Threshold Photoelectron Photoion Coincidence Spectroscopy Research Articles

Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy.

The unimolecular isomerisation of the prompt propargyl + propargyl "head-to-head" adduct, 1,5-hexadiyne, to fulvene and benzene by the 3,4-dimethylenecyclobut-1-ene (DMCB) intermediate (all C6H6) was studied in the high-pressure limit by threshold photoelectron (TPE) spectroscopy. TPE spectra (TPES) were recorded with photoelectron photoion coincidence spectroscopy using synchrotron vacuum ultraviolet radiation. Reference TPES, obtained using pure compounds or judiciously extracted from the pyrolysis data, served as basis functions for pyrolysis quantification. From these spectra, we measured a revised fulvene ionisation energy of 8.401 ± 0.005 eV. Temperature-dependent pyrolysis spectra were decomposed using these basis functions. The basis function coefficients were converted to product yields relying on assumed integral threshold photoionisation cross sections obtained by three, partially mutually exclusive sets of assumptions. Thus, the product yields of DMCB, fulvene, and benzene have been established, as well as their uncertainty. The derived mole fractions are consistent with modeling based on the C6H6 potential and RRKM master equation model of Miller and Klippenstein [J. Phys. Chem. A, 2003, 107, 7783]. Although our results are fully consistent with the parallel isomerisation pathways to benzene and fulvene found by Miller and Klippenstein, we observe the onset of fulvene at a lower temperature than that of the onset of benzene, in agreement with the master equation model but in contrast to the previous experiments of Stein et al. [Proc. Combust. Inst., 1990, 23, 85]. This work promotes the use of photoion mass-selected threshold photoelectron spectroscopy as a rapid, sensitive, isomer-selective, and quantitative detection tool among the panoply of established analytical techniques.

Photoionization studies of reactive intermediates using synchrotron radiation.

Photoionization of reactive intermediates with synchrotron radiation has reached a sufficiently advanced stage of development that it can now contribute to a number of areas in gas-phase chemistry and physics. These include the detection and spectroscopic study of reactive intermediates produced by bimolecular reactions, photolysis, pyrolysis or discharge sources, and the monitoring of reactive intermediates in situ in environments such as flames. This review summarises advances in the study of reactive intermediates with synchrotron radiation using photoelectron spectroscopy (PES) and constant-ionic-state (CIS) methods with angular resolution, and threshold photoelectron spectroscopy (TPES), taking examples mainly from the recent work of the Southampton group. The aim is to focus on the main information to be obtained from the examples considered. As future research in this area also involves photoelectron-photoion coincidence (PEPICO) and threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, these methods are also described and previous related work on reactive intermediates with these techniques is summarised. The advantages of using PEPICO and TPEPICO to complement and extend TPES and angularly resolved PES and CIS studies on reactive intermediates are highlighted.

Ionic dissociation dynamics and energetics of hexamethyldigermanium, (CH3)6Ge2, by threshold photoelectron-photoion coincidence spectroscopy

Threshold Photoelectron-Photoion Coincidence Spectroscopy (TPEPICO) has been used to study the ionic dissociation dynamics and energetics of Me6Ge2 (Me=CH3). Ions are energy-selected and the 0K dissociation onsets for methyl and Me3Ge loss are obtained from the breakdown diagram and mass spectra distributions, both of them analyzed and modeled with statistical SSACM theory and quantum chemical (DFT, MP2) calculations. An updated value for the enthalpy of formation, ΔfHm0(g) of Me6Ge2, is used to derive ΔfHm0(Me3Ge,g)=70.8±8.9kJ·mol−1. The bond dissociation enthalpies, BDE (GeX), of Me3GeX (X=Me, GeMe3, Cl and Br) were also derived in this study.

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO).

The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed. Emphasis is placed on instrumentation and the extraction of dynamical information about energy selected ion dissociation, not on the detailed spectroscopy of certain molecules. Three important advances have expanded greatly the power of the technique, and permitted its implementation on modern synchrotron radiation beamlines. The use of velocity focusing of threshold electrons onto an imaging detector in the 1990s simultaneously improved the sensitivity and electron energy resolution, and also facilitated the subtraction of hot electron background in both threshold electron spectroscopy and TPEPICO studies. The development of multi-start multi-stop collection detectors for both electrons and ions in the 2000s permitted the use of the full intensity of modern synchrotron radiation thereby greatly improving the signal-to-noise ratio. Finally, recent developments involving imaging electrons in a range of energies as well as ions onto separate position-sensitive detectors has further improved the collection sensitivity so that low density samples found in a variety of studies can be investigated. As a result, photoelectron photoion coincidence spectroscopy is now well positioned to address a range of challenging problems that include the quantitative determination of compositions of isomer mixtures, and the detection and spectroscopy of free radicals produced in pyrolysis or discharge sources as well as in combustion studies.

