available as Supporting Information. Comparing the Sn-H value with those in the literature requires some care. For instance, Martinho Simoes et al.3 list this value as 322.7 ( 4.2 kJ mol-1. However, a careful inspection of their table indicates that their error limits did not include the uncertainty of 17 kJ mol-1 in the Me3Sn• radical heat of formation. Because the bond dissociation enthalpy depends directly on this value, we have listed this BDE with its proper uncertainty. Brinkman et al.47 derived a BDE of 314 ( 15 kJ · mol-1 based on the gas-phase acidity of Me3SnH and the electron affinity of Me3Sn•. This value, based on the negative ion cycle,48 is one of the few independent experimental numbers.

Figure 7. Bond dissociation enthalpies (BDE) at 298.15 K for Me3X-H (solide line), Me3X-Me (dotted line), and Me3X-Me3X (dashed line).

Another Me3Sn-H BDE in the literature is one listed in the Comprehensive Handbook of Chemical Bond Energies by Luo.49 However, a careful inspection of this value shows that it is taken from a solution-phase study by Laarhoven et al.50 and refers to the Bu3Sn-H BDE. The effect of the solvent or the replacement of methyl by the bulky butyl group is not discussed. In 1979, Jackson reviewed the bond energies (no error limits) of a number of group IV metals, including those of Sn. Interestingly, he lists the Me3Sn-H BDE as 309 kJ mol-1, in precise agreement with our value. However, this value is based on a Me3Sn• heat of formation of 104.6 kJ mol-1 (compared to our value of 116.6 kJ mol-1), which means that he used an older heat of formation for Me3SnH so that the two errors compensated each other. The three modern values listed with error limits in Table 2, which are obtained by very different methods, agree quite well within the limits of the rather large uncertainties. The Me3Sn-SnMe3 bond energies have large errors. Our uncertainty of 14.6 kJ mol-1 includes the error in the Me3Sn-SnMe3 heat of formation as well as the error associated with the two Me3Sn• radicals. As with the Me3Sn-H BDE, Martinho Simoes3 listed the error in the Sn-Sn bond energy to be 8.4 kJ mol-1, completely neglecting the (17 kJ mol-1 uncertainty in the trimethyltin radical. When we include this uncertainty (twice), along with the 8.4 kJ mol-1, we obtain a total uncertainty of 25 kJ mol-1. Table 2 and Figure 7 show the BDE values for Me3X-Y (where X ) element of Group 14 and Y ) H, Me, XMe3) calculated, by eq 7, using the enthalpy of formation of each specie derived or determined in this work (Table 1) as well as that from the experimental literature values. From these results we can deduce the following observations. (a) BDE values for Me3X-H reflect expected trends of decreasing bond enthalpy down the periodic column, C-H g Si-H > Ge-H > Sn-H. It is remarkable that the BDE difference between C-H and Si-H is only 7.6 kJ · mol-1, while it decreases by about 37 kJ · mol-1 for heavier elements. This result is consistent with the increased metallic character of the

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heavier elements, which can be explained by the stronger shielding exerted by the core on valence electrons. (b) The BDE trend for Me3X-Me and Me3X-XMe3 are not uniform; they show a “cusp” in X ) Si, especially in the first case. This observation was widely discussed by Davalos and Baer,1 where the small alkyl substituent effects in organosilanes and the opposite effects in carbon compounds (“methyl group inductive effect”) were rationalized in terms of differences in the Pauling electronegativities of Si and C and the relation to the stability of their corresponding radicals. Thus, the lack of methyl substitution effect in Me3Si-H, according to Wetzel et al.,53 would be a consequence of the stability of the trimethylsilyl radical, which is essentially unaffected by alkyl substitution. (c) The methyl substituent effect on BDE of Me3Ge-H and Me3Sn-H decreases by almost 17 and 26 kJ · mol-1, respectively. This effect is greater for Me3Ge and Me3Sn substituents, in that the decreasing of BDE is 84 and 57 kJ · mol-1, respectively. These results are consistent with the increased metallic character of Ge and Sn, as was mentioned before. 5. Conclusions This dissociative photoionization study of tetramethyltin, which dissociates by methyl loss, has permitted the determination of an accurate 0 K onset energy of E0 ) 9.382 ( 0.020 eV, which in turn permits us to calculate an accurate heat of formation of ∆fH°m(Me3Sn+(g)) ) 746.3 ( 2.9. This value is based on an updated heat of formation of Me4Sn(l) (see text). Because the dissociation of Me6Sn2+ ions is slow on the time scale of the experiment, it was necessary to measure the dissociation rate for the methyl loss channel as a function of the ion internal energy and to extrapolate the k(E) function to the onset, thereby increasing the error considerably. The extrapolated methyl loss onset was 8.986 ( 0.050 eV. At a slightly higher energy, the loss of the Me3Sn• radical occurred at an onset energy of 9.153 ( 0.075 eV, with the additional error introduced by the fact that this reaction is in competition with the lower-energy methyl loss step. These onset energies in combination with the updated values for the heats of formation of the neutral Me4Sn and Me6Sn2 are used to derive the following 298.15 K gas-phase standard heat of formation, in kJ · mol-1: ∆fH°m(Me3Sn+(g)) ) 746.3 ( 2.9; ∆fH°m(Me5Sn2+(g)) ) 705.1 ( 7.5; ∆fH°m(Me3Sn•(g)) ) 116.6 ( 9.7; ∆fH°m(Me2Sn(g)) ) 123.0 ( 16.5; ∆fH°m(MeSn+(g)) ) 877.8 ( 16.4. These heats of formation lead to the following 298.15 K bond dissociation enthalpies, in kJ · mol-1: (Me3Sn-Me) ) 284.1 ( 9.9; (Me3Sn-SnMe3) ) 252.6 ( 14.8. Acknowledgment. This paper is dedicated to Dr. Pilar Jimenez, Research Scientist of CSIC-IQFR, on the occasion of her retirement. J.Z.D. gratefully acknowledges the support of the Spanish Project MICINN/CTQ2009-13652. This work was supported by a grant to UNC from the U.S. Department of Energy. Supporting Information Available: Computational results, diagram of energies of Me5Sn2+ and its dissociation products, DSC measurements, heat of sublimation, heat of vaporization at T ) 298.15 K, and theoretical enthalpy of formation of Me3SnH. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Davalos, J. Z.; Baer, T. J. Phys. Chem. A 2006, 110, 8572–8579.

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