Dissociation kinetics of energy-selected Cp(2)Mn(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the manganocene ion, Cp(2)Mn(+) (Cp = eta(5)-cyclopentadienyl). The Cp loss reaction was found to be extremely slow over a large ion internal energy range. By simulating the measured asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K thermochemical dissociation limit for CpMn(+) production was determined to be 9.55 +/- 0.15 eV. A CpMn(+)-Cp bond energy of 3.43 eV was obtained by combining this CpMn(+) + Cp dissociation limit with the Cp(2)Mn adiabatic ionization energy of 6.12 +/- 0.07 eV. Combining the measured onset with known heats of formation of Cp and Mn(+), the Cp-Mn(+) bond energy was determined to be 3.38 +/- 0.15 eV. These results lead to 298 K heats of formation of Cp(2)Mn(+) and CpMn(+) of 863 +/- 7 and 935 +/- 16 kJ/mol, respectively. Finally, by combining these results with a previous measurement of the CpMn(CO)(3) --> CpMn(+) + 3CO + e(-) dissociation limit, we arrive at a new value for Delta(f)H degrees (298K)(CpMn(CO)(3)) of -424 +/- 17 kJ/mol.     

Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH(3)Cl(+) ions was investigated in the excitation energy range of 11.0-18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH(3)(+) dissociated from CH(3)Cl(+)(A(2)A(1) and B(2)E) ions were recorded. CH(3)(+) was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH(2)Cl(+) fragment was very low. For dissociation of CH(3)Cl(+)(A(2)A(1)) ions, a series of homocentric rings was clearly observed in the CH(3)(+) image, which was assigned as the excitation of umbrella vibration of CH(3)(+) ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH(3)(+)(1(1)A') provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH(3)Cl(+)(B(2)E) ions, total kinetic energy released distribution for CH(3)(+) fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B(2)E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH(3)Cl(+), CH(3)(+) formation from CH(3)Cl(+)(A(2)A(1)) ions was a rapid direct fragmentation, while CH(3)Cl(+)(B(2)E) ions statistically dissociated to CH(3)(+) + Cl via internal conversion to the high vibrational states of X(2)E.     

The dissociation kinetics of energy-selected CpMn(CO)(3)(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the cyclopentadienyl manganese tricarbonyl ion, CpMn(CO)(3)(+). The ionization energy of CpMn(CO)(3) was measured from the threshold photoelectron spectrum to be 7.69 +/- 0.02 eV. The dissociation of the CpMn(CO)(3)(+) ion proceeds by the sequential loss of three CO molecules. The first and third CO loss reactions were observed to be slow (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, 0 K onsets for three product ions were determined to be 8.80 +/- 0.04, 9.43 +/- 0.04, and 10.51 +/- 0.06 eV, respectively. Combined with the adiabatic ionization energy, the three successive Mn-CO bond energies in the CpMn(CO)(3)(+) were found to be alternating with values of 1.11 +/- 0.04, 0.63 +/- 0.04, and 1.08 +/- 0.06 eV, respectively. Using a scaled theoretical Cp-Mn(+) bond energy of 3.10 +/- 0.10 eV and the combined results from theory and experiment, the 298 K gas-phase heat of formation of CpMn(CO)(3) is suggested to be -419 +/- 15 kJ/mol. Based on this value, the 298 K heats of formation of CpMn(CO)(3)(+), CpMn(CO)(2)(+), CpMnCO(+), and CpMn(+) are 325 +/- 15, 546 +/- 15, 719 +/- 15, and 938 +/- 15 kJ/mol, respectively. By scaling theoretical calculated neutral bond energies with the experimental information derived in this study, the successive Mn-CO bond energies were estimated to be 1.88, 1.10, and 1.03 eV, respectively, while the Cp-Mn bond energy was found to be 2.16 eV. Comparison between the quantum chemical calculations and experimental values shows that the loss of CO groups follows the lowest energy adiabatic path, in which electronic spin on the metal center is not conserved.     

