J. Phys. Chem. 1996, 100, 19659-19665
19659
Dissociation Thermochemistry of Tetramethylsilane Ion. Comparative Determination by Thermal Dissociation and Threshold Collisional Dissociation Chuan-Yuan Lin and Robert C. Dunbar* Chemistry Department, Case Western ReserVe UniVersity, CleVeland, Ohio 44106
Chris L. Haynes and P. B. Armentrout* Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112
D. Scott Tonner and Terrance B. McMahon Chemistry Department, UniVersity of Waterloo, Waterloo, Ontario, Canada ReceiVed: August 15, 1996X
The dissociation thermochemistry of tetramethylsilane cation was investigated by two new methods, threshold collision-induced dissociation (CID) and zero-pressure thermal infrared radiation-induced dissociation (ZTRID). For the loss of a methyl group from SiMe4+, the two methods gave 0 K bond dissociation energies of 0.72 eV (CID) and 0.67 eV (ZTRID). These values were supported by an indirect thermochemical calculation and by an ab initio (MP2) bond strength calculation. A best value for this bond strength of 0.69 ( 0.06 eV is assigned. From the CID measurements, further bond strengths were assigned for SiMe2+-Me (5.32 ( 0.28 eV), SiMe+-Me (1.27 ( 0.50 eV), and Si+-Me (4.29 ( 0.06 eV). New or reevaluated heat of formation values are given for the SiMex+ species (x ) 0-4). TABLE 1: Bond Dissociation Energies (kJ mol-1) (0 K)
Introduction The determination of bond strengths in gas-phase ionic molecules by observing the energy required to bring about fragmentation has been an extensive enterprise. Traditional approaches are based on dissociative ionization of the parent neutral molecule, using electron or photon impact for energy deposition. Although enormously successful, this approach always involves challenging problems of accounting for the energy carried off by the departing electron, and has additional complications when the geometry of the parent ion differs greatly from the geometry of the initial neutral molecule (poor Franck-Condon overlap.) More recently, attractive alternative approaches have been developed in which the ground state (or thermally relaxed) parent ion itself is the starting point for the experiment. These experiments then involve a determination of the additional energy input required to induce the fragmentation of interest. In the present work, two such methods are brought to bear on a particularly challenging ion, tetramethylsilane (TMS) ion. Precise determination of the dissociation thermochemistry of TMS ion by dissociative ionization of TMS is difficult for at least two reasons. First, the parent ion is severely distorted from the symmetrical neutral TMS structure, so the Franck-Condon factors for ionization in the threshold region are particularly bad. Second, the silicon-methyl bond in the TMS parent ion is quite weak. Consequently, a determination of this small bond energy by taking the difference between the ionization energy of the parent ion and the appearance energy of the fragment ion involves subtraction of two large numbers, with the expectation that the resulting uncertainty will be large. Dissociative photoionization results indicate that the dissociation of a methyl group from SiMe4+ is facile, requiring less than 1 eV, although the thermochemistry is not yet definitive * Authors to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)02523-3 CCC: $12.00
species +-Me
SiMe3
SiMe2+-Me SiMe+-Me Si+-Me
BDE (eV) 0.48 ( 0.04 0.67 ( 0.06 0.72 ( 0.06 0.67 ( 0.05 0.70 0.69 ( 0.06 5.32 ( 0.28 1.27 ( 0.50 4.29 ( 0.06
ref a
PEPICO direct thermochemicalb this study, CID this study, ZTRID this study, ab initio calculation this study, overall best assignment this study, CID this study, CID ref 46, revised (see text)
a Difference between IE(SiMe +) (ref 3) and AE (SiMe )+ (ref 2). 4 3 Thermochemical bond strength using the present best values for ∆fH°0 (SiMe4+) and ∆fH°0(SiMe3+) (Table 2). The heat of formation values given in ref 3 yield a BDE of 0.64 eV. b
for the reasons mentioned above. Several dissociation energy values are listed in Table 1. An early photoionization appearance measurement1 gives a dissociation energy of about 0.25 eV; however, this result is ambiguous because of the difficulty of accounting for thermal energy effects. A later PEPICO study2 apparently carefully considered the thermal energy effects and obtained a 0 K appearance energy for SiMe3+ of 10.28 ( 0.01 eV, which can be combined with the ionization energy of TMS, 9.80 ( 0.04 eV,1 to give a (0 K) dissociation energy of 0.48 ( 0.09 eV. However, an indirect thermochemical calculation, using the literature values for ∆fH°0(SiMe4+) (743 ( 5 kJ mol-1, ref 3) and ∆fH°0 (SiMe3+)(656 kJ mol-1, ref 2, 3), gives a value of 0.64 eV. (Actually a minor revision of these literature heats of formation seems appropriate, and our best values are collected and justified in Table 2. With these values, the calculated thermochemical dissociation energy becomes 0.67 ( 0.06 eV, as indicated in Table 1.) For such a weakly bound ion, an error of 0.19 eV is an important fraction of the total bond strength. This discrepancy may reflect the problems with dissociative ionization measurements noted above. Thus, this dissociation offers a case where new approaches to measuring dissociation © 1996 American Chemical Society
19660 J. Phys. Chem., Vol. 100, No. 50, 1996 TABLE 2: Heats of Formation at 0 and 298 K (kJ mol-1) species
∆fH°0
∆fH°298a
Me• SiMe4 Si+ SiMe+ SiMe2+ SiMe3+ SiMe4+
149.8 ( 0.4 -202 1236 ( 3 972 ( 7 1014 ( 30 656.3 ( 3.6 741 ( 5b
146.4 ( 0.4 -233.2 ( 3.0 1246 ( 3 635.7 ( 3.6 719 ( 5c
ref or source ref 48 ref 3 ref 49 ref 21, revised (see text) this study, CID ref 2 derived from values in ref 3
a All cation values use the thermal electron convention. For comparison, 298 K cation values from refs 2 and 3 need to be converted from the ion convention by adding 6.2 kJ mol-1, as has been done in the values tabulated here. b Corrected from the 298 K value using the MP2 ab initio vibrational frequencies of Table 3, which give a vibrational enthalpy of 20.0 kJ mol-1 for SiMe4+ at 298 K. c Combining ∆fH°298(TMS) for the neutral with the ionization energy of 9.80 ( 0.04 eV from ref 1.