Synchrotron threshold photoelectron photoion coincidence spectroscopy of radicals produced in a pyrolysis source: The methyl radical

We present here a flash pyrolysis source coupled with a threshold photoelectron photoion coincidence (TPEPICO) spectrometer and vacuum ultraviolet synchrotron radiation to investigate the spectroscopy and photochemistry of free radicals. The radicals are produced from pyrolysis in a heated silicon carbide tube, and the TPEPICO scheme provides a strategy to obtain pure spectra of the radicals without contamination from other byproducts. As a representative example, the methyl radical was studied, and its threshold photoelectron spectrum shows a series of umbrella vibrational transitions. The adiabatic ionization energy of the methyl radical was determined to be 9.84±0.01eV.

Rotamers and Migration: Investigating the Dissociative Photoionization of Ethylenediamine.

The unimolecular dissociation of energy-selected ethylenediamine cations was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) in the photon energy range of 8.60-12.50 eV. Modeling the breakdown diagram and time-of-flight distributions with rigid activated complex RRKM theory yielded 0 K appearance energies for eight dissociation channels, leading to NH2CHCH2(+)(•) at 9.120 ± 0.010 eV, CH3C(NH2)2(+) at 9.200 ± 0.012 eV, NH2CHCH3(+) at 9.34 ± 0.08 eV, CH2NH2(+) at 9.449 ± 0.025 eV, CH2NH3(+) at 9.8 ± 0.1 eV, c-C2H4NH2(+) at 10.1 ± 0.1 eV, CH3NHCHCH2(+) at 10.2 ± 0.1 eV, and the reappearance of CH2NH2(+) at 10.2 ± 0.1 eV. The CBS-QB3-calculated pathways highlighted the influence of intramolecular hydrogen attractions on the dissociation processes, presenting novel isomers and low-energy van der Waals intermediates that led to fragments in good agreement with experimental results. While most of the dissociation channels take place through reverse barriers, the 0 K heat of formation of (•)CH2NH2 was determined to be 147.6 ± 3.7 kJ mol(-1), in excellent agreement with literature, and the 0 K heat of formation of CH2NH3(+) at 844 ± 10 kJ mol(-1) is the first experimentally measured value available and is in good agreement with theory.

Theoretical Study of the Dissociation Energy of First-Row Metallocenium Ions.

The bond dissociation energy of a series of metallocenium ions, i.e., the energy difference of the reaction MCp2(+) → MCp(+) + Cp· (with M = Ti, V, Cr, Mn, Fe, Co, and Ni), was studied by means of multiconfigurational perturbation theory (CASPT2, RASPT2, NEVPT2) and restricted coupled cluster theory (CCSD(T)). From a comparison between the results obtained from these different methods, and a detailed analysis of their treatment of electron correlation effects, a set of MCp(+)-Cp binding energies are proposed with an accuracy of 5 kcal/mol. The computed results are in good agreement with the experimental data measured by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy but disagree with the more recent threshold collision-induced dissociation (TCID) experiments.

Photoionization and pyrolysis of a 1,4-azaborinine: retro-hydroboration in the cation and identification of novel organoboron ring systems.

The photoionization and dissociative photoionization of 1,4-di-tert-butyl-1,4-azaborinine by means of synchrotron radiation and threshold photoelectron photoion coincidence spectroscopy is reported. The ionization energy of the compound was determined to be 7.89 eV. Several low-lying electronically excited states in the cation were identified. The various pathways for dissociative photoionization were modeled by statistical theory, and appearance energies AE0K were obtained. The loss of isobutene in a retro-hydroboration reaction is the dominant pathway, which proceeds with a reverse barrier. Pyrolysis of the parent compound in a chemical reactor leads to the generation of several yet unobserved boron compounds. The ionization energies of the C4 H6 BN isomers 1,2- and 1,4-dihydro-1,4-azaborinine and the C3 H6 BN isomer 1,2-dihydro-1,3-azaborole were determined from threshold photoelectron spectra.