Dissociation dynamics and thermochemistry of tin species, (CH3)4Sn and (CH3)6Sn2, by threshold photoelectron-photoion coincidence spectroscopy

The dissociative photoionization of tetramethyltin (Me?Sn) and hexamethylditin (Me?Sn?) 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. Me?Sn(+) dissociates rapidly by methyl loss, with a 0 K onset of E? = 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 Me?Sn(?) 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 Me?Sn and Me?Sn? are used to derive the following 298.15 K gas-phase standard heats of formation, in kJ·mol?1: Δ(f)H(m)(o)(Me?Sn(+),g) = 746.3 ± 2.9; Δ(f)H(m)(o)(Me?Sn?(+),g) = 705.1 ± 7.5; Δ(f)H(m)(o)(Me?Sn(?),g) = 116.6 ± 9.7; Δ(f)H(m)(o)(Me?Sn,g) = 123.0 ± 16.5; Δ(f)H(m)(o)(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(Me?Sn-Me) = 284.1 ± 9.9; BDE(Me?Sn-SnMe?) = 252.6 ± 14.8.     

The decay of 1-chloropropyne cation studied by photoelectron-photoion coincidence spectroscopy

The decay of internal energy selected 1-chloropropyne cations is investigated using the fixed wavelength (He-Iα) photoelectron-photoion coincidence technique. The breakdown curves of the molecular ion and the C₃H₂Cl⁺, C₃HCl⁺, CCl⁺, C₃H⁺₃, C₃H⁺₃, C₃H⁺ fragment ions are reported. For 1-chloropropyne cations initially formed in their $\text{A}\text{?}$²E state it is found that four fragmentation channels compete with a non-dissociative relaxation pathway. The average kinetic energies released on formation of C₃H⁺₃ and C₃H⁺₃ are deduced from the time-of-flight distributions of these fragment ions measured at different internal energies of the molecular ion. The coincidence data are supplemented by electron impact appearance energies. The obtained decay pattern of 1-chloropropyne cation is compared with the breakdown diagrams reported for the C₃H⁺₄ isomers, i.e. allene-, propyne- and cyclopropene cations.     

Dissociation of energy-selected c-C2H4S+ in a region 10.6-11.8 eV: threshold photoelectron-photoion coincidence experiments and quantum-chemical calculations

Dissociation of energy-selected c-C2H4S+ was investigated in a region of 10.6-11.8 eV with a threshold photoelectron-photoion coincidence technique and a synchrotron as a source of vacuum ultraviolet radiation. Branching ratios and average releases of kinetic energy in channels of formation of c-C2H4S+, CH3CS+, and HCS+ were obtained from well-resolved time-of-flight peaks in coincidence mass spectra. Measured average releases of kinetic energy for channel CH3CS+ + H of least energy are substantial and much greater than calculated with quasiequilibrium theory; in contrast, small releases of kinetic energy near the appearance onset for channel HCS+ + CH3 agree satisfactorily with statistical calculations. Calculations of molecular electronic structures and energetics of c-C2H4S+ and C2H3S+ isomers and various fragments and transition states were also performed with Gaussian 3 method to establish dissociation mechanisms. A predicted dissociation energy of 11.05 eV for c-C2H4S --> HCS+ + CH3 agrees with a linearly extrapolated threshold at 10.99+/-0.04 eV and a predicted dissociation mechanism that c-C2H4S+ isomerizes to CH3CHS+ before dissociating to HCS+ + CH3 supports the experimental results. The large releases of kinetic energy for channel CH3CS+ + H might result from a dissociation mechanism according to which c-C2H4S+ isomerizes to a local minimum CH3CSH+ and then dissociates through a transition state to form CH3CS+ + H.     