energies of weakly-bound ions can contribute to resolving a contradictory thermochemical picture. Threshold CID measurements of dissociation energies have been applied to a number of systems, such as proton-bound water clusters4 and organometallics which can include virtually any metal (or a cluster of metals)5 and ligands ranging from familiar species such as CO,6-12 H2O,13-17 NO,9 alkenes,18,19 and benzene20,21 to unusual but potentially important ligands such as N2,9 CH4,15,22 and other alkanes.23,24 Extensive comparisons with thermochemistry obtained by other methods have begun to establish threshold CID as a proven technique.7,9,10,12,20,22,24,25 The most difficult aspect of quantifying this approach for measurement of accurate bond strengths is that the shape of the observed CID cross section in the region of the energy threshold is partly determined by a collisional energy deposition function that is not easy to specify. Further distortions involve thermal and kinetic shift effects. These complications are now being overcome by a combination of high-quality threshold data obtained using good guided ion beam instrumentation and detailed understanding and application of the theoretical basis of the threshold shape to extract the intrinsic 0 K dissociation onset energy from the observed cross section data.7,25,26 ZTRID is a new approach to quantitative ion dissociation measurements which has been applied to only a few systems but shows promise for weakly bound ions. It is based on observing the dissociation of thermalized parent ions induced by the steady, accurately known flux of blackbody infrared radiation which fills the ion-trapping region of a thermally equilibrated vacuum chamber. Both the dissociation rate and its temperature dependence can be analyzed to give reliable values for the dissociation energy.27 This approach has been successfully applied to weakly bound cluster ions28 and to the tetraethylsilane ion.29 Its application here to the TMS ion is particularly interesting because of the opportunity to compare the results with other accurate techniques. Experimental Section Collision-Induced Dissociation. The CID experiments were performed on a guided-ion beam tandem mass spectrometer30,31 equipped with a flow tube ion source. The ions are generated as described below, extracted from the source, accelerated, and passed through a magnetic sector for mass analysis. The massselected ions are decelerated to the desired kinetic energy where they are focused into an octopole ion trap. This device uses radio-frequency electric fields to trap the ions in the radial direction and ensure complete collection of reactant and product ions.32 The octopole passes through a gas cell that contains
Lin et al. the neutral reaction partner at a pressure sufficiently low (0.050.1 mTorr) that multiple ion-molecule collisions are improbable. It was verified that the results presented here exhibit no dependence on pressure and thus correspond to single ionmolecule collisions at these pressures. The unreacted parent and product ions drift to the end of the octopole where they are extracted, passed through a quadrupole mass filter for mass analysis, and detected with a secondary electron scintillation ion detector using standard pulse counting techniques. The raw ion intensities are converted to cross sections, as described previously.30 We estimate our absolute cross sections to be accurate to (20%. Laboratory (lab) energies are converted to energies in the center-of-mass (CM) frame by using the conversion ECM ) ElabM/(M + m), where m and M are the ion and neutral masses, respectively. The absolute energy scale and corresponding full width at half-maximum (fwhm) of the ion-beam kinetic energy distribution are determined by using the octopole as a retarding energy analyzer as described previously.30 The absolute uncertainty in energy scale is (0.05 eV (lab). The energy distributions are nearly Gaussian and have a typical fwhm of 0.3-0.5 eV (lab). Ion Source. SiMe4+ and SiMe3+ ions are made in our flowtube ion source, described in detail previously.33 The carrier gas is passed through a trap filled with liquid nitrogen to remove impurities and is introduced through a microwave discharge, mounted at the end of the flow tube, at a flow rate of about 8000 sccs (standard cubic centimeters per second), leading to a pressure of about 0.6-0.7 Torr. The source creates He+ and He* metastable states that ionize about 1-2 mTorr of tetramethylsilane which is introduced about 60 cm downstream of the microwave discharge source. Under the high-pressure conditions used in this source, the ions undergo >104 collisions with the flow gas and should therefore be thermalized to 300 K, the temperature of the flow gas. No evidence for excited species is found in the present studies. Beam intensities for the SiMe4+ reactant ion were about 5 × 105 s-1 and those for SiMe3+ were over 3 × 106 s-1. These intensities are clearly consistent with a SiMe4+ complex that decomposes easily to the SiMe3+ ion. Intensities for the SiMe2+ and SiMe+ ions were