An experimental and theoretical study on structural parameters and energetics in ionization and dissociation of cobalt tricarbonyl nitrosyl

In a combined theoretical and experimental study we calculate the geometrical parameters, dipole moments and rotational constants for the two lowest spin states of Co(CO)3NO, i.e., the 1A1 and 3A states of the neutral as well as their positively charged counterparts; the 2A′ and 4A″ states of [Co(CO)3NO]+. The geometrical parameters are also calculated for the two lowest spin states of all possible (Co containing) positively charged fragments, resulting from ligand loss through dissociative ionization (DI) of Co(CO)3NO. From the lowest total energies resulting from the geometrical optimizations, we calculate the ionization energy for Co(CO)3NO and the threshold energies for the two lowest spin states of all possible fragments, as well as the respective bond dissociation energies (BDE) describing the sequential ligand loss. Further, through electron bombardment, we determine the ionization energy for Co(CO)3NO, the appearance energies (AEs) of all DI products observed and the BDE for individual ligand loss. Finally, the theoretical and experimental IEs, AEs and BDEs derived are compared and discussed in context to earlier electron ionization and threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) studies.

Metal–Carbonyl Bond Energies in Phosphine Analogue Complexes of Co(CO)3NO by Photoelectron Photoion Coincidence Spectroscopy

Threshold photoelectron photoion coincidence spectroscopy was used to study a series of cobalt–organic complexes with phosphine and phosphine analogue ligands: PMe3Co(CO)2NO, PEt3Co(CO)2NO, AsMe3Co(CO)2NO, and SbMe3Co(CO)2NO. The two lowest energy dissociative photoionization channels were sequential carbonyl losses in all four cases. Nitrosyl loss was also observed as a minor channel from the molecular ion and as a major competitive dissociation from the first (carbonyl loss) daughter ion. Further sequential CO and NO losses lead to the LCo+ (L = PMe3, PEt3, AsMe3, SbMe3) ions, which, similarly to an earlier threshold collision-induced dissociation (TCID) mass spectrometry study on the phosphine complexes,(1) exhibited parallel ethene loss and methane loss dissociation reactions, although the bare metal ion was not observed. Unimolecular statistical rate theory (RRKM) calculations were performed to model the first two carbonyl loss channels and relate cobalt–carbonyl bond energy trends to the electron donor and acceptor properties of the phosphine analogue ligands. Co–CO bond energies of 0.90 ± 0.09, 0.84 ± 0.08, 1.13 ± 0.08, and 1.15 ± 0.09 eV were obtained in LCo(CO)2NO+ (L = PMe3, PEt3, AsMe3, SbMe3, respectively) and 0.82 ± 0.11, 0.74 ± 0.11, 0.95 ± 0.10, and 0.94 ± 0.09 in the first daughter ions, respectively.

Tunneling in H loss from energy selected ethanol ions

The H/D loss and CH(3)/CD(3) loss reactions from energy selected ethanol isotopologue ions C(2)H(5)OH(+), C(2)D(5)OD(+), CD(3)CH(2)OH(+), and CH(3)CD(2)OH(+) have been studied by imaging threshold photoelectron photoion coincidence (iPEPICO) spectroscopy. In the lowest energy dissociation channel, the α-carbon loses a hydrogen or a deuterium atom. Asymmetry in the daughter ion time-of-flight (TOF) peaks, an ab initio study of the reaction rates, and shifts in the phenomenological onsets between isotopologues revealed that H/D loss is slow at its onset. Tunneling through a reverse barrier along the reaction coordinate was found to play a significant role. Modeling the data with an Eckart barrier suggests that H loss from light ethanol ions proceeds via a reverse barrier of 151 meV, which agrees very well with the ab initio result of 155 meV. The higher energy methyl loss channel appears at its thermochemical threshold, but the branching ratios for methyl and H loss as a function of the ion internal energy are not entirely consistent with statistical theory. The methyl-loss signal cannot completely outcompete the hydrogen atom loss process. The shape of the photoelectron spectrum as well as our calculations indicate that the lowest energy ethanol ion structure lies considerably below the reported IE of 10.48 eV. Franck-Condon factors are favorable for ionization to a metastable ion state, which can rearrange to a more stable equilibrium structure. Combining theoretical results with previous experimental ones yields a revised ethanol adiabatic ionization energy of 10.37 eV. This applies to all isotopologues, as the isotope effect on the ionization energy is not more than a few meV.