Ethene loss kinetics of methyl 2-methyl butanoate ions studied by threshold photoelectron-photoion coincidence: The enol ion of methyl propionate heat of formation

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl 2-methyl butanoate ions [CH₃CH₂CH(CH₃)COOCH₃^·+]. This ester ion isomerizes to a lower energy distonic ion [CH₂CH₂CH(CH₃)COHOCH₃^·+] prior to dissociating by the loss of C₂H₄. The asymmetric time of flight distributions, which arise from the slow rate of dissociation at low ion energies, provide information about the ion dissociation rates. By modeling these rates with assumed k(E) functions, the thermal energy distribution for room temperature sample, and the analyzer function for threshold electrons, it was possible to extract the dissociative photoionization threshold for methyl 2-methyl butanoate which at 0 K is 9.80 ± 0.01 eV as well as the dissociation barrier of the distonic ion of 0.86 ± 0.01 eV. By combining these with an estimated heat of formation of methyl 2-methyl butanoate, we derive a 0 K heat of formation of the distonic ion CH₂CH₂CH(CH₃)COHOCH₃^·+ of 101.0 ± 2.0 kcal/mol. The product ion is the enol of methyl propionate, CH₃CHCOHOCH₃^·+, which has a derived heat of formation at 0 K of 106.0 ± 2.0 kcal/mol.     

Metal-benzene and metal-CO bond energies in neutral and ionic C6H6Cr(CO)3 studied by threshold photoelectron-photoion coincidence spectroscopy and density functional theory

photoelectron-photoion coincidence spectroscopy and density functional theory calculations have been used to investigate the dissociation kinetics of the benzene chromium tricarbonyl ion, BzCr(CO)3+ (Bz = C6H6). The dissociation of the BzCr(CO)3+ ion proceeds by the sequential loss of three CO and benzene ligands. The first and third CO and the benzene loss reactions were associated with metastable precursor ions (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K appearance energies of the four product ions were determined to be 8.33 +/- 0.05, 8.93 +/- 0.05, 9.97 +/- 0.06, and 11.71 +/- 0.06 eV, respectively. Combined with the ionization energy of BzCr(CO)3, 7.30 +/- 0.05 eV, the three successive Cr-CO bond energies in the BzCr(CO)3+ were found to alternate, with values of 1.03 +/- 0.05, 0.60 +/- 0.05, and 1.04 +/- 0.05 eV, respectively, and the Bz-Cr bond energy in BzCr+ is 1.74 +/- 0.05 eV, a trend confirmed by the density functional theory (DFT) calculations. Using the heats of formation of the fully dissociated products, C6H6, Cr+, and CO, the 298 K heats of formation the ionic BzCr(CO)n+ (n = 03) species were determined. By scaling the DFT calculated bond energies for the neutral molecules, the heats of formation of the neutral BzCr(CO)n (n = 03) were also obtained.     

Thermochemistry and dissociative photoionization of Si(CH3)4, BrSi(CH3)3, ISi(CH3)3, and Si2(CH3)6 studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to study the dissociation kinetics and thermochemistry of Me(4)Si, Me(6)Si(2), and Me(3)SiX, (X = Br, I) molecules. Accurate 0 K dissociative photoionization onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distribution, both of them analyzed and simulated in terms of the statistical RRKM theory and DFT calculations. The average enthalpy of formation of trimethylsilyl ion, Delta fH(o)298K(Me(3)Si(+)) = 617.3 +/- 2.3 kJ/mol, has been determined from the measured onsets for methyl loss (10.243 +/- 0.010 eV) from Me(4)Si, and Br and I loss from Me(3)SiBr (10.624 +/- 0.010 eV) and Me(3)SiI (9.773 +/- 0.015 eV), respectively. The methyl loss onsets for the trimethyl halo silanes lead to Delta fH(o)298K(Me(2)SiBr(+)) = 590.3 +/- 4.4 kJ/mol and Delta fH(o)298K(Me(5)Si(2)(+)) = 487.6 +/- 6.2 kJ/mol. The dissociative photoionization of Me(3)SiSiMe(3) proceeds by a very slow Si-Si bond breaking step, whose rate constants were measured as a function of the ion internal energy. Extrapolation of this rate constant to the dissociation limit leads to the 0 K dissociation onset (9.670 +/- 0.030 eV). This onset, along with the previously determined trimethylsilyl ion energy, leads to an enthalpy of formation of the trimethylsilyl radical, Delta fH(o)298K(Me(3)Si(*)) = 14.0 +/- 6.6 kJ/mol. In combination with other experimental values, we propose a more accurate average value for Delta fH(o)298K(Me(3)Si(*)) of 14.8 +/- 2.0 kJ/mol. Finally, the bond dissociation enthalpies (DeltaH(298K)) Si-H, Si-C, Si-X (X=Cl, Br, I) and Si-Si are derived and discussed in this study.     