Binding Energies and Isomerization in Metallocene Ions from Threshold Photoelectron Photoion Coincidence Spectroscopy

Metallocene ions (Cp(2)M(+), M = Cr, Co, Ni) were studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) to investigate the mechanism, energetics, and kinetics of the ionic dissociation processes. The examined energy-selected Cp(2)M(+) ions fragment by losing the neutral cyclopentadienyl ligand. In addition, CH and C(2)H(2) losses appear as minor channels, while the cobaltocene ion also loses an H atom. A possible isomerization pathway has also been observed for Cp(2)Ni(+), yielding a complex with pentafulvalene (C(10)H(8)) with a loss of H(2). In order to determine the 0 K appearance energies for the CpM(+) fragment ions, the asymmetric time-of-flight peak shapes and the breakdown diagrams of the energy-selected metallocene ions were modeled by both the rigid activated complex (RAC) Rice-Ramsperger-Kassel-Marcus (RRKM) theory and the simplified statistical adiabatic channel model (SSACM). The following appearance energies were obtained with SSACM, which is more reliable for loose transition states: 10.57 ± 0.14, 11.01 ± 0.13, and 10.18 ± 0.13 eV for M = Cr, Co, and Ni, respectively. These values combined with the corresponding adiabatic ionization energies yield M-Cp bond dissociation energies in Cp(2)M(+) ions of 5.04 ± 0.16, 5.77 ± 0.15, and 3.96 ± 0.15 eV. Density functional calculations at the B3LYP/6-311G(d,p) level of theory were used to determine the structures of these complexes and to provide parameters necessary for the analysis of the experimental data. The trends in the M-Cp bond energies can be related to the electronic structures of the metallocene ions based on a simple molecular orbital picture.

Tunneling in a Simple Bond Scission: The Surprising Barrier in the H Loss from HCOOH+

The dissociation dynamics of gas phase formic acid ions (HCOOH(+), DCOOD(+), HCOOD(+), DCOOH(+)) are investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy and high level ab initio calculations. The slow rate constants for this seemingly simple H loss reaction and the large onset energy shifts due to isotopic substitution point to a substantial exit barrier through which the H or D atoms tunnel. Modeling of the HCOOH(+) experimental data using RRKM theory with tunneling through an Eckart potential are best fitted with a barrier of about 17 kJ mol(-1). High level ab initio calculations support the experimental findings with a computed barrier of 15.9 kJ mol(-1), which is associated with the substantial geometry change between the product HOCO(+) cation and the corresponding HCOOH(+) molecular ion. Because of this exit channel barrier, the formic acid ion dissociation does not provide a route for determination of the HOCO(+) heat of formation. Rather, the most accurate value comes from the calculations employing the high accuracy extrapolated ab initio thermochemistry (HEAT) scheme, which yields a Δ(f)H(o)(0K)[HOCO(+)] = 600.3 ± 1.0 kJ mol(-1) (Δ(f)H(o)(298K)[HOCO(+)] = 597.3 ± 1.0 kJ mol(-1)). The calculated proton affinity of CO(2) is thus 534.7 ± 1.0 kJ mol(-1) at 0 K and 539.3 ± 1.0 kJ mol(-1) at 298.15 K.

Dissociation of Energy-Selected 1,1-Dimethylhydrazine Ions

The unimolecular dissociation of 1,1-dimethylhydrazine ions was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Time-of-flight distributions and breakdown curves were recorded in the photon energy range of 9.5-10.4 eV. The 0 K appearance energies of the fragment ions were extracted by modeling the experimental data with rigid activated complex (RAC-) RRKM theory. It was found that the data could be well-reproduced with a single TS for each dissociation channel if two different H-loss channels were assumed, one corresponding to a C-H and the other to a N-H bond dissociation. Once the appearance energies were established, heats of formation of the fragment ions could be derived. The heat of formation of the neutral molecule was computed by applying composite ab initio methods (G3, CBS-APNO, W1U) on a series of isodesmic reactions between methyl hydrazines and methyl amines.