The study of ionic fragmentation by photoelectron-photoion coincidence spectroscopy

Since its invention, a decade ago, photoelectron-photoion coincidence spectroscopy has continuously provided extremely important information on ionic fragmentation processes. The outstanding reputation of the technique is essentially founded on the fact that internal energy selected cations can be studied. In striking contrast to the importance of the method and to the reliability of the data obtained, only about 150 coincidence studies have been published up to now. This is due to the rather intricate experimental technique involved and, probably even more, to the main drawback of this method, i.e. the enormous length of time involved in the measurements. The systems investigated so far are comprehensively compiled in the present article. The two experimental variants currently in use, termed fixed and variable wavelength technique, are compared. Evidence for the complementary character of the two experimental versions is presented. It is recalled that the primary source of information is in any case a time-of-flight distribution. The different ways of analysing these distributions are summarized and the accuracy of the derived results is discussed. The current state of the art is exemplified by reviewing the outcomes for a number of selected examples. The presented results on di-, tri- and tetra-atomic cations reveal clearly the more recently achieved experimental progress. Data on such smaller ionic species are particularly useful for studying predissociation mechanisms, a domain which becomes more and more accessible to modern ab initio calculations of fragmentation pathways. The reported results for O₂⁺ and CO₂⁺ reveal that branching ratios for the formation of electronically and vibrationally excited dissociation products can now be quantified for sufficiently small systems. The possibility of distinct behaviour of isoenergetic molecular ions depending on the method of preparation is outlined in the case of formaldehyde cation. The same cation is used to exemplify a puzzling isotope effect originating in a rate-determining radiationless transition that precedes a significantly faster dissociative step. A variety of data on larger systems grouped according to topic are critically reviewed. The determination and accuracy of rate-energy functions is discussed, at first for the case of a single decay channel; then the analogous analysis for a series of competing fragmentations is outlined. The importance of competitive radiative and non-radiative/dissociative depletion of an excited electronic state is pointed out.     

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

The multiphoton multichannel photodynamics of NO(2) has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and "slow" charged particle extraction using additional electron and ion optics. The NO(2) photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm. The multiphoton excitation produces both NO(2) (+) parent ions and NO(+) fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO(2) (+) or NO(+) and the (NO(+),e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the A (2)B(2) state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the NO(2) (+) parent ion. Furthermore, Rydberg states are believed to be used as "stepping" states responsible for the rather narrow and well-separated photoelectron spectra in the NO(2) (+) parent ion. Slow statistical and fast direct fragmentation of NO(2) (+) after prompt photoelectron ejection is observed leading to formation of NO(+)+O. Fragmentation from both the ground state and the electronically excited a (3)B(2) and b (3)A(2) states of NO(2) (+) is observed. At short pump probe delay times, the dominant multiphoton pathway for NO(+) formation is a 3x400 nm+1x266 nm excitation. At long delay times (>500 fs) two multiphoton pathways are observed. The dominant pathway is a 1x400 nm+2x266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3x400 nm photon absorption to NO(2) Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(A (2)Sigma,v=1) fragments, ionized by one 266 nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO(2). We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.     