Self-Consistent Heats of Formation for the Ethyl Cation, Ethyl Bromide, and Ethyl Iodide from Threshold Photoelectron Photoion Coincidence Spectroscopy

The dissociative photoionization onsets of Br and I loss reactions were measured for C(2)H(5)Br and C(2)H(5)I, respectively, by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy to establish the heats of formation of these two basic ethyl halides. The appearance energy of ethyl cation from ethyl bromide was found to be 1074.2 +/- 0.8 kJ mol(-1), and that from ethyl iodide was found to be 1016.4 +/- 0.8 kJ mol(-1). The heats of formation of ethyl bromide and ethyl iodide are interconnected through the ethyl cation. In establishing the thermochemistry of the ethyl halides, the ethyl cation heat of formation was concluded to be 915.5 +/- 1.3 kJ mol(-1) on the basis of a recent value for ethyl radical heat of formation and the well-established ionization energy and on the basis of ab initio isodesmic calculations using recent PEPICO data. Using this anchor, we obtained the following heats of formation: Delta(f)H(0K) degrees[EtBr] = -40.8 +/- 1.5 kJ mol(-1) and Delta(f)H(0K) degrees[EtI] = 6.3 +/- 1.5 kJ mol(-1). These results are more consistent with the higher EtBr heats of formation values in the literature, contrary to recent findings. For ethyl iodide, the latest calorimetric value does not agree within the claimed accuracy.

Threshold photoelectron photoion coincidence spectroscopy sheds light on the dissociation of pyrrole and thiophene molecular ions

Threshold photoelectron photoion coincidence spectroscopy was employed to examine the loss of ethyne from ionized pyrrole and thiophene near their dissociation thresholds as a function of internal energy and reaction time. The B3-LYP/6-311+G(3df,3pd) minimum energy reaction pathways for ethyne loss from both ions are presented. The experimental molecular ion breakdown curves were modeled with RRKM theory. For ionized pyrrole, the fitting results in confirmation of the transition state energies in the decomposition pathways. For ionized thiophene, the modeling suggests that ethyne loss is a concerted process rather than a true step-wise reaction. If so, the fitting yields a Δ fH 298 for CH 2CS + of 1033 ± 2 kJ mol −1.

Does Tetrahydrofuran Ring Open upon Ionization and Dissociation? A TPES and TPEPICO Investigation

The threshold photoelectron spectrum (TPES) of tetrahydrofuran (THF) is compared to that of the unsaturated furan molecule. In general, there is a similarity in the orbital ionization profile for the two species, though unlike furan, THF exhibits (modest) vibrational detail only in the (9b)(-1) X (2)B band. An adiabatic ionization energy of 9.445 +/- 0.010 eV has been derived from the onset of the TPES spectrum. Threshold photoelectron photoion coincidence spectroscopy was used to explore the loss of a hydrogen atom from ionized THF over the photon energy range of 9.9-10.4 eV. RRKM fitting of the resulting breakdown curves yields an E(0) of 0.85 +/- 0.03 eV (82 +/- 3 kJ mol(-1)) (AE = 10.30 +/- 0.04 eV). If the G3 IE of 9.48 eV is used to convert the experimental data from photon energy to THF ion internal energy, E(0) = 0.81 +/- 0.01 eV (78 +/- 1 kJ mol(-1)). The latter value is closer to the G3 E(0) of 72 kJ mol(-1) for the formation of the cyclic ion 1. A variety of ring-opening reactions were also probed at the B3-LYP/6-31+G(d) and G3 levels of theory. The distonic isomer (*)CH(2)CH(2)CH(2)OCH(2)(+) lies 70 kJ mol(-1) higher than ionized THF, which places it within 1 kJ mol(-1) of the threshold for the dissociation to 1. All of the probed H-loss products from the distonic isomer (which includes singlet and triplet species) lie significantly higher in energy than ion 1, eliminating the possibility that ionized THF dissociates to m/z 71 via a ring-opening reaction in the present experiment. The derived Delta(double dagger)S value for the dissociation, 8 +/- 5 J K(-1) mol(-1), is also consistent with the formation of 1. The experimentally derived E(0) values can be used to derive the Delta(f)H(o)(0) for ion 1. Together with the Delta(f)H(o)(0) values for the THF ion (752.0 +/- 2 kJ mol(-1), derived from the neutral Delta(f)H(o)(0) of -154.9 +/- 0.7 kJ mol(-1) and experimental IE of 9.445 +/- 0.010 eV) and H atom (218.5 kJ mol(-1)) our E(0) of 82 +/- 3 kJ mol(-1) yields a Delta(f)H(o)(0) for ion 1 of 620 +/- 4 kJ mol(-1) (Delta(f)H(o)(298) = 594 +/- 4 kJ mol(-1)), in good agreement with the G3 Delta(f)H(o)(0) of 621 kJ mol(-1). Appearance energies for all fragment ions up to photon energies of 34 eV are also reported and discussed in comparison with the available literature.

Heat of Formation of the Allyl Ion by TPEPICO Spectroscopy

The 0 K onset of C(3)H(6) --> C(3)H(5)(+) + H(*) is measured by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. From the onset (11.898 +/- 0.025 eV) the heat of formation of the allyl ion (CH(2)CHCH(2)(+)) is determined to be DeltaH degrees (f,0K) = 967.2; DeltaH degrees (f,298K) = 955.4 +/- 2.5 kJ mol(-1). The value is significantly more positive than prior determinations, and resolves a discrepancy between measurements of the allyl radical and allyl ion heats of formation and recent highly precise measurements of the allyl radical adiabatic ionization energy. The new allyl ion heat of formation leads to a new proton affinity for propadiene (allene) of 765.0 +/- 2.6 kJ mol(-1). An attempt is made to determine the CH(3)CCH(2)(+) heat of formation by measuring the 0 K onset of 2-ClC(3)H(5) --> C(3)H(5)(+) + Cl(*). However, C(3)H(5)(+) appears at too low an energy to be the higher energy CH(3)CCH(2)(+) structure. Rather, 2-ClC(3)H(5)(+) undergoes a concerted hydrogen transfer and Cl-loss via an intramolecular S(N)2 like mechanism to produce the allyl ion. The 0 K onset of 3-ClC(3)H(5) --> C(3)H(5)(+) + Cl(*) (11.108 +/- 0.010 eV) is measured to determine the 3-ClC(3)H(5) heat of formation (DeltaH degrees (f,0K) = 14.9; DeltaH degrees (f,298K) = 1.1 +/- 2.7 kJ mol(-1)). 3-ClC(3)H(5)(+) is suggested to readily isomerize to trans 1-ClC(3)H(5)(+) prior to dissociation.

Experimental Thermochemistry of SiCl3R (R = Cl, H, CH3, C2H5, C2H3, CH2Cl, SiCl3), SiCl3+, and SiCl3•

The 0 K onsets (E(0)) of a series of trichlorosilane derivatives SiCl(3)R --> SiCl(3)(+)+ R(*) (R = Cl, H, CH(3), C(2)H(5), C(2)H(3), CH(2)Cl, SiCl(3)) are measured by threshold photoelectron-photoion coincidence spectroscopy. The well-known heat of formation of SiCl(4) is used as an anchor to determine the heat of formation of SiCl(3)(+), which is, in turn, used as an anchor to determine the heats of formation of the other alkyltrichlorosilanes investigated. A series of isodesmic reactions at the G3 and CBS-QB3 levels are shown to accurately reproduce the experimental heats of formation, and this scheme is used to calculate the heat of formation of Si(2)Cl(6), from which the measured E(0) determines the SiCl(3)(*) heat of formation. The measured values then determine the IE of SiCl(3)(*) along with the Si-R bond dissociation enthalpies of the six neutral species investigated. The experimental heats of formation are also used in a series of isodesmic reaction calculations to determine the heats of formation of SiH(3)R (R = H, CH(3), C(2)H(5), C(2)H(3), CH(2)Cl, SiCl(3)).

Dissociative Photoionization of X(CH3)3 (X = N, P, As, Sb, Bi): Mechanism, Trends, and Accurate Energetics

Threshold photoelectron photoion coincidence spectroscopy is used to study the dissociation of energy-selected X(CH(3))(3)(+) ions (X = As, Sb, Bi) by methyl loss, the only process observed up to 2 eV above the ionization energy. The ion time-of-flight distributions and the breakdown diagrams are analyzed in terms of the statistical RRKM theory to obtain accurate ionic dissociation energies. These experiments complement previous studies on analogous trimethyl compounds of the N group where X = N and P. However, trimethylamine was observed to lose only an H atom, whereas trimethylphosphine was shown to lose methyl radical, H atom, and, to a lesser extent, methane in parallel dissociation reactions. Both kinetic and thermodynamic arguments are needed to explain these trends. The methyl radical loss has two channels: either a H transfer to the central atom, followed by CH(3) loss, or a direct homolytic bond cleavage. However, the H transfer channel is blocked in trimethylamine by an H loss channel with an earlier onset, and, thus, the methyl loss is not observed. Bond energies are defined based on ab initio reaction energies and show that the main thermodynamic reason behind the trends in the energetics is the significantly weakening C=X double bond in the ion in the N --> As direction. The first adiabatic ionization energies of Sb(CH(3))(3) and Bi(CH(3))(3) have also been measured by ultraviolet photoelectron spectroscopy to be 8.02 +/- 0.05 and 8.08 +/- 0.05 eV, respectively.