Formation of multi-charged Kr ions through photoionization of 2p electrons studied with a coincidence technique

Multi-charged Kr ions have been measured using monochromatized undulator radiation combined with a coincidence technique. The coincidence measurements between multi-charged ions and energy-selected Auger electrons have clarified decay processes, as follows. The Auger final states formed through L₃M₄₅M₄₅ decays turn significantly into Kr⁴⁺ and those through L₃M₂₃M₄₅ decays generate Kr⁵⁺ mainly. The Auger decays of L₃M₂₃M₂₃ types yield Kr⁶⁺ dominantly. These findings are consistent with the consideration on energy levels of Kr ions.     

Dissociative photoionization and thermochemistry of dihalomethane compounds studied by threshold photoelectron photoion coincidence spectroscopy

The dissociative photoionization studies have been performed for a set of dihalomethane CH(2)XY (X,Y = Cl, Br, and I) molecules employing the threshold photoelectron photoion coincidence (TPEPICO) technique. Accurate dissociation onsets for the first and second dissociation limits have been recorded in the 10-13 eV energy range, and ionization potentials have been measured for these compounds. By using our experimental dissociation onsets and the known heat of formation of CH(2)Cl(2) molecule, it has been possible to derive the 0 and 298 K heats of formation of all six neutral dihalomethanes as well as their ionic fragments, CH(2)Cl(+), CH(2)Br(+), and CH(2)I(+), to a precision better than 3 kJ/mol. These new measurements serve to fill the lack of reliable experimental thermochemical information on these molecules, correct the old literature values by up to 19 kJ/mol, and reduce their uncertainties. From our thermochemical results it has also been possible to derive a consistent set of bond dissociation energies for the dihalomethanes.     

Heats of formation of HCCl3, HCCl2Br, HCClBr2, HCBr3, and their fragment ions studied by threshold photoelectron photoion coincidence

The dissociative photoionization onsets for Cl and Br loss reactions were measured for HCCl3, HCCl2Br, HCClBr2, and HCBr3 by threshold photoelectron photoion coincidence (TPEPICO) in order to establish the heats of formation of the mixed halides as well as the following fragment ions: HCCl2(+), HCClBr(+), HCBr2(+). The first zero Kelvin onsets were measured with a precision of 10 meV. The second onsets, which are in competition with the lower energy onsets, were established with a precision of 60 meV. Because both the chloroform and bromoform have relatively well established heats of formation, these measurements provide a route for establishing the heats of formation of the mixed halomethanes within uncertainties of less than 5 kJ mol(-1).     

Fragmentation of the valence electronic states of NF3 studied by threshold photoelectron–photoion coincidence spectroscopy

The valence threshold photoelectron spectrum of NF₃ is reported for the first time in the literature, and threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has measured, state-selectively, the decay dynamics of the valence states of NF₃⁺ in the range 13–23 eV. Vacuum–UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions and the breakdown diagram to be obtained. A comparison of the integrated threshold photoelectron and the total ion signals as a function of energy suggests that, in the range 16–19 eV, autoionisation via Rydberg states of NF₃ makes a significant contribution to the production of threshold electrons. The 50% crossover energy for production of NF₂⁺ from NF₃⁺ is determined to be 14.10±0.05 eV. The first onsets for NF₂⁺ and NF⁺ production are 13.95±0.05 and 17.6±0.1 eV, respectively. The majority of the Franck–Condon region of the ground state of NF₃⁺ is stable with respect to dissociation to NF₂⁺, whereas the unresolved states and most of the state dissociate exclusively to NF₂⁺. The and states dissociate to NF⁺. Translational kinetic energy releases have been measured in NF₂⁺ and NF⁺ at the energies of the Franck–Condon maxima of the valence states of NF₃⁺. The results are compared with models assuming statistical and impulsive dissociation. The Ã/ states of NF₃⁺ dissociate directly from the excited-state potential energy surface to NF₂⁺, whereas the higher-lying state probably dissociates off the ground-state surface following rapid internal conversion. It is not possible to correlate unambiguously the formation of NF⁺ with either F₂ or 2F, although on energetic grounds the latter products are more likely. Assuming that the neutral products are 2F, no information is obtained whether the two N–F bonds break simultaneously or sequentially.     

Dissociation dynamics of phenetole cations by photoelectron photoion coincidence

The dissociation rates of phenetole ions have been measured as a function of the ion internal energy by the method of photoelectron photoion coincidence spectrometry. The loss of ethylene to produce the phenol ion is the only dissociation pathway from its onset at 9.17 eV up to at least 12 eV. An activation energy of 1.64 ± 0.06 eV with an assumed activation entropy of + 1.8 ± 4 cal/mol-K is derived from fitting the statistical theory decay rates to the measured rates. The transition state energy lies ≈ 1 eV below the thermochemical dissociation limit to the C₂H⁺₅ + C₆H₅O· products. It is thus unlikely that an ion-radical complex is involved in the production of the phenol ion.     

Dissociative photoionization of mono-, di- and trimethylamine studied by a combined threshold photoelectron photoion coincidence spectroscopy and computational approach

Energy selected mono-, di- and trimethylamine ions were prepared by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Below 13 eV, the main dissociative photoionization path of these molecules is hydrogen atom loss. The ion time-of-flight (TOF) distributions and breakdown diagrams for H loss are analyzed in terms of the statistical RRKM theory, which includes tunneling. Experimental evidence, supported by quantum chemical calculations, indicates that the reverse barrier along the H loss potential energy curve for monomethylamine is 1.8 +/- 0.6 kJ mol(-1). Accurate dissociation onset energies are derived from the TOF simulation, and from this analysis we conclude that Delta(f)H degrees (298K)[CH(2)NH(2)(+)] = 750.4 +/- 1.3 kJ mol(-1) and Delta(f)H degrees (298K)[CH(2)NH(CH(3))(+)] = 710.9 +/- 2.8 kJ mol(-1). Quantum chemical calculations at the G3, G3B3, CBS-APNO and W1U levels are extensively used to support the experimental data. The comparison between experimental and ab initio isodesmic reaction heats also suggests that Delta(f)H degrees (298K)[N(CH(3))(3)] = -27.2 +/- 2 kJ mol(-1), and that the dimethylamine ionization energy is 8.32 +/- 0.03 eV, both of which are in slight disagreement with previous experimental values. Above 13 eV photon energy, additional dissociation channels appear besides the H atom loss, such as a sequential C(2)H(4) loss from trimethylamine for which a dissociation mechanism is proposed.     

Molecular alignment of ammonia studied by electron-ion-ion coincidence spectroscopy

Electron-ion-ion coincidence measurements carried out at discrete resonances near the N 1s threshold in ammonia are reported. The measured coincidence spectra show clear alignment of the molecule upon resonant core-electron excitation. The coincidence data are analyzed to extract information about the molecule in the excited state by simulating the alignment and the dissociation processes. Dynamic changes in molecular geometry are found as the photon energy is scanned through the N 1s-->4a(1) resonance, whereas for the N 1s-->2e state the geometry and kinetic energy released upon dissociation remain unchanged. The alignment of the core-excited molecules is found to be preserved even in two-step dissociation processes.     

Dissociation of cyclobutane,studied with HAM/3

The possibility to obtain unoccupied orbitals with correct energies in the new semi-empirical method HAM/3 implies new means to study frontier orbitals of chemical reactions. The transition state can often be identified as the state where the frontier orbitals are degenerate. The coplanar rectangular decomposition of cyclobutane into two ethylenes is chosen as a representative example. It is found that the activation energy is smaller than according to previous studies and thus not much different from the experimental value. It is pointed out that with a better parametrization of the HAM method it will be possible to study chemical reactions in detail and to calculate reliable activation energies.